Vol. 87 (1995) ACTA PHY SICA POLONICA A No. 2 Proceedings of the XXIII International School of Semiconducting Compounds, Jaszowiec 1994 EPITAXIAL LAYERS VERSUS BULK SINGLE CRYSTALS OF GaN. TEMPERATURE STUDIES OF LATTICE PARAMETERS AND ENERGY GAP* H. TEISSEYRE, P. PERLIN, M. LESZCZYŃSKI, T. SUSKI, L. DMOWSKI, I. GRZEGORY, S. POROWSKI, J. JUN High Pressure Research Center "Unipress", Polish Academy of Sciences Sokołowska 29/37, 01-142 Warszawa, Poland AND T.D. MOUSTAKAS Boston University, Molecular Beam Epitaxy Laboratory Department of Electrical, Computer and System Engineering Boston MA 02215, USA Gallium nitride epitaxial layer grown by molecular beam epitaxy and bulk crystal grown at high pressure were examined by using X-ray diffrac- tion methods, and by optical absorption at a wide temperature range. The free electron concentration was 6 x 10 17 cm-3 for the layer and about 5 x 10 19 cm -3 for the bulk crystal. The experiments revealed a different po- sition of the absorption edge and its temperature dependence for these two kinds of samples. The structural examinations proved a significantly higher crystallographic quality of the bulk sample. However, the lattice constants of the samples were nearly the same. This indicated that a rather different electron concentration was responsible for the different optical properties via Burstein—Moss effect. PACS numbers: 65.70.+y, 78.50.Ge, 78.66.Fd 1. Introduction The wide (3.5 eV) direct energy gap of GaN makes thiS compound a very promising material for optoelectronics (blue lasers, detectorS, light emitting diodes) and for high power and high temperature devices. The possible applications stimu- late the research on the basic properties of this semiconductor. Most of the experi- mental works have been performed on epitaxial layers grown on foreign substrates as sapphire, gallium arsenide or silicon carbide. A high lattice mismatch between *This work is supported by grants no. PB0288/P4/94/06 and PB066/P4/93/05 of the State Committee for Scientific Research (Republic of Poland). (403)