X. C. Wang

Effects of rapid thermal annealing on structure and luminescence of self-assembled InAs/GaAs quantum dots

Postgrowth rapid thermal annealing was used to modify the structural and optical properties of the self-assembled InAs quantum dots grown on GaAs substrates by molecular beam epitaxy. It is found that significant narrowing of the luminescence linewidth (from 78.9 to 20.5 meV) from the InAs dot layer occurs together with about 260 meV blueshift at annealing temperatures up to 850u2009°C. Observation of high-resolution transmission electron microscopy shows the existence of the dots under lower annealing temperatures but disappearance of the dots annealed at 850u2009°C. The excited-state-filling experiments for the samples show that the luminescence of the samples annealed at 850u2009°C exhibits quantum well-like behavior. Comparing with the reference quantum well, we demonstrate significant enhancement of the interdiffusion in the dot layer.

Characteristics of InGaAs quantum dot infrared photodetectors

A quantum dot infrared photodetector (QDIP) consisting of self-assembled InGaAs quantum dots has been demonstrated. Responsivity of 3.25 mA/W at 9.2 μm was obtained for nonpolarized incident light on the detector with a 45° angle facet at 60 K. The QDIPs exhibit some unique electro-optic characteristics such as a strong negative differential photoconductance effect and blueshift of the response peak wavelength.

Observation of optically-active metastable defects in undoped GaN epilayers

Optically-active defects of undoped GaN epilayers grown on sapphire by metalorganic chemical vapor epitaxy was investigated with photoluminescence. A new metastable defect emitting blue light was found, besides the well-known yellow luminescence centers. With excitation by the 325 nm He–Cd laser, this metastable defect, at low temperature, exhibits the luminescence fatigue effect with the decay time determined to be about 6 min. When the temperature is increased to room temperature, it recovers its optically-active state. The yellow band emission increases in intensity as the blue band emission decreases in intensity. Analysis shows that this metastable center is a hole trap, and Ga vacancy is its most probable candidate.

Widely tunable intersubband energy spacing of self-assembled InAs/GaAs quantum dots due to interface intermixing

In this article, we showed the significant reduction of the energy spacing between ground state and excited state emissions from InAs/GaAs quantum dots ~QDs! due to interface interdiffusion induced by thermal treatment. In addition, the strong narrowing of the luminescence linewidth of the ground state and excited state emissions from the InAs dot layers for the annealed samples indicates an improvement of the size distribution of the QDs. Large blueshift of the energy positions of both emissions was also observed. High resolution x-ray diffraction experiments give strong evidence of the interface atom interdiffusion in the annealed samples. This work shows the ability to tune the wavelength for applications like infrared detectors and lasers based on intrasubband transitions of self-assembled QDs.

Polarization dependence of intraband absorption in self-organized quantum dots

Photoluminescence and intraband absorption were investigated in n-doped self-organized InAs and In0.35Ga0.65As quantum dots grown on a GaAs substrate. Intraband absorption of the dots is strongly polarized along the growth axis in the mid infrared spectral range. The absorption is maximum at around 120 meV for InAs dots and at 130 meV for In0.35Ga0.65As dots. The experimental results on InAs dots are in agreement with published theoretical calculations.

X-ray diffraction and optical characterization of interdiffusion in self-assembled InAs/GaAs quantum-dot superlattices

We report on the characterization of thermally induced interdiffusion in InAs/GaAs quantum-dot superlattices with high-resolution x-ray diffraction and photoluminescence techniques. The dynamical theory is employed to simulate the measured x-ray diffraction rocking curves of the InAs/GaAs quantum-dot superlattices annealed at different temperatures. Excellent agreement between the experimental curves and the simulations is achieved when the composition, thickness, and stress variations caused by interdiffusion are taken in account. It is found that the significant In-Ga intermixing occurs even in the as-grown InAs/GaAs quantum dots. The diffusion coefficients at different temperatures are estimated

SILICA CAPPING FOR AL0.3GA0.7AS/GAAS AND IN0.2GA0.8AS/GAAS QUANTUM WELL INTERMIXING

Spin-on silica capping has been demonstrated to be an effective dielectric encapsulant layer for quantum well (QW) intermixing at temperatures significantly lower than for conventionally deposited silica. A blueshift of up to 125 meV was observed in the photoluminescence (PL) peak energy of both GaAs and InGaAs QWs after annealing for less than 60 s at 850u2009°C, without noticeable degradation in the PL emission intensity. A threshold temperature was identified below which no significant QW disordering took place. The activation energy for Al diffusion in Al0.3Ga0.7As/GaAs QWs was about 2.55 eV. Broadly similar effects were seen for In0.2Ga0.8As/GaAs QWs but, in addition, strain effects appear to enhance disordering during the early stages of the anneal.

Laser induced copper electroless plating on polyimide with Q-switch Nd:YAG laser

Abstract The results of laser induced deposition of copper on polyimide substrate from copper electrolyte solution are reported. Unlike most work reported in the literatures where CW Ar+ lasers were used, a second harmonic (532xa0nm wavelength) Q-switch Nd:YAG laser was used for our experiments. The deposition process was conducted by laser-catalyzing of the polyimide surface and subsequent photothermal-accelerated reduction of copper-complex ions in an alkaline reducing environment. The characteristics of the deposited copper line were investigated in terms of laser beam scanning speed, and the number of scans. The surface morphology and chemical composition of the deposited copper were analyzed using field emission scanning electron microscope (FESEM) and energy dispersive spectrometer (EDX). The optimum processing conditions have been identified. The copper deposit was found to adhere well to the substrate.

Process study for laser-induced surface coloration

A KrF excimer laser beam is used to induce controllable and high-contrast colors on a stainless-steel surface under different processing conditions. The laser beam acts as a localized heating source that promotes the formation of semitransparent oxide films of varying thicknesses and morphologies on the sample surface. Ellipsometry and atomic force microscopy were employed to understand the laser-induced colors by studying the surface morphology and optical properties of the oxide films and their thickness. It is found that the laser-induced colors vary with the oxide layer thickness. The laser-treated areas have granular structures, which affect oxide growth and light scattering.

STRONG INFLUENCE OF SIO2 THIN FILM ON PROPERTIES OF GAN EPILAYERS

In this letter, we report strong degradation of photoluminescence (PL) performance of GaN epilayers due to SiO2 layers that were deposited on GaN surfaces by electron-beam evaporation. Secondary ion mass spectrometry measurements show that the oxygen concentration of GaN with SiO2 layers is one order of magnitude more than that of as-grown GaN. This fact indicates that oxygen can very easily replace nitrogen in GaN. It was also found that rapid thermal processing can recover and improve the optical quality of GaN with SiO2 layer. As a reference, SixNy was found to have little effect on PL performance of GaN.