Structural properties, crystal quality and growth modes of MOCVD-grown AlN with TMAl pretreatment of sapphire substrate

Abstract

The growth of high quality AlN epitaxial films relies on precise control of the initial growth stages. In this work, we examined the influence of the trimethylaluminum (TMAl) pretreatment of sapphire substrates on the structural properties, crystal quality and growth modes of heteroepitaxial AlN films on (0001) sapphire substrates. Without the pretreatment, the AlN films nucleated on the smooth surface but exhibited mixed crystallographic Al-(N-) polarity, resulting in rough AlN film surfaces. With increasing the pretreatment time from 1 to 5 s, the N-polarity started to be impeded. However, small islands were formed on sapphire surface due to the decompostion of TMAl. As a result, small voids became noticeable at the nucleation layer (NL) because the growth started as quasi three-dimensional (3D) but transformed to 2D mode as the film grew thicker and got coalesced, leading to smoother and Al-polar films. On the other hand, longer pretreatment time of 40 s formed large 3D islands on sapphire, and thus initiated a 3D-growth mode of the AlN film, generating Al-polar AlN nanocolumns with different facets, which resulted into rougher film surfaces. The epitaxial growth modes and their correlation with the AlN film crystal quality under different TMAl pretreatments are also discussed.

Recent Progress on Aluminum Gallium Nitride Deep Ultraviolet Lasers by Molecular Beam Epitaxy

“…Moreover, the MBE growth takes place at lower substrate temperatures compared with other growth techniques for the same material. For example, the substrate temperature for growing AlGaN thin films by MBE is generally below 800 C, [88][89][90][91][92] whereas by MOCVD it requires 1000-1200 C. [24,34,[93][94][95] Lastly, the MBE growth chamber is equipped with RHEED, which allows an in situ monitoring of the growth and an instantaneous adjustment on the growth parameters, beneficial to the growth of ultrathin layers, e.g., the monolayer (ML)-thick GaN QWs. [96][97][98] In the next two subsections, we will show that benefited from these merits, high-quality AlGaN alloys including both thin films and nanowires can be grown by MBE, which provides a promising path to the electrically injected AlGaN DUV lasers by MBE.…”

Deep-ultraviolet integrated photonic and optoelectronic devices: A prospect of the hybridization of group III–nitrides, III–oxides, and two-dimensional materials

“…By embedding the regrown AlN layer, dislocations were observed on the surface as 3D growth that assisted in relaxing the resultant compressive strain. Conversely, without the AlN regrowth layer, 3D growth arises because of the roughness of surfaces at grown layer interface, and hence the majority of dislocations were bent [436][437][438] . It was detected that while growing thick Al x Ga 1−x N layers, the growth process followed 2D be- [439] .…”

Time–Energy Quantum Uncertainty: Quantifying the Effectiveness of Surface Defect Passivation Protocols for Low-Dimensional Semiconductors

“…Nonetheless, in this work we stress the fact that quantifying the quality of molecular beam epitaxy (MBE)-grown nanowires incorporating only the terms is insufficient. Indeed, with significant reduction in common impurities and vacancy centers, such as the growth-induced formation of carbon impurities − and nitrogen-vacancy related defects, surface recombination predominantly affects the performance of nanowire-based devices because the quantum efficiency of nanowire-based optoelectronic devices can be limited by the increased density of surface trap states (Shockley–Read–Hall recombination), which also manifests itself by introducing lower-energy defect-induced photoemission because of band bending that distorts the shape of the photoemission spectrum as described earlier. The introduction of such emission centers can be accounted for by eq .…”