Timothy A. Gessert

High-mobility, sputtered films of indium oxide doped with molybdenum

Thin films of molybdenum-doped indium oxide, an n-type transparent conducting oxide, were deposited on glass substrates by a large-area deposition technique, radio-frequency magnetron sputtering, and their electrical properties were examined. Molybdenum content was varied from 1 to 4 wt%, and the highest mobility achieved was 83 cm2 V−1 s−1 at a carrier concentration of 3.0×1020 cm−3 without any postdeposition treatment for one of the films made from the target with 2 wt% Mo. Temperature-dependent Hall analysis indicated that this high mobility is limited by phonon scattering, whereas the method of four coefficients analysis showed that the conduction band is parabolic.

Development of rf sputtered, Cu-doped ZnTe for use as a contact interface layer to p-CdTe

Cu-doped ZnTe films deposited by rf-magnetron sputtering have been analyzed with the intention to use this material as a contact interface in CdS/CdTe thin-film photovoltaic solar-cell devices. It is observed that unless careful attention is made to the pre-deposition conditioning of the ZnTe target, the electrical resistivity of thin films (∼70 nm) will be significantly higher than that measured on thicker films (∼1.0 μm). It is determined that N contamination of the target during substrate loading is likely responsible for the increased film resistivity. The effect of film composition on the electrical properties is further studied by analyzing films sputtered from targets containing various Cu concentrations. It is determined that, for targets fabricated from stoichiometric ZnTe and metallic Cu, the extent of Zn deficiency in the film is dependent on both sputtering conditions and the amount of metallic Cu in the target. It is observed that the carrier concentration of the film reaches a maximum value of ∼3 × 1020 cm−3 when the concentrations of Te and (Zn+Cu) are nearly equal. For the conditions used, this optimum film stoichiometry results when the concentration of metallic Cu in the target is ≈6 at.%.

Transparent conducting zinc oxide thin films doped with aluminum and molybdenum

Undoped ZnO, ZnO:Al (0.5, 1, and 2wt% Al2O3), and ZnO:Mo (2wt% Mo) films were deposited by radio-frequency magnetron sputtering. Optimal deposition temperature was found to be ∼200°C for all films. Electron mobilities of 48cm2V−1s−1 were achieved for undoped ZnO films using a sputtering gas with H2∕Ar ratio of 0.3%; corresponding carrier concentrations were ∼3×1019cm−3. A target incorporating 0.5wt% Al2O3 in ZnO yielded films with mobility of 36cm2V−1s−1 and carrier concentration of 3.4×1020cm−3. These films present comparable conductivity and lower free-carrier absorption than films grown from a target containing 2wt% Al2O3. Mo was found to be an n-type dopant of ZnO, though electrical and optical properties were inferior to those of ZnO:Al. Temperature-dependent Hall measurements of ZnO:Al films show evidence of a different scattering mechanism than ZnO:Mo films.

Intrinsic surface passivation of CdTe

Recombination is critically limiting in CdTe devices such as solar cells and detectors, with much of it occurring at or near the surface. In this work, we explore different routes to passivate p-type CdTe surfaces without any intentional extrinsic passivation layers. To provide deeper insight into the passivation routes, we uniquely correlate a set of characterization methods: surface analysis and time-resolved spectroscopy. We study two model systems: nominally undoped single crystals and large-grain polycrystalline films. We examine several strategies to reduce surface recombination velocity. First, we study the effects of removing surface contaminants while maintaining a near-stoichiometric surface. Then we examine stoichiometric thermally reconstructed surfaces. We also investigate the effects of shifting the surface stoichiometry by both “subtractive” (wet chemical etches) and “additive” (ampoule anneals and epitaxial growth) means. We consistently find for a variety of methods that a highly ordered ...

Charge-carrier transport and recombination in heteroepitaxial CdTe

We analyze charge-carrier dynamics using time-resolved spectroscopy and varying epitaxial CdTe thickness in undoped heteroepitaxial CdTe/ZnTe/Si. By employing one-photon and nonlinear two-photon excitation, we assess surface, interface, and bulk recombination. Two-photon excitation with a focused laser beam enables characterization of recombination velocity at the buried epilayer/substrate interface, 17.5 μm from the sample surface. Measurements with a focused two-photon excitation beam also indicate a fast diffusion component, from which we estimate an electron mobility of 650 cm2 (Vs)−1 and diffusion coefficient D of 17 cm2 s−1. We find limiting recombination at the epitaxial film surface (surface recombination velocity Ssurface = (2.8 ± 0.3) × 105 cm s−1) and at the heteroepitaxial interface (interface recombination velocity Sinterface = (4.8 ± 0.5) × 105 cm s−1). The results demonstrate that reducing surface and interface recombination velocity is critical for photovoltaic solar cells and electronic d...

Spectrally and time resolved photoluminescence analysis of the CdS/CdTe interface in thin-film photovoltaic solar cells

Light absorption and charge separation in thin-film polycrystalline cadmium telluride (CdTe) photovoltaic (PV) solar cells largely occur in the vicinity of the CdS/CdTe interface. Sulfur alloying at this interface to form CdSxTe1-x and doping with Cu appear to be important for efficient PV devices. Based on the different band gaps of CdSxTe1-x and CdTe, we apply spectroscopic and computational photoluminescence (PL) analysis to characterize this interface. We find that Cu concentration changes the dynamics of charge separation and PL emission intensities from the CdSxTe1-x and CdTe regions. We have determined charge separation lifetime and minority carrier lifetime, and we have estimated minority carrier mobility as <100 cm2 V−1 s−1.

Carrier diffusion and radiative recombination in CdTe thin films

We employed cathodoluminescence spectroscopy and imaging to investigate the carrier diffusion and radiative recombination in CdTe thin films. We observed that carriers excited by the electron beam diffuse by excitons or by free electrons via donor states at low temperatures. The distribution and concentration of these states in the CdTe films were estimated from the dependence on the excitation level of the donor-to-acceptor radiative recombination.

Evaporated Te on CdTe: A vacuum-compatible approach to making back contact to CdTe solar cell devices

A commonly used process for forming low-resistance contacts to thin-film p-type CdTe involves the formation of a Te layer by etching the CdTe film in a concentrated mixture of nitric and phosphoric acids. The authors compare evaporated Te back contacts with ‘control’ back contacts formed by the usual etching process, and demonstrate that evaporating Te onto a CdTe thin film is a viable process for forming a low-resistance contact. The best efficiency achieved for a CdTe solar cell made with an evaporated Te back contact is 12.1%, whereas the efficiency of the device made with the control back contact was 11.9%. The evaporation process offers numerous advantages over acid etching, most notably vacuum compatibility amenable to large-scale production of CdTe solar cell modules.

Discussion of some “trap signatures” observed by admittance spectroscopy in CdTe thin-film solar cells

Considerable ambiguity and controversy exist concerning the defect signatures (H1, H2, and H3) frequently observed in admittance spectroscopy of thin-film CdTe solar cells. We prove that the commonly labeled H1 defects, observed in all devices in this study, are actually due to the freeze-out of the majority carriers in the neutral CdTe absorber. This freeze-out is evident in the temperature dependencies of capacitance, carrier concentration, and depletion region width. Contrary to intuitive expectation, the activation energy of freeze-out is less than, not identical to, that of the conductivity. In some other cases, H2 or H3 are observed and attributed to the back-contact potential barrier, rather than to the carrier emission from the traps. We extract the back-contact barrier height from the activation energy of the saturation current determined from the temperature-dependent current-voltage curves using the back-to-back diode model. The back-contact barrier height agrees well with the H2 or H3 energy d...

Effects of hydrogen ambient and film thickness on ZnO:Al properties

Undoped ZnO and ZnO:Al (0.1, 0.2, 0.5, 1.0, and 2.0wt.% Al2O3) films were deposited by rf magnetron sputtering. Controlled incorporation of H2 in the Ar sputtering ambient for films grown at substrate temperatures up to 200°C results in mobilities exceeding 50cm2V−1s−1 when using targets containing 0.1 and 0.2wt.% Al2O3. Temperature-dependent Hall measurements show evidence of phonon scattering as the dominant scattering mechanism in these lightly Al-doped films, while ionized impurity scattering appears increasingly dominant at higher doping levels. A combination of compositional and structural analysis shows that hydrogen expands the ZnO lattice normal to the plane of the substrate and desorbs from ZnO at ∼250°C according to temperature-programmed desorption and annealing experiments.