MOCVD Growth and Characterization of High Quality Semi-polar (11-22) AlGaN Obtained with Overgrowth Technique

Epitaxial growth of semi-polar (11-22) AlGaN layers with different Al composition has been performed by using the metal organic chemical vapour deposition (MOCVD) technique. A two-step overgrowth technique has been developed to both improve the crystal quality and address the cracking issue simultaneously for the AlGaN growth. Comprehensive studies on the structural and optical properties of the AlGaN overgrowth layers have been performed.
Semi-polar (11-22) AlGaN layers with various Al compositions have been obtained with two different approaches. The first one is the standard AlGaN growth on the planar (10-10) m-plane sapphire substrates with either AlN or GaN buffer layer. Semi-polar AlGaN multiple quantum wells (MQWs) with various QW thickness grown on this standard template show no clear blueshift in optical emission wavelength with increasing excitation power, indicating the absence of quantum-confined Stark effect (QCSE). However, this method results in inadequate crystal quality and serious wafer cracking in the epilayers.
To address both issues, another approach - a two-step overgrowth technique - has been developed. With such overgrowth technique, we have achieved thick (> 2 μm) and crack-free semi-polar (11-22) AlGaN layers grown on the top of nearly but not yet fully-coalesced GaN overgrown on micro-rod arrayed templates. These overgrowth layers with the Al composition up to 55.3% exhibit the best crystal quality ever achieved compared to other reports.
A comprehensive investigation on the reduction of defects including the basal-plane stacking faults (BSFs) and strain relaxation has been carried out on the AlGaN overgrowth layers. Detailed X-ray diffraction measurements in on-axis and off-axis planes for different AlGaN structures indicate significantly reduced dislocation density in the AlGaN overgrowth samples. The BSF density is also found to be greatly reduced with the overgrowth technique by performing both the (11-22) reciprocal space mapping (RSM) and the low temperature (LT) photoluminescence (PL) measurements.
The in-plane and out-of-plane strain in the overgrown AlGaN layers have been calculated by adopting a triclinic unit cell model, exhibiting anisotropic strain along two primary in-plane directions and even compressive instead of tensile in-plane strain. The great strain relaxation has been attributed to the residual voids formed within the underlying GaN during the overgrowth process. It has been found that large lattice tilts between the AlGaN and the GaN are present, which also confirms dramatic strain relaxation along the in-plane direction.
Furthermore, the optical properties of the AlGaN samples with Al composition ranging from 32.0% to 55.3% have been investigated systematically. Both the near-band-edge (NBE) and BSFs-related emission are studied by LT PL and room temperature cathodoluminescence (CL) measurements. The energy separation between the two emission peaks increases with increasing Al composition, which is mainly ascribed to the compositional discrepancy within BSF regions and enhanced QCSE also within the BSF regions.
Temperature-dependent PL measurements have also been performed to study the exciton localization effect in both emission regions, showing the localization depth of the BSFs-related emission is larger than that of the NBE in each sample. More importantly, a detailed comparison study has been performed on these semi-polar AlGaN samples and their c-plane AlGaN counterparts with similar Al composition. All the semi-polar AlGaN samples show a clear reduction in exciton localization depth and PL linewidth compared with their c-plane counterparts. These results presented demonstrate that semi-polar (11-22) AlGaN may be more favourable to be employed towards deep UV laser diodes than c-plane AlGaN.