K.L. Bhatia

Study of effects of dopants on structure of vitreous semiconductors (GeSe3.5)100-xMx (M = Bi, Sb) using high pressure techniques

An investigation of the problem of controlled doping of amorphous chalcogenide semiconductors utilizing a Bridgman anvil high pressure technique, has been undertaken. Bulk amorphous semiconducting materials (GeSe3.5)100-x doped with M = Bi (x = 2, 4, 10) and M = Sb (x = 10) respectively are studied up to a pressure of 100 kbar down to liquid nitrogen temperature, with a view to observe the impurity induced modifications. Measurement of the electrical conductivity of the doped samples under quasi-hydrostatic pressure reveals that the pressure induced effects in lightly doped (2 at % Bi) and heavily doped (x = 4, 10) semiconductors are markedly different. The pressure effects in Sb-doped semiconductors are quite different from those in Bi-doped material.

Carrier-sign reversal in Bi-doped bulk amorphous semiconductors Ge20Te

A p-n transition in the electronic conduction in Bi-doped bulk amorphous semiconductors Ge 2 Te 80-x Bi x at about x=3.5 has been observed for the first time. n-type conduction has been established by the measurement of thermoelectric power that becomes negative at x~3.5. Measurement of the electrical transport under high quasihydrostatic pressure (p≤ 120 kbar) has also been studied in x=0,2,3,4,6 compositions. Electrical resistivity (ρ ), thermal activation energy (Δ E), and (dlog 10ρ /dp) show a sudden drop in their values in the ρ -n transition region (x=3-4). Results are discussed in the light of Phillips model of microclusters and constraint theory.

Frequency-dependent electrical conductivity of bismuth-modified amorphous semiconductors (GeSe3.5)100−xBix☆

Abstract An investigation of the AC conductivity and dielectric relaxation of Bi-modified amorphous germanium selenide semiconductors (Ge Se 3.5 ) 100− x Bi x in the frequency range 100 Hz–10 KHz and in the temperature span 180 K–440 K has been undertaken for the first time. The AC conductivity is found to be proportional to ω s . The temperature dependence of σ AC ( ω ) and the parameter s in all the compositions is reasonably interpreted by the correlated barrier hopping model (CBH). The analyis of the results reveals that the electronic conduction in x = 0, 2, 4 compositions takes place via bipolaron hopping. Addition of Bi in higher concentration ( x = 10) induces a new type of microscopic environment and new defects which take part in the single polaron hopping conduction process.

Role of dopants in pressure-induced effects in glassy GeSe3·5 containing Bi and Sb

Abstract The Bridgman anvil high-pressure technique has been used to investigate the role of dopants in pressure-induced effects in the electrical resistivities of amorphous (GeSe3·5)100-xMx (M = Bi, Sb). The results for Sb-doped glasses are found to be entirely different from those of Bi-doped glasses. This difference is discussed on the basis of the presence of different types of molecular units in the two glassy systems.

Pressure induced effects in bulk amorphous n-type semiconductors (GeSe3.5)100−xBix

The effect of pressure on the electrical resistivity of amorphous n-type (GeSe3.5)100�xBix been studied in a Bridgeman anvil system up to a pressure of 90 kbar down to liquid nitrogen temperature. A continuous amorphous semiconductor to metal-like solid transition in the undoped GeSe3.5 is observed at room temperature. Incorporation of Bi in the GeSe3.5 network is found to significantly disturb the behaviour of the resistivity with pressure. With increasing Bi concentration a much broader variation in resistivity with pressure is observed. The temperature dependence of the resistivity and activation energy at different pressures is also measured and they are found to be composition dependent. Results are discussed in the light of the Phillips Model of ordered clusters in chalcogenide semiconductors.

Electronic conduction in vitreous semiconductors in the pseudo-binary system (As2S3)1-x(PbS)x

AC conductivity and dielectric relaxation measurements of the bulk amorphous compositions in the pseudo-binary system (As2S3)1-x(PbS)x (x = 0, 0.1, 0.4 and 0.5) in the frequency range 500 Hz-10 kHz and in the temperature span 180–450 K are reported. The temperature dependence of the ac conductivity, σac(ω), has a broad structure at all frequencies in compositions with x = 0.1, 0.4 and 0.5 whose peak position is not thermally activated. A similar structure was also observed in the data on the dielectric constant, ϵ1, which peaked at a frequency of 1 kHz in the composition with x = 0.5. Analysis of the results using the correlated barrier hopping model revealed that the electronic conduction takes place by single polaron and bipolaron hopping processes at high and low temperatures, respectively, in compositions containing Pb. The microstructure and phase-separation in the glasses containing Pb influence the electrical transport and dielectric dispersion. This study has revealed the possible presence of a phase transformation at around 300 K at a frequency of 1 kHz in the dielectric dispersion behaviour of composition with x = 0.5.

On the structural features of doped amorphous chalcogenide semiconductors

A study of Bi-doped amorphous (Ge42S58)100−xBix and Ge20S80−xBix has been carried out by differential thermal analysis (DTA) and X-ray diffraction methods so as to elucidate the impurity-induced modifications in the semiconductors. Thermal analysis reveals the presence of complex structural units in the modified material. An interesting feature of this study is the existence of a double glass transition in Ge20S80−xBix, which is reported for the first time in this system.

On the glass formation tendency in (PbS)x·(M2S3)1−x (M = As, Sb)

Abstract Materials in the system (PbS)x·(M2S3)1−x (M = As, Sb, Bi) form alloy compounds over a wide range of compositions. PbAsS forms glasses upto x = 0.625, PbSbS produces glassy compounds only near eutectic composition and PbBiS only forms crystalline compounds using the conventional liquid quenching technique. The vitrification characteristics of this series of solids is investigated using the basic crystal chemistry of the two participating systems: PbS and M2S3. Further, disorder induced defects in PbAsS glasses, are studied using the long term relaxation of photoconductivity.

Effect of addition of Zn impurities on the electronic conduction in semiconducting Se80-xTe20Znx glasses

Abstract A study of the effect of the addition of Zn impurities on the electronic transport properties of melt-quenched semiconducting glasses in the system Se80-xTe20Znx (x = 0, 1, 2 and 4) has been undertaken for the first time. Measurements of the temperature dependence of the d.c. conductivity and the frequency and temperature dependence of the d.c. conductivity [sgrave]a.c.(ω) and dielectric relaxation have been made in the temperature interval 180–450 K and frequency range 500 Hz–10 kHz. It is revealed that [sgrave]a.c.(ω) is proportional to ω s . The temperature dependence of [sgrave]a.c.(ω) and the parameter s is reasonably interpreted by the correlated-barrier-hopping model. The analysis of the results shows that the electronic conduction in compositions with x = 0 and 1 takes place by a bipolaron-hopping process in the whole temperature range of study. The addition of higher concentrations of Zn impurities (x = 2 and 4) induces new types of defects which contribute predominantly to single-polaro...

Electrical and optical properties of thermally evaporated thin films of (As2S3)1−x(PbS)x

Abstract An investigation of the electrical and optical properties of amorphous thin films of alloys in the pseudo-binary system (As2S3)1−x(PbS)x (x = 0, 0.1, 0.4 and 0.5) prepared by flash evaporation has been carried out. The synthesized films were characterized by X-ray diffraction, electron spectroscopy for chemical analysis and scanning electron microscopy techniques. The study shows that the stoichiometric composition (x = 0.5 ) exhibits electronic properties which are quite different from those of the other compositions. The optical energy gap decreases with the addition of PbS. The electrical transport takes place predominantly in the extended states. The alternating current (a.c.) conductivity results show that, at lower temperatures, conduction occurs via bipolaron hopping, while, at higher temperatures, it predominantly takes place by single polaron hopping. In contrast to the bulk glasses, no broad peak is observed in the temperature dependence of the a.c. conductivity of the thin films. The thin films appear to be homogeneous, while the corresponding bulk compositions show phase separation. The structural and electronic features of these semiconductors arc affected by the increasing Pb coordination number with the increase in the PbS content; the coordination number is 9 in the composition with x = 0.5.