HIGH-PRESSURE RAMAN SCATTERING OF BIAXIALLY STRAINED GaN ON GaAs H. SIEGLE*, A. R. GONI*, C. THOMSEN*, C. ULRICH**, K. SYASSEN**, B. SCHOTTKER***, D. J. AS***, D. SCHIKORA*** *Institut ftir Festk6rperphysik, TU Berlin, Hardenbergstrale 36, 10623 Berlin, Germany **Max-Planck-Institut ffir Festk6rperforschung, Stuttgart, Germany ***Institut f'ir Optoelektronik, GHS Paderborn, Germany ABSTRACT We present results of high-pressure Raman-scattering experiments on bulk GaN and GaN grown on GaAs. We determined the Griineisen parameters of both the cubic TO and LO phonon modes and the hexagonal A,, El and E 2 modes. Our measurements reveal that the Grtineisen parameters for the GaAs substrate are about 30% smaller than those of bulk GaAs. This is a consequence of the lower compressibility of GaN compared to GaAs, which results in a pressure-induced biaxial strain on the substrate. From the pressure behavior of the GaAs modes and by comparing with our results for bulk GaN we obtained information about the biaxial strain in the GaN epitaxial layer. INTRODUCTION A major problem in growing GaN layers on standard substrates as, e.g., sapphire, 6H- SiC, or GaAs is the large lattice-mismatch and the difference in the thermal expansion coefficients between layer and substrate. This causes a large biaxial stress in the layers, which is either compressive for hexagonal GaN on sapphire or tensile for GaN on 6H-SiC [1]. In order to handle thin-film heterostructures based on GaN a knowledge of the internal stress is necessary. Experimental information about the main strain/pressure parameters in particular for cubic GaN is still lacking. We therefore performed Raman-scattering experiments on predominantly cubic GaN layers grown on GaAs and bulk hexagonal GaN crystals at high hydrostatic pressures up to 6 GPa. We determined the Grilneisen parameters of the cubic and hexagonal phonon modes. The application of hydrostatic pressure on the GaN/GaAs sample resulted in a tensile stress of the GaAs substrate while the GaN layer became compressively strained. This is a consequence of the different bulk moduli of GaN and GaAs. From the pressure dependence of the different phonon modes we could estimated the stress distribution. While the GaAs substrate was heavily biaxially strained due to the applied hydrostatic pressure, we found only a weak biaxial stress in the GaN layer. This can partly be explained by stress relaxation at the interface to the substrate. 225 Mat. Res. Soc. Symp. Proc. Vol. 468 @ 1997 Materials Research Society