Photoluminescence ͑PL͒, Raman spectroscopy, and x-ray diffraction are employed to demonstrate the coexistence of a biaxial and a hydrostatic strain that can be present in GaN thin films. The biaxial strain originates from growth on lattice-mismatched substrates and from post-growth cooling. An additional hydrostatic strain is shown to be introduced by the presence of point defects. A consistent description of the experimental results is derived within the limits of the linear and isotropic elastic theory using a Poisson ratio ϭ0.23Ϯ0.06 and a bulk modulus Bϭ200Ϯ20 GPa. These isotropic elastic constants help to judge the validity of published anisotropic elastic constants that vary greatly. Calibration constants for strain-induced shifts of the near-bandedge PL lines with respect to the E 2 Raman mode are given for strain-free, biaxially strained, and hydrostatically contracted or expanded thin films. They allow us to extract differences between hydrostatic and biaxial stress components if present. In particular, we determine that a biaxial stress of one GPa would shift the near-band-edge PL lines by 27Ϯ2 meV and the E 2 Raman mode by 4.2Ϯ0.3 cm Ϫ1 by use of the listed isotropic elastic constants. It is expected from the analyses that stoichiometric variations in the GaN thin films together with the design of specific buffer layers can be utilized to strain engineer the material to an extent that greatly exceeds the possibilities known from other semiconductor systems because of the largely different covalent radii of the Ga and the N atom. ͓S0163-1829͑96͒03148-7͔

We present structural and electronic data which indicate that the dominant defects in GaAs grown at low temperatures (LT GaAs) by molecular beam epitaxy (MBE) are As antisites (AsGa) and Ga vacancies (VGa), with negligible amounts of As interstitials (Asi). We show that the change of lattice parameter correlates with the concentration of AsGa, and that AsGa alone can account for the lattice expansion. We also show that the total concentration of AsGa has a characteristic second power dependence on the concentration of AsGa in the positive charge state for the material grown at different temperatures. This can be understood provided that VGa defects are the acceptors responsible for the carrier compensation. Our results are consistent with most experimental results and the theoretical expectation from the calculation of defect formation energies. We find that the conclusion may also be true in As-rich bulk GaAs.

Transmission electron microscopy has been applied to characterize the structure of Ti/Al and Ti/Al/Ni/Au Ohmic contacts on n-type GaN (∼1017 cm−3) epitaxial layers. The metals were deposited either by conventional electron-beam or thermal evaporation techniques, and then thermally annealed at 900 °C for 30 s in a N2 atmosphere. Before metal deposition, the GaN surface was treated by reactive ion etching. A thin polycrystalline cubic TiN layer epitaxially matched to the (0001) GaN surface was detected at the interface with the GaN substrate. This layer was studied in detail by electron diffraction and high resolution electron microscopy. The orientation relationship between the cubic TiN and the GaN was found to be: {111}TiN//{00.1}GaN, [110]TiN//[11.0]GaN, [112]TiN//[10.0]GaN. The formation of this cubic TiN layer results in an excess of N vacancies in the GaN close to the interface which is considered to be the reason for the low resistance of the contact.