The stress and wafer bending of (112¯0) a-plane GaN layers of different thicknesses grown on (11¯02) r-plane sapphire substrates by hydride vapor phase epitaxy were studied by high-resolution x-ray diffraction and photoluminescence and photoreflectance spectroscopies. The layers are found to be under compression in the growth plane and under tension in the growth direction. The elastic and thermal anisotropies of the GaN and the sapphire crystal result in an in-plane stress and a wafer curvature, both of which are different in the two in-plane directions parallel and perpendicular to the GaN c axis. The GaN unit cell is no longer hexagonal but orthorhombic. The stress relaxes with increasing GaN layer thickness while the curvature of the wafer increases. Different stress relief mechanisms are considered, and the stresses in the layer and the curvature of the wafer are calculated using standard models on wafer bending. The results suggest that the wafer bending is the dominant stress relief mechanism. In addition, the redshift of the near-band-edge photoluminescence and the free exciton photoreflectance peaks with increasing layer thickness is correlated with the strain data determined by x-ray diffraction.

We use photoreflectance (PR) spectroscopy to study the electronic band structure modification of GaN films grown along different nonpolar orientations due to biaxial, anisotropic in-plane strain. The exciton transition energies of an unstrained, high-quality C-plane GaN film are used to accurately determine the crystal-field and spin-orbit splitting energies. For films with a nonpolar orientation, the resonant features observed in the PR spectra exhibit a strong in-plane polarization anisotropy and different transition energies from the ones measured in the C-plane GaN film. The deformation potential D5 is accurately determined from four GaN films with a nonpolar orientation using the measured energies together with the polarization properties and out-of-plane strain.

GaAsSb bandgap, surface fermi level, and surface state density studied by photoreflectance modulation spectroscopy Appl.The photoreflectance signal from GaN films is sensitive to the ambient medium. A large decrease in the photoreflectance amplitude is observed, when the ambient medium is changed from air to vacuum. This effect is attributed to ultraviolet-light-induced desorption of oxygen from the sample surface leading to a reduction of the surface barrier height. The effect is absent, when a thin Ti layer is deposited on top of the GaN film. A simple model is used to demonstrate that the surface photovoltage can be strongly reduced with a decrease of the surface barrier height.

The amplitude and the signal-to-noise ratio of photoreflectance (PR) spectra are experimentally and theoretically investigated as a function of the pump and probe intensity. The model calculations of the PR amplitude and the signal-to-noise ratio based on a simple transport model taking the shot noise of the photodetector as the only noise source confirm the experimentally observed dependencies. Increasing the probe light intensity leads to a decrease of the absolute PR background noise. At the same time, the PR amplitude decreases. This may, in particular for comparable probe and pump intensities, result in a decrease of the signal-to-noise ratio.

Below-gap structures in photoreflectance spectra of semi-insulating GaAs were observed originating from electroabsorption of light reflected from the back surface. The peak amplitude of these structures was strongly temperature dependent within the range (78-290) K. The different lineshape at low temperature in comparison to that at room temperature suggested different physical mechanisms. This conclusion was proven experimentally using a two phase lock-in technique by which a fast (3 and slsw (Yj component of the spectra could be separated. A fit procedure provides the pure back surface reflection effect for both components. The X mode dominating at low temperatures is attributed to electromodulation of the field-broadened excitor& absorption tail whereas the Y mode originates from the Franz-Keldysh electroabsorption.