ISSN 1063-7834, Physics of the Solid State, 2018, Vol. 60, No. 3, pp. 499–503. © Pleiades Publishing, Ltd., 2018. Original Russian Text © S.M. Asadov, S.N. Mustafaeva, 2018, published in Fizika Tverdogo Tela, 2018, Vol. 60, No. 3, pp. 495–498.
SEMICONDUCTORS
Dielectric Losses and Charge Transfer in Antimony-Doped TlGaS2 Single Crystal S. M. Asadova and S. N. Mustafaevab, * a Institute of Catalysis and Inorganic Chemistry named after Academician M. Nagiyev, Azerbaijan National Academy of Sciences, pr. H. Javid 113, Baku, AZ1143 Azerbaijan b Institute of Physics, Azerbaijan National Academy of Sciences, pr. H. Javid 131, Baku, AZ1141 Azerbaijan *e-mail:
[email protected]
Received September 19, 2017; in final form, September 25, 2017
Abstract—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. DOI: 10.1134/S1063783418030034
1. INTRODUCTION TlGaS2 crystals belong to triple compounds of the TlGaX2 type (X = S, Se, and Te) with a layered structure. The quasi-two-dimensionality, wide energy bands, structural anisotropy, phase transitions, optical and photoconducting properties, and other peculiarities attract the attention of researchers to understand the physics of TlGaS2 better. The optical activity and high photosensitivity in a wide spectral region [1, 2] distinguish TlGaS2 from other semiconductors and make it promising from a practical point of view [3–6]. TlGaS2 was proposed to be used in designing visible and infrared light sensors [7] and highly sensitive laser radiation sensors [8–11]. TlGaS2-based materials with continuously varying properties in the dependence on compositions can be obtained. For this reason and also because of possible applications of TlGaS2 in optoelectronic devices in the optical range, a significant attention has been given recently to the study of the electrical, dielectric, photovoltaic, and photoelectric [12, 13] properties of pure TlGaS2 [14], and that doped with transition and rare-earth metals [15–19]. It was shown that doping of TlGaS2 crystals led to a substantial change in the dielectric coefficients of the single crystals and changed the nature of dielectric losses in them. Semimetallic antimony (Sb) is known to be one of main impurities in semiconductors. However, when solving a problem related to selective doping of semiconductors with antimony, a strong segregation of Sb
hampers the obtainment of materials with reproducible characteristics. The segregation coefficient of Sb, for example, in silicon varies by almost five orders of magnitude in the temperature range 573–823 K. The effect of doping of TlGaS2 crystals with semimetallic impurities on their physical properties is scantily known. The photoconductivity of single-crystal TlGa0.8Sb0.2S2 solid solution grown by the Bridgman–Stockbarger method was studied in [20]. The photoconductivity spectrum of the single crystal was measured at 20 K. The spectrum contained four peaks at 504 nm (2.460 eV), 525 nm (2.361 eV), 571 nm (2.171 eV), and 584 nm (2.123 eV). It was noted that the peak at 504 nm corresponds to energy gap. The peaks at 571 and 584 nm can be assigned to the transition of electrons from the valence band to donor levels. In [20], comparison with the data for undoped TlGaS2 was not performed, and the influence of antimony on the physical properties was not considered. Taking into account the aforementioned, we present in this work the results of studying the influence of semimetallic antimony (0.5 mol % Sb) on the dielectric properties and the ac electrical conductivity of TlGaS2-based single crystals. The aim of this work is to reveal the nature of the dielectric losses in TlGa0.995Sb0.005S2 and the charge transfer mechanism. 2. EXPERIMENTAL To solve the problem noted above, we used singlecrystal TlGaS2 and TlGa0.995Sb0.005S2 samples. The
499
500
ASADOV, MUSTAFAEVA
estimation of the solubility of antimony in the TlGaS2 crystal lattice with the inclusion of the effective ionic radii of the elements indicates that the Sb3+ radius (0.76 Å) is closer to ionic radius of Ga3+ (0.62 Å) than to the radius of Tl+ (1.5 Å) [21]. Thus, a partial substitution of antimony for gallium in TlGaS2 crystals corresponds to the condition of the formation of the substitutional solid solution. The samples were prepared using high-pure chemical elements: Tl (Tl00), Ga (Ga 5N), S (high pure 165), and Sb (SU0). The samples were synthesized from the elements taken in stoichiometric proportions by their immediately melting in quartz ampoules evacuated to 10–3 Pa at 1000 ± 3 K for 5–7 h. The samples were homogenized at 750 K for 120 h, and the ampoules were cooled to room temperature in the regime of the turned-off furnace. The completeness of the synthesis of the TlGaS2-based crystals and their individuality was controlled by X-ray phase analysis (XPA). XPA was carried out using a Bruker D8 ADVANCE diffractometer using CuKα radiation at room temperature. The synthesized TlGaS2 and TlGa0.995Sb0.005S2 polycrystals were used to grow these single crystals by the Bridgman–Stockbarger method [14, 16]. To do this, the polycrystals were ground and placed in a quartz ampoule 8–10 cm long with a sharp end and internal diameter of 1 cm. To grow the single crystal, the ampoules evacuated to 10−3 Pa filled with the polycrystals were placed in a two-temperature furnace. The temperature in the upper zone of the furnace was maintained to be 1170 ± 3 K (i.e., higher than the melting temperature of TlGaS2 that is 1165 K) and, in the lower zone, the temperature was 1110 ± 3 K. The optimal velocity of displacement of the ampoule in the furnace was 0.3–0.5 cm/h, and the temperature gradient near the crystallization front was 25 ± 3 K. As is known, single crystals TlGaS2 exhibit a strong anisotropy of the physical characteristics and are prone to polytypism. Because of this, the physical parameters of these crystals were measured on the samples taken from the same technological series. The dielectric coefficients of the TlGaS2 and TlGa0.995Sb0.005S2 single crystals were measured by the resonance method [22, 23]. The frequency range of the alternating electric field was 5 × 10 4–3.5 × 107 Hz. The single-crystal TlGaS2 and TlGa0.995Sb0.005S2 samples for the electrical measurements were prepared as flat capacitors, whose plane was perpendicular to the crystallographic C axis of the samples. Silver paste was used as electrode material. The sample thickness was 80–100 μm and the plate area was 5 × 10–2 cm2. All the dielectric measurements were carried out at 300 K. The reproducibility of the resonance position was ±0.2 pF in capacity and ±1.0–1.5 scale division in Q factor (Q = 1/tanδ). In this case, the largest deviations from the average values were 3–4% for ε' and 7%
for tanδ [22, 23]. The experimental study of the TlGaS2-based samples showed that the segregation redistribution of Sb in the grown TlGa0.995Sb0.005S2 does not influence the reproducibility of the physical properties. 3. RESULTS AND DISCUSSION The XPA data for the TlGaS2-based samples showed that the unit cell parameters of the crystal lat6 tice of monoclinic system (space group C2h (C2/m)) had the values as follows: a = 10.299 Å, b = 10.284 Å, c = 15.175 Å, and β = 99.603°. These characteristics agree with the data of [24]. The samples, in which the antimony concentration was x = 0.005, were also single-phase, and, in this case, the lattice parameters were almost unchanged. The X-ray diffraction patterns of the antimony-doped TlGa0.995Sb0.005S2 sample contain only diffraction peaks of the TlGaS2 phase and does not contain additional X-ray diffraction maxima. This indicates that Sb isomorphously dissolves in TlGaS2. Figure 1 shows the frequency dependences of the real component of the complex dielectric permittivity (ε' of the TlGaS2 and TlGa0.995Sb0.005S2 samples. It is seen that there is no substantial dispersion of ε' observed in TlGaS2 (curve 1) over the entire frequency range under study. In TlGa0.995Sb0.005S2, ε' decreased from 14.4 to 11.6 as the frequency was changed from 5 × 10 4 to 3.5 × 107 Hz (curve 2). The monotonic decrease in the dielectric permittivity of the TlGa0.995Sb0.005S2 single crystal as the frequency from 5 × 10 4 to 3.5 × 107 Hz observed experimentally demonstrates the relaxation dispersion [25]. The antimony-doping of the TlGaS2 crystals led to a marked decrease in ε'. So, the value of ε' of TlGa0.995Sb0.005S2 was lower than ε' of TlGaS2 by a factor of 1.6 at f = 5 × 10 4 Hz and by a factor of 2 at f = 3.5 × 107 Hz. Figure 2 depicts the frequency dependences of the imaginary part of the complex dielectric permittivity ε'' of the (curve 1) TlGaS2 and (curve 2) TlGa0.995Sb0.005S2 single crystals. The values of the dielectric loss tangent (tanδ) in the single crystals TlGa0.995Sb0.005S2 were substantially higher than tanδ in TlGaS2 (Fig. 3). The hyperbolic decrease in tanδ with an increase in the frequency in the single crystals under study demonstrates the loss of the through conductivity [25]. Figure 4 depicts the results of the study of the frequency-dependent ac conductivity of the (curve 1) TlGaS2 and (curve 2) TlGa0.995Sb0.005S2 single crystals at 300 K. The ac conductivity of the TlGaS2 single crystal was changed by the law σac ~ f 0.6 in the frequency region 5 × 10 4–2 × 105 Hz and by the law σac ~ f 0.8 at f = 2 × 105–2 × 107 Hz; σac ~ f 2 at f > 2 ×
PHYSICS OF THE SOLID STATE
Vol. 60
No. 3
2018
DIELECTRIC LOSSES AND CHARGE TRANSFER
28
501
0.4
24
1
0.3
ε''
ε'
20 16
0.2 1
12 105
106
107
2
0.1
2 108
f, Hz
0
105
106
107
108
f, Hz
Fig. 1. Dispersion curves ε'( f ) for the (1) TlGaS2 and (2) TlGa0.995Sb0.005S2 single crystals at 300 K.
Fig. 2. Frequency dependences of the imaginary component of the complex dielectric permittivity of the (1) TlGaS2 and (2) TlGa0.995Sb0.005S2 single crystals.
300
10−5 1 σac, Ω−1 cm−1
2
tanδ × 104
200
10
−6
10−7
2
100
10−8
1 105
106
107
108
105
106
f, Hz
107
108
f, Hz
Fig. 3. Dependences of the dielectric loss tangent (tanδ) in the (1) TlGaS2 and (2) TlGa0.995Sb0.005S2 single crystals on the frequency of the applied electric field.
Fig. 4. Frequency-dependent conductivity of the (1) TlGaS2 and (2) TlGa0.995Sb0.005S2 single crystals at T = 300 K.
107 Hz. Dispersion curve σac( f ) of the TlGa0.995Sb0.005S2 sample over entire frequency range obeyed the law σac ~ f 0.8. The band-type ac-conductivity is, as is known, mainly frequency-independent up to 1010–1011 Hz. The experimental dependence σac ~ f 0.8 that we observed in the TlGaS2-based crystals shows that it is due charge carriers jumps between states localized in the energy gap. They can be the states localized near the allowed band edges or the states localized near the Fermi level [26]. However, since, in experimental conditions, the conductivity over states near the Fermi
level always dominate under the conductivity over the states near the allowed band edges, the obtained law σac ~ f 0.8 demonstrates the hopping mechanism of charge transfer over the states localized in the vicinity of the Fermi level. The formula for this conductivity proposed in [27] has the form
PHYSICS OF THE SOLID STATE
Vol. 60
No. 3
2018
⎡ ⎛ ν ph ⎞⎤ (1) ⎢⎣ln ⎜⎝ f ⎟⎠⎦⎥ , where e is electron charge, k is the Boltzman, NF is the density of states near the Fermi level, al = 1/α is the localization radius, α is the decay constant of the wave 3 σac( f ) = π e2 kTN F2al5 f 96
4
502
ASADOV, MUSTAFAEVA
function of a localized charge carrier ψ ~ e–αr, and νph is the phonon frequency. According to Eq. (1) the ac conductivity is dependent on frequency as f [ln(νph/f )]4, i.e., at f ≪ νph, σac is proportional to f 0.8. The density of states at the Fermi level was calculated from the values of σac( f ) of TlGaS2 and TlGa0.995Sb0.005S2 samples found experimentally using Eq. (1). The calculated values of NF for TlGaS2 and TlGa0.995Sb0.005S2 samples were 5.9 × 1018 and 6.8 × 1018 eV–1 cm–3, respectively. Thus, the doping of the TlGaS2 single crystal with antimony led to an increase in the density of states near the Fermi level. The quantity NF was calculated using the localization radius al = 14 Å [14]. The value of νph for TlGaS2 is of 1012 Hz [28]. According to the theory of the hopping ac conductivity, the average jump distance (R) is determined by the formula [26]
⎛ν ⎞ (2) R = 1 ln ⎜ ph ⎟ . 2α ⎝ f ⎠ In Eq. (2), the value of f corresponds to the average frequency, at which the f 0.8 law is observed. The values of R calculated by Eq. (2) for the TlGaS2 and TlGa0.995Sb0.005S2 single crystals were 81 and 77 Å, respectively. These values of R are approximately six times higher than the average distance between the localization centers of charge carriers in these single crystals. The value of R allowed us to determine the average jump times in the TlGaS2 and TlGa0.995Sb0.005S2 single crystals by the formula
τ−1 = ν ph exp(−2αR).
(3)
s, We obtained the values τ = 9.9 × 10 and 4.4 × respectively. The energy scatters of the states localized near the Fermi level in the TlGaS2 and TlGa0.995Sb0.005S2 single crystals (ΔE = 150 and 154 meV, respectively) were estimated by the formula –8
10–8
3 . (4) 2πR3N F The concentrations of deep traps responsible for the ac conductivity in these samples determined by the formula ΔE =
N t = N F ΔE , where Nt = 8.8 × 1017 and 1018 cm–3.
(5)
4. CONCLUSIONS For TlGaS2 and TlGa0.995Sb0.005S2 single crystals, we have studied the reproducible frequency dependences of the dielectric loss tangent (tanδ), real (ε') and imaginary (ε'') components of the complex dielectric permittivity and ac conductivity (σac) across the
crystal layers in the frequency range f = 5 × 10 4–3.5 × 107 Hz. The doping of the TlGaS2 single crystals with antimony leads to the modification of dispersion curves ε'( f ) and ε''( f ). The conductivity losses in the TlGaS2 and TlGa0.995Sb0.005S2 take place over the entire frequency range. At high frequencies, the ac conductivity of the TlGaS2 and TlGa0.995Sb0.005S2 single crystals obeyed the law σac ~ f 0.8 characterized of the hopping mechanism of charge transfer over the states localized near the Fermi level. We estimated the density and energy scatter of the states lying in the vicinity of the Fermi level, the average time and distance of jumps in the TlGaS2 and TlGa0.995Sb0.005S2 samples. The comparison shows that the antimonydoping of the TlGaS2 single crystal led to substantial change in the dielectric characteristics of the TlGa0.995Sb0.005S2 single crystals, the increase in the density of states near the Fermi level (from 5.9 × 1018 to 6.8 × 1018 eV–1 cm–3), the decrease in the average time (from 9.9 × 10–8 to 4.4 × 10–8 s) and the average distance (from 81 to 77 Å) of the jumps. In this case, the concentration of deep traps responsible for the ac conductivity in these samples increases from 8.8 × 1017 to 1018 cm–3. Thus, it was found that the dielectric coefficients and the ac conductivity can be controlled due to the alloying of the layered TlGaS2 single crystal with antimony.
ACKNOWLEDGMENTS This work supported by the Science Development Foundation under the President of the Republic of Azerbaijan (project 5.EİF-BGM-3-BRFTF-2+/2017). REFERENCES 1. I. G. Stamov, N. N. Syrbu, V. V. Ursaki, and V. V. Zalamai, Opt. Commun. 298–299, 145 (2013). 2. L. Nemerenco, N. N. Syrbu, V. Dorogan, N. P. Bejan, and V. V. Zalamai, J. Luminesc. 172, 111 (2016). 3. T. Kawabata, Y. Shim, K. Wakita, and N. Mamedov, Thin Solid Films 571, 589 (2014). 4. B. Abay, H. S. Güder, H. Efeoǧlu, and Y. K. Yoǧurtçu, Phys. Status Solidi B 227, 469 (2001). 5. B. Gürbulak, S. Duman, and A. Ateş, Czechoslov. J. Phys. 55, 93 (2005). 6. Y. Shim, W. Okada, K. Wakita, and N. Mamedov, J. Appl. Phys. 102, 083537 (2007). 7. A. F. Qasrawi and N. M. Gasanly, Cryst. Res. Technol. 39, 439 (2004). 8. A. F. Qasrawi and N. M. Gasanly, Phys. Status Solidi A 202, 2501 (2005). 9. I. M. Ashraf, J. Phys. Chem. B 108, 10765 (2004). 10. A. Kato, M. Nishigaki, N. Mamedov, M. Yamazaki, S. Abdullayeva, E. Kerimova, H. Uchiki, and S. Iida, J. Phys. Chem. Solids 64, 1713 (2003).
PHYSICS OF THE SOLID STATE
Vol. 60
No. 3
2018
DIELECTRIC LOSSES AND CHARGE TRANSFER 11. A. A. Al Ghamdi, A. T. Nagat, F. S. Bahabri, R. H. Al Orainy, and S. E. Al Garni, Appl. Surf. Sci. 257, 3205 (2011). 12. C.-D. Kim and M.-S. Jin, New Phys.: Sae Mulli 65, 1068 (2015). 13. M. Açıkgöz, P. Gnutek, and C. Rudowicz, Solid State Commun. 150, 1077 (2010). 14. S. N. Mustafaeva, Phys. Solid State 46, 1008 (2004). 15. S. N. Mustafaeva, Zh. Radioelektron. 8, 1 (2008). 16. S. N. Mustafaeva, Izv. Akad. Nauk, Neorg. Mater. 42, 530 (2006). 17. S. N. Mustafaeva, Zh. Radioelektron. 4, 1 (2009). 18. S. N. Mustafaeva, M. M. Asadov, E. M. Kerimova, and N. Z. Gasanov, Inorg. Mater. 49, 1175 (2013). 19. V. G. Hurtavy, A. U. Sheleg, S. N. Mustafaeva, E. M. Kerimova, and S. G. Dzhafarova, Phys. Solid State 59, 1501 (2017). 20. M.-S. Jin and H.-J. Song, Curr. Appl. Phys. 3, 409 (2003).
PHYSICS OF THE SOLID STATE
Vol. 60
No. 3
2018
503
21. J. E. Huheey, Inorganic Chemistry: Principles of Structure and Reactivity (Pearson, New Delhi, 2000). 22. S. N. Mustafaeva, D. M. Babanly, M. M. Asadov, and D. B. Tagiyev, Phys. Solid State 57, 1963 (2015). 23. S. N. Mustafaeva, Vse Mater., No. 10, 74 (2016). 24. G. E. Delgado, A. J. Mora, F. V. Perezb, and J. Gonzalez, Phys. B (Amsterdam, Neth.) 391, 385 (2007). 25. V. V. Pasynkov and V. S. Sorokin, Materials for Electronic Technique, 6th ed. (Lan’, St. Petersburg, Moscow, Krasnodar, 2004) [in Russian]. 26. N. F. Mott and E. Davis, Electronic Processes in NonCrystalline Materials (Clarendon, Oxford, 1971). 27. M. Pollak, Philos. Mag. 23, 519 (1971). 28. K. R. Allakhverdiev, E. A. Vinogradov, and R. Kh. Nani, in Physical Properties of COmplex Semiconductors (Elm, Baku, 1982), p. 55.
Translated by Yu. Ryzhkov
