Investigation of Structural Phase Transitions on Wurtzite Gallium Nitride Surfaces

Surface structures of wurtzite gallium nitride (w-GaN) have been investigated previously,[1][2] and it is well known that above 300K there exist order-disorder phase transitions. For N-polar w-GaN (000-1) at 300K, a family of surface reconstructions occurs, including 1$\times $1, 3$\times $3, 6$\times $6, and c(6$\times $12). Not much is known, however, about what happens to these structures as they are cooled below 300K. We have recently developed a new epitaxy/analysis system, including a sample stage which can be both heated and cooled. The N-polar w-GaN surfaces are prepared using rf N-plasma-assisted molecular beam epitaxy, and monitored in-situ using reflection high energy electron diffraction (RHEED). The approach is to monitor the [11-20] and [10-10] RHEED diffractions during cryogenic cooling, starting with the 1$\times $1 or 3$\times $3 structures. A critical issue to explore is the interrelationship between surface gallium concentration and structural deformation. This study may provide the missing link to new reconstructions of w-GaN recently observed using LT scanning tunneling microscopy.[3] This work is supported by NSF (Grant No. 0730257). [1] A. R. Smith et al., Phys. Rev. Lett. \textbf{79}, 3934 (1997). [2] A. R. Smith et al., Surface Science \textbf{423}, 70 (1999). [3] D. Acharya, S.-W. Hla et al., unpublished.