Stephen R. Lee

Effect of dislocation density on efficiency droop in GaInN/GaN light-emitting diodes

Measurements of light-output power versus current are performed for GaInN∕GaN light-emitting diodes grown on GaN-on-sapphire templates with different threading dislocation densities. Low-defect-density devices exhibit a pronounced efficiency peak followed by droop as current increases, whereas high-defect-density devices show low peak efficiencies and little droop. The experimental data are analyzed with a rate equation model to explain this effect. Analysis reveals that dislocations do not strongly impact high-current performance; instead they contribute to increased nonradiative recombination at lower currents and a suppression of peak efficiency. The characteristics of the dominant recombination mechanism at high currents are consistent with processes involving carrier leakage.

Internal quantum efficiency and nonradiative recombination coefficient of GaInN/GaN multiple quantum wells with different dislocation densities

Room-temperature photoluminescence measurements are performed on GaInN/GaN multiple quantum wells grown on GaN-on-sapphire templates with different threading-dislocation densities. The internal quantum efficiencies as a function of carrier concentration and the non-radiative coefficients are obtained.

Effect of threading dislocations on the Bragg peakwidths of GaN, AlGaN, and AlN heterolayers

We develop a reciprocal-space model that describes the (hkl) dependence of the broadened Bragg peakwidths produced by x-ray diffraction from a dislocated epilayer. We compare the model to experiments and find that it accurately describes the peakwidths of 16 different Bragg reflections in the [010] zone of both GaN and AlN heterolayers. Using lattice-distortion parameters determined by fitting the model to selected reflections, we estimate threading-dislocation densities for seven different GaN and AlGaN samples and find improved agreement with transmission electron microscopy measurements.

Room-temperature direct current operation of 290 nm light-emitting diodes with milliwatt power levels

Ultraviolet light-emitting diodes (LEDs) have been grown by metalorganic vapor phase epitaxy using AlN nucleation layers and thick n-type Al0.48Ga0.52N current spreading layers. The active region is composed of three Al0.36Ga0.64N quantum wells with Al0.48Ga0.52N barriers for emission at 290 nm. Devices were designed as bottom emitters and flip-chip bonded to thermally conductive submounts using an interdigitated contact geometry. The ratio of quantum well emission to 330 nm sub-band gap emission is as high as 125:1 for these LEDs. Output power as high as 1.34 mW at 300 mA under direct current operation has been demonstrated with a forward voltage of 9.4 V. A peak external quantum efficiency of 0.18% has been measured at an operating current of 55 mA.

The band-gap bowing of AlxGa1−xN alloys

The band gap of AlxGa1−xN is measured for the composition range 0⩽x 800 °C usually lead to stronger apparent bowing (b>+1.3 eV); while growths initiated using low-temperature buffers on sapphire, followed by high-temperature growth, lead to weaker bowing (b<+1.3 eV). Extant data suggest that the intrinsic band-gap bowing parameter for AlGaN alloys is b=+0.62(±0.45) eV.

Precipitation and relaxation in strained Si1−yCy/Si heterostructures

We have studied the thermal stability of Si1−yCy/Si (y=0.007 and 0.014) heterostructures formed by solid phase epitaxial regrowth of C implanted layers. The loss of substitutional C was monitored over a temperature range of 810–925 °C using Fourier transform infrared absorbance spectroscopy. Concurrent strain measurements were performed using rocking curve x‐ray diffraction to correlate strain relaxation with the loss of substitutional C from the lattice. Loss of C from the lattice was initiated immediately without an incubation period, indicative of a low barrier to C clustering. The activation energy as calculated from a time to 50% completion analysis (3.3±5 eV) is near the activation energy for the diffusion of C in Si. Over the entire temperature range studied, annealing to complete loss of substitutional C resulted in the precipitation of C into β‐SiC. The precipitates are nearly spherical with diameters of 2–4 nm. These precipitates have the same crystallographic orientation as the Si matrix but th...

Metastable SiGeC formation by solid phase epitaxy

We report the synthesis and detailed structural characterization of SiGeC metastable alloys formed by solid phase epitaxial regrowth. Epitaxial layers with 0.7 and 1.4 at. % C are formed by 700 °C regrowth of multiple energy carbon implants into preamorphized Si0.86Ge0.14 layers on Si substrates. Transmission electron microscopy and Rutherford backscattering spectrometry show heteroepitaxial regrowth of Si1−x−yGexCy layers into the metastable diamond cubic phase. Fourier transform infrared spectroscopy verifies that the carbon occupies substitutional lattice sites. Double crystal x‐ray diffraction measurements of Si1−x−yGexCy and Si1−yCy reference layers quantify the C‐induced tensile strain component. This strain compensates for the compressive strain in the SiGe layers, and indicates a change in lattice constant per atomic fraction C in agreement with Vegard’s law.

Control and Elimination of Cracking of AlGaN Using Low-Temperature AlGaN Interlayers

We demonstrate that the insertion of low-temperature AlGaN interlayers is effective in reducing mismatch-induced tensile stress and suppressing the formation of cracks during growth of high-temperature AlGaN directly upon GaN epilayers. Stress evolution and relaxation is monitored using an in situ optical stress sensor. The combination of in situ and ex situ characterization techniques enables us to determine the degree of pseudomorphism in the interlayers. It is observed that the elastic tensile mismatch between AlGaN and GaN is mediated by the relaxation of interlayers; the use of interlayers offers tunability in the in-plane lattice parameters.

Brittle-ductile relaxation kinetics of strained AlGaN/GaN heterostructures

The authors have directly measured the stress evolution during metal organic chemical vapor deposition of AlGaN/GaN heterostructures on sapphire. In situ stress measurements were correlated with ex situ microstructural analysis to directly determine a critical thickness for cracking and the subsequent relaxation kinetics of tensile-strained Al{sub x}Ga{sub 1{minus}x}N on GaN. Cracks appear to initiate the formation of misfit dislocations at the AlGaN/GaN interface, which account for the majority of the strain relaxation.

Optical performance of top-down fabricated InGaN/GaN nanorod light emitting diode arrays.

Vertically aligned InGaN/GaN nanorod light emitting diode (LED) arrays were created from planar LED structures using a new top-down fabrication technique consisting of a plasma etch followed by an anisotropic wet etch. The wet etch results in straight, smooth, well-faceted nanorods with controllable diameters and removes the plasma etch damage. 94% of the nanorod LEDs are dislocation-free and a reduced quantum confined Stark effect is observed due to reduced piezoelectric fields. Despite these advantages, the IQE of the nanorod LEDs measured by photoluminescence is comparable to the planar LED, perhaps due to inefficient thermal transport and enhanced nonradiative surface recombination.