Thermal expansion of GaN at low temperatures - a comparison of bulk and homo- and heteroepitaxial layers

The thermal expansion of different GaN samples is studied by high-resolutio n Xray diffraction within the temperature range of 10 to 600 K. GaN bulk cr ystals, a homoepitaxial layer and different heteroepitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were investigated. Below 100 K the thermal expansion coefficients (TE C) were found to be nearly zero which has to be taken into account when est imating the thermal strain of GaN layers in optical experiments commonly pe rformed at low temperatures. The homoepitaxial layer and the underlying GaN substrate with a lattice mismatch of -6.10(4) showed identical thermal exp ansion. The comparison between the temperature behavior of lattice paramete rs of heteroepitaxial layers and bulk GaN points to a superposition of ther mally induced biaxial strain and compressive hydrostatic strain.