M. Stevens

Polychromatic light emission from single InGaN quantum wells grown on pyramidal GaN facets

We have studied the properties of InGaN∕GaN quantum wells grown on epitaxially laterally overgrown GaN stripes. The stripes have a triangular cross section due to {112¯2} crystalline facets. We have observed that the integrated light emission from such structures is uniformly polychromatic over a wide range of the visible spectrum. Using cathodoluminescence techniques, we find that the local emission wavelength increases steadily along the facets, in the direction away from the substrate. The gradient in the emission wavelength is related to the dependence of the quantum well width on the relative position along the facet. The continuous variation of the quantum well properties causes a uniform, polychromatic luminescence band. For some conditions, such distribution can resemble solar-white light emission. This approach can be used to produce an integrated white light source for monolithically integrated white light-emitting diodes.

Effect of layer thickness on the electrostatic potential in InGaN quantum wells

High-resolution electron holography in the transmission electron microscope has the capability to profile the spatial variation of the electrostatic potential in semiconductors, at subnanometer resolution. We have used electron holography to measure the internal electrostatic potential and fields across quantum wells with well thickness ranging from 2to10nm, at a nominal indium concentration of x=0.13. A comparison of field strength versus well width shows a precipitous drop in field strength beyond 6nm. A close look at the microstructure of the widest well shows additional contrast at the growth interface. An explanation consistent with both observations supports the possibility of compositional fluctuations in combination with strain relaxation at the early stages of growth of the quantum well.

Mapping the electrostatic potential across AlGaN/AlN/GaN heterostructures using electron holography

Electron holography has been used to study the properties of a nominally undoped AlGaN∕AlN∕GaN heterostructures. Important characteristics such as the electrostatic potential and the two-dimensional electron gas (2DEG) distribution have been determined with high spatial resolution across the thin film structure. The origin of 2DEG electrons is directly probed by analyzing the charge distribution in the AlGaN layer. It is shown that the contribution of residual donors is trivial and the ionized donorlike surface states are the major source of the 2DEG electrons.

Growth of AlN and GaN on 6H–SiC(0001) using a helium supersonic beam seeded with ammonia

We have grown AlN and GaN layers on 4° off-axis 6H–SiC (0001) substrates using He supersonic beams seeded with NH3. The AlN films were used as buffer layers for GaN growth at 800°C. We estimate 39% incorporation of the NH3 molecules impinging on the substrate surface during GaN film growth. High structural quality of the epitaxial GaN layers was confirmed by transmission electron microscopy and electron channeling patterns. The GaN films, which had a thickness of ∼105 nm, contained a defect density of ∼2×1010 cm−2.

Carrier dynamics and electrostatic potential variation in InGaN quantum wells grown on {112¯2} GaN pyramidal planes

InGaN quantum wells grown in the conventional [0001] direction are known to exhibit long carrier lifetimes that are attributed to strong internal electric fields due to spontaneous and piezoelectric polarization. These fields are expected to be considerably reduced in other crystal geometries. In this work, the authors have investigated the carrier dynamics of InGaN single quantum wells grown parallel to {112¯2} planes. The carrier lifetimes, measured by time-resolved cathodoluminescence, are considerably smaller than for quantum wells grown in the conventional c-plane geometry. In addition, the lifetime does not change significantly with varying indium composition or increasing well width. Electron holography measurements of the electrostatic potential across these quantum wells confirm that the internal fields in this geometry are negligible. These results are of interest for the development of higher-efficiency light-emitting diodes using alternative substrate orientations.

Electrostatic Fields and Compositional Fluctuations in InGaN Quantum Wells

The metastable nature of wurtzite InGaN alloys is exhibited in the numerous reports linking compositional fluctuations to electron microscopy and luminescence measurements. The electronic effects of these fluctuations can be simplified by a spatial variation of the bulk band gap. We have examined both the electrostatic potential and relative atomic density of In0.13Ga0.87N 10nm‐wide quantum wells containing In‐compositional fluctuations and electrostatic potential fluctuations, using electron holography (EH) and electron tomography (ET). EH has shown the potential fluctuations are localized near to the substrate, are 2–3nm in size, and have negatively charged cores. Our ET 3D density maps of the In distribution reveal the existence of quantum dot like regions (QDs) of high In% also localized to within 3nm of the substrate, 3–5nm in width, and spaced ∼8nm apart.

Localization versus carrier‐screening effects in InGaN quantum wells — A time‐resolved cathodoluminescence study

Time‐resolved cathodoluminescence (CL) has been used to separate the effects of internal fields and localization at indium fluctuations in InGaN quantum wells. Spatially‐time‐resolved CL measurements were recorded at regions of high and low indium content. Faster decay is observed in the local transients recorded at indium rich regions.