Defect physics of CuGaS2 (original) (raw)

Electronic and phononic properties of the chalcopyrite CuGaS2

Physical Review B, 2011

The availability of ab initio electronic calculations and the concomitant techniques for deriving the corresponding lattice dynamics have been profusely used for calculating thermodynamic and vibrational properties of semiconductors, as well as their dependence on isotopic masses. The latter have been compared with experimental data for elemental and binary semiconductors with different isotopic compositions. Here we present theoretical and experimental data for several vibronic and thermodynamic properties of CuGaS2, a canonical ternary semiconductor of the chalcopyrite family. Among these properties are the lattice parameters, the phonon dispersion relations and densities of states (projected on the Cu, Ga, and S constituents), the specific heat and the volume thermal expansion coefficient. The calculations were performed with the ABINIT and VASP codes within the LDA approximation for exchange and correlation and the results are compared with data obtained on samples with the natural isotope composition for Cu, Ga and S, as well as for isotope enriched samples.

First-principles study of defects at Σ3 grain boundaries in CuGaSe2

Solid State Communications, 2021

We present a first-principles computational study of cation-Se Σ3 (112) grain boundaries in CuGaSe 2. We discuss the structure of these grain boundaries, as well as the effect of native defects and Na impurities on their electronic properties. The formation energies show that the defects will tend to form preferentially at the grain boundaries, rather than in the grain interiors. We find that in Ga-rich growth conditions Cu vacancies as well as Ga at Cu and Cu at Ga antisites are mainly responsible for having the equilibrium Fermi level pinned toward the middle of the gap, resulting in carrier depletion. The Na at Cu impurity in its +1 charge state contributes to this. In Ga-poor growth conditions, on the other hand, the formation energies of Cu vacancies and Ga at Cu antisites are comparatively too high for any significant influence on carrier density or on the equilibrium Fermi level position. Thus, under these conditions, the Cu at Ga antisites give rise to a p-type grain boundary. Also, their formation energy is lower than the formation energy of Na at Cu impurities. Thus, the latter will fail to act as a hole barrier preventing recombination at the grain boundary, in contrast to what occurs in CuInSe 2 grain boundaries. We also discuss the effect of the defects on the electronic properties of bulk CuGaSe 2 , which we assume reflect the properties of the grain interiors.

Germanium doping of wider-band-gap CuGaSe2 chalcopyrites: Local and electronic structure

We present here a complementary study on germanium doping of the wider-band-gap CuGaSe2 (CGS) chalcopyrite. In photoluminescence studies, the occurrence of a new emission line was identified as Ge related and explained as a donor-acceptor-pair recombination. The precise role the Ge is playing in this doping of CGS is revealed by x-ray absorption spectroscopy and ab initio calculations based on the density-functional theory. Extended x-ray absorption fine-structure spectroscopy (EXAFS) as well as x-ray absorption near-edge spectroscopy performed at the Ge K-, Cu K-, and Ga K-edge show that the Ge dopants occupy the cationic sites of GeCu or GeGa of the host lattice. The complementary ab initio calculations support the EXAFS results. They further indicate that the incorporated Ge atoms preferentially occupy Ga sites when relaxation around the dopant is taken into account. Additionally, our corresponding theoretical band-structure model predicts the existence of additional localized electronic acceptor and donor defect bands within the band gap of CuGaSe2 originating from a strong covalent interaction between Ge 4s and Se 4p states for Ge atoms tetrahedrally surrounded by the Se nearest-neighbor atoms. A theoretically predicted antibonding Ge-Se 4sp3 defect band appearing well above the Fermi level for the GeGa 1+ point-defect system can be directly linked to a Ge-dopantrelated donor-acceptor-pair transition as observed in our photoluminescence spectra.

Band Alignment of the CuGaS2 Chalcopyrite Interfaces Studied by First-Principles Calculations

ACS Omega, 2020

The valence and conduction band offsets for both polar and nonpolar CuGaS 2 / CuAlSe 2 and CuGaS 2 /ZnSe interfaces were studied here by the state-of-the-art first-principles calculations. Using the hybrid functional calculations, we show that the CuGaS 2 /CuAlSe 2 and CuGaS 2 /ZnSe heterostructures in all interfaces form type II band alignment. The difference of valence and conduction band offsets is mainly due to lattice mismatch, generating stress in the interface and affecting the electronic properties of each material; meanwhile, the polarity configuration does not play an important role in these values. From the local density of states and the charge density, we can determine how the nature of the band alignments changes when the semiconductor conforms to each interface. This allows us to localize the electrons and holes at different sites of the interface.

Defect physics of the CuInSe2 chalcopyrite semiconductor

Journal of Physics and Chemistry of Solids, 1999

The activation energies of acceptor E A and donor levels E D in the chalcopyrite compound CuInSe 2 are calculated by using a simpler model based in the effective-mass theory for the case of single, double and triple point defect centers. Despite of the simplicity of this model, it is found that the values of E A and E D thus calculated for shallow and deep levels are in reasonable agreement with those reported from the experimental data. In the case of not shallow donor levels values of E D in good agreement with these data are calculated by using the free electron mass m 0 instead of the effective electron mass. From the analysis of the results, most of these levels have been identified as due to the presence of several native point defects.

A systematic approach to investigate electronic and optical property of CuGaS 2 using DFT

Present paper is a systematic analysis of electronic and optical properties of CuGaS 2 ternary chalcopyrite semiconductor which having wide applications in area of optoelectronics. Complete knowledge of electronic structural behavior of these materials is a challenging part for researchers. All the calculations presented here are preformed using mBJ exchange correlation potential under density functional theory (DFT) framework and embodied in WIEN2K computer code. Our investigation indicates that CuGaS 2 have direct band gap of 2.071eV and there absorption spectra shows good absorptive nature over wide solar spectrum range (0-5eV) which indicates suitability of this material in optoelectronic applications mostly in solar cell. All electronic and optical properties calculated in this paper poses good agreement with experimental result.

Defect levels in the epitaxial and polycrystalline CuGaSe2 by photocurrent and capacitance methods

Journal of Applied Physics, 2011

The defect levels in epitaxial and polycrystalline wide bandgap chalcopyrite CuGaSe 2 with various stoichiometry deviations were investigated using modulated photocurrent spectroscopy. The results were analyzed as a function of light intensity and Fermi-level position. Comparison of the results from epitaxial and polycrystalline material distinguished levels belonging to intrinsic defects and their correlation with the material stoichiometry. We also compared the fingerprints of defect levels by MPC to the results derived from capacitance spectroscopy performed on Schottky diodes fabricated on both epitaxial and polycrystalline layers. This allowed us to attribute unambiguously levels observed in the capacitance response to bulk point defects. In the final conclusions we provide information on the electronic parameters of nine defect levels observed in CuGaSe 2 and their correlation with the material stoichiometry. These results should help to identify intrinsic defects that are important for the photovoltaic performance of solar cells based on chalcopyrites.

The effect of Na on the defect structure in CuGaSe/sub 2/ grown by molecular beam epitaxy

Conference Record of the Thirty-first IEEE Photovoltaic Specialists Conference, 2005., 2005

Epitaxial CGS films were gram on (100) GaAs as a function of CulGa ratio with and without Na dosing. All asgrown films showed good crystalline quality as evidenced by the FWHM value of the *rocking curve for the (008) reflection being less than 300 arcsec for all films. In particular, the lowest value ever reported for CGS (80 arcsec) was measured for a Nadoped, Cu-rich CGS film. TEM analysis revealed pseudomorphic growth for all films with the strain believed to be released by the formation of twins in localized regions in the sample. From PL spectra at 10 K, donor-acceptor transitions were associated with Vm-Vs, and CUG-VS~ defects as confirmed from their activation energies. A new defect level related to Na, which was tentatively assigned to Naa, was observed for Cu-rich, Nadosed CGS film and its activation energy was proposed as 20 meV above the valence band edge.