Maria Gonzalez

Characterization of Metamorphic GaAsP/Si Materials and Devices for Photovoltaic Applications

GaAsyP1-y anion-sublattice compositionally graded buffers and device structures were grown directly on Si(100) substrates by way of a high-quality GaP integration layer, yielding GaAsP target layers having band gaps of photovoltaic interest (1.65-1.8 eV), free of antiphase domains/borders, stacking faults, and microtwins. GaAsyP1-y growths on both Si and GaP substrates were compared via high-resolution X-ray diffractometry of the metamorphic buffers and deep-level transient spectroscopy (DLTS) of p+-n diodes that are lattice matched to the final buffer layer. Structural analysis indicates highly efficient epitaxial relaxation throughout the entire growth structure for both types of samples and suggests no significant difference in physical behavior between the two types of samples. DLTS measurements performed on GaAsP diodes fabricated on both Si and GaP substrates reveal the existence of identical sets of traps residing in the n-type GaAsP layers in both types of samples: a single majority carrier (electron) trap, which is located at EC - 0.18 eV, and a single minority carrier (hole) trap, which is located at EV + 0.71 eV. Prototype 1.75-eV GaAsP solar cell test devices grown on GaAsyP1-y/Si buffers show good preliminary performance characteristics and offer great promise for future high-efficiency III-V photovoltaics integrated with Si substrates and devices.

Role of subsurface defects in metal-ZnO(0001¯) Schottky barrier formation

The authors fabricated diodes of Au, Al, Ni, Pt, Pd, Mo, Ta, and Ir on single crystal ZnO(0001¯) surfaces from different vendors and measured their Schottky barriers, idealities, and reverse currents on as-received and remote oxygen (20% O2∕80%He) plasma-treated surfaces. Using low temperature nanoscale depth-resolved cathodoluminescence spectroscopy (DRCLS) under the metal, the authors identified the presence of defect transitions at 2.1, 2.5, and 3.0eV that change dramatically depending on the process steps and choice of metal. I‐V measurements exhibited transitions from Ohmic to Schottky and lower idealities for Pt, Au, Ir, and Pd with plasma treatment. ZnO with low defect densities yield lower idealities and reverse currents. Deep level optical and transient spectroscopies correlated bulk and surface defects, showing deep levels at 2.54 and 0.53eV, while DRCLS shows that these densities can increase by >100 times at the surface. The magnitude of the metal’s influence correlates directly to the relativ...

Deep level defects in proton radiated GaAs grown on metamorphic SiGe/Si substrates

The effect of 2MeV proton radiation on the introduction of deep levels in GaAs grown on compositionally graded SiGe∕Si substrates was investigated using deep level transient spectroscopy (DLTS). Systematic comparisons were made with identical layers grown on both GaAs and Ge substrates to directly assess the influence of threading dislocations on radiation-related deep levels for both n-type and p-type GaAs. DLTS revealed that for p+n structures, proton irradiation generates electron traps at Ec−0.14eV, Ec−0.25eV, Ec−0.54eV, and Ec−0.72eV in the n‐GaAs base, and, for n+p structures, radiation-induced hole traps appear at Ev+0.18eV, Ev+0.23eV, Ev+0.27eV, and Ev+0.77eV in the p-type GaAs base, irrespective of substrate choice for both polarities. The primary influence of substituting SiGe∕Si substrates for conventional GaAs and Ge substrates is on the introduction rates of the individual traps as a function of proton radiation fluence. Substantially reduced concentrations are found for each radiation-induce...

Impact of threading dislocations on both n/p and p/n single junction GaAs cells grown on Ge/SiGe/Si substrates

Single junction GaAs solar cells having an n/p polarity were grown on p-type Ge/SiGe/Si substrates for the first time. The cell performance and material properties of these n/p cells were compared with p/n cells grown on n-type Ge/SiGe/Si substrates for which record high minority carrier hole lifetimes of 10 ns and open circuit voltages (V/sub oc/) greater than 980 mV (AM0) were achieved. The initial n/p experimental results and correlations with theoretical predictions have indicated that for comparable threading dislocation densities (TDD), n/p cells have longer minority carrier diffusion lengths, but reduced minority carrier lifetimes for electrons in the p-type GaAs base layers. This suggests that a lower TDD tolerance exists for n/p cells compared to p/n cells, which has implications for the optimization of n/p high efficiency cell designs using alternative substrates.

Determination of bandgap states in p-type In0.49Ga0.51P grown on SiGe/Si and GaAs by deep level optical spectroscopy and deep level transient spectroscopy

The presence and properties of traps in p-type In0.49Ga0.51P grown on low dislocation density, metamorphic Ge/SiGe/Si substrates and GaAs substrates were determined using deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) leading to the quantification of trap behavior throughout the entire 1.9 eV bandgap of the In0.49Ga0.51P material as a function of substrate. Thermal emission-based DLTS revealed a single hole trap at Ev + 0.71 eV for growth on both lattice matched and mismatched substrates with similar concentrations. Complementary, optical emission-based DLOS measurements detected bandgap states at Ev + 1.18 eV, Ev + 1.36 eV, and Ev + 1.78 eV for p-type In0.49Ga0.51P grown on both substrate types. The total concentration of the DLOS-detected states was found to comprise approximately 80% of the entire trap concentration in p-type In0.49Ga0.51P bandgap. This relatively high concentration of above midgap levels may be of great significance for minority carrier devices tha...

Impact of annealing and V:III ratio on properties of MBE grown wide-bandgap AlGaInP materials and solar cells

The growth and properties of wide bandgap (Al/sub x/Ga/sub 1-x/)/sub 0.51/In/sub 0.49/P layers and solar cells grown by solid source molecular beam epitaxy were examined to correlate the impact of growth conditions and in-situ annealing on photovoltaic performance. A P/sub 2/:III flux ratio of 12 was found to optimize the optical qualities of Ga/sub 0.51/In/sub 0.49/P epilayers. Ga/sub 0.51/In/sub 0.49/P solar cells were grown and subjected to different in-situ annealing conditions. The effect of annealing on material quality and device performance was characterized through deep level transient spectroscopy (DLTS) and photoluminescence (PL), which revealed a trend in non-radiative recombination. The results suggest that removal of a deep level near E/sub C/ - 0.78 eV in the n-type base is responsible for the observed improvement in current collection seen after anneal. After refinement of the Ga/sub 0.51/In/sub 0.49/P growth, wider bandgap material was investigated for future use in a high temperature/high intensity solar cell. A range of (Al/sub x/Ga/sub 1-x/)/sub 0.51/In/sub 0.49/P materials, with direct bandgaps from 2.09 eV to 2.26 eV have been successfully demonstrated using digital alloying and conventional bulk growth.

1EV GAN x AS 1-x-y SB y material for lattice-matched III–V solar cell implementation on GaAs and Ge

The effect of different arsenic species (As<inf>2</inf> or As<inf>4</inf>) on the quality of molecular beam epitaxy (MBE) grown GaNAsSb materials (samples A and B) and GaAs/ GaNAsSb/GaAs p⁺ n⁻ n⁺ devices (samples C and D) were investigated. The improvement in material quality in sample B, as well as the improvement in diode and solar cell characteristics in sample C, may suggest a successful defect density manipulation using As<inf>2</inf> overpressure for GaNAsSb growth.