We measured the lattice constants of bulk aluminum nitride crystals at various temperatures by high resolution X-ray diffraction. By the use of a high temperature chamber and a X-ray cryostat a temperature regime from 20 to 1210 K was available. Furthermore, the measured data were fitted by Einstein- and Debye models which yield reliable parameters for the calculation of the thermal expansion coefficients of AlN.
An important feature of zinc oxide and gallium nitride materials are their similar physical properties. This allows to use them as a p-n junction materials for applications in optoelectronics. In earlier work we presented use of ZnO as a transparent contact to GaN, which may improve external efficiency of LED devices. In this work we discuss properties of a n-ZnO/p-GaN heterostructure and discuss its optimization. The heterostructure is investigated by us for possible applications, e.g. in a new generation of UV LEDs or UV light detectors.
Cathodoluminescence is applied for evaluation of in-depth and in-plane variations of light emission from two types of GaN-based laser diode structures. We evaluate in-depth properties of the laser diode emission and demonstrate that potential fluctuations still affect emission of laser diodes for e-beam currents above thresholds for a stimulated emission.
Scanning and spot-mode cathodoluminescence investigations of homo- and hetero-epitaxial GaN films indicate a surprisingly small influence of their microstructure on overall intensity of a light emission. This we explain by a correlation between structural quality of these films and diffusion length of free carriers and excitons. Diffusion length increases with improving structural quality of the samples, which, in turn, enhances the rate of nonradiative recombination on structural defects, such as dislocations.
Monocrystalline films of zinc oxide were grown at 300C by atomic layer deposition. ZnO layers were grown on various substrates like ZnO bulk crystal, GaN, SiC and Al_2O_3. Electrical properties of the films depend on structural quality. Structural quality, surface morphology and optical properties of ZnO films were characterized using X-ray diffraction, scanning electron microscopy, and photoluminescence, respectively. High resolution X-ray diffraction spectra show that the rocking curve FWHM of the symmetrical 00.2 reflection equals to 0.058° and 0.009° for ZnO deposited on a gallium nitride template and a zinc oxide substrate, respectively. In low temperature photoluminescence sharp excitonic lines in the band-edge region with a FWHM equal to 4 meV, 5 meV and 6 meV, for zinc oxide deposited on gallium nitride, zinc oxide and sapphire substrate, respectively.
Instabilities of light emission and also of stimulated emission in series of GaN epilayers and InGaN quantum well structures, including laser diode structures, are studied. A stimulated emission is observed under electron beam pumping. This enabled us to study light emission properties from laser structures and their relation to microstructure details. We demonstrate large in-plane fluctuations of light emission and that these fluctuations are also present for excitation densities larger than the threshold densities for the stimulated emission.
The luminescence of In_{x}Ga_{1-x}N is studied for thick epitaxial layers and quantum wells. Using spatially resolved cathodoluminescence spectroscopy the commonly observed broad integral photoluminescence spectra were found to result from spectral and lateral inhomogeneous emission across the samples. Moreover, the integral photoluminescence and absorption spectra show different temperature dependences. The effects can be explained assuming fluctuations of the composition associated with a variation of the band gap.
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