Wei-Tsai Liao

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.

Characteristics and interactions of threading dislocations in GaN films grown on (0001) sapphire substrates with or without short-period superlattice insertion

In this study, transmission electron microscopy was employed to investigate the characteristics of threading dislocations (TDs) in GaN films grown on the (0001) sapphire substrates with or without the insertion of Al0.3Ga0.7N (2 nm)/GaN (2 nm) short period superlattices (SPSLs). By using gb = 0 invisibility criterion, it was found that most of the TDs were type a TDs in a GaN film either containing SPSL or having no SPSL insertion. Type a + c TDs were found to nucleate through the interactions between type a and type c TDs in GaN near the GaN/sapphire interface. Some of the type a TDs were observed to bend along GaN basal plane because of the influence of biaxial strain near the GaN/SPSL interface.

Comparison of the performance of InGaN/AlGaN MQW LEDs grown on c-plane and a-plane sapphire substrates

We report a comparative study on the performance of InGaN/AlGaN multiple quantum well (MQW) light emitting diodes (LEDs) fabricated on c- and a-plane sapphire substrates, respectively. It was found that the LEDs grown on a-plane sapphire substrates exhibited enhanced electroluminescence intensity, decreased double crystal X-ray diffraction linewidth, reduced etching pit density, and smaller ideality factor compared to those deposited on c-plane sapphire substrates. The improved LED characteristics are attributed to threading dislocation density decrement inside the LEDs due to the reduced mismatch between LED structure and a-plane sapphire substrate.

Post-annealing Induced Formation of ZnO Nanowires on the ZnO Films in the N2O Ambient

ZnO films were grown on (0001) sapphire substrates by atomic layer deposition (ALD) using diethylzinc (DeZn) and nitrous oxide (N 2 O) in an inductively heated reactor operated at atmospheric pressure. Low-temperature (LT) ZnO buffer layers having various thicknesses were deposited at 400¢J followed by subsequent growth of ZnO films at 600¢J. Some of the ZnO films were then post-annealed at 1000¢J in the N 2 O flow. Under certain growth conditions, ZnO nanowires were formed on the post-annealed ZnO samples. Room temperature (RT) photoluminescence (PL) spectra of the ZnO nanowires show strong ultraviolet (UV) near band edge emissions at 3.27 eV with a typical full width at half-maximum ( FWHM ) of ~130 meV and quenched defect luminescence at 2.8 eV. 10 K PL spectra of the post-annealed ZnO all exhibit sharp excitonic emissions with the dominant emission being located at 3.36 eV having a FWHM of 4.6 meV.

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.