Self-separated freestanding GaN using a NH4Cl interlayer

Abstract

Thick GaN films were grown on void buffer layer by hydride vapor phase epitaxy. The void-buffer layers were consisted of NH4Cl, GaN dots, and low-temperature (LT) GaN buffer layer. Instead of GaN, NH4Cl was easily synthesized in NH3 and HCl atmospheres by simply lowering the growth temperature to 500°C, and LT GaN buffer growth was followed during increasing substrate temperature to 600°C. The LT GaN buffer acted as a protecting layer against evaporation of the NH4Cl and a seeding layer for the high temperature (HT) GaN. The NH4Cl layer between a sapphire substrate with GaN dots and the LT GaN buffer were fully evaporated during the HT GaN growth at 1040°C. Many voids were formed at interface caused by evaporation of the NH4Cl layer, which strongly assisted self-separation of thick HT GaN during cooldown after the growth resulting in a 200μm thick freestanding (FS) GaN. The FS GaN showed smooth surface morphology and absence of any crack. The a-axis and c-axis lattice constants of FS GaN were 3.189 and 5.185Å, respectively, which well agrees with those of strain-free bulk GaN. The observed donor-bound exciton emission peak at 3.4718eV agreed with the peak position of bulk GaN. All these features indicate that the obtained FS GaN through the self-separation process is nearly strain-free.