CuGaS2 Crystals Research Articles

CUGAS2 КРИСТАЛЛДАРЫ, АЛАРДЫН МҮНӨЗДӨМӨЛӨРҮ ЖАНА ИШТЕТИЛИШИ

In this article, the characteristics of CuGaS2 crystals grown are studied by the gas transport method and the two-temperature synthesis method. The described materials, primarily original ones, as well as the presented materials, allow us to conclude that a wide range of compounds represents an unexplored class of substances, especially in the field of experimental determination of their physic-chemical properties, methods of crystal growth and correlation of material characteristics with technological parameters of the production processes. The growing practical application of semiconductors stimulates research into new materials. In recent years, much attention has been paid to studying the physical properties of multicomponent systems, especially crystals of groups I-II-VI2, which are promising for many technical applications. For a number of crystals of group I-II-VI2, methods for selecting p- and n-type samples have been developed, on the basis of which heterojunctions have been created.

Structural, optical and thermal properties of CuGaS2 crystals by chemical vapor transport (CVT) method

The ternary CuGaS2 single crystals were grown by chemical vapor transport (CVT) method, using iodine as a transporting agent in a closed system. The as grown CuGaS2 crystals exhibit dark green color. The concentration of transporting agent was 12mg/cm3. The maximum size of single crystal has the dimension of 6×2×1mm3. XRD studies show that the grown crystal belongs to tetragonal (chalcopyrite) system with (112) as dominant orientation. High resolution scanning electron microscope (HR-SEM) analysis disclosed layer growth pattern of the CuGaS2 crystal. Energy Dispersive X-ray (EDX) analysis revealed the composition of the grown CuGaS2 crystal. Raman spectrum of the grown CuGaS2 crystal exhibited a sharp peak of the A1 mode at 308cm−1. The emission spectrum showed that the crystal has a band gap emission of 2.32eV (534nm) when excited with 490nm. The melting point of the grown CuGaS2 crystal was found to be 1236°C using differential thermal analysis.

Chemical vapor transport of chalcopyrite semiconductors: CuGaS2 and AgGaS2

Crystals of CuGaS2 and AgGaS2 with different isotopic compositions have been grown by chemical vapor transport (CVT) using iodine as the transport agent. Before performing the CVT growth, sulfur and copper were purified by sublimation and etching, respectively. 109Ag and the etched 71Ga isotopes were purified from oxides by vacuum annealing. Transparent yellow orange crystals of CuGaS2 and greenish yellow crystals of AgGaS2 were obtained in the shape of platelets, chunks, rods and needles in sizes of up to 8mm (CuGaS2) and 30mm (AgGaS2).These crystals were used to study their electronic, vibrational and thermodynamic properties. Higher excitonic states (n=2,3) were observed at low temperatures with wavelength-modulated reflectivity spectroscopy, thus proving an excellent surface and crystal quality. In addition, the experimentally determined non-monotonic temperature dependence of the excitonic energies can be well fitted by using two Bose–Einstein oscillators and their statistical factors, corresponding to characteristic acoustic and optical phonon frequencies. Isotopic shift of excitonic energies has also been successfully observed in these crystals.

Conducting behavior of chalcopyrite-type CuGaS₂ crystals under visible light.

Millimeter size high quality crystals of CuGaS2 were grown by chemical vapor transport. The highly ordered chalcopyrite structure is confirmed by X-ray diffraction and Raman spectroscopy. According to energy dispersive X-ray spectroscopy the composition of the crystals is very close to the formula CuGaS2. Room temperature photoluminescence measurements indicate the presence of an emission peak at about 2.36 eV that can be related to a donor-acceptor pair transition. The electrical resistance as a function of temperature is very well described by the Mott variable range hopping mechanism. Room temperature complex impedance spectroscopy measurements were performed in the alternating current frequency range from 40 to 10(7) Hz in the dark and under normal light. According to the impedance spectroscopy data the experimental results can be well described by two circuits in series, corresponding to bulk and grain boundary contributions. An unusual positive photoresistance effect is observed in the frequency range between 3 and 30 kHz, which we suggest to be due to intrinsic defects present in the CuGaS2 crystal.

Synchrotron reflectivity spectra and the optical properties of CuGaS2 crystal

For a CuGaS2 crystal, the polarized spectra of complete sets of optical functions in the energy range of 0 to 24 eV are obtained for the first time. The calculations are performed using synchrotron reflectivity spectra for E ∥ c and E ⊥ c polarizations in the energy ranges of 0 to 24 eV and of 1 to 5 eV by means of computer programs using the Kramers-Kronig relations. The main features of the spectra of permittivity, characteristic electron loss, and other optical functions are determined. The band maximum energies for the volume and surface plasmons are obtained, and their independence from the type of polarization is found.

Growth of CuGaS2 Single Crystals by Chemical Vapor Transport and Characterization

Single crystals of CuGaS2 (CGS) compounds were grown by a chemical vapor transport technique in a closed system using iodine as a transporting agent at different growth conditions. The grown crystals exhibit different colors such as yellow, green, and orange depending on the temperature of the growth zone. Single-crystal X-ray diffraction studies confirmed the chalcopyrite structure of the grown crystals. The powder X-ray diffraction showed the presence of the secondary phase in the yellow (Cu2S) and orange (Ga2S3) crystals. Scanning electron microscope analysis disclosed step growth, lateral expansion, and layer growth patterns on the yellow-, orange-, and green-colored single crystals, respectively. Energy dispersive X-ray analysis revealed the stoichiometric composition of the green crystals, whereas rich Cu and Ga were found in yellow and orange crystals, respectively. Raman spectra of green-colored CGS single crystals exhibited a high-intensity peak of the A1 mode at 315 cm-1. For yellow and orange c...

Resonance impurity radiation in CuGaS2 crystals

In this work the reflectivity spectra of the CuGaS2 crystals have been investigated in the EIIc polarization and the absorption spectra were studied in the E⊥c polarization at the 10K. The parameters of the excitons and the band gaps have been determined. The low temperature (10K) luminescence spectra of the CuGaS2 crystals have been investigated under the excitation by 4880Å and 4965Å Ar+ laser lines. A model of optic transitions between the levels of excitons bound on neutral donor is proposed.

Interference of resonance luminescence of exciton polaritons in CuGaS2 crystals

The nonmodulated and wavelength-modulated reflection spectra of CuGaS2 crystals for the polarization EIIc of 10K are studied. The states n=1, 2 and 3 of the excitons Γ4 (A-excitons) and n=1, n=2 of B- and C-excitons are found. The nonmodulated absorption spectra for the polarization E⊥c at 10K have been studied. The states n=1, 2 and 3 of Γ5 excitons are found. The main parameters of the A (Γ4, Γ5) and B, C exciton series at the energies of the longitudinal and transverse excitons Γ4 for the states n=1 and n=2, the effective masses of electrons (me1∗) and holes (mv1∗,mv2∗,mv3∗) are determined. The photoluminescence peaks were observed at n=3 and n=4 of the excitons Γ5 in the luminescence spectra excited by the line 4880Å of Ar+ laser. In the luminescence spectra the interference is found.

Zero-field Splitting and Local Lattice Distortions for Fe3+ Ions in Some Ib-IIIb-VI2 Semiconductors

The EPR zero-field splitting D for Fe3+ ions in some Ib-IIIb-VI2 semiconductors is calculated with the superposition model. The calculated D values, when using the local rotation angles τ (Fe3+) for Fe3+ in CuGaS2 and AgGaS2 crystals, are consistent with the observed values, whereas for Fe3+ in CuAlS2 crystal they are not. The calculated results are discussed. The local lattice distortions except the local rotation angles τ for Fe3+ in CuAlS2 are suggested.

Resonance Raman scattering by excitonic polaritons in CuGaS2

Resonance Raman scattering by exciton polaritons in crystals of CuGaS2 under excitation with the 4880 and 4765 A lines of an Ar+ laser at 9 K is studied. Lines of one-and two-phonon scattering of excitonic polaritons are found and studied. It is shown that the 1LO and 2LO phonons are arranged in accordance with their energies as the Stokes shifts move farther away from the excitation energy.

Spatial dispersion in polariton spectra of CuGaS2 crystals

The unmodulated and wavelength-modulated reflectivity spectra of CuGaS2 crystals for the polarization E ‖ c, k ⊥ c at 77 and 8 K have been studied. The states n=1, 2, and 3 of A excitons and n=1 and 2 of B and C excitons are established. The luminescence spectra from the surface at k ‖ c and k ⊥ c are obtained. The fine structure of the reflectivity spectra of excitons are analyzed with due regard for the normal and oblique incidence of light onto the crystal surface. The main parameters of the A, B, and C excitonic series are determined such as the energies of the longitudinal and transverse excitons Γ4 (E ‖ c) for states n=1 and 2, the longitudinal and the transverse mass of excitons in CuGaS 2, and the effective masses of electrons (m c1*) and holes (m v1*, m v2*, m v3*). It is shown that the mass m v1* in the upper valence band at k ‖ c equals (0.7–0.8)m 0 and at k ⊥ c, 1.87m 0.

Optical functions and electronic structure ofCuInSe2,CuGaSe2,CuInS2,andCuGaS2

We report on the complex dielectric tensor components of four chalcopyrite semiconductors in the optical energy range (1.4-5.2 eV, from 0.9 eV for CuInSe2) determined at room temperature by spectroscopic ellipsometry. Our results were obtained on single crystals of CuInSe2, CuGaSe2, CuInS2, and CuGaS2. Values of refractive indices n, extinction coefficients k and normal-incidence reflectivity R in the two different polarizations are given and compared with earlier data where available. We analyze in detail the structures of the dielectric function observed in the studied energy region. Critical-point parameters of electronic transitions are obtained from fitting of numerically calculated second-derivative spectra. Experimental energies and polarizations are discussed on the basis of published band structure calculations.

Photoluminescence of CuGaS2 epitaxial layers grown by metalorganic vapor phase epitaxy

High quality epitaxial layers of CuGaS2 have been grown on GaAs and GaP substrates by low-pressure metalorganic vapor phase epitaxy using cycropentadienylcoppertriethylphosphine, normal-tripropylgallium, and ditertiallybutyl sulphide precursors. Photoluminescence (PL) studies at 8 K have been carried out, and exciton-related PL peaks at 2.489 and 2.477 eV have been observed in addition to PL peaks at 2.43 (free-to-bound), 2.39 (donor-acceptor pair), and broad band at 1.65 eV. Temperature and excitation intensity dependences of the PL peaks have been studied in detail, and PL properties are compared with those for the bulk CuGaS2 crystals grown by the chemical vapor transport (CVT) method. PL spectra for nearly stoichiometric CuGaS2 epilayers exhibited the intense free exciton-related PL peak over the entire temperature range (8–300 K), which are in contrast to PLs dominated by defect-related deep peaks for Ga-rich or Cu-rich epilayers and the impurity related near-band edge PL for the CVT bulk crystal. These PL results show that the nearly stoichiometric CuGaS2 epilayers grown in this study are the highest quality reported so far.

Picosecond Time-Resolved Spectroscopy of Exciton Systems in CuGaS2 Bulk Single Crystals

The decay times of bound-exciton emissions in CuGaS2 bulk crystals grown by iodine transport were measured by picosecond time-resolved spectroscopy as a function of temperature. The CuGaS2 crystals examined in this study showed two bound-exciton emission lines at 2.493 and 2.496 eV at low temperatures. Experimental results were analyzed with rate equations on the exciton kinetics using two models of free-exciton oscillator strength: strength per excitonic volume and strength per CuGaS2 molecule. Reasonable values of capture center densities and capture cross sections for the free exciton for the exciton-binding centers were obtained when using the oscillator strength per free-excitonic volume. The capture cross sections were ∼10-14 cm2 and the capture center densities were ∼1016 cm-3.

Investigation of the Raman spectra of CuxAgl−xGaS2 solid solutions

Raman spectra in (┴, ┴) and (║, ║) polarizations are investigated on crystals of CuGaS2, AgGaS2 compounds and CuxAgl−xGaS2 solid solutions. The frequencies of longitudinal and transverse optical phonons are determined, their concentration dependences are constructed, and the behavior of the vibrational modes in CuxAgl−xGaS2 solid solutions is established.

Yb-Related Photoluminescence in CuGaS2, AgGaSe2 and AgGaS2

Near-infrared photoluminescence (PL) has been studied in CuGaS2, AgGaSe2 and AgGaS2 crystals doped with Yb. Sharp PL lines are observed in the 1.0 µm region for these crystals. These PL lines are assigned to 2F5/2 → 2F7/2 transition of the spin-orbit split states of the Yb3+ ion in the chalcopyrite crystal lattice.

Low Temperature Resonant Secondary Emission in CuGaS2 by Γ Point Excitation

Resonant secondary emissions are examined on the (100) surface of CuGaS2 crystal. Two bound-exciton luminescence lines and 27 sharp resonant-Raman lines are observed under polarizing excitation of 4765 Å and 4880 Å in <X|ZZ| \\barX> configuration. Resonant enhancement and breakdown of the Raman selection rule for longitudinal E modes are attributed to the intraband Fröhlich interaction. High resolution measurement and accurate polarization evaluation can be performed using a line rejection filter and a polarization compensator, respectively.

Single Crystal Growth of Cu–III–VI2 Semiconductors by THM

The traveling heater method (THM) has been applied to the single crystal growth of Cu–III–VI2 chalcopyrite semiconductors. Up to this time, bulk single crystals of CuGaS2, CuGaSe2, CuInS2 and CuInSe2 ternary compounds have been obtained by the THM technique using In solvent. The THM growth of bulk single crystals of CuGa0.6In0.4S2 quaternary alloy, which is lattice-matched to ZnS, has been investigated. By the use of In-solution zones containing 60 mol% CuGa0.37In0.63S2 solute, single crystals with a uniform composition of CuGa0.6In0.4S2 were obtained.

Photoreflectance studies in CuGaS2 single crystals

Photoreflectance (PR) measurements have been performed near the fundamental absorption edge of CuGaS2 single crystals grown by an iodine-transport method. The PR spectra at room temperature are analyzed as the superposition of exciton and band-to-band transition. The exciton transition energies at 290 and 77 K are obtained to be 2.463 and 2.503 eV, respectively. The quality of CuGaS2 crystals grown with different amounts of iodine is characterized with respect to energies and broadening of the PR spectra.

Absorption and Photoluminescence Spectra of Heavily Zn-Doped CuGaS2 Crystals

In heavily Zn-doped (up to 5%) CuGaS2 crystals, reductions of excitonic absorption and above-band-gap absorption were observed for optical transitions of the polarization parallel to the optic axis, probably related to a change in the exciton oscillator strength ratio of the transitions of polarizations parallel and perpendicular to the optic axis. An impurity absorption whose concentration dependence is consistent with electron transitions from ZnGa acceptors to the conduction band is reported for the first time. An emission appearing at 2.1∼2.3 eV is considered to be due to the electron transitions from a donor level of sulfur vacancies or their complexes with ZnGa to the ZnGa acceptor level, involving the effect of broadening of the latter level. The existence of isoelectronic-trap ZnCu-ZnGa pairs is suspected from time-resolved emission spectra.