Abstract:
A method of making a bulk crystal substrate of a GaN single crystal includes the steps of forming a molten flux of an alkali metal in a reaction vessel and causing a growth of a GaN single crystal from the molten flux, wherein the growth is continued while replenishing a compound containing N from a source outside the reaction vessel.

Description:
This is a divisional of application Ser. No. 09/590,063 filed Jun. 8, 2000, now U.S. Pat. No. 6,592,663. 

   BACKGROUND OF THE INVENTION 
   The present invention generally relates to semiconductor devices and more particularly to a semiconductor device having a GaN bulk crystal substrate. 
   GaN is a III-V compound semiconductor material having a large bandgap of blue to ultraviolet wavelength energy. Thus, intensive investigations are being made with regard to development of optical semiconductor devices having a GaN active layer for use particularly in optical information storage devices including a digital video data recorder (DVD). By using such a light emitting semiconductor device producing blue to ultraviolet wavelength optical radiation for the optical source, it is possible to increase the recording density of optical information storage devices. 
   Conventionally, a laser diode or light-emitting diode having a GaN active layer has been constructed on a sapphire substrate in view of the absence of technology of forming a GaN bulk crystal substrate. 
     FIG. 1  shows the construction of a conventional GaN laser diode according to Nakamura, S., et al., Jpn. J. Appl. Phys. vol. 36 (1997) pp. L1568–L1571, Part 2, No. 12A, 1 Dec. 1997, constructed on a sapphire substrate  1 . 
   Referring to  FIG. 1 , the sapphire substrate  1  has a (0001) principal surface covered by a low-temperature GaN buffer layer  2 , and includes a GaN buffer layer  3  of n-type grown further on the buffer layer  2 . The GaN buffer layer  3  includes a lower layer part  3   a  and an upper layer part  3   b  both of n-type, with an intervening SiO 2  mask pattern  4  provided such that the SiO 2  mask pattern  4  is embedded between the lower layer part  3   a  and the upper layer part  3   b . More specifically, the SiO 2  mask pattern  4  is formed on the lower GaN buffer layer part  3   a , followed by a patterning process thereof to form an opening  4 A in the SiO 2  mask pattern  4 . 
   After the formation of the SiO 2  mask pattern  4 , the upper GaN layer part  3   b  is formed by an epitaxial lateral overgrowth (ELO) process in which the layer  3   b  is grown laterally on the SiO 2  mask  4 . Thereby, desired epitaxy is achieved with regard to the lower GaN layer part  3   a  at the opening  4 A in the SiO 2  mask pattern  4 . By growing the GaN layer part  3   b  as such, it is possible to prevent the defects, which are formed in the GaN layer part  3   a  due to the large lattice misfit between GaN and sapphire, from penetrating into the upper GaN layer part  3   b.    
   On the upper GaN layer  3   b, a  strained super-lattice structure  5  having an n-type Al 0.14 Ga 0.86 N/GaN modulation doped structure is formed, with an intervening InGaN layer  5 A of the n-type having a composition In 0.1 Ga 0.9 N interposed between the upper GaN layer part  3   b  and the strained superlattice structure  5 . By providing the strained superlattice structure  5  as such, dislocations that are originated at the surface of the sapphire substrate  1  and extending in the upward direction are intercepted and trapped. 
   On the strained superlattice structure  5 , a lower cladding layer  6  of n-type GaN is formed, and an active layer  7  having an MQW structure of In 0.01 Ga 0.98 N/In 0.15 Ga 0.85 N is formed on the cladding layer  6 . Further, an upper cladding layer  8  of p-type GaN is formed on the active layer  7 , with an intervening electron blocking layer  7 A of p-type AlGaN having a composition of Al 0.2 Ga 0.8 N interposed between the active layer  7  and the upper cladding layer  8 . 
   On the upper cladding layer  8 , another strained superlattice structure  9  of a p-type Al 0.14 Ga 0.86 N/GaN modulation doped structure is formed such that the superlattice structure  9  is covered by a p-type GaN cap layer  10 . Further, a p-type electrode  11  is formed in contact with the cap layer  10  and an n-type electrode  12  is formed in contact with the n-type GaN buffer layer  3   b.    
   It is reported that the laser diode of  FIG. 1  oscillates successfully with a practical lifetime, indicating that the defect density in the active layer  7  is reduced successfully. 
   On the other hand, the laser diode of  FIG. 1  cannot eliminate the defects completely, and there remain substantial defects particularly in correspondence to the part on the SiO 2  mask  4  as represented in  FIG. 2 . See Nakamura S. et al., op cit. It should be noted that such defects formed on the SiO 2  mask  4  easily penetrate through the strained superlattice structure  5  and the lower cladding layer  6  and reach the active layer  7 . 
   In view of the foregoing concentration of the defects in the central part of the SiO 2  mask pattern  4 , the laser diode of  FIG. 1  uses the part of the semiconductor epitaxial structure located on the opening  4 A of the SiO 2  mask  4 , by forming a mesa structure M in correspondence to the opening  4 A. However, the defect-free region formed on the opening  4 A has a lateral size of only several microns, and thus, it is difficult to construct a high-power laser diode based on the construction of  FIG. 1 . When the laser diode of  FIG. 1  is driven at a high power, the area of optical emission in the active region extends inevitably across the defects, and the laser diode is damaged as a result of optical absorption caused by the defects. Further, the laser diode of  FIG. 1  having such a construction has other various drawbacks associated with the defects in the semiconductor epitaxial layers, such as large threshold current, limited lifetime, and the like. Further, the laser diode of  FIG. 1  has a drawback, in view of the fact that the sapphire substrate is an insulating substrate, in that it is not possible to provide an electrode on the substrate. As represented in  FIG. 1 , it is necessary to expose the top surface of the n-type GaN buffer layer  3  by an etching process in order to provide the n-type electrode  12 , while such an etching process complicates the fabrication process of the laser diode. In addition, the increased distance between the active layer  7  and the n-type electrode  12  causes the problem of increased resistance of the current path, while such an increased resistance of the current path deteriorates the high-speed response of the laser diode. 
   Further, the conventional laser diode of  FIG. 1  suffers from the problem of poor quality of mirror surfaces defining the optical cavity. Due to the fact that the sapphire single crystal constituting the substrate  1  belongs to hexagonal crystal system, formation of the optical cavity cannot be achieved by a simple cleaving process. It has been therefore necessary to form the mirror surfaces, when fabricating the laser diode of  FIG. 1  by conducting a dry etching process, while the mirror surface thus formed by a dry etching process has a poor quality. 
   Because of the foregoing reasons, as well as because of other various reasons not mentioned here, it is desired to form the substrate of the GaN laser diode by a bulk crystal GaN and form the laser diode directly on the GaN bulk crystal substrate. 
   With regard to the art of growing a bulk crystal GaN, there is a successful attempt reported by Porowski (Porowski, S., J. Crystal Growth 189/190 (1998) pp. 153–158, in which a GaN bulk crystal is synthesized from a Ga melt under an elevated temperature of 1400–1700° C. and an elevated N 2  pressure of 12–20 kbar (1.2–2 GPa). This process, however, can only provide an extremely small crystal in the order of 1 cm in diameter at best. Further the process of Porowski requires a specially built pressure-resistant apparatus and a long time is needed for loading or unloading a source material, or increasing or decreasing the pressure and temperature. Thus, the process of this prior art would not be a realistic solution for mass-production of a GaN bulk crystal substrate. It should be noted that the reaction vessel of Porowski has to withstand the foregoing extremely high pressure, which is rarely encountered in industrial process, under the temperature exceeding 1400° C. 
   Further, there is a known process of growing a GaN bulk crystal without using an extremely high pressure environment for growing a GaN bulk crystal as reported by Yamane, H., et al., Chem. Mater. 1997, 9, 413–416. More specifically, the process of Yamane et al. successfully avoids the use of the extremely high-pressure used in Porowski, by conducting the growth of the GaN bulk crystal from a Ga melt in the presence of a Na flux. 
   According to the process of Yamane, a metallic Ga source and a NaN 3  (sodium azide) flux are confined in a pressure-resistance reaction vessel of stainless steel together with a N 2  atmosphere, and the reaction vessel is heated to a temperature of 600–800° C. and held for a duration of 24–100 hours. As a result of the heating, the pressure inside the reaction vessel is elevated to the order of 100 kg/cm 2  (about 10 MPa), which is substantially lower than the pressure used by Porowski. As a result of the reaction, GaN crystals are precipitated from the melt of a Na—Ga system. In view of the relatively low pressure and low temperature needed for the reaction, the process of Yamane et al. is much easier to implement. 
   On the other hand, the process of Yamane relies upon the initially confined N 2  molecules in the atmosphere and the N atoms contained in the NaN 3  flux for the source of N. Thus, when the reaction proceeds, the N 2  molecules in the atmosphere or the N atoms in the Na—Ga melt are depleted with the precipitation of the GaN crystal, and there appears a limitation in growing a large bulk crystal of GaN. The GaN crystals obtained by the process of Yamane et al. typically have a size of 1 mm or less in diameter. Thus, the process of Yamane et al. op cit., while being successful in forming a GaN bulk crystal at a relatively low pressure and temperature, cannot be used for a mass production of a GaN substrate in the industrial base. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is a general object of the present invention to provide a novel and useful GaN semiconductor device having a bulk crystal substrate wherein the foregoing problems are eliminated. 
   Another and more specific object of the present invention is to provide a process of making a bulk crystal substrate of a GaN single crystal. 
   Another object of the present invention is to provide a process of fabricating a GaN semiconductor device having a bulk crystal substrate of a GaN single crystal. 
   Another object of the present invention is to provide a bulk crystal substrate of a single crystal GaN. 
   Another object of the present invention is to provide an optical semiconductor device having a bulk crystal substrate of a GaN single crystal. 
   Another object of the present invention is to provide an electron device having a bulk crystal substrate of a GaN single crystal. 
   Another object of the present invention is to provide an apparatus for making a bulk crystal substrate of a GaN single crystal. 
   According to the present invention, a high-quality GaN bulk crystal substrate is obtained with a process suitable for mass-production, by continuously supplying N so as to compensate for the depletion of N occurring in the system in which precipitation of a GaN single crystal takes place. By using the GaN bulk crystal substrate thus obtained, it is possible to fabricate an optical semiconductor device that produces an optical radiation of blue to ultraviolet wavelength with a large optical power. Further, the GaN bulk crystal substrate can be used as a substrate of an electron device such as HEMT. 
   Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing the construction of a conventional laser diode constructed on a sapphire substrate; 
       FIG. 2  is a diagram showing the problem associated with the laser diode of  FIG. 1 ; 
       FIG. 3  is a diagram showing the construction of a growth apparatus used in a first embodiment of the present invention for growing a GaN bulk crystal; 
       FIGS. 4A and 4B  are diagrams showing a part of the apparatus of  FIG. 3  in detail; 
       FIG. 5  is a diagram showing a cathode luminescent spectrum of a GaN bulk crystal obtained in the first embodiment; 
       FIG. 6  is a diagram showing a control of GaN composition in the growth apparatus of  FIG. 3 ; 
       FIG. 7  is a diagram showing the construction of a growth apparatus used in a second embodiment of the present invention for growing a GaN bulk crystal; 
       FIG. 8  is a diagram showing the construction of a growth apparatus used in a third embodiment of the present invention for growing a GaN bulk crystal; 
       FIG. 9  is a diagram showing the construction of a growth apparatus used in a fourth embodiment of the present invention for growing a GaN bulk crystal; 
       FIG. 10  is a diagram showing the construction of a growth apparatus used in a fifth embodiment of the present invention for growing a GaN bulk crystal; 
       FIG. 11  is a diagram showing the construction of a growth apparatus used in a sixth embodiment of the present invention for growing a GaN bulk crystal; 
       FIG. 12  is a diagram showing the construction of a growth apparatus used in a seventh embodiment of the present invention for growing a GaN bulk crystal; 
       FIG. 13  is a diagram showing the construction of a seed crystal used in the growth apparatus of  FIG. 12 ; 
       FIG. 14  is a diagram showing the construction of a growth apparatus used in an eighth embodiment of the present invention for growing a GaN bulk crystal; 
       FIGS. 15A and 15B  are diagrams showing a part of the growth apparatus of  FIG. 14 ; 
       FIG. 16  is a diagram showing the growth apparatus of  FIG. 14  in the state in which a growth of the GaN bulk crystal has been made; 
       FIG. 17  is a diagram showing the construction of a growth apparatus used in a ninth embodiment of the present invention for growing a GaN bulk crystal; 
       FIG. 18  is a diagram showing the construction of a growth apparatus used in a tenth embodiment of the present invention for growing a GaN bulk crystal; 
       FIG. 19  is a diagram showing X-ray diffraction data obtained for a GaN bulk crystal according to an eleventh embodiment of the present invention; 
       FIG. 20  is a diagram showing the construction of a laser diode having a GaN bulk crystal substrate according to a twelfth embodiment of the present invention; and 
       FIG. 21  is a diagram showing the construction of a HEMT having a GaN bulk crystal substrate according to a thirteenth embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   [First Embodiment] 
     FIG. 3  shows the construction of a growth apparatus  100  used in a first embodiment of the present invention for growing a GaN bulk crystal. 
   Referring to  FIG. 3 , the growth apparatus  100  includes a pressure-resistant reaction vessel  101  typically of a stainless steel having an inner diameter of about 75 mm and a length of about 300 mm and accommodates therein a crucible  102  of Nb or BN. As will be explained later, the crucible  102  is loaded with a starting material of metallic Ga and a NaN 3  flux and is confined in the reaction vessel  101  together with an N 2  atmosphere  107 . Further, the reaction vessel  101  is supplied with N 2  or a gaseous compound of N from an external source via a regulator valve  109  and an inlet  108 . The reaction vessel  101  thus loaded with the starting material in the crucible  102  is heated by energizing heaters  110  and  111  to a temperature of 650–850° C., and the pressure inside the reaction vessel is regulated to a moderate value of about 5 MPa by controlling the valve  109 . By holding the temperature and the pressure, a precipitation of GaN bulk crystal takes place from a Na—Ga melt, which is formed in the crucible  102  as a result of the melting of the starting material. 
     FIG. 4A  shows the loading of the starting material in the crucible  102 , while  FIG. 4B  shows the state in which the source material has caused a melting. 
   Referring to  FIG. 4A , a high-purity metallic Ga and a high-purity metallic Na are weighed carefully and loaded into the crucible  102 , wherein the foregoing process of weighing and loading are conducted in the N 2  atmosphere. It is also possible to use high-purity NaN 3  in place of high-purity metallic Na source. 
   In the state of  FIG. 4B , on the other hand, there appears a melt  102 A of the Na—Ga system in the crucible  102  and crystallization of GaN takes place from various parts of the melt  102 A including a free surface of the melt and a sidewall or bottom wall of the crucible  102 . There, it was observed that a large single crystal  102 B of GaN grows on the melt free surface contacting with the atmosphere and fine needle-like GaN crystals  102 C grow on the sidewall or bottom wall of the crucible  102 . 
   With the growth of the GaN crystals, particularly with the growth of the GaN single crystal  102 B, N in the atmosphere is consumed and the pressure inside the reaction vessel gradually falls as a result of depletion of N in the atmosphere. Thus, in the present embodiment, the depletion of N in the atmosphere  107  is compensated for by replenishing N 2  or a compound of N such as NH 3  from an external source. Thereby, the growth of the GaN single crystal  102 B continues at the melt free surface and a large GaN single crystal suitable for use in an optical semiconductor device such as a laser diode or light-emitting diode as a GaN bulk crystal substrate is obtained. The construction of  FIG. 3  can easily produce the GaN single crystal  102 B with a thickness of 100 μm or more. The GaN single crystal  102 B thus formed at the temperature of 650–850° C. has a hexagonal crystal symmetry. 
     FIG. 5  shows the cathode luminescent spectrum of the GaN single crystal  102 B thus obtained in comparison with the cathode luminescent spectrum of a GaN thick film grown on a sapphire substrate or an SiC substrate. 
   Referring to  FIG. 5 , it can be seen that the GaN crystal  102 B of the present embodiment shows a distinct and strong peak corresponding to the band edge of GaN at the wavelength of about 360 nm. Further, it can be seen that no other peak exists in the GaN single crystal  102 B of the present embodiment. The result of  FIG. 5  indicates that the GaN crystal  102 B thus formed has a defect density of less than 10 2 –10 3  cm −2 . Thus, the GaN single crystal  102 B is suitable for use as a bulk GaN substrate of various optical semiconductor devices including a laser diode and a light-emitting diode as noted already. Hereinafter, the GaN single crystal  102 B will be called a GaN bulk crystal in view of application to a GaN bulk crystal substrate. 
   Contrary to the present embodiment, the GaN thick film formed on the sapphire substrate or formed on the SiC substrate shows a remarkable peak at the wavelength of about 600 nm corresponding to deep impurity levels. This clearly indicates that the GaN thick film thus formed on a sapphire substrate or an SiC substrate contains a substantial amount of defects. Associated with the high level of defects, it can be seen that the peak strength for the band edge is substantially smaller than the case of the GaN bulk crystal  102 B of the present embodiment. 
   In the growth process of  FIG. 4B , it should be noted that there appears also an intermetallic compound  102 D of GaNa along the sidewall and bottom surface of the crucible  102  indicated in  FIG. 4B  by a broken line. Thus, the region represented in  FIG. 4B  by the broken line in fact includes the fine GaN crystals  102 C and the GaNa intermetallic compound  102 D in the form of a mixture. The GaN fine crystals  102 C or the GaNa intermetallic compound  102 D thus formed releases Ga into the melt  102 A, and the Ga atoms thus released contribute to the growth of the GaN bulk crystal  102 B when transported to the melt surface. 
   Thus, by continuously replenishing N 2  or NH 3 , the growth process of the GaN bulk crystal  102 B continues until Ga in the melt  102 A is used up. 
     FIG. 6  shows the control of the N 2  pressure in the atmosphere  107  with the growth of the GaN bulk crystal  102 B from the melt  102 A. 
   Referring to  FIG. 6 , it can be seen that the N 2  pressure a necessary for maintaining the stoichiometric composition for the GaN bulk crystal  102 B changes depending on the Ga content in the melt  102 A represented in the horizontal axis. When the N 2  pressure in the atmosphere  107  is fixed (a 1 =a 2 ), it is not possible to maintain the stoichiometric composition for the GaN bulk crystal  102 B. Thus, the present invention changes the N 2  pressure a in the atmosphere  107  with the progress of growth of the GaN bulk crystal  102 B as represented as a 1 ≠a 2 . 
   [Second Embodiment] 
     FIG. 7  shows the construction of a growth apparatus  200  according to a second embodiment of the present invention, wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description there of will be omitted. 
   Referring to  FIG. 7 , the present embodiment uses heaters  111 A and  111 B in place of the heater  111  and induces a temperature gradient in the melt  102 A for facilitating transport of Ga from the GaN fine crystals  102 C or the GaNa intermetallic compound  102 D to the melt surface. 
   More specifically, the heater  111 B is provided in correspondence to the bottom part of the crucible  102  and controls, together with the heart  11 A, the melt temperature at the bottom part of the crucible  102  lower than the melt surface. As a result of energization of the heaters  111 A and  111 B, a temperature gradient shown in  FIG. 7  is induced. 
   Due to the increased temperature at the bottom part of the crucible  102 , undesirable precipitation of GaN crystals on bottom surface of the crucible  102  is minimized, and the growth of the GaN bulk crystal  102 B on the melt surface is promoted substantially. When a GaN fine crystal  102 C is formed, such a GaN fine crystal  102 C is immediately dissolved into the melt  102 A and no substantial deposition occurs on the bottom part of the crucible  102 . Further, the intermetallic compound of GaNa, formed at a temperature lower than about 530° C., acts also as the source of Ga and Na in the melt  102 A. 
   Similarly to the first embodiment, the GaN bulk crystal  102 B formed according to the present embodiment has a defect density in the order of 10 2 –10 3  cm −2  or less. Thus, the GaN bulk crystal  102 B is suitable for a bulk GaN substrate of various optical semiconductor devices including a laser diode and a light-emitting diode. 
   [Third Embodiment] 
     FIG. 8  shows the construction of a growth apparatus  300  according to a third embodiment of the present invention, wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
   Referring to  FIG. 8 , the present embodiment is a modification of the embodiment of  FIG. 7  and uses the heaters  110  and  111 , described with reference to the growth apparatus  100  for inducing the desired temperature gradient. As other aspects of the present embodiment are substantially the same as those of the previous embodiment, further description will be omitted. 
   [Fourth Embodiment] 
     FIG. 9  shows the construction of a growth apparatus  400  according to a fourth embodiment of the present invention, wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
   Referring to  FIG. 9 , the growth apparatus  400  has a construction similar to that of  FIG. 3 , except that there is provided a container  103  holding a metallic Ga source  104  inside the reaction vessel  101 . The container  103  is provided at a first end of a tube  103 A extending outside of the reaction vessel  101 , and there is provided a pressure regulator  106  at a second, opposite end of the tube  103 . The pressure regulator  106  is supplied with a pressurized N 2  gas from an external source and causes a molten Ga, formed in the container  103  as a result of heating, to drip to the Na—Ga melt  102 A in the crucible  102  via a hole  105  formed at a bottom part of the container  103 . 
   According to the construction of  FIG. 9 , depletion of Ga in the melt  102 A is replenished from the Ga source  104  and a thickness of 300 μm or more is obtained for the GaN bulk crystal  102 B as a result of the continuous crystal growth. 
   Similarly to the previous embodiments, the GaN bulk crystal  102 B formed according to the present embodiment has a defect density of 10 2 –10 3  cm −2  or less. Thus, the GaN bulk crystal  102 B of the present embodiment is suitable for use as a bulk GaN substrate of various optical semiconductor devices including a laser diode and a light-emitting diode. 
   [Fifth Embodiment] 
     FIG. 10  shows the construction of a growth apparatus  500  according to a fifth embodiment of the present invention, wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
   Referring to  FIG. 10 , the growth apparatus  500  has a construction similar to that of the growth apparatus of  FIG. 9 , except that there is provided an outer pressure vessel  112  outside the reaction vessel  101 , and the space between the reaction vessel  101  and the outer pressure vessel  112  is filled with a pressurized gas such as N 2 , which is introduced via a regulator  114  and an inlet  113 . 
   By providing the pressure vessel  112  outside the reaction vessel  101 , the pressurized reaction vessel  101  is supported from outside and the design of the reaction vessel  101  becomes substantially easier. As represented in  FIG. 10 , there is provided a thermal insulator  115  between the heater  110  or  111  and the outer pressure vessel  112  and the temperature rise of the pressure vessel  112  is avoided. Thereby, the pressure vessel  112  maintains a large mechanical strength even when the inner, reaction vessel  101  is heated to the temperature exceeding 600 or 700° C. In order to avoid the decrease of mechanical strength, it is possible to provide a water cooling system (not shown) on the outer pressure vessel  112 . 
   The outer pressure vessel  112  can be provided also to the growing apparatuses  100 – 300  explained before as well as to the growing apparatuses to be described hereinafter. 
   As other features of the present embodiment are substantially the same as those of the previous embodiments, further description thereof will be omitted. 
   [Sixth Embodiment] 
     FIG. 11  shows the construction of a growing apparatus  600  according to a sixth embodiment of the present invention, wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
   Referring to  FIG. 11 , the growing apparatus  600  has a construction similar to that of the growing apparatus  100  of  FIG. 3 , except that there is provided a holder  601  holding a Ga—Na melt outside the reaction vessel  101  and the Ga—Na melt in the holder  601  is supplied into the reaction vessel  101  and to the melt  102 A in the crucible  102  via a tube  601 A penetrating through a wall of the reaction vessel  101 , in response to a pressurization of the holder  601  by a pressurized gas such as an N 2  gas supplied via a line  602 . 
   According to the present embodiment, the depletion of Ga in the melt  102 A is replenished together with the Na flux, and the growth of the GaN bulk crystal  102 B at the free surface of the melt  102 A is conducted continuously. It should be noted that depletion of N in the system is also replenished by the external N source similarly to the previous embodiments. As a result, a high-quality GaN bulk crystal suitable for use as a substrate of various optical semiconductor devices is obtained with a thickness well exceeding 100 μm, generally about 300 μm or more. 
   [Seventh Embodiment] 
     FIG. 12  shows the construction of a growth apparatus  700  according to a seventh embodiment of the present invention, wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
   Referring to  FIG. 12 , the growing apparatus  700  has a construction similar to that of the growing apparatus  600  of the previous embodiment, except that there is provided a rod  702  carrying a seed crystal  701  at a tip end thereof in contact with the free surface of the melt  102 A in the crucible  102 . Further there is provided a motor  703  for pulling up the rod  702 , and there occurs a continuous growth of the GaN bulk crystal  102 B at the melt surface with the pulling up of the rod  702 . Thereby, an ingot of a GaN bulk crystal is obtained. 
   By slicing the GaN bulk crystal ingot thus obtained, it is possible to mass produce the GaN bulk crystal substrate for use in various optical semiconductor devices including a laser diode and a light-emitting diode. 
     FIG. 13  shows an example of the seed crystal  701  provided at the tip end of the rod  702 . 
   Referring to  FIG. 13 , the seed crystal  702  is formed to have a slab shape with a width w and a thickness d corresponding to the width and thickness of the GaN substrate to be formed. Thus, by pulling up the rod  702  straight in the upward direction, a slab-shaped GaN bulk crystal is grown continuously. Thus, by merely polishing the surface of the GaN bulk crystal slab, followed by a cleaving process, it is possible to mass-produce the GaN bulk crystal substrate having a quality suitable for use in various optical semiconductor devices including a laser diode and a light-emitting diode. 
   As other features of the present embodiment is more or less the same as those of the previous embodiments, further description thereof will be omitted. 
   [Eighth Embodiment] 
     FIG. 14  shows the construction of a growing apparatus  800  according to an eighth embodiment of the present invention, wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
   Referring to  FIG. 14 , the growing apparatus  800  has a construction similar to that of the growing apparatus  700  of the previous embodiment, except that a cover member  803  is provided so as to cover the free surface of the melt  102 A. Further, the container  601  of a Na—Ga melt is eliminated and a container  801  having a heating mechanism  801 A and containing therein a molten Na is provided outside the reaction vessel  101 . Thereby, a vapor of Na is supplied from the container  801  into the interior of the reaction vessel  101  via a tube  802  and the Na vapor is added to the atmosphere  107  therein. 
   According to the present embodiment, uncontrolled precipitation of the GaN fine crystals  102 C on the sidewall or bottom surface of the crucible (see  FIG. 4B ) is minimized, by controlling the vapor pressure of Na from the container  801 . Further, no GaN precipitation occurs on the melt free surface, as the free surface of the melt  102 A is covered by the cover member  803 , except for a central part of the melt where there is formed an opening  803 A in the cover member  803  for allowing the seed crystal  701  on the rod  702  to make a contact with the surface of the melt  102 A. 
   Thus, according to the construction of  FIG. 14 , the Na vapor flux acts selectively at the part of the melt  102 A where the growth of the bulk GaN ingot is made, and the uncontrolled precipitation of the GaN fine crystals  102 C is effectively suppressed. 
   It should be noted that cover member  803  has a variable geometry construction formed of a number of small, fan-shaped members, in which the opening  803 A can be changed with the growth of the GaN bulk crystal  102 B in the form of ingot by moving the fan-shaped members in a direction of an arrow Q as represented in  FIGS. 15A and 15B , wherein  FIG. 15A  shows the state in which the central opening  803 A of the cover member  803  is closed while  FIG. 15B  shows the state in which the opening  803 A has been expanded for allowing the growth of the GaN bulk crystal ingot  102 B as represented in  FIG. 16 . It should be noted that  FIG. 16  shows the growing apparatus  800  in the state that there occurred a growth of the GaN bulk crystal  102 B in the form of ingot. 
   [Ninth Embodiment] 
     FIG. 17  shows the construction of a growing apparatus  900  according to a tenth embodiment of the present invention, wherein those parts corresponding to the parts described previously are designated by the same reference numeral and the description thereof will be omitted. 
   Referring to  FIG. 17 , the growing apparatus  900  has a construction similar to that of the growing apparatus  800  of the previous embodiment, except that the tube  802  supplying the Na vapor flux has a sleeve part  802 A surrounding the rod  702 . The sleeve part  802 A extends along the rod  702  and has an opening  802 C in correspondence to the surface of the melt  102 A where the opening  803 A is formed in the cover member  803  for the growth of the GaN bulk crystal  102 B. 
   According to the construction of  FIG. 17 , the Na flux is supplied selectively to the part where the growth of the GaN bulk crystal  102 B takes place and an efficient growth becomes possible. 
   As other aspects of the present embodiment are the same as those of the previous embodiment, further description thereof will be omitted. 
   [Tenth Embodiment] 
     FIG. 18  shows the construction of a growing apparatus  1000  according to a ninth embodiment of the present invention wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
   Referring to  FIG. 18 , the growing apparatus  1000  has a construction similar to that of the growing apparatus  700  of  FIG. 12 , except that the rod  702  driven by the motor  703  and pulling up the seed  701  in the upward direction is replaced by a rod  702 ′ driven by a motor  703 ′ and pulls down a seed  701 ′ in the downward direction. Thus, as represented in  FIG. 18 , the GaN bulk crystal  102 B forms an ingot grown inside the melt  102 A. As other aspects of the present invention is the same as those described before, further description of the present embodiment will be omitted. 
   [Eleventh Embodiment] 
   In any of the foregoing first through tenth embodiments, the grown of the GaN bulk crystal  102 B has been achieved at the temperature of 650–850° C. under the presence of a Na flux. As mentioned before, the GaN bulk crystal  102 B thus obtained has a symmetry of hexagonal crystal system. 
   On the other hand, the inventor of the present invention has discovered that a cubic GaN crystal is obtained as the bulk GaN crystal  102 B provided that the growth is made at a temperature of less than 600° C. under the presence of Na, or when the growth is made at a temperature of 650–850° C. under the presence of K. K may be introduced into the system in the form of a high-purity metallic K starting material, similarly to the case represented in  FIG. 4A . 
   From the x-ray diffraction peak position data, it was confirmed that the cubic GaN bulk crystal  102 B thus formed has a cubic lattice constant a o  of 4.5063±0.0009 Å.  FIG. 19  shows x-ray diffraction intensity data obtained for a GaN bulk crystal grown by the apparatus of  FIG. 3  as the bulk crystal  102 B at a temperature of 750° C. under the total pressure of 7 MPa in the reaction vessel  101 . In  FIG. 19 , it should be noted that the Fo represents the structural factor obtained from the diffraction intensity data for each of the reflections (h k 1) and s represents the error factor of the measurement, while Fc represents the structural factor calculated from a cubic zinc blende structure. A reliability factor, R, of 2.1% demonstrates the well agreement of Fo and Fc, where R is defined as: 
   
     
       
         
           R 
           = 
           
             ∑ 
             
               
                  
                 
                   F0 
                   - 
                   Fc 
                 
                  
               
               / 
               
                 ∑ 
                 
                   F0 
                   . 
                 
               
             
           
         
       
     
   
   Referring to  FIG. 19 , it can be seen that there is an excellent agreement between the observed structural factor and the calculated structural factor assuming the cubic zinc blende structure for the obtained GaN bulk crystal  102 B. It can be safely concluded that the GaN bulk crystal  102 B obtained in the present embodiment is a 100% cubic GaN crystal. From the X-ray diffraction analysis, existence of hexagonal GaN crystal was not detected. Further it was confirmed that the cubic GaN bulk crystal  102 B thus formed provides a cathode luminescent peak substantially identical with the spectrum of  FIG. 5 . In other words, the cubic GaN bulk crystal of the present embodiment contains little deep impurity levels or defects and has an excellent quality characterized by a defect density of 10 2 –10 3  cm −2  or less. 
   In view of increasing defect density in the GaN crystals grown at low temperatures, and further in view of the fact that a mixture of cubic GaN and hexagonal GaN appears when the growth of the GaN bulk crystal is conducted at the temperature of 600° C. or lower under presence of Na flux, it is preferred to grow a cubic GaN bulk crystal at the temperature of 650–850° C. under presence of a K flux. 
   [Twelfth Embodiment] 
     FIG. 20  shows the construction of a laser diode  150  of edge-emission type according to a twelfth embodiment of the present invention. 
   Referring to  FIG. 20 , the laser diode  150  is constructed on a GaN bulk crystal substrate  151  produced in any of the process explained before. More specifically, the GaN bulk crystal substrate  151  has a high crystal quality characterized by a defect density of 10 2 –10 3  cm −2  or less. 
   On the GaN bulk crystal substrate  151 , there is provided a lower cladding layer  152  of n-type AlGaN epitaxially with respect to the substrate  151  and an optical waveguide layer  153  of n-type GaN is formed on the lower cladding layer  152  epitaxially. 
   On the optical waveguide layer  153 , there is provided an active layer  154  of MQW structure including an alternate stacking of quantum well layers of undoped InGaN having a composition represented as In x Ga 1−x N (x=0.15) and barrier layers of undoped InGaN having a composition represented as In y Ga 1−y N (y=0.02). The active layer  154  is covered by an optical waveguide layer  155  of p-type GaN, and an upper cladding layer  156  of p-type AlGaN is formed epitaxially on the optical waveguide layer  155 . Further, a contact layer  157  of p-type GaN is formed on the upper cladding layer  156 . 
   The contact layer  157  and the underlying upper cladding layer  156  are subjected to a patterning process to form a loss-guide structure extending in the axial direction of the laser diode  150  and the loss-guide structure thus formed is covered by an SiO 2  film  158 . The SiO2 film  158  is formed with an opening  158 A extending in the laser axial direction for exposing the contact layer  157 , and a p-type electrode  159  is provided on the SiO 2  film  158  in contact with the contact layer  157  at the opening  158 A. 
   Further, an n-type electrode  160  is provided at a bottom surface of the GaN bulk crystal substrate  151 . 
   After forming the laser structure as such, the layered semiconductor body including the GaN substrate  151  and the epitaxial layers  151 – 157  is subjected to a cleaving process to form mirror surfaces M 1  and M 2  defining an optical cavity. Thereby, the laser diode produces a blue to ultraviolet optical beam as a result of stimulated emission and optical amplification occurring in the optical cavity, as represented in  FIG. 20  by an arrow. 
   According to the present invention, the optical cavity is formed by a simple cleaving process and the quality of the mirror surfaces M 1  and M 2  defining the optical cavity is improved substantially. Thereby, threshold of laser oscillation is lowered substantially. Further, the laser diode  150  carries the n-type electrode on the bottom surface of the GaN bulk crystal substrate  151  and the process of fabricating the laser diode is improved substantially. As the epitaxial layers, particularly the GaN optical waveguide layers  153  and  155  and the active layer  154  sandwiched between the layer  153  and  155  are formed epitaxially on the GaN bulk crystal substrate containing only a very small amount of defects, the quality of the crystal constituting the foregoing layers  153 – 155  is improved substantially over the conventional laser diode of  FIG. 1  and the laser diode  150  of  FIG. 20  can be driven with a large power. Further, the laser diode  150  of the present embodiment has an improved lifetime over the conventional laser diode of  FIG. 1 . 
   It should be noted that the GaN bulk crystal substrate  151  may be any of the hexagonal type or cubic type. In view of the easiness of cleaving process, on the other hand, it is preferable to form the GaN bulk crystal substrate  151  according to the process of the eleventh embodiment by using a K flux. 
   Based on the structure of  FIG. 20 , it is also possible to construct a light-emitting diode. Further, it is possible to construct a vertical cavity laser diode, which produces a laser beam in a direction vertical to the epitaxial layers, also by using the GaN bulk crystal substrate of the present invention. 
   In the case of a vertical cavity laser diode, a pair of mirror surfaces defining an optical cavity are formed by the epitaxial layers on the GaN bulk crystal substrate  151 , and an optical window is formed in the electrode  159 . In such a case, the GaN substrate  151  may have a thickness larger than 100 μm such as 300 μm or more. 
   In the laser diode of  FIG. 20 , it is also possible to form the mirror surfaces M 1  and M 2  by a dry etching process. 
   [Thirteenth Embodiment] 
     FIG. 21  shows the construction of an electron device  170  constructed on a GaN bulk crystal substrate  171  according to a thirteenth embodiment of the present invention. 
   Referring to  FIG. 21 , the electron device  170  is an FET, and the GaN bulk crystal  102 B of any of the foregoing first through twelfth embodiments is used for the GaN substrate  171 . 
   On the substrate  171 , there is provided a high-resistance epitaxial layer  172  of AlN, and a buffer layer  173  of undoped GaN is formed epitaxially on the AlN high-resistance layer  172 . 
   On the buffer layer  173 , a lower barrier layer  174  of undoped AlGaN is formed epitaxially, and a channel layer  175  of undoped GaN is formed on the lower barrier layer  174  such that the channel layer  175  is sandwiched between the lower barrier layer  174  and an upper barrier layer  176  of undoped AlGaN formed epitaxially on the channel layer  175 . 
   The upper barrier layer  176  is covered by a contact layer  177  of n-type GaN wherein the layers  174 – 177  are patterned to form a mesa region for device isolation. Further, the contact layer  177  is patterned to expose the upper barrier layer  176  in correspondence to the channel region, and a Schottky electrode  178  of a Ni/Au structure is provided in contact with the exposed upper barrier layer  176  as the gate electrode. Further, ohmic electrodes  179  and  180  of a Ti/Al structure are formed on the contact layer  177  at both lateral sides of the gate electrode  178  as a source electrode and a drain electrode, respectively. 
   In operation, a two-dimensional electron gas is induced in the channel layer  175  in response to application of a gate voltage to the gate electrode  178 . In this state, the FET is turned on. 
   According to the present invention, it is thus possible to construct an active device such as an FET on a GaN substrate, by using the GaN bulk crystal for the substrate. As the GaN bulk crystal produced according to the present invention has an high crystal quality characterized by a defect density of 10 2 –10 3  cm −2  or less, the problem of severe leakage current that would occur when an FET is constructed on a conventional GaN epitaxial layer formed on a sapphire substrate or an SiC substrate, is successfully eliminated. Further, the construction of  FIG. 21  is advantageous in view of the fact that the electron density of the two-dimensional electron gas induced in the channel layer  175  is increased due to enhanced piezoelectric effect and associated increase of degree of electron confinement into the channel layer. When the channel layer contains a high concentration of defects, there occurs a lattice relaxation and the effect of carrier confinement is degraded inevitably. 
   Further, the GaN bulk crystal of the present invention can be used also as the GaN substrate of other various electron devices including a HEMT, MESFET and an HBT. In fact, the structure of  FIG. 21  can be modified to form a HEMT by employing an n-type AlGaN layer for the upper barrier layer  176 . 
   Further, the present invention is by no means limited to the embodiments described heretofore, but various variations and modifications may be made without departing from the scope of the invention.