Rongzhen Chen

Dielectric function spectra at 40 K and critical-point energies for CuIn0.7Ga0.3Se2

We report ellipsometrically determined dielectric function e spectra for CuIn0.7Ga0.3Se2 thin film at 40 and 300 K. The data exhibit numerous spectral features associated with interband critical points (CPs) in the spectral range from 0.74 to 6.43 eV. The second-energy-derivatives of e further reveal a total of twelve above-bandgap CP features, whose energies are obtained accurately by a standard lineshape analysis. The e spectra determined by ellipsometry show a good agreement with the results of full-potential linearized augmented plane wave calculations. Probable electronic origins of the CP features observed are discussed.

Band-edge density-of-states and carrier concentrations in intrinsic and p-type CuIn1−xGaxSe2

The electronic structures of chalcopyrite CuIn1−xGaxSe2 have recently been reported to have strongly anisotropic and non-parabolic valence bands (VBs) even close to the Γ-point VB maximum. Also, the lowest conduction band (CB) is non-parabolic for energies 50–100 meV above the CB minimum. The details in the band-edge dispersion govern the materials electrical properties. In this study, we, therefore, analyze the electronic structure of the three uppermost VBs and the lowest CB in CuIn1−xGaxSe2 (x = 0, 0.5, and 1). The parameterized band dispersions are explored, and the density-of-states (DOS) as well as the constant energy surfaces are calculated and analyzed. The carrier concentration and the Fermi energy EF in the intrinsic alloys as functions of the temperature is determined from the DOS. The carrier concentration in p-type materials is modeled by assuming the presence of Cu vacancies as the acceptor type defect. We demonstrate that the non-parabolicity of the energy bands strongly affects the total ...

Electronic and optical properties of Cu2XSnS4 (X = Be, Mg, Ca, Mn, Fe, and Ni) and the impact of native defect pairs

Reducing or controlling cation disorder in Cu2ZnSnS4 is a major challenge, mainly due to low formation energies of the anti-site pair ( Cu Zn − +  Zn Cu +) and the compensated Cu vacancy ( V Cu − +  Zn Cu +). We study the electronic and optical properties of Cu2XSnS4 (CXTS, with X = Be, Mg, Ca, Mn, Fe, and Ni) and the impact of defect pairs, by employing the first-principles method within the density functional theory. The calculations indicate that these compounds can be grown in either the kesterite or stannite tetragonal phase, except Cu2CaSnS4 which seems to be unstable also in its trigonal phase. In the tetragonal phase, all six compounds have rather similar electronic band structures, suitable band-gap energies Eg for photovoltaic applications, as well as good absorption coefficients α(ω). However, the formation of the defect pairs ( C u X +  X Cu) and ( V Cu +  X Cu) is an issue for these compounds, especially considering the anti-site pair which has formation energy in the order of ∼0.3 eV. The ( ...