Kp O'Donnell

Direct evidence of spontaneous quantum dot formation in a thick InGaN epilayer

We report a direct observation of quantum dots formed spontaneously in a thick InGaN epilayer by high resolution transmission electron microscopy. Investigation of a (280 nm thick) In0.22Ga0.78N single layer, emitting in the blue/green spectral region, reveals quantum dots with estimated sizes in the range of 1.5–3 nm. Such sizes are in very good agreement with calculations based on the luminescence spectra of this specimen.

The dependence of the optical energies on InGaN composition.

A wide-ranging experimental approach reveals a linear relationship between photoluminescence band peak energy and measured indium fraction for In[x]Ga[1] N epilayers with 0 < x < 0.40. We examine the dependence of the emission spectrum on composition using local measurements of the average indium content by Rutherford backscattering spectrometry, energy dispersive X-ray analysis, extended X-ray absorption fine structure and wavelength dispersed electron probe micro-analysis. Corresponding absorption and photoluminescence excitation data reveal the existence of a supplementary linear relationship between the optical bandgap and the indium fraction. Our observations provide definitive and conclusive evidence that the optical properties of InGaN do not conform to current theoretical descriptions of alloy band structure.

Intrinsic Infrared Luminescence from InGaN Epilayers

The extension of nitride technology to longer wavelengths is a considerable technical challenge that poses fundamental questions about the origin of luminescence from semiconductor solid solutions. Among the commercial suppliers of nitride semiconductor devices, Nichia Company alone has fabricated amber InGaN LEDs which peak at 594 nm, while some of the present authors have reported 650 nm, red, photoluminescence (PL) from an epilayer of ‘high’ indium content. Here we show further that indium gallium nitride is capable of emitting not only red but also infrared radiation in a band-to-band process. By using a Stokes shift model we predict a limiting peak, for intrinsic InGaN emission, near a wavelength of one micron, which is 0.7 eV lower in energy than the band gap of pure indium nitride. In confirmation of this prediction, we have measured intrinsic luminescence with peak wavelengths up to 950 nm in a selected InGaN epilayer.

The optical linewidth of InGaN light emitting diodes

A comparative study of the optical linewidths of high-quality InGaN epilayers and commercial single quantum well light emitting diode structures was undertaken using photo- and electroluminescence. Optical linewidths show a linear increase with increasing indium concentration in both cases. We assess the contribution of three mechanisms to the luminescence linewidth: alloy fluctuations, well width fluctuations and strain effects. It is found that the broadening of the emission line is an intrinsic property of InGaN alloys. The piezoelectric effect in wurtzite semiconductors is proposed as a novel line-broadening mechanism.

Cathodoluminescence from an InGaN/GaN MQW Grown on an Epitaxially Laterally Overgrown GaN Epilayer

In this paper we report room temperature CL studies of a 5 period InGaN (3 nm)/GaN (7 nm) multiple quantum well (MQW) with a nominal indium concentration of 15%, grown on and off a 4.8 μm epitaxially lateral overgrown GaN (ELOG) epilayer. The ELOG epilayer was overgrown through a SiO2 striped mask with 3 μm window and SiO2 stripe widths oriented in the GaN 〈11-00〉 direction by metalorganic vapour phase epitaxy (MOVPE). We show that the luminescence efficiency of both the GaN and the MQW is improved for material lying above the stripes of the SiO2 mask compared to that above the windows of the mask or unpatterned material. Monochromatic CL images were acquired from both the GaN and MQW layers with the emission from the MQW layer being much brighter than that from the GaN. Marked differences are observed between corresponding CL images acquired at different electron beam energies.

Comparative study of structural properties and photoluminescence in InGaN layers with a high In content

Rutherford backscattering and channeling spectrometry (RBS), photoluminescence (PL) spectroscopy and transmission electron microscopy (TEM) have been used to investigate macroscopic and microscopic segregation in MOCVD grown InGaN layers. The PL peak energy and In content (measured by RES) were mapped at a large number of distinct points on the samples. An indium concentration of 40%, the highest measured in this work, corresponds to a PL peak of 710 nn strongly suggesting that the light-emitting regions of the sample me very indium-rich compared to the average measured by RES. Cross-sectional TEM observations show distinctive layering of the InGaN films. The TEM study further reveals that these layers consist of amorphous pyramidal contrast features with sizes of order 10 nm The composition of these specific contrast features is shown to be In-rich compared to the nitride matrix.

Probing nitride thin films in 3-dimensions using a variable energy electron beam

In this paper we illustrate the application of electron beam techniques to the measurement of strain, defect and alloy concentrations in nitride thin films. We present brief comparative studies of CL spectra of AlGaN and InGaN epilayers and EBSD patterns obtained from two silicon-doped 3 mum thick GaN epilayers grown on an on-axis (0001) sapphire substrate and a sapphire substrate misoriented by 10 degrees toward the m-plane (10 (1) over bar0).