Photoluminescence degradation in GaN induced by light enhanced surface oxidation

The exponential degradation of the photoluminescence (PL) intensity at the near-band-gap was observed in heavily doped or low-quality GaN with pristine surface under continuous helium-cadmium laser excitation. In doped GaN samples, the degradation speed increased with doping concentration. The oxidation of the surface with laser irradiation was confirmed by x-ray photoemission spectroscopy measurements. The oxidation process introduced many oxygen impurities and made an increase of the surface energy band bending implied by the shift of Ga 3d binding energy. The reason for PL degradation may lie in that these defect states act as nonradiative centers and/or the increase of the surface barrier height reduces the probability of radiative recombination.

Calculation of energy levels in InGaAs/GaAs quantum dot array

The subbands of the ground state E-c1, the first excited state E-c2 and heavy hole state E-HH1 are calculated by solving the eigenvalues of effective-mass Hamiltonian H-0 which is derived from eight-band k . p theory and the calculations are performed at k(x) = k, = k = 0 for the three-dimensional array of InGaAs/GaAs quantum dots (QDs). With indium content in InGaAs QDs gradually increasing from 30% to 100%,the intersubband transition wavelength of E-c2 to E-c1, blue-shifts from 18.50 to 11.87 mu m,while the transition wavelength of E-c1, to E-HH1, red-shifts from 1. 04 to 1. 73 mu m. With the sizes of Ir-0.5 Ga-0.5 As and InAs QDs increasing from 1.0 to 5.0 nm, the intersubband transition from E-c1, to E-C2 transforms from bound-state-to-continuum-state to bound-state-to-bound-state, and the corresponding intersubband transition wavelengths red-shift from 8.12 pm (5.90 pm) to 53.47 mu m (31.87 pm), respectively, and the transition wavelengths of E-C1 to E-HH1 red-shift from 1. 13 mu m (1.60 mu m) to 1.27 mu m (2.01 mu m), respectively.

Simulation research of nonlinear behavior induced by the charge-carrier effect in resonant-cavity-enhanced photodetectors

In this paper, we have calculated and discussed in detail the nonlinear effect induced by three carrier effects: free-carrier absorption, bandgap filling, and bandgap shrinkage. The central wavelength of response of resonant-cavity-enhanced (RCE) photodetectors shifts according to the change of the refractive index, and the response of a given optical wavelength simultaneously changes.With an increasing As composition of ln(1-x)Ga(x)As(y)P(1-y) and the spacer thickness, the nonlinear effect increases, but the -1-dB input saturation optical power and the -1-dB saturation photocurrent decrease. Bistable-state operation occurs when the input optical power is in the proper bistable region.

Characterization of free-standing GaN substrate grown through hydride vapor phase epitaxy with a TiN interlayer

Large-scale GaN free-standing substrate was obtained by hydride vapor phase epitaxy directly on sapphire with porous network interlayer. The bottom surface N-face and top surface Ga-face showed great difference in anti-etching and optical properties. The variation of optical and structure characteristics were also microscopically identified using spatially resolved cathodoluminescence and micro-Raman spectroscopy in cross-section of the GaN substrate. Three different regions were separated according to luminescent intensity along the film growth orientation. Some tapered inversion domains with high free carrier concentration of 5 x 10(19) cm(-3) protruded up to the surface forming the hexagonal pits. The dark region of upper layer showed good crystalline quality with narrow donor bound exciton peak and low free carrier concentration. Unlike the exponential dependence of the strain distribution, the free-standing GaN substrate revealed a gradual increase of the strain mainly within the near N-polar side region with a thickness of about 50 mu m, then almost kept constant to the top surface. (c) 2007 Elsevier B.V. All rights reserved.

Weak anti-localization in InAlAs/InGaAs/InAlAs high mobility two-dimensional electron gas systems

Magneto-transport measurements have been carried out on three heavily Si delta-doped In-0.52 Al-0.48 As/In-0.53 Ga-0.47 As/In-0.52 A(10.48) As single quantum well samples in which two subbands were occupied by electrons. The weak anti-localization (WAL) has been found in such high electron mobility systems. The strong Rashba spin-orbit (SO) coupling is due to the high structure inversion asymmetry (SIA) of the quantum wells. Since the WAL theory model is so complicated in fitting our experimental results, we obtained the Rashba SO coupling constant alpha and the zero-field spin splitting Delta(0) by an approximate approach. The results are consistent with that obtained by the Shubnikov-de Haas (SdH) oscillation analysis. The WAL effect in high electron mobility system suggests that finding a useful approach for deducing alpha and Delta(0) is important in designing future spintronics devices that utilize the Rashba SO coupling.

Enhanced performance of p-GaN by Mg delta doping

The electrical and structural properties of Mg delta-doped GaN epilayers grown by MOCVD were investigated. Compared to uniform Mg-doping GaN layers, it has been shown that the delta-doping (delta-doping) process could suppress the dislocation density and enhance the p-type performance. The influence of pre-purge step on the structural properties of GaN was also investigated. The hole concentration of p-GaN decreases when using a pre-purge step. These results can be explained convincingly using a simple model of impurity incorporation under Ga-free growth condition. (C) 2007 Elsevier B.V. All rights reserved.

Al compositional inhomogeneity of AlGaN epilayer with a high Al composition grown by metal-organic chemical vapour deposition

The A1 compositional distribution of A1GaN is investigated by cathodoluminescence (CL). Monochromatic CL images and CL spectra reveal a lateral A1 compositional inhomogeneity, which corresponds to surface hexagonal patterns. Cross-sectional CL images show a relatively uniform Al compositional distribution in the growth direction, indicating columnar growth mode of A1GaN films. In addition, a thin A1GaN layer with lower Al composition is grown on top of the buffer A1N layer near the bottom of the A1GaN epilayer because of the larger lateral mobility of Ga adatoms on the growth surface and their accumulation at the grain boundaries.

Effects of rapid thermal annealing on the emission properties of highly uniform self-assembled InAs/GaAs quantum dots emitting at 1.3 mu m

The authors report the effects of rapid thermal annealing (RTA) on the emission properties of highly uniform self-assembled InAs quantum dots (QDs) emitting at 1.3 mu m grown on GaAs substrate by metal organic chemical vapor deposition. Postgrowth RTA experiments were performed under N-2 flow at temperatures ranging from 600 to 900 degrees C for 30 s using GaAs proximity capping. Surprisingly, in spite of the capping, large blueshifts in the emission peak (up to about 380 meV at 850 degrees C) were observed (even at low annealing temperatures) along with enhanced integrated photoluminescence (PL) intensities. Moreover, pronounced peak broadenings occurred at low annealing temperatures (< 700 degrees C), indicating that RTA does not always cause peak narrowing, as is typically observed with traditional QDs with large inhomogeneous PL linewidths. The mechanism behind the large peak blueshift was studied and found to be attributed to the as-grown QDs with large size, which cause a larger dot-barrier interface and greater strain in and near the QD regions, thereby greatly promoting Ga-In intermixing across the interface during RTA. The results reported here demonstrate that it is possible to significantly shift the emission peak of the QDs by RTA without any additional procedures, even at lower annealing temperatures. (c) 2007 American Institute of Physics.

Anomalous photoluminescence of InAs quantum dots implanted by Mn ions

The photoluminescence (PL) of Mn-implanted quantum dot (QD) samples after rapid annealing is studied. It is found that the blue shift of the PL peak of the QDs, introduced by the rapid annealing, decreases abnormally as the implantation dose increases. This anomaly is probably related to the migration of Mn atoms to the InAs QDs during annealing, which leads to strain relaxation when Mn atoms enter InAs QDs or to the suppression of the inter-diffusion of In and Ga atoms when Mn atoms surround QDs. Both effects will suppress the blue shift of the QD PL peaks. The temperature dependence of the PL intensity of the heavily implanted QDs confirms the existence of defect traps around the QDs. (c) 2006 Elsevier B.V. All rights reserved.

Surface plasmon modulated nano-aperture vertical-cavity surface-emitting laser

We have fabricated surface plasmon modulated nano-aperture vertical-cavity surface-emitting lasers (VCSELs) from common 850 nm VCSELs using focus ion beam etching with Ga+ ion source. The far-field output power is about 0.3 mW at a driving current of 15 mA with a sub-wavelength aperture surrounded by concentric periodic grooves. The enhancement of transmission intensity can be explained by diffraction and enhanced fields associated with surface plasmon. This structure also exhibits beaming properties.

Influence of defects in n(-)-GaN layer on the responsivity of Schottky barrier ultraviolet photodetectors

The influence of defects on the responsivity of GaN Schottky barrier ultraviolet photodetectors with n(-)-GaN/n(+)-GaN layer structures is investigated. It is found that employing undoped GaN instead of Si-doped GaN as the n(-)-GaN layer brings about a higher responsivity due to a lower Ga vacancy concentration. On the other hand, the dislocations may increase the recombination of electron-hole pairs and enhance the surface recombination in the photodetectors. Employing undoped GaN and reducing the dislocation density in the n(-)-GaN layer are necessary to improve the responsivity of Schottky barrier photodetectors. (c) 2007 American Institute of Physics.

Identification of vacancies in electron irradiated GaSb by coincidence Doppler broadening spectroscopy

In this paper we present the results of coincidence Doppler broadening (CDB) measurements and positron lifetime spectroscopy (PLS) on the semiconductor material GaSb. Gallium vacancy with positron lifetime of about 283 ps (V-Ga, (283 ps)) was identified in as-grown sample by CDB technique and PAS technique. For electron irradiated samples with dosages of 10(17) cm(-2) and 10(18) cm(-2), the PAS showed almost the same defectrelated positron lifetime of about 285 ps. CDB experiments indicated that defects in irradiated samples were related to Ga vacancies. (c) 2006 Published by Elsevier B.V.

Effect of Al incorporation on the AlGaN growth by metalorganic chemical vapor deposition

The effect of Al incorporation on the AlGaN growth by metalorganic chemical vapor deposition is investigated. With the increase of trimethylalluminum (TMAl) flux, the crystal quality becomes worse, and the epilayer surface becomes rougher. An interesting phenomenon is that the growth rate of AlGaN decrease with increasing TMAl flux, which is opposite to the AlN growth rate dependence on the TMAl flux. All these effects are attributed to the different properties of At atoms due to the higher bond strength of Al-N compared with Ga-N, which lead to lower surface mobility and stronger competitive ability of Al atoms during the growth. The enhancement of the surface mobility of Al is especially important for improving the quality of AlGaN. (c) 2006 Elsevier B.V. All rights reserved.

Self-organized hexagonal ordering of quantum dot arrays

We report a structure of (In, Ga)As/GaAs quantum dots which are vertically correlated and laterally aligned in a hexagonal way thus forming three-dimensionally ordered arrays. The growth pathway is based on a mechanism of self-assembly by strain-mediated multilayer vertical stacking on a planar GaAs(100) substrate, rather than molecular-beam epitaxy on a prepatterned substrate. The strain energy of lateral island-island interaction is minimum for the arrangement of hexagonal ordering. However, realization of hexagonal ordering not only depends on a complicated trade-off between lateral and vertical island-island interaction but is also related to a delicate and narrow growth kinetics window.

Effect of oxidation on the optical and surface properties of AlGaN

The chemical properties of AlxGa1-xN surfaces exposed to air for different time periods are investigated by atomic force microscopy (AFM), photoluminescence (PL) measurement and X-ray photoelectron spectroscopy (XPS). PL and AFM results show that AlxGa1-xN samples exhibit different surface characteristics for different air-exposure times and Al contents. The XPS spectra of the Al 2p and Ga 2p core levels indicate that the peaks shifted slightly, from an Al-N to an Al-O bond and from a Ga-N to a Ga-O bond. All of these results show that the epilayer surface contains a large amount of Ga and Al oxides. (c) 2006 Elsevier B.V. All rights reserved.