Low energy electron beam induced vacancy activation in GaN

Experimental evidence on low energy electron beam induced point defect activation in GaN grown by metal-organic vapor phase epitaxy(MOVPE) is presented. The GaN samples are irradiated with a 5–20 keV electron beam of a scanning electron microscope and investigated by photoluminescence and positron annihilation spectroscopy measurements. The degradation of the band-to-band luminescence of the irradiatedGaN films is associated with the activation of point defects. The activated defects were identified as in-grown Ga-vacancies. We propose that MOVPE-GaN contains a significant concentration of passive VGa-H n complexes that can be activated by H removal during low energy electron irradiation.Peer reviewe

Electrical measurement of internal quantum efficiency and extraction efficiency of III-N light-emitting diodes

We propose a direct electrical measurement method for determining the extraction efficiency (EXE) and internal quantum efficiency(IQE) of III-Nitride light-emitting diodes(LEDs). The method is based on measuring the optical output power as a function of injection current at current densities near the external quantum efficiency (EQE) maximum and extracting IQE and EXE from the measurement data. In contrast to conventional methods, our method requires no low temperaturemeasurements or prior knowledge of the device structure. The method is far more convenient than commonly used methods because it enables measuring the EXE and IQE of different LED structures at room temperature directly in a repeatable and consistent way. This enables convenient comparison of LED structures. We apply the method to determine the IQE and EXE of one commercial LED and selected self-grown planar LED chips to compare the effects of different LED structure designs. Our results are in line with published experimental results and also give more insight to our earlier findings regarding the effects of growth parameters on the quantum efficiency. In addition, our measurement method allows estimating the Shockley-Read-Hall and radiative recombination parameters if the Auger parameter is known.Peer reviewe

Identification of the VAl-ON defect complex in AlN single crystals

In this Rapid Communication, we report positron annihilation results on in-grown and proton irradiation-induced vacancies and their decoration in aluminium nitride (AlN) single crystals. By combining positron lifetime and coincidence Doppler measurements with ab initio calculations, we identify in-grown VAl−ON complexes in the concentration range 10 exp 18 cm exp −3 as the dominant form of VAl in the AlN single crystals, while isolated VAl were introduced by irradiation. Further, we identify the UV absorption feature at around 360 nm that involves VAl.Peer reviewe

Enhanced light extraction from InGaN/GaN quantum wells with silver gratings

We demonstrate that an extraction enhancement by a factor of 2.8 can be obtained for a GaN quantum well structure using metallic nanostructures, compared to a flat semiconductor. The InGaN/GaN quantum well is inserted into a dielectric waveguide, naturally formed in the structure, and a silver grating is deposited on the surface and covered with a polymer film. The polymer layer greatly improves the extraction compared to a single metallic grating. The comparison of the experiments with simulations gives strong indications on the key role of weakly guided modes in the polymer layer diffracted by the grating.Peer reviewe

Impurity breakdown and terahertz luminescence in n-GaN epilayers under external electric field

We report on the observation and experimental studies of impurity breakdown and terahertz luminescence in n-GaN epilayers under external electric field. The terahertz electroluminescence is observed in a wide range of doping levels (at noncompensated donor density from 4.5×10[sup 16] to 3.4×10[sup 18] cm[sup −3]). Spectra of terahertz luminescence and photoconductivity are studied by means of Fourier transform spectrometry. Distinctive features of the spectra can be assigned to intracenter electron transitions between excited and ground states of silicon and oxygen donors and to hot electron transitions to the donor states.Peer reviewe

Surface Morphology Evolution Mechanisms of InGaN/GaN Multiple Quantum Wells with Mixture N2/H2-Grown GaN Barrier

This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.Surface morphology evolution mechanisms of InGaN/GaN multiple quantum wells (MQWs) during GaN barrier growth with different hydrogen (H2) percentages have been systematically studied. Ga surface-diffusion rate, stress relaxation, and H2 etching effect are found to be the main affecting factors of the surface evolution. As the percentage of H2 increases from 0 to 6.25%, Ga surface-diffusion rate and the etch effect are gradually enhanced, which is beneficial to obtaining a smooth surface with low pits density. As the H2 proportion further increases, stress relaxation and H2 over- etching effect begin to be the dominant factors, which degrade surface quality. Furthermore, the effects of surface evolution on the interface and optical properties of InGaN/GaN MQWs are also profoundly discussed. The comprehensive study on the surface evolution mechanisms herein provides both technical and theoretical support for the fabrication of high-quality InGaN/GaN heterostructures.Peer reviewe

Mechanism of disyllabic tonal reduction in Taiwan Mandarin

This study was designed to test the hypothesis that time pressure is a direct cause of tonal reduction in Taiwan Mandarin. Tonal reduction refers to the phenomenon of the tones of a disyllabic unit being contracted into a monosyllabic unit. An experiment was carried out in which six native Taiwan Mandarin male speakers produced sentences containing disyllabic compound words /ma/+/ma/ with varying tonal combinations at different speech rates. Analyses indicated that increasing time pressure led to severe tonal reductions. Articulatory effort, measured by the slope of F0 peak velocity of unidirectional movement over F0 movement amplitude, is insufficient to compensate for duration-dependent undershoot (in particular, when time pressure exceeds certain thresholds). Mechanisms of tonal reduction were further examined by comparing F0 velocity profiles against the Edge-in model, a rule-based phonological model. Results showed that the residual tonal variants in contracted syllables are gradient rather than categorical—as duration is shortened, the movement towards the desired targets is gradually curtailed

Stress distribution of GaN layer grown on micro-pillar patterned GaN templates

High-resolution Raman mapping of the stress distribution in an etched GaN micro-pillar template and a 5 μm thick GaN layer grown on a micro-pillar patterned GaN template is investigated. Raman mapping of the E2 (high) phonon shows differences in stress between the coalescing boundary, the top surface of the pillar region and around the GaN micro-pillar. Increased compressive stress is observed at the coalescing boundary of two adjacent GaN micro-pillars, when compared to the laterally grown GaN regions. The electron channeling contrast image reveals the reduction of threading dislocation density in the GaN layer grown on the micro-pillar patterned GaN template

Infrared absorption of hydrogen-related defects in ammonothermal GaN

Polarization controlled Fourier transform infrared (FTIR) absorption measurements were performed on a high quality m-plane ammonothermal GaN crystal grown using basic chemistry. The polarization dependence of characteristic absorption peaks of hydrogen-related defects at 3000-3500 cm(-1) was used to identify and determine the bond orientation of hydrogenated defect complexes in the GaN lattice. Majority of hydrogen was found to be bonded in gallium vacancy complexes decorated with one to three hydrogen atoms (V-Ga-H-1,H-2,H-3) but also hydrogenated oxygen defect complexes, hydrogen in bond-center sites, and lattice direction independent absorption were observed. Absorption peak intensity was used to determine a total hydrogenated V-Ga density of approximately 4 x 10(18) cm(-3), with main contribution from V-Ga-H-1,H-2. Also, a significant concentration of electrically passive V-Ga-H-3 was detected. The high density of hydrogenated defects is expected to have a strong effect on the structural, optical, and electrical properties of ammonothermal GaN crystals. Published by AIP Publishing.Peer reviewe