Method of growing group iii nitride crystals using high pressure vessel

Abstract

Present invention discloses a high-pressure vessel of large size formed with a limited size of e.g. Ni—Cr based precipitation hardenable superalloy. Vessel may have multiple zones. For instance, the high-pressure vessel may be divided into at least three regions with flow-restricting devices and the crystallization region is set higher temperature than other regions. This structure helps to reliably seal both ends of the high-pressure vessel, at the same time, may help to greatly reduce unfavorable precipitation of group III nitride at the bottom of the vessel. Invention also discloses novel procedures to grow crystals with improved purity, transparency and structural quality. Alkali metal-containing mineralizers are charged with minimum exposure to oxygen and moisture until the high-pressure vessel is filled with ammonia. Several methods to reduce oxygen contamination during the process steps are presented. Back etching of seed crystals and a new temperature ramping scheme to improve structural quality are disclosed.

What is claimed is: 
     
         1 . A method for growing group III nitride crystals in an ammonothermal method and in a manner that exposes an alkali-metal mineralizer to minimal oxygen and moisture comprising:
 a. placing at least one group III-nitride seed crystal in a crystallization region of a high-pressure vessel;   b. placing a group III-containing nutrient in the high-pressure vessel;   c. placing an airtight container into the high-pressure vessel, wherein the airtight container comprises mineralizer and a metal covering which protects the mineralizer from water and oxygen;   d. reducing pressure to less than 1×10 −5  mbar after placing the airtight container into the high-pressure vessel;   e. filling the high-pressure vessel with ammonia;   f. releasing the mineralizer into the ammonia surrounding the metal covering;   g. maintaining (i) a temperature of the crystallization region above 500° C. and (ii) a pressure sufficiently high that the ammonia is supercritical for a sufficient time to grow a group III-nitride crystal on said seed crystal.   
     
     
         2 . The method of  claim 1  wherein the airtight container was formed by pouring molten mineralizer into the metal covering and solidifying the mineralizer within the covering. 
     
     
         3 . The method of  claim 2  wherein the metal covering is sealed with a metal foil seal. 
     
     
         4 . The method of  claim 3  wherein the airtight container consists essentially of the mineralizer, the metal foil seal, and the metal covering. 
     
     
         5 . The method of  claim 4  wherein the metal covering and the metal foil seal are each formed of a metal that is stable in supercritical ammonia. 
     
     
         6 . The method of  claim 5  wherein the mineralizer is sodium and the group III nitride is GaN. 
     
     
         7 . The method of  claim 1  wherein the act of releasing the mineralizer comprises rupturing a metal foil seal on said covering so that the mineralizer exits the covering and dissolves in the ammonia. 
     
     
         8 . The method of  claim 7  wherein metal foil seal is ruptured using pressure. 
     
     
         9 . The method of  claim 8  wherein the ammonia and the mineralizer form a supercritical ammonobasic solution. 
     
     
         10 . The method of  claim 9  wherein the mineralizer is sodium and the group III nitride is GaN. 
     
     
         11 . The method of  claim 1  further comprising heating the high-pressure vessel during step (d) of  claim 1 . 
     
     
         12 . The method of  claim 1  further comprising adding, in addition to the mineralizer, an additive containing at least one of Ce, Ca, Mg, Al, Mn, Fe, B, In, Zn, Sn, and Bi. 
     
     
         13 . The method of  claim 1  wherein said group III-nitride seed crystal was back-etched before starting crystal growth in step (g) of  claim 1 . 
     
     
         14 . The method of  claim 13  wherein said group III nitride seed crystal is back etched by maintaining the temperature of the nutrient region higher than 400° C. and the temperature in the crystallization region is maintained at least 50° C. lower than the temperature of the nutrient region before starting crystal growth in step (g) of  claim 1 . 
     
     
         15 . The method of  claim 14  wherein said group III nitride seed crystal is back etched by more than 1 micron. 
     
     
         16 . The method of  claim 13  wherein said group III nitride seed crystal is back etched by maintaining the temperature of crystallization region lower than 400° C. and the temperature in the crystallization region is maintained at least 50° C. higher than the temperature of the nutrient region before starting crystal growth in step (g) of  claim 1 . 
     
     
         17 . The method of  claim 16  wherein said group III nitride seed crystal is back etched by more than 1 micron. 
     
     
         18 . The method of  claim 1  further comprising
 a. back etching the seed crystal by maintaining the temperature of the nutrient region higher than 400° C. and the temperature in the crystallization region is maintained at least 50° C. lower than the temperature of the nutrient region; and 
 b. ramping the temperature of the crystallization region above 500° C. prior to the step of maintaining the temperature of the crystallization region above 500° C. to grow the group III-nitride crystal on the seed crystal. 
 
     
     
         19 . The method of  claim 18  wherein the seed crystal is back etched by more than 1 micron. 
     
     
         20 . The method of  claim 1  further comprising
 a. back etching the seed crystal by maintaining the temperature of the crystallization region lower than 400° C. and the temperature in the crystallization region is maintained at least 50° C. higher than the temperature of the nutrient region; and 
 b. ramping the temperature of the crystallization region above 500° C. prior to the step of maintaining the temperature of the crystallization region above 500° C. to grow the group III-nitride crystal on the seed crystal. 
 
     
     
         21 . The method of  claim 20  wherein the seed crystal is back etched by more than 1 micron before starting crystal growth in step (g) of  claim 1 .