Shingo Sakakibara

Strong luminescence from dislocation-free GaN nanopillars

GaN nanostructures were prepared on Si(111) by a hot-wall epitaxy technique employing the modified two-step growth method. Isolated hexagonal pillar-like GaN nanostructures (GaN nanopillars) with the typical diameter, height, and density of 200–300nm, 0.5–1μm, and 3–4×108cm−2, respectively, are self-organized without intentional pre-processing to the Si substrate. The photoluminescence and cathodoluminescence (CL) measurements show the strong near-band-edge emissions without the yellow band at room temperature. Stronger CL is obtained from the GaN nanopillars in comparison to single-crystalline GaN. The obtained strong CL is related to high crystal quality of the dislocation-free GaN nanopillars.

Growth of Pillarlike GaN Nanostructures

We have characterized the growth of GaN nanostructures on Si (111). Hexagonal faceted pillarlike GaN nanostructures (GaN nanopillars) were grown by hot wall epitaxy. The GaN nanopillars were self-assembled. Scanning electron microscopy and atomic force microscopy were used to characterize the samples. The typical diameter of the GaN nanopillars was 200 nm. It was found that GaN nanopillars are grown only on a low-temperature GaN island buffer layer. By changing the annealing temperature of the buffer layer, the density of the GaN nanopillars was controlled from 0.4×108 to 3.5×108 cm-2.

Growth and Characterization of Hot-Wall Epitaxial InGaN Films Using Mixed (Ga+In) Source

A simple mixed-source (metallic gallium and indium) method has been used successfully in a hot-wall epitaxial system to grow high-quality InGaN films on sapphire with GaN buffer layers. The InGaN films exhibit sharp and strong near-band-edge-emissions, ranging from violet (384 nm) to blue (448 nm), with photoluminescence (PL) peak widths ranging from 12–33 nm at room temperature. The influence of the mixed-source In/Ga ratio on the In incorporation and crystal quality is investigated, and it is found that a relatively low In/Ga ratio, 1/4–1/5 in the source or 2–3 in the vapor is desirable for improving the crystal quality and enhancing In incorporation. The PL spectra are obtained from 10 K to 300 K and the activation energy, deduced from the thermal quenching of the PL intensity, is about 30.5 meV, suggesting a rather small compositional fluctuation. PL peaks also exhibit anomalous redshifts (10–70 K) and blue shifts (80–140 K).

SrS:Ce Thin Films on Different Insulating under Layers

SrS:Ce thin films were prepared on several kinds of insulating underlayers, Ta2O5, SiO2, Si3N4 and their composite films, by hot wall deposition. The crystallinity of the SrS:Ce films was found to depend strongly upon the underlayers, although no crystal structures were confirmed for the underlayers. The SrS:Ce films grown directly on Ta2O5 layers gave the best crystallinity, and better luminance levels were also obtained for their electroluminescent devices, due to both the large dielectric constant of Ta2O5 and good crystallinity of the SrS:Ce films.

HIGH-QUALITY INGAN FILMS GROWN BY HOT-WALL EPITAXY WITH MIXED (GA+IN) SOURCE

A simple mixed source (gallium and indium metals) method was used in a hot-wall epitaxial system to grow InGaN films on sapphire with thick (~1.5 µm) and thin (~3 nm) GaN buffer layers. Indium incorporation was controlled independently by the substrate temperature, the N2 partial pressure and the mixed source temperature. High-quality InGaN films were obtained, showing strong near-band-edge emission peaks ranging from 370 to 465 nm and narrow X-ray rocking curve full-width at half maximum for InGaN (0002) of 7.03 arcmin. Non-resonant Raman shift of InGaN layers was clearly observed for the first time.

Preparation of GaN film of low carrier density by the HWE system with Ga and TMG sources

GaN films with low carrier density of the order of 10 16 cm -3 , smooth and mirror like surface morphology, and good crystalline quality were prepared on a (0001)sapphire substrate by the hot wall epitaxy system using Ga metal and TMG gas source, respectively. To investigate the dependence of the quality on the thickness of the GaN buffer layer, several ramping rates were performed between the growth of the buffer and the epi-layer. A simple model is proposed to explain the results.

Secondary ion mass spectrometry analysis of Mg doped GaN films prepared by hot wall epitaxy

Abstract Mg doped GaN films were prepared by hot wall epitaxy and Mg and oxygen concentrations were measured by secondary ion mass spectrometry (SIMS). Two kinds of doping methods were performed: one was modulation doping of Mg and another was continuous doping. The modulation doping was performed by flip-flop method in which growth was interrupted on the hot wall for impurity after every 15 nm thick growth of GaN and the continuous growth was performed by introducing Mg impurity into the GaN hot wall using Mg 3 N 2 source. SIMS analysis of modulation doped GaN films showed clear periodic Mg distribution even though the growth temperature is as high as 1000°C. Background oxygen concentration of modulation doped GaN films were of order of 10 18 cm −3 and that of continuous growth was one order of magnitude lower than that of modulation doping. The oxygen concentration increased with the increase of Mg impurity concentration both for modulation doping and continuous doping. The oxygen concentration for continuous growth was restricted to one order of magnitude lower than that of the Mg impurity concentration.