THERMAL-EXPANSION OF GAN BULK CRYSTALS AND HOMOEPITAXIAL LAYERS

Thermal expansion of gallium nitride was measured using high resolutio n X-ray diffraction. The following samples were examined: (i) single m onocrystals grown at pressure of about 15 kbar, (ii) homoepitaxial lay ers. The main factor influencing both, the lattice parameters and the thermal expansion coefficient, are free electrons related to the nitro gen vacancies. The origin of an increase in the lattice constants by f ree electrons is discussed in terms of the deformation potential of th e conduction-band minimum. An increase of the thermal expansion by fre e electrons is explained by a decrease of elastic constants.