N. Newman

Defect energy levels and electronic behavior of Ni-, Co-, and As-doped synthetic pyrite (FeS2)

This work investigates the properties of Ni, Co, and As dopants in pyrite. Optical transmission spectroscopy and temperature-dependent Hall measurements were performed on doped pyrite crystals grown by chemical-vapor transport (CVT). The energy position(s) of the defect levels within the bandgap were determined from the optical spectrum. These values were then used to infer the concentration and occupancy of the defect levels from a statistical model fit to the temperature-dependent Hall concentration results. Doping pyrite with Ni atoms introduces partly filled, NiFe donor levels at 0.37 and 0.42u2009eV below the conduction band minimum (CBM). Doping with Co introduces a partially filled CoFe donor level at 0.09u2009eV below the CBM. Doping with As modifies the valence bands, which are of Fe d character. It reduces the gap slightly, and adds a hole. The Ni-doped, As-doped and undoped pyrite all have an absorption peak at 0.13u2009eV. Self-consistent GW electronic-structure calculations provide reliable conduction an...

ZnGeAs 2 thin films properties: A potentially useful semiconductor for photovoltaic applications

We have studied the chalcopyrite compound ZnGeAs<inf>2</inf> to access its potential as a novel photovoltaic material. Thin films of ZnGeAs2 have been produced by pulsed laser depositionThe films are deposited at 315 °C and are amorphous. They crystallize above 450 °C and improve in crystallinity up to and including 600 °C. At that temperature the Hall mobility is 55 ± 2 cm²/Vs , which is within uncertainty to the highest mobility ever reported for this material. We find a rather high carrier concentration, of order 10¹⁸ – 10¹⁹ for the annealed films, presumably due to the presence of Zn vacancies. The material is an effective light absorber, with an absorption coefficient of order 10⁴ 1/cm at 1.2 eV. These properties suggest ZnGeAs<inf>2</inf> may be used to produce cost effective and efficient solar cells.

Characterization of ZnGeAs 2 thin films produced by pulsed laser deposition

We have characterized the properties of chalcopyrite ZnGeAs2 thin films produced over a wide range of growth conditions using pulsed laser deposition. By using a Zn-enriched target, stoichiometric films could be produced up to a substrate temperature of 315° C; above which the films were Zn and As deficient. Optical absorption measurements indicate that bandgap of the ZnGeAs2 thin films is direct with a value of ∼1.15 eV. Hot point probe indicate that the as-deposited and annealed thin films are both p-type. Hall measurements confirm this and also indicate that the carrier mobility, μp, is over 50 cm2/V·sec in the 600° C annealed samples. Channeling Rutherford Backscattering Spectroscopy (RBS) indicates that the structurally best films are achieved after 450 °C annealing with a channeling yield, ?min, of 50%. Our results, in combination with the observation that the constituents are abundant elements, suggest that ZnGeAs2 is an ideal candidate for photovoltaic applications.