Michael A. Scarpulla

Diluted II-VI Oxide Semiconductors with Multiple Band Gaps

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%.

Effects of sodium on electrical properties in Cu2ZnSnS4 single crystal

We have studied the effect of sodium on the electrical properties of Cu2ZnSnS4 (CZTS) single crystal by using temperature dependence of Hall effect measurement. The sodium substitution on the cation site in CZTS is observed from the increasing of unit-cell size by powder X-ray diffraction. Sodium increases the effective hole concentration and makes the thermal activation energy smaller. The degree of compensation decreases with sodium incorporation, thus the hole mobility is enhanced. We revealed that sodium is important dopant in CZTS to control the electrical properties.

Ferromagnetic Ga1−xMnxAs produced by ion implantation and pulsed-laser melting

The work at the Lawrence Berkeley National Laboratory was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. The work at Harvard was supported by NASA Grant No. NAG8-1680. One of the authors ~M.A.S.! acknowledges support from an NSF Graduate Research Fellowship.

Temperature dependent conductivity of polycrystalline Cu2ZnSnS4 thin films

The temperature-dependent conductivity of Cu2ZnSnS4 (CZTS) thin films prepared by sulfurization of different sputtered ZnS/Cu/Sn stacks and also of the same stack annealed for different times was investigated from 30-300 K. Fitting of the through-thickness conductivity requires a model including Mott variable-range hopping (M-VRH), nearest-neighbor hopping (NNH), and thermionic emission over grain boundary (GB) barriers. The GB barrier height varies sensitively from 50–150 (±5) meV with annealing and especially with [Cu]/([Zn] + [Sn]) ratio but is independent of [Zn]/[Sn] ratio. These results are critical for understanding the behavior of solar cells based on polycrystalline CZTS absorber layers.

Air shear driven flow of thin perfluoropolyether polymer films

We have studied the wind driven movement of thin perfluoropolyether (PFPE) polymer films on silicon wafers and CNx overcoats using the blow-off technique. The ease with which a liquid polymer film moves across a surface when sheared is described by a shear mobility χS, which can be interpreted both in terms of continuum flow and in terms of wind driven diffusion. Generally, we find that the movement of PFPE films can be described as a flow process with an effective viscosity, even when the film thickness is smaller than the polymer’s diameter of gyration. Only in the special case of sparse coverage of a polymer with neutral end groups is the motion better described by a wind driven diffusion process. The addition of alcohol end groups to the PFPE polymer chain results in strong interactions with the substrate, creating a restricted layer having an effective viscosity an order of magnitude larger than the mobile layer that sits on top of the restricted layer.

Synthesis of GaNxAs1−x thin films by pulsed laser melting and rapid thermal annealing of N+-implanted GaAs

We present a systematic investigation on the formation of the highly mismatched alloy GaN{sub x}As{sub 1-x} using N{sup +}-implantation followed by a combination of pulsed laser melting and rapid thermal annealing. Thin films of GaN{sub x}As{sub 1-x} with x as high as 0.016 and an activation efficiency of the implanted N up to 50% have been synthesized with structural and optical properties comparable to films grown by epitaxial deposition techniques with similar substitutional N content. The effects of N{sup +} implantation dose, laser energy fluence and rapid thermal annealing temperature on the N incorporation as well as optical and structural properties of the GaN{sub x}As{sub 1-x} films are discussed.

Poly(fumaric–co-sebacic anhydride): A degradation study as evaluated by FTIR, DSC, GPC and X-ray diffraction

The degradation of three poly(fumaric-co-sebacic anhydride) [P(FA:SA)] copolymers is examined in a composition of microspheres made by the hot melt encapsulation process. The emergence of low molecular weight oligomers occurs during degradation of the copolymer microspheres, as evidenced by a variety of characterization methods. Characterization was conducted to determine the extent of degradation of the polyanhydride microspheres using Fourier-transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and X-ray diffraction. It is demonstrated that degradation of P(FA:SA) is greatly accelerated at basic pH, yet there is little difference between degradation in neutral and acidic buffers. A good correlation exists between the results of each characterization method, which allows a better understanding of the degradation process and the resulting formation of low molecular weight oligomers in poly(fumaric-co-sebacic anhydride).

New methodologies for measuring film thickness, coverage, and topography

We describe how the techniques of X-ray reflectivity (XRR), electron spectroscopy for chemical analysis (ESCA), and atomic force microscopy (AFM) can be used to obtain the structural parameters-thickness, coverage, and topography-of thin films used on magnetic recording disks. We focus on ultra-thin amorphous nitrogenated carbon (CNx) overcoats on disks. Each technique has its own strengths: XRR measures film thickness absolutely, ESCA determines the chemical composition of the films, and AFM maps topography accurately. For the CNx overcoats investigated, we find incomplete coverage for thicknesses less than 20 /spl Aring/, and we find a small surface roughness with rms roughness /spl les/11 /spl Aring/.

Lubricant spin-off from magnetic recording disks

As the rotation rate of magnetic recording disks increases over the next few years, lubricant spin-off from the disk surface may be significant. Lubricant thickness was measured as a function of spin time at 10 000 rpm on typical carbon overcoated magnetic recording disks initially lubricated with 10–135 Å of perfluoropolyether Zdol. The viscosity of the lubricant film increased as the film thickness decreased with spin time. Lubricant spin-off in response to air shear stress on the free surface was approximately described by viscous flow. The rate of lubricant removal by evaporation was compared to the spin-off removal rate in films between 10 and 50 Å thick. Dispersion interaction and chemisorption are expected to retain a molecularly thin film of lubricant on the disk surface.

Model of native point defect equilibrium in Cu2ZnSnS4 and application to one-zone annealing

We report a quasichemical model for point defect equilibrium in Cu2ZnSnS4 (CZTS). An ab initio calculation was used to estimate the changes in the phonon spectrum of CZTS due to trial point defects and further vibrational free energy, which in turn influences the final defect concentrations. We identify the dominant point defects and estimate the free carrier concentrations as functions of the Zn, Cu, and Sn chemical potentials, the sulfur chemical potential being set by the vapor-solid equilibrium with elemental S at the same temperature as the sample (one-zone annealing). As hinted by calculated low formation enthalpies, either the Cu vacancy (VCu−) or Cu on Zn antisite (CuZn−) acceptors are expected to dominate over a wide range of cation chemical potentials. However, the sulfur vacancy (VS2+) becomes a dominant compensating donor especially for one-zone annealing conditions. We also find that different native defects induce distinct perturbations to the vibrational free energy, resulting in non-trivia...