Brian Hawkins

Near-field scanning optical microscopy cross-sectional measurements of crystalline GaAs solar cells

Near-field scanning optical microscopy (NSOM) was used to study cleaved edges of GaAs solar cell devices. Using visible light for excitation, the NSOM acquired spatially resolved traces of the photocurrent response across the various layers in the device. For excitation energies well above the band gap, carrier recombination at the cleaved surface had a strong influence on the photocurrent signal. Decreasing the excitation energy, which increased the optical penetration depth, allowed the effects of surface recombination to be separated from collection by the pn junction. Using this approach, the NSOM measurements directly observed the effects of a buried minority carrier reflector/passivation layer.

Characteristics of GaAsN∕GaAsSb type-II quantum wells grown by metalorganic vapor phase epitaxy on GaAs substrates

Pseudomorphic four-period GaAs0.978N0.022∕GaAs0.78Sb0.22 type-II multiquantum well structures were grown on (100) GaAs substrates by metalorganic vapor phase epitaxy at 530°C. The GaAs0.978N0.022 layers were grown at a V/III ratio of 685 and N∕V ratio of 0.96, whereas the GaAs0.78Sb0.22 was grown at a V/III ratio of 3.8 and Sb∕V ratio of 0.8. The superlattice peaks in the x-ray diffraction θ-2θ scans around the (400) GaAs peak were fitted using a dynamical simulation model to determine layer thickness and alloy compositions. The GaAsN and GaAsSb thicknesses were ∼8nm and ∼5nm, respectively. The photoluminescence (PL) spectra were obtained at 30K and the PL peak energy was found to match the type-II transition energy obtained from a 10-band k∙p model. Postgrowth annealing under arsine-H2 with a N2 cooldown was found to increase the low temperature PL intensity and result in the appearance of luminescence at room temperature.

Towards intersubband quantum box lasers: Electron-beam lithography update

We report on the progress in the patterning and fabrication of the intersubband quantum-box (QB) laser structure. From a patterning point of view our goal is to make 30-nm-diameter SiO2 and/or hydrogen silsesquioxane (HSQ) disks on 60–80nm centers on a GaAs surface to serve as masks for in situ etch and regrowth of QBs. Electron-beam lithography with high-resolution negative resist HSQ was used, and two processes have been investigated. The first process is to pattern HSQ directly on the GaAs surface, while the second one involves putting down an intermediate oxide layer first, followed by the e-beam lithography and the transfer of the pattern into the oxide. Problems were encountered with the e-beam patterning of HSQ directly on the GaAs surface because of the broad scattering from the substrate and not very good adhesion. Excellent patterning was demonstrated when the intermediate oxide layer was present between the GaAs substrate and the HSQ resist.

Demonstration of near-field scanning photoreflectance spectroscopy

A near-field scanning optical microscope (NSOM) was developed to perform photoreflectance (PR) spectroscopy experiments at high spatial resolution (∼1 μm). Representative PR spectra are shown, along with an image illustrating the capability of observing contrast in images due to the strength of a PR feature. It was found that sufficiently high intensity light from the NSOM tip can produce photovoltages large enough to limit the spatial resolution of the electric field determination by PR. The photovoltage effect is measured as a function of light intensity, and the results are discussed in terms of a simple photovoltage expression.

Lateral Epitaxial Overgrowth of InAs on (100) GaAs Substrates

The microstructure of epitaxial InAs thin films grown by MOCVD on mask-patterned “LEO” (lateral epitaxial overgrowth) GaAs and on unpatterned GaAs substrates was studied using double-crystal x-ray diffraction, scanning electron microscopy and cross-sectional transmission electron microscopy. This paper describes the improvement in crystal quality (factor of 20 reduction in x-ray rocking curve width), the order of magnitude reduction in dislocation density, and the rearrangement of the remaining extended defects that were observed in the LEO material when compared to the film grown on the unpatterned wafer.