Mixed conduction and anisotropic single oscillator parameters in low dimensional TlInSe2 crystals

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Highlights

  • The anisotropic transport mechanism in low dimensional TlInSe2 has been investigated.

  • The conduction is dominated by thermionic, mixed and variable range hopping.

  • The anisotropic dispersive optical properties of TlInSe2 are studied.

  • The anisotropy in the refractive index and dielectric constant are determined.

Abstract

Due to the importance of the TlInSe2 crystal as neutron and γ-ray detectors, its electrical and dispersive optical parameters have been investigated. Particularly, the anisotropic current conduction mechanism in the temperature region of 100–350 K and the room temperature anisotropic dispersive optical properties were studied by means of electrical conductivity and optical reflectance, respectively. It has been shown that the mixed conduction is the most dominant transport mechanism in the TlInSe2 crystals. Particularly, when the electric field is applied perpendicular to the crystal's c-axis, the main dominant current transport mechanism is due to the mixed conduction and the variable range hopping above and below 160 K, respectively. When the electric field is applied parallel to the crystal's c-axis, the electrical conductivity is dominated by the thermionic emission, mixed conduction and variable range hopping at high, moderate and low temperatures, respectively. The optical reflectivity analysis in the wavelength range 210–1500 nm revealed a clear anisotropy effect on the dispersive optical parameters. Particularly, the static refractive index, static dielectric constant, dispersion energy and oscillator energy exhibited values of 2.50, 6.24, 20.72 eV and 3.96 eV, and values of 3.05, 9.33, 39.27 eV and 4.72 eV for light propagation parallel and perpendicular to the crystal's c-axis, respectively. Moreover, the frequency dependence of the dielectric constant, ε(ω), reflected strong dielectric anisotropy that exhibit maximum ε(ω) value of 38.80 and 11.40 at frequencies of 11.07 × 1014 Hz for light propagation parallel and perpendicular to the crystal's c-axis, respectively. The anisotropy in the ε(ω) makes the TlInSe2 crystals attractive to be used as nonvolatile static memory devices.

Introduction

The atomic arrangement of low-dimensional materials usually restricts the conduction electrons or holes to move in only one or two dimensions. This restricted motion generally leads to strange directional physical properties like anisotropy. TlInSe2 is an example of these anisotropic low-dimensional crystals [1], [2], [3], [4], [5], [6]. Due to its interesting characteristics this material has attracted much attention of researchers. Previously, a heterojunction based on semiconductors with a chain crystal structure p-TlSe–p-TlInSe2 was obtained. For this purpose, liquid-phase epitaxy from molten TlSe on the natural (110) cleavage surface of a TlInSe2 crystal was used. It was reported that the structure obtained is sensitive to light and hard radiation [7]. Recently, a new semiconductor detector of neutron radiation based on a TlInSe2 crystal has been investigated [8]. The detector is produced from a homogeneous semiconductor sample with two electric contacts and operates in an integrating mode. It was reported that, owing to its high sensitivity (∼10−13 A/(neutron cm−2 s−1)) and small size (the volume of the sensitive crystal element is ∼7 mm3), the detector is capable of monitoring spatial, time, and intensity distributions of γ rays and neutrons in pulse research reactors.

Some of the physical properties of the TlInSe2 crystal system have been investigated. Particularly, reports on: specific heat capacities [9], piezoelectric properties [10], optical and electrical properties [11], [12], thermoelectric power [4] and dielectric measurements [5] have been published. In the present study, we will concentrate on the anisotropy effects on optical properties of TlInSe2 single crystals. In other words, we are going to analyze the reflectivity data reported in Ref. [3] to investigate the anisotropy effect on the dispersive optical parameters parallel and perpendicular to the c-axis of the crystal. In addition, the current transport mechanism along and perpendicular to the c-axis of the crystal will be also reported. Particularly, the conductivity activation energy, the Mott's variable range hopping parameters, the static dielectric constant as well as the oscillator and dispersion energy, the static refractive index along both axes will be calculated, compared and discussed.

Section snippets

Experimental details

TlInSe2 single crystals were grown by the Bridgman method from the stoichiometric melts of the starting materials sealed in evacuated (10−5 Torr) silica tubes with a tip at the bottom. The ampoule was moved in a vertical furnace through a thermal gradient of 20 °C cm−1, between the temperatures 375 and 150 °C at a rate of 6 mm h−1. The X-ray powder diffraction technique was used to identify the crystalline nature of TlInSe2 compound. For this purpose, a Philips PW1740 diffractometer with a

Results and discussion

Accurate electrical conductivity (σ) measurements (parallel (σ//) and perpendicular (σ) to the crystal's c-axis) on TlInSe2 were possible within the temperature range of 100–350 K. The temperature dependencies of electrical conductivities are displayed in Fig. 1. For both conductivities, the figure reflects a decrease in the conductivity values with temperature down to 270 and 300 K for parallel and perpendicular measurements, respectively. In the temperature region of 260–210 K, the σ// − T

Conclusions

The electrical and optical properties of TlInSe2 crystals have been studied. Particularly, the electrical conductivity is measured in the temperature region of 100–350 K. The data are analyzed using the thermionic emission, mixed conduction and variable range hopping of charge carrier's theories. It is concluded that the mixed conduction and the thermally assisted Mott's variable range hopping are the most dominant transport mechanisms in the crystal above and below 170 K, respectively. The

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