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High temperature electrical conductivity in ZnSe:In and in CdSe:In under selenium vapor pressure
Abstract
High temperature electrical conductivity (HTEC) isotherms and isobars of ZnSe:In and of CdSe:In are compared. There are differencies in In-doping mechanisms of II–VI compounds. When HTEC isotherms and isobars of ZnSe:In and of CdSe:In, measured under metal component vapour pressure give both n-type conductivity then differences appear in the results of measurements under the selenium vapor pressure (p). ZnSe:In isotherms in the last case are characterized by the conductivity type conversion but no such drastic change of HTEC type is observed on CdSe:In isotherms. Under the conditions of p, the activation energy of HTEC isobars for ZnSe:In is ΔE ≈ 1.3–1.6 eV and for CdSe:In is ΔE ≈ 1.2 eV. The onefold ionized substitutional In at Zn place is proposed to be compensated by native defects in ZnS:In and in CdSe:In under high p. This native defect may be onefold ionized zinc vacancy for ZnSe:In and twofold ionized cadmium vacancy for CdSe:In. Association of defects occur at lower temperatures. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
... Although ZnSe is a very promising compound for the thin film applications of solar cells [8][9][10][11], there are some difficulties in the control of atomic ratios of the constituent elements and as a result of the nature of the defects in the structure, and common high resistivity problem of the ZnSe films obstruct their photovoltaic applications. Therefore, alloying polycrystalline wide band gap thin film semiconductors with elements in group III is a familiar application to optimize the resistivity of these materials [12,13]. In view of the fact that interests on ZnSe, Zn-In-Se (ZIS) has a high band gap value with low resistivity values to meet the expectations as a promising buffer layer. ...
In this study, structural properties of the Zn-In-Se (ZIS) thin films deposited by thermal evaporation method were investigated. The as-grown and annealed ZIS films were found in polycrystalline structure with the main orientation in (112) direction. The compositional analysis of the films showed that they were in Zn-rich behavior and there was a slight change in the elemental contribution to the structure with annealing process. Raman analysis was carried out to determine the crystalline structure and the different vibration modes of ZIS thin films. According to these measurements, the highest Raman intensity was in the LO mode which was directly proportional to the crystallinity of the samples. The atomic force microscopy (AFM) analyses were done in order to obtain detailed information about the morphology of the thin film surface. The surface of the films was observed as nearly-smooth and uniform in as-grown and annealed forms. X-ray photoelectron spectroscopy (XPS) measurements were analyzed to get detailed information about surface and near-surface characteristics of the films. The results from the surface and depth compositional analyses of the films showed quite good agreement with the energy dispersive X-ray spectroscopy (EDS) analysis.
Temperature-dependent transmission experiments of ZnInSe thin films deposited by thermal evaporation method were performed in the spectral range of 550–950 nm and in temperature range of 10–300 K. Transmission spectra shifted towards higher wavelengths (lower energies) with increasing temperature. Transmission data were analyzed using Tauc relation and derivative spectroscopy. Analysis with Tauc relation was resulted in three different energy levels for the room temperature band gap values of material as 1.594, 1.735 and 1.830 eV. The spectrum of first wavelength derivative of transmittance exhibited two maxima positions at 1.632 and 1.814 eV and one minima around 1.741 eV. The determined energies from both methods were in good agreement with each other. The presence of three band gap energy levels were associated to valence band splitting due to crystal-field and spin–orbit splitting. Temperature dependence of the band gap energies were also analyzed using Varshni relation and gap energy value at absolute zero and the rate of change of gap energy with temperature were determined.
ZnInSe2/Cu0.5Ag0.5InSe2 diode structures have been fabricated by thermal evaporation of stacked layers on indium tin oxide-coated glass substrates. Temperature-dependent dark current–voltage measurements were carried out to extract the diode parameters and to determine the dominant conduction mechanisms in the forward- and reverse-bias regions. The heterostructure showed three order of magnitude rectifying behavior with a barrier height of 0.72 eV and ideality factor of 2.16 at room temperature. In the high forward-bias region, the series and shunt resistances were calculated with the help of parasitic resistance relations, yielding room-temperature values of 9.54 × 10² Ω cm² and 1.23 × 10³ Ω cm², respectively. According to the analysis of the current flow in the forward-bias region, abnormal thermionic emission due to the variation of the ideality factor with temperature and space-charge-limited current processes were determined to be the dominant conduction mechanisms in this heterostructure. In the reverse-bias region, the tunneling mechanism was found to be effective in the leakage current flow with trap density of 10⁶ cm⁻³. Spectral photocurrent measurements were carried out to investigate the spectral working range of the device structure. The main photocurrent peaks observed in the spectrum corresponded to the band-edge values of the active thin-film layers.
The diffusion of In and Cu in ZnS single crystals was investigated by means of radioactive tracers. The diffusion constant D0 and the activation energy Ea of the diffusion equation D = D0 • exp(- Ea/kT) were determined by analysing the temperature dependence of the concentration profiles. The result for In (EA = 2,2 eV, D0 = 30cm2s_1) is interpreted as a diffusion via Zn vacancies and that for Cu (EA = 0,79 eV, D0 = 2,6 • 10- 3 cm2 s-1) as a diffusion via interstitial sites which probably is influenced by Zn vacancies. - Preceding doping with In significantly retards the Cu diffusion, consistent with the model of Cu-In pair formation. The reasons for some deviations of the experimental data from the calculated concentration profiles are discussed. - Diffusion measurements with high Cu-concentrations yield a Cu-solubility of 300 ppm at 840 K and 1000 ppm at 950 K.
High-temperature electrical measurements are explained in the framework of the theory of quasi-chemical reactions between defects in solids using solubility data for In in CdTe. The free carrier density is dependent on the solubility of dopant and the electrons are compensated by both native point defects (Cd vacancies) and the In-containing associate. Calculated point defect diagrams give electron densities in good agreement with the measured values.
Electrical and optical properties of the conductive n-type skin layer prepared in high-resistivity CdTe:In by annealing at 400-600°C under Cd-rich overpressure were investigated. Slightly compensated donor level with the concentration N<sub>D</sub>≈1.3×10<sup>16</sup> cm<sup>-3</sup> and ionization energy E<sub>D</sub>≈10 meV was evaluated from the Hall effect measurement. The electron mobility reached a maximum of 1×10<sup>4</sup> cm<sup>2</sup>/Vs at 35 K after annealing at 600°C. Purification of the n-type layer was found out by photoluminescence measurement, where a reduction of the intensities of the emission lines related to alkali and silver acceptors was observed. The emission line at 1.584 eV, which is usually expected to characterize high-resistivity material, was detected in as-grown high-resistivity samples as well as in the conductive n-type layer.
Given the difficulties arising from the high ionic character of their chemical bond—low thermal conductivity, associated melts, large nonstoichiometry, tendency to Polymorphism—and because of their high melting points, Zn chalcogenides have been submitted to a wide variety of growth techniques. Some of these techniques are melt growth, solution growth using either homo- or hetero-solvent, growth from the vapor phase in open, semi-closed and closed systems by sublimation or by chemical vapor transport, and hydrothermal growth. Melt growth is generally preferred for the preparation of large volume single crystals. ZnTe solution growth that allows a lower melting temperature, has been reported by the vertical Bridgman technique and by the traveling solvent method according to two variants, the traveling heater method and temperature gradient solution zoning using Te as the solvent. Green emitting diodes are recently realized on Bridgman grown bulk ZnTe crystals on which p/n junctions are obtained by diffusion of a donor through the vapor phase.
High temperature electrical conductivity (HTEC) and solubility measurements of Ga and In were performed in ZnS. To investigate Ga and In solubility in ZnS under the controlled vapour pressure of Zn, a special construction of quartz ampoules were used. Ga and In solubility isotherms in ZnS are given. At high temperatures and at low concentrations, Ga and In act as a single donor. At high zinc pressures, outdiffusion of gallium and indium occurs. The change of the solubility of Ga or In is reflected in ZnS HTEC isotherms. If doped at the same doping conditions, ZnS:Ga and ZnS:In had the same HTEC absolute values despite the differences in Ga and In solubilities.
The issue of modelling of point defect structure in In(Ga)-doped CdTe crystals has been analyzed. Contrary to previous papers, it has been shown that high-temperature point defect equilibrium modelling can be successfully achieved if sufficient experimental data are available and appropriate calculation methods are used. Room temperature free electron densities were computed for CdTe:In crystals and compared with experimental data. Since the density of point defects changes during cooling from the preparation to operational temperatures, it cannot be estimated. Therefore, a new approach, including calculation of a certain fitting Δ parameter, is proposed. Its determination allows preparation/cooling conditions necessary to prepare material with the desired electric properties to be defined.
The system of equations for the description of the high temperature equilibrium contains linear balance equations for impurities and charges and also nonlinear equations for connecting the defect concentrations via equilibrium constants. To calculate the high temperature defect equilibrium, a method for the solution of this system of equations by the iteration method is presented. For the calculation of frozen-in defect equilibrium, all defects are divided into three groups. The concentrations of all defects are expressed via the total concentration of the same type of defects.The equation is solved, to calculate the electroneutrality and the validity of the material balance condition is checked. An example of the calculation is given for ZnS:Cu:Al:Bi:Cl.
The electrical conductivity σ of undoped and Cu-doped CdSe crystals is measured over the range 600 to 1000°C as a function of cadmium and selenium vapour pressure and temperature. The formation enthalpy of the doubly ionized native donor defect (V or Cd) is found to be (1.86 ± 0.09) eV at constant cadmium vapour pressure and ⪆ 4.95 eV at constant selenium vapour pressure. For Cu-doped samples the conductivity varies as pCd1/2 and the electron concentration is almost independent of temperature above 600°C in Cd vapour. The results are interpreted in terms of a distribution of Cu on the substitutional and interstitial sites. The association between Cu and the native defects is taken into account.[Russian Text Ignored].
Dopants of group III and group VII elements act as donors (D) in II–VI materials. Here we report the results of our investigations of donors in some II–VI donor-doped compounds (CdSe:Al, CdSe:Ga, CdSe:In, ZnSe:In, ZnS:Al, ZnS:Ga and ZnS:In) using the method of high temperature electrical conductivity (HTEC). Our interests lie in the activation energies determined from HTEC isobars and slopes of HTEC isotherms in the temperature region from 600 to 1200 °C where the donor action of dopant appears. We assumed the existence of near-zero activation energies of free carrier concentrations determined from HTEC isobars Arrhenius plots for the donor action regions. In some cases, this is realized as the result of formation of compensation through electroneutrality condition n = [D•]. In this region, the best results to obtain high n-type conductivity in frozen-in crystals can be achieved. Unexpected negative HTEC activation energies with non-zero slopes of HTEC isotherms were obtained for the systems ZnSe:In, ZnS:Al and ZnS:Ga in the donor action region. The appearance of negative activation energies of HTEC and the models for high temperature defect equilibrium of these systems are analyzed.
High temperature electrical conductivity (HTEC) as a function of temperature and component vapour pressure has been investigated in ZnS:Al and in CdSe:Al single crystals using a two zone resistance furnace and a vacuum sealed quartz ampoule with four tungsten or graphite electrodes. The activation energies of HTEC isobars and the slope values of HTEC isotherms in the large temperature region (500–1200 °C) and in a large component vapour pressure range were determined. The region of compensation of electrons with onefold ionized substitutional Al exists up to change with the region of self-compensation of native defects at high Cd vapour pressures in HTEC isotherms of CdSe:Al. These two electroneutrality conditions were never achieved in ZnS:Al crystals under investigation. The common feature for ZnS:Al isotherms at zinc vapour pressure and for CdSe:Al isotherms at selenium vapour pressure is the existence of self-compensation of Al defects. The equilibrium constant of the association of aluminium with cadmium vacancies in CdSe:Al was determined. The increasing role of holes appears similarly at low Zn vapour pressure values in ZnS:Al and at high selenium vapour pressure values in CdSe:Al.
Deep Level Transient Spectroscopy (DLTS) and Optical Deep Level Transient Spectroscopy (ODLTS) experiments have been conducted on a series of In-doped CdTe crystals grown by the Bridgman or the travelling heater (THM) methods using Te as the solvent. The THM samples are n-type but strongly compensated. Annealing at 700°C under high Cd vapour pressure leads to a decompensation of the crystals. The electron concentration is then a measure of the donor (In) concentration, which was in the range 3 × 1016−1.5 × 1018cm−3. Six and eight electron traps are, respectively detected in the non-annealed and annealed samples at a concentration level 102–103 times below the net electron concentration. They cover the energy range 0.2–0.8 eV. Similar traps are found in both types of crystals, the concentration of which increases with In content and after annealing. The presence of In interstitial-type defects is suggested. A main hole trap at 0.12eV is detected by ODLTS in compensated samples with a concentration close to the donor concentration. Low temperature electrical measurements show that this trap is ionized under equilibrium conditions. It appears to be the main compensating acceptor centre. A plausible microscopic structure is IncdVcd. This study shows that In-doped CdTe grown by THM is electrically compensated and that In-containing neutral associates or precipitates play a minor role in the compensation mechanism.
The high temperature defect equilibria and self-compensation behavior of ZnTe were investigated by measuring the high temperature electrical transport properties under equilibrium conditions of Al-doped single crystals. The conductivity type was changed from p-type to low resistivity n-type between 700 and 925°C in Zn-rich atmospheres by the addition of 6 × 1018 cm−3 Al donors. A two-carrier treatment of the transport measurements yields nαP+0·4Zn. It is proposed that the impurity donors are compensated by doubly ionized native acceptors.
High-temperature electrical conductivity (HTEC) as a function of temperature T and metal component vapour pressure pMe has been studied in ZnS:Ga and CdSe:Ga single crystals. The dependence of the concentration of free carriers n, on pMe and T may be expressed as n∼pMeαexp(-ΔE/kT). The activation energies ΔE from HTEC isobars and slope α values from HTEC isotherms were determined. CdSe:Ga HTEC isotherm is independent of pCd at temperatures lower than 850°C. Ga acts in CdSe as a donor with electroneutrality condition (ENC) and Ga mass balance condition (MBC) approximation n=[GaCd]≈[Ga]total. ZnS:Ga isotherms characterize with α≈0.5 at temperatures higher than 1070°C. ENC and Ga MBC approximation [GaZn]=[(GaZnVZn)/]≈1/2[Ga]total characterize Ga self-compensation in ZnS:Ga.
We propose a materials design using In species as both coactivators to obtain blue-Ag emission and reactive codopants for N acceptors to realize low-resistivity p-type ZnS. In codoping causes a change in the N-impurity state from a localized state, for ZnS:N, to a delocalized one, with a substantial decrease in electrostatic energy.
Measurements of Hall effect and Zn self-diffusion on single crystals of ZnSe doped with various amounts of Al or As were performed at 800 degree , 900 degree , and 1000 degree C when the crystals were in equilibrium with atmospheres of well-defined zinc pressures; Hall effect measurements were also performed on crystals cooled after high temperature equilibration. Analysis of the results leads to the conclusion that Schottky disorder is the main type of atomic disorder. Values of the parameters for various equilibrium constants are determined.
Molecular complexes of four phenols (hydroquinone, resorcinol, phloroglucinol and 4-hydroxybenzoic acid) with isonicotinamide are characterized by X-ray diffraction. (Hydroquinone)0.5·(isonicotinamide)
1 and (resorcinol)·(isonicotinamide)22 have tapes of O–HN and amide N–HO dimer synthons. In (phloroglucinol)·(isonicotinamide)2·(H2O)23, six component self-assembly results in stacked dimers with four O–HN and four amide N–HO hydrogen bonds and the third phenol OH aggregates with two water molecules in a supramolecular chair cyclohexane array. Phenolpyridine and amide N–HO dimers are robust synthons in 1–3 for the self-assembly of 0D discrete aggregates and infinite 1D chains. Hydrogen bonding of amide dimers via N–HOphenol/water is ascribed to cooperative effects. The crystal structure of (4-hydroxybenzoic acid)·(isonicotinamide)
4 establishes the robustness of phenolpyridine hydrogen bonding in the presence of carboxylic acid groups. Molecular components in 4 are arranged as zigzag tapes of O–HN hydrogen bonds and acidamide heterodimers. Supramolecular synthesis with phenol–isonicotinamide adducts expands the versatility of isonicotinamide as a co-crystallizing agent in crystal engineering.
The defect structure of donor-doped Te-rich CdTe is studied theoretically within quasi-chemical formalism and experimentally in heavily In-doped CdTe by in situ high temperature galvanomagnetic measurements in the temperature interval 900–200 °C. The experimental data are evaluated within defect model optimized to recent high temperature experiments and assuming doping-induced band gap renormalization. We show that a proper thermal treatment can be conveniently used for the optimization of room temperature electric properties and for a preparation of the semi-insulating detector grade material with a deep level doping below the limit 1013 cm–3 demanded in the detector industry. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
The electrical and optical properties of the conductive n-type surface layer created in high resistivity CdTe:In by annealing at 400-600°C under Cd-rich overpressure was investigated. Slightly compensated donor level with concentration N<sub>D</sub>∼1.3×10<sup>16</sup> cm<sup>-3</sup> and ionization energy E<sub>D</sub>∼10 meV was obtained from the Hall effect measurement and the electron mobility reaches a maximum of 1×10<sup>4</sup> cm<sup>2</sup>/Vs at 35 K after annealing at 600°C. Purification of the n-type layer was confirmed by photoluminescence measurement, where decreasing of the emission lines related to alkali and silver acceptors were observed. The emission line at 1.854 eV, which is expected to characterize high resistivity material, was detected in as grown high resistivity samples as well as in the conductive n-type surface layer.
- K Lott
- T Nirk
- O Volobujeva
K. Lott, T. Nirk, and O. Volobujeva, Cryst. Eng. 5, 164 (2003).
- A K Ray
- F A Kröger
A. K. Ray and F. A. Kröger, J. Electrochem. Soc. 125, 1348 (1978).
- R Grill
- P Fochuk
- J Franc
- B Nahlovskyy
- P Höschl
- P Moravec
- Z Zakharuk
- Ye Nykonyuk
- O Panchuk
R. Grill, P. Fochuk, J. Franc, B. Nahlovskyy, P. Höschl, P. Moravec, Z. Zakharuk, Ye. Nykonyuk, and O. Panchuk, phys. stat. sol. (b) 243, 787 (2006).
- T Yamamoto
- S Kishimoto
- S Iida
T. Yamamoto, S. Kishimoto, and S. Iida, phys. stat. sol. (b) 229, 371 (2002).
- P Fochuk
- O Panchuk
- O Korovyanko
P. Fochuk, O. Panchuk, and O. Korovyanko, J. Alloys Compd. 371, 10 (2004).
- E Belas
- R Grill
- A L Toth
- P Moravec
- P Horodysky
- J Franc
- P Höschl
- H Wolf
- Th Wichert
E. Belas, R. Grill, A. L. Toth, P. Moravec, P. Horodysky, J. Franc, P. Höschl, H. Wolf, and Th. Wichert, IEEE
Trans. Nucl. Sci. 52, 1932 (2005).
- T Nirk
- M Nõges
- J Varvas
T. Nirk, M. Nõges, and J. Varvas, phys. stat. sol. (a) 10, K27 (1972).
- J Varvas
- T Nirk
J. Varvas and T. Nirk, phys. stat. sol. (a) 33, 75 (1976).
- H Nelkowski
- G Bollmann
H. Nelkowski and G. Bollmann, Z. Nat.forsch. A 24, 1302 (1969).
- K Lott
- O Volobujeva
- A Öpik
- T Nirk
- L Türn
- M Nõges
K. Lott, O. Volobujeva, A. Öpik, T. Nirk, L. Türn, and M. Nõges, phys. stat. sol. (c) 0, 618 (2003).