S. Zamir

Thermal microcrack distribution control in GaN layers on Si substrates by lateral confined epitaxy

GaN epitaxial layers grown uniformly on Si substrates suffer from randomly distributed thermal cracks. The growth on prepatterned Si substrates is demonstrated as an efficient way to control the geometrical distribution of the thermal cracks. In order to study this effect and to find the maximum crack-free lateral dimension of a GaN patterned unit on Si, a simple procedure termed lateral confined epitaxy (LCE) was developed. This procedure confines the growth of GaN to separate mesas of Si, which are defined on the Si substrate prior to the growth. The growth is performed by a single, continuous metalorganic chemical vapor deposition run. LCE enables the variation of mesa lateral size, while keeping the growth rate nearly unchanged. By performing a set of LCE growth runs of ∼0.7 μm GaN, on Si mesas of varying lateral dimensions, we specified the maximum crack-free range of GaN on Si as 14.0±0.3 μm, for that GaN thickness. A reduction of random crack density is observed with decreasing GaN pattern size and...

The effect of AlN buffer layer on GaN grown on (111)-oriented Si substrates by MOCVD

GaN films were grown by metal organic chemical vapor deposition on (1 1 1)Si substrates, using AlN as a buffer layer. The influence of the AlN buffer layer growth temperature and growth duration on the morphology and preferred orientation of GaN films was studied. Drastic enhancement of epitaxial registration was observed with increasing buffer growth temperature. A sharp transition in the growth mode occurred at 760°C. For that temperature, an optimal buffer layer growth duration was found. The use of March parameter as a figure of merit in X-ray diffraction testing of textured GaN films is proposed.

ELECTRICAL ISOLATION OF GAN BY ION IMPLANTATION DAMAGE : EXPERIMENT AND MODEL

Electrical and optical isolation of unintentionally doped GaN layers due to the damage created by H+ and He+ ions passing through the layer are demonstrated. As a result of the irradiation, the sample resistance increases by 11 orders of magnitude and the band-to-band photoluminescence (PL) emission is totally quenched. Following annealing (1000 °C, 30 s), the conductivity can be nearly completely recovered, whereas only partial recovery of the PL emission is obtained. A model is proposed which invokes the presence of potential barriers for electronic transport across extended defects as the major factor limiting carrier mobility. Radiation defects increase these barriers, thus affecting the sample resistivity. This model fits the experimental results for both H and He induced damage extremely well and thus proves that defects created by nuclear collisions of the ions traversing the layer are responsible for the observed effects.

Lateral confined epitaxy of GaN layers on Si substrates

We developed a novel, simple procedure for achieving lateral confined epitaxy (LCE). This procedure enables the growth of uncracked GaN layers on a Si substrate, using a single, continuous metalorganic chemical vapor deposition (MOCVD) run. The epitaxial growth of GaN is confined to mesas, defined by etching into the Si substrate prior to the growth. The LCE-GaN layers exhibit improved morphological and optical properties compared to the plain GaN-on-Si layers grown in the same MOCVD system. By performing a set of LCE growth runs on mesas of varying lateral dimensions, we specified the crack-free range of GaN on Si as 14.0 ± 0.3 μm.

Enhanced photoluminescence from GaN grown by lateral confined epitaxy

Lateral confined epitaxy (LCE) of GaN on Si substrates results in a reduction of thermal crack density with decreasing the lateral dimensions of the growth pattern. Below a critical size, crack-free GaN on Si is obtained. The intensity of band-to-band photoluminescence (PL) peak in LCE GaN is strongly enhanced with respect to uniformly grown GaN on Si. The present study rules out the effect of crack density, internal reflections (microcavity effects), as well as enhanced light extraction efficiency, and excitation or emission through preferred facets (shape effects) as the main factor in PL enhancement. It is shown that the reduction in threading dislocation density (TDD) along the edges of the LCE patterns improves the luminescence efficiency. The relative increase in high quality material (low TDD) with the reduction of LCE unit size is, thus, the main reason for the enhanced PL intensity.

Multiparameter Statistical Design of Experiments for GaN Growth Optimization

A statistical multi-parameter Design of Experiments for growth optimization of GaN is presented. According to the obtained statistical model, increasing the buffer layer V/III ratio is beneficial for minimizing the FWHM of the X-ray diffraction rocking curve for the (002) reflection. Statistical models were obtained also for background electron concentration and Hall mobility, but further electrical measurements lead us to the conclusion that those models are disturbed by the presence of a highly conductive layer near the low temperature buffer layer. Inclusion of an AlxGa1—xN isolation layer shows a reduction by two orders of magnitude in the measured background concentration, as well as a significant increase in Hall mobility, without degradation of the crystalline quality.

Simulation of x-ray diffraction profiles in imperfect multilayers by direct wave summation

A novel simulation program in the MATLAB format for x-ray diffraction profiles in multilayers was developed, which can be applied to any multilayered structure with no limitations. The simulation algorithm (nickname DIWAS) is based on direct summation of waves, scattered by individual atomic planes. It takes into account strain and concentration-induced fluctuations of interplanar spacings, interface roughness and buried amorphous layers, and enables the addition of diffuse scattering profiles.The program allows handling of asymmetric reflections as well as symmetric ones, taking special care of the effective layer thickness. The summation over individual layers can be done coherently or incoherently, depending on the interface structure. To make the fitting procedure comprehensible, the contribution of every layer can be plotted separately.In this paper, the direct wave summation (DIWAS) routine is described in detail and is applied to fit experimental diffraction profiles taken from MOVPE grown heterostructures and superlattices of practical importance, such as InGaN/GaN/AlGaN/sapphire, GaAsN/GaAs, InGaAsP/InP and InGaAs/InP.

Coupled waveguides in GaN-based lasers

GaN-based lasers include, apart from the separate confinement waveguide, a parasitic GaN buffer waveguide. The effect of coupling between the active laser waveguide and the buffer waveguide on the lasing modes is studied in terms of a coupled-mode formalism and a round-trip analysis. Assuming a low-reflectivity internal mirror, Fabry–Perot modes with an intensity envelope that resembles experimental measurements is obtained. The gradual change in the modes, taking part in the round-trip model, is presented as a function of wavelength and temperature. The influence of temperature changes on the peak wavelength of the output spectrum predicted by this analysis is in good agreement with experimental reports.