Search

InGaN/GaN multiple-quantum-well (MQW) blue and green-light-emitting diodes (LEDs) were grown on sapphire substrates using metalorganic vapor phase epitaxy. High-resolution transmission microscopy shows that a much larger density of stacking faults exist in the quantum-well region of the blue LEDs than in the green LEDs. In the green LEDs, the blueshift in the electroluminescence (EL) emission energy at larger driving currents is more prominent than in the blue LEDs, which is explained by different strength of quantum confined Stark effect as a result of different piezoelectric field intensity by different scales of strain relaxation in the blue and green MQWs. The steady broadening of the EL emission energy linewidth on the higher energy side with the increase of the driving current was observed in both blue and green LEDs, which is attributed to the band filling effect. (c) 2005 American Institute of Physics.

The correlation between the resistivity of an undoped GaN/Al2O3 interfacial layer and in-situ reflectance spectrum in metalorganic chemical vapor deposition and the mechanism of this correlation were investigated. The first minimum reflectance during the initial high-temperature GaN growth was found to be a good indicator of the resistivity of the GaN buffer. The background electron concentration and mobility were both higher in the samples with higher indicative reflectance at that point. The resistivity of the GaN buffer layer was predominantly determined by an similar to0.25-mum-thick layer near the GaN/Al2O3 interface. Atomic force microscope (AFM) and high-resolution x-ray diffraction (HRXRD) results showed that the samples with higher indicative reflectance had smaller sized but higher density nuclei before the high-temperature GaN growth and lower screw threading dislocation (TD) density in the initially grown GaN. The difference in the background electron concentration and mobility of the interfacial layer was related to the relatively higher concentration of the 0 and Al diffused from Al2O3, which is also dependent on the size and density of the nuclei. These differences were found not to affect the structural and electrical properties or the surface morphology of AlGaN/GaN high electron-mobility transistors (HEMTs, except for the buffer conduction) when the GaN buffer is thick enough (e.g., similar to2.5 mum).

We have studied the temperature-dependent carrier transfer processes in InGaNGaN multi-quantum-well light-emitting devices using various optical techniques such as photoluminescence, electroluminescence, and photoluminescence excitation spectra. The role of the defects in the GaN barrier neighboring to the InGaN region was demonstrated clearly in capturing carriers only at low temperatures. The physical origin of the defects was most possibly attributed to the stacking faults at the interface according to the high-resolution transmission electron spectroscopy pictures. © 2005 American Institute of Physics.

GaN-based blue light-emitting diodes (LEDs) with InGaN multi-quanturn-wells were grown on patterned sapphire substrates by metalorganic vapor-phase epitaxy (MOVPE). The patterned substrates with 60-nm-deep parallel grooves of different dimensions along < 1 1 2 0> and < 1 0 1 0> orientations were etched by BCl3/Cl-2-based inductively coupled plasma reactive ion etching (ICP-RIE). Enhancement of the electroluminescence (EL) peak at 465 nm from LEDs grown on these grooves has been observed, in comparison with LEDs grown on unpatterned sapphire substrates. A maximum power increase of 25\% was measured for LEDs grown on patterned substrate with 2 mum (ridge) x 4 pm (trench) grooves along the < 1 0 1 0 > orientation at a forward current of 20 mA at room temperature. Stronger light emission in the trench regions of the stripes at very low drive currents has been observed using optical microscopy. Several possible mechanisms are proposed to interpret this groove-induced EL enhancement. (C) 2004 Elsevier B.V. All rights reserved.

Temperature dependent electroluminescence (EL) and photoluminescence (PL) spectra were studied for blue, green and yellow-green InGaN/GaN light emitting devices (LEDs). Selective excitation has unambiguously proven that the origin of the 3.27 eV defect peak was radiative recombination at the defects in the GaN barriers adjacent to the QDs. Temperature dependent photoluminescence excitation (PLE) spectra demonstrated a still clearer evidence that at low temperatures the carrier transfer efficiency to the luminescent InGaN QDs is poor, due to carriers being captured by the defects in the GaN barrier. At high temperatures however those carriers generated inside the GaN barriers contribute more effectively to and finally dominate the PL emission of the InGaN QDs, showing a thermally activated carrier transfer process from the GaN layer to the QDs. High resolution transmission electron microscopy (HRTEM) pictures were used to reveal that the origin of the defects is most probably due to stacking faults in the GaN barrier.

Light emitting spatial inhomogeneity in InGaN/GaN multiple quantum well (MQW) blue and green LEDs were observed directly with a microscope. Electroluminescence (EL) spectra of InGaN/GaN QW blue and green LEDs were studied in the temperature range of 10-300 K at an injection current of 5mA. Some anomalous behavior was observed. Intensity of the EL main peak increased monotonically with temperature from 10 to 200 K and slightly decreased with further temperature increase in the 200 K range. This is in contrast with the monotonic decrease of EL with increasing temperature for conventional AlGaInP QW red LEDs. The anomalous behavior can only be observed on InGaN/GaN systems. The origin of such behaviors was discussed using a model of quantum dot clusters in the InGaN/GaN psudo-quantum wells, with a small potential barrier at the boundary of the quantum dots.

The Schottky barrier characteristics of Ni on p-GaN have been investigated using current-voltage-temperature (I-V-T) and capacitance-voltage characteristics (C-V) measurements. Barrier height values ranging from 2.68 to 2.87 eV were obtained from C-V measurements. The temperature dependence of I-V characteristics clearly indicated the dominance of tunneling current in the transport mechanism of the diodes, therefore, barrier height determination using I-V measurements can lead to erroneous results, as indicated by the wide range of barrier heights reported in the literature. Acceptor concentration, deduced from C-V measurements, was found to be of 10(19)/cm(3) within 200 A of the sample surface, and tapered off to be similar to 10(18)/cm(3). These values are 10-100 times higher than the hole concentration of similar to 10(17)/cm(3) obtained from Hall measurements. (C) 2001 American Institute of Physics.