CN118742677A - GaN substrate - Google Patents

Abstract

The present invention provides an SI substrate having a large resistivity even in a high temperature environment or an SI substrate having excellent semi-insulation and excellent crystal quality. The GaN substrate of the present invention is a GaN substrate doped with manganese, wherein the activation energy Ea of the carrier is 0.7 eV or more when the carrier concentration is expressed by the formula (I): carrier concentration (atoms/cm ³ ) = A×EXP(-Ea/kT), or a GaN substrate having a carrier mobility that is positively correlated with temperature.

Description

GaN substrate

Technical Field

The present invention relates to a gallium nitride (GaN) substrate, and more particularly to a semi-insulating GaN substrate used for high-frequency devices.

Background Art

In recent years, GaN-based high electron mobility transistors (HEMTs) (hereinafter sometimes referred to as "GaN-HEMTs") have been actively developed as devices for high-frequency applications ranging from tens of GHz to hundreds of GHz. GaN-HEMTs are formed by crystal growth of GaN on a substrate. As substrates for GaN-HEMTs, silicon, silicon carbide (SiC), GaN, etc. have been studied, but GaN-HEMTs using GaN substrates can be expected to have superior performance compared to those using other substrates. In addition, although GaN-HEMTs repeatedly turn on/off the current at a high frequency, it is ideal that the current does not flow at all in the OFF state. Therefore, the GaN substrate used for GaN-HEMTs is required to have a high resistance, and the GaN substrate is semi-insulated to form a semi-insulating substrate (SI substrate).

As a method for making a GaN substrate semi-insulating, a method of doping with transition metal elements is known. Transition metal elements act as acceptors, and therefore have the effect of compensating for background donors that are unintentionally introduced into the GaN substrate and reducing the carrier concentration in the GaN substrate. Such transition metal elements are called compensating impurities.

For example, in the invention described in Non-Patent Document 1, a GaN substrate doped with iron (Fe) and manganese (Mn) as compensation impurities is obtained.

Prior art literature

Non-patent literature

Non-patent literature 1: M Iwinska et al., "Iron and manganese as dopants used in the crystallization of highly resistive HVPE-GaN on native seeds", JapaneseJournal of Applied Physics, Volume 58 SC1047, 2019

Summary of the invention

Problems to be solved by the invention

In the past, the resistivity of SI substrates was discussed based on values near room temperature, but it is known that the resistivity decreases as the temperature increases. Non-patent document 1 also shows that the resistivity of a GaN substrate doped with Mn decreases as the temperature increases (Fig. 8. (a)).

In recent years, the application fields of high-frequency devices have been expanding, and with this, the opportunities for use in harsh areas such as high-temperature environments have also increased. Therefore, realizing high-frequency devices that can withstand high-temperature environments is one of the important issues for improving the reliability of operations in a wide range of application environments and the processing/controllability of a large amount of information.

Therefore, a first object of the present invention is to provide an SI substrate having a high resistivity even in a high temperature environment.

In addition, as mentioned above, the SI substrate is required to have excellent semi-insulating properties, and at the same time, it is also important to improve the crystal quality of the GaN single crystal that constitutes the SI substrate. In the case of poor crystal quality of the SI substrate, there is a risk of causing adverse effects when the GaN buffer layer is epitaxially grown on the SI substrate, or the performance of the device deteriorates due to defects in the SI substrate.

Therefore, a second object of the present invention is to provide an SI substrate having excellent semi-insulating properties and excellent crystal quality.

Solutions to the problem

The present inventors have conducted intensive studies on the above-mentioned problems.

Then, the present inventors found that the first problem can be solved by forming a GaN substrate from a crystal doped with Mn and having a large activation energy (Ea) of carriers, thereby completing the present invention.

In addition, the inventors have repeatedly conducted in-depth research on the second subject and found that when Mn is used as a compensating impurity, a GaN substrate with excellent semi-insulating properties can be obtained compared with the case where other compensating impurities are used. In addition, the inventors found that by focusing on the carrier mobility and carrier type of the Mn-doped GaN substrate, a Mn-doped GaN substrate with excellent crystal quality can be stably obtained, thereby completing the present invention.

That is, the gist of the present invention is as follows.

<1> A GaN substrate doped with manganese, wherein when the carrier concentration is represented by the following formula (I), the activation energy Ea of the carrier is 0.7 eV or more,

Carrier concentration (atoms/cm ³ ) = A×EXP (-Ea/kT) (I)

In formula (I), A is a proportionality constant, EXP is an exponential function, Ea is the activation energy of carriers (eV), k is the Boltzmann constant (8.617×10 ^-5 eV/K), and T is the temperature in Kelvin (K).

<2> The GaN substrate according to <1> above, wherein

The activation energy Ea of the above carriers is 0.7 to 1.2 eV.

<3> The GaN substrate according to <1> above, wherein

The activation energy Ea of the carrier is determined by the least square method, and the coefficient of determination ^R2 of the approximate curve when exponential approximation is performed by the least square method is 0.9 or more.

<4> The GaN substrate according to <1> above, wherein

The activation energy Ea of the carriers is obtained by fitting the carrier concentration measured by changing the temperature at intervals of 50 K in the temperature range of 450 K to 950 K to the above formula (I).

<5> A GaN substrate, which is a GaN substrate doped with manganese, wherein a carrier concentration of the GaN substrate at 900K is less than 1×10 ¹³ atoms/cm ³ .

<6> A GaN substrate having a resistivity of 5×10 ⁴ Ωcm or more at 900K.

<7> The GaN substrate according to <6>, which is a GaN substrate doped with manganese.

<8> The GaN substrate according to any one of <1> to <7> above,

The type of carrier is p-type.

<9> The GaN substrate according to any one of <1> to <8>, wherein the dislocation density is 1×10 ⁶ cm ^-2 or less.

<10> A GaN substrate doped with manganese, wherein carrier mobility of the GaN substrate is positively correlated with temperature.

<11> A GaN substrate doped with manganese, wherein a ratio μ _H /μ _L of a carrier mobility μ _H at 800 K to a carrier mobility μ _L at 650 K of the GaN substrate is 1 or more.

<12> A GaN substrate, which is a GaN substrate doped with manganese, wherein the type of carriers in the GaN substrate is p-type.

<13> A GaN substrate, which is a semi-insulating GaN substrate, wherein the carrier mobility of the GaN substrate at 900K is 50 cm ² /Vs or more.

<14> The GaN substrate according to <13>, which is a GaN substrate doped with manganese.

<15> The GaN substrate according to any one of <10> to <14>, wherein the dislocation density is 1×10 ⁶ cm ^-2 or less.

Effects of the Invention

The GaN substrate of the present invention has a superior crystal quality compared to the existing Mn-doped GaN substrate, and thus has a large activation energy of carriers. Therefore, the GaN substrate of the present invention can maintain high resistance even in a high temperature environment.

In addition, the GaN substrate of the present invention has a high specific resistance due to Mn doping, and the carrier mobility and carrier type show specific behaviors, which means that the crystal quality of the GaN single crystal constituting the GaN substrate is very excellent.

Therefore, the GaN substrate of the present invention is very suitable as a substrate used in a nitride semiconductor device having a horizontal device structure such as a GaN-HEMT.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a case where the (0001) surface of a GaN substrate according to an embodiment is divided into cells of 5 mm×5 mm by a square lattice.

FIG. 2 is a cross-sectional view showing a single crystal GaN substrate according to an embodiment.

FIG. 3 is a cross-sectional view showing a single crystal GaN substrate according to an embodiment.

FIG. 4 is a cross-sectional view showing the steps of explaining the method for manufacturing the GaN substrate according to the present embodiment.

FIG. 5 is a cross-sectional view showing the steps of explaining the method for manufacturing the GaN substrate according to the present embodiment.

FIG. 6 is a schematic diagram showing the basic configuration of an HVPE apparatus.

FIG. 7 is a graph showing the measurement results of the carrier concentration in the GaN substrate of Example 1-1.

FIG. 8 is a graph showing the measurement results of the resistivity of the GaN substrate of Example 1-1.

FIG. 9 is a graph showing the measurement results of carrier mobility in GaN substrates of Examples 2-1 and 2-2.

Explanation of symbols

1 Seed wafer

2GaN thick film

3c-plane GaN substrate seed

5c-surface GaN substrate seed

6GaN layer

10Mn doped GaN crystal

11(0001) Surface

20HVPE device

21 Reactor

22 Gallium storage battery

23 Base

24 First Heater

25 Second Heater

100Mn doped GaN substrate

101(0001) surface

102(000-1) Surface

110 First Area

120 Second Area

DETAILED DESCRIPTION

Hereinafter, the present invention will be described in detail, but the present invention is not limited to the following embodiments and can be implemented with various modifications within the scope of the gist of the invention.

In the present disclosure, the crystal axis parallel to the [0001] axis is referred to as the c-axis, the crystal axis parallel to the <10-10> axis is referred to as the m-axis, and the crystal axis parallel to the <11-20> axis is referred to as the a-axis. The crystal plane orthogonal to the c-axis is referred to as the c-plane, the crystal plane orthogonal to the m-axis is referred to as the m-plane, and the crystal plane orthogonal to the a-axis is referred to as the a-plane.

The Miller index (hkil) of the hexagonal crystal has the relationship of h+k=-i, and therefore, it is sometimes expressed as (hkl) using a three-digit number. For example, if (0004) is expressed as (004) using a three-digit number.

In this specification, when referring to a crystal axis, a crystal plane, a crystal orientation, etc., unless otherwise specified, they refer to the crystal axis, crystal plane, crystal orientation, etc. in a GaN substrate or a GaN layer, respectively.

In this specification, the Mn concentration and the C concentration at a specific position are values determined from respective detection amounts using secondary ion mass spectrometry (SIMS).

In this specification, unless otherwise specified, "(0001) surface" means "a surface whose inclination is 10 degrees or less (including 0 degrees) relative to the (0001) crystal plane". Unless otherwise specified, "(000-1) surface" means "a surface whose inclination is 10 degrees or less (including 0 degrees) relative to the (000-1) crystal plane".

In the present specification, when the expression "to" is used, it is used as an expression including the numerical values or physical property values before and after it. That is, "A to B" means A or more and B or less.

In addition, in this specification, "mass" and "weight" have the same meaning.

＜First Method＞

(GaN substrate)

The GaN substrate of the first embodiment (embodiment 1-1) in the first aspect is a GaN substrate doped with manganese (Mn), and when the carrier concentration is represented by the following formula (I), the activation energy Ea of the carrier is 0.7 or more.

Carrier concentration (atoms/cm ³ ) = A×EXP (-Ea/kT) (I)

(In formula (I), A is a proportional constant, EXP is an exponential function, Ea is the activation energy of the carrier (eV), k is the Boltzmann constant (8.617×10 ^-5 eV/K), and T is the temperature in Kelvin (K).)

When the GaN crystal is made highly resistive by doping with Mn and the activation energy Ea of the carriers of the GaN substrate is above 0.7 eV, there is a tendency for the carrier concentration to decrease at high temperatures. As a result, the increase in the carrier concentration of the GaN substrate of the first embodiment at high temperatures becomes smaller, and the resistivity in a high-temperature environment can be maintained at a high level.

The GaN substrate of the first embodiment in the first aspect has a Mn doped layer doped with Mn in the GaN substrate. The thickness of the Mn doped layer in the c-axis direction does not necessarily coincide with the thickness of the GaN substrate in the c-axis direction.

For example, the thickness of the GaN substrate as a whole is 400 μm, whereas the thickness of the Mn doped layer is sufficient as long as it is 100 μm on the surface of the GaN substrate. Of course, the thickness of the Mn doped layer is not limited to the above thickness, and the case where it is thicker or thinner than the above thickness is not excluded. In addition, the GaN substrate may be composed of the Mn doped layer as a whole in the thickness direction of the GaN substrate.

The Mn concentration of the GaN substrate is preferably 1.0×10 ¹⁶ atoms/cm ³ or more and 1.0×10 ²⁰ atoms/cm ³ or less. Mn as a compensating impurity contributes to high resistance. When the Mn concentration is 1.0×10 ¹⁶ atoms/cm ³ or more, the resistance of the GaN crystal can be increased. When the Mn concentration is 1.0×10 ²⁰ atoms/cm ³ or less, the crystal quality can be well maintained.

In addition, from the perspective of impurity energy levels, doping with Mn is superior to doping with other compensating impurities such as Fe (iron). This is because the energy level of Mn is deeper than that of Fe, so a larger energy is required to return the captured electrons to the conduction band.

Regarding the Mn concentration of the GaN substrate, the upper limit is preferably 6.0×10 ¹⁹ atoms/cm ³ or less, 5.0×10 ¹⁹ atoms/cm ³ or less, 3.0×10 ¹⁹ atoms/cm ³ or less, 1.0×10 ¹⁹ atoms/cm ³ or less, and 5.0×10 ¹⁸ atoms/cm ³ or less, and the lower limit is preferably 1.0×10 ¹⁶ atoms/cm ³ or more, 3.0×10 ¹⁶ atoms/cm ³ or more, 5.0×10 ¹⁶ atoms/cm ³ or more, 1.0×10 ¹⁷ atoms/cm ³ or more, 3.0×10 ¹⁷ atoms/cm ³ or more, and 5.0×10 ¹⁷ atoms/cm ³ or more.

As impurities that act as donors to GaN, O (oxygen), Si (silicon), S (sulfur), Ge (germanium), Sn (tin), etc. are known. Donor impurities hinder the effect of high resistance brought about by compensating impurities, so the total donor impurity concentration of the GaN substrate is preferably less than 5.0×10 ¹⁶ atoms/cm ^3. When the total donor impurity concentration is less than 5.0×10 ¹⁶ atoms/cm ³ , the resistivity of the GaN substrate is not easily affected by the fluctuation of the donor impurity concentration. In addition, even with a relatively low Mn concentration, a high resistivity can be achieved. It should be noted that the total donor impurity concentration refers to the sum of the concentrations of the donor impurities contained in the GaN substrate.

The total donor impurity concentration is preferably less than 4.0×10 ¹⁶ atoms/cm ³ , and more preferably less than 2.0×10 ¹⁶ atoms/cm ³ . The less the donor impurity concentration, the better, and therefore, the lower limit is not particularly limited.

As described later, the Mn-doped GaN crystal used to obtain the GaN substrate can be grown by hydride vapor phase epitaxy (HVPE), so even if O (oxygen) and Si (silicon) are not intentionally added to the GaN substrate, they are contained at a concentration of 10 ¹⁵ atoms/cm ³ or more. On the other hand, only when the above-mentioned donor impurities are intentionally doped, other donor impurities other than O and Si are contained in the GaN substrate at a non-negligible concentration. It should be noted that "intentional doping" refers to the case where the element as a target element is added as a raw material in the form of a simple substance or a compound in order to dope the GaN crystal.

Therefore, unless the GaN substrate is intentionally doped with donor impurities other than O and Si, the total donor impurity concentration of the GaN substrate can be considered to be equal to the sum of the O concentration and the Si concentration. Whether the GaN substrate is doped with donor impurities other than O and Si can be confirmed by elemental analysis or the like.

The GaN substrate may contain C (carbon) as one of the compensating impurities at a concentration not less than the lower limit of detection by secondary ion mass spectrometry (SIMS) (about 5×10 ¹⁵ atoms/cm ³ ), preferably less than 1×10 ¹⁷ atoms/cm ³ , and more preferably less than 5×10 ¹⁶ atoms/cm ³ .

Furthermore, the GaN substrate may contain compensating impurities other than Mn and C, such as Fe (iron), Co (cobalt), Ni (nickel), etc., as long as there is no problem in practical use.

In addition to the impurities mentioned above, the GaN substrate may further contain H (hydrogen), and the concentration thereof may be in the range of, for example, 10 ¹⁶ to 10 ¹⁷ atoms/cm ³ .

The threading dislocation density (hereinafter sometimes referred to as "dislocation density") of the GaN substrate is usually less than 1×10 ⁷ cm ^-2 , and from the viewpoint of crystallinity, is preferably 5×10 ⁶ cm ^-2 or less, more preferably 1×10 ⁶ cm ^-2 or less, further preferably 5×10 ⁵ cm ^-2 or less, and particularly preferably 1×10 ⁵ cm ^-2 or less.

In addition, when the (0001) surface of the GaN substrate is divided into 5 mm×5 mm units by a square lattice as shown in FIG1 , it is particularly preferred that at least one 100 μm×100 μm square region without threading dislocations exists in each 5 mm×5 mm unit.

The dislocation density can be adjusted by the GaN crystal growth method such as the vapor phase method or the liquid phase method, the crystal properties of the seed crystal substrate used to grow the GaN crystal, the crystal growth conditions, the selection of the growth surface, the impurity content, etc.

The dislocation density has the same meaning as the dark spot density based on the cathodoluminescence (CL) method.

In addition, there are three types of threading dislocations: edge, screw, and mixed, but in this specification, these are not distinguished and are collectively referred to as threading dislocations.

The presence or absence of threading dislocations in the GaN substrate and their density can be investigated by etching in 89% sulfuric acid heated to 270°C for 1 hour. The etch pits formed on the (0001) surface by the above etching correspond to threading dislocations, and their density is equivalent to the threading dislocation density. This can be confirmed by investigating the correspondence between the etch pits formed when etching a conductive GaN crystal grown by HVPE under the same conditions and the dark spots that appear in the cathode luminescence (CL) image.

The GaN substrate of the first embodiment in the first mode may be formed of the above-mentioned Mn-doped GaN crystal, or may be configured by stacking a seed layer and a Mn-doped layer. In the case of having a Mn-doped layer and a seed layer, as shown in FIG. 2 , the GaN substrate of the first embodiment includes a first region 110 and a second region 120 .

FIG. 2 shows a cross section of the Mn-doped GaN substrate 100 cut along a plane perpendicular to the (0001) surface 101 .

In the Mn-doped GaN substrate 100 , the first region 110 including the (0001) surface 101 is preferably formed of Mn-doped GaN crystal.

The Mn-doped GaN substrate 100 has a second region 120 on the (000-1) surface 102 side. The second region 120 may be formed of a GaN crystal having a room temperature resistivity of less than 1×10 ⁵ Ωcm (ie, a GaN crystal that is not semi-insulating).

The total concentration of compensating impurities in the second region 120 is generally lower than that in the first region 110. The second region 120 may have a region near a boundary with the first region 110 where the total concentration of compensating impurities increases stepwise or continuously as it approaches the first region 110.

The Mn-doped GaN substrate 100 having the first region 110 and the second region 120 may be manufactured by forming the second region 120 on the first region 110 by epitaxial growth, or by forming the first region 110 on the second region 120 by epitaxial growth.

The Mn-doped GaN substrate of the first embodiment in the first mode has a (0001) surface 101 of Ga polarity and a (000-1) surface 102 of N polarity.

When the Mn-doped GaN substrate has a seed layer and a Mn-doped layer, as shown in FIG. 2 , the surface of the first region 110 as the Mn-doped layer becomes a (0001) surface 101 , and the surface of the second region 120 as the seed layer becomes a (000-1) surface 102 .

When the Mn-doped GaN substrate is formed of a Mn-doped GaN crystal, as shown in FIG. 3 , one surface of the Mn-doped layer becomes a (0001) surface 101 , and the other surface becomes a (000-1) surface 102 .

The diameter of the Mn-doped GaN substrate is usually greater than 20 mm, and can be set to any size such as 25-27 mm (about 1 inch), 50-55 mm (about 2 inches), 100-105 mm (about 4 inches), 150-155 mm (about 6 inches).

The thickness t of the Mn-doped GaN substrate can be set to a value that does not make handling of the Mn-doped GaN substrate difficult according to the diameter. For example, when the diameter of the Mn-doped GaN substrate is about 2 inches, the thickness of the Mn-doped GaN substrate is preferably 250 to 500 μm, more preferably 300 to 450 μm.

When the Mn-doped GaN substrate has a structure in which a seed layer and a Mn-doped layer are stacked as shown in FIG. 2 , the thickness _t1 of the first region 110 as the Mn-doped layer is preferably 80 to 200 μm, more preferably 100 to 150 μm.

Of the two large-area surfaces of the Mn-doped GaN substrate, the (0001) surface 101 is used as the front surface for epitaxial growth of the nitride semiconductor layer. The (0001) surface 101 is mirror-finished, and its root mean square (RMS) roughness measured by AFM is usually less than 2 nm, preferably less than 1 nm, and more preferably less than 0.5 nm in a measurement range of 2 μm×2 μm.

The (000-1) surface 102 is the back surface, and therefore, can be subjected to mirror finish or matte (rough surface) finish.

The edges of the Mn-doped GaN substrate may be chamfered.

The Mn-doped GaN substrate may be provided with various markings as needed, such as an orientation flat or cutout for indicating the orientation of the crystal, an index flat for making it easy to distinguish the front and back surfaces, and the like.

The shape of the Mn-doped GaN substrate is not particularly limited. The shapes of the (0001) surface and the (000-1) surface may be disc-shaped, square, rectangular, hexagonal, octagonal, elliptical, etc., or may be irregular shapes.

When the carrier concentration of the Mn-doped GaN substrate of the first embodiment in the first aspect is represented by the following formula (I), the activation energy Ea of the carriers is in the range of 0.7 eV or more, preferably in the range of 0.7 to 1.2 eV.

Carrier concentration (atoms/cm ³ ) = A×EXP (-Ea/kT) (I)

(In formula (I), A is a proportional constant, EXP is an exponential function, Ea is the activation energy of the carrier (eV), k is the Boltzmann constant (8.617×10 ^-5 eV/K), and T is the temperature in Kelvin (K).)

By making the activation energy Ea of the carrier within the above range, the decrease of the specific resistance at high temperature can be suppressed, and a GaN substrate with high resistance maintained in a high temperature environment can be obtained. From the same point of view, the activation energy Ea of the carrier is preferably 0.8 eV or more, more preferably 0.9 eV or more, and preferably 1.1 eV or less, more preferably 1.0 eV or less.

The activation energy Ea of the carriers is determined by the least square method based on the scatter diagram of the carrier concentration and 1/T. The temperature range for determining the activation energy Ea of the carriers is not particularly limited. From the perspective of data reliability, it is preferred to set the temperature range to a range above 450°C to determine the above Ea. Since the resistivity of the manganese-doped substrate is very high, it is difficult to accurately measure it near room temperature. However, when it reaches above 450°C, the resistivity decreases to a measurable region.

In addition, it is desirable that the coefficient of determination ^R2 of the approximation curve when performing exponential approximation by the least square method is 0.9 or more. If it is less than this number, the value of the activation energy Ea becomes unreliable.

Specifically, the activation energy Ea of the carriers can be obtained by fitting the carrier concentration measured by varying the temperature at intervals of 50 K in the temperature range of 450 K to 950 K to the above formula (I) by the least square method.

The inventors of the present invention have repeatedly conducted research, focusing on the relationship between the activation energy of carriers and the resistivity at high temperatures, and have come up with the possibility of obtaining a GaN substrate that maintains high resistance even in a high-temperature environment by controlling the activation energy of carriers to a specific range. Here, it can be considered that the value of the activation energy of carriers in GaN crystals is affected by the quality of the GaN crystals. For example, in GaN crystals with poor crystal quality, there are multiple carrier capture energy levels due to defects in the crystals, etc., and a large carrier activation energy cannot be achieved. In particular, since the existing GaN crystals doped with Mn are not ideal in terms of crystal quality, it can be considered that a Mn-doped GaN substrate with a large carrier activation energy cannot be obtained. As described later, the inventors of the present invention successfully obtained a GaN crystal with excellent crystal quality by reconsidering the manufacturing method of Mn-doped GaN crystals, etc. As a result, a Mn-doped GaN substrate with a large carrier activation energy is achieved.

The carrier concentration of the Mn-doped GaN substrate of the first embodiment in the first aspect at 900 K is preferably less than 1×10 ¹³ atoms/cm ³ . When the carrier concentration at 900 K is within the above range, the resistivity is inversely proportional to the carrier concentration, so a sufficiently large resistivity can be obtained even in a high temperature environment.

The carrier concentration at 900 K is more preferably 5×10 ¹² atoms/cm ³ or less, further preferably 2×10 ¹² atoms/cm ³ or less, and particularly preferably 1×10 ¹² atoms/cm ³ or less. From the viewpoint of resistivity, the lower the carrier concentration at 900 K, the better. Therefore, the lower limit is not particularly limited.

In addition, it can be considered that the carrier concentration is also affected by the crystal quality of the GaN crystal. According to the research of the inventors, the carrier concentration at a high temperature such as 900K tends to reflect the influence of the quality of the GaN crystal more strongly than the carrier concentration near room temperature. It can be considered that by improving the crystal quality of the Mn-doped GaN crystal, a GaN substrate with a small carrier concentration at 900K can be obtained.

It should be noted that the carrier concentration, specific resistance, and carrier type can be determined using Hall effect measurement.

The Mn-doped GaN substrate of the first embodiment in the first aspect preferably has a resistivity of 5×10 ⁴ Ωcm or more at 900 K. When the resistivity at 900 K is within the above range, the resistivity does not decrease even when used in a high temperature environment, and the substrate can operate in a wide range of application environments.

The specific resistance at 900K is more preferably 1×10 ⁵ Ωcm or more, further preferably 2×10 ⁵ Ωcm or more, particularly preferably 5×10 ⁵ Ωcm or more. The higher the specific resistance, the more preferred. Therefore, the upper limit is not particularly limited.

The Mn-doped GaN substrate of the first embodiment of the first mode preferably has a p-type carrier type. In the case of poor crystal quality of the GaN substrate, even if doped with Mn, it cannot fully compensate for the electrons caused by unexpected impurities such as Si and O, and there is a tendency to become n-type. Therefore, the p-type carrier type means that the crystallinity of the GaN single crystal constituting the GaN substrate is good.

The GaN substrate of the second embodiment (Embodiment 1-2) in the first aspect is a GaN substrate doped with manganese, and its carrier concentration at 900K is less than 1×10 ¹³ atoms/cm ³ .

As in the first embodiment, the GaN crystal is made more resistive by doping with Mn. Furthermore, when the carrier concentration of the GaN substrate at 900K is less than 1×10 ¹³ atoms/cm ³ , the increase in the carrier concentration of the Mn-doped GaN substrate at high temperature in the second embodiment becomes smaller, and the resistivity in a high temperature environment can be maintained at a high level.

The GaN substrate of the second embodiment in the first aspect may be a substrate composed of only a Mn doped layer, or may be a substrate in which a seed layer and a Mn doped layer are stacked, similarly to the first embodiment.

The characteristics and features of the Mn-doped GaN substrate of the second embodiment in the first aspect are the same as the characteristics and features of the Mn-doped GaN substrate of the first embodiment.

The Mn-doped GaN substrate of the second embodiment of the first aspect is characterized in that the carrier concentration at 900 K is less than 1×10 ¹³ atoms/cm ^3. When the carrier concentration of the Mn-doped GaN substrate at 900 K is less than 1×10 ¹³ atoms/cm ³ , the resistivity is inversely proportional to the carrier concentration, and therefore a sufficiently large resistivity can be obtained even in a high temperature environment.

The carrier concentration at 900 K is preferably 5×10 ¹² atoms/cm ³ or less, more preferably 2×10 ¹² atoms/cm ³ or less, and further preferably 1×10 ¹² atoms/cm ³ or less. From the viewpoint of resistivity, the lower the carrier concentration at 900 K, the better. Therefore, the lower limit is not particularly limited.

In addition, it can be considered that the carrier concentration is also affected by the crystal quality of the GaN crystal. According to the research of the inventors, the carrier concentration at a high temperature such as 900K tends to reflect the influence of the quality of the GaN crystal more strongly than the carrier concentration near room temperature. It can be considered that by improving the crystal quality of the Mn-doped GaN crystal, a GaN substrate with a small carrier concentration at 900K can be obtained.

The layer composition, size, shape, activation energy of carriers, specific resistance, etc. of the Mn-doped GaN substrate of the second embodiment in the first mode are respectively the same as those of the first embodiment.

The GaN substrate of the third embodiment (Embodiment 1-3) in the first aspect has a resistivity of 5×10 ⁴ Ωcm or more at 900 K. When the GaN substrate has a resistivity of 5×10 ⁴ Ωcm or more at 900 K, it can maintain high resistance in a high temperature environment and is therefore suitable for use in high-frequency devices.

From the viewpoint of increasing resistance, the resistivity of the GaN substrate at 900 K is preferably 1×10 ⁵ Ωcm or more, more preferably 2×10 ⁵ Ωcm or more, and further preferably 5×10 ⁵ Ωcm or more. The higher the resistivity, the better, and therefore the upper limit is not particularly limited.

The GaN substrate of the third embodiment in the first aspect may be a substrate composed of a layer consisting of GaN crystal alone, or may be a substrate composed of a stack of a seed crystal layer and a layer consisting of GaN crystal.

In order to make the resistivity of a GaN substrate at 900K be 5×10 ⁴ Ωcm or more, for example, there is a method of doping a GaN crystal with Mn.

Among them, the GaN substrate of the third embodiment in the first aspect preferably has a Mn doping layer doped with Mn. By having the Mn doping layer, the resistance of the GaN crystal is increased, and a desired specific resistance can be obtained.

The characteristics and features of the Mn-doped GaN crystal in the GaN substrate of the third embodiment in the first method are the same as the characteristics and features of the Mn-doped GaN crystal in the first embodiment. In addition, the layer composition, size, shape, carrier activation energy, resistivity, etc. of the GaN substrate of the third embodiment are the same as the layer composition, size, shape, carrier activation energy, resistivity, etc. in the first embodiment.

＜Method 2＞

(GaN substrate)

The GaN substrate of the first embodiment (Embodiment 2-1) in the second aspect is a GaN substrate doped with manganese (Mn), and its carrier mobility is positively correlated with temperature.

By doping with Mn, the GaN crystal has a high resistance. At the same time, the carrier mobility of the GaN substrate is positively correlated with the temperature, indicating that the GaN single crystal constituting the GaN substrate has very few microscopic defects inside and excellent crystal quality. Therefore, the GaN substrate of the first embodiment in the second mode can achieve both high specific resistance and excellent crystal quality.

The GaN substrate of the first embodiment in the second mode has a Mn-doped layer doped with Mn in the GaN substrate. The thickness of the Mn-doped layer in the c-axis direction is not necessarily consistent with the thickness of the GaN substrate in the c-axis direction. For example, the overall thickness of the GaN substrate is 400μm, whereas the thickness of the Mn-doped layer is sufficient as long as it is 100μm on the surface of the GaN substrate. Of course, the thickness of the Mn-doped layer is not limited to the above thickness, and does not exclude the case where it is thicker or thinner than the above thickness. In addition, it may also be a GaN substrate in which the entire thickness direction of the GaN substrate is composed of a Mn-doped layer.

The Mn concentration of the GaN substrate is preferably 1.0×10 ¹⁶ atoms/cm ³ or more and 1.0×10 ²⁰ atoms/cm ³ or less. Mn as a compensating impurity contributes to high resistance. When the Mn concentration is 1.0×10 ¹⁶ atoms/cm ³ or more, the resistance of the GaN crystal can be increased. When the Mn concentration is 1.0×10 ²⁰ atoms/cm ³ or less, the crystal quality can be well maintained.

In addition, from the perspective of impurity energy levels, doping with Mn is superior to doping with other compensating impurities such as Fe (iron). This is because the energy level of Mn is deeper than that of Fe, so a larger energy is required to return the captured electrons to the conduction band.

Regarding the Mn concentration of the GaN substrate, the upper limit is preferably 6.0×10 ¹⁹ atoms/cm ³ or less, 5.0×10 ¹⁹ atoms/cm ³ or less, 3.0×10 ¹⁹ atoms/cm ³ or less, 1.0×10 ¹⁹ atoms/cm ³ or less, and 5.0×10 ¹⁸ atoms/cm ³ or less, and the lower limit is preferably 1.0×10 ¹⁶ atoms/cm ³ or more, 3.0×10 ¹⁶ atoms/cm ³ or more, 5.0×10 ¹⁶ atoms/cm ³ or more, 1.0×10 ¹⁷ atoms/cm ³ or more, 3.0×10 ¹⁷ atoms/cm ³ or more, and 5.0×10 ¹⁷ atoms/cm ³ or more.

As impurities that act as donors to GaN, O (oxygen), Si (silicon), S (sulfur), Ge (germanium), Sn (tin), etc. are known. Donor impurities will hinder the effect of high resistance brought about by compensating impurities, so it is preferred that the total donor impurity concentration of the GaN substrate is less than 5.0×10 ¹⁶ atoms/cm ^3. When the total donor impurity concentration is less than 5.0×10 ¹⁶ atoms/cm ³ , the resistivity of the GaN substrate is not easily affected by the fluctuation of the donor impurity concentration. In addition, even with a relatively low Mn concentration, a high resistivity can be achieved. It should be noted that the total donor impurity concentration refers to the sum of the concentrations of the donor impurities contained in the GaN substrate.

The total donor impurity concentration is preferably less than 4.0×10 ¹⁶ atoms/cm ³ , and more preferably less than 2.0×10 ¹⁶ atoms/cm ³ . The less the donor impurity concentration, the better, and therefore, the lower limit is not particularly limited.

As described later, the Mn-doped GaN crystal used to obtain the GaN substrate can be grown by hydride vapor phase epitaxy (HVPE), so even if O (oxygen) and Si (silicon) are not intentionally added to the GaN substrate, they are contained at a concentration of 10 ¹⁵ atoms/cm ³ or more. On the other hand, only when the above-mentioned donor impurities are intentionally doped, other donor impurities other than O and Si are contained in the GaN substrate at a non-negligible concentration. It should be noted that "intentional doping" refers to the case where the element as a target element is added as a raw material in the form of a simple substance or a compound in order to dope the GaN crystal.

Therefore, unless the GaN substrate is intentionally doped with donor impurities other than O and Si, the total donor impurity concentration of the GaN substrate can be considered to be equal to the sum of the O concentration and the Si concentration. Whether the GaN substrate is doped with donor impurities other than O and Si can be confirmed by elemental analysis or the like.

The GaN substrate may contain C (carbon) as one of the compensating impurities at a concentration not less than the lower limit of detection by secondary ion mass spectrometry (SIMS) (about 5×10 ¹⁵ atoms/cm ³ ), preferably less than 1×10 ¹⁷ atoms/cm ³ , and more preferably less than 5×10 ¹⁶ atoms/cm ³ .

Furthermore, the GaN substrate may contain compensating impurities other than Mn and C, such as Fe (iron), Co (cobalt), Ni (nickel), etc., as long as there is no problem in practical use.

In addition to the impurities mentioned above, the GaN substrate may further contain H (hydrogen), and the concentration thereof may be in the range of, for example, 10 ¹⁶ to 10 ¹⁷ atoms/cm ³ .

The threading dislocation density (hereinafter sometimes referred to as "dislocation density") of the GaN substrate is usually less than 1×10 ⁷ cm ^-2 , and from the viewpoint of crystallinity, is preferably 5×10 ⁶ cm ^-2 or less, more preferably 1×10 ⁶ cm ^-2 or less, further preferably 5×10 ⁵ cm ^-2 or less, and particularly preferably 1×10 ⁵ cm ^-2 or less.

In addition, when the (0001) surface of the GaN substrate is divided into 5 mm×5 mm units by a square lattice as shown in FIG1 , it is particularly preferred that at least one 100 μm×100 μm square region without threading dislocations exists in each 5 mm×5 mm unit.

The dislocation density can be adjusted by the GaN crystal growth method such as the vapor phase method or the liquid phase method, the crystal properties of the seed crystal substrate used to grow the GaN crystal, the crystal growth conditions, the selection of the growth surface, the impurity content, etc.

The dislocation density has the same meaning as the dark spot density based on the cathodoluminescence (CL) method.

In addition, there are three types of threading dislocations: edge, screw, and mixed, but they are not distinguished in this specification and are collectively referred to as threading dislocations.

The presence or absence of threading dislocations in the GaN substrate and their density can be investigated by etching in 89% sulfuric acid heated to 270°C for 1 hour. The etch pits formed on the (0001) surface by the above etching correspond to threading dislocations, and their density is equivalent to the threading dislocation density. This can be confirmed by investigating the correspondence between the etch pits formed when etching a conductive GaN crystal grown by HVPE under the same conditions and the dark spots that appear in the cathode luminescence (CL) image.

The GaN substrate of the first embodiment in the second embodiment may be formed of the above-mentioned Mn-doped GaN crystal, or may be configured to have a seed layer and a Mn-doped layer stacked. In the case of having a Mn-doped layer and a seed layer, as shown in FIG. 2 , the GaN substrate of the first embodiment in the second embodiment includes a first region 110 and a second region 120.

FIG. 2 shows a cross section of the Mn-doped GaN substrate 100 cut along a plane perpendicular to the (0001) surface 101 .

In the Mn-doped GaN substrate 100 , the first region 110 including the (0001) surface 101 is preferably formed of Mn-doped GaN crystals.

The Mn-doped GaN substrate 100 has a second region 120 on the (000-1) surface 102 side. The second region 120 may be formed of a GaN crystal having a room temperature resistivity of less than 1×10 ⁵ Ωcm (ie, a GaN crystal that is not semi-insulating).

The total concentration of compensating impurities in the second region 120 is generally lower than that in the first region 110. The second region 120 may have a region near a boundary with the first region 110 where the total concentration of compensating impurities increases stepwise or continuously as it approaches the first region 110.

The Mn-doped GaN substrate 100 having the first region 110 and the second region 120 may be manufactured by forming the second region 120 on the first region 110 by epitaxial growth, or by forming the first region 110 on the second region 120 by epitaxial growth.

The Mn-doped GaN substrate of the first embodiment in the second aspect has a (0001) surface 101 of Ga polarity and a (000-1) surface 102 of N polarity.

When the Mn-doped GaN substrate has a seed layer and a Mn-doped layer, as shown in FIG. 2 , the surface of the first region 110 as the Mn-doped layer becomes a (0001) surface 101 , and the surface of the second region 120 as the seed layer becomes a (000-1) surface 102 .

When the Mn-doped GaN substrate is formed of a Mn-doped GaN crystal, as shown in FIG. 3 , one surface of the Mn-doped layer becomes a (0001) surface 101 , and the other surface becomes a (000-1) surface 102 .

The diameter of the Mn-doped GaN substrate is usually greater than 20 mm, and can be set to any size such as 25-27 mm (about 1 inch), 50-55 mm (about 2 inches), 100-105 mm (about 4 inches), 150-155 mm (about 6 inches).

The thickness t of the Mn-doped GaN substrate can be set to a value that does not make handling of the Mn-doped GaN substrate difficult according to the diameter. For example, when the diameter of the Mn-doped GaN substrate is about 2 inches, the thickness of the Mn-doped GaN substrate is preferably 250 to 500 μm, more preferably 300 to 450 μm.

When the Mn-doped GaN substrate has a structure in which a seed layer and a Mn-doped layer are stacked as shown in FIG. 2 , the thickness _t1 of the first region 110 as the Mn-doped layer is preferably 80 to 200 μm, more preferably 100 to 150 μm.

Of the two large-area surfaces of the Mn-doped GaN substrate, the (0001) surface 101 is used as the front surface for epitaxial growth of the nitride semiconductor layer. The (0001) surface 101 is mirror-finished, and its root mean square (RMS) roughness measured by AFM is usually less than 2 nm, preferably less than 1 nm, and more preferably less than 0.5 nm in a measurement range of 2 μm×2 μm.

The (000-1) surface 102 is the back surface, and therefore, can be subjected to mirror finish or matte (rough surface) finish.

The edges of the Mn-doped GaN substrate may be chamfered.

The Mn-doped GaN substrate may be provided with various markings as needed, such as an orientation flat or cutout for indicating the orientation of the crystal, an index flat for easily distinguishing the front and back surfaces, and the like.

The shape of the Mn-doped GaN substrate is not particularly limited. The shapes of the (0001) surface and the (000-1) surface may be disc-shaped, square, rectangular, hexagonal, octagonal, elliptical, etc., or may be irregular shapes.

The carrier mobility of the Mn-doped GaN substrate of the first embodiment in the second mode is positively correlated with temperature. In general, except for the low temperature region below 300K, the carrier mobility of the semiconductor decreases as the temperature rises. That is, the carrier mobility is negatively correlated with the temperature. The behavior of this carrier mobility can be explained as follows: the higher the temperature, the greater the lattice vibration of the atoms constituting the crystal, and the carriers are scattered and hindered from moving. On the other hand, in the high temperature region above 300K, the mechanism of the positive correlation between the carrier mobility and the temperature has not been theoretically identified. It is very rare for GaN crystals to have a positive correlation at such a high temperature above 650K. In addition, the absolute value of the carrier mobility is surprising. Unexpectedly, the carrier mobility of the GaN crystal doped with the acceptor has a particularly high value of more than ^50cm2 /Vs even at 900K. As mentioned above, at high temperatures, carriers are affected by lattice scattering. In addition, if there are various defects such as Ga vacancies, carriers will be captured and the mobility will be reduced. The higher the temperature, the greater the mobility is, indicating that the carriers have a positive effect that outweighs the negative effects of lattice scattering and trapping.

Therefore, the carrier mobility is positively correlated with temperature and has a large absolute value, which becomes an indicator for judging the quality of the crystal. As an indicator of the quality of the crystal of the existing known GaN single crystal, for example, the dislocation density can be cited. However, it does not necessarily mean that the lower the dislocation density, the better the operation of all GaN devices. This is because, for example, in horizontal electronic devices, carriers move vertically relative to dislocations.

As described above, since the temperature dependence of carrier mobility more keenly reflects the number of microscopic defects in the GaN crystal of the semi-insulating substrate, etc., by focusing on the temperature dependence of carrier mobility, GaN substrates with excellent crystal quality can be stably manufactured.

The carrier mobility is determined by the Hall effect measurement. In addition, the correlation of the carrier mobility with respect to the temperature can be obtained by measuring the carrier mobility each time the sample temperature is changed. The temperature range when determining the correlation of the carrier mobility with respect to the temperature is not particularly limited. From the perspective of the reliability of the data, the temperature range is preferably determined to be a range of more than 650K. This is because the specific resistance of the manganese-doped substrate is very high, and therefore, it is difficult to accurately measure near room temperature. However, when reaching more than 650K, the specific resistance will be reduced to a measurable region.

The above-mentioned Mn-doped GaN substrate having a positive correlation between carrier mobility and temperature can be obtained by, for example, a method for producing a Mn-doped GaN crystal to be described later.

In the second aspect, the ratio μ _H / μ _L of the carrier mobility μ _H at 800 K to the carrier mobility μ _L at 650 K of the Mn-doped GaN substrate of the first embodiment is preferably greater than 1. When the ratio μ _H / μ _L of the carrier mobility μ _H at 800 K to the carrier mobility μ _L at 650 K is within the above range, the crystal quality of the GaN substrate is excellent and is suitable as an SI substrate for GaN-HEMT.

The ratio μ _H /μ _L of the carrier mobility μ _H at 800 K to the carrier mobility μ _L at 650 K is more preferably 5 or more, further preferably 7 or more, particularly preferably 10 or more, and the upper limit is not particularly limited but is usually 100 or less.

The carrier mobility of the Mn-doped GaN substrate of the first embodiment in the second aspect at 900 K is preferably 50 cm ² /Vs or more. When the carrier mobility at 900 K is within the above range, the crystal quality of the GaN substrate is excellent and it is suitable as an SI substrate for GaN-HEMT. The carrier mobility at 900 K is more preferably 70 cm ² /Vs or more, further preferably 80 cm ² /Vs or more, and particularly preferably 100 cm ² /Vs or more. The upper limit is not particularly limited, but is usually 10000 cm ² /Vs or less.

The Mn-doped GaN substrate of the first embodiment in the second mode preferably has a p-type carrier type. In the case of poor crystal quality of the GaN substrate, even if doped with Mn, it cannot fully compensate for the electrons caused by unexpected impurities such as Si and O, and there is a tendency to become n-type. Therefore, the p-type carrier type means that the crystallinity of the GaN single crystal constituting the GaN substrate is good. It should be noted that the type of carrier can be experimentally determined using Hall effect measurement.

The Mn-doped GaN substrate of the first embodiment in the second aspect preferably has a resistivity of 5×10 ⁴ Ωcm or more at 900 K. When the resistivity at 900 K is within the above range, the resistivity does not decrease even when used in a high temperature environment, and the substrate can operate in a wide range of application environments.

The specific resistance at 900K is more preferably 1×10 ⁵ Ωcm or more, further preferably 2×10 ⁵ Ωcm or more, particularly preferably 5×10 ⁵ Ωcm or more. The higher the specific resistance, the more preferred. Therefore, the upper limit is not particularly limited.

The GaN substrate of the second embodiment (embodiment 2-2) in the second aspect is a GaN substrate doped with manganese, and the ratio of its carrier mobility _μH at 800K to its carrier mobility _μL at 650K _μH / _μL is 1 or more.

The GaN substrate of the second embodiment in the second mode is similar to the first embodiment, and the GaN crystal is made high-resistance by doping with Mn. Furthermore, when the ratio _μH / _μL of the carrier mobility μH at 800K to the carrier mobility _μL at _650K is 1 or more, the crystal quality of the GaN substrate is excellent for the Mn-doped GaN substrate of the second embodiment, and it is suitable as an SI substrate for GaN-HEMT.

The GaN substrate of the second embodiment in the second aspect may be a substrate composed of only a Mn doped layer, or may be a substrate in which a seed layer and a Mn doped layer are stacked, similarly to the first embodiment.

The characteristics and features of the Mn-doped GaN substrate of the second embodiment in the second aspect are the same as the characteristics and features of the Mn-doped GaN substrate of the first embodiment.

The Mn-doped GaN substrate of the second embodiment in the second mode is characterized in that a ratio μ _H / μ L of the carrier mobility μ _H at 800 K to the carrier mobility μ _L at 650 K is greater than 1. When the ratio μ _H / μ _L of the carrier mobility μ _H at 800 K to the carrier mobility μ _L at 650 _K is within the above range, the crystal quality of the GaN substrate is excellent and it is suitable as an SI substrate for GaN-HEMT.

The ratio μ _H /μ _L of the carrier mobility μ _H at 800 K to the carrier mobility μ _L at 650 K is more preferably 5 or more, further preferably 7 or more, particularly preferably 10 or more, and the upper limit is not particularly limited but is usually 100 or less.

Generally speaking, except for the low temperature region below 300K, the carrier mobility of the GaN substrate doped with compensating impurities decreases as the temperature rises. Therefore, the ratio μ _H / μ _L of the carrier mobility μ _H at 800K and the carrier mobility μ _L at 650K becomes less than 1. The behavior of this carrier mobility can be explained as follows: the higher the temperature, the greater the lattice vibration of the crystal constituent atoms, and the carriers are scattered and hindered from moving. On the other hand, the reason why the ratio μ _H / μ _L of the carrier mobility μ _H at 800K and the carrier mobility μ _L at 650K reaches more than 1 has not been found out in theory. It is very rare that the ratio μ _H / μ _L of the carrier mobility μ _H at 800K and the carrier mobility μ _L at 650K of the GaN crystal reaches more than 1. In addition, it is surprising that the absolute value of the mobility. Surprisingly, the acceptor-doped GaN crystal has a particularly high carrier mobility of more than 50 ^cm2 /Vs even at 900K. As mentioned above, at high temperatures, carriers are affected by lattice scattering. In addition, if there are various defects such as Ga vacancies, carriers will be trapped and mobility will decrease. The higher the temperature, the greater the mobility, indicating that the carriers have a positive effect that exceeds the negative effects of lattice scattering and capture.

Therefore, the ratio of the carrier mobility _μH at 800K to the carrier mobility _μL at 650K, _μH / _μL, reaches 1 or more, and the carrier mobility has a large absolute value, which becomes an indicator for judging the quality of the crystal. As an indicator of the quality of the crystal of the existing known GaN single crystal, for example, the dislocation density can be cited. However, it is not necessarily the case that the lower the dislocation density, the better the operation of all GaN devices. This is because, for example, in horizontal electronic devices, carriers move vertically relative to dislocations.

As described above, since the ratio μ _H /μ _L of the carrier mobility μ _H at 800 K to the carrier mobility μ L at 650 K more keenly reflects the number of microscopic defects in the GaN crystal of the semi-insulating substrate, etc., by focusing on the ratio μ _H /μ _L of the carrier mobility μ _H at 800 K to the carrier mobility μ _L at 650 _K , it is possible to stably manufacture GaN substrates with excellent crystal quality.

The layer composition, size, shape, carrier mobility and its temperature dependence, carrier type, resistivity, etc. of the Mn-doped GaN substrate of the second embodiment in the second method are respectively the same as the layer composition, size, shape, carrier mobility and its temperature dependence, carrier type, resistivity, etc. in the first embodiment.

In order to make the ratio μ _H /μ _L of the carrier mobility μ _H of the GaN substrate at 800K and the carrier mobility μ _L at 650K greater than 1, for example, a method of growing a Mn-doped GaN crystal to obtain a GaN substrate by the method described below can be cited.

The GaN substrate of the third embodiment (Embodiment 2-3) in the second aspect is a GaN substrate doped with manganese, and the type of carriers thereof is p-type.

The GaN substrate of the third embodiment in the second mode is similar to the first embodiment, in which the GaN crystal is made high-resistance by doping with Mn. Furthermore, when the type of carrier is p-type, the Mn-doped GaN substrate of the third embodiment has excellent crystal quality of the GaN substrate and is suitable as an SI substrate for GaN-HEMT.

In the case of poor crystal quality of Mn-doped GaN substrates, there is a tendency for the type of carriers to be n-type. It can be considered that this is because even if Mn is doped, the electrons caused by unexpected impurities such as Si and O cannot be fully supplemented. Therefore, the type of carriers being p-type means that the crystallinity of the GaN single crystal constituting the GaN substrate is good. As an indicator of the quality of the crystal of the existing known GaN single crystal, for example, dislocation density can be cited, but the dislocation density does not necessarily reflect all the microscopic defects in the crystal. In particular, when judging the quality of semi-insulating substrates, it is not necessarily corresponding. Semi-insulation refers to electrical properties. However, when the dislocation density reaches a certain value or less, it has no effect on the electrical properties. On the other hand, the type of carriers more keenly reflects the electrical quality of the semi-insulating GaN crystal. Therefore, by focusing on the type of carriers, GaN substrates with excellent crystal quality can be stably manufactured.

It should be noted that the type of carriers can be experimentally determined using Hall effect measurement.

The layer composition, size, shape, carrier mobility and temperature dependence of the Mn-doped GaN substrate of the third embodiment in the second mode are respectively the same as those of the first embodiment.

The GaN substrate of the third embodiment in the second aspect may be a substrate composed of only a Mn doped layer, or a substrate in which a seed layer and a Mn doped layer are stacked, similarly to the first and second embodiments.

The characteristics and features of the Mn-doped GaN substrate of the third embodiment in the second aspect are the same as the characteristics and features of the Mn-doped GaN substrate of the first embodiment or the second embodiment.

The GaN substrate of the fourth embodiment (Embodiment 2-4) in the second aspect is a semi-insulating GaN substrate having a carrier mobility of 50 cm ² /Vs or more at 900 K. When the carrier mobility of the GaN substrate at 900 K is 50 cm ² /Vs or more, the crystal quality of the GaN substrate is excellent and it is suitable as an SI substrate for GaN-HEMT.

The carrier mobility of the GaN substrate at 900K is preferably 70 cm ² /Vs or more, further preferably 80 cm ² /Vs or more, and particularly preferably 100 cm 2 /Vs or more, from the viewpoint that the larger the carrier mobility, the better the crystal quality. The upper limit is not particularly limited, but is preferably 10000 ^{cm 2 /} Vs or less.

In order to make the carrier mobility of the GaN substrate at 900K 50 cm ² /Vs or more, for example, there is a method of growing a Mn-doped GaN crystal to obtain a GaN substrate by the method described later.

The GaN substrate of the fourth embodiment in the second mode is a semi-insulating substrate. That is, the resistivity at room temperature (300K) is 1×10 ⁵ Ωcm or more. In addition, the resistivity at 900K is preferably 1×10 ⁴ Ωcm or more. When the resistivity at 900K is within the above range, the resistivity will not decrease even when used in a high temperature environment, and it can operate in a wide range of application environments. The resistivity at 900K is more preferably 5×10 ⁴ Ωcm or more, and further preferably 1×10 ⁵ Ωcm or more. In addition, the higher the resistivity, the better, so the upper limit is not particularly limited.

The GaN substrate of the fourth embodiment in the second aspect may be a substrate composed only of a layer formed of GaN crystal, or may be a substrate in which a seed layer and a layer formed of GaN crystal are stacked.

Among them, the GaN substrate of the fourth embodiment in the second aspect is preferably a GaN substrate doped with Mn. By using a Mn-doped GaN substrate, it is possible to achieve both high resistance and excellent crystal quality of the GaN substrate.

The characteristics and features of the Mn-doped GaN crystal in the GaN substrate of the fourth embodiment in the second aspect are the same as the characteristics and features of the Mn-doped GaN crystal in the first embodiment.

(Method for manufacturing GaN substrate)

The GaN substrates of the first to third embodiments (embodiments 1-1 to 1-3) in the first embodiment and the GaN substrates of the first to fourth embodiments (embodiments 2-1 to 2-4) in the second embodiment can be produced by, for example, a vapor phase growth method or a liquid phase growth method.

As a vapor phase growth method, for example, HVPE method can be cited, and as a liquid phase growth method, for example, acidic ammonothermal method, alkaline ammonothermal method, Na flux method, etc. can be cited. Among them, from the viewpoint of ease of production, HVPE method is preferred, and from the viewpoint of mass production and improvement of crystal quality, ammonothermal method is preferred.

Hereinafter, the method for manufacturing a GaN substrate according to the present embodiment will be described by taking the HVPE method as an example.

The manufacturing method of the GaN substrate of this embodiment based on the HVPE method preferably has the following first step (i). This method can be applied to the manufacturing of a GaN substrate having a region on the N polarity side and a region on the Ga polarity side sandwiching a regrown interface, and more preferably improves the resistivity at least in a portion on the Ga polarity side.

(i) In the first step, a GaN film is formed by growing an oriented thick GaN film on a c-plane GaN substrate seed crystal by the HVPE method (0001).

It should be noted that, in this specification, “on a substrate” and “on the surface of a substrate” have the same meaning.

Hereinafter, the above-mentioned first step will be described in more detail.

In the first step, as shown in (a) and (b) of FIG. 4 , a (0001) oriented GaN layer 6 is grown on the Ga polarity plane of the c-plane GaN substrate seed crystal 5 by HVPE to obtain a GaN substrate as a stacked structure. The growth thickness t _6g of the GaN layer 6 is preferably greater than 50 μm. At this time, a regrown interface is formed between the c-plane GaN substrate seed crystal 5 and the GaN layer 6.

The method for obtaining the c-plane GaN substrate seed crystal used in the above-mentioned first step is not particularly limited. For example, after preparing the seed crystal wafer 1 shown in (a) of Figure 5, a GaN thick film 2 formed of undoped GaN and oriented (0001) is intentionally grown thereon by the HVPE method as shown in (b) of Figure 5. Furthermore, by processing the GaN thick film 2 as shown in (c) of Figure 5, at least one c-plane GaN substrate seed crystal 3 as a substrate can be obtained. An example of the seed crystal wafer 1 is a c-plane sapphire wafer, and a peeling layer can preferably be provided on the main surface. In addition, a GaN thick film is further grown on the c-plane GaN substrate seed crystal 3 obtained in this way by the HVPE method, and the c-plane substrate thus obtained can be used as the c-plane GaN substrate seed crystal used in the above-mentioned first step.

Usually, the Ga polar surface of the GaN substrate seed crystal 5 is processed to be flat by a flattening process, that is, by appropriately applying grinding, polishing, a processing technique using a chemical polishing agent and a polishing pad called CMP, etc. before growing the GaN layer 6. The Ga polar surface after the flattening can be processed to a rough surface by a roughening process, that is, a process of etching to make it rough, and then the GaN layer 6 can be grown.

In the first step, at least a portion of the GaN layer 6 is doped with Mn as a compensating impurity.

It should be noted that, in addition to Mn, doping with Fe and C as compensating impurities is not excluded.

Mn doping can be performed by a known method, such as HVPE. In the case of HVPE, the raw material gas may include manganese dichloride, manganese vapor, etc. From the perspective of stable crystal growth, manganese dichloride is more preferred. In the case of using manganese dichloride, GaN crystals with better crystal quality can be obtained.

In view of high productivity, when manganese dichloride is used as the raw material gas, the growth rate is preferably 20 μm/hr or more, and in view of stable crystal growth, it is preferably 100 μm/hr or less.

The partial pressure of manganese dioxide is adjusted so as to achieve the above-mentioned manganese concentration.

The growth thickness t _6g of the GaN layer 6 may be set according to the designed thickness of the Ga polarity side region of the GaN substrate to be manufactured.

In the case of a GaN substrate, even when the diameter of the wafer is 4 inches or 6 inches, the growth thickness t _6g of the GaN layer 6 can be suppressed to 500 μm or less.

After the first step, a thinning step of thinning the GaN layer 6 as a stacked structure may be performed as necessary, as shown in FIG. 4( c ).

In (c) of Figure 4, the thickness of the GaN substrate seed crystal 5 is reduced from the initial thickness _t5i to the final thickness _t5f , and the thickness of the GaN layer 6 is reduced from the initial thickness _t6i to the final thickness _t6f , but in the thinning step, only one of the GaN substrate seed crystal 5 and the GaN layer 6 may be processed.

When the offcut orientation of the GaN substrate to be manufactured is the same as the offcut orientation of the GaN substrate seed crystal 5 , the back surface (N-polar surface of the stacked structure) of the GaN substrate seed crystal 5 can be used as a reference for the surface orientation during the thinning process.

When the off-angle orientation of the GaN substrate to be manufactured is different from the off-angle of the GaN substrate seed crystal 5, that is, when at least one of the off-angle angle and the off-angle direction is different, the crystal orientation of the GaN layer that becomes the stacked structure can be confirmed by an X-ray diffraction device before thinning.

The processing technique used in the thinning step can be appropriately selected from grinding, polishing, CMP, dry etching, wet etching, and the like.

By using the manufacturing method described above, the GaN substrate of this embodiment can be produced with a good yield.

Next, an example of an HVPE apparatus that can be used in the first step included in the above-mentioned production method will be described below with reference to FIG. 6 .

6 includes a hot wall reactor 21, a gallium storage cell 22 and a susceptor 23 disposed in the reactor, and a first heater 24 and a second heater 25 disposed outside the reactor. The first heater 24 and the second heater 25 surround the reactor 21 in an annular shape.

The reactor 21 is a quartz tube chamber and has a first zone Z1 mainly heated by a first heater 24 and a second zone Z2 mainly heated by a second heater 25. An exhaust pipe PE is connected to the reactor end on the second zone Z2 side.

The gallium storage cell 22 disposed in the first zone Z1 is a quartz container having a gas inlet and a gas outlet.

The susceptor 23 disposed in the second zone Z2 is formed of, for example, graphite. Any mechanism for rotating the susceptor 23 may be provided.

In order to grow GaN using the HVPE device 20, after a seed crystal is placed on the susceptor 23, the inside of the reactor 21 is heated by the first heater 24 and the second heater 25, and NH ₃ (ammonia) diluted with a carrier gas is supplied to the second region Z2 through the ammonia introduction pipe P1, and HCl (hydrogen chloride) diluted with a carrier gas is supplied to the gallium storage tank 22 through the hydrogen chloride introduction pipe P2. The HCl reacts with the metal gallium in the gallium storage tank 22, and the generated GaCl (gallium chloride) is transported to the second region Z2 through the gallium chloride introduction pipe P3.

In the second zone Z2 , NH ₃ reacts with GaCl, and the generated GaN is crystallized on the seed crystal placed on the susceptor 23 .

When the grown GaN is intentionally doped, a doping gas diluted with a carrier gas is introduced into the second zone Z2 in the reactor 21 through the dopant introduction pipe P4 .

The ammonia introduction pipe P1 , the hydrogen chloride introduction pipe P2 , the gallium chloride introduction pipe P3 , and the dopant introduction pipe P4 are formed of quartz at portions disposed in the reactor 11 .

As a carrier gas for diluting each of NH ₃ , HCl, and the doping gas, H ₂ (hydrogen), N ₂ (nitrogen), or a mixed gas of H ₂ and N ₂ can be preferably used.

Preferred conditions for growing GaN using the HVPE apparatus 20 are as follows.

The temperature of the gallium storage cell is, for example, 500 to 1000° C., preferably 700° C. or higher, and preferably 900° C. or lower.

The susceptor temperature is, for example, 900 to 1100° C., preferably 930° C. or higher, more preferably 950° C. or higher, and preferably 1050° C. or lower, more preferably 1020° C. or lower.

The ratio of the NH ₃ partial pressure to the GaCl partial pressure in the reactor, that is, the V/III ratio, is, for example, 1 to 20, preferably 2 or more, more preferably 3 or more, and preferably 10 or less.

If the V/III ratio is too large or too small, the morphology of the growth surface of GaN will be deteriorated. The deterioration of the morphology of the growth surface will cause the crystal quality to decrease.

For certain impurities, the efficiency of introduction into GaN crystals is highly dependent on the crystal orientation of the growth surface. The uniformity of the concentration of the impurities is reduced inside the GaN crystal grown under the condition of poor growth surface morphology. This is caused by the presence of facets of various orientations on the growth surface with poor morphology.

A typical example of an impurity whose introduction efficiency into GaN crystal varies significantly depending on the crystal orientation of the growth surface is O (oxygen). Since O is a donor impurity, a decrease in the uniformity of its concentration will cause a decrease in the uniformity of the resistivity.

In addition, using too low a V/III ratio will increase the nitrogen vacancy concentration of the grown GaN crystal. The effect of nitrogen vacancies on GaN crystals, GaN substrates using the GaN crystals, or nitride semiconductor devices formed on the GaN substrates is still unclear, but since they are point defects, it can be considered that the concentration should be as low as possible.

The growth rate of GaN is preferably 40 to 200 μm/h, and can be controlled by taking the product of the NH ₃ partial pressure and the GaCl partial pressure in the reactor as a parameter. Too high a growth rate will deteriorate the surface morphology of the grown GaN.

When the GaN layer 6 is doped in the first step described above, in order to prevent the morphology of the growth surface from being deteriorated, it is preferred that the supply rate of the doping gas be gradually increased to a given value over several minutes or tens of minutes from the start of supply.

For the same reason, it is preferred to start supplying doping gas when the GaN layer 6 has grown for at least several μm. Specifically, when forming the GaN layer through the first step, it is preferred that doping gas is not supplied for at least 5 μm or more of the initial growth of the GaN layer, preferably at least 10 μm or more, so that the GaN layer grows as an undoped layer. Thus, no new dislocations are generated at the interface between the undoped layer and the doped layer, and the quality of the seed crystal can be maintained.

The method of adding compensating impurities such as Mn to the GaN layer 6 is not limited, but a method of introducing a doping gas into an HVPE apparatus is generally used.

The doping gas used for Mn doping is as described above. For example, when manganese dichloride is used as the raw material gas, metal Mn is set in the inlet tube, and manganese dichloride gas is prepared on the spot by the circulation of hydrogen chloride gas while heating it. In the case of preparing manganese dichloride gas on the spot, in order to make the concentration of manganese dichloride gas uniform, it is preferred to increase the contact area between metal Mn and hydrogen chloride gas. For example, metal Mn is finely crushed to a diameter of several millimeters, loaded into a quartz tube, and diluted hydrogen chloride gas is circulated therein, thereby increasing the contact area of gas/solid. In this case, the uniformity of the crystal quality of GaN crystals doped with Mn can be improved, so it is preferred.

When Mn is doped after the undoped layer, it is preferred to gradually increase the concentration of hydrogen chloride. By gradually increasing the concentration of hydrogen chloride, it is possible to prevent the formation of a steep undoped layer/manganese-doped layer interface in the crystal, thereby improving the quality of the crystal. Specifically, it is expected that it will take more than 3 minutes to linearly increase the concentration of hydrogen chloride required for the desired manganese-doping concentration. If an undoped layer/manganese-doped layer interface with a steep Mn concentration is generated in the crystal, there is a hidden danger of deteriorating the quality of the crystal, but by adopting such a method, the quality of the Mn-doped crystal can be improved. The ratio of the Mn concentration at a position 10 μm away from the undoped layer/manganese-doped layer interface in the [0001] direction in the crystal to the Mn concentration at the undoped layer/manganese-doped layer interface is preferably more than 2 times, or can be more than 5 times. In this case, it means that the concentration gradient in the GaN crystal around the undoped layer/manganese-doped layer interface is not steep, and the concentration changes gently.

In the case where Fe is doped as a compensating impurity in addition to Mn, the doping gas for Fe doping can be, for example, vaporized ferric chloride. Ferric chloride vapor can be produced by a method in which heated metallic iron is brought into contact with HCl under a carrier gas flow, or by a method in which ferrocene (bis(cyclopentadienyl) iron) vaporized by heating under a carrier gas flow is reacted with HCl in a dopant introduction tube. Here, ferrocene can also be replaced by other organic compounds containing iron.

When other transition metal elements are added to GaN, vapor of the transition metal element or vapor of a chloride of the transition metal element may be used as a doping gas.

GaN grown using the HVPE apparatus 20 may contain O and Si at a concentration detectable by SIMS (secondary ion mass spectrometry) even when it is not intentionally doped.

The Si source is quartz (SiO ₂ ) used for the reactor and the piping in the reactor, and the O source is either or both of the above quartz and moisture remaining in or intruding into the reactor.

6 includes components not shown in the figure, and components arranged in the reactor 21 may be made of SiC (silicon carbide), SiNx (silicon nitride), BN (boron nitride), aluminum oxide, W (tungsten), Mo (molybdenum), etc., in addition to quartz and carbon. Therefore, the concentration of impurity elements other than Si, O and H in GaN grown using the HVPE apparatus 20 can be independently 5×10 ¹⁵ atoms/cm ³ or less, as long as no intentional doping is performed.

Hereinabove, an example of a method for manufacturing a GaN substrate as a stacked structure has been described as a method for manufacturing a GaN substrate according to the present embodiment. However, the GaN substrate is not limited to a GaN substrate as a stacked structure.

For example, such a GaN substrate can also be obtained by growing the GaN layer 6 to a certain thickness or more in the first step to form a GaN thick film, and cutting out a GaN substrate as a c-plane GaN substrate from the GaN thick film. In this case, the obtained GaN substrate is composed only of the GaN layer 6 described above.

When the specific doping region with a high Mn concentration is selected as the region for slicing the GaN substrate as the c-plane GaN substrate, the Mn concentration of the entire region of the obtained GaN substrate becomes high. That is, a GaN substrate consisting only of the specific doping region without the GaN substrate seed crystal 5 can be produced.

(Application of GaN substrate)

The GaN substrate of the present invention can be preferably used for the manufacture of nitride semiconductor devices, especially nitride semiconductor devices with horizontal device structures. Specifically, the nitride semiconductor device can be manufactured by a method having the following steps: a step of preparing the above-mentioned GaN substrate, and a step of epitaxially growing one or more nitride semiconductor layers on the prepared substrate.

Nitride semiconductors are also called nitride III-V compound semiconductors, III-nitride compound semiconductors, GaN-based semiconductors, etc. In addition to GaN, they also include compounds in which part or all of the gallium in GaN is replaced by other elements from Group 13 of the periodic table (B, Al, In, etc.).

A typical example of a nitride semiconductor device with a horizontal device structure is GaN-HEMT, but the horizontal device structure can also be used in electronic devices other than HEMT such as bipolar transistors, and can also be used in light-emitting devices such as light-emitting diodes (LEDs) and laser diodes (LDs).

Example

Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited thereto.

<Test Example 1>

(Example 1-1)

1. Epitaxial growth of GaN crystals

A GaN crystal was epitaxially grown on a self-supporting GaN seed crystal having a diameter of 62 mm and a thickness of 400 μm by the HVPE method using a vapor phase growth apparatus equipped with a hot wall reactor made of quartz.

In this process, (1) a temperature raising step, (2) a GaN layer growing step, and (3) a cooling step are sequentially performed as described below.

(1) Heating step

First, a self-supporting GaN seed crystal is placed in a reactor.

Next, while ammonia and a carrier gas were supplied to the self-supporting GaN seed crystal, the reactor temperature was raised from room temperature to 985° C. Hydrogen and nitrogen were used as carrier gases. A GaN substrate seed crystal was obtained through this step.

(2) Mn-doped GaN layer growth steps

While the reactor temperature was maintained at 1000° C., a mixed gas containing ammonia and gallium chloride as shown in the following growth conditions was supplied as a raw material gas, thereby growing a Mn-doped GaN layer to a thickness of 400 μm on the GaN substrate seed crystal.

The growth rate was set to 50 μm/hr, and the growth conditions were set to reactor pressure 101 kPa, ammonia partial pressure 3.172 kPa, GaCl partial pressure 0.847 kPa, hydrogen partial pressure 43.4 kPa, nitrogen partial pressure 55.581 kPa, and manganese dichloride partial pressure 5.37×10 ⁻⁴ kPa.

It should be noted that manganese dichloride was not flowed for the initial 30 minutes of growth to form an undoped layer, and 30 minutes later, diluted hydrogen chloride gas was flowed through the metal Mn heated to 800° C. to generate manganese dichloride.

Here, the concentration of the diluted hydrochloric acid was linearly increased from 0 vol % to 0.023 vol % over 10 minutes so as not to generate a steep interface between the undoped layer and the Mn-doped layer.

This confirmed that the dislocation density of the Mn-doped layer was substantially the same as the dislocation density of the seed crystal, and that no new dislocations were generated at the undoped/Mn-doped interface.

In addition, metal Mn is finely pulverized to a diameter of several millimeters, placed in a quartz tube, and diluted hydrogen chloride gas is circulated therein, thereby increasing the gas/solid contact area.

The gas partial pressure ( _PG ) here is a value obtained by multiplying the ratio (r) of the volume flow rate of the gas to the total volume flow rate of all gases supplied to the reactor by the reactor pressure ( _PR ), that is, _PG = r × _PR .

Next, the temperature of the reactor was raised to 1010° C. while the raw material gas and the carrier gas were continuously supplied.

(3) Cooling step

After the Mn-doped GaN layer growth step (2) is completed, the supply of gallium chloride to the GaN substrate seed crystal is stopped, and the heating of the reactor is stopped to lower the reactor temperature to room temperature. Until the temperature drops to 600°C, the gas flowing in the reactor is set to ammonia and nitrogen, and then to only nitrogen.

The entire surface of the as-grown GaN crystal taken out from the reactor was mirror-like and flat.

2. Fabrication of c-plane GaN substrate

The as-grown GaN crystal obtained in 1. above was laser scored to obtain a square epitaxial substrate with a side length of 7 mm. Then, the +c plane and -c plane were ground and polished to complete the c-plane GaN substrate doped with Mn. Then, the seed portion of the -c plane was completely removed.

The total film thickness of the c-plane GaN substrate is 300 μm, which is entirely composed of Mn-doped layers.

3. Evaluation of c-plane GaN substrate

<Measurement of Mn concentration>

The Mn concentration of the c-plane GaN substrate was measured by SIMS.

As a result, the Mn concentration was 8×10 ¹⁷ atoms/cm ³ .

<Measurement of carrier concentration, specific resistance, and carrier type>

Next, for Hall effect measurement, Ti 30nm and Au 100nm were continuously vacuum-deposited on the surface of the c-plane GaN substrate. The 4-terminal Vnv der Pauw method was used in the Hall effect measurement. In the Hall effect measurement, the carrier concentration, resistivity, and carrier type were measured while changing the measurement temperature.

The measurement results of the carrier concentration are shown in Fig. 7. In the graph of Fig. 7, 1000/T (1/K) is plotted on the x-axis and the carrier concentration (atoms/cm ³ ) is plotted on the y-axis.

When the temperature is increased, the carrier concentration increases. The carrier concentration is fitted by the following formula (I), and the activation energy Ea of the carrier is 0.952 eV ([Mn] = 8E17 (8×10 ¹⁷ ) cm ⁻³ ).

Carrier concentration (atoms/cm ³ ) = A×EXP (-Ea/kT) (I)

(In formula (I), A is a proportionality constant, EXP is an exponential function, Ea is the activation energy of carriers (eV), k is the Boltzmann constant (8.617×10 ^-5 eV/K), and T is the temperature in Kelvin (K).) It should be noted that the coefficient of determination R ² when performing exponential approximation is 0.9266.

In addition, the measurement results of the specific resistance are shown in Fig. 8. From the results of Fig. 8, it can be seen that the specific resistance at 900K is 5×10 ⁴ Ωcm or more.

It should be noted that the type of carrier determined by the Hall effect measurement in all temperature ranges was p-type.

The above evaluation shows that the c-plane GaN substrate of Example 1 has a very large activation energy Ea, and it is considered that this large value contributes to the large specific resistance of the Mn-doped GaN substrate.

<Measurement of dislocation density>

The dislocation density of the c-plane GaN substrate was measured by the CL method. The c-plane GaN substrate for CL measurement and the c-plane GaN substrate for Hall effect measurement were prepared separately. This is because the luminescence near the band edge of the Mn-doped substrate is very weak, so the dislocation cannot be observed as a dark spot. After the Mn-doped GaN layer was grown to 400μm by the above-mentioned methods of "1. Epitaxial growth of GaN crystal" and "2. Preparation of c-plane GaN substrate", only the manganese dichloride gas was turned off, and the undoped GaN layer was continuously grown to 50μm. The cooling step and the like are the same as above. The obtained c-plane GaN substrate for CL measurement was directly subjected to CL measurement in the generated state. Assuming that the dislocation density in the Mn-doped layer increases, the dislocation density of the GaN layer grown thereon should also increase in the same way. Therefore, this method is appropriate as a method for evaluating the dislocation density of the Mn-doped layer. The number of dark spots was measured by CL observation under the conditions of 3kV, 100pA, and 1000 times the field of view, and the density was calculated to evaluate the dislocation density.

As a result, the dislocation density was 8×10 ⁵ cm ^-2 , which was equivalent to that of the seed crystal used, indicating that dislocations did not increase due to Mn doping.

<Test Example 2>

(Example 2-1)

1. Epitaxial growth of GaN crystals

A GaN crystal was epitaxially grown on a self-supporting GaN seed crystal having a diameter of 62 mm and a thickness of 400 μm by the HVPE method using a vapor phase growth apparatus equipped with a hot wall reactor made of quartz.

In this process, (1) a temperature raising step, (2) a GaN layer growing step, and (3) a cooling step are sequentially performed as described below.

(1) Heating step

First, a self-supporting GaN seed crystal is placed in a reactor.

Next, while ammonia and a carrier gas were supplied to the self-supporting GaN seed crystal, the reactor temperature was raised from room temperature to 985° C. Hydrogen and nitrogen were used as carrier gases. A GaN substrate seed crystal was obtained through this step.

(2) Mn-doped GaN layer growth steps

While the reactor temperature was maintained at 1000° C., a mixed gas containing ammonia and gallium chloride and shown in the following growth conditions was supplied as a raw material gas, thereby growing a Mn-doped GaN layer to a thickness of 400 μm on the GaN substrate seed crystal.

The growth rate was set to 50 μm/hr, and the growth conditions were set to reactor pressure 101 kPa, ammonia partial pressure 3.172 kPa, GaCl partial pressure 0.847 kPa, hydrogen partial pressure 43.4 kPa, nitrogen partial pressure 55.581 kPa, and manganese dichloride partial pressure 5.37×10 ⁻⁴ kPa.

It should be noted that manganese dichloride was not flowed for the initial 30 minutes of growth to form an undoped layer, and 30 minutes later, diluted hydrogen chloride gas was flowed through the metal Mn heated to 800° C. to generate manganese dichloride.

Here, the concentration of the diluted hydrochloric acid was linearly increased from 0 vol % to 0.023 vol % over 10 minutes so as not to generate a steep interface between the undoped layer and the Mn-doped layer.

This confirmed that the dislocation density of the Mn-doped layer was substantially the same as the dislocation density of the seed crystal, and that no new dislocations were generated at the undoped/Mn-doped interface.

Furthermore, metal Mn is finely pulverized to a diameter of several millimeters, placed in a quartz tube, and diluted hydrogen chloride gas is flowed therethrough, thereby increasing the gas/solid contact area.

The gas partial pressure ( _PG ) here is a value obtained by multiplying the ratio (r) of the volume flow rate of the gas to the total volume flow rate of all gases supplied to the reactor by the reactor pressure ( _PR ), that is, _PG = r × _PR .

Next, the temperature of the reactor was raised to 1010° C. while the raw material gas and the carrier gas were continuously supplied.

(3) Cooling step

After the Mn-doped GaN layer growth step (2) is completed, the supply of gallium chloride to the GaN substrate seed crystal is stopped, and the heating of the reactor is stopped to lower the reactor temperature to room temperature. Until the temperature drops to 600°C, the gas flowing in the reactor is set to ammonia and nitrogen, and then to only nitrogen.

The entire surface of the as-grown GaN crystal taken out from the reactor was mirror-like and flat.

2. Fabrication of c-plane GaN substrate

The as-grown GaN crystal obtained in 1. above was laser-scored to obtain a square epitaxial substrate with a side length of 7 mm. Then, the +c and -c planes were ground and polished in sequence to complete the Mn-doped c-plane GaN substrate. Then, the seed portion of the -c plane was completely removed.

The total film thickness of the c-plane GaN substrate is 300 μm, which is entirely composed of Mn-doped layers.

3. Evaluation of c-plane GaN substrate

<Measurement of Mn concentration>

The Mn concentration of the c-plane GaN substrate was measured by SIMS.

As a result, the Mn concentration was 9×10 ¹⁸ atoms/cm ³ .

<Measurement of carrier mobility and carrier type>

For Hall effect measurement, Ti30nm and Au100nm were continuously vacuum-deposited on the surface of the c-plane GaN substrate. The 4-terminal van der Pauw method was used in the Hall effect measurement. In the Hall effect measurement, the carrier mobility and carrier type were measured while changing the measurement temperature.

The measurement results of the carrier mobility are shown in Fig. 9. In the graph of Fig. 9, temperature (K) is plotted on the x-axis and carrier mobility ( ^cm2 /Vs) is plotted on the y-axis.

<Measurement of dislocation density>

The dislocation density of the c-plane GaN substrate was measured by the CL method. The c-plane GaN substrate for CL measurement and the c-plane GaN substrate for Hall effect measurement were prepared separately. This is because the luminescence near the band edge of the Mn-doped substrate is very weak, so the dislocation cannot be observed as a dark spot. After the Mn-doped GaN layer was grown to 400μm by the above-mentioned methods of "1. Epitaxial growth of GaN crystal" and "2. Preparation of c-plane GaN substrate", only the manganese dichloride gas was turned off, and the undoped GaN layer was continuously grown to 50μm. The cooling step and the like are the same as above. The obtained c-plane GaN substrate for CL measurement was directly subjected to CL measurement in the generated state. Assuming that the dislocation density in the Mn-doped layer increases, the dislocation density of the GaN layer grown thereon should also increase in the same way. Therefore, this method is appropriate as a method for evaluating the dislocation density of the Mn-doped layer. The number of dark spots was measured by CL observation under the conditions of 3kV, 100pA, and 1000 times the field of view, and the density was calculated to evaluate the dislocation density.

As a result, the dislocation density was 8×10 ⁵ cm ^-2 , which was equivalent to that of the seed crystal used, indicating that dislocations did not increase due to Mn doping.

(Example 2-2)

In Example 2-1, a c-plane GaN substrate was produced and evaluated in the same manner as in Example 2-1 except that the Mn doping concentration was changed.

As a result, the Mn concentration was 1×10 ¹⁹ atoms/cm ³ and the dislocation density was 8×10 ⁵ cm ^-2 .

In addition, the results of measuring carrier mobility are shown in FIG. 9 .

In embodiments 2-1 and 2-2, the temperature region in which the measurement results of stable carrier mobility can be obtained is above 650K. In this region, the carrier mobility is positively correlated with the temperature. In addition, the ratio of the carrier mobility μ _H at 800K to the carrier mobility μ _L at 650K is μ _H /μ _L or more than 1. In any embodiment, the type of carriers determined by the Hall effect measurement in all temperature ranges is p-type.

Generally speaking, for GaN crystals, mobility is negatively correlated with temperature in the temperature range above 400K. This is due to the lattice scattering mechanism of carrier mobility. Therefore, the temperature dependence of the carrier mobility of the Mn-doped GaN substrate obtained in this experiment is very special, which shows that the crystal quality of the Mn-doped GaN substrate is very excellent.

In addition, the present invention is described in detail with reference to specific embodiments, but it should be clear to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on Japanese patent applications (Japanese Patent Application No. 2022-026202 and Japanese Patent Application No. 2022-026203) filed on February 22, 2022, the contents of which are hereby introduced as reference.

Claims (15)

1. A GaN substrate, which is a GaN substrate doped with manganese, wherein when the carrier concentration is represented by the following formula (I), the activation energy Ea of the carrier is greater than 0.7 eV, Carrier concentration (atoms/cm ³ ) = A×EXP (-Ea/kT) (I) In formula (I), A is a proportionality constant, EXP is an exponential function, Ea is the activation energy of carriers (eV), k is the Boltzmann constant (8.617×10 ^-5 eV/K), and T is the temperature in Kelvin (K). 2. The GaN substrate according to claim 1, wherein: The activation energy Ea of the carrier is 0.7 to 1.2 eV. 3. The GaN substrate according to claim 1, wherein: The activation energy Ea of the carrier is determined by the least square method, and the determination coefficient ^R2 of the approximate curve when the exponential approximation is performed by the least square method is greater than 0.9. 4. The GaN substrate according to claim 1, wherein: The activation energy Ea of the carrier is obtained by fitting the carrier concentration measured by changing the temperature at intervals of 50K in the temperature range of 450K to 950K to the formula (I). 5. A GaN substrate, which is a GaN substrate doped with manganese, wherein the carrier concentration of the GaN substrate at 900K is less than 1×10 ¹³ atoms/cm ³ . 6. A GaN substrate having a resistivity of 5×10 ⁴ Ωcm or more at 900K. The GaN substrate according to claim 6 , which is a GaN substrate doped with manganese. 8. The GaN substrate according to any one of claims 1 to 7, wherein The type of carrier is p-type. 9 . The GaN substrate according to claim 1 , wherein the dislocation density is 1×10 ⁶ cm ⁻² or less. 10. A GaN substrate, which is a GaN substrate doped with manganese, wherein the carrier mobility of the GaN substrate is positively correlated with temperature. 11. A GaN substrate doped with manganese, wherein a ratio of a carrier mobility _μH at 800K to a carrier mobility _μL at 650K of the GaN substrate, _μH / _μL, is 1 or more. 12. A GaN substrate, which is a GaN substrate doped with manganese, wherein the carrier type of the GaN substrate is p-type. 13. A GaN substrate, which is a semi-insulating GaN substrate, wherein the carrier mobility of the GaN substrate at 900K is 50 ^cm2 /Vs or more. The GaN substrate according to claim 13 , which is a GaN substrate doped with manganese. 15 . The GaN substrate according to claim 10 , wherein the dislocation density is 1×10 ⁶ cm ⁻² or less.