Yu-Li Tsai

Observation of compositional pulling phenomenon in AlxGa1-xN (0.4<x<1.0) films grown on (0001) sapphire substrates

High Al content AlxGa1−xN (0.4<x<1.0) films were grown at 1050 °C either on (0001) sapphire substrates or on AlN films by alternative supply of group III metalorganics (trimethylaluminum and trimethylgallium) and NH3 in an inductively heated quartz reactor operated at atmospheric pressure. θ–2θ x-ray diffraction data show a compositional separation in the AlxGa1−xN films grown on low-temperature (LT) aluminum nitride (AlN) buffer layer-coated (0001) sapphire substrates. Absorption spectroscopic measurements also reveal double cutoff edges in the AlxGa1−xN sample grown directly on a LT AlN-coated (0001) sapphire substrate while only one absorption edge having higher energy was observed in the AlxGa1−xN film grown on a 0.1-μm-thick high-temperature (HT) AlN intermediate layer, which was deposited on top of the (0001) sapphire substrate. Cross-sectional transmission electron microscopic observations show the bilayer nature of the AlxGa1−xN films grown directly on the LT AlN-coated sapphire substrate. Such a ...

Effects of Al-containing Intermediate III-nitride Strained Multilayers on the Threading Dislocation Density and Optical Properties of GaN Films

We report the influence of intermediate AlN/GaN, AlGaN/GaN or AlN/AlGaN strained multilayer structures on the threading dislocation (TD) density and optical properties of GaN films. A series of GaN films were deposited at 1050°C on (0001) sapphire substrates using intermediate AlN/GaN, AlGaN/GaN or AlN/AlGaN strained multilayer structures. Etching pit density (EPD) counts and transmission electron microscopic (TEM) studies show that the implementation of AlN/GaN and AlGaN/GaN in GaN films enables the blocking of TDs in the films very efficiently. Photoluminescence (PL) measurements exhibit the increment of near band edge emission intensity in the GaN films having Al0.5Ga0.5N/GaN or AlN/GaN intermediate strained multilayer structures. Cross-sectional TEM observations show that TD annihilation and de-multiplication processes play important roles in the TD density reduction in the GaN films having Al-containing strained multilayer structures.

Optical Characteristics of GaN Films Overgrown on Wet-etched GaN Templates

GaN films were deposited on wet-etched GaN templates having various etch pit sizes. Improved optical characteristics of the overgrown GaN films were observed including a remarkable increment in photoluminescence (PL) intensity and a considerable reduction in emission linewidth of the near bandedge (BE) emission. Improvement of the optical property of an overgrown GaN film is attributed to a selective elimination of threading dislocations (TDs) which results in a reduction of etching pit density (EPD) count of the overgrown GaN film.

Compositional pulling effect in high Al-content AlGaN films grown on (0001) sapphire substrates

In this presentation, we report strain induced compositional pulling phenomenon in high Al content AlGaN films grown on AlN-coated sapphire substrates. It was found that Al constituent in a high Al content AlGaN film was expelled out of the film in the initial stage of the growth followed by the growth of a AlGaN film having higher Al content. The AlGaN films were characterized by X-ray diffractometry (XRD) for composition evaluation. Absorption spectroscopy was employed for the measurements of absorption spectra of the AlGaN films.

Effect of high Al-content AlGaN/GaN short period strained-layer superlattices on the threading dislocations in GaN films

In this paper, we report the role played by AlGaN/GaN short period strained-layer superlattices (SPSLS) on the threading dislocation (TD) density reduction in GaN films. TD annihilation and de-multiplication processes were observed to account for the etching pit densities (EPD) reduction in GaN films involving AlGaN/GaN intermediate SPSLS structures. Transmission electron microscopy (TEM) was employed to characterize the microstructural properties of GaN films.