# Influence of the Composition of (TlGaS2)1- (TlInSe2)x Alloys on Their Physical Properties

S.N Mustafaeva, S.G. Jafarova, E.M Kerimova, N.Z Gasanov, S.M Asadov

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S.N Mustafaeva, S.G. Jafarova, E.M Kerimova, N.Z Gasanov, S.M Asadov. Influence of the Composition of (TlGaS2)1- (TlInSe2)x Alloys on Their Physical Properties. Mechanics, Materials Science & Engineering Journal, 2017, 7, 10.13140/RG.2.2.29609.600. hal-01508604

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# Influence of the Composition of (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> Alloys on Their Physical Properties

Mustafaeva S.N.<sup>1,a</sup>, Jafarova S.G.<sup>1</sup>, Kerimova E.M.<sup>1</sup>, Gasanov N.Z.<sup>1</sup>, Asadov S.M.<sup>2</sup>

1. Institute of Physics, National Academy of Sciences of Azerbaijan, AZ-1143, G. Javid Pr., 131, Baku, Azerbaijan

2. Institute of Catalysis and Inorganic Chemistry, National Academy of Sciences of Azerbaijan, AZ-1143, G. Javid Pr., 113, Baku, Azerbaijan

a. solmust@gmail.com

<figure><img src="image_2.png" /></figure>

DOI 10.13140/RG.2.2.29609.600

**Keywords:** TlGaS<sub>2</sub>, TlInSe<sub>2</sub>, alloys physical properties, roentgensensitivity, photoresistors.

**ABSTRACT.** The single crystals of (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> (x = 0-0.5) solid solutions have been grown up. The photoelectric, roentgendosimetric, dielectric and optical characteristics of the (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> solid solutions with various compositions have been determined. The maximum and spectral range of photosensitivity were found to redshift as x increases from 0 to 0.5. Both the photo- and roentgensensitivity of the solid solutions are higher than those of pure TlGaS<sub>2</sub>. The nature of dielectric losses and the hopping mechanism of charge transport in the (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> solid solutions were established from the experimental results on high-frequency dielectric measurements. The temperature dependences of exciton peak position for various compositions (x = 0-0.3) are investigated in 77-180 K temperature interval. It was established that with increasing x in (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> solid solutions the width of their forbidden gap decreases.

PACS: 71.20.Nr; 71.35.Cc; 72.15.Rn; 72.20.Ee; 72.20.Jv; 72.30.+q; 72.40.+w; 73.20.At

**Introduction.** Ternary layer-chain TlGaS<sub>2</sub> and TlInSe<sub>2</sub> single crystals exhibit high photo- and roentgensensitivity making them well-suited for photoresistors and roentgendetectors [1-4]. The study of physical properties of the TlGaS<sub>2</sub>, TlInSe<sub>2</sub> compounds and solid solutions on their base are very important for establishing the relations between their compositions and properties. This offers the possibility of controlling the band gap, energy position of emission bands and electrical conductivity of such semiconductors. In [5-7] the results of investigation of *ac* – electric and dielectric properties of TlGaS<sub>2</sub>, TlInSe<sub>2</sub> and diluted (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> solid solutions (x = 0.005 and 0.02) are given.

The purpose of present work was to investigate the influence of (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> solid solutions compositions (x = 0-0.5) on their photo- and roentgensensitivity, *ac* – electric, dielectric and optical properties.

**Experiment.** The synthesis of (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> solid solutions was carried out in an ampule evacuated to pressure 10<sup>-3</sup> Pa. The ampule was fabricated from a fused silica tube. In this case, (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> samples were prepared through the interaction of initial components (TlGaS<sub>2</sub> and TlInSe<sub>2</sub>). In order to prevent the ampule filled with reactants from explosion, the furnace temperature was raised to the melting temperature of selenium (T = 493 K) and the ampule was held at this temperature for 3h. Then, the furnace temperature was raised to T = 1080 K at a rate of 50 K/h and the ampule was held at this temperature for 4 h, after which it was cooled to 300 K at a rate of 20 K/h. Phase purity of (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> was established by differential thermal analysis and powder X-ray diffraction. Each sample was used as the charge for Bridgman crystal growth. The crystal data for (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> are presented in the Table 1.
<table><caption>Table 1. Crystal data for TlGaS<sub>2</sub> and (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub>.</caption><thead><tr><th>Solid solution composition</th><th>a(Å)</th><th>b(Å)</th><th>c(Å)</th><th>β, deg</th><th>Z</th><th>Sp.gr.</th></tr></thead><tbody><tr><td>TlGaS<sub>2</sub></td><td>10.40</td><td>10.40</td><td>15.17</td><td>100.06</td><td>16</td><td>P2<sub>1/n</sub></td></tr><tr><td>(TlGaS<sub>2</sub>)<sub>0.9</sub>(TlInSe<sub>2</sub>)<sub>0.1</sub></td><td>10.40</td><td>10.40</td><td>15.18</td><td>100.06</td><td>16</td><td>P2<sub>1/n</sub></td></tr><tr><td>(TlGaS<sub>2</sub>)<sub>0.8</sub>(TlInSe<sub>2</sub>)<sub>0.2</sub></td><td>10.41</td><td>10.41</td><td>15.18</td><td>100.06</td><td>16</td><td>P2<sub>1/n</sub></td></tr><tr><td>(TlGaS<sub>2</sub>)<sub>0.7</sub>(TlInSe<sub>2</sub>)<sub>0.3</sub></td><td>10.43</td><td>10.43</td><td>15.181</td><td>100.06</td><td>16</td><td>P2<sub>1/n</sub></td></tr><tr><td>(TlGaS<sub>2</sub>)<sub>0.6</sub>(TlInSe<sub>2</sub>)<sub>0.4</sub></td><td>10.435</td><td>10.435</td><td>15.20</td><td>100.06</td><td>16</td><td>P2<sub>1/n</sub></td></tr><tr><td>(TlGaS<sub>2</sub>)<sub>0.5</sub>(TlInSe<sub>2</sub>)<sub>0.5</sub></td><td>10.452</td><td>10.452</td><td>15.245</td><td>100.06</td><td>16</td><td>P2<sub>1/n</sub></td></tr></tbody></table>

The spectral characteristics were recorded with a GIBI-TIBI potentiometer; the samples were illuminated with a 400-W incandescent lamp through a DMR-4 monochromator.

In roentgendosimetry measurements, we used a URS-55 X-ray generator. The variation in sample resistance under X-ray irradiation was followed with an R-4053 bridge. X-ray dose rates were measured with a DRGZ-02 dosimeter.

Measurements of the dielectric properties of (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> (x = 0.1; 0.2) single crystals were performed at fixed frequencies in the range 5×10<sup>4</sup>-3.4×10<sup>7</sup> Hz by the resonant method using a TESLA BM560 Qmeter. The single-crystal samples for dielectric measurements had the form of planar capacitors normal to the C-axis of the crystals, with silver-paste electrodes. The thickness of the crystal samples was 90-120 µm, and the area of the capacitor plates was 8×10<sup>-2</sup>-2×10<sup>-1</sup> cm<sup>2</sup>. All dielectric measurements were performed at T = 300 K. The accuracy in determining the resonance capacitance and the quality factor Q=1/tanδ of the measuring circuit was limited by errors related to the resolution of the device readings. The accuracy of the capacitor graduation was ±0.1 pF. The reproducibility of the resonance position was ±0.2 pF in capacitance and ±(1.0-1.5) scale divisions in quality factor. The largest deviations were 3-4% in ε and 7% in tan δ.

Optical absorption spectra were measured using samples in the form of platelets 10-100 µm thick, cleaved from the single-crystal ingots. Light was incident along the normal to the layers of the samples, that is, along the crystallographic axis C of the crystals. Optical transmission spectra were measured as functions of temperature using an experimental setup built around a KSVU-6M system and UTREKS helium cryostat, which ensured temperature stabilization with an accuracy of ± 0.01 K. The setup included an MDR-6 double monochromator and FEU-100 photomultiplier tube. The spectral resolution of the experimental configuration was = 2 Å.

**Results and discussion.** We measured the spectral dependences of photoconductivity and photosensitivity R<sub>d</sub>/R<sub>ph</sub> (R<sub>d</sub> is the dark resistance, and R<sub>ph</sub> is the resistance of the sample under above-gap illumination) at a steady illumination, as well as the roentgensensitivity and other photoelectric parameters. Table 2 and fig.1 give the photoelectric properties of the (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> solid solutions.
<table><caption>Table 2. Photoelectric and roentgenosimetric characteristics of the (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> solid solutions.</caption><thead><tr><th>Solid solution composition</th><th>Δλ<sub>max</sub>,<br>μm</th><th>R<sub>d</sub>, Ohm</th><th>R<sub>d</sub>/R<sub>ph</sub><br>at 200 lx</th><th>K<sub>σ</sub>, min/R</th></tr></thead><tbody><tr><td>TlGaS<sub>2</sub></td><td>0.46-0.57</td><td>(3-5)×10<sup>10</sup></td><td>5-8</td><td>0.063-0.159</td></tr><tr><td>(TlGaS<sub>2</sub>)<sub>0.9</sub>(TlInSe<sub>2</sub>)<sub>0.1</sub></td><td>0.50-0.62</td><td>(1-2)×10<sup>10</sup></td><td>10-25</td><td>0.075-0.178</td></tr><tr><td>(TlGaS<sub>2</sub>)<sub>0.8</sub>(TlInSe<sub>2</sub>)<sub>0.2</sub></td><td>0.55-0.66</td><td>(3-4)×10<sup>9</sup></td><td>15-30</td><td>0.089-0.198</td></tr><tr><td>(TlGaS<sub>2</sub>)<sub>0.7</sub>(TlInSe<sub>2</sub>)<sub>0.3</sub></td><td>0.59-0.71</td><td>(2-3)×10<sup>8</sup></td><td>21-37</td><td>0.098-0.213</td></tr><tr><td>(TlGaS<sub>2</sub>)<sub>0.6</sub>(TlInSe<sub>2</sub>)<sub>0.4</sub></td><td>0.64-0.76</td><td>(1-2)×10<sup>7</sup></td><td>23-42</td><td>0.107-0.219</td></tr><tr><td>(TlGaS<sub>2</sub>)<sub>0.5</sub>(TlInSe<sub>2</sub>)<sub>0.5</sub></td><td>0.69-0.81</td><td>(3-5)×10<sup>6</sup></td><td>25-46</td><td>0.142-0.252</td></tr></tbody></table>

From fig. 1 one can see, that the photosensitivity maximum (λ<sub>max</sub>) linearly shifts from 0.50 to 0.73 μm as x increases from 0 to 0.5. This shift is associated with a decrease in the band gap with increasing x. Increasing x leads to a redshift of the sensitivity range Δλ and a considerable rise in R<sub>d</sub>/R<sub>ph</sub> at 200 lx. For example, the R<sub>d</sub>/R<sub>ph</sub> of (TlGaS<sub>2</sub>)<sub>0.5</sub>(TlInSe<sub>2</sub>)<sub>0.5</sub> is 5 to 6 times greater than that of pure TlGaS<sub>2</sub> (table 2). The rise in R<sub>d</sub>/R<sub>ph</sub> with increasing x is apparently related to an increase in both the lifetime and mobility of the photogenerated carriers.

<figure><img src="image_3.png" /><figcaption>Fig. 1. Composition dependence of the photosensitivity maximum in (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> solid solutions.</figcaption></figure>

Roengenosensitivity K<sub>σ</sub> of (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> was characterized by relative change in conductivity per unit dose rate,

$$K_{\sigma} = \frac{\Delta\sigma_{E,0}}{\sigma_{0} \cdot E} \qquad (1)$$

where σ<sub>0</sub> – is the conductivity of the unirradiated crystal;

Δσ<sub>E,0</sub> = σ<sub>E</sub> – σ<sub>0</sub> is the change in the conductivity under X-ray irradiation with dose rate E (R/min).
Table 2 lists K<sub>σ</sub> values at accelerating voltages from 25 to 30 keV and dose rates from 0.75 to 10 R/min. One can see that the K<sub>σ</sub> of (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> solid solutions exceeds that of pure TlGaS<sub>2</sub>. As the TlInSe<sub>2</sub> content increases, K<sub>σ</sub> rises to 0.142–0.252 min/R at x = 0.5.

We measured also the electric capacitance of (TlGaS<sub>2</sub>)<sub>0.9</sub>(TlInSe<sub>2</sub>)<sub>0.1</sub> and (TlGaS<sub>2</sub>)<sub>0.8</sub>(TlInSe<sub>2</sub>)<sub>0.2</sub> samples in the frequency range 5×10<sup>4</sup>–3.4×10<sup>7</sup> Hz. Using the measured capacities of these samples, we calculate the permittivity ε at different frequencies. The ε values of (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> single crystals vary from 9.5 to 12.7 for x = 0.1 and from 9.8 to 11.6 for x = 0.2 over the entire frequency range studied, with no significant dispersion (the ε of TlGaS<sub>2</sub> single crystal, as it was shown in [5], varies from 26 to 30 at f = 5×10<sup>4</sup>–3×10<sup>7</sup> Hz).

In contrast to what was reported for TlGaS<sub>2</sub> [5], the frequency dependences of the loss tangent for the (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> (x = 0.1; 0.2) single crystals have maxima, which points to relaxation losses [8].

The ac-conductivity of investigated samples varies as f<sup>0.8</sup> at f = 5×10<sup>4</sup>–2×10<sup>6</sup> Hz for x = 0.1 and at f = 5×10<sup>4</sup>–6×10<sup>6</sup> Hz for x = 0.2. At more high frequencies σ<sub>ac</sub>(f) – dependence of these crystals was superlinear (~f<sup>1.4</sup>).

The σ<sub>ac</sub>~f<sup>0.8</sup>-dependence indicates that the mechanism of charge transport is hopping over localized states near the Fermi level [9].

$$ \sigma_{ac}(f) = \frac{\pi^3}{96} \cdot e^2 k T N_F^2 a^5 f \left[ \ln \frac{\nu_{ph}}{f} \right]^4 \quad (2) $$

where e – is the elementary charge;

k – is the Boltzmann constant;

N<sub>F</sub> – is the density of localized states near the Fermi level;

a = 1/α – is the localization length, α is the decay parameter of the wave function of a localized charge carrier, ψ ~ e<sup>-αr</sup>;

ν<sub>ph</sub> – is the phonon frequency.

Using expression (2), we can calculate the density of states at the Fermi level from the measured values of the conductivity σ<sub>ac</sub>(f). Calculated values of N<sub>F</sub> for (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> solid solutions (x = 0.1; 0.2) single crystals were given in Table 3 (localization radius is chosen as 14 Å, in analogy with the TlGaS<sub>2</sub> single crystal [5]).

<table><caption>Table 3. Parameters of (TlGaS<sub>2</sub>)<sub>1-x</sub>(TlInSe<sub>2</sub>)<sub>x</sub> single crystals obtained from high-frequency dielectric measurements.</caption><thead><tr><th>Crystal composition</th><th>N<sub>F</sub>, eV<sup>-1</sup>·cm<sup>-3</sup></th><th>τ, s</th><th>R, Å</th><th>N<sub>t</sub>, cm<sup>-3</sup></th></tr></thead><tbody><tr><td>TlGaS<sub>2</sub></td><td>2.1×10<sup>18</sup></td><td>2×10<sup>-6</sup></td><td>103</td><td>4.2×10<sup>17</sup></td></tr><tr><td>(TlGaS<sub>2</sub>)<sub>0.9</sub>(TlInSe<sub>2</sub>)<sub>0.1</sub></td><td>6.8×10<sup>18</sup></td><td>9.8×10<sup>-7</sup></td><td>98</td><td>5.1×10<sup>17</sup></td></tr><tr><td>(TlGaS<sub>2</sub>)<sub>0.8</sub>(TlInSe<sub>2</sub>)<sub>0.2</sub></td><td>7.7×10<sup>18</sup></td><td>3.3×10<sup>-7</sup></td><td>90</td><td>6.5×10<sup>17</sup></td></tr></tbody></table>

According to the theory of hopping conduction we calculate the mean hop distance (R) and mean hop time (τ) in an applied ac-electric field using the formula [9]:
$$ \tau^{-1} = \nu_{ph} \exp(-2R\alpha) \qquad (3) $$

where R – is the average hopping distance.

$$ R = \frac{1}{2\alpha} \ln \frac{\nu_{ph}}{f} \qquad (4) $$

These values also are presented in the table 3.

Knowing $N_F$ and $R$ from [9]:

$$ \frac{4\pi}{3} R^3 N_F \cdot \frac{\Delta E}{2} = 1 \qquad (5) $$

We estimate energetic scattering of trap states near the Fermi level ($\Delta E$): $\Delta E = 0.075$ eV for $x = 0.1$ and $0.085$ eV for $x = 0.2$. Evaluated concentrations of deep traps determining the ac-conductivity of $(TlGaS_2)_{1-x}(TlInSe_2)_x$ single crystals ($N_t = N_F \cdot \Delta E$) are given in last column of the table 3. It is seen from the table 3 that with increasing of $x$ from 0 to 0.2 in $(TlGaS_2)_{1-x}(TlInSe_2)_x$ single crystals the values of $N_F$ and $N_t$ increased, but $R$ decreased.

Optical properties of $(TlGaS_2)_{1-x}(TlInSe_2)_x$ ($x = 0-0.3$) single crystals have been studied in 77–180 K temperature interval. The thicknesses of crystals under study were 20–50 µm. Light was incident on the crystals in direction parallel to their crystallographic axis C.

The present data on the optical properties of the $(TlGaS_2)_{1-x}(TlInSe_2)_x$ demonstrate that, at temperatures from 77 to 180 K crystals have an absorption band near fundamental absorption edge, which is due to transitions to a direct exciton state. We examined the temperature dependence of the energy position of the exciton peak for $(TlGaS_2)_{1-x}(TlInSe_2)_x$ crystals in the temperature range 77–180 K (fig. 2). It is seen that the peak position of the exciton band of $(TlGaS_2)_{1-x}(TlInSe_2)_x$ solid solutions has a positive temperature coefficient. Given that the exciton energy is a weak function of temperature, this indicates that the band gap of $(TlGaS_2)_{1-x}(TlInSe_2)_x$ crystals increases with temperature.

<figure><img src="image_4.png" /><figcaption>Fig. 2. Temperature dependences of the energy position of the exciton peak at the absorption edge of $(TlGaS_2)_{1-x}(TlInSe_2)_x$ solid solutions: (1) $x = 0$; (2) $x = 0.02$; (3) $x = 0.1$; (4) $x = 0.2$; (5) $x = 0.3$.</figcaption></figure>
The temperature variation of the band gap of semiconductors $E_g$, is known to be determined by a combined effect of the thermal expansion of their lattice and electron-phonon interaction. Semiconductors rarely have a positive temperature coefficient of their band gap. In particular, such an experimental fact in TiGaS$_2$ and TiGaS$_2$-based single crystals [10, 11] is thought to be caused by the significant contribution of the thermal expansion of their lattice to the temperature variation of $E_g$. Thus, the TiGaS$_2$ and (TiGaS$_2$)$_{1-x}$(TiInSe$_2$)$_x$ crystals were found to be similar in the structure of their absorption edge, formed by direct interband transitions.

**Summary.** The results of photoelectric, roentgenodosimetric and high-frequency dielectric measurements on obtained (TiGaS$_2$)$_{1-x}$(TiInSe$_2$)$_x$ solid solutions provided an opportunity to increase photo- and roentgenosensitivity, to determine the mechanism of dielectric losses and charge transport, and also to evaluate the density of localized states at the Fermi level, the average time of charge carrier hopping between localized states, average hopping distance, scattering of trap states near the Fermi level and concentration of deep traps responsible for *ac*-conductivity. The temperature dependences of exciton peak position for (TiGaS$_2$)$_{1-x}$(TiInSe$_2$)$_x$ solid solutions were investigated in 77–180 K temperature interval. It is established that the edge of optical absorption of these solid solutions is formed by straight line exciton with the positive temperature coefficient.

## References

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