Abstract:
When a crystal layer of III-V Group nitride compound semiconductor is formed, a nitride compound semiconductor layer is first overlaid on a substrate to form a base layer and a III-V Group nitride compound semiconductor represented by the general formula In x Ga y Al z N (where 0≦x≦1, 0≦y≦1, 0≦z≦1, x+y+z=1) is epitaxially grown on the base layer by hydride vapor phase epitaxy at a deposition pressure of not lower than 800 Torr. By making the deposition pressure not lower than 800 Torr, the crystallinity of the III-V Group nitride compound semiconductor can be markedly improved and its defect density reduced.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method of manufacturing III-V nitride compound semiconductor by hydride vapor phase epitaxy (HVPE).  
           [0003]    2. Background Art  
           [0004]    III-V Group nitride compound semiconductors represented by the general formula In x Ga y Al z N (where 0≦x≦1, 0≦y≦1, 0≦z≦1, x+y+z=1) can be adjusted in direct band gap energy by varying the III Group element content. As this makes them adaptable to optical energies of wavelengths from ultraviolet to red, they are usable as high-efficiency light-emitting element materials over a range extending from ultraviolet to visible light. Moreover, since they have a larger band gap than Si, GaAs and other such semiconductors widely used up to now, they can maintain the characteristics of a semiconductor up to high temperatures at which conventional semiconductors cannot operate. This property is basically utilizable to fabricate electronic devices with excellent environmental resistance.  
           [0005]    Owing to the very high vapor pressure of the III-V Group nitride compound semiconductors in the vicinity of the melting point, however, growth of a large crystal is extremely difficult and a crystal of a size practical for use as a substrate for fabricating semiconductor chips cannot be obtained. The general practice in fabricating compound semiconductors is therefore to use a substrate of Si, GaAs, SiC, sapphire, ZrB 2  or other material that has a crystal structure similar to the compound semiconductor and enables large crystal fabrication and to epitaxially grow a desired single crystal thin-film layer on the substrate. Relatively good quality crystals of these compound semiconductors have now become obtainable by using this method. By using the metalorganic vapor phase epitaxial (MOVPE) method to conduct growth using a buffer layer, it is generally possible to achieve a half-value width for (0004) of around 200 seconds as measured from the x-ray rocking curve.  
           [0006]    On the other hand, hydride vapor phase epitaxy (HVPE) compares favorably with other methods of growing compound semiconductor in the points of high growth rate and ability to achieve high-purity crystal growth by suppressing incorporation of impurities. However, HVPE lags behind MOVPE and other methods in the establishment of a heteroepitaxial method enabling two-stage growth using a buffer layer. In actual practice, a thin film of around 3 μm thickness grown by MOVPE or the like is used as a base layer and a thick layer of the compound semiconductor is homoepitaxially grown on the base layer by HVPE. As cracking occurs even when homoepitaxial growth is conducted by HVPE, however, it is difficult to obtain high-quality crystal over a wide area. Other drawbacks have also come to light, such as that crystallinity of the grown layer is inferior to that of the base layer, as evidenced by the fact that the half-value width of the x-ray rocking curve (XRC) of the grown layer is broader than that of the base layer.  
         SUMMARY OF THE INVENTION  
         [0007]    An object of the present invention is to provide a method for obtaining high-quality III-v Group compound semiconductor crystal.  
           [0008]    Another object of the present invention is to provide a method of growing III-v Group compound semiconductor crystal excellent in crystallinity by HVPE.  
           [0009]    Another object of the present invention is to provide a method of manufacturing III-V Group compound semiconductor that enables epitaxial growth of III-V Group compound semiconductor excellent in crystallinity at low cost.  
           [0010]    The inventors conducted numerous and various tests and experiments to achieve this object. These were premised on the thinking that, while growth of GaN crystal by HVPE is usually conducted at atmospheric pressure, it is conceivable that crystallinity may be affected by growth pressure change. This approach led to the discovery that crystal quality improves markedly when the deposition pressure is slightly higher than atmospheric pressure. The present invention was accomplished based on this finding.  
           [0011]    The method of manufacturing III-V Group nitride compound semiconductor by HVPE according to the present invention is characterized in that the deposition pressure is set to not lower than 800 Torr. The deposition pressure is preferably not lower than 850 Torr, more preferably not lower than 900 Torr. The effect of this invention cannot be obtained when the deposition pressure is lower than 800 Torr.  
           [0012]    In accordance with another feature of the present invention, when a crystal layer of III-V Group nitride compound semiconductor represented by the general formula In x Ga y Al z N (where 0≦x≦1, 0≦y≦1, 0≦z≦1, x+y+z=1) is formed by by hydride vapor phase epitaxy, a crystal layer of III-V Group compound semiconductor excellent in crystallinity can be obtained by preparing a substrate and forming the III-V Group nitride compound semiconductor on the substrate under a deposition pressure of not lower than 800 Torr.  
           [0013]    The substrate can be appropriately selected from among materials such as Si, GaAs, SiC, ZrB 2  and sapphire. Use can also be made of a substrate of nitride compound semiconductor grown on one of the foregoing substrates beforehand by MOVPE, molecular beam epitaxy (MBE) or the like. When a III-V Group nitride compound semiconductor is epitaxially grown on such a substrate using HVPE, the crystallinity of the III-V Group nitride compound semiconductor obtained can be markedly improved over that using atmospheric pressure deposition by setting the deposition pressure slightly higher than atmospheric pressure. The reason for this is not certain but is considered to be that the pressurization alters the growth mechanism in a manner that upgrades the crystallinity. It is also possible to first form a nitride compound semiconductor layer on the substrate and thereafter grow the required III-V Group nitride compound semiconductor crystal layer on the nitride compound semiconductor layer by hydride vapor phase epitaxy under a deposition pressure of not lower than 800 Torr. 
       
    
    
     BRIEF EXPLANATION OF THE DRAWINGS  
       [0014]    FIGS.  1 A- 1 D are process diagrams for explaining a method that is an embodiment of the present invention.  
         [0015]    [0015]FIG. 2 is a schematic diagram showing the principal parts of a vapor deposition semiconductor manufacturing system used to conduct the processes of FIGS.  1 A- 1 D.  
         [0016]    [0016]FIG. 3 is a graph showing a temperature profile in the reactor of the system shown in FIG. 2.  
         [0017]    [0017]FIG. 4 is a graph showing how XRC half-value width varies as a function of deposition pressure in specimens manufactured according to the present invention.  
         [0018]    [0018]FIG. 5 is a micrograph showing the GaN thin film surface state of a specimen manufactured according to the present invention.  
         [0019]    [0019]FIG. 6 is a micrograph showing the GaN thin film surface state of a specimen manufactured by a conventional method. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    FIGS.  1 A- 1 D are process diagrams for explaining a method that is an embodiment of the present invention. First, a sapphire substrate  1  of appropriate size is prepared by washing to remove oil and water (FIG. 1A). The washing can be done using any of various known cleansing fluids.  
         [0021]    Next, the washed sapphire substrate  1  is placed in an MOVPE reactor and surface etching is conducted at a high temperature of around 1000° C. A buffer layer  2  of, for example, AlN, GaN, AlGaN or SiC is then grown at a temperature of about 500° C. (FIG. 1B). After formation of the buffer layer  2 , the temperature is returned to about 1000° C. and a III-V Group nitride compound semiconductor layer  3  is formed (FIG. 1C). The multilayered semiconductor crystal obtained in this manner is used as a base layer B in this invention. The obtained base layer B is set in an HVPE reactor to epitaxially grow a III-v Group nitride compound semiconductor represented by the general formula In x Ga y Al z N (where 0≦x≦1, 0≦y≦1, 0≦z≦1, x+y+z=1). The compound semiconductor grown in this embodiment is GaN. As a result, there is obtained a GaN thin film  4  (FIG. 1D). The epitaxial growth of the GaN thin film  4  on the base layer B can be conducted under temperature conditions appropriately selected from among those known to the art but the deposition pressure must be not lower than 800 Torr. The use of a deposition pressure of 800 Torr or higher, i.e., one slightly higher than atmospheric pressure, markedly improves the crystal quality of the GaN thin film  4  over that obtained at atmospheric pressure.  
         [0022]    [0022]FIG. 2 is a schematic diagram showing the principal parts of a vapor deposition semiconductor manufacturing system  10  that is an example of an HVPE semiconductor manufacturing system usable for implementing the method of the present invention.  
         [0023]    The vapor deposition semiconductor manufacturing system  10  is equipped with a reactor  12  that is supplied through material supply lines  11  with source gases from a starting material supply system (not shown). A susceptor  17  is provided in the reactor  12  for placement and heating of the base layer B. The starting material supply lines  11  consist of a first line  11 A for supplying N 2  purging gas, a second line  11 B for supplying NH 3  and carrier gas, and a third line  11 C for supplying HCl and carrier gas. The carrier gas supplied by the second line  11 B can be N 2  or H 2  alone or a mixture of N 2  and H 2 . The carrier gas supplied by the third line  11 C can also be N 2  or H 2  alone or a mixture of N 2  and H 2 .  
         [0024]    An exhaust line  13  is connected to an exhaust port  12 A of the reactor  12 . The flow rate of exhaust gas discharged through the exhaust line  13  is regulated by a flow rate control valve VL provided in the exhaust line  13 . This enables control of the pressure in the reactor  12 . The deposition pressure of the reactor  12  can therefore be controlled to the desired value by regulating the flow rate control valve VL.  
         [0025]    The pressure in the reactor  12  is detected by a pressure sensor  14 . The output of the pressure sensor  14  is sent to pressure display  15  by which the deposition pressure in the reactor  12  can be monitored. In this embodiment, the pressure sensor  14  is attached to the exhaust line  13  between the exhaust port  12 A and the flow rate control valve VL to enable detection of the pressure in the reactor  12 .  
         [0026]    An electric furnace  16  is provided for heating the susceptor  17 . The reactor  12  can be heated to establish the required deposition temperature profile by supplying the electric furnace  16  with heating current from a power source (not shown). FIG. 3 shows an example of the deposition temperature profile in the reactor  12 .  
         [0027]    In the vapor deposition semiconductor manufacturing system  10  of the foregoing configuration, source gases supplied to the reactor  12  through the starting material supply lines  11  react on the base layer B (omitted in FIG. 2) placed on the susceptor  17  heated by the electric furnace  16  to vapor deposit the desired GaN thin film on the base layer B. The used gas is discharged to the exterior through the exhaust port  12 A to be sent through the exhaust line.  13  to an exhaust gas treatment apparatus (not shown).  
         [0028]    This process will be explained more specifically. The base layer B obtained in the process of FIG. 1C is placed on the susceptor  17  in the reactor  12 , source gas is supplied into the reactor  12 , and the GaN thin film  4  is grown on the base layer B using a profile of the deposition temperature like that shown in FIG. 3, for example (FIG. 1D).  
         [0029]    While keeping an eye on the pressure display  15 , the operator regulates the flow rate control valve VL to control the flow rate of the exhaust gas passing through the exhaust line  13  and thereby establish a flow rate of the source gas that maintains the deposition pressure in the reactor  12  at 800 Torr, for instance.  
         [0030]    A high-quality GaN thin film  4  is thus gown on the base layer B by, as explained with reference to FIG. 1D, maintaining the deposition pressure slightly higher than atmospheric pressure, i.e., at 800 Torr or higher. When a III-V Group nitride compound semiconductor is epitaxially grown by HVPE, a III-V Group nitride compound semiconductor having very good crystallinity can be manufactured merely by setting the deposition pressure slightly higher than atmospheric pressure. The present invention thus offers a method of outstanding industrial value for its ability to manufacture high-quality III-V Group compound semiconductor at low cost.  
         [0031]    The embodiment of the invention explained in the foregoing relates to the case of growing a GaN thin film  4  on the base layer B. However, the present invention is not limited to this embodiment and can, for example, provide the same excellent effect when applied to regrow a GaN layer by ELO (epitaxial layer overgrowth). The present invention can also be directly applied when a substrate of a material with a lattice constant relatively close to that of the compound semiconductor to be produced is used as the base layer.  
       WORKING EXAMPLES  
     Example 1  
       [0032]    The vapor deposition semiconductor manufacturing system  10  shown in FIG. 2 was used as set out below to epitaxially grow a GaN thin film on a base layer of GaN formed on a sapphire substrate by MOVPE.  
         [0033]    The temperature in the reactor  12  was controlled by regulating the flow rate control valve VL concomitantly with temperature increase, and the temperature profile in the reactor  12  was established as shown in FIG. 3. The starting materials were supplied at a constant V/III ratio of 25 (NH 3  partial pressure: 0.2 atm, GaCl partial pressure: 8×10 −3  atm) and growth was conducted with the internal pressure of the reactor  12  controlled to 910 Torr. As the flow rate was constant, the carrier gas flow rate was varied with pressure. The experiment was conducted using a mixed carrier gas composed of equal amounts of N 2  and H 2  (N 2 :H 2 =1:1). Specimens for comparison were produced at pressures of 300 Torr and 760 Torr.  
         [0034]    The invention specimen produced at a deposition pressure of 910 Torr and the specimens for comparison produced at deposition pressures of 300 Torr and 760 Torr were evaluated for crystallinity by x-ray diffraction analysis. Change in the half-value width of XRC (x-ray rocking curve) for (0004) and (10-10) obtained by the analysis is plotted as a function of the deposition pressure in FIG. 4. In both the (0004) and (10-10) diffraction results, the half-value width (FWHM: full width at half-maximum) was smallest when the deposition pressure was 910 Torr, and the improvement in crystallinity over the specimens obtained at deposition pressures of 300 Torr and 760 Torr was pronounced. These results demonstrate that the crystallinity of GaN thin film improves when the deposition pressure is increased.  
         [0035]    [0035]FIG. 5 shows the surface state of the GaN thin film grown at a deposition pressure of 910 Torr. FIG. 6 shows the surface state of a GaN thin film grown at a conventional deposition pressure of 760 Torr. Observation of the two surfaces confirmed that crack occurrence could be suppressed to obtain a markedly better surface state solely by increasing the deposition pressure from 760 Torr to 910 Torr.  
       Example 2  
       [0036]    A specimen was manufactured by growing a GaN thin film on a base layer of GaN formed on a sapphire substrate by MOVPE. The same method as in Example 1 was used expect for changing the deposition pressure to 836 Torr. The surface morphology of the specimen was observed with an optical microscope. The surface was found to be free of cracks and of excellent quality like that of Example 1. The half-value width of the (0004) and (10-10) diffractions were measured and found to be 175 seconds and 220 seconds, indicating good crystallinity on a par with that in Example 1.  
         [0037]    As explained in the foregoing, the present invention enables manufacture of a III-V Group nitride compound semiconductor exhibiting excellent crystallinity simply by setting the deposition pressure slightly higher than atmospheric pressure when epitaxially growing a III-V Group nitride compound semiconductor by HVPE. The present invention thus offers a method of outstanding industrial value for its ability to manufacture high-quality III-V Group compound semiconductor at low cost.