Patent Application
20110177678
1. Field of the Invention
The present invention relates to a method for manufacturing a nitride semiconductor device composed of a group III-V nitride-based semiconductor. More specifically, the present invention relates to a method for manufacturing an excellent nitride semiconductor device through simple processes.
2. Background Art
As a material for light-emitting elements or electronic devices, such as semiconductor laser elements and light-emitting diodes, group III-V nitride-based semiconductors have been actively studies and developed. Utilizing the characteristics thereof, blue light-emitting diodes, green light-emitting diodes; and blue-violet semiconductor lasers as the light sources for high-density optical disks have already been in practical use.
As a group-V material gas in the crystal growth of nitride-based semiconductors ammonia (NH3) is widely used. InGaN used for the active layer of a light-emitting element is not crystallized unless it is grown at about 900° C. or lower, because In is easily re-vaporized from the surface. Since the decomposition efficiency of NH3 is extremely low in this temperature range, a large quantity of NH3 is required. Moreover, since the V/III ratio must be practically elevated, and the growing speed must be lowered, there has been a problem wherein unintended impurities are mixed in the crystal.
When blue to green visible light is emitted, the In component of InGaN used in the active layer must be 20% or more. In this case, InGaN must be grown at about 800° C. or lower, and a more quantity of NH3 is required. Furthermore, since InGaN having 20% or more In component is easily deteriorated by heat, the active layer is deteriorated in the growing process of the clad layer and the contact layer grown on the InGaN active layer, or by heat treatment performed in the wafer processing to lower the light-emitting efficiency, there has been the problem wherein device characteristics are worsened.
In order to solve the above-described problems, methods wherein hydrazine, which has a high decomposition efficiency, is used as a group-V material gas in place of NH3, have been disclosed (for example, refer to Japanese Patent Application Laid-Open No. 2001-144325). Furthermore, in order to reduce the thermal damage of active layers, methods wherein p-layers are grown at a temperature of 900° C. or lower, have been disclosed (for example, refer to Japanese Patent Application Laid-Open No. 2004-87565).
However, by only using hydrazine as the group-V material gas, there has been a problem wherein the quality improvement of the InGaN active layer is insufficient, and especially light-emitting characteristics are deteriorated when visible light of blue to green is emitted. Moreover, even if the p-layer is grown at a temperature of 900° C. or lower, there has been another problem wherein the active layer is thermally damaged during high-temperature annealing at 800 to 1000° C. for activating Mg used as a p-type dopant, and light-emitting characteristics are degraded. This problem has significantly occurred in the region where the active layer wavelength exceeds blue color.
In view of the above-described problems, an object of the present invention is to provide a method for manufacturing an excellent nitride semiconductor device through simple processes.
According to the present invention, a method for manufacturing a nitride semiconductor device comprises: forming an n-type nitride-based semiconductor layer on a substrate; forming an active layer of a nitride-based semiconductor having In on the n-type nitride-based semiconductor layer by using ammonia and hydrazine derivative as group-V materials and a carrier gas having hydrogen; and forming a p-type nitride-based semiconductor layer on the active layer by using ammonia and hydrazine derivative as group-V materials.
The present invention makes it possible to manufacture an excellent nitride semiconductor device through simple processes.
Other and further objects, features and advantages of the invention will appear more fully from the following description.
The embodiments of the present invention will be described below referring to the drawings. The same constituents will be donated by the same numerals and characters, and the description thereof will be omitted.
On the major surface (0001) of an n-type GaN substrate 10, an n-type Al0.03Ga0.97N clad layer 12 having a thickness of 2.0 μm, an n-type GaN light guide layer 14 having a thickness of 0.1 μm, an active layer 16, a p-type Al0.2Ga0.8N electron barrier layer 18 having a thickness of 0.02 μm, a p-type GaN light guide layer 20 having a thickness of 0.1 μm, a p-type Al0.03Ga0.97N clad layer 22 having a thickness of 0.5 μm, and a p-type GaN contact layer 24 having a thickness of 0.06 μm are sequentially formed.
The p-type Al0.03Ga0.97N clad layer 22 and the p-type GaN contact layer 24 constitute a waveguide ridge 26. The waveguide ridge 26 is formed on the central portion in the width direction of a resonator, and extends between the both cleaved surfaces that become the end surfaces of the resonator.
An SiO2 film 28 is disposed on the sidewall of the waveguide ridge 26 and the exposed surface of the p-type GaN light guide layer 20. An opening 30 of the SiO2 film 28 is disposed on the upper surface of the waveguide ridge 26, and the surface of the p-type GaN contact layer 24 is exposed from the opening 30. A p-side electrode 32 is formed on the exposed p-type GaN contact layer 24. An n-side electrode 34 is formed on the back face of the n-type GaN substrate 10.
A method for manufacturing a nitride semiconductor device according to the first embodiment will be described. As the method for growing crystals, an MOCVD method is used. As group-III materials, trimethyl gallium (TMG), trimethyl aluminum (TMA), and trimethyl indium (TMI), which are organic metal compounds, are used. As group-V materials, ammonia (NH3) and 1,2-dimethylhydrazine (hydrazine derivative) are used. As an n-type impurity material, monosilane (SiH4) is used; and as a p-type impurity material, cyclopentadienyl magnesium (CP2Mg) is used. As a carrier gas for these materials, hydrogen (H2) gas or nitrogen (N2) gas is used. However, as the p-type impurity, Zn or Ca may also be used in place of Mg.
First, the n-type GaN substrate 10, whose surface has been previously cleaned by thermal cleaning or the like, is prepared. Then, after placing the n-type GaN substrate 10 in the reaction furnace of an MOCVD apparatus, the temperature of the n-type GaN substrate 10 is elevated to 1000° C. while supplying NH3. Next, the supply of TMG, TMA, and SiH4 is started to form the n-type Al0.03Ga0.97N clad layer 12 on the major surface of the n-type GaN substrate 10. Next, the supply of TMA is stopped to form the n-type GaN light guide layer 14. Next, the supply of TMG and SiH4 is stopped, and the temperature of the n-type GaN substrate 10 is lowered to 750° C.
Next, as the carrier gas, a small quantity of H2 gas is mixed to N2 gas, and ammonia, 1,2-dimethylhydrazine, TMG, and TMI are supplied to form the In0.2Ga0.8N well layer 16a. Then, the supply of TMI is stopped, and ammonia, 1,2-dimethylhydrazine, and TMG are supplied to form the GaN barrier layer 16b. Two pairs of these are alternately laminated to form the active layer 16 having the multiple quantum well (MQW) structure. Here, the flow rate of H2 gas is within a range between 0.1% and 5% of the flow rate of the total gas.
Next, the temperature of the n-type GaN substrate 10 is elevated again from 750° C. to 1000° C. while supplying NH3 having a flow rate of 1.3×10−1 mol/min and nitrogen gas having a flow rate of 20 L/min. Then, using a 1:1 mixed gas of hydrogen gas and nitrogen gas as the carrier gas, TMG having a flow rate of 2.4×10−4 mol/min, TMA having a flow rate of 4.4×10−5 mol/min, and CP2Mg having a flow rate of 3.0×10−7 mol/min as group-III materials; and 1,2-dimethylhydrazine having a flow rate of 1.1×10−3 mol/min in addition to NH3 as group-V materials to form the p-type Al0.2Ga0.6N electron barrier layer 18. In this case, the mole ratio of 1,2-dimethylhydrazine supplied to the group-III materials is 3.9, and the mole ratio of NH3 supplied to 1,2-dimethylhydrazine is 120.
Next, the supply of TMA is stopped, and TMG having a flow rate of 1.2×10−4 mol/min and CP2Mg having a flow rate of 1.0×10−7 mol/min are supplied as group-III materials; and 1,2-dimethylhydrazine having a flow rate of 1.1×10−3 mol/min are supplied in addition to NH3 as group-V materials together with the carrier gas to form the p-type GaN light guide layer 20.
Next, the supply of TMA is started again, and TMG having a flow rate of 2.4×10−4 mol/min, TMA having a flow rate of 1.4×10−5 mol/min, and CP2Mg having a flow rate of 3.0×10−7 mol/min are supplied as group-III materials; and NH3 and 1,2-dimethylhydrazine are supplied as group-V materials to form the p-type Al0.03Ga0.37N clad layer 22. In this case, the mole ratio of 1,2-dimethylhydrazine supplied to the group-III materials is 4.3, and the mole ratio of NH3 supplied to 1,2-dimethylhydrazine is 120. The carbon concentration in the p-type Al0.03Ga0.97N clad layer 22 is 1×1018 cm−5 or less.
Next, the supply of TMA is stopped, and TMG having a flow rate of 1.2×10−4 mol/min and CP2Mg having a flow rate of 9.0×10−7 mol/min are supplied as group-III materials; and 1,2-dimethylhydrazine having a flow rate of 1.1×10−5 mol/min are supplied in addition to NH3 as group-V materials together with the carrier gas to form a p-type GaN contact layer 24. In this case, the mole ratio of 1,2-dimethylhydrazine supplied to the group-III materials is 9.4, and the mole ratio of NH3 to 1,2-dimethylhydrazine is 120.
Next, the supply of TMG, which is the group-III material, and CP2Mg, which is the p-type impurity material, is stopped, and the system is cooled to about 300° C. while supplying group-V materials. Then, the supply of the group-V materials is stopped, and the system is cooled to a room temperature. When the supply of TMG and CP2Mg is stopped, the system may be cooled to about 300° C. while stopping the supply of NH3, and while supplying 1,2-dimethylhydrazine alone; or the supply of NH3 and 1,2-dimethylhydrazine may be simultaneously stopped.
After the above-described crystal growing, a resist is applied onto the entire surface of the p-type GaN contact layer 24, and by lithography, a resist pattern corresponding to the shape of the mesa-like portion is formed. By reactive ion etching (RIE) using the resist pattern as a mask, the area from the p-type GaN contact layer 24 to the middle of the p-type Al0.03Ga0.97N clad layer 22 is etched to form the waveguide ridge 26 that becomes a light waveguide structure. As an etching gas for RIE, for example, a chlorine-based gas is used.
Next, leaving the resist pattern, the SiO2 film 28 having a thickness of 0.2 μm is formed on the entire surface of the n-type GaN substrate 10 using, for example, CVD, vacuum vapor deposition, and sputtering. Then, at the same time of the removal of the resist pattern, the SiO2 film 28 on the waveguide ridge 26 is removed by a method referred to as a liftoff method. Thereby, the opening 30 is formed in the SiO2 film 28 on the waveguide ridge 26.
Next, a Pt film and an Au film are sequentially formed on the p-type GaN contact layer 24 using vacuum vapor deposition. Thereafter, a resist (not shown) is applied and lithography and wet etching or dry etching are carried out to form the p-side electrode 32 in ohmic contact with the p-type GaN contact layer 24.
Next, a Ti film, a Pt film, and an Au film are sequentially formed on the back face of the n-type GaN substrate 10 by vacuum vapor deposition to form the n-side electrode 34. Next, the n-type GaN substrate 10 is processed into bar-shaped portions by cleavage or the like to form both end planes of the resonator. Then, after applying coating onto the end planes of the resonator, and the bar portions are cleaved into chips to manufacture the nitride semiconductor device according to the first embodiment.
In the above-described manufacturing method, a mixed gas of ammonia and 1,2-dimethylhydrazine as group-V materials to form the active layer 16. Thereby, an effective V/III ratio can be achieved even at a low growing temperature of 900° C. or lower, and the generation of N holes, which is a crystal fault, can be suppressed, and the mixing of impurities can by reduced. The manufacturing method according to the present embodiment can be applied not only to the InGaN quantum well structure, but also to In-containing active layers.
In addition, by adding several percent H2 to the carrier gas, an etching function works in the growth of InGaN, and the segregation of In can be reduced to grow a quantum well structure having favorable optical properties.
Also, SiH4 may be introduced into the InGaN active layer. For example, by doping Si to have a carrier concentration of 1×1018 cm−3, Si acts so as to bury N holes, and spot defects to become non-light emitting centers are reduced and the mixing of impurities are suppressed to further improve crystallinity. When emitting of light having shorter wavelength than blue wherein the In composition exceeds 20%, since growing is performed at lower temperatures, the efficiency of decomposing the group-V materials is lowered, and since N holes are easily generated in the InGaN crystals, the effect of crystallinity improvement by Si doping becomes further significant.
Next, the reason why both NH3 and hydrazine derivatives (e.g., 1,2-dimethylhydrazine) are used in forming the p-type nitride-based semiconductor layer will be described.
When the p-type nitride-based semiconductor layer is formed, if only NH3 is used as a group-V material, H radicals formed from NH3 is incorporated in crystals in the p-type nitride-based semiconductor layer, and the H radicals react with the p-type impurities to generate H passivation (lowering of the activation of p-type impurities). Therefore, a heat treatment process for activation is required, and there is a problem wherein the escape of N occurs from the outermost surface of the crystals by heat treatment, and the quality of crystals is lowered. In addition, there is another problem wherein the active layer is damaged by heat treatment, and light-emitting characteristics are lowered.
Therefore, when the group-V material is changed from ammonia gas to dimethylhydrazine (UDMHy), CH3 radicals produced from UDMHy react with H radicals, and the H radicals produced from UDMHy are not incorporated in the crystals in the p-type nitride-based semiconductor layer.
However, since trimethyl gallium (TMGa), which is an organic metal compound, is used as the group-III material, CH3 radicals are liberated from TMGa, and unless the CH3 radicals are exhausted as CH4, the CH3 radicals are incorporated in the crystals to elevate carbon concentration of the crystals, and elevate the resistivity of the p-type nitride-based semiconductor layer.
Therefore, when the V-group material is completely changed from ammonia gas to dimethylhydrazine, since H radicals, which are required in producing CH4 from CH3 radicals, become insufficient, in the present embodiment, a prescribed quantity of NH3 that can supply the required quantity of H radicals to produce CH4 is added.
Specifically, when the p-type nitride-based semiconductor layer is formed from dimethylhydrazine, firstly in order to lower the concentration of carbon incorporated in the crystals, in other words, in order to suppress the incorporation of compensated carbon by the accepter, H radicals required to exhaust CH3 radicals liberated from dimethylhydrazine as CH4 is supplied from NH3.
However, if the quantity of H radicals produced from NH3 is excessively large, H-passivation occurs; therefore, the supply quantity of NH3, which is the supply source of H radicals, is made to be requisite minimum.
As described above, by supplying a prescribed flow rate of a mixed gas of NH3 and 1,2-dimethylhydrazine as group-V materials when the p-type nitride-based semiconductor layer is formed, the occurrence of H-passivation can be suppressed, and the p-type nitride-based semiconductor layer having a low concentration of contained carbon and having a low electrical resistivity in an as-grown state can be formed. Therefore, since heat treatment processes for activating Mg used as a p-type dopant can be omitted, and thermal damage to the active layer can be reduced, an excellent nitride semiconductor device can be manufactured using simple processes.
As a result, when the NH3/hydrazine supply mole ratio becomes 10 or less, since the supply of the H radicals becomes insufficient and the carbon concentration in the crystal is elevated, the resistance is elevated. On the other hand, when the NH3/hydrazine supply mole ratio is between 500 and 1000, the resistivity is sharply elevated. This is because H passivation is caused when H is incorporated into the crystal by the excessive supply of NH3. Therefore, the range of the NH3/hydrazine supply mole ratios is preferably between 10 and 1000 inclusive, and more preferably between 20 and 500 inclusive.
As a result, resistivity is sharply elevated between the hydrazine/group-III material supply mole ratios 20 and 25. This is caused by the elevation of carbon concentration in the crystal. On the other hand, if the hydrazine/group-III material supply mole ratio is less than 1, group-V holes are produced in the crystal, and crystal defection is caused. Therefore, when the p-type GaN layer is formed, the supply mole ratio of hydrazine to organic metal compound is preferably 1 or more and less than 25, more preferably 3 or more to 15 or less.
As a result, the carbon concentration in the crystal was sharply lowered at temperatures between 800° C. and 900° C. In addition, when the growing temperature was lowered, the decomposition of NH3 was reduced, and no CH3 radicals were released as CH4. This is considered that the CH3 radicals are incorporated in the crystals. On the other hand, the temperature at which the crystal growth of p-type GaN is feasible is lower than 1200° C. Therefore, when the p-type GaN layer is formed, the temperature of the n-type GaN substrate 10 is preferably 800° C. or higher and lower than 1200° C., and more preferably 900° C. or higher and lower than 1100° C.
As a result, although the PI, intensity of the active layer is little changed until the growing temperature of the p-type clad layer was 1100° C., the PI, intensity was sharply lowered when the growing temperature exceeded 1100° C. This was because the active layer was damaged by heat exceeding 1100° C., and light-emitting characteristics were degraded. In addition, the thermal damage to the active layer and the carbon concentration in the p-type nitride-based semiconductor layer must be taken in consideration. Therefore, the growing temperature of the p-type nitride-based semiconductor layer is preferably 800° C. or higher and lower than 1100° C., more preferably 900° C. or higher and lower than 1100° C.
Although it is preferable that no carbon is contained in the p-type GaN layer, when hydrazine is used, even a small quantity of carbon is incorporated in the p-type GaN layer. However, the carbon concentration of the p-type GaN layer can be made to be 1×1018 cm−3 or lower by selecting the manufacturing conditions according to the present embodiment.
Also when the p-type nitride-based semiconductor layer is formed, a mixed gas of hydrogen gas and nitrogen gas, wherein the volume composition ratio of the hydrogen gas is x (0≦x≦1), and the volume composition ratio of the nitrogen gas is 1−x, is used as the carrier gas. Specifically, the carrier gas for forming the p-type nitride-based semiconductor layer may be any of nitrogen gas alone, a mixed gas of nitrogen gas and hydrogen gas, and hydrogen gas alone. Here, when the temperature of the substrate is about 1000° C., hydrogen gas is not dissociated and present as the state of hydrogen molecules, and is not incorporated in the crystal. Therefore, H radicals incorporated in the crystal are considered to be mainly H radicals dissociated from NH3, so the p-type nitride-based semiconductor layer having a low resistivity can be formed even if the carrier gas is hydrogen gas alone. For example, a mixed gas of a flow rate of 10 L/min of hydrogen gas and a flow rate of 10 L/min of nitrogen gas in the ratio of 1:1 can be used ad the carrier gas.
The active layer 36 is a multiple quantum well structure wherein 2 pairs of Al0.01In0.21Ga0.78N well layers 36a each having a thickness of 3.0 nm and Al0.01In0.015Ga0.975N barrier layers 36b each having a thickness of 16.0 nm are alternately laminated.
A method for manufacturing the active layer 36 will be described. First, the temperature of the n-type GaN substrate 10 is elevated to 750° C. while supplying NH3 gas. Next, as the carrier gas, a small quantity of H2 gas is mixed to N2 gas, and ammonia, 1,2-dimethylhydrazine, TMG, TMI, and TMA are supplied to form the Al0.01In0.21Ga0.78N well layer 36a and the Al0.01In0.015Ga0.975N barrier layer 36b. By alternately laminating 2 pairs of these, the active layer 36, which is a multiple quantum well (MOW) structure, is formed.
In the present embodiment, since the active layer 36 is composed of AlInGaN, and the bonding force of the crystal is improved in comparison with the active layer composed of InGaN, the degradation of the crystal by heat can be prevented. As for the rest, the same effects as those in the first embodiment can be achieved.
The active layer 38 is a multiple quantum well structure wherein 2 pairs of In0.2Ga0.8N well layers 38a each having a thickness of 3.0 nm and Al0.03In0.002Ga0.968N barrier layers 38b each having a thickness of 16.0 nm are alternately laminated.
A method for manufacturing the active layer 38 will be described. First, the temperature of the n-type GaN substrate 10 is elevated to 750° C. while supplying NH3 gas. Next, as the carrier gas, a small quantity of H2 gas is mixed to N2 gas, and ammonia, 1,2-dimethylhydrazine, TMG, and TMI are supplied to form the In0.2Ga0.8N well layer 38a. Then, ammonia, 1,2-dimethylhydrazine, TMG, TMI, and TMA are supplied to form the Al0.03In0.02Ga0.968N barrier layer 38b. By alternately laminating 2 pairs of these, the active layer 38, which is a multiple quantum well (MQW) structure, is formed.
Since the well layer 38a is composed of InGaN, which is a ternary mixed crystal having excellent crystallinity, and the barrier layer 38b is composed of AlInGaN, which is a quaternary mixed crystal having excellent heat resistance, a light-emitting element having more excellent light-emitting characteristics can be obtained. As for the rest, the same effects as those in the first embodiment can be achieved.
In addition, it is preferable that the well layer 38a has a compressive strain because of being composed of InGaN having an a-axis length longer than GaN of the substrate 10; and the barrier layer 38b has a tensile strain because of being composed of InAlGaN having an a-axis length shorter than GaN of the substrate 10. Normally, the well layer of the active layer that emits blue-violet or blue light has a large compressive strain, and the strain quantity is enlarged when the wavelength becomes longer. Then, when the wavelength becomes longer than the wavelength of blue, misfit dislocation is significantly occurred by strain. Whereas, by using the barrier layer 38b having a tensile strain, the occurrence of misfit dislocation can be reduced.
Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
The entire disclosure of a Japanese Patent Application No. 2010-009288, filed on Jan. 19, 2010 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, are incorporated herein by reference in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-009288 | Jan 2010 | JP | national |
