This paper describes the preliminary results obtained from our study of optical and electrical properties of BiI3 crystals. The bismuth iodine polycrystals were grown using commercial starting material by vertical Bridgman method. For our measurements we used only single crystal samples that were cut out from grown crystals perpendicular to C6-axis.

Bismuth iodide crystals as a detector material: some optical and electrical properties

To realize effective van der Waals (vdW) transistors, vdW dielectrics are needed in addition to vdW channel materials. We study the dielectric properties of 32 exfoliable vdW materials using first principles methods. We calculate the static and optical dielectric constants and discover a large out-of-plane permittivity in GeClF, PbClF, LaOBr, and LaOCl, while the in-plane permittivity is high in BiOCl, PbClF, and TlF. To assess their potential as gate dielectrics, we calculate the band gap and electron affinity, and estimate the leakage current through the candidate dielectrics. We discover six monolayer dielectrics that promise to outperform bulk HfO2: HoOI, LaOBr, LaOCl, LaOI, SrI2, and YOBr with low leakage current and low equivalent oxide thickness. Of these, LaOBr and LaOCl are the most promising and our findings motivate the growth and exfoliation of rare-earth oxyhalides for their use as vdW dielectrics. Developments in the field of two-dimensional van der Waals materials offer big promise for device applications. This study reports a first-principle investigation on the dielectric properties of 32 exfoliable two-dimensional layered dieletrics for assessing the prospects of these materials in devices.

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Identification of Two-Dimensional Layered Dielectrics from First Principles

Etude des constantes dielectriques et de la conductivite fonction de la frequence et de la temperature, et estimation de la concentration. Les defauts a l'aide de differents mecanismes de polarisation

Dielectric properties of mercuric iodide (HgI2) single crystals

Lead iodide is a layered structure that experiences polytypism. The 2H polytype contains high rates of iodine vacancies but retains its stoichiometry. This characteristic feature makes it potentially useful for many practical applications. The hopping of iodine ions among vacancies is the dominant motion in the 2H polytype. We present a preliminary 127I and 207Pb solid-state nuclear magnetic resonance (NMR) spectroscopy and spin–lattice relaxation study of the 2H polytype below 400 K. We present reasonable models for the solid-state NMR results in terms of the effects of iodine hopping and lattice vibrations.

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127I and 207Pb Solid-State NMR Spectroscopy and Nuclear Spin Relaxation in PbI2: A Preliminary Study

Deep levels in PbI2 have been investigated by photoinduced current transient spectroscopy for the first time. By separating the signal processing from the data acquisition it was possible to analyze the transient using different methods, in particular a four‐gate treatment which allows to clear the detrapping current of uncontrolled changes of the recombination lifetime of thermally released carriers. Three hole traps located at 0.30, 0.47, and 0.66 eV have been detected and the corresponding thermal capture cross sections evaluated. In addition a photomemory effect has been evidenced. The main features of the excited state of the crystal are increased photosensitivity, neutralization of hole traps, and residual conductivity. It is believed that macroscopic recombination barriers acting as minority carrier traps under optical excitation are responsible for this phenomenon. The presence of these extended defects is probably related to the layered structure of PbI2. An energy model is proposed on the basis of which the observed logarithmic time dependence of the photosensitization at room temperature can be predicted. The model allows also to give a plausible interpretation for all the characteristic features of the photomemory effect and for various results previously reported by other authors.

Investigation of deep levels in PbI2 by photoinduced current transient spectroscopy

Dielectric constant (K), loss (tan δ), and conductivity (σ) of single crystals of PbI2 are measured (E ∥ c-axis) as a function of frequency in the range 102 to 107 Hz and at temperatures between 77 to 560 K. At room temperature (≈ 303 K), the K-value at 106 Hz is 6.7. The dielectric constant of this crystals increases slowly with temperature up to about 300 K (this increase being practically frequency-independent) beyond which the K increase is fast and frequency-dependent; this later stage of increase of K is attributed to an appreciable space charge polarisation (due to crystal defects) at such temperatures. The activation energy for intrinsic conduction (beyond 450 K) is calculated to be 0.44 eV from the lg σ versus 1/T plots at different frequencies. The lg (ΔK) versus 1/T plots at different frequencies for this crystal (where ΔK is the difference between the value of the dielectric constant at any temperature T and that value at room temperature) are straight lines from about 400 K, having the same slope for all frequencies; the activation energy is 0.41 eV. This agreement between the two activation energy values suggests that the same charge carriers are responsible for the two processes.

Dielectric properties of PbI2 single crystals

The dielectric constant (K), loss (tan δ), and conductivity (σ) of PbI2 single crystals, after quenching and X-ray irradiation are measured in the frequency range 102 to 107 Hz and in the temperature region 30 to 300 °C. X-ray irradiation of as-grown crystals increases only the low frequency K and tan δ. Quenching (from 370 °C) and X-ray irradiation of these crystals produces the same effect; the changes in K and tan δ at room temperature (≈ 30 °C) are largest in the latter samples. K and tan δ of the samples subjected to different treatments (like quenching and X-ray irradiation) increase with temperature, the rate of increase is smaller at higher frequencies; the largest rate of increase is observed in quenched and X-ray irradiated samples. The plot, log δ versus 1/T for these samples give activation energies for conduction in the temperature region 200 to 280 °C as 0.44, 0.41, 0.39, and 0.35 eV for as-grown, quenched, X-ray irradiated and quenched, and X-ray irradiated PbI2 crystals, respectively. An attempt is made to understand these results.



Es werden die Dielektrizitatskonstante (K), der Verlustfaktor (tg δ), und die Leitfahigkeit (σ) von PbI2-Einkristallen nach Abschrecken und Rontgenbestrahlung im Frequenzbereich 102 bis 107 Hz und bei Temperaturen zwischen 30 bis 300 °C gemessen. Rontgenbestrahlung der ursprung-lich gewachsenen Kristalle ergibt nur einen Anstieg der Niederfrequenz-Dielektrizitatskonstante (K) und des Verlustfaktors tg δ. Abschreckn (von 370 °C) und Rontgenbestrahlung dieser Kristalle ruft den gleichen Effekt hervor; die Anderungen in K und tg δ sind bei Zimmertemperatur (≈ 30 °C) in den letzteren Kristallen am grosen. K und tg δ der unterschiedlichen Behandlungen (Abschrecken und Rontgenbestrahlung) ausgesetzten Proben nehmen mit der Temperatur zu, wobei die Anstiegsgeschwindigkeit bei hoheren Frequenzen kleiner ist; die groste Anstiegsgeschwindigkeit wird in abgeschreckten und rontgenbestrahlten Proben beobachtet. Die log δ−1/T- Kurven dieser Proben ergeben Werte fur die Aktivierungsenergie der Leitfahigkeit im Temperatur-bereich 200 bis 280 °C von 0,44; 0,41; 0,39 und 0,35 eV fur ursprungliche, abgeschreckte, rontgenbestrahlte und abgeschreckte bzw. rontgenbestrahlte PbI2-Kristalle. Es wird der Versuch gemacht, diese Ergebnisse zu verstehen.

H. N. Acharya

Papers

Dielectric properties of PbI2 single crystals

Effect of quenching and X-ray irradiation on the dielectric properties of PbI2 single crystals

Structural and thermal properties of (Sm1−xNdx)2(MoO4)3

Inversion Layer Formation and Surface States at the GaAs-Electrolyte Interface