Patent Publication Number: US-2018033907-A1

Title: Nitride semiconductor template and method for manufacturing same

Description:
TECHNICAL FIELD 
     The invention relates to a nitride semiconductor template and a method for manufacturing the nitride semiconductor template. 
     BACKGROUND ART 
     A nitride semiconductor template is known in which a nitride semiconductor layer is formed on a Ga 2 O 3  substrate via an AlN buffer layer (see, e.g., PTL 1) 
     According to PTL 1, appropriately selecting a plane orientation of a main surface of the Ga 2 O 3  substrate allows the nitride semiconductor layer to have a mirror surface. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] 
     JP-A 2014-199935 
     SUMMARY OF INVENTION 
     Technical Problem 
     When forming a nitride semiconductor on the Ga 2 O 3  substrate, however, the conditions to prevent pits or cracks on the nitride semiconductor are different depending on the amount of the Al composition of the nitride semiconductor. Therefore, the optimal method needs to be chosen for each composition to obtain a higher-quality nitride semiconductor. 
     In recent years, ultraviolet LEDs in the wavelength range of 315 to 360 nm have been developed as an alternative to high-pressure mercury lamps used for curing, etc. 
     It is an object of the invention to provide a transparent nitride semiconductor template that includes a high-quality nitride semiconductor, is suitable for use in an ultraviolet LED and has an electrical conductivity, as well as a manufacturing method that allows simple manufacture of the transparent nitride semiconductor template. 
     Solution to Problem 
     To achieve the above-mentioned object, an aspect of the invention provides a nitride semiconductor template described in the following [1] to [5] and a method for manufacturing a nitride semiconductor template described in the following [6] to [8]. 
     A nitride semiconductor template, comprising: a Ga 2 O 3  substrate; a buffer layer formed on the Ga 2 O 3  substrate and comprising AlN as a principal component; a first nitride semiconductor layer formed on the buffer layer and comprising Al x Ga 1-x N (0.2&lt;x≦1) as a principal component; and a second nitride semiconductor layer formed on the first nitride semiconductor layer and comprising Al y Ga 1-y N (0.2≦y≦0.55, y&lt;x) as a principal component. 
     The nitride semiconductor template described in [1], wherein the buffer layer is not more than 10 nm in thickness. 
     The nitride semiconductor template described in [1] or [2], wherein the second nitride semiconductor layer has no crack on a surface thereof. 
     The nitride semiconductor template described in [1] or [2], wherein the second nitride semiconductor layer has no pit on a surface thereof. 
     The nitride semiconductor template described in [1] or [2], wherein the second nitride semiconductor layer has a dislocation density of not more than 2.0×10 10  cm −2 . 
     A method for manufacturing a nitride semiconductor template, comprising: a step of forming a Ga 2 O 3  substrate; a step of forming a buffer layer comprising AlN as a principal component on the Ga 2 O 3  substrate; a step of forming a first nitride semiconductor layer comprising Al x Ga 1-x N (0.2&lt;x≦1) as a principal component on the buffer layer; and a step of forming a second nitride semiconductor layer comprising Al y Ga 1-y N (0.2≦y≦0.55, y&lt;x) as a principal component on the first nitride semiconductor layer. 
     The method for manufacturing a nitride semiconductor template described in [6], wherein the buffer layer is not more than 10 nm in thickness. 
     The method for manufacturing a nitride semiconductor template described in [6] or [7], wherein a growth temperature of the second nitride semiconductor layer is more than 1100° C., and a growth temperature of the first nitride semiconductor layer is less than 1100° C. 
     Advantageous Effects of Invention 
     According to the invention, a transparent nitride semiconductor template can be provided that includes a high-quality nitride semiconductor, is suitable for use in an ultraviolet LED and has electrical conductivity, as well as a manufacturing method that allows simple manufacture of the transparent nitride semiconductor template. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a vertical cross-sectional view showing a nitride semiconductor template in an embodiment. 
         FIG. 2A  is an image of a surface of a second nitride semiconductor layer of Sample 1 observed under an optical microscope. 
         FIG. 2B  is an image of a surface of a second nitride semiconductor layer of Sample 4 observed under an optical microscope. 
         FIG. 2C  is an image of a surface of a second nitride semiconductor layer of Sample 5 observed under an optical microscope. 
         FIG. 3  is an X-ray diffraction pattern of the nitride semiconductor template as Sample 5 obtained using a symmetrical reflection method. 
         FIG. 4  shows photoluminescence spectra of the nitride semiconductor template as Sample 5. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Embodiment 
     (Structure of Nitride Semiconductor Template) 
       FIG. 1  is a vertical cross-sectional view showing a nitride semiconductor template  10  in the embodiment. The nitride semiconductor template  10  is a template suitable for use in an ultraviolet LED with an emission wavelength of 315 to 360 nm. 
     The nitride semiconductor template  10  includes a Ga 2 O 3  substrate  11 , a buffer layer  12  on the Ga 2 O 3  substrate  11 , a first nitride semiconductor layer  13  on the buffer layer  12 , and a second nitride semiconductor layer  14  on the first nitride semiconductor layer  13 . 
     The Ga 2 O 3  substrate  11  is formed of a β-Ga 2 O 3  single crystal. The main surface of the Ga 2 O 3  substrate  11  is a (−201) plane, a (101) plane, a (310) plane, a (3-10) plane or planes inclined from these planes within a range of about ±2°, which can be a base for growth of high-quality nitride semiconductor crystal. The Ga 2 O 3  substrate  11  is, e.g., a circular substrate having a diameter of 50.8 mm (2 inches), but the shape and size thereof are not limited. 
     Since Ga 2 O 3  hardly absorbs light with a wavelength of 315 to 360 nm, the Ga 2 O 3  substrate  11  is excellent as a substrate of the nitride semiconductor template  10  which is used to form an UV LED with an emission wavelength of 315 to 360 nm. By contrast, e.g., GaN absorbs light with a wavelength of 315 to 360 nm well. Therefore, GaN substrates are not suitable as UV LED templates, and to prevent a decrease in light extraction efficiency, the GaN substrates need to be removed after manufacturing LEDs. 
     In addition, the Ga 2 O 3  substrate  11 , which contains a dopant such as Si or Sn and has excellent conductivity, is excellent as an LED substrate. On the other hand, in case that a low-conductivity substrate, e.g., a sapphire substrate, is used, it is not possible to form vertical-type LEDs, and horizontal-type LEDs, even when formed, have high electrical resistance since an electric current flows through a thin nitride semiconductor layer on the substrate. 
     The buffer layer  12  is formed of a crystal consisting mainly of AlN. The buffer layer  12  may partially cover the upper surface of the Ga 2 O 3  substrate  11  as shown in  FIG. 1 , or may cover the entire upper surface. To obtain higher crystal quality, the thickness of the buffer layer  12  is preferably not more than 10 nm, more preferably, not more than 5 nm. 
     The second nitride semiconductor layer  14  is used as a cladding layer in a UV LED which is formed using the nitride semiconductor template  10 . To form a UV LED with an emission wavelength of 315 to 360 nm, the second nitride semiconductor layer  14  to be a cladding layer need to have a composition roughly represented by Al y Ga 1-y N (0.2≦y≦0.55). 
     The Al composition of the first nitride semiconductor layer  13  is greater than that of the second nitride semiconductor layer  14 . In other words, the composition of the first nitride semiconductor layer  13  is expressed by Al x Ga 1-x N (0.2&lt;x≦1), and the Al composition-x of the first nitride semiconductor layer  13  and the Al composition-y of the second nitride semiconductor layer  14  satisfy the relation of y&lt;x. The first nitride semiconductor layer  13  having such a composition allows the second nitride semiconductor layer  14  to have a mirror surface and generation of cracks and pits to be suppressed. 
     The first nitride semiconductor layer  13  and the second nitride semiconductor layer  14  may contain a dopant such as Si. The thickness of the first nitride semiconductor layer  13  is, e.g., 100 to 300 nm. The thickness of the second nitride semiconductor layer  14  is, e.g., 1 to 2μm. 
     The surface of the second nitride semiconductor layer  14  is a mirror surface and hardly contains, or does not contain cracks and pits (hole-like defects) at all. 
     If the second nitride semiconductor layer  14  is formed on the buffer layer  12  without providing the first nitride semiconductor layer  13 , cracks are generated on the surface of the second nitride semiconductor layer  14 . Meanwhile, when only the first nitride semiconductor layer  13  is formed on the buffer layer  12  without providing the second nitride semiconductor layer  14 , a mirror surface cannot be obtained. 
     (Method for Manufacturing Nitride Semiconductor Template) 
     An example method for manufacturing the nitride semiconductor template  10  will be described below. 
     Firstly, the Ga 2 O 3  substrate  11  treated by CMP (Chemical Mechanical Polishing) is cleaned with an organic solvent and SPM (Sulfuric acid/hydrogen peroxide mixture). 
     Next, the Ga 2 O 3  substrate  11  is conveyed to a chamber of a MOCVD (Metal Organic Chemical Vapor Deposition) apparatus. 
     Next, the buffer layer  12  is formed on the Ga 2 O 3  substrate  11 . An AlN crystal is grown on the Ga 2 O 3  substrate  11  by supplying source gases and N 2  gas as a carrier gas into the chamber in a state that the temperature inside the chamber is maintained at 400 to 600° C., thereby forming the buffer layer  12  in the form of film. 
     The source gases used to form the buffer layer  12  are, e.g., trimethylaluminum (TMA) gas as an Al source and NH 3  gas as an N source. The carrier gas may be alternatively H 2  gas, etc. 
     Next, the first nitride semiconductor layer  13  is formed on the buffer layer  12 . In detail, for example, source gases for the first nitride semiconductor layer  13  and H 2  gas as a carrier gas are supplied into the chamber with pressure maintained at 100 mbar and temperature maintained at not less than 885° C., thereby growing the first nitride semiconductor layer  13 . 
     The source gases used to form the nitride semiconductor layer  13  are, e.g., trimethylaluminum (TMA) gas as an Al source, trimethylgallium (TMG) gas as Ga source and NH 3  gas as an N source. The carrier gas may be alternatively N 2  gas, etc. 
     Next, the second nitride semiconductor layer  14  is formed on the first nitride semiconductor layer  13 . In detail, for example, source gases for the second nitride semiconductor layer  14  and H 2  gas as a carrier gas are supplied into the chamber with temperature maintained at not less than 1100° C., thereby growing the second nitride semiconductor layer  14 . 
     Here, generation of pits can be suppressed when the second nitride semiconductor layer  14  is grown at a growth temperature of more than 1100° C. Furthermore, generation of pits can be suppressed more reliably when the second nitride semiconductor layer  14  is grown at a growth temperature of not less than 1120° C. 
     The source gases for the second nitride semiconductor layer  14  may be the same as those for the first nitride semiconductor layer  13 . The carrier gas may be alternatively N 2  gas, etc. 
     (Evaluation of Surface State of Second Nitride Semiconductor Layer) 
     Table 1 below shows the growth conditions of each layer and the results of evaluating the surface state of the second nitride semiconductor layers. 
     Each of the Ga 2 O 3  substrates of seven types of nitride semiconductor templates (Samples 1 to 7) used for evaluation was a 2 inch-diameter circular substrate having a (−201) plane as the main surface. Trimethylaluminum (TMA) gas, trimethylgallium (TMG) gas and NH 3  gas were respectively used as the Al source, the Ga source and the N source for the first and second nitride semiconductor layers. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                   
                 First nitride 
                 Second nitride 
                   
               
               
                   
                 Buffer layer 
                 semiconductor layer 
                 semiconductor layer 
                 Surface state 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Sam- 
                 Film 
                 Growth 
                   
                 Growth 
                   
                 Growth 
                 Check of 
                   
                   
               
               
                 ple 
                 thickness 
                 temperature 
                   
                 temperature 
                   
                 temperature 
                 etched Ga 2 O 3   
               
               
                 No. 
                 [nm] 
                 [° C.] 
                 Composition 
                 [° C.] 
                 Composition 
                 [° C.] 
                 substrate 
                 Pit 
                 Crack 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 5 
                 550 
                 N/A 
                 N/A 
                 Al 0.3 Ga 0.7 N 
                 1100 
                 Etched 
                 Observed 
                 Observed 
               
               
                 2 
                 5 
                 800 
                 N/A 
                 N/A 
                 Al 0.3 Ga 0.7 N 
                 1100 
                 Etched 
                 Observed 
                 Observed 
               
               
                 3 
                 5 
                 550 
                 Al 0.3 Ga 0.7 N 
                 1020 
                 Al 0.3 Ga 0.7 N 
                 1100 
                 Not etched 
                 Observed 
                 Observed 
               
               
                 4 
                 5 
                 550 
                 AlN 
                 1020 
                 Al 0.3 Ga 0.7 N 
                 1100 
                 Not etched 
                 Observed 
                 Not observed 
               
               
                 5 
                 5 
                 550 
                 AlN 
                 1020 
                 Al 0.3 Ga 0.7 N 
                 1120 
                 Not etched 
                 Not observed 
                 Not observed 
               
               
                 6 
                 5 
                 550 
                 Al 0.8 Ga 0.2 N 
                 1020 
                 Al 0.3 Ga 0.7 N 
                 1120 
                 Not etched 
                 Not observed 
                 Not observed 
               
               
                 7 
                 5 
                 550 
                 Al 0.8 Ga 0.2 N 
                 1020 
                 Al 0.45 Ga 0.55 N 
                 1120 
                 Not etched 
                 Not observed 
                 Not observed 
               
               
                   
               
            
           
         
       
     
     The second nitride semiconductor layer of each of Samples 1 to 7 was grown at a growth rate of 2 μm/h. 
     In Samples 1 and 2, the second nitride semiconductor layer was directly formed on the buffer layer without forming the first nitride semiconductor layer. Pits and cracks were generated on the surface of the second nitride semiconductor layer of Sample 1 and a mirror surface was obtained only in a 25 mm-diameter region. Likewise, pits and cracks were generated also on the surface of the second nitride semiconductor layer of Sample 2. It is considered that this is because the first nitride semiconductor layer was not formed. 
     Meanwhile, in Sample 1, the Ga 2 O 3  substrate was partially etched. The reason is considered as follows: since the second nitride semiconductor layer to be grown at a higher temperature than the first nitride semiconductor layer was directly formed on the buffer layer, the buffer layer migrated (or crystallized) too much and thus did not sufficiently protect some portion of the surface of the Ga 2 O 3  substrate. On the other hand, in Sample 2, the Ga 2 O 3  substrate was etched when forming the buffer layer since the growth temperature of the buffer layer was too high. 
     In Sample 3, cracks were generated on the surface of the second nitride semiconductor layer. It is considered that this is because the Al composition of the first nitride semiconductor layer was the same as that of the second nitride semiconductor layer. 
     In Sample 4, pits were generated on the surface of the second nitride semiconductor layer. It is considered that this is because growth of the crystal in the lateral direction was insufficient when the second nitride semiconductor layer was grown at a temperature of 1100° C. 
     In Sample 5, none of cracks and pits were generated on the surface of the second nitride semiconductor layer. It is considered that this is mainly because the first and second nitride semiconductor layers were both formed and the Al composition of the second nitride semiconductor layer was smaller than that of the first nitride semiconductor layer. The reason why pits were not generated is considered that the second nitride semiconductor layer was grown at a temperature of 1120° C., i.e., higher than 1100° C. 
     Sample 6 was the same as Sample 5, except that the material of the first nitride semiconductor layer was changed to Al 0.8 Ga 0.2 N from AlN to decrease electrical resistance of the first nitride semiconductor layer. Also in Sample 6, none of cracks and pits were generated. 
     In Sample 7, the Al composition of the second nitride semiconductor layer was increased to more than that of Samples 5 and 6 for use in LEDs with a short wavelength. Also in Sample 7, none of cracks and pits were generated. 
     In all of Samples 1 to 7, dislocation density in the second nitride semiconductor layer was suppressed to not more than 2.0×10 10  cm −2 . 
     It is understood from the evaluation results of Samples 1 to 7 that the conditions to obtain the second nitride semiconductor layer with a good surface state are that the first and second nitride semiconductor layers are both formed, that the Al composition of the second nitride semiconductor layer is smaller than that of the first nitride semiconductor layer, and that the growth temperature of the second nitride semiconductor layer is more than 1100° C. 
       FIGS. 2A, 2B and 2C  are images of the surfaces of the respective second nitride semiconductor layers of Samples 1, 4 and 5 observed under an optical microscope. As shown in Table 1, cracks are observed on the surface of the second nitride semiconductor layer of Sample 1 shown in  FIG. 2A , and pits are observed on the surface of the second nitride semiconductor layer of Sample 4 shown in  FIG. 2B . On the other hand, none of cracks and pits are observed on the surface of the second nitride semiconductor layer of Sample 5 shown in  FIG. 2C . 
       FIG. 3  is an X-ray diffraction pattern of the nitride semiconductor template as Sample 5. 
     The X-ray diffraction pattern in  FIG. 3  only has peaks of diffraction from the Ga 2 O 3  substrate at a (−201) plane and planes parallel to the (−201) plane, from AlN as the first nitride semiconductor layer at a plane parallel to a (0001) plane and from Al 0.3 Ga 0.7 N as the second nitride semiconductor layer at planes parallel to a (0001) plane, and shows that the second nitride semiconductor layer does not have a phase grown in a different direction. Note that, the Al composition is shown as Al 0.29 Ga 0.71 N in  FIG. 3  since the Al composition of Al 0.3 Ga 0.7 N was actually 0.29 as a result of calculation based on complete lattice relaxation derived from the peak position. 
     Meanwhile, as a result of x-ray rocking curve measurement on the nitride semiconductor template as Sample 5, the full width at half maximum of diffraction peak from a (0002) plane was 1164 arcseconds and the full width at half maximum of diffraction peak from a (1-102) plane was 1536 arcseconds. 
       FIG. 4  shows photoluminescence spectra of the nitride semiconductor template as Sample 5. This spectrum was obtained by photoluminescence measurement using excitation light with a wavelength of 244 nm at room temperature, and the peak at a wavelength of 305 nm probably due to band edge emission is shown as a main peak. 
     (Effects of the Embodiment) 
     In the embodiment, it is possible to obtain a nitride semiconductor template which has a high-quality nitride semiconductor on a Ga 2 O 3  substrate and is suitable for use in a UV LED with an emission wavelength of 315 to 360 nm. 
     Although the embodiment of the invention has been described, the invention is not intended to be limited to the embodiment, and the various kinds of modifications can be implemented without departing from the gist of the invention. 
     In addition, the invention according to claims is not to be limited to embodiment. Further, it should be noted that all combinations of the features described in the embodiment are not necessary to solve the problem of the invention. 
     INDUSTRIAL APPLICABILITY 
     Provided is a transparent nitride semiconductor template that includes a high-quality nitride semiconductor, is suitable for use in an ultraviolet LED and has electrical conductivity, as well as a manufacturing method that allows simple manufacture of the transparent nitride semiconductor template. 
     REFERENCE SIGNS LIST 
       10  NITRIDE SEMICONDUCTOR TEMPLATE 
       11  Ga 2 O 3  SUBSTRATE 
       12  BUFFER LAYER 
       13  FIRST NITRIDE SEMICONDUCTOR LAYER 
       14  SECOND NITRIDE SEMICONDUCTOR LAYER