JP2011258782A - Nitride semiconductor substrate - Google Patents

Abstract

【Task】
A nitride semiconductor layer having a normally-off action is formed between the intermediate layer and the device active layer, which can further improve the threshold voltage.
[Solution]
An intermediate layer formed on a Si single crystal substrate and having a laminated structure of nitride semiconductors, and formed on the intermediate layer, composition Al _x Ga _1-x N (0 ≦ x ≦ 0.05), thickness of 200 nm to 2000 nm Hereinafter, a region 1 made of a nitride semiconductor having a carbon concentration of 1'10 ¹⁸ atoms / cm ³ or more and 1'10 ²¹ atoms / cm ³ or less, and a composition Al _y Ga _1-y N (0. 1 ≦ y ≦ 1), a region 2 made of a nitride semiconductor having a thickness of 0.2 nm to 100 nm and a carbon concentration of 1′10 ¹⁸ atoms / cm ³ or more and 1′10 ²¹ atoms / cm ³ or less; A nitride semiconductor substrate comprising a nitride semiconductor device active layer to be formed.
[Selection] Figure 1

Description

  The present invention relates to a structure of a nitride semiconductor substrate used for a nitride semiconductor suitable as a high-speed and high-voltage electronic device.

Nitride semiconductors such as gallium nitride (GaN) and aluminum nitride (AlN) have excellent characteristics such as high electron mobility and high heat resistance, and are used particularly in regions that require high-frequency operation and high breakdown voltage. Applications to electronic devices such as high electron mobility transistors (HEMTs) and heterojunction field effect transistors (HFETs) are expected.

As a structure and manufacturing method of a nitride semiconductor substrate used for this nitride semiconductor, for example, a method of forming a nitride semiconductor crystal on an Si single crystal substrate through an intermediate layer made of a nitride semiconductor layer is a high quality. This can be said to be preferable in that crystals can be stably supplied and a large-area substrate can be obtained at a low price.

By the way, a field effect transistor using a heterojunction uses a heterojunction in which an AlGaN layer is used as an electron supply layer and a GaN layer is used as a channel layer. It is difficult to produce an off-type element. Further, if the object is simply to be a normally-off type, it is in contradiction to the reduction of the on-resistance, which is a characteristic of this material system. That is, in order to produce a normally-off type element, carriers must be suppressed, but this increases the on-resistance of the element, which is not preferable. Therefore, in order for a normally-off operation to be realized without impairing the low on-resistance, the current is cut off only by the controllable gate portion, and the other source-gate channel and the gate-drain channel are low. Resistance must be ensured.

As a method for solving this problem, for example, Patent Document 1 discloses that in a nitride semiconductor field effect transistor, Al _{x is applied in} the + c direction of the crystal orientation in order to obtain an enhancement type operation capable of controlling a threshold voltage. Ga _1-x N layer, GaN layer, and Al _y Ga _1-y N layer are stacked in this order, and the gate portion has a channel having a double heterostructure that is depleted when x ≧ y. A technique called a nitride semiconductor field effect transistor is disclosed.

Further, in Patent Document 2, as a semiconductor device capable of realizing a normally-off operation, a first nitride semiconductor layer and a first nitride semiconductor layer provided on the first nitride semiconductor layer are provided. A second nitride semiconductor layer having a large band gap; a source electrode provided on the second nitride semiconductor layer; a drain electrode provided on the second nitride semiconductor layer; An insulating layer provided between the source electrode and the drain electrode on the surface of the nitride semiconductor layer, a p-type third nitride semiconductor layer provided on the insulating layer, and a third nitride semiconductor A technique of providing a gate electrode provided on a layer is disclosed.

JP 2008-10803 A JP 2009-71061 A

With the technique described in Patent Document 1, it is possible to control the threshold voltage, but it cannot be said that the request for obtaining a higher threshold voltage is sufficiently met.

In the technique described in Patent Document 2, a p-type nitride semiconductor layer is provided under the gate electrode, and problems that occur when the p-type nitride semiconductor layer is provided simply in contact with the electron supply layer are suppressed. For this purpose, an insulating film for electrically insulating and separating the p-type nitride semiconductor layer and the channel layer is formed. Therefore, the formation of the insulating film causes an increase in cost. Also, it is not enough to improve the threshold voltage.

The present invention has been made in view of such technical problems, and provides a nitride semiconductor substrate suitable for a normally-off type high breakdown voltage device capable of realizing a higher threshold voltage with a simple structure. The purpose is to do.

A nitride semiconductor substrate according to the present invention includes a substrate made of a Si single crystal, an intermediate layer formed on the substrate and having a nitride semiconductor multilayer structure including at least one nitride semiconductor layer containing Al, and Formed on the intermediate layer, composition Al _x Ga _1-x N (0 ≦ x ≦ 0.05), thickness 200 nm or more and 2000 nm or less, carbon concentration 1′10 ¹⁸ atoms / cm ³ or more 1′10 ²¹ atoms / cm ^A region 1 made of a nitride semiconductor having a single-layer or multilayer structure of ³ or less, and a composition Al _y Ga _1-y N (0.1 ≦ y ≦ 1) and a thickness of 0.2 nm or more formed on the region 1 A region 2 made of a single-layer or multi-layer nitride semiconductor having a carbon concentration of 100 nm or less and a carbon concentration of 1′10 ¹⁸ atoms / cm ³ or more and 1′10 ²¹ atoms / cm ³ or less, and a nitride formed on the region 2 It is characterized by comprising a semiconductor device active layer. By adopting such a configuration, it is possible to obtain a normally-off type nitride semiconductor substrate in which the threshold voltage can be increased as compared with the prior art.

In the nitride semiconductor substrate according to the present invention, the carbon concentration of any one or both of the intermediate layer and the device active layer is a carbon concentration of 1′10 ¹⁸ atoms / cm ³ or more and 1′10 ²¹ atoms / cm. It is preferable that it is ³ or less. By adopting such a configuration, it is possible to obtain a normally-off type nitride semiconductor substrate that has the effect of achieving normally-off in addition to high withstand voltage and can have a higher threshold voltage than conventional ones. It becomes.

  The nitride semiconductor substrate according to the present invention can be applied to a high breakdown voltage device, particularly a normally-off switching device, because the threshold voltage can be improved by manufacturing a device using the nitride semiconductor substrate. can do.

  Hereinafter, embodiments of the present invention will be described in detail. FIG. 1 is a conceptual diagram showing one embodiment of a nitride semiconductor substrate according to the present invention.

The conceptual diagram which shows one form of the nitride semiconductor substrate which concerns on this invention.

A nitride semiconductor substrate according to the present invention includes a substrate made of a Si single crystal, an intermediate layer formed on the substrate and having a nitride semiconductor multilayer structure including at least one nitride semiconductor layer containing Al, and Formed on the intermediate layer, composition Al _x Ga _1-x N (0 ≦ x ≦ 0.05), thickness 200 nm or more and 2000 nm or less, carbon concentration 1′10 ¹⁸ atoms / cm ³ or more 1′10 ²¹ atoms / cm ^A region 1 made of a nitride semiconductor having a single-layer or multilayer structure of ³ or less, and a composition Al _y Ga _1-y N (0.1 ≦ y ≦ 1) and a thickness of 0.2 nm or more formed on the region 1 A region 2 made of a single-layer or multi-layer nitride semiconductor having a carbon concentration of 100 nm or less and a carbon concentration of 1′10 ¹⁸ atoms / cm ³ or more and 1′10 ²¹ atoms / cm ³ or less, and a nitride formed on the region 2 It consists of a semiconductor device active layer.

A substrate made of Si single crystal is used as a base substrate for forming a nitride semiconductor. As this Si single crystal, various materials manufactured by known techniques can be widely used. For example, a single crystal manufactured by the Czochralski method (CZ method) or the floating zone method (FZ method), or a substrate manufactured by various manufacturing methods such as a vapor phase growth method or a bonding method can be applied. In addition, considering the characteristics of the nitride semiconductor substrate, the thickness and surface state of the Si single crystal substrate, dopants such as oxygen, nitrogen, carbon, phosphorus and boron contained in the substrate, and the concentration and distribution of various crystal defects It is also possible to use with timely control.

An intermediate layer having a laminated structure of nitride semiconductors including at least one nitride semiconductor layer containing Al is formed on the Si single crystal substrate. The single crystal of Si and the nitride semiconductor have different lattice constants and thermal expansion coefficients, which causes large warpage in the nitride semiconductor substrate and frequent occurrence of crystal defects such as slip. In order to suppress this, an intermediate layer having various structures is provided between the device active layer forming the device. In the present invention, since a high substrate characteristic can be easily obtained, at least a nitride semiconductor layer containing Al is provided. An intermediate layer having a laminated structure of nitride semiconductors including one or more layers is preferably applied.

On the intermediate layer, a composition Al _x Ga _1-x N (0 ≦ x ≦ 0.05), a thickness of 200 nm to 2000 nm, a carbon concentration of 1′10 ¹⁸ atoms / cm ^{3 to} 1′10 ²¹ atoms / A region 1 made of a nitride semiconductor having a single-layer or multilayer structure of cm ³ or less is formed. Here, in the case of multiple layers, the total thickness of each layer is defined as the thickness of region 1.

As an example of a normally-off type nitride semiconductor substrate, an electron transit layer made of GaN and an electron supply layer made of AlGaN are formed in the device active layer, and further, a layer made of AlGaN between the intermediate layer and the electron transit layer A structure in which is inserted. By adjusting the thickness and characteristics of each layer, it is possible to achieve normally-off and further improve the threshold voltage.

As a method for improving the threshold voltage, the energy difference between the conduction band and the Fermi level in an AlGaN layer (hereinafter referred to as a normally-off layer) formed between the intermediate layer and the electron transit layer. It is conceivable to increase In order to keep the Fermi level low, it is necessary to reduce the density of electrons in the normally-off layer. For example, the generation of defects in the normally-off layer can be suppressed.

By the way, AlGaN itself is a material in which defects are likely to occur when a film is formed by a vapor deposition method. For this reason, the number of electrons contained in the deposited layer increases, the Fermi level increases, and it becomes difficult to improve the threshold voltage.

When AlGaN is used as the normally-off layer, a certain degree of film thickness is required to achieve the normally-off layer. However, if the film thickness is increased, the amount of electrons contained in the deposited layer also increases in proportion to this, so that the threshold voltage cannot be improved.

Therefore, in the present invention, the normally-off layer is configured to have two different structures, region 1 and region 2, so that the normally-off and the threshold voltage improvement, which are mutually contradictory characteristics, can be simultaneously performed at a high level. Make it possible to achieve. Here, since the region 1 is a region in which defects are hardly generated, the region 1 plays a role of bringing about an effect of improving the threshold voltage.

_X of Al _x Ga _1-x N in the region 1 is preferably 0 or more and 0.05 or less, and more preferably 0 or more and 0.02 or less. Compared to AlGaN, GaN is less prone to defects during film formation, so GaN containing no Al is ideal in improving the threshold voltage. However, the effects of the present invention can be obtained even if Al is contained within a range that does not significantly inhibit the threshold voltage improvement. Substantially in the vapor phase growth process, a small number of layers in which the Al composition is not completely zero are formed in the region 1. Nevertheless, there is no particular problem if _{x of} Al _x Ga _1-x N is in the range of 0 ≦ x ≦ 0.05.

The thickness of Al _x Ga _1-x N in region 1 is preferably 200 nm or more and 2000 nm or less, and more preferably 1000 nm or more and 1500 nm or less. If it is less than 200 nm, it is not preferable because the film thickness is remarkably insufficient in order to obtain an amount of electrons with which the effect of the present invention can be obtained. However, if it exceeds 2000 nm, the total film thickness of the nitride semiconductor layer including the intermediate layer and the device active layer becomes too thick, and the warpage of the entire nitride semiconductor substrate increases, which is also not preferable.

The region 1 is composed of a single-layer or multi-layer nitride semiconductor of Al _x Ga _1-x N (0 ≦ x ≦ 0.05), but has a composition Al _x Ga _1-x N (0 ≦ x ≦ 0.05), a multilayer structure in which a plurality of Al _x Ga _1-x N layers having different values of the Al composition x may be formed. Furthermore, a structure in which the Al concentration changes uniformly as it increases or decreases in the thickness direction also falls within the scope of the present invention, since pseudo layers with different x can be regarded as a multilayer continuous structure.

The carbon concentration in region 1 is preferably 1′10 ¹⁸ atoms / cm ³ or more and 1′10 ²¹ atoms / cm ³ or less, more preferably 1′10 ¹⁹ atoms / cm ³ or more and 5′10 ¹⁹ atoms / cm ³ or less. is there. By containing carbon at a high concentration in the nitride semiconductor, an increase in the Fermi level due to defects can be suppressed and the conduction band of the GaN layer formed as the device active layer can be increased. However, at a carbon concentration lower than 1′10 ¹⁸ atoms / cm ³ , the Fermi level increase suppression due to defects and the effect of raising the conduction band of the GaN layer formed as the device active layer cannot be sufficiently obtained. If the concentration is higher than 10 ²¹ atoms / cm ³ , it is difficult to dope carbon.

Here, the carbon concentration of 1′10 ¹⁸ atoms / cm ³ or more and 1′10 ²¹ atoms / cm ³ or less is a known concentration such as a spreading resistance (SR) measurement method or a secondary ion mass spectrometry (SIMS) method. Using the measurement method, the average value when measured along the thickness direction of the substrate is used. Further, unless otherwise specified, it is represented by a measurement value at one central point on one main surface of the substrate, but a value obtained by measuring multiple points on one main surface of the substrate may be used as necessary.

Next, on the region 1, the composition Al _y Ga _1-y N (0.1 ≦ y ≦ 1), the thickness is 0.2 nm or more and 100 nm or less, and the carbon concentration is 1′10 ¹⁸ atoms / cm ³ or more and 1′10. ^A region 2 made of a nitride semiconductor having a single layer or multilayer structure of ²¹ atoms / cm ³ or less is formed.

In the AlGaN layer, when the Al composition is large, the energy difference from the conduction band to the Fermi level becomes large, so that the threshold voltage can be increased. Since it is difficult to sufficiently increase the threshold voltage only in the region 1, the threshold voltage can be further increased by combining with the region 2.

When the Al composition in the region 2 is Al _y Ga _1-y N, y is preferably 0.1 or more and 1 or less, and more preferably 0.2 or more and 1 or less. If the amount of Al is small, the energy difference from the conduction band to the Fermi level cannot be made sufficiently large, so that it is difficult to sufficiently achieve the purpose of increasing the threshold voltage. As in the case of region 1, the upper limit of the Al composition in region 2 is not particularly limited as long as the effect of the present invention is not significantly impaired.

The thickness of Al _y Ga _1-y N in the region 2 is preferably 0.2 nm or more and 100 nm or less, and more preferably 0.2 nm or more and 10 nm or less. If the thickness is less than 0.2 nm, the film is thin enough to be smaller than one molecular layer, so that it is difficult to form a film uniformly on the entire surface of the substrate, and there is a concern about characteristic deterioration due to non-uniform film thickness maintenance, which is not preferable. . However, if it exceeds 100 nm, the Fermi level becomes high and the effect of improving the threshold voltage is reduced, which is also not preferable.

The carbon concentration in the region 2 is preferably 1′10 ¹⁸ atoms / cm ³ or more and 1′10 ²¹ atoms / cm ³ or less, more preferably 1′10 ¹⁹ atoms / cm ³ or more and 5′10 ¹⁹ atoms / cm ³ or less. is there. Similar to the region 1, the high concentration of carbon contained in the nitride semiconductor has the effect of suppressing the increase of the Fermi level due to defects and increasing the conduction band of the GaN layer formed as the device active layer. However, if the carbon concentration is lower than 1′10 ¹⁸ atoms / cm ³ , the increase in the Fermi level due to defects and the effect of raising the conduction band of the GaN layer formed as the device active layer cannot be sufficiently obtained, which is not preferable. On the other hand, when the concentration is higher than 1′10 ²¹ atoms / cm ³ , it is difficult to dope carbon itself, which is also not preferable.

Similarly to the region 1, the region 2 is composed of a single-layer or multi-layer nitride semiconductor of Al _y Ga _1-y N (0.1 ≦ y ≦ 1). However, not only a single layer of composition Al _y Ga _1-y N (0.1 ≦ y ≦ 1) but also a multilayer structure in which a plurality of Al _y Ga _1-y N layers having different values of the composition y of Al are formed. There may be. Furthermore, a structure in which the Al concentration changes uniformly by increasing or decreasing in the thickness direction can also be regarded as a structure in which layers having different y are pseudo-multilayered, and thus falls within the scope of the present invention.

A nitride semiconductor device active layer is formed on the region 2. As a preferred example of the present invention, a laminated structure of a nitride semiconductor layer composed of an electron transit layer and an electron supply layer can be mentioned. At this time, a nitride semiconductor layer corresponding to the region 2 of the present invention may be inserted between the electron transit layer and the electron supply layer. This is because the effect of improving the threshold voltage can be added to this portion by interposing the region 2.

In the nitride semiconductor substrate according to the present invention, the intermediate layer has a carbon concentration of 1′10 ¹⁸ atoms / cm ³ or more and 1′10 ²¹ atoms / cm ³ or less, and the device active layer has a carbon concentration of 5′10 ¹⁷ atoms / cm ^3. It is preferable that it is cm ³ or less.

Similar to regions 1 and 2, carbon is contained in the intermediate semiconductor in a high concentration in the nitride semiconductor, thereby suppressing an increase in Fermi level due to defects and a conduction band of a GaN layer formed as a device active layer Has the effect of pulling up. However, if the carbon concentration is lower than 1′10 ¹⁸ atoms / cm ³ , the increase in the Fermi level due to defects and the effect of raising the conduction band of the GaN layer formed as the device active layer cannot be sufficiently obtained, which is not preferable. On the other hand, if the concentration is higher than 1'10 ²¹ atoms / cm ³ , it is difficult to dope carbon, so that neither is preferable. In addition, if the carbon layer concentration in the device active layer is higher than 5'10 ¹⁷ atoms / cm ³ , electrons are scattered, and the decrease in electron mobility becomes remarkable, which is not preferable.

As described above, the nitride semiconductor substrate according to the present invention includes the normally-off layer that can further improve the threshold voltage between the intermediate layer and the device active layer with a simple configuration. As a result, a normally-off type nitride semiconductor substrate that can have a higher threshold voltage than the conventional one can be obtained.

Hereinafter, preferred embodiments of the present invention will be described based on examples, but the present invention is not limited to these examples.

A nitride semiconductor substrate having a layer structure as shown in FIG. 1 was produced by the following steps.
As a common manufacturing method, a Si single crystal substrate 1 having a diameter of 4 inches is set in an MOCVD apparatus, and is laminated using trimethyl gallium (TMG), trimethyl aluminum (TMA), NH ₃ , and methane as raw materials for nitride semiconductors. These raw materials were properly used according to the film, and each layer was formed by vapor phase growth at a vapor phase growth temperature of 1000 ° C. The composition and thickness of each layer were adjusted by selecting each raw material and adjusting the flow rate and supply time.

Example 1
An AlN single crystal layer 21 having a thickness of 20 nm and a carbon concentration of 5′10 ¹⁹ atoms / cm ³ is formed on the Si single crystal substrate 1, and subsequently having a thickness of 80 nm and a carbon concentration of 5′10 ¹⁹ atoms / cm ³ . The Al _0.2 Ga _0.8 N single crystal layer 22 was laminated, and these were alternately repeated in the same process, and the intermediate layer 2 was formed by laminating 10 layers each for a total of 20 layers. On this intermediate layer 2, a region 1 consisting of a single layer having a thickness of 1400 nm and a composition of Al _x Ga _1-x N (x = 0) and a carbon concentration of 5′10 ¹⁹ atoms / cm ³ , and a thickness on the region 1 A region 2 composed of a single layer having a carbon concentration of 5′10 ¹⁹ atoms / cm ³ and a composition of Al _y Ga _1-y N (y = 0.2) was formed. On the region 2, a GaN single crystal layer having a thickness of 500 nm and a carbon concentration of 5′10 ¹⁷ atoms / cm ³ is used as the electron transit layer 41, and subsequently, Al having a thickness of 30 nm and a carbon concentration of 5′10 ¹⁷ atoms / cm ³ is used. _The nitride active layer 4 was formed by laminating the electron supply layers 42 of _0.25 Ga _0.75 N single crystals. Through the above steps, a nitride semiconductor substrate was obtained.

(Comparative Example 1)
The same processes as in Example 1 were performed until the formation of the intermediate layer 2. On the intermediate layer 2, only the region 1 composed of a single layer having a thickness of 1430 nm and a composition of Al _x Ga _1-x N (x = 0) and a carbon concentration of 5′10 ¹⁹ atoms / cm ³ was produced. On the region 1, the same nitride active layer 4 as in Example 1 was formed. Through the above steps, a nitride semiconductor substrate was obtained.

(Reference example)
As a reference example, an example of a structure without the region 1 in the present invention is shown. First, it produced similarly to Example 1 until formation of the intermediate | middle layer 2. FIG. On the intermediate layer 2, only a region 2 composed of a single layer having a thickness of 30 nm and a composition of Al _y Ga _1-y N (y = 0.2) and a carbon concentration of 5′10 ¹⁹ atoms / cm ³ was produced. On the region 2, the same nitride active layer 4 as in Example 1 was formed. Through the above steps, a nitride semiconductor substrate was obtained.

The threshold voltages were measured and compared for these three nitride semiconductor substrates, respectively. Note that the threshold voltage is measured by forming a recess gate Schottky electrode (Ni / Au) and an ohmic electrode (Ti / Al) as source / drain on the device active layer of the nitride semiconductor substrate produced, respectively. After element isolation and forming a field effect transistor device, it was carried out by performing IV measurement with a curve tracer at room temperature.

The threshold voltage was 0.7V in the reference example and 2.2V in the example 1 with respect to the comparative example 1.0V. As a result, it was confirmed that the threshold voltage was improved by using the configuration of the present invention.

Next, based on the nitride semiconductor substrate having the form of Example 1, each threshold value of the nitride semiconductor substrate manufactured by changing the Al composition, the film thickness, and the carbon concentration of the region 1 and the region 2 is changed. The voltage was measured and compared. Table 1 shows the parameter values and threshold voltages of region 1 and region 2 at this time. In addition, in the evaluation results of the example and the comparative example in which each parameter was changed, those in which the characteristic was changed except for the threshold voltage were described as special notes.

From the results in Table 1, it can be seen that the threshold voltage is 1.5 V or more in the implementation range of the present invention, and the effect of improving the threshold is exhibited.

  The present invention is suitable as a nitride semiconductor substrate used for a nitride semiconductor suitable as a high-current control inverter or a high-speed and high-voltage electronic device.

1 ... Si single crystal substrate, 2 ... intermediate layer, 21 ... first buffer layer, 22 ... second buffer layer, 3 ... normally off layer, 31 ... region 1, 32 ... region 2, 4 ... device activity Layer, 41 ... electron transit layer, 42 ... electron supply layer.

Claims (2)

A substrate formed of Si single crystal, an intermediate layer formed of a nitride semiconductor stacked structure including at least one nitride semiconductor layer including Al formed on the substrate, and a composition Al _x formed on the intermediate layer. Ga _1-x N (0 ≦ x ≦ 0.05), thickness of 200 nm to 2000 nm, carbon concentration of 1′10 ¹⁸ atoms / cm ^{3 to} 1′10 ²¹ atoms / cm ³ of single layer or multilayer structure A region 1 made of a physical semiconductor; a composition Al _y Ga _1-y N (0.1 ≦ y ≦ 1); a thickness of 0.2 nm to 100 nm; a carbon concentration of 1′10 ¹⁸ atoms. / Cm ³ or more and 1'10 ²¹ atoms / cm ³ or less of a region 2 made of a nitride semiconductor having a single layer or a multilayer structure, and a nitride semiconductor device active layer formed on the region 2 The nitride semiconductor substrate characterized by the above-mentioned. The intermediate layer has a carbon concentration of 1′10 ¹⁸ atoms / cm ³ or more and 1′10 ²¹ atoms / cm ³ or less, and the device active layer has a carbon concentration of 5′10 ¹⁷ atoms / cm ³ or less. The nitride semiconductor substrate according to claim 1.