We report the use of Te as an n-type dopant in GaAs core-shell p-n junction nanowires for use in photovoltaic devices. Te produced significant change in the morphology of GaAs nanowires grown by the vapor-liquid-solid process in a molecular beam epitaxy system. The increase in radial growth of nanowires due to the surfactant effect of Te had a significant impact on the operating characteristics of photovoltaic devices. A decrease in solar cell efficiency occurred when the Te-doped GaAs growth duration was increased.

GaAs nanowire (NW)-based p-n photovoltaic devices, with two distinct p and n spatial distributions and where Te was the n-dopant, have been studied by impedance spectroscopy in the 10(3)-10(7) Hz frequency range and the - 1.5-1.5 V bias range. For a large n-core/p-shell overlap region within NWs in a coaxial geometry, the p-n junction properties (DC rectification and p-n depletion capacitance) are found to prevail. The impedance data at low bias for both NW devices show large frequency dispersions with relaxation frequencies that are compatible with carrier re-emission times from traps due to GaAs surface states. An increasing conductance with increasing frequency for low bias is observed, suggesting hopping transport through localized states. For large bias the conductance increases exponentially with bias and is frequency independent, indicating conduction through extended states in this regime.

The lateral composition modulation that is present in InGaAs quantum wells (QWs) produces an asymmetry in the plane of the QW with two regions where transition to the heavy hole (HH) can occur. Polarization resolved photoluminescence shows that transitions between the conduction band to HH subbands in both the In-rich and Ga-rich regions produce anisotropic features. Also, changing the relative separation of the light hole and HH subbands has little impact on the separation of the anisotropic features of the photoluminescence curves.

The use of an InP epitaxial layer grown at low temperatures before the growth of a step-graded InAsP metamorphic buffer has been shown to provide a large improvement in the crystal quality of the final metamorphic layer. The improvement is evidenced by over an order of magnitude increase in photoluminescence intensity as well as a large reduction of the mosaic spread and the overall tilt of the relaxed layers.

A low-temperature InP-cap layer is used to enhance quantum well intermixing (QWI) following rapid thermal annealing (RTA) on an InGaAsP quantum well (QW) structure. The influence of different proximity caps (Si, InP and GaAs) used during the RTA step has been investigated. A combination of cross-sectional transmission electron microscopy and high-resolution TEM together with energy-dispersive x-ray analysis directly reveals compositional and morphological changes resulting after QWI. Room temperature photoluminescence is used to establish the changes in emission wavelength and intensity resulting from the QW modification. Use of a Si proximity cap leads to the formation of a sharply defined QW with the least amount of QW modification, while GaAs proximity capping results in the broadest QW, broadened QW/barrier interfaces and the highest concentration of phosphorous in the well. Use of an InP proximity cap results in intermediate QW modification. In all cases, after QWI the bottoms of the QWs are flat and the well is square-like with broadened sidewalls. It has also been found that group V rich precipitates are produced on the top two layers of the structure. The concentration and size of the precipitates are minimum with Si proximity capping, while the use of GaAs produces the highest concentration and largest precipitates. The concentration and size of the precipitates are intermediate for the InP capping. The choice of proximity cap material affects the intermixing through changes of the effective P diffusivity which has been calculated from obtained profiles.