High-pressure vessel for growing group iii nitride crystals and method of growing group iii nitride crystals using high-pressure vessel and group iii nitride crystal

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 a mineralizer having a covering protecting the mineralizer from oxygen into the high-pressure vessel;   d. filling the high-pressure vessel with ammonia;   e. releasing the mineralizer from its covering; and   f. maintaining a temperature of the crystallization region above 500° C. for a sufficient time to grow a group III-nitride crystal on said seed crystal.   
     
     
         2 . The method of  claim 1 , wherein the mineralizer comprises an alkali metal mineralizer and the covering comprises a metal. 
     
     
         3 . The method of  claim 2  wherein said metal of the covering comprises Ni. 
     
     
         4 . The method of  claim 2  wherein the alkali metal mineralizer comprises Na. 
     
     
         5 . The method of  claim 1  wherein the mineralizer is contained in a metal container into which molten mineralizer was poured and solidified. 
     
     
         6 . The method of  claim 5  wherein a top surface of the mineralizer is covered with a foil. 
     
     
         7 . The method of  claim 1  wherein a top surface of the mineralizer is covered with a metal which is stable in supercritical ammonia. 
     
     
         8 . The method of  claim 1  wherein the covering releases the mineralizer while the high-pressure vessel is self-pressurized by heating. 
     
     
         9 . The method of  claim 1  wherein the covering protecting the mineralizer encapsulates the mineralizer and is oxygen- and water-impermeable. 
     
     
         10 . The method of  claim 9  wherein the act of releasing the encapsulant comprises rupturing the encapsulant using pressure. 
     
     
         11 . The method of  claim 1  wherein the high-pressure vessel is filled with reducing gas and successively pumped to a pressure less than 1×10 −5  mbar at least once prior to maintaining the temperature of the crystallization region above 500° C. to grow the group III-nitride crystal on said seed crystal. 
     
     
         12 . The method of  claim 11  wherein the high-pressure vessel is heated during the step described in  claim 11 . 
     
     
         13 . The method of  claim 11  wherein the reducing gas contains ammonia. 
     
     
         14 . The method of  claim 11  wherein the reducing gas contains hydrogen. 
     
     
         15 . The method of  claim 1  wherein an additive containing at least one of Ce, Ca, Mg, Al, Mn, and Fe is added in addition to the mineralizer. 
     
     
         16 . The method of  claim 1  wherein an additive containing at least one of B, In, Zn, Sri, and Bi is added in addition to the mineralizer. 
     
     
         17 . The method of  claim 1  wherein said group III-nitride seed crystal is back etched before starting crystal growth in step (f). 
     
     
         18 . The method of  claim 17  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 (f) of  claim 1 . 
     
     
         19 . The method of  claim 17  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 (f) of  claim 1 . 
     
     
         20 . The method of  claim 17  wherein the group III nitride seed crystal is thermally back etched at temperature higher than 800° C. in an atmosphere containing etching gas selected from hydrogen fluoride, fluorine, hydrogen chloride, chlorine, hydrogen bromide, bromine, hydrogen iodide, iodine, ammonia, and hydrogen before step (a) of  claim 1 . 
     
     
         21 . The method of  claim 17  wherein said group III nitride seed crystal is back etched by more than 1 micron. 
     
     
         22 . The method of  claim 1  further comprising maintaining the temperature of the crystallization region lower than the temperature of the nutrient region before starting crystal growth in step (f) of  claim 1 . 
     
     
         23 . The method of  claim 22  wherein the duration of the step of maintaining the temperature of the crystallization region lower than the temperature of the nutrient region is more than 1 hour. 
     
     
         24 . The method of  claim 1  wherein ammonobasic solution is used and the temperature of the crystallization region is set at or tower than the temperature of the nutrient region during the step (f) of  claim 1 . 
     
     
         25 . The method of  claim 24  wherein the temperature difference between the crystallization region and the nutrient region is less than 30° C. 
     
     
         26 . 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. tower 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. 
 
     
     
         27 . The method of  claim 26  wherein the seed crystal is back etched by more than 1 micron. 
     
     
         28 . 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. 
 
     
     
         29 . The method of  claim 28  wherein the seed crystal is back etched by more than 1 micron in the step (f). 
     
     
         30 . The method of  claim 1  wherein the group III-nitride crystal comprises GaN.