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

Mechanics, Materials Science & Engineering, December 2016

ISSN 2412-5954

Influence of the Composition of (TlGaS2)1- (TlInSe2)x Alloys on Their Physical Properties4 Mustafaeva S.N.1,a, Jafarova S.G. 1, Kerimova E.M. 1, Gasanov N.Z. 1, Asadov S.M.2 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 DOI 10.13140/RG.2.2.29609.600

Keywords: TlGaS2, TlInSe2, alloys physical properties, roentgensensitivity, photoresistors.

ABSTRACT. The single crystals of (TlGaS 2)1- (TlInSe2) ( = 0 0,5) solid solutions have been grown up. The photoelectric, roentgendosimetric, dielectric and optical characteristics of the (TlGaS 2)1- (TlInSe2) 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 TlGaS2. The nature of dielectric losses and the hopping mechanism of charge transport in the (TlGaS 2)1- (TlInSe2) 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 (TlGaS2)1- (TlInSe2) 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 TlGaS2 and TlInSe2 single crystals exhibit high photo- and roentgensensitivity making them well-suited for photoresistors and roentgendetectors [1-4]. The study of physical properties of the TlGaS2, TlInSe2 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 TlGaS2, TlInSe2 and diluted (TlGaS2)1- (TlInSe2) solid solutions (x = 0.005 and 0.02) are given.

The purpose of present work was to investigate the influence of (TlGaS2)1- (TlInSe2) solid solutions compositions (x = 0-0.5) on their photo- and roentgensensitivity, ac electric, dielectric and optical properties.

Experiment. The synthesis of (TlGaS2)1- (TlInSe2) solid solutions was carried out in an ampule evacuated to pressure 10-3 Pa. The ampule was fabricated from a fused silica tube. In this case, (TlGaS2)1- (TlInSe2) samples were prepared through the interaction of initial components (TlGaS2 and TlInSe2). 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 The Authors. Published by Magnolithe GmbH. This is an open access article under the CC BY-NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/

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Mechanics, Materials Science & Engineering, December 2016

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K/h. Phase purity of (TlGaS2)1- (TlInSe2) 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 (TlGaS2)1- (TlInSe2) are presented in the Table 1.

Table 1. Crystal data for TlGaS2 and (TlGaS2)1- (TlInSe2) .

Solid solution composition

b

c

, deg

Z

Sp.gr.

TlGaS2

10.40

10.40

15.17

100.06

16

P21/n

(TlGaS2)0.9(TlInSe2)0.1

10.40

10.40

15.18

100.06

16

P21/n

(TlGaS2)0.8(TlInSe2)0.2

10.41

10.41

15.18

100.06

16

P21/n

(TlGaS2)0.7(TlInSe2)0.3

10.43

10.43

15.181

100.06

16

P21/n

(TlGaS2)0.6(TlInSe2)0.4

10.435

10.435

15.20

100.06

16

P21/n

(TlGaS2)0.5(TlInSe2)0.5

10.452

10.452

15.245

100.06

16

P21/n

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 (TlGaS2)1- (TlInSe2) (x = 0.1; 0.2) single crystals were 4 7 perf 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-paster electrodes. The thickness of the -2 -1 crystal samples was 90 cm2. All dielectric measurements were performed at T = 300 K. The accuracy in determining the resonance capacitance and the quality factor Q=1/tan of the measuring circuit was limited by errors related to 1.5) scale divisions in quality factor. The largest deviations were 3

.

Optical absorption spectra were measured using samples in the form of platelets 10 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 ens K. The setup included an MDR-6 double monochromator and FEU-100 photomultiplier tube. The Results and discussion. We measured the spectral dependences of photoconductivity and photosensitivity Rd/Rph (Rd is the dark resistance, and Rph is the resistance of the sample under abovegap 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 (TlGaS2)1- (TlInSe2) solid solutions.

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Table 2. Photoelectric and roentgendosimetric characteristics of the (TlGaS2)1- (TlInSe2) solid solutions.

Solid solution composition TlGaS2 (TlGaS2)0.9(TlInSe2)0.1 (TlGaS2)0.8(TlInSe2)0.2 (TlGaS2)0.7(TlInSe2)0.3 (TlGaS2)0.6(TlInSe2)0.4 (TlGaS2)0.5(TlInSe2)0.5

Rd, Ohm

max,

0.46-0.57 0.50-0.62 0.55-0.66 0.59-0.71 0.64-0.76 0.69-0.81

(3(1(3(2(1(3-

10 10 9 8 7 6

Rd/Rph at 200 lx 5-8 10-25 15-30 21-37 23-42 25-46

K , min/R 0.063-0.159 0.075-0.178 0.089-0.198 0.098-0.213 0.107-0.219 0.142-0.252

From fig. 1 one can see, that the photosensitivity maximum ( max) linearly shifts from 0.50 to 0.73 x increases from 0 to 0.5. This shift is associated with a decrease in the band gap with increasing x. Increasing x and a considerable rise in Rd/Rph at 200 lx.

For example, the Rd/Rph of (TlGaS2)0.5(TlInSe2)0.5 is 5 to 6 times greater than that of pure TlGaS2 (table 2). The rise in Rd/Rph with increasing x is apparently related to an increase in both the lifetime and mobility of the photogenerated carriers.

Fig. 1. Composition dependence of the photosensitivity maximum in (TlGaS2)1- (TlInSe2 ) solid solutions.

Roengenosensitivity K of (TlGaS2)1- (TlInSe2) was characterized by relative change in conductivity per unit dose rate,

(1)

where

0 E,0

is the conductivity of the unirradiated crystal; =

E

0

is the change in the conductivity under X-ray irradiation with dose rate E

(R/min).

Table 2 lists K 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 of (TlGaS2)1- (TlInSe2) solid solutions exceeds that of pure TlGaS2. As the TlInSe2 content increases, K rises to 0.142 0.252 min/R at x = 0.5.

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Mechanics, Materials Science & Engineering, December 2016

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We measured also the electric capacitance of (TlGaS2)0.9(TlInSe2)0.1 and (TlGaS2)0.8(TlInSe2)0.2 4 7 Hz. Using the measured capacities of these samples, values of (TlGaS2)1- (TlInSe2) 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 2 single crystal, as it was shown in [5], 4 7 varies from 26 to 30 at f Hz).

In contrast to what was reported for TlGaS2 [5], the frequency dependences of the loss tangent for the (TlGaS2)1- (TlInSe2) (x = 0.1; 0.2) single crystals have maxima, which points to relaxation losses [8].

The ac-conductivity of investigated samples varies as f 0.8 at f 4 6 f Hz for x = 0.2. At more high frequencies ac(f) 1.4 superlinear (~f ).

4

6

Hz for x = 0.1 and at dependence of these crystals was

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

(2) where e is the elementary charge; k is the Boltzmann constant; NF is the density of localized states near the Fermi level; a charge carrier, ph

~ e- ;

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 ac(f). Calculated values of NF for (TlGaS2)1- (TlInSe2) solid solutions (x = 0.1; 0.2) single crystals were given in Table 3 ( the TlGaS2 single crystal [5]).

Table 3. Parameters of (TlGaS2)1- (TlInSe2) single crystals obtained from high- frequency dielectric measurements.

Crystal composition NF, eV-1 cm-3 18 TlGaS2 18 (TlGaS2)0.9(TlInSe2)0.1 18 (TlGaS2)0.8(TlInSe2)0.2

,s -6 -7 -7

R 103 98 90

Nt, cm-3 17 17 17

According to the theory of hopping conduction we calculate the mean hop distance (R) and mean hop ac-electric field using the formula [9]: (3) where R is the average hopping distance.

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These values also are presented in the table 3.

Knowing NF and R from [9]: (5) We estimate energetic scattering of trap states near the Fermi level ( E): 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 (TlGaS2)1- (TlInSe2) single crystals ( ) 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 (TlGaS2)1- (TlInSe2) single crystals the values of NF and Nt increased, but R decreased.

Optical properties of (TlGaS2)1- (TlInSe2) (x = 0 0.3) single crystals have been studied in 77 180 K temperature interval. The thicknesses of crystals under study were 20 the crystals in direction parallel to their crystallographic axis C. The present data on the optical properties of the (TlGaS2)1- (TlInSe2) 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 (TlGaS2)1- (TlInSe2) 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- (TlInSe2) 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 (TlGaS2)1- (TlInSe2) crystals increases with temperature.

Fig. 2. Temperature dependences of the energy position of the exciton peak at the absorption edge of (TlGaS2 )1- (TlInSe2) solid solutions: (1) x = 0; (2) x = 0.02; (3) x = 0.1; (4) x = 0.2; (5) x = 0.3.

The temperature variation of the band gap of semiconductors Eg, 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 TlGaS2 and TlGaS2-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 Eg. Thus, the TlGaS2 and (TlGaS2)1- (TlInSe2) crystals were found to be similar in the structure of their absorption edge, formed by direct interband transitions.

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Summary. The results of photoelectric, roentgenodosimetric and high-frequency dielectric measurements on obtained (TlGaS2)1- (TlInSe2) 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 (TlGaS2)1- (TlInSe2) 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 [1] Mustafaeva, S.N., Kerimova, E.M., Ismailova, P.G., and Asadov, M.M., Roentgendosimetric characteristics of detectors on the base of TlGaS2 Yb single crystals, Fizika, 2004, no. 4, p. 108.

[2] E.M. Kerimova, S.N.Mustafaeva, Yu.G.Asadov, R.N.Kerimov. Synthesis, growth and properties of TlGa1 xYbxS2 crystals, Crystallography Reports, 2005, V.50, Suppl. 1, P.S122 S123.

[3] S.N. Mustafaeva, Photoelectric and x-ray dosimetric properties of TlGaS2 Physics of the Solid State, 47, 2015 (2005), doi:10.1134/1.2131137

Yb

single crystals

[4] S.N. Mustafaeva, E.M. Kerimova, M.M. Asadov, R.N. Kerimov, Roentgenodetectors on the base of TlInSe <Li+>, Fizika, Vol. 9, 62 (2003).

[5] S.N. Mustafaeva, Frequency dispersion of dielectric coefficients of layered TlGaS single crystals Physics of the Solid State, Vol. 46, 1008 (2004).

[6] S.N. Mustafaeva, Frequency dependence of real and imaginary parts of complex dielectric permittivity and conductivity of TlInSe single crystal at relaxation processes, Journal of Radioelectronics, 7, 8 (2013).

[7] Mustafaeva, S.N. Frequency effect on the electrical and dielectric properties of (TlGaS2)1- x(TlInSe2)x (x = 0.005, 0.02) single crystals, Inorg Mater (2010) 46: 108. doi:10.1134/S0020168510020032 [8] V.V. Pasynkov, V.S. Sorokin, Materials of electron techniques, Sankt-Petersburg- Moscow, 2004.368 p.

[9] N. Mott and E. Davis, Electron processes in noncrystalline materials, Clarendon Press, Oxford, 1971. 472 p.

[10] Mustafaeva, S.N., Asadov, M.M., Kyazimov, S.B. et al. T-x phase diagram of the TlGaS2-TlFeS2 system and band gap of TlGa FexS2 doi:10.1134/S0020168512090117.

[11] Mustafaeva, S.N., Asadov, M.M., Kerimova, E.M. et al. Dielectric and optical properties of TlGa ErxS2 (x = 0, 0.001, 0.005, 0.01) single crystals, Inorg Mater (2013) 49: 1175. doi:10.1134/S0020168513120121

Cite the paper Mustafaeva S.N., Jafarova S.G., Kerimova E.M., Gasanov N.Z., Asadov S.M. (2016). Influence of the Composition of (TlGaS2)1- (TlInSe 2)x Alloys on Their Physical Properties. Mechanics, Materials Science & Engineering, Vol 7. doi:10.13140/RG.2.2.29609.600

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