Abstract:
A method for producing a Group III nitride semiconductor includes reacting a molten mixture containing at least a Group III element and an alkali metal with a gas containing at least nitrogen, to thereby grow a Group III nitride semiconductor crystal on the seed crystal. The method includes forming a template substrate including a sapphire substrate and a first Group III nitride semiconductor layer as the seed crystal which is formed by vapor phase growth and which includes a c-plane as a main plane is employed, and the template substrate is placed and maintained in the molten mixture under conditions where crystal growth of the Group III nitride semiconductor is inhibited, to thereby partially melt back a plurality of separated parts of the first Group III nitride semiconductor layer to such a depth that the sapphire substrate is partially exposed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for producing a Group III nitride semiconductor by the flux method. More particularly, the invention relates to a method of separating a Group III nitride semiconductor layer from a sapphire substrate while cracking of the Group III nitride semiconductor layer grown by the flux method is prevented, in the case where a template substrate formed of the sapphire substrate and the Group III nitride semiconductor layer is employed as a seed crystal. 
     2. Background Art 
     The Na flux method is a known method for producing a Group III nitride semiconductor such as GaN. In this method, a molten mixture of Na (sodium) and Ga (gallium) is reacted with nitrogen at about 800° C. and a pressure of some ten atmospheres, to thereby grow GaN crystals. 
     The Na flux method employs, as a seed crystal, for example, a template substrate formed of a sapphire substrate and a GaN layer grown thereon by HVPE, or a GaN substrate. 
     Japanese Patent Application Laid-Open (kokai) No. 2006-169024 discloses a method for growing a Group III nitride crystal, which method includes employing a GaN substrate as a seed crystal; placing the seed crystal in a molten mixture of Na and Ga at such a temperature and a pressure that crystal growth of GaN is inhibited, to thereby melt back a part of the surface of the GaN substrate and form grooves; and subsequently, growing a GaN crystal on the GaN substrate at such a temperature and a pressure that crystal growth of GaN is allowed, to thereby fill the grooves with the molten mixture. In this method, since the grooves are filled with the molten mixture, the heat radiation performance of the GaN crystal is effectively improved. 
     Japanese Patent Application Laid-Open (kokai) No. 2004-247711 discloses a method for producing a Group III nitride free-substrate, which method includes employing, as a seed crystal, a template substrate formed of a sapphire substrate and a stripe-pattern GaN layer formed thereon; growing GaN on the a stripe-pattern GaN layer through the Na flux method; and subsequently, cooling the stacked product, to thereby generate stress attributable to the difference in linear expansion coefficient between sapphire and GaN and separate the seed crystal, whereby a high-quality GaN free-standing substrate is produced. The patent document also discloses that the stripe-pattern GaN layer may be formed through dry-etching a part of the GaN layer to such a depth that the surface of the sapphire substrate is exposed. 
     However, the second method is very cumbersome, since the method includes the dry-etching step of forming the stripe-pattern GaN layer and the step of growing GaN crystal by the Na flux method, separately. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, an object of the present invention is to readily produce a high-quality Group III nitride semiconductor substrate by the method for producing a Group III nitride semiconductor by the flux method employing a template substrate as a seed crystal. 
     Accordingly, in a first aspect of the present invention, there is provided a method for producing a Group III nitride semiconductor, including reacting a molten mixture containing at least a Group III element and an alkali metal with a gas containing at least nitrogen, to thereby grow a Group III nitride semiconductor crystal on the seed crystal, wherein the method comprises: 
     a first step in which a template substrate formed of a sapphire substrate and a first Group III nitride semiconductor layer which is formed on the sapphire substrate by vapor phase growth and which has a c-plane as a main plane is employed as the seed crystal; and the template substrate is placed and maintained in the molten mixture under the conditions where crystal growth of the Group III nitride semiconductor is inhibited, to thereby melt back a part of the first Group III nitride semiconductor layer to such a depth that the sapphire substrate is exposed, so that the remaining part of the first Group III nitride semiconductor layer is left in the form of a plurality of upright columns; 
     a second step subsequent to the first step in which a second Group III nitride semiconductor layer having a c-plane as a main plane is grown on the first Group III nitride semiconductor layer under the conditions where crystal growth of the Group III nitride semiconductor is permitted, so that the spaces between the columns are not filled with the second Group III nitride semiconductor; and 
     a third step subsequent to the second step in which cracking is induced in the first Group III nitride semiconductor layer through lowering of temperature, to thereby separate the second Group III nitride semiconductor layer from the sapphire substrate. 
     As used herein, the term “Group III nitride semiconductor” refers to a semiconductor represented by Al x Ga y In z N (x+y+z=1, 0≦x, y, z≦1) and also encompasses those in which Al, Ga, and In atoms are partially substituted by another group 13 element (Group 3B element) such as B or Tl, and N atoms are partially substituted by another group 15 element (Group 5B element) such as P, As, Sb, or Bi. More specifically, the semiconductor is a semiconductor essentially containing Ga; i.e., GaN, InGaN, AlGaN, or AlGaInN. 
     The Group III element is at least one species selected from Ga, Al, and In. Particularly, the second Group III nitride semiconductor layer is preferably GaN containing only Ga as a Group III element. The chemical composition of the first Group III nitride semiconductor layer is preferably the same as that of the second Group III nitride semiconductor layer. 
     The alkali metal is generally Na (sodium), but K (potassium) or a mixture of Na and K may be used. Alternatively, the alkali metal may contain Li (lithium) or an alkaline earth metal. The molten mixture may contain a dopant for controlling physical properties (e.g., conduction type or magnetic property) of Group III nitride semiconductor to be grown, promoting crystal growth, preventing formation of miscellaneous crystals, controlling the growth direction, etc. For example, when C (carbon) is added, formation of miscellaneous crystals can be prevented, and crystal growth can be promoted. As an n-type dopant, Ge (germanium) or the like may be used, and as a p-type dopant, Zn (zinc) or the like may be used. 
     The gas containing nitrogen is a gas of nitrogen molecules, a gas of a compound containing nitrogen as a component element (e.g., ammonia), a mixture thereof, or a mixture with inert gas such as rare gas. 
     The percent area of the first Group III nitride semiconductor layer in the form of a plurality of upright columns in the sapphire substrate surface is preferably 50% or more and less than 100%. When the percent area falls within the above range, there can be produced a high-quality second Group III nitride semiconductor layer in which cracking is more effectively prevented. 
     Whether the crystal growth of the Group III nitride semiconductor is permitted or not is determined by factors such as the temperature of molten mixture, the pressure, and the saturation state of nitrogen in the molten mixture. By controlling the factors, the conditions under which crystal growth of Group III nitride semiconductor is inhibited (first step) can be changed to those under which crystal growth of Group III nitride semiconductor is permitted (second step). 
     Particularly when the temperature of the molten mixture in which the first Group III nitride semiconductor layer is melted back in the first step is equalized to the temperature of the molten mixture in which the second Group III nitride semiconductor layer is grown through the flux method in the second step; the pressure in the first step is adjusted to be lower than the pressure at which the Group III nitride semiconductor is grown; and the pressure is elevated in the second step so as to allow crystal growth of the Group III nitride semiconductor, the first step can be shifted to the second step by controlling only pressure without controlling temperature, whereby the production steps can be further streamlined. 
     Also, when the pressure at which the first Group III nitride semiconductor layer is melted back in the first step is equalized to the pressure at which the second Group III nitride semiconductor layer is grown by the flux method in the second step; the temperature of the first step is adjusted to be higher than the crystal growth temperature of the Group III nitride semiconductor; and the temperature is lowered in the second step to a temperature at which the Group III nitride semiconductor is grown, the first step can be shifted to the second step through controlling only temperature without controlling pressure, whereby the production steps can be further streamlined. 
     Alternatively, the nitrogen level of the molten mixture may be adjusted to a non-saturation level which is a level close to but under the saturation level (hereinafter may be referred to as a “sub-saturation level”) in the first step, and then the nitrogen level of the molten mixture may be elevated to a super-saturation level by melting back the first Group III nitride semiconductor layer in the first step, whereby the conditions under which crystal growth of the Group III nitride semiconductor is permitted are attained. Through this alternative method, the first step can be shifted to the second step without controlling temperature or pressure, whereby the production steps can be further streamlined. Needless to say, the nitrogen-sub-saturation level may be shifted to the nitrogen-super-saturation level by controlling temperature and pressure. As used herein, a non-saturation level which is a level close to but under the saturation level, i.e., the sub-saturation level, means a starting nitrogen concentration level, where nitrogen is dissolved in the molten mixture at a level almost equal to the saturation level, in the first step and a level which reaches the saturation level when that the first Group III nitride semiconductor layer is not completely melted back and a plurality of upright columns are formed. 
     A second aspect of the invention is directed to a specific embodiment of the method for producing a Group III nitride semiconductor of the first aspect, wherein the second Group III nitride semiconductor layer is grown to a thickness of 0.5 mm or more. 
     A third aspect of the invention is directed to a specific embodiment of the method for producing a Group III nitride semiconductor of the first or second aspect, wherein the crystal growth of the Group III nitride semiconductor in the first step is inhibited by adjusting the pressure to be lower than the pressure at which crystal growth of the Group III nitride semiconductor is permitted, and the crystal growth of the Group III nitride semiconductor in the second step is permitted through elevating a pressure from of the pressure of the first-step to the pressure at which crystal growth of the Group III nitride semiconductor is permitted. 
     A fourth aspect of the invention is directed to a specific embodiment of the method for producing a Group III nitride semiconductor of the third aspect, wherein the molten mixture in the first step has a temperature which is equal to a temperature of the molten mixture of the second step. 
     A fifth aspect of the invention is directed to a specific embodiment of the method for producing a Group III nitride semiconductor of the first or second aspect, wherein the crystal growth of the Group III nitride semiconductor in the first step is inhibited through elevating a temperature of the molten mixture to be higher than a temperature at which crystal growth of the Group III nitride semiconductor is permitted, and the crystal growth of the Group III nitride semiconductor in the second step is permitted by lowering the temperature of the molten mixture in the first step to a temperature at which crystal growth of the Group III nitride semiconductor is permitted. 
     A sixth aspect of the invention is directed to a specific embodiment of the method for producing a Group III nitride semiconductor of the fifth aspect, wherein the first step employs a pressure which is equal to a pressure of the second step. 
     A seventh aspect of the invention is directed to a specific embodiment of the method for producing a Group III nitride semiconductor of the first or second aspect, wherein the crystal growth of the Group III nitride semiconductor in the first step is inhibited through adjusting a nitrogen level of the molten mixture to a non-saturation level which is a level close to but under the saturation level, and the crystal growth of the Group III nitride semiconductor in the second step is permitted by adjusting the nitrogen level of the molten mixture to a super-saturation level by melting back the first Group III nitride semiconductor layer in the first step. 
     According to the first aspect of the invention, the second Group III nitride semiconductor layer which has been grown by the flux method and which has a c-plane as a main plane can be readily separated from the sapphire substrate without generating cracks in the semiconductor layer. 
     According to the second aspect, the present invention is advantageous in the case where the second Group III nitride semiconductor layer is grown through the flux method to a thickness of 0.5 mm or more. When the thickness is 0.5 mm or more in a simple deposition, the stress generated due to the differences in linear expansion coefficient and lattice constant between sapphire and GaN increases, to thereby readily generate cracks in the GaN layer. However, according to the present invention, generation of cracks can be effectively prevented in the growing second Group III nitride semiconductor, whereby a crack-free second Group III nitride semiconductor layer having a thickness of 0.5 mm or more can be readily produced. 
     According to the third aspect of the invention, through controlling only pressure, the conditions where crystal growth of the Group III nitride semiconductor is inhibited can be shifted to the conditions where crystal growth of the Group III nitride semiconductor is permitted. Particularly, according to the fourth aspect of the invention, the first step can be shifted to the second step through controlling only pressure, whereby the production steps can be streamlined. 
     According to the fifth aspect of the invention, through controlling only the temperature of the molten mixture, the conditions where crystal growth of the Group III nitride semiconductor is inhibited can be shifted to the conditions where crystal growth of the Group III nitride semiconductor is permitted. Particularly, according to the sixth aspect of the invention, the first step can be shifted to the second step through controlling only temperature, whereby the production steps can be streamlined. 
     According to the seventh aspect of the invention, a nitrogen concentration of the molten mixture at a non-saturation level which is a level close to but under the saturation level is converted to a super-saturation level through melting back of the semiconductor layer, whereby the conditions where crystal growth of the Group III nitride semiconductor is inhibited can be shifted to the conditions where crystal growth of the Group III nitride semiconductor is permitted. Thus, the production steps can be streamlined. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1D  show the steps of producing a GaN crystal; 
         FIG. 2  is a cross-section of a production apparatus; and 
         FIGS. 3A and 3B  show the steps of preparing the template substrate. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will next be described in detail with reference to the drawings attached hereinbelow, which should not be construed as limiting the invention thereto. 
     Embodiment 1 
       FIGS. 1A to 1D  show the steps of producing a GaN crystal by the Na flux method performed in Embodiment 1.  FIG. 2  shows an apparatus for producing the GaN crystal. 
     The configuration of the production apparatus will next be described. The production apparatus comprises a pressure container  10 , a reaction vessel  11 , a crucible  12 , a heating apparatus  13 , supply pipes  14 ,  16 , and discharge pipes  15 ,  17 . 
     The pressure container  10  is a pressure-resistant hollow cylinder made of stainless steel. To the pressure container  10 , the supply pipe  16  and the discharge pipe  17  are connected. In the pressure container  10 , the reaction vessel  11  and the heating apparatus  13  are disposed. The reaction vessel  11  has heat resistance. In the reaction vessel  11 , the crucible  12  is placed. The crucible  12  holds a molten mixture containing Ga and Na, and a template substrate  102  is maintained in the molten mixture. To the reaction vessel  11 , the supply pipe  14  and the discharge pipe  15  are connected. Through operation of valves (not illustrated) attached to the supply pipe  14  and the discharge pipe  15 , there are performed aeration in and feeding nitrogen into the reaction vessel  11 , and controlling the pressure inside the reaction vessel  11 . Nitrogen is also supplied to the pressure container  10  via the supply pipe  16 . Through operation of valves (not illustrated) attached to the supply pipe  16  and the discharge pipe  17 , the nitrogen flow rate and discharge rate are controlled, whereby the pressure inside the pressure container  10  is equalized to that of the reaction vessel  11 . The temperature inside the reaction vessel  11  is controlled by means of the heating apparatus  13 . 
     Meanwhile, when the reaction vessel  11  has pressure resistance, the pressure container  10  is not necessarily employed. 
     The steps of producing a GaN crystal will next be described with reference to  FIGS. 1A to 1D . 
     Firstly, a GaN layer  101  having a c-plane as a main plane was formed by HVPE on a sapphire substrate  100 , to thereby provide a template substrate  102  ( FIG. 1A ). The GaN layer  101  has a thickness of 10 μm. 
     Then, the template substrate  102  was placed in the crucible  12  with Na, Ga, and C, such that the sapphire substrate  100  faces the bottom of the crucible  12 . Na and Ga of the solid form may be placed in the crucible  12 , or Na and Ga in the liquid form may be placed in the crucible  12 . Alternatively, a mixture of Na liquid and Ga liquid may be fed to the crucible  12 . C was added in order to promote crystal growth of GaN and prevent formation of miscellaneous crystals. 
     Subsequently, the crucible  12  was heated by means of the heating apparatus  13 , to thereby form a Ga—Na molten mixture, and the temperature of the molten mixture was adjusted to 850° C. Nitrogen was fed into to the reaction vessel  11  via the supply pipe  14  and the discharge pipe  15 , and the pressure inside the reaction vessel  11  was adjusted to 2.5 MPa. The template substrate  102  was maintained in the molten mixture. The temperature of the molten mixture was adjusted to permit crystal growth of GaN, but the pressure was adjusted to be lower than the pressure at which crystal growth of GaN is permitted. Thus, the GaN layer  101  was melted back. Melting back was performed until the surface of the sapphire substrate  100  was exposed. However, a part of the GaN layer  101  was not melted back to the surface of the sapphire substrate  100 , whereby the GaN layer  101  remained as a plurality of upright columns ( FIG. 1B ). A conceivable reason for formation of such a plurality of upright columns of the GaN layer  101  is that a distribution in Ga concentration in the molten mixture contained in the crucible  12  provides variation in melting back rate. 
     Then, while the molten mixture was maintained at 850° C., the inside of the reaction vessel  11  was pressurized to 4 MPa by controlling the nitrogen flow rate and discharge rate via the supply pipe  14  and the discharge pipe  15 . Through this pressurization, both the temperature and pressure met the conditions where crystal growth of GaN is permitted. Through maintaining the temperature and pressure for 100 hours, a GaN layer  103  was grown on the GaN layer  101  to a thickness of 0.5 mm ( FIG. 1C ). Notably, the GaN layer  103  was grown without filling spaces  104  between upright columns of the GaN layer  101 . 
     The feature of the growth is attained through the following conceivable mechanism. Specifically, the nitrogen concentration (level) of the molten mixture gradually increases toward the liquid surface (i.e., the interface between the molten mixture and nitrogen). Thus, within the entire surface of a column of the GaN layer  101 , which surface is in contact with the molten mixture, the upper face  101   a  of the column of the GaN layer  101  is a part where the nitrogen concentration is higher than in the other parts. Accordingly, crystal growth of GaN starts in the vicinity of the upper face  101   a  of the GaN layer  101 , and propagates from the start point as a nucleus in the plane direction of the template substrate  102  and in the vertical direction thereto. Once GaN is grown, the nitrogen concentration of the molten mixture is higher at the upper surface of the grown GaN layer than in the other parts. Thus, GaN is continue to grow on the upper face  101   a  of the GaN layer  101  but does not fill intercolumnar spaces  104 . Eventually, GaN grown layers formed from a plurality of upper faces  101   a  are integrated, to thereby form the continuous GaN layer  103  having a c-plane as a main plane. 
     In this embodiment, the temperature of the molten mixture during melting back of the GaN layer  101  was adjusted to be equal to the temperature at which the GaN layer  103  was grown. However, so long as the temperature and pressure in a process of the melt back are set in a range in which GaN can be melted back and the temperature and pressure in a process of the crystal growth are set in a range in which GaN can be grown, the two temperatures of the two processes may be different from each other. However, by employment of the same temperature, the step of melting back the GaN layer  101  can be shifted to the step of growing the GaN layer  103  by controlling only pressure, whereby the production steps can be streamlined. 
     Subsequently, heating and pressurization were stopped, and the temperature and pressure were returned to ambient conditions, whereby crystal growth of the GaN layer  103  was terminated. In the course of lowering the temperature, stress was generated due to the differences in linear expansion coefficient and lattice constant between sapphire and GaN. The generated stress concentrated mainly in the GaN layer  101 , to thereby generate cracks in the GaN layer  101 . As a result, the sapphire substrate  100  was separated from the GaN layer  103  ( FIG. 1D ). Notably, since the stress generated due to the differences in linear expansion coefficient and lattice constant between sapphire and GaN does not concentrate in the GaN layer  103 , cracking of the GaN layer  103  can be prevented. 
     Thereafter, the GaN layer  103  was removed from the substrate  100 , and the parts of the GaN layer  101  remaining on the GaN layer  103  were removed through polishing, thereby yielding a high-quality GaN free-standing substrate free from cracking. 
     Embodiment 2 
     The embodiment 2 is related with a template substrate  102  used in the method of the present invention for producing a Group III nitride semiconductor.  FIGS. 3A and 3B  show the steps of preparing the template substrate  102 . An AlN buffer layer  201  was uniformly formed on a sapphire substrate  200 . Subsequently a GaN layer  202  with a thickness of 2 μm was uniformly formed on the AlN buffer layer  201 . A photoresist was uniformly formed on the GaN layer  202 . Subsequently a periodic pattern mask of the photoresist was formed on the surface of the GaN layer  202  by a photolithography. Next a maskless portion was dry etched until the sapphire substrate  200  was exposed as shown in  FIG. 3A . After removing the mask and cleaning the substrate, a GaN layer  203  was grown again by HVPE or MOVPE as shown in  FIG. 3B . In this way, the template substrate  102  was formed. The template substrate which is formed by such a process can be used as the template substrate  102  of the embodiment 1. 
     The steps of producing a GaN crystal by the Na flux method was performed by the same method as the embodiment 1 as shown in  FIGS. 1A to 1D . The template substrate  102  of the  FIG. 1A  is corresponding to the template substrate  102  of the  FIG. 3B . 
     The GaN layer grown on the sapphire substrate  200  exposed by dry etching had a high defect density, which was easier to be melted back than that grown on the remaining portion of the GaN layer  202  with a thickness of 2 μm. In  FIG. 3B , the portion A of the GaN layer  203  has poor crystalline quality, which is easily etched. The portion B of the GaN layer  203  has superior crystalline quality, which is not easily etched, because the portion B of the GaN layer  203  is grown on the GaN layer  202  having superior crystalline quality formed on the AlN buffer layer  201 . 
     In Embodiments 1 and 2, the thickness of the GaN layer  101  of the template substrate was adjusted to 10 μm. However, the thickness of the GaN layer  101  is preferably 5 to 30 μm. When the thickness is less than 5 μm, the GaN layer  101  might be completely melted back, whereas when the thickness is larger than 30 μm, an excessive period of time is required for melting back to expose the surface of the sapphire substrate  10 . Thus, the thickness of the GaN layer  101  is more preferably 10 to 30 μm. 
     In Embodiments 1 and 2, the GaN layer  103  was grown to a thickness of 0.5 mm. The present invention is particularly advantageous in the case where the GaN layer  103  is grown to a thickness of 0.5 mm or more. Generally, when the thickness is 0.5 mm or more, the stress generated due to the differences in linear expansion coefficient and lattice constant between sapphire and GaN increases, to thereby readily generate cracks in the GaN layer  103 . However, according to the present invention, generation of cracks can be effectively prevented. 
     In Embodiments 1 and 2, the temperature of the molten mixture during melting back of the GaN layer  101  was equalized to the temperature of the molten mixture during crystal growth of the GaN layer  103  by the flux method; the pressure during melting back of the GaN layer  101  was adjusted to be lower than the pressure during the crystal growth of GaN; and then the pressure was elevated to a pressure at which crystal growth of GaN is permitted, whereby the GaN layer  103  was grown. However, the following alternative methods may be employed. 
     In one alternative method, the pressure during melting back of the GaN layer  101  is equalized to the pressure during crystal growth of the GaN layer  103  by the flux method; the temperature of the molten mixture during melting back of the GaN layer  101  is adjusted to be higher than the temperature at which crystal growth of GaN is permitted; and then the temperature is lowered to a temperature at which crystal growth of GaN is permitted, whereby the GaN layer  103  is grown. Through employment of this alternative method, only temperature control is required without controlling pressure, whereby the production steps can be streamlined. 
     In another alternative method, the nitrogen concentration of the molten mixture is adjusted to a non-saturation level which is a level close to but under the saturation level at the start of melting back of the GaN layer  101 ; and then the nitrogen concentration of the molten mixture is adjusted to a super-saturation level by melting back of the GaN layer  101 , whereby the GaN layer  103  is grown. Through employment of this alternative method, no control of temperature or pressure is required, whereby the production steps can be further streamlined. 
     Embodiments 1 and 2 are directed to a method for producing a GaN crystal. However, the present invention may be applicable to production of a Group III nitride semiconductor other than GaN such as AlGaN, InGaN, or AlGaInN. 
     The Group III nitride semiconductor produced according to the present invention may be employed as a substrate for producing Group III nitride semiconductor light-emitting devices or the like.