JP2000306854A - Activation method of gallium nitride based p-type compound semiconductor layer - Google Patents

Abstract

(57) [Problem] To reduce the resistance of a gallium nitride based p-type compound semiconductor layer. A method for activating a p-type layer (6, 7) included in a gallium nitride-based compound semiconductor device is to emit light having a wavelength within a range from ultraviolet to visible light by 200 to 200 μm.
Irradiate the p-type layer (6, 7) at a temperature in the range of 500 ° C., thereby separating and removing hydrogen bonded to the p-type dopant contained in the p-type layer (6, 7). It is characterized by promoting activation of a dopant as an acceptor.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for improving the activity of a dopant by separating and neutralizing and removing hydrogen bonded to a dopant added to a gallium nitride-based compound semiconductor crystal layer. In particular, the present invention relates to reducing the resistance of a p-type conductive layer by improving the activation rate of an acceptor dopant.

[0002]

2. Description of the Related Art GaN, GaInN, GaAlN, Ga
In gallium nitride based semiconductors such as InAlN, the energy transition of electrons is a direct transition, and the energy band gap can be changed in the range of 1.95 to 6.2 eV by controlling the composition, and the thermal stability is further improved. Therefore, gallium nitride based semiconductors are expected to be used as materials for semiconductor light emitting devices, particularly as materials for light emitting diodes and semiconductor lasers for emitting blue to ultraviolet light. However, according to the gallium nitride-based semiconductor, it is not easy to obtain a low-resistance layer even if a p-type dopant is added, and it is difficult to commercialize a high-luminance light-emitting element using the gallium nitride-based p-type semiconductor layer. It has become.

Under ordinary conditions for growing a semiconductor crystal layer by the MOCVD method, hydrogen contained in a source gas for crystal growth and a carrier gas is combined with a crystal constituent element and a dopant to form a composite. Such a phenomenon that the original activity of the dopant bonded to hydrogen is hindered is known in semiconductor materials such as Si, GaAs, ZnSe, and GaN. P with high carrier concentration
In order to realize an n-type or n-type conductive layer, the dopant constituting the composite and hydrogen must be separated by some means, and the separated hydrogen must be neutralized and removed.

In gallium nitride based compound semiconductors,
Generally, the activation rate of the acceptor dopant is low, and it is difficult to obtain a high hole concentration. In particular, when Mg having a relatively shallow impurity energy level is used as a dopant, its activation rate generally becomes low. As described above, the reason for such a low activation rate is that the proton (H ⁺⁾ bonds to the acceptor (A ⁻⁾ added during the growth of the semiconductor crystal layer, and the complex (A
^- -H ⁺⁾ is formed, it is that the activity of the acceptor is inhibited. As a result of prior art efforts to solve such problems, hydrogen is separated by subjecting the composite (A ⁻ −H ⁺ ) to an annealing treatment at a relatively high temperature in an inert gas atmosphere, By discharging the hydrogen out of the crystal, the activation rate of the high-concentration p-type conductive layer is improved, and an improved pn junction can be formed. Such a technique is disclosed in Japanese Unexamined Patent Publication No. 2-25767.
9 and JP-A-5-183189.

Japanese Patent Application Laid-Open No. Hei 2-257679 discloses a technique in which an i-type gallium nitride-based compound semiconductor layer is irradiated with an electron beam. It is stated that a low-resistance gallium nitride-based p-type semiconductor layer can be realized by applying a technique of annealing at a temperature.
In these prior art documents, when a gallium nitride-based semiconductor crystal layer is formed by a vapor phase growth method, ammonia as a nitrogen source and hydrogen or a hydrogen compound as a carrier gas are generally used. It is stated that the hydrogen bonded to the acceptor added to the gallium nitride based semiconductor prevents the p-type dopant from acting as an acceptor.

The above-mentioned JP-A-2-257679 and JP-A-5-257679
Certainly, a low-resistance p-type layer of a gallium nitride-based semiconductor can be formed by the technique disclosed in US Pat. No. 5,183,189. In the former technique, high-energy electrons are used to irradiate electrons to a deep region of the p-type semiconductor layer. In this case, the crystal of the semiconductor layer is damaged. Further, according to the latter prior art, the optimum annealing temperature is as high as about 700 ° C., so that the generation of donor levels due to the elimination of nitrogen in the semiconductor crystal, especially the gallium indium nitride-based semiconductor Since the dissociation temperature of is low, the generation amount of nitrogen vacancies acting as a shallow donor such as 40 meV increases, and the acceptor is canceled by that much, which results in the inhibition of activation.

[0007] Therefore, the growth stripe of the normal semiconductor crystal layer
The semiconductor layer obtained according to the above conditions is treated by the aforementioned prior art.
The hole concentration obtained is 3 × 10¹⁷~ 8 ×
10 ¹⁷/ Cm^ThreeAnd using such a semiconductor layer
Enough for the pn junction and the ohmic properties of the electrodes
Performance cannot be obtained and light emitting devices such as blue laser
This is the reason that practical application is difficult.

Recently, a current injection method effective for lowering the annealing temperature for activating the acceptor has been proposed in the 45th Applied Physics Association Related Conference Proceedings 30a-ZR.
-5. According to this report, if a light emitting diode including a GaN layer doped with Mg as an acceptor is formed and then annealed with a forward current applied to the diode, hydrogen is dissociated at about 400 ° C. This means that the Mg dopant is activated. However, although this method has the effect of activating dopants by low-temperature annealing, considering the practical use as a semiconductor device manufacturing process, the process becomes considerably complicated, and the manufacturing cost of the semiconductor device increases. There is a problem of getting high.

[0009]

As described above in relation to a gallium nitride based semiconductor, the phenomenon that the activation of the p-type dopant is hindered by hydrogen is caused by zinc selenide,
It is also observed in gallium arsenide, silicon and the like, and is a problem common to various semiconductor materials. As described above, the reduction of the resistance of the p-type layer is insufficient at present, and has become an important issue in the production of semiconductor devices.

For example, in the case of GaN, GaAlN, GaInN, etc., which are currently attracting attention as materials for high-luminance light-emitting elements, when growing a p-type conductive layer in a crystal,
Generally, Mg having a level of 160 meV is added as a dopant for the purpose of forming a shallower acceptor level. If the acceptor A ^- is associated with the protons H ⁺ that have entered the crystal layer during the crystal growth by MOCVD, the acceptor A ^- is electrically coupled to generate a composite MgH.

Japanese Patent Application Laid-Open No. 5-183189 describes that after the growth of a p-type layer, the Mg dopant can be activated by annealing this layer at a temperature of 400 ° C. or more at which the composite MgH is decomposed. However, the decomposition method using thermal energy cannot sufficiently decompose the composite MgH, and the residual proton increases due to insufficient neutralization of the separated proton H ⁺ . Therefore, the Mg dopant cannot be sufficiently activated as an acceptor by decomposition only with thermal energy, and its activation rate is at most about 1%, and there is still a problem concerning the reduction of the resistance of the p-type conductive layer. .

On the other hand, when a crystal layer of a gallium nitride-based semiconductor is grown by a vapor phase growth method, an n-type conductive layer is generally easily formed. Therefore, when forming a p-type conductive layer, Be, Mg, C
There is a problem that a sufficiently high carrier concentration cannot be obtained even if a dopant such as a, Zn, and Cd is added. The main reason for this is that the semiconductor crystal layer is exposed to a relatively high temperature atmosphere of about 1000 ° C. during its growth,
If the nitrogen partial pressure of the atmosphere is low, dissociation of nitrogen occurs in the grown crystal to form a vacancy at the nitrogen lattice point, and this nitrogen vacancy serves as a donor. The amount of nitrogen vacancies generated in this manner changes significantly depending on the growth conditions of the semiconductor crystal layer, and is about 1 × 10 ¹⁷ to 1 × 10 ^22.
/ Cm ³ is said to change. In general, in the formation of a p-type conductive layer, it should be possible to compensate the donor of the nitrogen vacancy with an acceptor generated by adding a large amount of a p-type dopant, thereby increasing the acceptor concentration. However, actually, the acceptor concentration in the p-type conductive layer grown by vapor phase is 10 ¹⁸ / c
Saturation is reached on the order of m ³ .

In view of the above-mentioned problems in the prior art, the present invention provides a method for simply and efficiently activating an acceptor dopant in a p-type conductive layer of a gallium nitride-based compound semiconductor crystal, whereby a low-resistance p-type The purpose is to obtain a conductive layer.

[0014]

According to the present invention, a method for activating a p-type layer included in a gallium nitride-based compound semiconductor device comprises the steps of: providing light having a wavelength within a range from ultraviolet to visible light; P at a temperature in the range of 200-500 ° C.
Irradiation is performed on the mold layer, whereby hydrogen bonded to the p-type dopant contained in the p-type layer is separated and removed to promote activation of the p-type dopant as an acceptor.

Light irradiation for activating a p-type dopant as an acceptor may be performed by periodic or aperiodic intermittent irradiation of light of a constant intensity.

The light irradiation of the p-type layer can be performed in an inert gas atmosphere. Further, the light irradiation on the p-type layer can be performed before or after the step of forming an electrode included in the semiconductor device.

That is, in the present invention, for example, when forming a p-type light-emitting layer or a p-type conductive layer as a p-type clad layer included in a semiconductor light-emitting device, such a p-type conductive layer is visible from ultraviolet rays. By irradiating light containing a wavelength included in the light, protons are directly separated from the complex of the dopant and hydrogen. At this time, the electrons generated by the light irradiation also function to separate protons from the composite MgH and neutralize the protons by the electric energy, so that activation of the p-type dopant is effectively promoted, and the p-type dopant is effectively activated. An improvement in performance of a semiconductor device that includes a conductive layer is provided.

Means for generating electrons in the p-type conductive layer before forming an anode and a cathode for energizing a semiconductor device include light having an energy larger than the band gap energy of the gallium nitride-based semiconductor, that is, ultraviolet light. It is sufficient to irradiate light including a wavelength within the range of visible light.
At this time, it is necessary to generate electrons more than at least the number of protons bonded to the dopant. However, considering the recombination ratio with holes generated at the same time as the generation of electrons, one to two orders of magnitude higher than protons. It is desired to promote the dissociation and neutralization of hydrogen from the composite (Mg ⁻ -H ⁺ ) by irradiating light energy and light amount that can generate as many electrons as possible in the p-type conductive layer.

In other words, the method for activating the p-type layer according to the present invention does not require the formation of an electrode for the activation, and is a very simple method which can be achieved only by irradiating light at a relatively low temperature. However, there is a problem that holes are generated simultaneously with electrons due to light irradiation. However, in order to effectively dissociate hydrogen from the composite, the light irradiation intensity must be increased to increase the number of electron hole pairs,
In addition, the diffusion distribution of electrons and holes can be controlled by irradiating the p-type conductive layer with light non-uniformly. In this way, the probability of annihilation of electrons due to direct recombination of electrons and holes can be reduced, thereby increasing the number of electrons that should contribute to the dissociation of hydrogen from the composite. Although the detailed mechanism of such a phenomenon is not always clear, the generated electrons are converted to a complex (Mg ⁻ −H ⁺ ).
Proton H ⁺ electrically coupled to Mg ⁻ when approaching a certain range of H ⁺ therein is neutralized by electron charge e ⁻ , and its hydrogen is neutralized. Is considered to be a phenomenon.

[0020]

Embodiments of the present invention will be described below. However, the following embodiments are examples for embodying the technical idea of the present invention, and the method of the present invention is limited by the exemplified material composition, crystal growth conditions, type of vapor growth gas, and the like. Not something.

FIG. 1 is a schematic sectional view showing an example of a gallium nitride-based compound semiconductor light emitting device to which the present invention can be applied. In such a light emitting device, sapphire, MgAl ₂ O ₄ , GaN, MgO,
Materials such as SiC, Si, ZnO can be used, but usually sapphire is used. When MOCVD is used to grow the GaN buffer layer 2 on the substrate 1, hydrogen may be used as a carrier gas, and ammonia (NH ₃ ) and trimethylgallium (TMG) may be used as source gases.

On the GaN buffer layer 2, n ⁺ -type GaN
The layer 3, the n-type GaAlN layer 4, the n-type GaInN layer 5, the p-type GaAlN layer 6, and the p ⁺ -type GaN layer 7 are layers having respective carrier concentrations and thicknesses suitable for forming a light emitting device. Grow continuously. In the present invention, p
P-type GaAlN layer 6 and p ⁺ -type GaN layer 7 as p-type conductive layers
In order to obtain a p-type conductive layer having a high carrier concentration, hydrogen can be dissociated from a hydrogen-bonded acceptor to increase the activation rate as an acceptor.

In a gallium nitride-based semiconductor, the activation efficiency of a p-type dopant such as Be, Mg, Ca, Zn, or Cd, which should serve as an acceptor, is low, and it is difficult to obtain a high hole concentration. In particular, when Mg, which is a p-type dopant capable of forming a shallow impurity level of 160 meV, is used, the activation rate is about 1% even when a thermal annealing treatment is performed, and the obtained hole concentration is low. About 5 × 10
It is as low as about ¹⁷ / cm ³ .

According to the method of the present invention, 200 to 500 ° C.
At a relatively low temperature, for example, about 300 ° C.
A group of electrons can be excited and generated by irradiating the conductive layer with light. In order to enhance the generation of excited electrons, it is sufficient to irradiate light having energy corresponding to the band gap of the p-type conductive layer. Generally, in a semiconductor device, a plurality of P-type conductive layers having a plurality of band gaps different from each other are used. In such a case, it is preferable to irradiate mixed light containing a plurality of different wavelengths that are appropriate within a range from ultraviolet to red.

In the treatment method of the present invention, hydrogen is separated from the hydrogen-bonded dopant by hydrogen and neutralized by the electron. Therefore, the heating temperature for promoting the activation of the dopant is a relatively low temperature of about 300 ° C. It is enough. The relatively low processing temperature of about 300 ° C.
It has an important significance in activating a p-type conductive layer of a gallium nitride based semiconductor. In other words, compared to the conventional activation method at a relatively high temperature, it is possible to suppress the desorption of nitrogen having a high vapor pressure and to suppress the generation of nitrogen vacancies acting as donors due to the desorption, and to control the p-type carrier concentration. Is enhanced.

The light intensity of the electron group excitation light source in the method of the present invention is preferably higher for activation of the dopant, but there is a concern that crystal damage may occur due to local absorption heat due to crystal defects and the like. Therefore, it is not preferable that the light intensity is too high. For example, a light intensity of about 250 mW / cm ² can be preferably used.

FIG. 2 is a graph showing that a gallium nitride layer doped with Mg as a p-type dopant is improved to a low-resistance p-type layer using the present invention. In this graph, the horizontal axis represents the temperature (° C.) at the time of light irradiation of the p-type layer,
The vertical axis represents the resistivity (Ω · cm) after light irradiation.
In the measurement of FIG. 2, MOCVD was performed on a sapphire substrate.
A GaN buffer layer is formed by a method, and a GaN layer is formed on the GaN buffer layer while adding Mg as a p-type dopant.
m. Then, 250 mW / c at various temperatures with respect to the p-type GaN layer in a nitrogen atmosphere.
m ² UV light was pulsed for 20 minutes. As apparent from FIG. 2, the resistivity of the p-type GaN layer starts to decrease from the light irradiation temperature of about 200 ° C., and becomes almost constant low by light irradiation at a temperature of about 400 ° C. or more. From this, it is apparent that the method for activating the p-type conductivity type layer according to the present invention has a remarkable effect.

An embodiment in which a gallium nitride-based compound semiconductor light emitting device is manufactured by using the method of activating a p-type dopant according to the present invention will be described below.

[0029]

(Embodiment 1) Referring to FIG. 1, a sufficiently cleaned sapphire substrate 1 is placed in an MOCVD reaction tube, and this sapphire substrate is heated to 1100 ° C. while flowing a carrier gas of hydrogen. Thermal cleaning was performed.
After the substrate temperature is lowered to 500 ° C., hydrogen as a carrier gas and TMG and NH as source gases
₃ is used to form a 20-nm thick Ga on the sapphire substrate 1.
An N buffer layer 2 was grown.

Thereafter, the TMG gas is stopped and the substrate temperature is set to 1
After the temperature was raised to 030 ° C, TMG and
And NH_ThreeTogether with Si as a donor dopant
Silane gas SiH as a dopant gas for addition
_FourIs supplied and 1 × 10¹⁹/ Cm^ThreeHas a carrier concentration of
N for negative contact⁺Type GaN layer 3 is 3.5 μm thick
I was grown up. Subsequently, the amount of Si added was reduced.
SiH to reduce _FourReduce the gas flow, 1 × 1
0¹⁸/ Cm^Three0.5 μm thick having a carrier concentration of
n-type Ga_0.8Al_0.2An N cladding layer 4 was grown.

After the growth of the n-type cladding layer 4, both the source gas and the dopant gas are stopped, the substrate temperature is lowered to 800 ° C., the carrier gas is switched from hydrogen to nitrogen, and TMG, TMI (trimethyl Indium), NH ₃ , and SiH ₄ as a dopant gas
Using the above, an n-type Ga _0.85 In _0.15 N layer 5 having a thickness of 10 nm was grown.

Further, the source gas and the dopant gas are stopped, the substrate temperature is raised to 1030 ° C., and TMG, NH ₃ , and TMA (trimethylaluminum) as the source gas, and Cp ₂ Mg (cyclo) as the dopant gas. A p-type Ga _0.9 Al _0.1 N layer 6 doped with Mg was grown to a thickness of 0.2 μm using pentadienyl magnesium). Thereafter, the TMA was stopped, and the flow rate of Cp ₂ Mg was increased to grow the p ⁺ -type GaN layer 7 for anode contact to a thickness of 0.3 μm.

After the growth of the p ⁺ -type GaN layer 7, the substrate 1
The semiconductor layer on the substrate was taken out of the CVD reaction tube and subjected to light irradiation. As the light irradiation conditions, a pulse oscillation N ₂ laser (pulse width 10 ns) that emits ultraviolet light having a wavelength of 337 nm, which substantially corresponds to the band gap energy of Ga _0.8 Al _0.2 N and GaN, is used as a light excitation light source. Has an intensity of 250 mW / cm ²
Under a temperature condition of 450 ° C. in a nitrogen atmosphere.
Minutes of light irradiation.

In the semiconductor laminated structure obtained as described above, the anode 9 is formed on the p ⁺ -type GaN contact layer 7 by a well-known method, and the n ⁺ -type GaN clad layer 3 is formed.
By forming the cathode 8 thereon, a 500 μm square light emitting diode was manufactured. In this light emitting diode, uniform light emission having an emission wavelength of 407 nm and an emission output of 1.7 mW was obtained in a state where the forward voltage was 3.4 V and the forward current was 20 mA.

(Example 2) In Example 2, a light emitting diode was manufactured in the same manner as in Example 1. The second embodiment differs from the first embodiment only in that two types of lasers are used as light sources for light excitation. That is, in the second embodiment, the light source for light excitation has a wavelength of 3
H ₂ pulsed laser and the wavelength 4 for emitting ultraviolet light of 37nm
A continuous wave Ar ion laser emitting blue light of 88 nm was used. 10n with respect to pulsed N ₂ laser
s and a light irradiation intensity of 150 mW / cm ² , and 100 mW for a continuous wave Ar ion laser.
/ Cm ² light intensity was used. As a result of measuring the characteristics of the light emitting diode of Example 2 thus obtained, 3.3 V was obtained.
Under a forward voltage of 20 mA and a forward current of 20 mA,
Uniform emission characteristics having an emission wavelength of 7 nm and an emission output of 1.8 mW were obtained.

[0036]

As described above, according to the present invention, gas nitride
Bands for p-type conductive layers of lithium-based compound semiconductors
Irradiates light with energy equivalent to gap energy
Deactivates the acceptor-type dopant
Compound (A ^--H⁺Separates hydrogen from
And electrically neutralize the p-type dopan
Activation can be significantly improved. In addition,
Different suitable as light for electronic excitation in light method
By using light containing multiple wavelengths, for example,
Band gaps different from each other as included in the semiconductor laser
Efficiently activates a plurality of p-type conductive layers
Can be. Further, utilizing the light irradiation of the present invention
The method for activating the p-type layer is simple in its processing step.
It has great value from the viewpoint of the production process.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing a structure of a semiconductor light emitting device to which the present invention can be applied.

FIG. 2 is a graph showing the effect of lowering the resistance of a p-type layer by light irradiation according to the method of the present invention.

[Explanation of symbols]

1 sapphire substrate 2 buffer layer 3 n ⁺ -type GaN layer 4 n-type Ga _1-y Al y N layer 5 n-type Ga _1-x In _x N layer 6 p-type Ga _1-z Al z N layer 7 p ⁺ -type GaN Layer 8 cathode 9 anode

Claims (4)

[Claims] 1. A method for activating a p-type layer included in a gallium nitride-based compound semiconductor device, wherein light having a wavelength within a range from ultraviolet to visible light is emitted from 200 to 50.
Irradiating the p-type layer at a temperature in the range of 0 ° C., thereby separating and removing hydrogen bonded to the p-type dopant contained in the p-type layer and activating the p-type dopant as an acceptor; A method characterized by promoting. 2. The method according to claim 1, wherein the light irradiation of the p-type layer is performed by periodic or aperiodic intermittent irradiation of light having a constant intensity. 3. The method according to claim 1, wherein the light irradiation of the p-type layer is performed in an inert gas. 4. The method according to claim 1, wherein the light irradiation of the p-type layer is performed before or after a step of forming an electrode included in the semiconductor device. .