Tayyar Dzhafarov

Formation of CdZnS thin films by Zn diffusion

The structural, composition and optical absorption properties of Cd1−xZnxS (CdZnS) thin films fabricated by the reactive diffusion of Zn in CdS have been investigated. These ternary compounds were characterized by x-ray diffraction (XRD), energy dispersive x-ray fluorescence and optical absorption measurements. It is established that thermal annealing of Zn/CdS structure at temperature (400 °C) lower than the melting point of Zn (418 °C) results in the concentration distribution of Zn in CdS film described by an erfc-curve (D = 5 × 10−14 cm2 s−1) and characterizing the free impurity diffusion from a constant source. In contrast to this, the concentration profile of Zn in CdS film at higher annealing temperature (570 °C) is not described by the erfc-curve and shows a nearly stepped form, which is characteristic of reactive diffusion. XRD patterns of Zn/CdS structures annealed at 570 °C showed diffraction peaks of ternary CdZnS compounds. Analysis of the absorption spectrum of such films indicates formation of CdZnS composition with the largest value of energy band gap up to 2.64 eV, exceeding the band gap of CdS (2.43 eV). It is concluded that interdiffusion in Zn/CdS structures at temperatures exceeding the melting point of Zn was accompanied by formation of Cd1−xZnxS ternary compounds. The band gap of this variable band structure changes from 2.64 eV in the near surface region to 2.43 eV (CdS) in the inner region of the film.

The effect of Ag photodiffusion on characteristics of Ag-CdS diode structures

The effect of illumination on silver diffusion in Ag-CdS diode structures at room temperature was investigated by using radiotracer technique. The effect of photostimulated Ag diffusion in CdS at room temperature is equivalent to thermal diffusion of silver at about . The influence of silver photodiffusion on electrical, photoelectrical and optical characteristics of Ag-CdS diode structures was studied. A decrease in the photosensitivity, rectifying characteristics of Ag-CdS diode structures and an increase in the electron concentration in the near-surface region of the CdS substrate and in the junction width and optical transmission of the structure were discovered after illumination. The possible mechanisms of photostimulated silver diffusion in CdS and degradation of characteristics of Ag-CdS diode structures are considered.

Porous Silicon-Based Gas Sensors and Miniature Hydrogen Cells

The current–voltage (I–V) characteristics of Au/porous silicon (PS)/Si Schottky-type structures in humid, CO, and H2S gas atmospheres were investigated. It was found that the gas atmosphere influences the I–V characteristics, particularly the reverse current in Au/PS/Si and Au/PS structures. The reverse current in the H2S atmosphere was 103 times larger than that in room-temperature (300 K) air [45% relative humidity (RH)]. The generation of an open-circuit voltage (up to 480 mV) at the Au/PS interface in humid, CO, and H2S atmospheres was detected. The humidity- and gas-stimulated effect of voltage generation in the Au/PS/Si structures was reversible with a response time of 60 s. The quite high sensitivities to humidity, CO, and H2S (about 9 mV/RH, 4 mV/ppm, and 2 mV/ppm, respectively) indicate the possibility of using the obtained Au/PS/Si structures as both gas sensors and miniature hydrogen fuel cells. The mechanism for the gas-stimulated generation of electricity in the Au/PS structures is discussed and is considered to be similar to that of the proton exchange membrane in hydrogen fuel cells.

Influence of Au diffusion on structural, electrical and optical characteristics of CdTe thin films

Diffusion of Au and its effects on structural, electrical and optical properties of CdTe films fabricated by the close-spaced sublimation technique have been investigated. Diffusion of Au was studied in the range 400?550??C using energy dispersive x-ray fluorescence analysis. Au-doped and un-doped CdTe films were characterized by x-ray diffraction (XRD), electrical and optical absorption measurements. The temperature dependence of the diffusion coefficient of Au in CdTe films is described as D = 4.4 ? 10?7exp(?0.54?eV/kT). The mechanism of Au diffusion in polycrystalline CdTe films is attributed to the fast migration of Au along grain boundaries with simultaneous penetration into grains and settling on Cd-vacancies. It is supposed that the weak influence of Au diffusion on XRD patterns of CdTe films can be explained by dispersal of Au atoms preferentially on Cd-vacancies owing to proximity of the covalent radius of Au and Cd. Au atoms, placed on Cd-vacancies (AuCd) during fast cooling from diffusion temperature to room temperature, show an acceptor behaviour with an energy level about of Ev + 0.2?eV. The nature of this level is discussed.

Modification of CdTe thin films by Zn reactive diffusion

Formation of Cd1−xZnxTe (CdZnTe) ternary compounds as a result of Zn diffusion in CdTe thin films, the structural and optical properties of CdZnTe compounds and I–V characteristics of CdZnTe/CdTe heterojunctions are presented. X-ray diffraction study of Zn/CdTe structures exposed to thermal annealing indicates the formation of CdZnTe compounds. Analysis of the absorption spectra of Zn/CdTe structures, annealed at 500 °C, and CdTe thin films show that the energy band gap of the former (1.65 eV) is larger than that of the latter (1.50 eV). Such an increase of the band gap of annealed two-layer Zn/CdTe is attributed to the reactive diffusion of Zn in CdTe films accompanied by the formation of Cd1−x ZnxTe compounds with an average value of x = 0.22. The temperature dependence of the effective diffusion coefficient of Zn in CdTe at 430–520 °C is described as D = 2.5 × 10−3 exp(−1.30 kT−1)~cm2 s−1.

Metal/Porous Silicon Schottky Diode Structures as Sensors

In this work we present data on investigation of the current-voltage and capacitance characteristics of Au/PS Schottky type structures in the presence of different hydrogen-containing solutions (glucose, ethanol, methanol, boric acid, sodium tetraborate pentahydrate, sodium borohydride, benzine, KOH). Generation of the open-circuit voltage and short-circuit current density and capacitance up to 0.55 V, 25 mA/cm2 and 1μF respectively on placing of Au/PS structures in these solutions was discovered. This effect is mainly caused by hydrogen component of solutions. The possible mechanism generation of voltage and capacitance in metal/PS sensors hydrogen-containing solutions is suggested. The advantage of metal/PS Schottky type sensors consists in working without applying external electricity.

Photostimulated Changes of Electrical Characteristics of Ag/CdTe Thin Film Structures

Electrical, optical and structural properties of Ag/CdTe structures exposed to thermal (in dark) and photoannealing (under illumination) have been studied. The effective diffusion coefficie nt of Ag in CdTe films have been estimated from resistance versus duration of annealing curves. In the range of 280-420°C the effective coefficient of thermal diffusion (D t ) and photodiffusion (D ph ) are described as Dt= 1.9x10 5 exp (-1.60/kT) and Dph =8.7x10 3 exp(-1.36/kT). The acceleration of Ag diffusion under illumination was tentatively attributed to photoionization of Ag increasing the interstitial flux of silver. Ag/CdTe structures exposed to annealing were characterized by X-ray diffraction (XRD), I-V, C-V, conductivity-temperature and optical transmission measurements. In XRD patterns of annealed Ag/CdTe structures, besides the intensive (111) peak of cubic CdTe, the weak peaks of Ag 2 Te phase are also present. The temperature dependence of conductivity of annealed Ag/CdTe structures showed the energy levels 0.13 eV.

Molybdenum diffusion in CuInSe2 thin films

Molybdenum diffusion in CuInSe2 (CIS) films was investigated in the temperature range 240-520°C using the energy-dispersive X-ray (EDX) and resistivity techniques. CuInSe2 thin films were deposited by single-source evaporation on glass and molybdenum-covered glass substrates. The temperature dependence of Mo diffusion coefficient is described by equation D=1.3×10-8exp (-0.53/kT). It is shown that the diffusion doping of p-type CIS by molybdenum is accompanied by the significant decrease of resistivity. The observed results of the diffusion and electrical measurements are attributed to migration of Mo in the polycrystalline CIS films by means of both along intergrain surfaces and into grains.