H. Neff

Bio: H. Neff is an academic researcher from North Carolina State University. The author has contributed to research in topics: Sputtering & Photoluminescence. The author has an hindex of 5, co-authored 7 publications receiving 108 citations.

Photoluminescence and photoconductivity of CuInSe2

Abstract: In this paper we report for the first time the results of photoluminescence (PL) and photoconductivity (PC) measurements on stoichiometric and well-defined off-stoichiometric copper indium diselenide. The energy gap at 10 K is 1.050\ifmmode\pm\else\textpm\fi{}0.005 eV and free-exciton emission at 1.030\ifmmode\pm\else\textpm\fi{}0.005 eV is the dominant feature in the PL spectra of stoichiometric nominally undoped ${\mathrm{CuInSe}}_{2}$. For Cu:In and metal:selenium ratios g1 recombination between Se-vacancy donors (-70 meV versus the conduction-band edge ${\mathrm{Cu}}_{\mathrm{In}}$ antisite acceptors [40 meV versus the valence-band edge (VBE)], and In-vacancy acceptors (80 meV versus the VBE), respectively, dominates the PL spectra. Excitation from the above two acceptor states contributes to the PC spectra with complimentary temperature dependence as compared to the deep emission.

Photoluminescence Studies of CuInS2-CuInSe2 Alloy Crystals

Abstract: Photoluminescence spectra are presented for single crystals of CuInS2, CuInSe2 and CuInS2ySe2-2y alloys. The PL spectrum of stoichiometric CuInSe2 is dominated by free exciton emission of 9 meV FWHM at 1.03 eV, but structure at 090, 0.94 and 0.97 eV is observed due to transitions involving residual native defect states. For pure CuInS2 broad deep luminescence bands are obtained that involve several deep native defect states, e.g. donors at 45 and 160 meV below the CBE and acceptors at 85 meV above the VBE. These defects persist in S-rich CuInS2/CuInSe2 alloys, but excitonic emission is observed, in addition to the deep luminescence. Surprisingly CuInS2ySe2-2y crystals at y ≳ l are totally dominated by free exciton emission. This result shows that an increase of at least 20% over the bandgap of CuInSe2 and excellent crystal quality can be achieved by partial substitution of Se by S.

Optical and electrochemical properties of CuInSe2 and CuInS2‐CuInSe2 alloys

Abstract: The fundamental optical transitions in single crystals of CuInS2xSe2−2x alloys have been studied by electrolyte electroreflectance (EER) spectroscopy. The band gap of the alloys increases nonlinearly with increasing sulphur content corresponding to a bowing parameter 0.14. The flatband potential derived from the EER spectra is in excellent agreement with differential capacitance measurements and determined as −0.350 V versus the saturated calomel electrode. CuInS2xSe2−2x liquid junction solar cells are reported that exhibit a long wavelength cutoff in the spectral response matching the EER data.

Sputtering and native oxide formation on (110) surfaces of Cd1−xMnxTe

Abstract: Native oxides on the surface of Cd(1-x)Mn(x)Te (X between 0 and 0.7) have been analyzed on the basis of X-ray photoemission spectroscopy measurements. Depth profile analysis revealed a significant increase in the thickness at higher Mn concentrations and a strong Mn segregation to the surface, respectively. Sputter-induced damage on cleaved (110)-oriented surfaces was analyzed by photoreflectance and photoluminescence measurements. The damage was found to be larger on CdTe than on the alloy. Thermal annealing showed nearly complete restoration for the surface of the alloy, while CdTe revealed irreversible modifications in the near-surface regime upon sputtering and post annealing.

The absolute energy positions of conduction and valence bands of selected semiconducting minerals

Abstract: The absolute energy positions of conduction and valence band edges were compiled for about 50 each semiconducting metal oxide and metal sulfide minerals. The relationships between energy levels at mineral semiconductor-electrolyte interfaces and the activities of these minerals as a catalyst or photocatalyst in aqueous redox reactions are reviewed. The compilation of band edge energies is based on experimental flatband potential data and complementary empirical calculations from electronegativities of constituent elements. Whereas most metal oxide semiconductors have valence band edges 1 to 3 eV below the H2O oxidation potential (relative to absolute vacuum scale), energies for conduction band edges are close to, or lower than, the H2O reduction potential. These oxide minerals are strong photo-oxidation catalysts in aqueous solutions, but are limited in their reducing power. Non-transition metal sulfides generally have higher conduction and valence band edge energies than metal oxides; therefore, valence band holes in non-transition metal sulfides are less oxidizing, but conduction band electrons are exceedingly reducing. Most transition-metal sulfides, however, are characterized by small band gaps (<1 eV) and band edges situated within or close to the H2O stability potentials. Hence, both the oxidizing power of the valence band holes and the reducing power of the conduction band electrons are lower than those of non-transition metal sulfides.

Quaternary CuIn(S1−xSex)2 Nanocrystals: Facile Heating-Up Synthesis, Band Gap Tuning, and Gram-Scale Production

Abstract: Tetragonal chalcopyrite CuIn-(S1−xSex)2 (0 ≤ x ≤ 1) nanocrystals were synthesized by reacting a mixture of CuCl, InCl3, S, and Se in the presence of oleylamine at 265 °C. The S/Se composition ratio in the CuIn(S1−xSex)2 could be tuned across the entire composition range of x from 0 to 1 by modulating the S/Se reactant mole ratio. The tetragonal lattice constants, that is, a and c, increase linearly with the increase of Se content, following Vegard’s law. The band gap energies of CuIn(S1−xSex)2 nanocrystals could be tuned in the range between 0.98 and 1.46 eV and change nonlinearly with respect to x, deriving a bowing parameter of 0.17 eV. In addition, the method developed in this study was scalable to achieve gram-scale production of stoichiometry-controlled CuIn(S1−xSex)2 and CuIn1−xGaxSe2 nanocrystals.

Phase Diagram and Optical Energy Gaps for CuInyGa1-ySe2 Alloys

Abstract: The T(y) phase diagram of the alloys CuInyGa1−ySe2, prepared by the chemical vapor deposition method, is obtained from X-ray diffraction and differential thermal analyses. It is found that in addition to the chalcopyrite structure, a zincblende In2Se3-rich phase and a zincblende Ga2Se3-rich plus liquid two-phase field, are obtained. Also it is found that the variation of the lattice parameter ratio c/a is not linear with composition but varies from 1.96 to 2.00 as y is increased from 0.0 to 0.6 and c/a being equal to 2.00 in the composition range 0.6 ⪅ y ⪅ 1.0. The energy gap values, obtained from optical absorption measurements, follow a second-order equation in y giving a downward bowing parameter of about 0.15 eV. Mittels Rontgenbeugung und differentieller Thermoanalyse wird das T(y)-Phasendiagramm von mit der CVD-Methode erhaltenen CuInyGa1−ySe2-Legierungen erhalten. Es wird zusatzlich zur Chalkopyritstruktur eine In2Se3-reiche Zinkblendephase und ein Zweiphasenfeld Ga2Se3-reiche Zinkblende plus Flussigkeit gefunden. Es wird ebenfalls gefunden, das die anderung des Gitterparameter-verhaltnisses c/a nicht linear in der Zusammensetzung ist, sondern von 1,96 bis 2,00 variiert, wenn y von 0,0 auf 0,6 ansteigt und c/a, gleich 2,00 im Zusammensetzungsbereich von 0,6 ⪅ y ⪅ 1,0 bleibt. Die Energieluckenwerte aus optischen Absorptionsmessungen folgen einer Gleichung zweiter Ordnung in y und ergeben einen nach unten gerichteten Bowingparameter von etwa 0,15 eV.