Showing papers by "Stacia Keller published in 1998"

“S-shaped” temperature-dependent emission shift and carrier dynamics in InGaN/GaN multiple quantum wells

Abstract: We report temperature-dependent time-integrated and time-resolved photoluminescence (PL) studies of InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition. We observed anomalous emission behavior, specifically an S-shaped (decrease–increase–decrease) temperature dependence of the peak energy (Ep) for InGaN-related PL with increasing temperature: Ep redshifts in the temperature range of 10–70 K, blueshifts for 70–150 K, and redshifts again for 150–300 K with increasing temperature. In addition, when Ep redshifts, the spectral width is observed to narrow, while when Ep blueshifts, it broadens. From a study of the integrated PL intensity as a function of temperature, it is found that thermionic emission of photocarriers out of local potential minima into higher energy states within the wells is the dominant mechanism leading to the thermal quenching of the InGaN-related PL. We demonstrate that the temperature-induced S-shaped PL shift is caused by a change in the carrier dyna...

Effective band gap inhomogeneity and piezoelectric field in InGaN/GaN multiquantum well structures

Abstract: The emission mechanisms of strained InxGa1−xN quantum wells (QWs) were shown to vary depending on the well thickness, L, and x. The absorption edge was modulated by the quantum confined Stark effect and quantum confined Franz-Keldysh effect (QCFK) for the wells, in which, for the first approximation, the product of the piezoelectric field, FPZ, and L exceed the valence band discontinuity, ΔEV. In this case, holes are confined in the triangular potential well formed at one side of the well producing the apparent Stokes-like shift. Under the condition that FPZ×L exceeds the conduction band discontinuity ΔEC, the electron-hole pair is confined at opposite sides of the well. The QCFK further modulated the emission energy for the wells with L greater than the three dimensional free exciton Bohr radius aB. On the other hand, effective in-plane localization of carriers in quantum disk size potential minima, which are produced by nonrandom alloy compositional fluctuation enhanced by the large bowing parameter and...

Structural origin of V-defects and correlation with localized excitonic centers in InGaN/GaN multiple quantum wells

Abstract: In the growth of InGaN/GaN multiple quantum well (MQW) structures, a novel defect (called the “V-defect”) initiates at threading dislocations in one of the first quantum wells in a MQW stack. This defect is common to almost all InGaN MQW heterostructures. The nature of the V-defect was evaluated using transmission electron microscopy (TEM), scanning TEM (STEM), and low-temperature cathodoluminescence (CL) on a series of In0.20Ga0.80N/GaN MQW samples. The structure of the V-defect includes buried side-wall quantum wells (on the {1011} planes) and an open hexagonal inverted pyramid which is defined by the six {1011} planes. Thus, in cross section this defect appears as an open “V”. The formation of the V-defect is kinetically controlled by reduced Ga incorporation on the pyramid walls ({1011} planes). The V-defect is correlated with the localized excitonic recombination centers that give rise to a long-wavelength shoulder in photoluminescence (PL) and CL spectra. This long-wavelength shoulder has the fol...

High-transconductance self-aligned AlGaN/GaN modulation-doped field-effect transistors with regrown ohmic contacts

Abstract: Self-aligned AlGaN/GaN modulation-doped field-effect transistors (MODFETs) with high transconductances have been demonstrated on a sapphire substrate. Source and drain were selectively regrown with ∼1700 A of n-GaN adjacent to the gate electrode. Source resistance was reduced to 0.95 Ω mm from 1.4 to 1.8 Ω mm with conventional GaN-based MODFETs. These self-aligned devices show a record high value of extrinsic transconductance ∼400 mS/mm for AlGaN/GaN MODFETs with a gate length of 1.2 μm.

Spiral Growth of InGaN Nanoscale Islands on GaN.

Abstract: Spiral growth was found to be the dominant growth mechanism for InGaN single quantum wells grown under low trimethylgallium flow rates by metal-organic chemical vapor deposition on GaN-on-sapphire films. This spiral growth around threading dislocations with a screw component led to the formation of flat, 1–2 nm high, InGaN islands of diameters in the range of 100–400 nm. The diameter of the InGaN islands increased with decreasing trimethylgallium flow, i.e., decreasing InGaN growth rate. The formation of spatially separated islands could be enhanced by disilane pre-treatment of the GaN surface prior to InGaN deposition. The InGaN island formation is discussed as a possible contribution to the development of spatial inhomogeneities in InGaN layers.

High quality, semi-insulating gallium nitride and method and system for forming same

Abstract: A method for growing high-quality gallium nitride over a substrate is disclosed. The method comprises growing first layer with a high dislocation density over the substrate, a second layer having a high number of point defects and a reduced dislocation density as compared to the dislocation density of the first layer over the first layer, and a third layer having a reduced number of point defects as compared to the second layer over the second layer. The resulting gallium nitride is semi-insulating, which inhibits parasitic current flow and parasitic capacitive effects, yet it not so insulating that electron flow in adjacent transistor channels is inhibited.

Optical Properties of InGaN/GaN Quantum Wells with Si Doped Barriers

Abstract: Optical properties of InGaN/GaN multi-quantum wells with Si doped barriers are studied using cw and time resolved photoluminescence (TRPL) and photoluminescence excitation (PLE) spectroscopy. The room temperature carrier lifetime depends strongly on the Si doping level in the quantum well barriers, decreasing from 10 ns to 1 ns as the doping level is increased from unintentionally doped to 5×1018 cm-3 (Si:GaN). The shift between the absorption edge and emission peak decreases from 220 meV to 110 meV as the doping is increased. Temperature dependent photoluminescence measurements indicate a higher density of non-radiative centers in the undoped structures.

Gallium nitride based materials and their application for light emitting devices

Abstract: There have recently been breakthroughs in the growth of group-III nitrides for high efficiency blue and green GaN light emitting diodes and laser diodes. GaN LEDs complete the primary color spectrum and have enabled the fabrication of bright and reliable full-color solid state displays.