CuInSe2 Single Crystals Research Articles

Near‐edge Optical Properties of CuInSe2 Studied by Photoacoustic Spectroscopy

AbstractOptical absorption spectra of n‐type CuInSe2 single crystals were evaluated by means of photoacoustic spectroscopy in the photon energy range from 0.94 to 1.02 eV. At photon energies below 0.99 eV the spectra exhibit exponential tails that are ascribed to the presence of shallow impurity states. At higher photon energies the absorption coefficients follow the relation for direct allowed optical transitions between parabolic bands. The parameters of this relation are determined and compared with previous measurements.

Electrical and Optical Properties of CuInSe2 Single Crystals with Various Deviations from Stoichiometry

The optical and electrical properties of CuInSe2 single crystals were measured for various deviations from stoichiometry. The absorption curve was found to shift toward the lower energy region with an increase in deviation from stoichiometry. It can be interpreted in terms of Urback rule in connection with the compositional disordering. We have determined the energy levels for the defects in CuInSe2 from the temperature dependence of the carrier concentration and the photoluminescence spectra.

Electron Spin Resonance Study of CuInSe2 Single Crystals

Imperfections relating to iron contamination in undoped CuInSe2 single crystals have been studied using Electron Spin Resonance at liquid He temperature. A fine structure of the S=5/2 state attributed to Fe3+ has been observed for crystals of Cu/In ∼ 1. On the other hand, some of indium-rich (Cu/In<1) crystals show a single line of geff∼10. Fe2+ is one of the possible candidates for this line. The two different ionization states of iron in CuInSe2 suggest that iron atoms substitute copper sites in Cu-deficient system rather than indium sites which are preferably occupied by Fe3+ in nearly stoichiometric system.

Studies of the Effects of Ion-Implantation and Electron Beam Irradiation on CuInSe2 Single Crystals

ABSTRACTA series of CuInSe2 single crystals which were grown by the vertical Bridgman technique have been implanted with oxygen and xenon ions. These implants tend to cause a change from n to p-type conductivity and an enhancement of the photoconductivity. We present HREM and SIMS characterisation of the microstructural effects caused by high dose ion implants on CuInSe2. We also correlate our data with calculated ion implant profiles. In addition, we show that CuInSe2 thin foils can undergo significant degradation under the electron beam irradiation conditions which are commonly encountered in electron microscopes.

Photoconductivity Spectra of n‐type CuInSe2 Single Crystals

AbstractPhotoconductivity spectra of n‐type CuInSe2 single crystals are measured in the photon energy range hv = 0.75 – 3.1 eV and as a function of temperature in the range T = 80 – 320 K. It is found that the photoconductivity of as‐grown crystals is a nearly pure surface effect, while sensitization of the crystal volume is observed only after sufficiently long annealing in the presence of powdered material. To explain the temperature dependence of the photoconductivity carrier trapping processes must be taken into account.

Annealing Effects on Photoconductivity Spectra of CuInSe2 Single Crystals

AbstractPhotoconductivity spectra of as‐grown n‐type and p‐type CuInSe2 single crystals and of crystals annealed under maximum and minimum selenium pressure are measured in the photon energy range hv = 0.75 – 3.0 eV and as a function of temperature in the range T = 80 – 320 K. A model with trapping of minority carriers is used to explain the temperature dependence of the photoconductivity. The trap energies present in the samples depend on their preparation conditions.

Infrared Optical Characterization of Thermally Oxidized CuInSe2 Single Crystals

AbstractInfrared reflectivity spectra of thermally oxidized CuInSe2 single crystals are measured at room temperature in the wavenumber range from 180 to 4000 cm−1. A Kramers‐Kronig analysis of the spectra reveals seven vibrational modes with frequencies which agree with mode frequencies in In2O3. No vibrational modes due to Cu–O and Se–O bonds could be observed. The results obtained are compared with previous studies of oxidized CuInSe2 crystals.

Diode n‐p CuInSe2 Structures Fabricated by Oxygen Implantation

AbstractThe influence of oxygen implantation on the electrical features of n‐type CuInSe2 single crystals has been studied. n‐p type conversion in the implanted region has been observed. Photoelectric characteristics of the fabricated structures have been examined. It has been shown that ion implantation allows to fabricate CuInSe2 photoconverters with the absolute current sensitivity up to Si = 33 mA/W at 300 K.

ChemInform Abstract: Defect Level Identification in CuInSe2 from Photoluminescence Studies.

AbstractA chemically and physically consistent interpretation of luminescent transitions in CuInSe2 is presented on the basis of photoluminescence data, electronic parameters, stoichiometries, and copper diffusion coeffients determined for a number of p‐type CuInSe2 single crystals.

Studies of the electrical and interface properties of the metal contacts to CuInSe2 single crystals

The electrical behavior of the metal contacts and ITO and CdS junctions to single crystals of CuInSe2 has been studied using I–V and electron beam induced current measurements, then correlated to the chemical composition and intrinsic defect states in the semiconductor. The results have indicated that the contact resistance, junction characteristics, and crystalline order of surfaces are controlled mainly by the type and relative concentration of the intrinsic defect states dominating the copper-indium-deselenide material; these states are very sensitive to heat treatments and surface preparation procedures. Correlation between the behavior of different samples (polycrystalline thin films or single crystals) should be based upon similarities in the type and relative concentration of the chemical composition.

Relation between electrical properties and composition in CuInSe2 single crystals

Abstract The electrical properties and the elemental composition of as-grown CuInSe2 single crystals are measured and compared with the predictions of an intrinsic defect chemistry model. It is found that the validity range of the intrinsic defect model is limited to small deviations from the ideal stoichiometry of the compound. The critical values for the deviations from molecularity and valence stoichiometry are given. Preliminary results are reported on changes of composition and electrical parameters during annealing.

Effect of Granularity on CuInSe2 Solar Cell Response

ABSTRACTPolycrystalline CuInSe2 solar cells, fabricated by evaporation or by selenization of metal films, are granular and relatively porous. Grains are a few hundred nanometers in dimension, and hence about 1% of the atoms are at a surface. Despite the granularity, quantum efficiency is quite high and implies a diffusion length exceeding the grain dimension. The primary photovoltaic loss is excessive forward recombination current. The proposed model consists of single crystal CuInSe2 granules with an indium rich surface layer. When properly passivated, the otherwise uncoordinated indium bonds are terminated by oxygen. However, residual non-passivated crystalline surface states distributed throughout the depletion region provide the paths for enhanced recombination.

Intrinsic defect states in CuInSe2 single crystals

High-resolution photoluminescence measurements at 10 K were performed on samples cut from n- and p-type single crystals of (CuInSe2), before and after annealing in air (at 220 and 400 °C, respectively), and in excess of selenium vapor (at 450 and 650 °C). These data provided the basis of the presented energy band diagram which correlates the intrinsic defect levels, their origins and relative concentration with the material electrical behavior and composition. A summary of the variation in the type and relative concentration of the dominant defect states and their electrical role induced by various heat treatments is also given.

Crystal growth of CuInSe2 by the Bridgman method

Single crystals of CuInSe2 have been fabricated by the vertical Bridgman method. A conventional Czochralski crystal-pulling system was adapted for this purpose. An accelerated crucible-rotation technique was employed for a better mixing of the melt during the growth. Void- and crack-free crystal grains with an area as large as 50 mm2 and a thickness of more than 5 mm could be selectively cut from the ingots. From room-temperature Hall-effect measurements, mobility values as large as 73 cm2 ∙ V−1 ∙ s−1 were obtained for the present samples. X-ray diffraction studies suggested that abrasive polishing might create an amorphous layer on the surface of the CuInSe2 crystals.

Optical absorption and phase transitions in CuInSe2 and CuInS2 single crystals at high pressure

The optical absorption edge of CuInSe2 and CuInS2 single crystals was measured as a function of hydrostatic pressure up to 10 GPa. In both cases the direct energy gap increases linearly with pressure at the rate of 30 and 24 meV GPa−1 for CuInSe2 and CuInS2 , respectively. From changes in the light-transmission characteristic of the samples under pressure, a structural first-order irreversible transition was observed at 7.1 GPa for CuInSe2 and 9.6 GPa for CuInS2 . Also, a new metastable semiconducting phase was formed in pressure-quenched CuInS2 samples.

Verification of CuInSe2 density of states features by reflection electron energy loss spectroscopy

Reflection electron energy loss measurements of CuInSe2 single crystals are reported and when possible, the losses are compared with former results by Kazmerski and Jamjoum et al. Their identification of the transitions is confirmed and refined by spectrum simulations with total and partial densities of states from band structure calculations of Jaffe, Bendt, Zunger, and Bullett.

Microhardness of CuInSe2

Abstract Vickers microhardness measurements have been performed on (112) faces of n-and p-type CuInSe2 single crystals at room temperature. In nearly stoichiometric samples, hardness values of 2·33 ± 0·06 GPa and 3·37 ± 009 GPa were found for n-and p-type material respectively. Preliminary results regarding hardness anisotropy and microhardness changes due to larger deviations from stoichiometry are given. At applied loads in the range 5–500 g no dependence of the microhardness on the load was found.

Optical Properties of CuInSe2 Single Crystals Grown by the Vertical Bridgman Technique

AbstractThe optical absorption near the fundamental edge of n‐type CuInSe2 single crystals was studied for samples having different impurity concentrations. It is found that with increasing impurity concentration the gap energy decreases whilst the tail absorption below the edge and its characteristic energy increase. It is concluded that band‐gap narrowing due to high ionized impurity concentrations plays an important role in CuInSe2 single crystals.

Quantitative studies of cleaved and sputtered CuInSe2 surfaces

CuInSe2 has emerged as an important photovoltaic semiconductor. The critical importance of stoichiometry changes, interfacial chemical reactions, and compositional variations on cell performance has made the acquisition of reliable quantitative data by surface analysis methods mandatory. In this study, single crystals of CuInSe2 were cleaved in UHV, exposing the (110) plane which is representative of the bulk material. Quantitative AES analysis was performed using both N(E) and dN(E)/dE data, yielding concentration results within 2 at. % of stoichiometric values. The method developed is based upon an independent evaluation of matrix effects using elemental standards and actual density considerations. The effects of Ar+ ion sputtering were investigated for beam energies 0.1&amp;lt;Ep&amp;lt;5.0 kV. Quantitative data for sputter-etched and virgin surfaces have been compared. Oxygen adsorption and In oxidation was observed during exposure to O2 and electron beam irradiation. Polycrystalline thin films of CuInSe2 were also analyzed, and quantitative results have been compared to the single-crystal values.

Phase relations in the Cu, In, Se system and the properties of CuInSe2 single crystals

Experimental data on the phase relations in the Cu, In, Se system are presented and the growth of single crystals of CuInSe2 from the melt is described. Hall measurements on both n- and p-type crystals reveal that impurity conduction dominates the low-temperature transport properties. Type conversion by annealing of initially n-type crystals in excess selenium vapor leads to a substantial increase in the compensation level.