CuInSe2 Single Crystals Research Articles

Near-infrared optical analysis of CuInSe2(1±x) crystals using transmission, photoacoustic and transmission-photoacoustic spectroscopies

We report on the near infrared optical absorption properties of CuInSe2(1±x) single crystals in which small deviations (x) are introduced into the valence stoichiometry prior to the crystal growth using three spectroscopic techniques: transmission, photoacoustic and transmission-photoacoustic. Monocrystalline samples were cut from ingots grown using the Bridgman technique with the compositions: CuInSe2, CuInSe1.787, CuInSe2.084 and CuInSe2.212. The phases and crystallographic structures were checked from their corresponding powders using X-ray diffraction. The optical properties were measured in photon energy range 0.7 to 1.2 eV. A comparison between the extracted absorption spectral distributions from different measurement methods is presented. Resolved peaks at photon energies higher than the compound bandgap close to the conduction/valence bands due to inter-bands transitions are detected from photoacoustic measurements. Also discussed are the defect levels observed in the spectra tail following the alloy starting composition change. It is shown that the sensitivity of photoacoustic operational modes to detect changes due to modification in samples’ composition is higher than the transmission technique. Differences in spectral behavior with particular attention being given to the detected defects from different measuring operational modes are discussed.

Deep defect levels in CuInSe2 single crystals using DLTS, MCTS and photoacoustic spectroscopy

In this paper, we investigate the presence of deep defect levels in as-grown p-conducting CuInSe2 single crystals using deep level transient spectroscopy, minority carrier transient spectroscopy and photoacoustic spectroscopy. The samples were cut from the middle of ingots grown by employing a vertical displaced Bridgman furnace. This material is attracting increasing interest in solar cells fabrication as the absorber layer. Aluminum Schottky contacts were deposited on the as-grown and etched material in an evaporation system pumped down to less than 10−6 mbar. Three majority carrier traps were observed using deep level transient spectroscopy, and one minority carrier trap was detected by minority carrier transient spectroscopy. The concentrations of the traps have been estimated. The results hence obtained are compared to deep defect levels observed in the measured spectral dependence of the samples prior to junction’s fabrication by using the technique of photoacoustic spectroscopy in the photon energy range 0.6 to 1.3 eV. The study shows a good agreement between the results of DLTS/MCTS to those of photoacoustic and to published data.

X‐Ray Fluorescence Holography Analysis of Local Structure in CuInSe2 and CuGaSe2

X‐ray fluorescence holography (XFH) is carried out using a synchrotron radiation on CuInSe2 and CuGaSe2 single crystals, which is powerful for the investigation of local atomic structure by three‐dimensional atom image. Incident X‐ray energies (9.7–12.7 keV) are used to excite the Cu Kα fluorescent X‐ray. Atom images are regenerated by Fourier transformation. In the cation plane in CuInSe2 (z = 0), the cation atom spots observed are the 2nd (In), 4th (Cu), 8th (Cu), and 10th (In) – nearest neighbor (NN) of the central Cu atom. For the anion plane (z = c/8) of CuInSe2, Se atoms spots, 1st and 3rd NN of Cu, are observed. Se atom spots are seen to deviate from the regular tetrahedral position due to the bond length difference between Cu‐Se bond and In‐Se bond. In the CuGaSe2 epitaxial layer (300 nm) grown on (100) GaAs, the cation plane exhibits clear atom spots of Cu and Ga atoms up to10th (Ga) NN of Cu. In the anion plane (z = c/8), only 1st NN Se atom spots are observed, and those of 3rd NN atom spots are not been observed. These results are discussed with relation to the perturbation acting on the Se atoms.

Detailed absorption analysis from photoacoustic measurements of post-annealed CuInSe2 in various environments

In this paper, we analyze the optical absorption properties of CuInSe2 single crystals in the photon energy range 0.8 to 1.1 eV from measurements carried out at ambient temperature using the photoacoustic spectroscopy technique following post-annealing processes at the temperature range 100 to 300 °C for a period of time of 30 min in various environments (vacuum, air and neutral). These are usually the most used media for thermal treatment. As-prepared thin films semiconductor materials usually require post-annealing in order to improve their structure and morphology to enhance their electronic properties. Despite the low temperatures and short period of the process, post-annealing is shown that it has an important impact on the absorption spectral behaviour of the compound. The potential of photoacoustic depth profiling operational mode carried out at two modulating frequencies (112 and 185 Hz) has been used to check on the samples’ homogeneity in respect to depth defect populations distributions. A shift by 65 meV in the upper limit of the absorption coefficient spectrum due to a higher inter-band transition has been observed following the processes of post-annealing in air and in vacuum, whereas the process when carried out in a neutral environment the samples keep their band edge as prior to the treatment.

Diffusion of sodium in single crystal CuInSe2

The incorporation of sodium from sodium fluoride in single-crystal CuInSe2 (CIS) is investigated to provide insight into the intra-granular aspects of sodium incorporation in CIS-based thin films. Sodium was incorporated by evaporating NaF onto two CIS crystals of varying compositions and defect structures followed by heating under vacuum. Diffusion profiles show a near-surface reaction before a deeper diffusion zone which follows a complementary error function, confirming Na diffusion into the crystals. Transmission electron microscopy analysis indicates that dislocations do not control the diffusion process. The activation energy of diffusion is ∼0.7 eV for both crystals. This low activation energy suggests that Na diffusion occurs rapidly through the bulk at temperatures as low as 300 °C and helps explain the uniform Na concentration often observed in grain interiors of polycrystalline Cu(InGa)Se2 thin films.

RBS-channeling study of radiation damage in Ar+ implanted CuInSe2 crystals

Chalcopyrite solar cells are reported to have a high tolerance to irradiation by high energy electrons or ions, but the origin of this is not well understood. This work studies the evolution of damage in Ar+-bombarded CuInSe2 single crystal using Rutherford backscattering/channeling analysis. Ar+ ions of 30 keV were implanted with doses in the range from 1012 to 3 × 1016 cm−2 at room temperature. Implantation was found to create two layers of damage: (1) on the surface, caused by preferential sputtering of Se and Cu atoms; (2) at the layer of implanted Ar, possibly consisting of stacking faults and dislocation loops. The damage in the second layer was estimated to be less than 2% of the theoretical prediction suggesting efficient healing of primary implantation defects.

A photoluminescence study of CuInSe2 single crystals ion implanted with 5 keV hydrogen

CuInSe2 single crystals ion implanted with 5 keV hydrogen at doses from 3 × 1014 to 1016 cm−2 are studied by photoluminescence (PL). The PL spectra before and after implantation reveal two bands, a main dominant band centred at 0.96 eV and a lower intensity band centred at 0.93 eV. Detailed analysis of the shape of these bands, their temperature and excitation intensity dependencies allow the recombination mechanisms to be identified as band-to-tail (BT) and band-to-impurity (BI), respectively. The implantation causes gradual red shifts of the bands increasing linearly with the dose. The average depth of potential fluctuations is also estimated to increase with the dose and saturates for doses above 1015 cm−2. A model is proposed which associates the potential fluctuations with the antisite defects copper on indium site and indium on copper site. The saturation is explained by full randomization of copper and indium atoms on the cation sub-lattice.

A photoluminescence study of excitonic grade CuInSe2 single crystals irradiated with 6 MeV electrons

High-quality single crystals of CuInSe2 with near-stoichiometric elemental compositions were irradiated with 6 MeV electrons, at doses from 1015 to 3 × 1018 cm−2, and studied using photoluminescence (PL) at temperatures from 4.2 to 300 K. Before irradiation, the photoluminescence spectra reveal a number of sharp and well resolved lines associated with free- and bound-excitons. The spectra also show broader bands relating to free-to-bound transitions and their phonon replicas in the lower energy region below 1.0 eV. The irradiation with 6 MeV electrons reduces the intensity of the free- and the majority of the bound-exciton peaks. Such a reduction can be seen for doses above 1016 cm−2. The irradiation induces new PL lines at 1.0215 eV and 0.9909 eV and also enhances the intensity of the lines at 1.0325 and 1.0102 eV present in the photoluminescence spectra before the irradiation. Two broad bands at 0.902 and 0.972 eV, respectively, are tentatively associated with two acceptor-type defects: namely, interstitial selenium (Sei) and copper on indium site (CuIn). After irradiation, these become more intense suggesting an increase in the concentration of these defects due to irradiation.

A study on the effect of oxygen implants in CuInSe2 by photoacoustic spectroscopy

This paper presents the results of an analysis on defect states changes following the irradiation of oxygen in CuInSe2 single crystals by using photoacoustic spectroscopy. CuInSe2 samples, n‐type conducting, of high quality grown by using the vertical Bridgman technique have been implanted at ambient temperature with O+ with the energy of 40 keV with doses of 1015 and 1016 ions/cm2. A theoretical model based on two‐layer samples has been used to extract the absorption spectrum of only the implanted layer from that of the bulk. Oxygen is found to create a shallow defect at 31meV and a deep one at 256±2 meV. It has also led to the disappearance of some other defect levels originally detected in the samples prior to implantation.

Ion channeling in CuInSe2 single crystals

CuInSe2 single crystals are studied by the Rutherford Backscattering of 2MeV helium ions involving axial channeling along the 〈221〉 direction of the chalcopyrite lattice. Experimental values of the critical angle and normalized minimum yield are measured and compared with theory. The concentration of point defects separately in the Cu, In, and Se sublattices, determined from the comparison of experimental and theoretical minimum yields, are estimated as 4.7×1020, 1.0×1020, and 5.7×1020cm−3, respectively.

A photoelectron spectroscopy study of the electronic structure evolution in CuInSe2-related compounds at changing copper content

Evolution of the valence-band structure at gradually increasing copper content has been analysed by x-ray photoelectron spectroscopy (XPS) in In2Se3, CuIn5Se8, CuIn3Se5, and CuInSe2 single crystals. A comparison of these spectra with calculated total and angular-momentum resolved density-of-states (DOS) revealed the main trends of this evolution. The formation of the theoretically predicted gap between the bonding and non-bonding states has been observed in both experimental XPS spectra and theoretical DOS.

Excitation power and temperature dependence of excitons in CuInSe2

Excitonic recombination processes in high quality CuInSe2 single crystals have been studied by photoluminescence (PL) and reflectance spectroscopy as a function of excitation powers and temperature. Excitation power dependent measurements confirm the identification of well-resolved A and B free excitons in the PL spectra and analysis of the temperature quenching of these lines provides values for activation energies. These are found to vary from sample to sample, with values of 12.5 and 18.4 meV for the A and B excitons, respectively, in the one showing the highest quality spectra. Analysis of the temperature and power dependent PL spectra from the bound excitonic lines, labelled M1, M2, and M3 appearing in multiplets points to a likely assignment of the hole involved in each case. The M1 excitons appear to involve a conduction band electron and a hole from the B valence band hole. In contrast, an A valence band hole appears to be involved for the M2 and M3 excitons. In addition, the M1 exciton multiplet seems to be due to the radiative recombination of excitons bound to shallow hydrogenic defects, whereas the excitons involved in M2 and M3 are bound to more complex defects. In contrast to the M1 exciton multiplet, the excitonic lines of M2 and M3 saturate at high excitation powers suggesting that the concentration of the defects involved is low.

Optical properties of xenon implanted CuInSe2 by photoacoustic spectroscopy

A theoretical relation is derived for the normalized photoacoustic amplitude signal of a gas-coupled cell for the case of double-layer solid samples with particular application given to ion implanted semiconductors. Numerical estimates for a solar cell of the type CdS/CuInSe2 based on experimental measured data of these compounds are given to illustrate the photoacoustic effect originating from double-layer samples. In application to ion implanted semiconductors, we show that the absorption coefficient of the implanted layer can be very easily extracted by photoacoustic spectroscopy if the absorption coefficient of the untreated substrate is known. We also present the optical properties results obtained from the analysis of the effect of xenon implantation into CuInSe2 single crystals with the energy of 40keV and a dose of 5×1016ions/cm2.

Xe irradiation-induced defects in CuInSe2 by phase resolved photoacoustic spectroscopy

We report a study on the optical properties of 40 keV Xe+ implants with a dose of 5 × 1016 ions/cm2 into p-type conducting CuInSe2 single crystals using the phase resolved method of the photoacoustic spectroscopy (PAS) technique. Photoacoustic spectra have been measured in the photon energy range 0.7 < hν < 1.4 eV prior and after implantation at various phase angles using a high resolution fully computerized spectrometer. Once the spectra separation is carried out, an analysis on the impact of Xe+ on the defect structure of CuInSe2 is presented. The results obtained here are discussed in the light of current reported literature.

Incorporation of hydrogen in CuInSe2: Improvements of the structure

CuInSe2 single crystals were ion implanted with a dose of 3 × 1016 cm−2 by 2.5 keV H+ at 150 and 250 °C. Before and after the implantation the crystals were analyzed by Rutherford backscattering/channeling (RBS/C) along the 〈112〉 axis using 2 MeV He+. The RBS/C spectra indicate that the implantation at 150 °C introduces a layer of radiation damage, whereas after the implantation at 250 °C no structural deterioration of the lattice can be seen. Quite the contrary, the RBS/C spectra reveal a considerable decrease in the dechanneling parameters suggesting improvements in the lattice structural quality attributed to the incorporation of hydrogen.

Excited States of the A and B Free Excitons in CuInSe2

CuInSe2 single crystals, grown by the vertical Bridgman technique were studied using polarisation resolved photoluminescence (PL) at cryogenic temperatures. The emission lines related to the first (n = 2) excited states for the A and B free excitons were observed in the PL spectra at 1.0481 and 1.0516 eV, respectively. The spectral positions of these lines were used to estimate accurate values for the A and B exciton binding energies (8.5 and 8.4 meV, respectively), Bohr radii (7.5 nm), band gaps (EgA = 1.050 eV and EgB = 1.054 eV), and the static dielectric constant (11.3) assuming the hydrogenic model.

Excited states of the free excitons in CuInSe2 single crystals

High-quality CuInSe2 single crystals were studied using polarization resolved photoluminescence (PL) and magnetophotoluminescence (MPL). The emission lines related to the first (n=2) excited states for the A and B free excitons were observed in the PL and MPL spectra at 1.0481 meV and 1.0516 meV, respectively. The spectral positions of these lines were used to estimate accurate values for the A and B exciton binding energies (8.5 meV and 8.4 meV, respectively), Bohr radii (7.5 nm), band gaps (EgA=1.050 eV and EgB=1.054 eV), and the static dielectric constant (11.3) assuming the hydrogenic model.

Mechanisms of charge transfer and magnetoresistance in CuInSe2

The electrical conductivity and magnetoresistance effect of n and p types of CuInSe2 single crystals were studied within the temperature range of 4.2–300 K and within magnetic fields of up to 6 T. It was found that the hopping mechanism of conductivity dominates in the temperature range of 4.2–100 K. A peculiarity of electrical conductivity behavior, accompanied by a change in the magnetoresistance sign in the vicinity of T ≈ 60 K, was revealed.

Temperature dependence of excitonic emission in CuInSe2

AbstractRadiative recombination processes in CuInSe2 (CIS) single crystals grown by the vertical Bridgman technique were studied using photoluminescence (PL) and reflectance (RF) spectroscopies at temperatures from 4.2 to 60 K, and excitation intensity from 0.6 to 30 W/cm2. Study of the quenching parameters of the A and B free‐ and first three bound‐excitons (M1, M2 and M3) in high‐quality CuInSe2 single crystal leads to estimates for the binding energy of the A (7.7 meV) and B (7.9 meV) free excitons as well as dissociation energies for the M1, M2 and M3 bound‐excitons. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effects of magnetic fields on free excitons in CuInSe2

AbstractThe effects of magnetic fields up to 20 T were studied in CuInSe2 single crystals using photoluminescence (PL) at 4.2 K. Diamagnetic shifts of the free A and B excitons measured in the PL spectra in CuInSe2 at 4.2 K under the magnetic fields were used to estimate the reduced masses (0.095m0 for the A and 0.098m0 for the B exciton), binding energies (7.0 meV for the A and 7.2 meV for the B exciton) and Bohr radii (7.6 nm for the A and 7.3 nm for the B exciton) of the free‐excitons in CuInSe2 assuming that both excitons are isotropic and hydrogen‐like. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)