R. M. Mehra

Highly conductive and transparent aluminum-doped zinc oxide thin films prepared by pulsed laser deposition in oxygen ambient

Highly conducting and transparent aluminum-doped zinc oxide films were prepared on quartz and corning glass 7059 substrate by ablating the sintered ZnO target containing 2 wtu200a% Al2O3 with a XeCl excimer laser (λ=308 nm). To grow the films, a repetition rate of 5 Hz and energy density of 1.5 J/cm2 was kept. The effect of substrate temperature from room temperature to 400u200a°C and oxygen pressure (0.1–5 mTorr) have been investigated by analyzing the optical and electrical properties of these films. The average transmittance was found to be in the range of 86%–92%, and a variable resistivity (ρ) 3.56×10−3–7.0×10−3u200aΩu200acm have been obtained. The lowest resistivity was found to be 1.4×10−4u200aΩu200acm at 300u200a°C in 1 mTorr of oxygen pressure. Structural changes in the films were also investigated by determining the full width at half maximum of (002) x-ray diffraction peak. These results show improvement in the crystallinity of films, which support our conductivity and transmittance data. The sharp decrease in the transmi...

p-type conduction in codoped ZnO thin films

p-type conduction in ZnO thin films was realized by the codoping method. Types of conduction and carrier density in codoped ZnO films were found to be dependent on the oxygen partial pressure ratios in the sputtering gas mixture. The lowest room temperature resistivity was found to be 11.77 Ωu200acm with a hole density of 9.0×1016u2009cm−3 for the films deposited in 60% of oxygen partial pressure ratio. Codoped films were found to be c-oriented and highly transparent.

n-Type conduction in Pb doped Se–In chalcogenide glasses

This paper reports on the p to n transition in Pb doped Se–In chalcogenide glasses. Measurements of thermoelectric power in the temperature range 300u2009K⩽T⩽315u2009K, dc conductivity in the temperature range 100⩽T⩽300u2009K, and optical band gap (Egopt) have been carried out for Se75In25−xPbx (x=0,5,10,15) samples. The p-n transition occurs with very low addition of Pb impurity (5 at. %). The conductivity and pre-exponential factor also change by five to six orders of magnitude with Pb doping. Results are explained on the basis of the formation of ionic Pb–Se bonds, instead of covalent bonds. Formation of ionic bonds disturbs the equilibrium between the charged defect states of Se–In glass and unpins the Fermi level and thus leads to n-type conduction in these glasses.

ELECTRICAL AND OPTICAL PROPERTIES OF AMORPHOUS (SE0.7TE0.3)100-XINX SYSTEM

The effect of In impurity on the dark and photoconductivity and the optical band gap of amorphous (Se0.7Te0.3)100−xInx has been studied. The activation energy ΔE and the pre‐exponential factor σ0(0,T) which appear in the dc conductivity are found to increase with increasing In content. The photocurrent, as a function of illumination intensity and time of illumination, has been measured at around room temperature. The bimolecular recombination mechanism is found to be predominant at steady state in all the samples near room temperature. The nonexponential decay, after stopping the illumination, has been explained in terms of localized‐localized recombination. The data have been analyzed in terms of the dispersive diffusion controlled recombination and monomolecular recombination is found to be dominant in the transient state. The optical band gap is found to decrease with increasing In content. This decrease has been explained on the basis of metallic bonding due to incorporation of In.

Kinetics of photodarkening in a-As2Se3 thin films

We report kinetics of photodarkening of a-As2Se3 films in terms of changes in absorption coefficient and optical band gap. These are estimated by in situ measurements of transmission spectra at intervals of 10 ms under laser illumination. The absorption coefficient increase with increasing time of laser illumination before saturating. The change in optical band gap, estimated from transmission spectra are studied as a function of time and intensity. A theoretical fitting of the change in optical parameters is done using photon-assisted site switching model, yielding the time constants for the changes in optical absorption and optical band gap. The change in absorption coefficient and optical band gap with incident laser intensity is also examined to investigate the saturation phenomenon.

PHYSICAL PROPERTIES OF AMORPHOUS GE20SB25-XBIXSE55 THIN FILMS

Abstract The influence of substitution of Sb atoms by Bi atoms on the electrical and optical properties of thin films of the Ge20Sb25−xBixSe55 [0≦x≦15] system are reported. Results of dc conductivity and thermoelectric power measurements between 150 and 450 K show that the Geue5f8Sbue5f8Se system is chemically modified by addition of large concentrations of Bi atoms between x=5 and x=10 at.%). A transition from p-type for Sb-doped to n-type for Bi-doped films and a decrease of resistivity is observed. The absorption edge shifts to shorter wavelength, thereby decreasing the optical band gap of the system. Compositional dependences of electrical conductivity, thermoelectric power, and the appearance of n-type conduction are discussed from the stand point of chemical bonds formed in the films and related to the defect states produced due to incorporation of Bi atoms in high concentrations. The coexistence of band and hopping conduction is proposed. The ac conductivity in 0.1–10.0 kHz frequency and 150–450 K temperature range was found to obey a power law σ(ω, T) = Aωs. The results were interpreted in terms of Elliotts theory, which assumes correlated barrier hopping (CBH) between the charged defect centres. It was found that computed results from the CBH model and experimental one are qualitative agreement for the present materials.

Study of thermal equilibration in selenium- and sulphur-doped a-Si:H

The present paper reports the thermal equilibration in selenium- and sulphur-doped hydrogenated amorphous silicon thin films deposited by plasma-enhanced chemical vapour deposition. The conductivity of Se- and S-doped a-Si:H is observed to be very sensitive to the rate at which the samples are cooled following the high temperature anneal. Arrhenius plots of conductivity for various doped films revealed thermal equilibration above the equilibration temperature, T E, thus accounting for larger activation energies. The barrier energy is lower for a-Si,Se:H than for a-Si,S:H, due to a higher defect density in S-doped films.

Tailoring refractive index of a-Si:H by TBP (C4H11P) doping

Abstract The refractive index, n , and extinction coefficient, k , of n-type TBP (tertiarybutylphosphine) doped a-Si:H films is reported. TBP is preferred over phosphine because of its lower toxicity and pyrophoricity. The films were deposited using glow discharge (plasma lab μ P) technique. The optical constants, n , k and absorption coefficient, α are determined as functions of wavelength, λ and doping concentration. The refractive index is found to decrease as the TBP/SiH 4 ratio increases from 0.1% to 3%. This decrease is due to increase in the internal strain in amorphous network. Also the refractive index and extinction coefficient decrease with increase in wavelength in the range of 600 to 900 nm.

Analysis of single polaron hopping in ac conductivity of amorphous Ge20SbxSe80−x glasses

The results for the ac conductivity over temperature range (298–500 K) and frequency range (102–107 Hz) for the Ge‐Sb‐Se glasses with different compositions are presented. These glasses exhibit a strong temperature dependence of ac conductivity and of its frequency exponent s. The results are discussed in the light of the correlated barrier hopping model for ac conduction. All the features observed are interpreted by taking into account the contribution of both single polarons and bipolarons. It is shown that the single polarons dominate the hopping transport process in the measured temperature range. The effect of the addition of Sb is also discussed in terms of the present model. The energy levels and densities of the defect for Ge‐Sb‐Se glasses are deduced from the comparison between the theoretical and experimental results.

Optical properties of large band gap Se- and S-doped amorphous hydrogenated silicon

Abstract Selenium- and sulfur-doped amorphous hydrogenated silicon films were prepared by the conventional plasma-enhanced chemical vapor deposition (PECVD) technique. Both Se- and S-doped films have band gaps that range from 1.7 to 2.0 eV, as a function of doping concentration. The photoresponse of the films was determined by illuminating with white light of 100 mW/cm2 intensity. The intensity variation of photoconductivity (PC) showed monomolecular recombination mechanism for a-Si:H films doped with Se (for H2Se/SiH4⩽10−1%) and S (for H2S/SiH4⩽10−2%) and bimolecular for other dopings. The persistent photoconductivity (PPC) measurements at room temperature showed that PPC increased with increasing irradiation time. PPC for Se-doped a-Si:H for a concentration of 10−3 at.% was of the same order as for 2.7×10−2 at.% S-doped sample. Photodegradation results indicate that a-Si,Se:H system would be more stable than the a-Si,S:H system as the compensation of the photodegradation due to PPC is larger in the case of Se-doped films.