Reduction of surface roughness by modification of step-bunched aluminum nitride layers towards step-flow morphology

Abstract

Conventional aluminum nitride (AlN) template fabrication techniques, like hydride vapour phase epitaxy or AlN growth on patterned sapphire substrates, usually lead to step-bunched template surfaces. The resulting macrosteps cause emission broadening or even multiple-peak characteristics of ultraviolet light-emitting diodes (UV LEDs) fabricated on such AlN templates. In order to reduce these macrosteps and to provide a smoother surface, even without the need of a thick AlN deposition, a two-layer growth procedure is reported here. A three-dimensional (3D) – twodimensional (2D) sequential growth initiates a significant modification of the previous AlN surface morphology and simultaneously limits the evolving tensile strain. A primarily step-bunched surface with a surface roughness root mean square of 1.8 nm is successfully reduced by the two-layer growth procedure down to 0.8 nm, without any film cracking. This distinct roughness reduction of more than 50% is achieved within an AlN thickness of only 1.3 µm. With a smoother surface, the electroluminescence characteristic of a UV LED structure is substantially improved. Instead of a double-peak emission, typical for LEDs grown on step-bunched templates, a single-peak emission and lower spectral width were achieved, indicating the high potential of the suggested two-layer technique for improving performance.

Influence of pressure on AlN thick films prepared by epitaxial lateral overgrowth through hydride vapor phase epitaxy

“…4(a), a large-step bunching morphology is observed with the grain stretching along [11−20] AlN and flowing in [10−10] AlN . 43,44 Due to the diffusion of threading dislocations, the stretching of AlN grains is interrupted and numerous unmerged holes appear, resulting in a large surface roughness of 5.73 nm. 45 The threading dislocation density is approximately 1.7 × 10 8 cm −2 based on the density of unmerged holes, which is consistent with the EPD in Fig.…”