Preparation and characterization of flash-evaporated CuInSe2 thin films

Thin films of CuInSe₂ have been evaporated onto glass substrates by flash evaporation. The as-deposited films are amorphous and annealing in selenium atmosphere produces polycrystalline films. The films were characterized bytem and x-ray diffraction techniques. The optical absorption of the films shows three energy gaps of 1·03, 1·07 and 1·22 eV. The crystal field and spin-orbit splitting are thus found to be 0·04 eV and 0·16 eV respectively. The percentaged-character of the valence band states is ∼35%. The Arrhenius plot of electrical conductivity of films showed impurity ionization ofE _A = 75 meV.     

Barrier characteristics of Pt/Ru Schottky contacts on <Emphasis Type="Italic">n</Emphasis>-type GaN based on <Emphasis Type="Italic">I–V–T</Emphasis> and <Emphasis Type="Italic">C–V–T</Emphasis> measurements

We have investigated the current–voltage (I–V) and capacitance–voltage (C–V) characteristics of Ru/Pt/n-GaN Schottky diodes in the temperature range 100–420 K. The calculated values of barrier height and ideality factor for the Ru/Pt/n-GaN Schottky diode are 0·73 eV and 1·4 at 420 K, 0·18 eV and 4·2 at 100 K, respectively. The zero-bias barrier height (Φ_b0) calculated from I–V characteristics is found to be increased and the ideality factor (n) decreased with increasing temperature. Such a behaviour of Φ_b0 and n is attributed to Schottky barrier (SB) inhomogeneities by assuming a Gaussian distribution (GD) of barrier heights (BHs) at the metal/semiconductor interface. The current–voltage–temperature (I–V–T) characteristics of the Ru/Pt/n-GaN Schottky diode have shown a double Gaussian distribution having mean barrier heights ( [`(F)]<sub>\textb0</sub> {\bar{{\Phi}}_{\text{b}0}} ) of 1·001 eV and 0·4701 eV and standard deviations (σ <sub>0</sub>) of 0·1491 V and 0·0708 V, respectively. The modified ln (J<sub>0</sub> /T<sup>2</sup> )-( q<sup>2</sup>s <sub>0</sub><sup>2</sup>/2k<sup>2</sup>T<sup>2</sup> ){ln} ({{J}_{0} /{T}^{2}} )-( {{q}^{2}{\sigma} _{0}^{2}/{2}{k}^{2}{T}^{2}} ) vs 10<sup>3</sup>/T plot gives [`(F)]_\textb0 \bar{{\Phi}}_{\text{b}0} and Richardson constant values as 0·99 eV and 0·47 eV, and 27·83 and 10·29 A/cm²K², respectively without using the temperature coefficient of the barrier height. The difference between the apparent barrier heights (BHs) evaluated from the I–V and C–V methods has been attributed to the existence of Schottky barrier height inhomogeneities.     

Iron and gold related defects in water quenched silicon

Thermally induced defects in heat-treated and then quenched in water n-silicon samples have been studied using deep level transient spectroscopy. Two deep levels at energies E _c – 0.48 eV and E _c – 0.25 eV are observed in high concentration. The emission rate signatures and annealing characteristics showed the DLTS signal due to level at energy position E _c – 0.48 eV is not only due to Au(A) but some other level also contributes to this signal. The energy state at E _c – 0.25 eV is identified to be pinned with E _c – 0.48 eV. The annealing characteristics also revealed the contribution of Au–Fe complex in DLTS signal of E _c – 0.25 eV level. A complementary behavior of these two levels in annealing characteristics has also been observed.     

Size quantization effects in optical and electrical properties of II–VI semiconductor films in nanocrystalline form

The electrical properties of CdTe and optical properties of ZnS in nanocrystalline thin film form are studied with a view to have a clearer understanding of the optical processes and the carrier transport mechanisms in nanocrystalline II–VI semiconductors, in general. Nanocrystalline ZnS and CdTe films were deposited by magnetron sputtering of respective targets in argon plasma. The optical absorption data of nanocrystalline ZnS films (thickness 10–40 nm) could be explained by the combined effects of phonon and inhomogeneity broadening along with optical loss due to light scattering at the nanocrystallites. The conductivity of CdTe (grain size within 4–4·7 nm) showed (T ₀/T) ^p dependence withp ∼ 0·5 indicating the presence of a Coulomb gap near the Fermi level. The width of the Coulomb gap varied within 0·02–0·04 eV depending on the deposition condition. The existing theoretical models were used for estimating hopping energy (0·02–0·04 eV) and hopping distance (2·8–5·1 nm) in nano CdTe films.     

Annealing kinetics of gold and iron–gold complex

Thermally induced defects in heat treated and then quenched in water n-silicon samples have been studied using deep level transient spectroscopy. Two deep levels at energies E _c-0.55 eV, and E _c-0.23 eV are observed in high concentration. The emission rate signature and annealing characteristics of energy state E _c-0.55 eV identify it as Au(A). During annealing a level emerges at energy position E _c-0.35 eV. This level has been identified as Au–Fe complex. Au(A) and Au–Fe showed an interesting reversible reaction in temperature range 175 °C–325 °C which follows the following theoretical relation that adds a new parameter in identifying Au(A) and Au–Fe complex.

Variation of electrical properties in sepiolite during dehydration

The dielectric constant (K), loss (tanδ) andac conductivity of a sepiolite sample have been measured as a function of frequency in the range 10²-10⁷ Hz and in the temperature region 30–400°C. Thedc conductivities of hot pressed pellets were measured in the temperature range 30–550°C.K and tanδ of these samples decrease with increasing frequency. Results of these measurements have been correlated with the structural transformation accompanying dehydration, making use ofdta, ir and x-ray data. The moderately large value of activation energy (∼ 2·2 eV) in the intrinsic range suggests that the conduction process is most probably associated with ions.     

Pressure-induced band cross-over in Pb1−x Sn x Te

Resistivity and thermoelectric power studies have been carried out on two semiconductor alloy systems viz Pb_0·8Sn_0·2Te and Pb_0·6Sn_0·4Te up to 35 kbar pressure. Thermoelectric power and resistivity data on Pb_0·8Sn_0·2Te indicate that the energy gapE _g=E _{L −} ⁶ −E _{L 6} ⁺ decreases with pressure resulting in a zero gap state near 35 kbar pressure. TEP studies on the alloy system Pb_0·6Sn_0·4Te provide direct evidence for a pressure induced L ₆ ⁻ →L ₆ ⁺ cross over transition.     

Piezoelectric properties,hysteresis behaviour and dielectric properties of PMN-PZT ceramics

Ferroelectric and piezoelectric properties ofxPb(Mg_1/3Nb_2/3)O₃−(1−x)Pb (Zr_0·55Ti_0·45)O₃ system have been investigated. X-ray diffraction patterns indicate rhombohedral and cubic structures. Maximum dielectric constant and piezoelectric properties are exhibited by 0·5–0·5 PMN-PZT composition.P _r is high in 0·6–0·4 PMN-PZT composition.     

Preparation and properties of (CdS) x -(PbS)1 −x thin-film composites

A modified chemical deposition process is employed for the preparation of thin-film (CdS) _x -(PbS)_{1 −x} composites with 0·2 ≤x ≤ 0·8. Cadmium sulphate, lead acetate and thiourea are used as the basic source materials. The electrical conductivity is found to decrease with increasing composition parameterx up to 0·5 and increase for further increase inx. The composites are polycrystalline as is revealed from XRD and microscopic observations and show phases of both cubic and hexagonal CdS, cubic CdO and PbS, and tetragonal PbO and PbO₂. Additional peaks of free elemental Cd and S have also been observed. For all the phases no significant variation in lattice parameters withx has been observed. The optical absorption studies show the presence of four well-defined absorption edges at approximately 0·45 eV, 1·05eV, 1·80 eV and 2·35 eV, at the same energies for allx values. The absorption coefficient is of the order of 10⁴ cm⁻¹ and mode of band-to-band transition is of the direct type.     

Characterization of La1 −x Sr1 +x Cu5 −x Fe x O12 + δ (0 ≤x ≤ 1·0) by dc electrical conductivity,magnetic susceptibility and EPR measurements(0 ≤x ≤ 1·0) by dc electrical conductivity,magnetic susceptibility and EPR measurements

The DC electrical resistivity results of La_{4 −x} Sr_{1 +x} Cu_{5 −x} Fe _x O_{12 + δ} (0 ≤x ≤ 1·0) showed that for S1 (x = 0) and S2 (x = 0·25) the temperature coefficient of resistivity (TCR), dρ/dT, is positive and slightly increases with increasing temperature in the range 20–270 K. This shows the metallic nature of S1 and S2. For the samples S3(x = 0·5) and S4 (x = 0·75), TCR slightly increases in the range 20–270 K, with change in sign from negative to positive at ∼ 80 K and ∼ 130 K, respectively. These results show the metal-insulator type transition in S3 and S4. For the sample S5 (x = 1·0), the TCR is negative and gradually increases in the range 20–270 K, which shows its semiconductor-like behaviour. The activation energy for S5 is found to be 0·21 × 10⁻² eV. Furthermore, the DC resistivity results of S1–S5 in the range 350–660 K are in conformity with the low temperature results. The very weak temperature dependence of magnetic susceptibility results of S1–S3 show Pauli-paramagnetic behaviour in the range 77K–400 K, while S4 and S5 exhibit Pauli-paramagnetic behaviour in the range 77–850 K. Long-range antiferromagnetic interaction is observed in S5 (x = 1·0) belowT _c ∼ 100 K. The room temperature EPR lineshapes gradually improve from metallic S1 (x = 0) to semiconductor-like S5(x = 1·0). Negativeg-shift is observed in the samples S2–S5 with increasing trend ing _iso-values of 1·880 in S2 to 1·961 in S5. However, theg _iso-value for S1 could not be observed due to very poor lineshape.     

Ellipsometric characterization of Cd1−x Mn x Te thin films in the presence of perturbative fields

Ellipsometric studies in the 1·5 to 5 eV range have been carried out on thin films of Cd_1−x Mn _x Te and exhibitE ₀,E ₁ andE ₁+Δ₁ transitions of the cubic semiconductors. The effect on these transitions upon irradiation by HeNe laser light, application of small alternating magnetic fields and the influence of both the fields, applied simultaneously, has been investigated. The observed critical point shifts have been interpreted on the basis of changes in the band structure of these thin films. This has been corroborated by theoretically calculating the effective number of electrons contributing to the transition per atom —N _eff. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995.     

Evidence of two ferroelectric PTCR components in valence-compensated ceramic system Ba1−x La x Ti1−x Co x O3

The electrical behaviour of valence-compensated ceramic system Ba_1−x La _x Ti_1−x Co _x O₃ has been studied as a function of temperature (300–600 K) and composition (x ⩽ 0·20), using the method of impedance spectroscopy. The necessary equivalent circuit models that represent the data best have been obtained using impedance and modulus formalisms and grain and grain boundary contributions have been separated out. The compositionsx = 0·20 and 0·10 show a negative temperature coefficient of resistance (NTCR) behaviour whereas a very small variation of the grain and grain boundary resistance with temperature is observed forx = 0·05. A positive temperature coefficient of resistance (PTCR) behaviour having two ferroelectric components is observed forx = 0·01. These results reveal limitations in current theories of the PTCR effect.     

Conduction mechanism in codeposited AgSe thin films

Thin films of AgSe of varying compositions and thicknesses have been formed on glass substrates employing the three-temperature method.I–V characteristics and thermoelectric power, α, of annealed samples have been measured as functions of composition, thickness and temperature of the films. Films exhibitn-type conductivity. Nonohmic conduction in films of Ag _x Se_1−x (0<x<0·5) and Ag _x Se_1−x (0>x>0·5) have been accounted for on the basis of the theory of Rose of defect insulator containing shallow traps and on Schottky emission respectively.     

Oxygen diffusion and defect mechanism in c-axis textured thin films of YBa2Cu3O7−x by resistivity measurements

Isothermal resistivity measurements have been carried out to study the dynamics of oxygen out- and in-diffusion in thin films of YBa₂Cu₃O_7−x in the temperature range 648–773 K. The activation energies for the out- and in-diffusion were determined to be 1·36 and 0·7 eV respectively. We have modelled the resistance-time plots for the oxygen in-diffusion using an equation for one dimensional diffusion into a plane. The microstructural defects such as low angle grain boundaries associated with the c-axis oriented grains are believed to provide the required diffusion paths in thin films of YBCO.     

In situ observation of the relaxation of conductivity in γ-irradiated <Emphasis Type="Italic">n</Emphasis>-type silicon under the action of ultrasound pulses

Reversible change of the electric conductivity σ_US in a temperature range of T = 110–180 K has been observed for the first time in gamma-irradiated and partly annealed (280°C) floating-zone grown silicon (n-Si-Fz) under the action of pulsed ultrasound (longitudinal wave) at a frequency of 6–10 MHz, intensity up to 4 × 10³ W/m², and pulse duration within 10⁻⁵–10⁻³ s. It is established that the temperature dependences of the parameters of acoustic-wave-induced change of σ_US (increase time, τ _i ; decay time, τ _d ) obey the Arrhenius law. Experimental τ _{i, d} (T) curves have been used to determine the corresponding activation energies (E _i ≈ 0.09 eV, E _d ≈ 0.13 eV) and preexponential factors (τ _i ⁰ ≈ 4 × 10⁻⁸ s, τ _d ⁰ ≈ 10⁻⁹ s). The observed phenomenon is interpreted as an acoustic-wave-induced transition between the states of a metastable structural defect.     

Effect of temperature on thermally induced defects in silicon

Deep level transient spectroscopy has been used to study thermally activated defects in silicon. It has been observed that different annealing temperatures activate different defects in silicon, which were lying on inactive sites before annealing. Two deep mid-gap levels at energy positions E _c −0.48 eV and E _c −0.55 eV were found to be introduced by different heat treatments. It is also noted that heat treatment at 1,250 °C suppresses the concentration of deep level at E _c −0.23 eV and enhances the concentration of deep level at E _c −0.25 eV, while heat treatment at 950 °C has an opposite effect. Annealing response of the level at E _c −0.48 eV is found different to the annealing response of the level at E _c −0.55 eV which suggests them two different levels.     

Thin film CdZnS/CuInSe2 solar cells by spray pyrolysis

Cd_1−xZn_xS/CuInSe₂ solar cells having efficiencies in the range of 2·3% were fabricated by spray pyrolysis. The best cell had the following parameters:V _oc = 305 mV,J _sc = 32 mA/cm², FF = 0·32 area = 0·4 cm² and efficiency = 3·149%.V _oc versus temperature measurements showed that the electron affinity difference was 0·22 eV. Forward dark current versus voltage curves were plotted and a possible current mechanism occurring in these cells has been proposed.     

Effect of cobalt substitution on magnetic and transport properties of Nd0·5Sr0·5Mn1?xCoxO3 (x = 0·1, 0·3 and 0·5)

The magnetic and transport properties of the compounds Nd_0·5Sr_0·5Mn_1-x_{{\rm 1}-{x}}Co_x_{{x}}O₃ (x = 0·1, 0·3 and 0·5), synthesized by citrate–gel route have been investigated. The spin transition in cobaltates at low temperatures affects the magnetic as well as transport properties. The irreversibility behaviour between the zero-field cooled (ZFC) and field cooled (FC) magnetization as a function of temperature becomes stronger with increasing Co content. This is understood on the basis of glassy behaviour, which becomes more robust with increasing Co substitution. The non-saturating M–H behaviour indicates strong magnetic inhomogeneities which may cause the magnetic phase separation at the nanoscopic length scale. The double exchange interaction is stronger between Mn^3 +–O^2 −–Mn^4 + as compared to Co^3 +–O^2 −–Co^4 + pairs. Co-substitution suppresses the double exchange which will lead to cluster/spin glass like behaviour as well as semiconducting features due to localization of charge carriers (mobile e_g{e}_{\rm g} electrons).     

Preparation and characterisation ofβ″-Al2O3

Beta alumina solid electrolyte is a potential candidate in the fabrication of Na-S batteries. In the present study, it has been prepared in the form of discs by uniaxially as well as isostatically pressing and sintering in the temperature range 1585–1630°C, the highest sintered density of 3·25 g/cm³ has been achieved in the samples isostatically pressed and sintered at 1630°C. X-ray analysis of the samples shows formation ofβ″-phase. Microstructure of the sintered samples reveals some darker regions which are attributed to low soda content. Resistivity at 300°C measured by the two-probe method at a frequency of 1 MHz on samples having vacuum-deposited silver electrodes is 15 ohm-cm, which is slightly higher than the value of 5–13 ohm-cm reported in literature using molten sodium electrodes. The activation energy of conductivity is 0·24 eV which is comparable to the literature value of 0·24–0·35 eV.     

Combustion synthesis and structural characterization of Li–Ti mixed nanoferrites

Polycrystalline samples of the mixed nanoferrites, Li_{0·5 + 0·5x} Ti _x Fe_{2·5 − 1·5x} O₄ (0·02 ≤ x ≤ 0·1), were prepared by combustion method at lower temperatures compared to the conventional high temperature sintering for the first time at low temperatures, using PEG which acts as a new fuel and oxidant. XRD patterns reveal a single-phase cubic spinel structure. The as synthesized Li–Ti ferrites are in nanocrystalline phase. The crystallite size was found to be in the range 16–27 nm. SEM images reveal rod-like morphology in all the samples with a discontinuous grain growth. The B–H loops have been traced using VSM technique, for all the compositions, at room temperature and the hysteresis parameters are calculated. Saturation magnetization decreases with increase in Ti content due to the fact that the Ti^4 + ion, which is a non-magnetic ion, replaces a magnetic Fe^3 + ion. The hysteresis loops show clear saturation at an applied field of ±10 kOe and the loops are highly symmetric in nature. The cation distribution is known indirectly by using saturation magnetization values.