Showing papers by "W. Shan published in 2003"

Superior radiation resistance of In1-xGaxN alloys: Full-solar-spectrum photovoltaic material system

Abstract: High-efficiency multijunction or tandem solar cells based on group III–V semiconductor alloys are applied in a rapidly expanding range of space and terrestrial programs. Resistance to high-energy radiation damage is an essential feature of such cells as they power most satellites, including those used for communications, defense, and scientific research. Recently we have shown that the energy gap of In1−xGaxN alloys potentially can be continuously varied from 0.7 to 3.4 eV, providing a full-solar-spectrum material system for multijunction solar cells. We find that the optical and electronic properties of these alloys exhibit a much higher resistance to high-energy (2 MeV) proton irradiation than the standard currently used photovoltaic materials such as GaAs and GaInP, and therefore offer great potential for radiation-hard high-efficiency solar cells for space applications. The observed insensitivity of the semiconductor characteristics to the radiation damage is explained by the location of the band edge...

Temperature dependence of the fundamental band gap of InN

Abstract: The fundamental band gap of InN films grown by molecular beam epitaxy have been measured by transmission and photoluminescence spectroscopy as a function of temperature The band edge absorption energy and its temperature dependence depend on the doping level The band gap variation and Varshni parameters of InN are compared with other group III nitrides The energy of the photoluminescence peak is affected by the emission from localized states and cannot be used to determine the band gap energy Based on the results obtained on two samples with distinctly different electron concentrations, the effect of degenerate doping on the optical properties of InN is discussed

Diluted II-VI Oxide Semiconductors with Multiple Band Gaps

Abstract: We report the realization of a new mult-band-gap semiconductor. Zn(1-y)Mn(y)OxTe1-x alloys have been synthesized using the combination of oxygen ion implantation and pulsed laser melting. Incorporation of small quantities of isovalent oxygen leads to the formation of a narrow, oxygen-derived band of extended states located within the band gap of the Zn(1-y)Mn(y)Te host. When only 1.3% of Te atoms are replaced with oxygen in a Zn0.88Mn0.12Te crystal the resulting band structure consists of two direct band gaps with interband transitions at approximately 1.77 and 2.7 eV. This remarkable modification of the band structure is well described by the band anticrossing model. With multiple band gaps that fall within the solar energy spectrum, Zn(1-y)Mn(y)OxTe1-x is a material perfectly satisfying the conditions for single-junction photovoltaics with the potential for power conversion efficiencies surpassing 50%.

Effect of oxygen on the electronic band structure in ZnOxSe1−x alloys

Abstract: The effect of alloying small amounts of ZnO with ZnSe on the electronic band structure has been studied. Optical transitions in molecular-beam-epitaxy-grown ZnOxSe1−x epitaxial films (0⩽x⩽1.35%) were investigated using photoreflectance and photoluminescence spectroscopies. The fundamental band-gap energy of the alloys was found to decrease at a rate of about 0.1 eV per atomic percent of oxygen. The pressure dependence of the band gap was also found to be strongly affected by O incorporation. Both the effects can be quantitatively explained by an anticrossing interaction between the extended states of the conduction band of ZnSe and the highly localized oxygen states located at approximately 0.22 eV above the conduction-band edge.

Hydrostatic pressure dependence of the fundamental bandgap of InN and In-rich group III nitride alloys

Abstract: We report studies of the hydrostatic pressure dependence of the fundamental bandgap of InN, In-rich In1−xGaxN (0

Band-gap bowing effects in BxGa1−xAs alloys

Abstract: Photomodulation spectroscopy studies of the transitions at the Γ point of the Brillouin zone of thin films of BxGa1−xAs alloys grown by metal-organic chemical-vapor deposition are presented. A very small increase of the fundamental band gap is found in samples with B content up to 3%. Under hydrostatic pressure, the band gap increases at a rate almost identical to the pressure dependence of the GaAs band gap. In contrast to the case of N incorporation at similar concentrations into GaAs, the experimental results show that B incorporation does not cause large modifications of the conduction-band structure in BxGa1−xAs alloys.

Mutual passivation of group IV donors and nitrogen in diluted GaNxAs1−x alloys

Abstract: This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, U.S. Department of Energy, under Contract No. DE-AC03-76SF00098. One of the authors ~M.A.S.! acknowledges support by an NSF graduate research fellowship.

Composition Dependence of the Hydrostatic Pressure Coefficients of the Bandgap of ZnSe(1-x)Te(x) Alloys

Abstract: Optical absorption experiments have been performed using diamond anvil cells to measure the hydrostatic pressure dependence of the fundamental bandgap of ZnSe(sub 1-xTe(sub x) alloys over the entire composition range. The first and second-order pressure coefficients are obtained as a function of composition. Starting from the ZnSe side, the magnitude of both coefficients increases slowly until x approx. 0.7, where the ambient-pressure bandgap reaches a minimum. For larger values of x the coefficients rapidly approach the values of ZnTe. The large deviations of the pressure coefficients from the linear interpolation between ZnSe and ZnTe are explained in terms of the band anticrossing model.

Mutual passivation of group IV donors and isovalent nitrogen in diluted GaNxAs1−x alloys

Abstract: We demonstrate the mutual passivation of electrically active group IV donors and isovalent N atoms in the GaN x As 1− x alloy system. This phenomenon occurs through the formation of a donor-nitrogen bond in the nearest neighbor IV Ga –N As pairs. In Si-doped GaInN 0.017 As 0.983 the electron concentration starts to decrease rapidly at an annealing temperature of 700°C from ∼3×10 19 cm −3 in the as-grown state to less than 10 16 cm −3 after an annealing at 900°C for 10 s. At the same time, annealing of this sample at 950°C increases the gap by about 35 meV, corresponding to a reduction of the concentration of the active N atoms by an amount very close to the total Si concentration. We also show that the formation of Si Ga –N As pairs is controlled by the diffusion of Si via Ga vacancies to the nearest N As site. The general nature of this mutual passivation effect is confirmed by our study of Ge-doped GaN x As 1− x layers formed by N and Ge co-implantation in GaAs followed by pulsed laser melting.

Pressure Dependence of Optical Transitions in In-rich Group III-Nitride Alloys

Abstract: The hydrostatic pressure dependence of the optical transitions in InN, In-rich In1−xGaxN (0 < x < 0.5) and In1−xAlxN (x = 0.25) alloys is studied using diamond anvil cells. The absorption edges and the photoluminescence peaks shift to higher energy with pressure. The pressure coefficient of InN is determined to be 3.0±0.1 meV/kbar. Together with previous experimental results, our data suggest that the pressure coefficients of group-III nitride alloys have only a weak dependence on the alloy composition. Photoluminescence gives much smaller pressure coefficients, which is attributed to emission involving highly localized states. This indicates that photoluminescence might not be an accurate method to study the pressure dependence of the fundamental bandgaps of group III-nitrides.