TlGaS2 Single Crystals Research Articles

Thermoluminescence properties and trapping parameters of TlGaS2 single crystals

TlGaS2 layered single crystals have been an attractive research interest due to their convertible characteristics into 2D structure. In the present paper, structural, optical and thermoluminescence properties of TlGaS2 single crystals were investigated. XRD pattern of the crystal presented five well-defined peaks associated with monoclinic unit cell. Band gap and Urbach energies were found to be 2.57 and 0.25 eV, respectively, from the analyses of transmittance spectrum. Thermoluminescence measurements were carried out above room temperature up to 660 K at various heating rates. One TL peak with peak maximum temperature of 573 K was obtained in the TL spectrum at 1.0 K/s. Curve fitting, initial rise and variable heating rate methods were used for analyses. All of those resulted in presence of a deep trapping level with activation energy around 0.92 eV. Heating rate dependence of the TL peak was also studied and it was indicated that peak maximum temperature shifted to higher temperatures besides decreasing TL intensity as the higher heating rates were employed.

Modifying the Dielectric Properties of the TlGaS2 Single Crystal by Electron Irradiation

The dielectric properties and ac conductivity of an electronically irradiated layered TlGaS2 single crystal are studied in the frequency range of 5 × 104–3.5 × 107 Hz. It is established that electron irradiation of TlGaS2 single crystal samples with doses of 2 × 10l2 to 2.4 × 1013 e/cm2 leads to a decrease in the real component (e') of the complex dielectric permittivity in a high-frequency region (f > 106 Hz) and an increase in its imaginary component (e''), the dielectric loss tangent (tanδ), and ac conductance (σac) across the layers in the entire studied frequency range. At irradiation doses of 2 × 10l2 to 2.4 × 1013 e/cm2, losses in the reach-through conductivity of TlGaS2 occur, and, as the electron irradiation dose accumulates, the dispersion of e'' and tanδ increases significantly. In the frequency range of f = 5 × 104–2 × 107 Hz, in the irradiated TlGaS2 samples, the ac conductivity changes according to the law σac ~ fn (where n = 0.7–0.8), which is typical of the hopping mechanism of charge transfer via localized states near the Fermi level. The parameters of localized states in TlGaS2 (the density of the states near the Fermi level and their energy spread) are estimated versus the electron irradiation dose.

Dielectric Losses and Charge Transfer in Antimony-Doped TlGaS2 Single Crystal

Effect of semimetallic antimony (0.5 mol % Sb) on the dielectric properties and ac-conductivity of TlGaS2-based single crystals grown by the Bridgman–Stockbarger method has been studied. The experimental results on the frequency dispersion of dielectric coefficients and the conductivity of TlGa0.995Sb0.005S2 single crystals allowed the revealing of the dielectric loss nature, the charge transfer mechanism, and the estimation of the parameters of the states localized in the energy gap. The antimony-doping of the TlGaS2 single crystal leads to an increase in the density of states near the Fermi level and a decrease in the average time and average distance of hopes.

Dielectric Properties and Conductivity of Ag-Doped TlGaS2 Single Crystals

The effect of silver ions (2 mol %) on the dielectric properties and electrical conductivity of TlGaS2 single crystals grown by the Bridgman–Stockbarger method is investigated. The experimental results of studying the frequency dispersion of the dielectric coefficients of TlGaS2 single crystals (2 mol % Ag) makes it possible to establish the nature of dielectric losses and the charge-transfer mechanism, to evaluate the density of states near the Fermi level, the spread of states, the average hopping time and length, and the concentration of deep traps responsible for ac conductivity. The Ag doping of the TlGaS2 single crystals results in an increase in the density of states near the Fermi level and in a decrease in the average hopping time and length.

Low-temperature X-ray studies of TlInS2, TlGaS2, and TlGaSe2 single crystals

The results of X-ray diffraction studies of the unit-cell parameters and thermal-expansion coefficients of TlInS2, TlGaS2, and TlGaSe2 crystals in the temperature range 100–300 K are described. It is shown that the unit-cell parameters of all the studied crystals gradually increase with increasing temperature. The temperature dependences of these parameters exhibit anomalies in the form of bends and kinks at temperatures corresponding to phase transitions in the crystals. The thermal-expansion coefficients along the [001] crystallographic direction of the crystals under study are determined. It is found that their values slightly change with increasing temperature.

Interband critical points in TlGaxIn1−xS2 layered mixed crystals (0 ⩽ x ⩽ 1)

The layered semiconducting TlGaxIn1−xS2 mixed crystals (0⩽x⩽1) were studied by spectroscopic ellipsometry measurements in the 1.2–6.2eV spectral range at room temperature. The spectral dependence of the components of the complex dielectric function, refractive index and extinction coefficient were revealed using an optical model. The interband transition energies in the studied samples were found from the analysis of the second-energy derivative spectra of the complex dielectric function. The variation of the obtained energies with composition were plotted to see the effect of the substitution of indium with gallium. Moreover, a simple diagram showing the revealed transitions in the available electronic band structure was given for TlGaS2 single crystals.

Multiphonon absorption processes in layered structured TlGaS2, TlInS2 and TlGaSe2 single crystals

The infrared transmittance and Raman scattering spectra in TlGaS2, TlInS2 and TlGaSe2 layered single crystals grown by Bridgman method were studied in the frequency ranges of 400–1500 and 10–400cm−1, respectively. Three, three and five bands observed at room temperature in IR transmittance spectra of TlGaS2, TlInS2 and TlGaSe2, respectively, were interpreted in terms of multiphonon absorption processes.

Effect of ionizing radiation on the dielectric characteristics of TlInS2 and TlGaS2 single crystals

The temperature dependences of the electrical conductivity and the permittivity of TlInSe2 and TlGaTe2 crystals unirradiated and irradiated with 4-MeV electrons at a doze of 1016 cm−2 have been investigated. It has been established that electron irradiation leads to a decrease in the electrical conductivity σ and the permittivity ɛ over the entire temperature range under study (90–320 K). It has been revealed that the TlInSe2 and TlGaTe2 single crystals undergo a sequence of phase transitions characteristic of crystals of this type, which manifest themselves as anomalies in the temperature dependences σ = f(T) and ɛ = f(T). Electron irradiation at a doze of 1016 cm−2 does not affect the phase transition temperatures of the crystals under investigation.

Analysis of Zero-Field Splitting Parameters of Fe3+ Doped TlGaS2 Crystal Using Spin Hamiltonian Separation (SHS) Method

The spin Hamiltonian separation (SHS) method is employed to investigate the local environment around the orthorhombic Fe3+ centers in TlGaS2 single crystal. In this analysis, we employ the spin-Hamiltonian parameters measured by electron paramagnetic resonance (EPR) at 15 K and room temperature. On the basis of the SHS analysis and the comparison with the experimental values at 15 K and room temperature, some anomalies have been found in the ratios of the zero-field splitting (ZFS) parameters (ZFSPs) \(B_{2}^{0}\) to the separated parameters B2a(1). Such anomalies may be attributed to the effects of the nearby Tl ions located on the cavities among the GaS4 tetrahedrons.

Electrical and optical properties of Co doped TlGaS2 crystals

AbstractIn this study, Co doped TlGaS2 single crystals which belongs to the class of AIIIBIIIX2VI have been investigated by means of XRD, temperature dependent dark and illuminated conductivity, Space Charge Limited Currents and absorption measurements. The room temperature conductivity and trap concentration values were about 10‐8 (Ω‐cm)‐1 and 7.5 × 1013 cm‐3, respectively. From the temperature dependent conductivity measurements, two activation energies namely 271 and 12 meV have been determined in the high and low temperature regions, respectively. The trap level at 271 meV that was determined by the dark temperature dependent conductivity measurement has also been verified by Space Charge Limited Currents analysis. The absorption measurements have showed that the layered compound had indirect and direct band gaps and the values were determined to be 2.49 and 2.56 eV, respectively. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of the switching phenomena of TlGaS2 single crystals

Single crystals of TlGaS2 were prepared by a special modified Bridgman technique and used to investigate the switching phenomena. The particular interest shown in switching studies of p-type TlGaS2 compound is associated with the possibility of its uses as an effective switching and memory elements in electronic devices. The switching effect observed in such crystal shows a memory character. Using a crystal holder and cryostat we measured the switching phenomenon at different ambient conditions such as temperature, light illumination as well as sample thickness. Pronounced parameters for switching for sample of thickness 0.17cm were determined from the experimental data such as threshold voltage Vth=400V, threshold current Ith=37μA, holding voltage Vh=350V, holding current Ih=42.3×10−4A, threshold power Pth=1.48×10−2W, threshold field Eth=196.429V/cm as well as the ratio between the resistance in the off state ROFF to the resistance in the conducting state RON as 130.253. The factors affecting these parameters have also been investigated.

Coexistence of Indirect and Direct Optical Transitions, Refractive, Index and Oscillator Parameters in TlGaS2 , TlGaSe2 , and TlInS2 Layered Single Crystals

Coexistence of Indirect and Direct Optical Transitions, Refractive, Index and Oscillator Parameters in TlGaS2 , TlGaSe2 , and TlInS2 Layered Single Crystals

Thermal and EPR investigations of thallium gallium disulphide single crystal

AbstractIn this research, the results of the differential scanning calorimetry (DSC) and electron paramagnetic resonance (EPR) investigations of TlGaS2 single crystal are presented. Specific heat capacity (Cp) anomalies of layered TlGaS2 have been obtained by using a new DSC technique for such crystals. Remarkable heat capacity anomalies have been revealed at the temperatures of 137.7 K, 174.5 K and 238.5 K. It is found that the anomalies appear at maximum with a small deviation (by 3‐4%) from the regular values, and Cp discontinuity amounted to approximately 5%. Additionally, EPR spectra of Fe doped TlGaS2 single crystals have been recorded at various temperatures down to 6 K for different orientations of the applied magnetic field. Transformations of present EPR spectra are not sufficient for the confirmation of structural phase transitions, in contrast to the cases in iso structural TlInS2 and TlGaSe2 compounds. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoelectric and X-ray dosimetric properties of TlGa0.97Mn0.03S2 single crystals

It is established that the partial substitution of gallium for manganese (3 mol %) in TlGaS2 single crystals leads to a decrease in width of the band gap (from 2.62 to 2.5 eV), broadening of the peak of intrinsic photocurrent, and appearance of a broad band of extrinsic photocurrent over the photon energy range hν = 1.7–2.4 eV. Upon the partial substitution Ga → Mn in TlGaS2, the X-ray sensitivity coefficient increases significantly (by a factor of 24–57) within an irradiation dose of 0.75–78 R/min, and the current-dose characteristics of TlGa0.97Mn0.03S2 have good reproducibility.

Low temperature EPR spectra of Fe3+ centers in ternary layered TlGaS2 crystal

AbstractThe results of low temperature electron paramagnetic resonance (EPR) study of Fe doped TlGaS2 single crystal in the temperature range of 5‐300 K are presented. Iron was added to the growth mixture in amounts corresponding to a molar ratio Fe/Ga of about 1%. The EPR signal due to Fe3+ centers located at the centers of GaS4 tetrahedrons has been observed. A transformation of EPR spectra has been observed on decreasing the temperature, which is attributed to lowering of the number of resonance lines. The simulation of rotational patterns of the resonance fields exhibits a decrease of the number of centers at low temperatures as a contrary to those of isostructural compounds of TlGaS2. On comparing the EPR spectra of Fe3+ centers in TlGaSe2, it has been revealed that the Tl atoms lose their zigzag alignment observed previously at room temperature and rearranged to equal structural positions on Tl chains on lowering the temperature. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optoelectronic and electrical properties of TlGaS2 single crystal

AbstractThe optoelectronic and electrical properties of TlGaS2 single crystals have been investigated by means of room temperature transmittance and reflectance spectral analysis, Hall coefficient, dark electrical resistivity and photocurrent measurements in the temperature range of 200–350 K. The optical data have revealed an indirect and direct allowed transition band gaps of 2.45 and 2.51 eV, an oscillator and dispersion energy of 5.04 and 26.45 eV, respectively, a static dielectric constant of 6.25 and static refractive index of 2.50. The dark Hall coefficient measurements have shown that the crystals exhibit a conductivity type conversion from p‐type to n‐type at a critical temperature of 315 K. Deep donor and acceptor energy levels of 0.37/0.36 eV and 0.66 eV has been calculated from the temperature dependence of Hall coefficient and resistivity, and photocurrent measurements, respectively. The photocurrent decreases with decreasing temperature. The analysis of the photocurrent data have revealed that, the recombination mechanism is linear and supralinear above and below 290 K, respectively. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature effect on dark electrical conductivity, Hall coefficient, space charge limited current and photoconductivity of TlGaS2 single crystals

The dark electrical conductivity, Hall coefficient, space charge limited current, and illumination and temperature dependences of the photocurrent of TlGaS2 single crystals in the temperature regions of 100–350, 200–350, 200–290 and 100–350 K, respectively, have been measured and analysed. The Hall coefficient measurements revealed the extrinsic type of conduction with conductivity-type conversion from p- to n-type at a critical temperature of 315 K. The temperature dependence of the dark electrical conductivity exhibits activation behaviour with activation energies (0.360 ± 0.005) eV and (0.240 ± 0.005) eV at high and low temperatures, respectively. The space charge limited current analysis has shown that the energy level of (0.240 ± 0.005) eV is a trapping state with trap density of (2.2–3.9) × 1012 cm−3. The data analysis of the photocurrent–temperature dependence has revealed two photoconductivity activation energies of (0.660 ± 0.005) eV and (0.360 ± 0.005) eV in the temperature regions of 290–350 K and 220–280 K, respectively. The illumination dependence of photoconductivity is found to exhibit linear and supralinear recombination mechanisms above and below 290 K, respectively.

The Urbach tails and optical absorption in layered semiconductor TlGaSe2 and TlGaS2 single crystals

TlGaSe2 and TlGaS2 single crystals were grown by the modified Bridgman-Stockbarger method. We report the result of an experimental study of the optical absorption of TlGaSe2 and TlGaS2 crystals. The absorption measurements were performed in steps of 10 K. The direct and indirect band gaps for TlGaSe2 and TlGaS2 samples were calculated as a function of temperature. The phonon energies in TlGaSe2 and TlGaS2 crystals were calculated as (39±4) and (9±4) meV at 240 K, respectively. At 10 K, direct and indirect band gaps were found as 2.294 and 2.148 eV for TlGaSe2, 2.547 and 2.521 eV for TlGaS2 crystals, respectively. The abrupt changes were observed in the direct and indirect band gaps in the some temperature ranges. These changes were interpreted as phase transformation temperatures. The steepness parameters and Urbach energy for TlGaSe2 and TlGaS2 samples increased with increasing sample temperature in the range (10–320) K.

Photoelectric and X-ray Dosimetric Properties of TlGaS[sub 2]〈Yb〉 Single Crystals

The photoelectric properties of TlGa1− xYbxS2 (x = 0, 0.01) single crystals are investigated. It is established that partial substitution of ytterbium for gallium leads to (i) an increase in the electrical resistivity of the samples, (ii) a shift in the maximum of the intrinsic photocurrent toward the long-wavelength range of the spectrum, (iii) a considerable broadening of the spectral sensitivity range, and (iv) an increase in the amplitude of the extrinsic photocurrent. Analysis of the x-ray dosimetric characteristics of the TlGa 1−xYbxS2 single crystals demonstrates that, upon partial substitution of ytterbium for gallium in TlGaS2, the x-ray sensitivity coefficient increases significantly and the current-dose characteristics ΔI E, 0 ∼ E α tend to linearity (α = 1) at low dose rates (E, R/min) of soft x rays. At high dose rates of hard x rays, α tends to 0.5 for both undoped and ytterbium-doped TlGaS2 single crystals.

Frequency dispersion of dielectric coefficients of layered TlGaS2 single crystals

Frequency dependence of the dissipation factor tanδ, the permittivity ɛ, and the ac conductivity σac across the layers in the frequency range f=5×104−3×107 Hz was studied in layered TlGaS2 single crystals. A significant dispersion in tanδ was observed in the frequency range 106−3×107 Hz. In the range of frequencies studied, the permittivity of TlGaS2 samples varied from 26 to 30. In the frequency range 5×104−106 Hz, the ac conductivity obeyed the f0.8 law, whereas for f>106 Hz σac was proportional to f2. It was established that the mechanism of the ac charge transport across the layers in TlGaS2 single crystals in the frequency range 5×104−106 Hz is hopping over localized states near the Fermi level. Estimations yielded the following values of the parameters: the density of states at the Fermi level NF=2.1×1018 eV−1 cm−3, the average time of charge carrier hopping between localized states τ=2 µs, and the average hopping distance R=103 A.