B. M. Keyes

Time-resolved photoluminescence studies of CdTe solar cells

We show that time-resolved photoluminescence measurements of completed polycrystalline CdTe solar cells provide a measure of recombination near the CdTe/CdS metallurgical interface that is strongly correlated to the open-circuit voltage in spite of complex carrier dynamics in the junction region. Oxygen in the growth ambient during close-spaced sublimation generally reduces this recombination rate; grain size does not have a strong effect.

Impact of GaAs buffer thickness on electronic quality of GaAs grown on graded Ge/GeSi/Si substrates

Minority carrier lifetimes and interface recombination velocities for GaAs grown on a Si wafer using compositionally graded GeSi buffers have been investigated as a function of GaAs buffer thickness using monolayer-scale control of the GaAs/Ge interface nucleation during molecular beam epitaxy. The GaAs layers are free of antiphase domain disorder, with threading dislocation densities measured by etch pit density of 5×105–2×106 cm−2. Analysis indicates no degradation in either minority carrier lifetime or interface recombination velocity down to a GaAs buffer thickness of 0.1 μm. In fact, record high minority carrier lifetimes exceeding 10 ns have been obtained for GaAs on Si with a 0.1 μm GaAs buffer. Secondary ion mass spectroscopy reveals that cross diffusion of Ga, As, and Ge at the GaAs/Ge interface formed on the graded GeSi buffers are below detection limits in the interface region, indicating that polarity control of the GaAs/Ge interface formed on GeSi/Si substrates can be achieved.

Hydrogen passivation effect in nitrogen-doped ZnO thin films

The role of hydrogen in nitrogen-doped ZnO thin films was studied by Fourier transform infrared (FTIR) absorption and modeled by first-principles calculations to understand the difficulty of doping ZnO p-type with nitrogen. Nitrogen-doped ZnO films were fabricated by low-pressure metal-organic chemical vapor deposition (MOCVD). High levels of nitrogen incorporation were observed, but the acceptor concentrations remained low. Theoretical analysis suggests there is a high probability that NO− and H+ charged defects combine to form the neutral defect complexes, thereby compensating the nitrogen-related acceptors. Calculated values of the vibrational frequencies of the related infrared modes agree well with the measured spectra. Thus, we believe the difficulty of achieving p-type doping in MOCVD-grown ZnO films is due, at least partially, to inadvertent passivation by hydrogen.

Combinatorial studies of Zn-Al-O and Zn-Sn-O transparent conducting oxide thin films

Abstract In this work, we discuss the development of combinatorial deposition and analysis tools for the investigation of and the optimization of transparent conducting oxides. Library deposition by co-sputtering followed by optical analysis is shown to be a facile way to achieve these goals. Initial work focused on Zn-Al-O libraries with low Al contents as a test case. Subsequent work has focused on the ZnO-SnO2 tie line. Local maxima in the composition dependence of the conductivity were found for Zn/Sn ≈2:1 (Zn2SnO4) and Zn/Sn ≈1:1 (ZnSnO3). For these two representative stoichiometries, constant composition films have also been grown by pulsed laser deposition.

High-mobility transparent conducting Mo-doped In2O3 thin films by pulsed laser deposition

Highly conductive and transparent Mo-doped indium oxide (IMO) thin films were grown on glass and (100) yttria-stabilized zirconia (YSZ) single-crystal substrates by pulsed laser deposition. The electrical, optical, and structural properties were measured for films grown from 0, 1, 2, and 4 wt % Mo-doped targets. Films grown from the 2 wt % Mo-doped target had the best overall properties. In particular, for biaxially textured 2 wt % Mo IMO films grown on (100) YSZ, the conductivity was ∼3000 S cm−1 with a mobility greater than 95 cm2 V−1 s−1. In the visible, the optical transmittance normalized to the substrate was greater than 90%.

High minority-carrier lifetimes in GaAs grown on low-defect-density Ge/GeSi/Si substrates

A high bulk minority-carrier lifetime in GaAs grown on Si-based substrates is demonstrated. This was achieved by utilizing a step-graded Ge/GeSi buffer (threading dislocation density 2×106 cm−2) grown on an offcut (001) Si wafer, coupled with monolayer-scale control of the GaAs nucleation to suppress antiphase domains. Bulk minority-carrier lifetimes (τp) were measured using room-temperature time-resolved photoluminescence applied to a series of Al0.3Ga0.7As/GaAs/Al0.3Ga0.7As double-heterojunction structures doped n=1.1×1017 cm−3 with GaAs thicknesses of 0.5, 1.0, and 1.5 μm. A lifetime τp=7.7 ns was determined for GaAs grown on Si. The extracted interface recombination velocity of 3.9×103 cm/s is comparable to recombination velocities found for Al0.3Ga0.7As/GaAs interfaces grown on both GaAs and Ge wafers, indicating that the crosshatch surface morphology characteristic of strain-relaxed Ge/GeSi surfaces does not impede the formation of high-electronic-quality interfaces. These results hold great promise...

INTENSITY-DEPENDENT MINORITY-CARRIER LIFETIME IN III-V SEMICONDUCTORS DUE TO SATURATION OF RECOMBINATION CENTERS

The minority‐carrier lifetime has been measured by time‐resolved photoluminescence in a variety of III‐V epitaxial material including GaAs and AlxGa1−xAs. In cases where Shockley–Read–Hall recombination is dominant, the measured lifetimes are dependent upon the intensity of the excitation source. These lifetime effects can be described by a Shockley–Read–Hall model that includes the injection dependence of the recombination. As the lifetimes increase with the injection level, we describe the effects as the saturation of recombination centers.

Ultralong minority-carrier lifetime epitaxial GaAs by photon recycling

The minority‐carrier lifetime has been measured by time‐resolved photoluminescence in epitaxial films of GaAs grown by metalorganic chemical vapor deposition. The measured lifetimes in thicker devices are 4 to 6 times the theoretical or radiative lifetime. These long lifetimes are the result of photon recycling or self‐generation of the self‐absorbed radiation.

BGaInAs alloys lattice matched to GaAs

We report the epitaxial growth of zinc-blende BxGa1−x−yInyAs and BxGa1−xAs on GaAs substrates with boron concentrations (x) up to 2%–4% by atmospheric-pressure metalorganic chemical vapor deposition. The band gap of BxGa1−xAs increases by only 4–8 meV/%B with increasing boron concentration in this concentration range. We demonstrate an epitaxial BxGa1−x−yInyAs layer deposited on GaAs with a band gap of 1.34 eV that is significantly less strained than a corresponding Ga1−yInyAs layer with the same band gap.

Impact of dislocations on minority carrier electron and hole lifetimes in GaAs grown on metamorphic SiGe substrates

The minority carrier lifetime of electrons (τn) in p-type GaAs double heterostructures grown on GaAs substrates and compositionally graded Ge/Si1−xGex/Si (SiGe) substrates with varying threading dislocation densities (TDDs) were measured at room temperature using time-resolved photoluminescence. The electron lifetimes for homoepitaxial GaAs and GaAs grown on SiGe (TDD∼1×106 cm−2) with a dopant concentration of 2×1017 cm−3 were ∼21 and ∼1.5 ns, respectively. The electron lifetime measured on SiGe was substantially lower than the previously measured minority carrier hole lifetime (τp) of ∼10 ns, for n-type GaAs grown on SiGe substrates with a similar residual TDD and dopant concentration. The reduced lifetime for electrons is a consequence of their higher mobility, which yields an increased sensitivity to the presence of dislocations in GaAs grown on metamorphic buffers. The disparity in dislocation sensitivity for electron and hole recombination has significant implications for metamorphic III-V devices.