M. V. Yakushev

Origin of the deep center photoluminescence in CuGaSe2 and CuInS2 crystals

Photoluminescence (PL) of CuGaSe2 and CuInS2 single crystals, either as grown or Cu annealed, reveals a broad and clear deep emission band at hν≈Eg−0.6u200aeV. In both of these as-grown materials this band has a similar doublet structure with the two D1,D2 subbands separated by about 100 meV. After the Cu annealing all samples became highly compensated and an additional deep PL band (W band) appeared on the high energy side of these D bands. This suggests a closely similar origin of the emission for the both materials. By a straightforward model calculation we show that the changes in the shape and intensity of these emission bands—due to variation of temperature, excitation intensity or due to the Cu annealing—are well explained if we assume that the D1 and D2 PL subbands originate in the recombination between the closest and the second closest donor–acceptor pairs, with the essential ingredient of the emission center being an interstitial donor defect, i.e., either Cui or Gai in CuGaSe2 and Cui or Ini in Cu...

The role of deep donor–deep acceptor complexes in CIS-related compounds

Abstract Deep PL emission bands are observed in several samples of CuGaSe 2 , CuIn 0.5 Ga 0.5 Se 2 and CuInS 2 . In all these materials these bands have a closely similar structure. The D1 and D2 bands centered at hν =1.148xa0 and 1.042xa0eV in CuGaSe 2, at hν =0.948xa0 and 0.857xa0eV in CuIn 0.5 Ga 0.5 Se 2, and at hν =0.954xa0eV and 0.864xa0eV in CuInS 2 , respectively, are concluded to result from a donor–acceptor pair (DAP) recombination, such that the donor atom of the DAP occupies an interstitial position within the chalcopyrite lattice and the acceptor atom resides at a cation site (either In or Ga) next to it. The probable donor defect is identified as an interstitial Cu atom and the associated acceptor defect as a cation vacancy, i.e. either V In or V Ga . On the basis of the simple Coulombic interaction Z A Z D e 2 /(er) between the components of the DAP, additional deep bands D3, D4, D5,...,are predicted. In the present work we find these additional emissions experimentally in CuInS 2 and CuIn 0.5 Ga 0.5 Se 2 , but not in our CuGaSe 2 samples.

On the Shape of the Close-to-Band-Edge Photoluminescent Emission Spectrum in Compensated CuGaSe2

Photoluminescence (PL) properties of compensated as-grown and air-annealed CuGaSe2 single crystals, grown by the vertical Bridgman technique, in the edge emission spectral region were studied. The intensity maximum of the broad asymmetrical PL band at T= 8 K was found to be at hν max = 1.586 eV. After air annealing at 673 K for 15 min the PL band shifts towards higher energies, and its intensity slightly decreases but the shape remains the same. It is shown that this typical asymmetric PL band is not associated with a certain acceptor level but originates from the bandtail recombination. The valence band tail is formed by the potential fluctuations of charged defects. The average depth of these fluctuations is determined by the Debye-Huckel correlation in the distribution of donors and acceptors. The low-temperature air-annealing reduces the concentration of charged defects, but the sample remains highly compensated.

Photoluminescence and photoluminescence excitation study of CuGaTe2

Photoluminescence (PL) and photoluminescence excitation measurements over a temperature range 12u2009K<T<300u2009K on high quality CuGaTe2 crystals grown by the vertical Bridgman method were completed. The whole PL spectrum consists of two regions. The first region includes PL bands E1 at 1.431 eV, E2 at 1.426 eV, and E3 at 1.417 eV, together with their phonon replicas while in the second region we have a PL band D0 at 1.338 eV with its well-resolved LO-phonon replicas (ℏωLO=26.5u2009meV). All these PL bands appear to be at higher energy than the lowest (fundamental) band gap energy EgA. The possible origin of observed PL bands is discussed.

Switching of deep levels in CuInSe2 due to electric field-induced Cu ion migration

To search for the origin of the 0.26 eV deep hole trap, which is well established in p-type CuInSe2 single crystals and thin films, we investigated the influence of electric field-induced Cu ion migration on this level. We show that this hole trap can be removed from the space charge region of a reverse-biased Schottky barrier in p-type CuInSe2 single crystals by annealing at 490 K for 10 min. Simultaneously, a new hole trap at 0.49 eV from the valence band is created in this region. Conversely, the application of a forward bias under the same annealing conditions causes a decrease in the concentration of the 0.49 eV hole trap and a recreation of the 0.26 eV hole trap to the original state. These two processes were found to be reversible. Our results are completely consistent with recent evidence of an electric field-induced Cu ion migration via a vacancy mechanism in CuInSe2. According to a recent theoretical paper on the defect physics of CuInSe2, a candidate for experimentally observed 0.26 eV deep hol...

A Rutherford backscattering-channelling and Raman study of CuInSe2 single crystal surfaces

The (1 1 2) surfaces of Bridgman-grown p- and n-type CuInSe2 single crystals were prepared using different polishing, etching and annealing regimes. The surfaces were subsequently studied using Rutherford backscattering-channelling (2 MeV He+ ions) and Raman techniques. A layer of damage below the polished surface was produced after polishing with 3 Μm and 1 Μm diamond pastes and fine 0.05 Μm alumina slurry. The thickness of the damaged layer depends on the polishing grade and was found to be 30 nm after 0.05 Μm grade polishing. Subsequent etching in a 1% Br-methanol solution removed the damaged layer after 30s. However, the etching process produced what appeared to be an Se excess in a layer close to the surface. This excess can be dissipated by annealing; also, heat treatment of unetched samples at 400 ‡C was found to repair polish-induced damage.

A PL study of CIGS thin films implanted with He and D ions

Cu(InGa)Se2 thin films deposited on Mo coated soda-lime glass substrates by co-evaporation, were implanted with He 1 and D 1 (energies between 2.5 and 40 keV, doses from 10 14 to 3 £ 10 15 cm 22 ) at room temperature. Implanted and non-implanted areas of the films were characterised using low temperature photoluminescence (PL). A single broad band (A1) at 1.1 eV dominated in the PL spectra from the nonimplanted material. Also a long, low energy and low intensity tail with a well-defined band (A2) at 0.78 eV was detected. Implantation of either He or D reduced the band A1 intensity. Three high intensity low energy peaks IH1, IH2, and IH3 were observed in the PL spectra in both cases. One of the peaks (IH3) was detected at 0.78 eV. An increase in either dose or energy of implantation was found to increase the intensity of IH2 and IH3 normalised to that of A1. Similarities between the low energy bands observed after either He or D implantation suggests that all the three bands have intrinsic origins. It is speculated that the implanted ions accumulate in small bubbles or diffuse to the surface and grain boundaries. The characteristics of bands A2 and IH3 suggest that the defects responsible for the emission were present in the films before implantation. Lattice recovery processes leading to an overall reduction in the primary implant-related defect population could produce additional defects, which are associated with the low-energy PL peaks. q 2000 Elsevier Science S.A. All rights reserved.

Characterization of CuInTe2 thin films prepared by pulsed laser deposition

Abstract In this paper initial results are presented for the growth and characterization of CuInTe 2 thin films prepared by pulsed laser deposition (PLD) using Nd:YAG laser and XeCl excimer laser. The films were analyzed using X-ray diffraction (XRD), energy dispersive X-ray analysis (EDAX) and Rutherford back scattering (RBS) measurements. The composition of the target material was largely maintained in the films. This suggests that PLD could be used as a technique for the fabrication of ternary semiconductor films. Electrical measurements indicated p-type conductivity with surface resistivity values in the range of 10 −1 –10 −2 Ω cm. The films had a high optical absorption of about 104 cm −1 and a band gap of 0.96 eV.

Photoacoustic spectroscopy use in the analysis of ion‐implanted CuInSe2 single crystals

A high‐resolution near‐infrared photoacoustic spectrometer of the gas–microphone type is used for room‐temperature analysis of the defect states in ion‐implanted CuInSe2 single crystals. A wide range of ions differing in their masses as well as in their electrical activities in the host crystal (O+, Ne+, Cu+, Xe+, and Li+) have been implanted at various energies and doses to assess the technique sensitivity to detect changes affecting the compound subgap absorption spectrum, which is characteristic of impurities. The detected changes (either by the appearance of new peaks or by a change in the impurity or defect concentrations) are shown to differ from one ion species to another. The depth profiling capability of the technique in analyzing implanted samples is also discussed. The results obtained here are correlated to existing published data. It is shown that photoacoustic spectrometry can be very useful if used to appraise the complex defect structure of ion‐implanted CuInSe2.

Photoluminescence study of deep levels in CuGaTe2 crystals

A deep photoluminscence band at 0.95 eV was studied in CuGaTe2 crystals. The shape of this band did not change with laser power and no j shift was detected. This band shifts towards higher energy with increasing temperature and its shape becomes more asymmetric. The activation energy of this band was ET=0.20u200aeV as measured with thermal quenching. The possible association of the 0.95 eV band with grain boundaries or dislocations is discussed.