A. Mascarenhas

Molecular beam epitaxy growth of GaAs1−xBix

GaAs1−xBix epilayers with bismuth concentrations up to x=3.1% were grown on GaAs by molecular beam epitaxy. The Bi content in the films was measured by Rutherford backscattering spectroscopy. X-ray diffraction shows that GaAsBi is pseudomorphically strained to GaAs but that some structural disorder is present in the thick films. The extrapolation of the lattice constant of GaAsBi to the hypothetical zincblende GaBi alloy gives 6.33±0.06 A. Room-temperature photoluminescence of the GaAsBi epilayers is obtained and a significant redshift in the emission of GaAsBi of ∼84 meV per percent Bi is observed.

Band gap of GaAs1−xBix, 0<x<3.6%

The band gap of GaAsBi epitaxial layers as a function of bismuth concentration up to 3.6% is determined. The optical transitions were measured by modulated electroreflectance. The energy of the band gap decreases at a linearized rate of 88 meV/% Bi, or 83 meV/% Bi for the heavy hole to conduction band transition for GaAsBi strained to GaAs. The valence-band splitting increases faster than that of GaAs under similar compressive strain whereas the temperature dependence of the observed GaAsBi transitions is similar to that of GaAs.

Highly enhanced Curie temperature in low-temperature annealed [Ga,Mn]As epilayers

We report Curie temperatures up to 150 K in annealed Ga1−xMnxAs epilayers grown with a relatively low As:Ga beam equivalent pressure ratio. A variety of measurements (magnetization, Hall effect, magnetic circular dichroism and Raman scattering) suggest that the higher Curie temperature results from an enhanced free hole density. The data also indicate that, in addition to the carrier concentration, the sample thickness limits the maximum attainable Curie temperature in this material, suggesting that the free surface of Ga1−xMnxAs epilayers may be important in determining their physical properties.

Electrochromic mechanism in a-WO3−y thin films

The electrochromic mechanism in amorphous tungsten oxide films is studied using Raman scattering measurements. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm−1 due to vibrations of the W6+–O and W6+=O bonds, respectively, and a weaker peak at 220 cm−1 that we attribute to the W4+–O bonds. When lithium or hydrogen ions and electrons are inserted, extra Raman peaks due to W5+–O and W5+=O bonds appear at 330 and 450 cm−1, respectively. Comparison of the Raman spectra of sputtered isotopic a-W16O3−y and a-W18O3−y films confirms these assignments. We conclude that the as-deposited films contain mainly the W4+ and W6+ states, and the W5+ states are generated as a result of reduction of the W6+ states when lithium or hydrogen ions and electrons are inserted. We propose that the optical absorption in the colored films is caused by transitions between the W6+ and W5+, and W5+ and W4+ states.

Band‐gap reduction and valence‐band splitting of ordered GaInP2

Low‐temperature photoluminescence excitation spectra are used to determine the order‐dependent parameters: valence‐band splitting and band‐gap reduction in spontaneously ordered GaInP2. Effects due to composition fluctuations between different samples and the associated strain, as well as the excitonic binding energies, have been properly taken into account to yield accurate band‐gap reduction and valence‐band splitting values. The results from recently published ab initio band structure calculations are used to extrapolate the band‐gap reduction from the strongest experimentally realized degree of ordering to perfect ordering. We find a total band‐gap reduction of 471±12 meV, which is very close to recent theoretical predictions.

Electrochromic coloration efficiency of a-WO3−y thin films as a function of oxygen deficiency

We report on how electrochromic coloration is affected by oxygen deficient stoichiometries in sputtered amorphous tungsten oxide (a-WO3−y) films. The electrochromic coloration efficiency increases with increasing oxygen deficiency in (a-WO3−y) films. No coloration is observed in nearly stoichiometric WO3 films. Raman spectroscopic studies reveal that the number of W5+ states generated with lithium insertion increases with the oxygen deficiency. Furthermore, there are no Raman peaks resulting from W5+ states in lithiated a-WO3−y films with near perfect stoichiometry, which is consistent with the absence of electrochromic coloration in those films. We conclude that the coloration efficiency of a-WO3−y films depends on the number of the W5+ states generated by lithium insertion and that the oxygen deficiency plays an important role in generating the W5+ states with lithium insertion.

Synthesis and photovoltaic effect of vertically aligned ZnO/ZnS core/shell nanowire arrays

A vertically aligned ZnO/ZnS core/shell nanowire array with type II band alignment was directly synthesized on an indium-tin-oxide glass substrate and the photovoltaic effect of the nanowire array was investigated. The epitaxial relationship, wurtzite (0001) matching zinc-blende (111), was observed in the ZnO/ZnS nano-heterostructure. ZnS coating is found to quench the photoluminescence of ZnO nanowires but enhance the photocurrent with faster response in the photovoltaic device, indicating improvement in charge separation and collection in the type II core/shell nanowire.

Effects of nitrogen on the band structure of GaNxP1−x alloys

We report that the incorporation of N in GaNxP1−x alloys (x⩾0.43%) leads to a direct band-gap behavior of GaNP. For N concentration lower than 0.43%, a series of sharp emission lines from the various N pair centers are observed for GaNP bulk layers. With increasing N concentration higher than 0.43%, a strong photoluminescence (PL) emission from GaNP bulk layers is observed at room temperature. While the PL peak redshifts with increasing N concentration to 3.1%, the PL intensity remains as intense. Absorption measurements show a direct band-gap behavior of GaNP alloys.

Raman spectroscopic studies of electrochromic a-WO3

We report on the eAect of electrochromic coloration on the Raman spectra of evaporated and sputtered a-WO3ˇy films. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm ˇ1 due to vibrations of the W 6+ ‐O and W 6+ .O bonds, respectively, and a weaker peak at 220 cm ˇ1 that we attribute to the vibrations of the W 4+ ‐O bonds. When lithium or hydrogen ions are inserted, extra Raman peaks due to W 5+ ‐O and W 5+ .O bonds appear at 330 and 450 cm ˇ1 , respectively. Comparison of the Raman spectra of sputtered a-W 16 O3ˇy and aW 18 O3ˇy isotopic films confirms these assignments. We conclude that the as-deposited films contain mainly the W 4+ and W 6+ states, and the W 5+ states are generated as a result of reduction of the W 6+ states when ions are inserted. We propose that the electrochromism in a-WO3ˇy films originates from reduction of the W 6+ states to W 5+ due to the double insertion/extraction of ions and electrons and the optical absorption in the colored films is caused by transitions between the W 6+ and W 5+ , and W 5+ and W 4+ states. # 1999 Elsevier Science Ltd. All rights reserved.

Alternating current impedance and Raman spectroscopic study on electrochromic a-WO3 films

The chemical diffusion of lithium ions in a-LixWO3 films is investigated using alternating current impedance spectroscopy and Raman scattering measurements. The diffusion coefficients increase with increasing x in a-LixWO3 up to x=0.072 and then decrease. Raman measurements show that the W6+=O/O–W6+–O ratio also increases at the early stage of lithium insertion and then decreases with further lithium insertion. We conclude that the diffusion kinetics of lithium ions in a-LixWO3 films is very closely related to the W6+=O/O–W6+–O ratio.