JP5644996B2 - Nitride optical semiconductor device - Google Patents

Description

  The present invention relates to a nitride optical semiconductor device.

  A nitride optical semiconductor element using a group III nitride semiconductor (GaN, AlGaN, AlInGaN system) is known. For example, it is known to grow a “C-plane” AlN layer on a “C-plane” sapphire substrate and to produce an effective optical semiconductor element thereon. In addition, as an optical semiconductor element using a group III nitride semiconductor, a light receiving element such as a photodiode (PD) is known in addition to a light emitting element such as a laser (LD) and a light emitting diode (LED). The characteristics and reliability of these nitride optical semiconductor elements depend on the crystallinity of the semiconductor used. That is, when a nitride optical semiconductor having excellent crystallinity is used, an optical semiconductor element having excellent characteristics is completed.

  In Patent Document 1, an AlN underlayer is temporarily formed on a C-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD), and then an AlN layer is formed by hydride vapor phase epitaxy (HVPE). The formed AlN layer is the C plane. The C plane is a plane (0001) plane perpendicular to the C axis, meaning the longitudinal direction of the crystal, and the A plane is a (11-20) plane perpendicular to the C plane. The thickness of the grown AlN layer is about 10 μm, but it is estimated that cracks have occurred after the growth. In particular, in the HVPE method, it is difficult to accurately control conditions at the interface portion. Therefore, when an AlN single crystal layer is directly grown on a single crystal substrate by the HVPE method without using an AlN underlayer, an AlN single layer is formed. The crystal quality of the crystal layer is greatly deteriorated, and the process becomes complicated. The surface roughness of this prior art AlN layer is 20 nm or less as measured by an atomic force microscope.

  Patent Document 2 is a technique related to a piezoelectric thin layer, and a crystal plane is a (0001) plane ((001) plane) on a sapphire (11-20) plane (also referred to as (110) plane) substrate (A plane). A technique for forming an AlN layer (C-plane AlN layer) is also disclosed.

  Non-Patent Document 1 discloses a similar compound semiconductor, but the half width (FWHM) of the X-ray rocking curve is 173 to 314 seconds with respect to the (0002) plane (also referred to as (002) plane), Regarding (10-12) plane (also referred to as (102) plane), it is 1574-1905 seconds. Note that, even if the crystallinity related to the (002) plane is good, if the crystallinity related to the (102) plane is poor, the light emitting element has low luminous efficiency, and the light receiving element has low sensitivity. The RMS value (corresponding to Ra) of the surface roughness (atomic force microscope) in this document is 2.34 nm when the smallest value is obtained. Further, when a surface photograph is observed, many large pits are confirmed on the surface, and the surface state is not preferable.

  Non-Patent Document 2 discloses an ultraviolet LED manufactured by the MOCVD method. A template in which a C-plane AlN layer is formed on a C-plane sapphire substrate is used, and the thickness of the AlN layer is 1 μm. The full width at half maximum (FWHM) of the X-ray rocking curve in this document is 60 seconds for the (0002) plane (also referred to as the (002) plane) and for the (10-12) plane (also referred to as the (102) plane). 1200 seconds.

  In a semiconductor crystal for an optical semiconductor element relating to light of a short wavelength (ultraviolet region), it is necessary to increase the Al composition of the AlGaN layer constituting the element, for example. At this time, since the crystal lattice spacing is greatly different, a large tensile strain is generated in the AlGaN layer, and many cracks are generated. In order to suppress this, it is desirable for realizing an optical semiconductor element that AlN is superior in crystal quality and has a lattice spacing smaller than that of AlGaN.

  Also, as described above, high crystallinity is required in the AlGaN layer in order to produce an optical semiconductor element with good characteristics. However, high-quality, large-sized AlN bulk substrates are not currently available, and a “C-plane” AlN layer grown on a “C-plane” sapphire substrate is about 0.8-1 μm. .

JP 2006-321705 A JP 05-327398 A

"Growth of Thick AlN Layer by Hydro Vapor Phase Epitaxy", Japan Journal of Applied Physics "Japanese Journal of Applied Physics", V. 44, no. 17, 2005, pp. L 505-L 507. '' AlGaN-Based Deep Ultraviolet Light-Emitting Diodes Grown on Epitaxic AlN / Sapphire Templates '', Japan Journal of Aps. 47, no. 1, 2008, pp. 43-46.

  Under such a background, there is a demand for the realization of a nitride optical semiconductor element having a high-quality AlGaN layer structure with high quality and no cracks even with a high Al composition. Certainly, although an LED using a substrate having a “C-plane” AlN layer formed on a “C-plane” sapphire substrate has been realized (see Non-Patent Document 1), the crystal quality is not sufficient. An LD requiring high quality crystallinity has not been realized using this substrate. That is, if the thickness of the AlN layer is increased to about 1 to 3 μm in order to improve the crystal quality, cracks are generated and an element having good crystallinity cannot be formed.

The present invention has been made in view of such problems, and an object thereof is crystalline to provide a good nitride compound optical semiconductor element.

In order to solve the above-described problems, a nitride optical semiconductor device according to the present invention is manufactured by using an A-plane sapphire substrate and a temperature T1 (1000 ° C. to 1800 ° C. ) by hydride vapor phase epitaxy (HVPE method) directly on the substrate. After the formation in step C, a C-plane AlN layer having a thickness of more than 1 μm formed at a temperature T2 (1400 ° C. to 1800 ° C.) higher than the temperature T1, and a group III of the first conductivity type formed on the AlN layer It is characterized by comprising: a nitride semiconductor layer; and a second conductivity type group III nitride semiconductor layer formed on the first conductivity type group III nitride semiconductor layer. In the present invention, it was discovered that by growing a C-plane AlN layer exceeding 1 μm on an A-plane sapphire substrate, a C-plane AlN layer excellent in flatness and crystallinity can be obtained without generating cracks. did. In this nitride optical semiconductor element, the cleavage plane (C plane) of the sapphire substrate and the cleavage plane (M plane) of the AlN layer have the same plane orientation.

  By growing a semiconductor element including the first conductive type and the second conductive type semiconductor layer on the AlN layer thus formed, a nitride optical semiconductor element having good characteristics can be obtained. In addition, in this structure, the cleavage plane of the sapphire substrate and the AlN layer can be made to coincide with each other. Therefore, when a laser element is employed as the optical semiconductor element, the cleavage plane that becomes the end face of the resonator is easily formed. There is also an advantage of being able to. The group III nitride semiconductor layer is preferably an AlGaN compound semiconductor layer.

  The first and second conductivity type Group III nitride semiconductor layers are preferably made of AlGaN, and the Al composition ratio is preferably 0.2 or more and 1.0 or less. In the present invention, even in the formation of the AlGaN layer having such a high Al composition ratio, a good crystal can be obtained.

  According to the nitride optical semiconductor device of the present invention, even if a plurality of semiconductor layers including a nitride semiconductor layer are grown thickly, the generation of cracks is suppressed and the compound semiconductor is excellent in crystallinity. ing. This provides a nitride optical semiconductor device having excellent characteristics in a high yield state that is not affected by cracks.

It is a graph which shows the value of the half value width (FWHM) of the X-ray rocking curve of the crystal | crystallization obtained when changing growth temperature. It is a graph which shows the value of the half value width (FWHM) of the X-ray rocking curve of the crystal | crystallization obtained when changing flow volume. It is a graph which shows the value of the half value width (FWHM) of the X-ray rocking curve of an initial stage. It is a figure of the electron micrograph of the crystal | crystallization surface obtained at various temperatures. It is a figure of the electron micrograph of the crystal surface obtained at various flow rates. It is a figure of the electron micrograph of the surface of an initial stage layer. It is sectional drawing of LD. It is a graph which shows the relationship between the wavelength (nm) of the emitted light of LD, and light intensity (arb. Unit). It is a graph which shows the relationship between the drive current (mA) of LD, and light intensity (arb. Unit). It is sectional drawing of LED. It is sectional drawing of PD. It is a time chart of manufacturing temperature.

  Hereinafter, the nitride optical semiconductor device according to the embodiment will be described. Note that the same reference numerals are used for the same elements, and redundant description is omitted.

FIG. 7 is a cross-sectional view of a semiconductor laser device (LD) as a nitride optical semiconductor. This element includes an AlN layer 2, a contact layer 3 made of n-type AlGaN, a clad layer 4 made of n-type AlGaN, a guide layer 5 made of AlGaN, and an active layer made of AlGaN, which are sequentially formed on an A-plane sapphire substrate 1. 6, a guide layer 7 made of AlGaN, a carrier block layer 8 made of p-type AlGaN, and a clad layer 9 made of p-type AlGaN, and an insulating layer (SiO ₂ ) 10 is formed on the clad layer 9, In the opening provided in the insulating layer 10, a contact layer 111 made of p-type GaN is formed together with the cladding layer 9, and a P-side electrode 112 is formed thereon. A part of the surface region of the contact layer 3 is etched, and an n-side electrode 113 is formed at the etched portion. Since the crystallinity of each layer depends on the crystallinity of the underlayer, the crystallinity of the underlayer is important.

  In the above-described device, when a group III nitride semiconductor crystal is formed, a chemical vapor deposition method such as a hydride vapor deposition method (HVPE method) or a metal organic chemical vapor deposition method (MOCVD method), or a molecular beam growth method is used. (MBE method), sublimation method, flux method and the like can be used. From the viewpoint of raw material cost and formation rate, the HVPE method is preferable. On the other hand, the MOCVD method is easy to control the composition of Al, Ga, and In and is suitable for manufacturing an n-type or p-type semiconductor crystal with high accuracy by doping with Si, Mg, or the like. In the following, an example in which an AlN layer is formed by HVPE and then laser crystal growth is performed by MOCVD will be described, but the present invention is not limited to this combination of growth methods. Further, various methods used for processing the element are not limited to those shown here.

The above laser device was manufactured. The details will be described below.
(First crystal growth)
First, a (11-20) A-plane sapphire single crystal substrate having a 2 inch diameter and a thickness of about 430 μm was prepared and placed on a susceptor of a reaction tube of an HVPE apparatus. In the case of the HVPE method, a group III metal such as Al and a hydride gas such as HCl gas are directly reacted at a temperature of about 500 ° C. to 700 ° C. to generate a group III source gas (for example, AlClx gas). And NH ₃ gas were reacted to form an AlN group III nitride, which was epitaxially grown on a substrate heated to, for example, temperature T1 (1150 ° C.). T1 can be selected from a temperature range of 1000 ° C to 1800 ° C. The boat in the reaction tube holds the metal Al raw material, and Al is used as the group III element. It should be noted that the flow rate of NH ₃ gas was 1.5 slm at each stage of the process unless otherwise stated. The flow rate of N ₂ gas is 1.1 slm.

In the growth of the first AlN layer, the pressure in the reaction tube is set to 30 Torr (4000 Pa), the temperature of the gas reaction region in the reaction tube is 550 ° C., and the substrate temperature is T1 (1150). The temperature was raised so as to be (° C.) (see FIG. 12). During the temperature increase (time 0 to t1: see FIG. 12), only NH ₃ and N ₂ gas were supplied. After the pressure and temperature in the reaction tube were stabilized, the surface of the sapphire substrate was cleaned (thermal cleaning) for 10 minutes with the NH ₃ and N ₂ gases supplied (time t1 to t2: see FIG. 12). The substrate temperature during thermal cleaning is preferably in the range of 900 to 1400 ° C.

Thereafter, by further supplying H ₂ gas and H ₂ diluted HCl gas to the vicinity of the boat through a predetermined gas introduction pipe, the generated AlClx gas is supplied to the vicinity of the substrate held by the susceptor, A first AlN layer (a lower half region of the AlN layer 2 in FIG. 7) was formed. The first AlN layer was formed for 1 minute (time t2 to t3: see FIG. 12), and the thickness of the first AlN layer was about 50 nm. 50 nm is preferred. The flow rates of H ₂ gas and HCl gas at this time are 0.2 slmm and 0.06 slm, respectively.

Next, supply of HCl gas is stopped, the substrate temperature is raised to T2 (1500 ° C.), and heat treatment is performed for 5 minutes (time t4 to t5: see FIG. 12) in a state where NH ₃ and N ₂ gases are supplied. (Nitriding treatment (*)). By this treatment, the crystal strain of the first AlN layer is further relaxed, and the surface is flattened. Note that the substrate temperature T2 is preferably in the range of 1400 to 1800 ° C. from the viewpoint of obtaining good crystallinity, but even without this treatment, good crystals can be obtained to some extent.

Note that the first AlN layer may be formed by nitriding with NH ₃ gas on the sapphire surface without supplying the AlClx gas.

Thereafter, the supply of HCl gas was started again, and the second AlN layer was grown on the first AlN layer for 20 minutes while maintaining the temperature T2 (time t5 to t6: see FIG. 12). The flow rate of HCl gas is 0.06 slm. By these steps, an AlN layer 2 (with a thickness exceeding 1 μm) having a total thickness of approximately 3 μm was obtained.
(Second crystal growth)
After the growth of the AlN layer, a laser layer structure that is most affected by the crystallinity as an optical semiconductor element characteristic was grown on the AlN layer. For the crystal growth, metal organic chemical vapor deposition (MOCVD) was used. Trimethylgallium (TMG) was used as the gallium (Ga) source, and ammonia (NH ₃ ) was used as the trimethylaluminum (TMA) nitrogen (N) source as the aluminum (Al) source. Hydrogen (H ₂ ) and nitrogen (N ₂ ) were used as carrier gases.

  After the substrate was introduced into the reaction chamber of the MOCVD growth apparatus, the temperature of the substrate was raised to 1075 ° C. in a hydrogen and ammonia atmosphere, and the substrate surface was cleaned by heat treatment for 5 minutes after the temperature rise.

Subsequently, the n-type Al _0.3 Ga _0.7 N layer (n-type contact layer 3) doped with Si was 2.8 μm, and the n-type Al _0.3 Ga _0.7 N layer (n-type cladding layer). 4) 600 nm, Al _0.16 Ga _0.84 N layer (first guide layer 5) 90 nm, and Al _0.06 Ga _0.94 N / Al _0.16 Ga _0.84 N quantum well layer (active Layer 6) was grown. Further, an Al _0.16 Ga _0.84 N layer (second guide layer 7) is grown to 120 nm, and an Mg-doped p-type Al _0.6 Ga _0.4 N layer (p-type electron blocking layer 8) is grown to 20 nm. Then, a p-type Al _0.3 Ga _0.7 N layer (p-type cladding layer 9) was grown to 500 nm, and a p-type GaN layer (p-type contact layer 111) was grown to 25 nm in this order.
(First element processing: semiconductor mesa portion formation)
Next, the crystal substrate grown up to the p-type contact layer 111 was taken out of the MOCVD chamber, and then an SiO ₂ layer having a thickness of about 300 nm was deposited over the entire surface of the p-type contact layer by plasma CVD or the like. Thereafter, an etching mask corresponding to the shape of the semiconductor mesa portion was formed on the SiO ₂ layer by a normal photolithography technique and etching technique. The etching mask has a shape extending on the p-type contact layer along the direction that will be the optical waveguide direction afterward with a predetermined width.

Next, dry etching with chlorine (Cl ₂ ) gas or the like was performed using the etching mask as a mask until the n-type contact layer 3 was exposed. By this etching, the portion from the p-type contact layer 1 to the n-type cladding layer 3 that is not covered with the etching mask is removed, so that a semiconductor mesa portion consisting of the n-type cladding layer 3 to the p-type contact layer 111 is formed. Is done. Thereafter, the etching mask was removed by etching.
(Second element processing: ridge structure formation)
Next, similarly, an insulating layer 10 made of SiO ₂ having a thickness of about 300 nm was deposited again on the entire surface of the crystal substrate by plasma CVD or the like. Thereafter, an etching mask was formed on the central region of the p-type contact layer 111 by a normal photolithography technique and etching technique on the insulating layer 10 in the same procedure as in the case of forming the etching mask. The etching mask has a shape that covers the central portion of the p-type contact layer 111 with a predetermined width.

Next, by using this etching mask as a mask, dry etching is performed using chlorine (Cl ₂ ) gas to the middle of the p-type cladding layer 9 to form a ridge structure having a width of 5 μm composed of the p-type contact layer 111 and the p-type cladding layer 10. Formed. Thereafter, the etching mask was removed by etching. Note that Ar gas can also be used for etching in addition to Cl ₂ gas.
(Third element processing: electrode formation)
Subsequently, an SiO ₂ layer having a thickness of about 300 nm was deposited again on the entire surface of the crystal substrate again by plasma CVD or the like. A resist pattern having a predetermined shape was formed on the SiO ₂ layer by photolithography to cover the region excluding the region where the negative electrode layer 113 was formed. Thereafter, using this resist pattern as a mask, the SiO ₂ layer was etched to form an opening in the formation region of the negative electrode layer 113.

Next, a titanium (Ti) layer and an aluminum (Al) layer were sequentially formed by a vacuum deposition method or the like. Subsequently, the resist pattern was peeled and removed together with the Ti layer and Al layer formed thereon with an organic solvent or the like. As a result, a negative electrode layer 113 in contact with the n-type contact layer through the opening of the SiO ₂ layer was formed. In FIG. 7, the description of the insulating layer around the electrode 113 is omitted for simplification.

Furthermore, the SiO ₂ layer at the top of the ridge structure is removed by the same process to expose the p-type contact layer, and then electrically connected to the p-type contact layer in the same procedure as the negative electrode layer. In addition, a positive electrode layer 112 made of a laminate of nickel (Ni) and gold (Au) was formed.
(Fourth element processing: resonance surface formation)
Next, the semiconductor substrate was cleaved at a plane perpendicular to the longitudinal direction of the ridge structure (a plane parallel to the paper surface) to form a resonator structure of the nitride semiconductor light emitting element. If necessary, the end face may be etched by dry etching using chlorine (Cl ₂ ) gas or the like, and this may be used for forming the resonator structure. A suitable resonator length is 900 μm, for example.

Thereafter, an end face coating may be applied to each end face of the resonator as necessary. At this time, for example, the front-side end surface reflectance is 50%, and the rear-side end surface reflectance is 99%, for example. The coating _{material, SiO 2, Al 2 O 3} , ZrO 2, AlN , and the like.

  Through the above steps, the optical semiconductor element (in this case, a laser element) of this embodiment made of a group III nitride semiconductor was formed.

When a group III nitride semiconductor is grown on the A-plane sapphire substrate 1, a C-plane AlN layer (semiconductor layer) is grown. At this time, the cleavage plane (C plane) of the sapphire substrate and the cleavage plane (M plane) of the semiconductor layer are in the same plane orientation, and in particular when the element to be fabricated is a laser element, a smooth resonant surface with excellent cleavage is obtained. As a result, an exceptional effect of lowering the laser oscillation threshold was obtained.
(Element characteristics)
FIG. 8 shows an emission spectrum obtained when the manufactured laser element is held at room temperature (27 ° C.) and a current of 680 mA is injected by pulse driving with a pulse width of 10 ns, and FIG. It is a graph which shows the change of the light output when it increases gradually. As is apparent from FIG. 8, ultraviolet laser oscillation of 337 nm, which cannot be easily achieved, was realized.

The threshold current value at which laser oscillation starts is about 550 mA, and the oscillation threshold current density calculated from the ridge width of 5 μm and the resonator length of 900 μm is about 12 kA / cm ² . A low value was obtained. Laser oscillation at a shorter wavelength could be realized at a current density comparable to the oscillation threshold of a conventional longer wavelength ultraviolet semiconductor laser.

  The improvement in the element characteristics described above is due to the improvement in the element crystal. The element crystal depends on the crystal state of the underlayer. Here, the crystalline states of the first AlN layer and the second AlN layer were evaluated.

  The AlN layer grown on the sapphire substrate 1 was evaluated by the X-ray rocking curve method (incident angle ω scan). The in-plane rotation angle φ was rotated as necessary. The X-ray rocking curves of the (002), (102), and (100) planes of the obtained AlN layer were measured.

FIG. 1 shows the half-value width (FWHM) (arcsec) of the X-ray rocking curve of the crystal obtained when the growth temperature T2 of the second AlN layer is changed (1450 ° C., 1500 ° C., 1550 ° C.). It is a chart. The flow rate of NH ₃ is 1.5 slm. FWHM related to the (002), (102), and (100) planes in the case of using an A-plane sapphire substrate and a C-plane sapphire substrate is described. In addition, in each column of FIG.1 and FIG.3, the thing where three numbers are located in order is φ from 30 °, 90 °, and 150 ° when using an A-plane sapphire substrate in order from the left. In the case of using a C-plane sapphire substrate, the FWHM is shown when φ is 0 °, 60 °, and 120 °.

  As is clear from the figure, when the A-plane sapphire substrate is used, the FWHM is smaller than when the C-plane sapphire substrate is used, and the crystallinity of the (102) plane is particularly improved at each temperature. You can see that From the viewpoint of crystallinity, the temperature T2 is particularly preferably around 1500 ° C., that is, greater than 1450 ° C. and less than 1550 ° C.

  The value of the half width (FWHM) of the X-ray rocking curve by the incident angle scan with respect to the (002), (102), and (100) planes of the first AlN layer (initial layer) formed on the above-described A-plane sapphire substrate is They were approximately 400 seconds, 500 seconds, and 650 seconds, respectively (right column in FIG. 3). In addition, the result regarding the initial layer when performing the nitriding process (*) is shown in the right column (Nitridation) of FIG. 3, and the result when not performing the nitriding process is shown in the left column of FIG. (Buffer). Further, in the evaluation of the surface flatness with an atomic force microscope, the Ra value of the surface roughness was about 0.3 nm. A C-plane AlN single crystal layer having no cracks on the surface and excellent crystallinity was formed on the A-plane sapphire substrate.

FIG. 2 shows the value of the full width at half maximum (FWHM) of the X-ray rocking curve of the crystal obtained when the temperature T2 = 1500 ° C. and the flow rate of NH ₃ is changed (1.0 slm, 1.5 slm, 2.0 slm). It is a chart which shows. As is clear from the figure, when the A-plane sapphire substrate is used, the FWHM is smaller than when the C-plane sapphire substrate is used, and the (102) plane crystallinity is obtained at each flow rate. You can see that it is improving. From the viewpoint of crystallinity, the flow rate of NH ₃ is particularly preferably in the vicinity of 1.5 slm, that is, greater than 1.0 slm and less than 2.0 slm.

FIG. 3 is a chart showing the value of the half width (FWHM) of the X-ray rocking curve of the first AlN layer (initial layer) in the case of NH ₃ flow rate = 1.5 slm. The Buffer column indicates the characteristics of the first AlN layer before the heat treatment, and the Nitridation column indicates the characteristics of the AlN layer after the heat treatment (time t4 to t5: see FIG. 12). As can be seen from the figure, when the A-plane sapphire substrate is used, the FWHM is smaller than when the C-plane sapphire substrate is used, and for the (102) plane and (100) plane, By the heat treatment at T2 = 1500 ° C., the numerical FWHM located in the center is reduced. From the viewpoint of improving crystallinity, the heat treatment time (time t4 to t5: see FIG. 12) is preferably 5 to 30 minutes.

FIG. 4 is a diagram in the case of using the first AlN (Buffer), and is an atomic force microscope image of the crystal surface (the surface of the second AlN) obtained at various temperatures. (A) shows T2 = 1450 ° C., (b) shows T2 = 1500 ° C., (c) shows T2 = 1550 ° C., and (d) shows the surface of the sapphire substrate. Each surface roughness Ra is 2.36 nm, 0.37 nm, 0.18 nm, and 0.18 nm as described in the upper part of the photograph. A minimum surface roughness of 0.18 nm was obtained, but a maximum surface roughness of 2.36 nm was obtained.

FIG. 5 is a diagram in the case of using the first AlN (Buffer), and is an atomic force microscope image of the crystal surface (second AlN surface) obtained at various flow rates with T2 = 1500 ° C. It is. (A) shows NH ₃ flow rate = 1.0 slm, (b) shows NH ₃ flow rate = 1.5 slm, (c) shows NH ₃ flow rate = 2.0 slm, and each surface roughness Ra is shown in the photograph. As described above, they are 0.31 nm, 0.37 nm, and 14.75 nm. A minimum surface roughness of 0.31 nm was obtained, but a maximum of 14.1 nm. A surface roughness of 75 nm was obtained.

FIG. 6 is an atomic force microscope image of the surface of the initial layer (first AlN layer) when NH ₃ flow rate = 1.5 slm. (A), (b) shows the surface of the initial layer before the heat treatment, and (A), (B) show the surface after the heat treatment at T2 = 1500 ° C. Each surface roughness Ra is 0.28 nm, 0.28 nm, 0.32 nm, and 0.21 nm as described in the upper part of the photograph. A minimum surface roughness of 0.21 nm was obtained, but a maximum surface roughness of 0.32 nm was obtained.

  As described above, by using an A-plane sapphire substrate, C-plane AlN excellent in crystallinity can be obtained by a single growth method (for example, only by the HVPE method). Also, the thickness of the AlN layer was at least 1 to 3 μm, and no cracks occurred, and the crystallinity was improved. Further, because of the anisotropy such as the thermal expansion coefficient in the plane of the A-plane crystal, it seems that no cracks are generated even if the thickness of the AlN layer is 3 μm or more. Further, the full width at half maximum of the X-ray rocking curves of the (102) plane and the (100) plane is remarkably reduced, and the high crystallinity can be confirmed. Moreover, many cracks have also arisen in AlN on C-plane sapphire.

  Note that it is preferable that an A-plane sapphire substrate having a groove or a concavo-convex structure on the surface and growing AlN on the substrate by the same method can be expected to reduce the strain of the crystal. Further, the crystal growth method and conditions, the process method and conditions, the layer composition and the structure of the optical semiconductor element, and the like shown in this embodiment are only examples, and the present invention is not limited thereto. The present invention can also be applied to LEDs (FIG. 10) and PDs (FIG. 11) as optical semiconductor elements.

  FIG. 10 is a cross-sectional view of the LED.

  This element comprises an AlN layer 12, a contact layer 13 made of n-type AlGaN, a clad layer 14 made of n-type AlGaN, an active layer 15 made of AlGaN, and a p-type AlGaN, which are sequentially formed on an A-plane sapphire substrate 11. A carrier block layer 16 and a clad layer 17 made of p-type AlGaN, and a contact layer 18 made of p-type GaN is formed, on which a P-side transparent electrode 19 and a P-side electrode 20 are formed. . A part of the surface region of the contact layer 13 is etched, and an electrode 121 is formed at the etched portion. The material and forming method of each layer are the same as in the case of the above-described LD, but a transparent electrode is used as a different material, and the transparent electrode 19 is made of Ni / Au or the like. In addition, when taking out light from the sapphire side, it is desirable to use a material with high reflectivity with respect to ultraviolet rays, such as Ni / Ag, instead of a transparent electrode.

  FIG. 11 is a cross-sectional view of a PD.

  This element includes an AlN layer 22, an n-type AlGaN contact layer 23, an AlGaN layer 24, a p-type AlGaN layer 25, a P-side transparent electrode 26, and a P-side transparent electrode 26, which are sequentially formed on an A-plane sapphire substrate 21. An electrode 27 is provided. Further, a part of the surface region of the contact layer 23 is etched, and an electrode 28 is formed at the etched portion. The material and forming method of each layer are the same as those of the above-described LED. Note that the above-described semiconductor conductivity types (n-type and p-type) can be operated even if they are interchanged.

  As described above, the above-described nitride optical semiconductor element includes an A-plane sapphire substrate (1, 11, 21) and a C-plane having a thickness of more than 1 μm provided on the substrate (1, 11, 21). An AlN layer (2, 12, 22), an n-type (first conductivity type) group III nitride semiconductor layer (4, 14, 23) formed on the AlN layer (2, 12, 22), and and a p-type (second conductivity type) group III nitride semiconductor layer (9, 17, 25) formed on the n-type group III nitride semiconductor layer. Between the n-type group III nitride semiconductor layer (4, 14, 23) and the p-type (second conductivity type) group III nitride semiconductor layer (9, 17, 25), a light emitting device is provided. Has a heterojunction for confining carriers and light, and the p-type and n-type semiconductor layers also function as a diode having a rectifying action.

  In the above example, the n-type and p-type group III nitride semiconductor layers are each made of AlGaN. However, in order to generate ultraviolet rays, the Al composition ratio is 0.2 or more and 1.0 or less. Even in this case, in the present invention, a device having good crystallinity can be obtained.

1,11,21 ... A-plane sapphire substrate, 2,12,22 ... C-plane AlN layer, 4,14,23 ... n-type group III nitride semiconductor layer, 9,17,25 ... p-type group III Nitride semiconductor layer.

Claims (3)

An A-plane sapphire substrate;
A thickness formed at a temperature T2 (1400 ° C. to 1800 ° C.) higher than the temperature T1 after formation at a temperature T1 (1000 ° C. to 1800 ° C. ) by direct hydride vapor phase epitaxy (HVPE method) on the substrate. A C-plane AlN layer having a thickness exceeding 1 μm;
A group III nitride semiconductor layer of the first conductivity type formed on the AlN layer;
A second conductivity type group III nitride semiconductor layer formed on the first conductivity type group III nitride semiconductor layer; and
A nitride optical semiconductor device comprising:   2. The nitride optical semiconductor device according to claim 1, wherein the cleavage plane (C plane) of the sapphire substrate and the cleavage plane (M plane) of the AlN layer have the same plane orientation.   The group III nitride semiconductor layer of the first and second conductivity types is made of AlGaN, and the composition ratio of Al is 0.2 or more and 1.0 or less. Nitride optical semiconductor device.