• Navdeep Goyal

      Articles written in Pramana – Journal of Physics

    • Electrical and photo-electrical properties of a chalcopyrite semiconductor AgInSe2

      Navdeep Goyal

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      In this paper we report the electrical and photoelectrical properties of AgInSe2. Nyquist plots for AgInSe2, obtained at different temperatures, are perfect arcs of a semicircle with their centres lying below the abscissa at an angleα. Finite values ofα (the distribution parameter) clearly indicate a multirelaxation behaviour. Transient and steady state photoconductivity of AgInSe2 has been studied at different temperatures and illumination levels. The lnIph vs lnF curves at different temperatures follow the empirical relation:IphFγ. Values of γ are close to 0.5 at all the temperatures, suggesting a bimolecular recombination. Decay of the photocurrent, when the illumination is switched off, shows that during decay, photocurrent has two components, i.e. a fast decay in the beginning and a slow decay thereafter. Decay time constant for slow decay process decreases with increasing temperature, suggesting that recombination is a thermally activated process in the temperature range studied.

    • Electrical properties of nickel-doped arsenic trisulphide

      Navdeep Goyal Rajni Shukla Manohar Lal

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      Results of temperature and frequency dependent a.c. conductivity of pure and nickel-doped a-As2S3 are reported. The a.c. conductivity of pure As2S3 obeys a well-known relationship: σacωs. Frequency exponents is found to decrease with increasing temperature. Correlated barrier hopping (CBH) model successfully explains the entire behaviour of a.c. conductivity with respect to temperature and frequency for pure As2S3. But a different behaviour of a.c. conductivity has been observed for the nickel doped As2S3. At higher temperatures, distinct peaks have been observed in the plots of temperature dependence of a.c. conductivity. The frequency dependent behaviour of a.c. conductivity (σacωs) for nickeldoped As2S3 is similar to pure As2S3 at lower temperatures. But at higher temperatures, ln σac vs lnf curves have been found to deviate from linearity. Such a behaviour has been explained by assuming that nickel doping gives rise to some neutral defect states (D0′) in the band gap. Single polaron hopping is expected to occur between theseD0‘ andD+ states. Furthermore, allD+,D0′ pairs are assumed to be equivalent, having a fixed relaxation time at a given temperature. The contribution of this relaxation to a.c. conductivity is found to be responsible for the observed peak in the plots of temperature dependence of a.c. conductivity for nickel-doped As2S3. The entire behaviour of a.c. conductivity with respect to temperature and frequency has been explained by using CBH and “simple pair” models. Theoretical results obtained by using these models, have been found to be in agreement with the experimental results.

    • Relation of coordination number to memory and threshold switching in chalcogenides

      Manohar Lal Navdeep Goyal Anil Vohra

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      The paper reports a structural study of some memory and threshold chalcogenides in terms of coordination numberC, defined byC=8−N, and is the average coordination number for covalently bonded materials. The average number of nearest neighbours surrounding a central atom, obtained for As-Ge-Te (memory) and Se-Ge-Te (threshold) systems have been used to estimate the cohesive energies, assuming simple additivity of bond energies. The bonding pattern so obtained, explains certain properties of these glasses.

    • Electrical properties of a-GexSe100-x

      Abdolali Zolanvari Navdeep Goyal S K Tripathi

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      In general, the conductivity in chalcogenide glasses at higher temperatures is dominated by band conduction (DC conduction). But, at lower temperatures, hopping conduction dominates over band conduction. A study at lower temperature can, eventually, provide useful information about the conduction mechanism and the defect states in the material. Therefore, the study of electrical properties of GexSe100-x in the lower temperature region (room temperature) is interesting. Temperature and frequency dependence of GexSe100-x (x = 15, 20 and 25) have been studied over different range of temperatures and frequencies. An agreement between experimental and theoretical results suggested that the behaviour of germanium selenium system (GexSe100-x) have been successfully explained by correlated barrier hopping (CBH) model.

    • Meyer–Neldel DC conduction in chalcogenide glasses

      S PraKash Kulbir Kaur Navdeep Goyal S K Tripathi

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      Meyer–Neldel (MN) formula for DC conductivity ($\sigma_{\text{DC}}$) of chalcogenide glasses is obtained using extended pair model and random free energy barriers. The integral equations for DC hopping conductivity and external conductance are solved by iterative procedure. It is found that MN energy ($\Delta E_{\text{MN}}$) originates from temperature-induced configurational and electronic disorders. Single polaron-correlated barrier hopping model is used to calculate $\sigma_{\text{DC}}$ and the experimental data of Se, As2S3, As2Se3 and As2Te3 are explained. The variation of attempt frequency $\upsilon_0$ and $\Delta E_{\text{MN}}$ with parameter $(r/a)$, where 𝑟 is the intersite separation and 𝑎 is the radius of localized states, is also studied. It is found that $\upsilon_0$ and $\Delta E_{\text{MN}}$ decrease with increase of $(r/a)$, and $\Delta E_{\text{MN}}$ may not be present for low density of defects.

    • Meyer–Neldel energy in Se-based binary and ternary chalcogenide glasses


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      The integral equations for DC conductivity and external conductance for the network of localised states in amorphous solids are solved by iteration method. The random free energy barriers and single polaron hoppingmodel are used to obtain the DC conductivity $\sigma_\rm{DC}$ and Meyer–Neldel energy $E_\rm{MN}$. The experimental estimates of optical band gap $E_\rm{g}$, dielectric function $\epsilon$, glass transition temperature $T_\rm{g}$ and $\sigma_\rm{DC}$ are used to calculate $E_\rm{MN}$ for Se-based binary and ternary chalcogenide glasses. The calculated values are found to be in agreement with the available experimental data. $E_\rm{MN}$ increases with increase of attempt frequency. The true pre-exponential factor $\sigma_\rm{00}$ is related to $E_\rm{MN}$ as ln $\sigma_\rm{00} = p − q E_\rm{MN}$, where $p$ is nearly 7.3 and $q$ is material-dependent. The calculated values of $E_\rm{MN}$ and $\sigma_\rm{00}$ suggest that DC conduction in these chalcogenides is due to acoustic and optical phonon-assisted polaron hopping.

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