The Peculiarities of Strain Relaxation in GaN/AlN Superlattices Grown on Vicinal GaN (0001) Substrate: Comparative XRD and AFM Study

Abstract

Superlattices (SLs) consisting of symmetric layers of GaN and AlN have been investigated. Detailed X-ray diffraction and reflectivity measurements demonstrate that the relaxation of built-up strain in the films generally increases with an increasing number of repetitions; however, an apparent relaxation for subcritical thickness SLs is explained through the accumulation of Nagai tilt at each interface of the SL. Additional atomic force microscopy measurements reveal surface pit densities which appear to correlate with the amount of residual strain in the films along with the appearance of cracks for SLs which have exceeded the critical thickness for plastic relaxation. These results indicate a total SL thickness beyond which growth may be limited for the formation of high-quality coherent crystal structures; however, they may indicate a growth window for the reduction of threading dislocations by controlled relaxation of the epilayers.

Integral Monolayer-Scale Featured Digital-Alloyed AlN/GaN Superlattices Using Hierarchical Growth Units

“…Numerous methods, including the modulation of the AlN layer by migration-enhanced epitaxy, optimization of the Ga/N ratio for the GaN epilayer, and alternate supply of groups III and V sources, have been developed for the preparation of digital-alloyed AlN/GaN structures. , Despite their benefits in improving AlN/GaN heteroepitaxy by coherent lattice match and atomically abrupt interfaces, preparing digital alloying structures through accurately stacking AlN and GaN monolayers down to one atomic layer thin in integral scale without compositional fluctuations remains challenging. In general, by precisely controlling growing units of AlN and GaN monolayer (ML)-scale epilayers below their equilibrium critical thickness, AlN/GaN structures can be coherently grown and the introduced strain in the alloyed substitution can be stabilized without relaxation . However, the difficulties for digital alloying AlN/GaN heteroepitaxial growth exist because of the nonequilibrium nature of the metal organic vapor-phase epitaxy (MOVPE) technique, and the limits of operating temperature lead to inevitable vapor-phase prereaction and surface roughness for single atomic layer growth.…”

Exciton Effects in Low-Barrier GaN/AlGaN Quantum Wells

“…Tilt angle in A has been found to be −0.003° (i.e. tilted to the opposite azimuth), thus showing the better relationship in that specimen between the tilt in the GaN substrate and that in the GaN/AlGaN layers [5].…”

“…However, as depicted in recent reviews [3,4], there are still several obstacles to tackle for this technology, hindering the achievement of high external quantum efficiency in AlGaN-based UV LEDs. Generally speaking, structural and, therefore, optical properties of nitride quantum wells strongly depend on the strain within the multistack -as a result of the mismatch in the (basal) unit cell parameters between different nitrides [5]. As an alternative, in the last years, attention has focused on gallium nitride layers with extremely reduced dimensionality, that is, ultrathin or quasi 2D GaN quantum wells, that can exhibit high quantum confinement, enhanced carrier localisation and blue-shift of their bulk optical emission if inserted in AlGaN matrix.…”

Point-Defect Distribution and Transformation Near the Surfaces of AlGaN Films Grown by MOCVD

“…AlGaN is one of the most promising group III nitride used for fabricating laser diodes, ultraviolet light-emitting diodes, solar-blind ultraviolet detectors, high electron mobility transistors, and gas-sensitive sensors due to their superior properties, such as their direct band gaps, high short wavelength photon emitting and detecting efficiencies, high electron mobilities, and high thermal conductivities. − To date, it has been still a great challenge in crystal engineering to obtain high-quality AlGaN films with low concentration of point defects using the epitaxial method due to the lack of low-cost GaN or AlN single-crystal freestanding substrates . The dominant point defects in AlGaN epitaxial layers arise from intrinsic defects, such as the vacancy types V Ga , V Al , and V N ; substituted types Ga N and N Ga ; and interstitial types Ga i , N i .…”