- Email: [email protected]
Determination of trapping parameters in Tl4Ga3InSe8 single crystals by thermally stimulated luminescence
Physica B 441 (2014) 37–41
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Determination of trapping parameters in Tl4Ga3InSe8 single crystals by thermally stimulated luminescence S. Delice n, N.M. Gasanly Physics Department, Middle East Technical University, 06800 Ankara, Turkey
art ic l e i nf o
a b s t r a c t
Article history: Received 28 December 2013 Accepted 6 February 2014 Available online 14 February 2014
Thermoluminescence (TL) measurements were performed on Tl4Ga3InSe8 layered single crystals grown by Bridgman method in the temperature range of 10–200 K. After illuminating the sample with blue light at 10 K and heating at a rate of 1.0 K s 1 in dark, TL curve exhibited peaks around 46 and 123 K. Thermal activation energies of the trap levels corresponding to the observed peaks were determined using curve fitting, initial rise and peak shape methods. Analyses have revealed the presence of two defect centers with activation energies of 7 and 41 meV. The consistency between the theoretical predictions for slow retrapping and experimental results showed that the retrapping process for the observed centers was negligible. Measurements at different heating rates and illumination temperatures were also carried out for the high-temperature peak. Distribution of traps has been established as a result of experiments. An increase of activation energy from 45 to 147 meV was revealed with the change of illumination temperature from 40 to 80 K. & 2014 Elsevier B.V. All rights reserved.
Keywords: Thermoluminescence Chalcogenides Defects Impurities
1. Introduction A wide variety of ternary and quaternary layered-structured thallium-based semiconductors attract much interest due to possible optoelectronic applications from ultraviolet to the infrared [1,2]. For the most part, optoelectronic properties of these materials are dominated by defects of various types and the interactions between them. In the recent years, a growing interest has been given to the physical properties of the layer- and chainlike structured materials such as TlGaSe2 and TlInSe2 crystals. There are large number of TlGaSe2 applications as memory switching elements, emission modulators and nonlinear optical transducers in nonlinear optics and photoelectronics [2,3]. The TlInSe2 compound exhibits, in its electrical behavior, many nonlinear effects, such as S-type characteristics with voltage oscillations in the negative differential resistance region, and switching and memory effects [3]. The quaternary compound Tl4Ga3InSe8 is a structural analog of TlGaSe2 [1] in which one quarter of gallium atoms are replaced by indium ones. Previously, we studied the optical properties of Tl4Ga3InSe8 crystals through the transmission measurements in the wavelength range of 500–1100 nm [4]. The analysis of the absorption
n
Corresponding author. Tel.: þ 90 312 2105059; fax: þ90 312 2105099. E-mail addresses: [email protected], [email protected] (S. Delice).
http://dx.doi.org/10.1016/j.physb.2014.02.007 0921-4526 & 2014 Elsevier B.V. All rights reserved.
data revealed the presence of both optical indirect and direct transitions with band gap energies of 1.94 and 2.20 eV, respectively. Transmission measurements carried out in the temperature range of 10–300 K established that the rate of change of the indirect band gap with temperature is γ¼ 4.1 10 4 eV K 1. Goksen et al. [5] investigated the low-temperature photoluminescence (PL) of Tl4Ga3InSe8 in the wavelength region of 600–750 nm. Radiative transitions from donor level (190 meV) to shallow acceptor level (30 meV) were suggested to be responsible for the observed PL band. The results of thermally stimulated current studies on Tl4Ga3InSe8 crystal in the temperature range 10–160 K were reported in Ref. [6]. Experimental evidence was found for two trapping centers with activation energies of 17 and 27 meV. Defects are crucial phenomena in order to understand the electrical and optical properties of semiconductor materials. Their influence can be decisive on the performance of the devices produced in the semiconductor industrial areas such as optoelectronic. For instance, in LEDs or lasers, defects may display behaviors as though they are tunneling and nonradiative recombination channels lowering the internal quantum efficiency, depending on defect density. In the case of electronic devices, defects introduce scattering centers lowering carrier mobility. Therefore, it is useful to get information on parameters of trapping centers in semiconductors in order to obtain high-quality devices. Among the several experimental methods to determine the properties of trapping centers, thermally stimulated
38
S. Delice, N.M. Gasanly / Physica B 441 (2014) 37–41
luminescence (TL) measurements provide valuable information on trap states [7]. The purpose of the present work is to get the detailed information about the trapping centers and their distribution in undoped Tl4Ga3InSe8 single crystals by TL experiments in the low-temperature range of 10–200 K.
2. Experimental details Single crystals of Tl4Ga3InSe8 were grown by the Bridgman method from the stoichiometric melt 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 30 1C cm 1 at a rate of 1.0 mm h 1. The resulting ingots (dark-red in color) showed good optical quality and were easily cleaved along the planes, which are perpendicular to the c-axis of the crystal. The chemical composition of Tl4Ga3InSe8 crystals was obtained by energy dispersive spectroscopic analysis using a JSM-6400 electron microscope. The composition of the studied samples (Tl:Ga:In:Se) was found to be 25.6:19.3:6.4:48.7, respectively. The crystal structure of the Tl4Ga3InSe8 was identified using X-ray diffraction experiments. Measurements were performed using “Rigaku miniflex” diffractometer with CuKα radiation (λ¼0.154049 nm). The scanning speed of the diffractometer was 0.021 s 1. Fig. 1 shows X-ray diffractogram of Tl4Ga3InSe8 crystal. Least-squares computer program “DICVOL 04” was used for indexing the registered diffractogram. The calculated Miller indices (hkl) are displayed in Fig. 1. The lattice parameters of the monoclinic unit cell were found as a¼0.7659, b¼0.7698, c¼1.0308 nm, and β¼94.251. TL measurements, performed in a temperature range of 10–300 K by means of “Advanced Research Systems” closed cycle helium gas cooling cryostat, were accomplished by illuminating the sample at low temperatures using LED creating light at a maximum peak of 2.6 eV. The sample with dimensions of 7 4 0.5 mm3 was illuminated for 600 s, which is experimentally determined time to completely fill the traps. Illuminated sample was left in dark for an expectation time (E 120 s) after the illumination was turned off and Lake-Shore 331 temperature controller was adjusted to increase the temperature of the sample with a constant heating rate. The temperature sensitivity of the system was about 10 mK. An optical system comprising lenses and mirrors was used to gather emitted photons which were focused on the photomultiplier tube (Hamamatsu R928; spectral response: 185–900 nm). Pulses from the photomultiplier were converted into TTL pulses using a photon-counting unit (Hamamatsu C3866) and counted by a counter (National instrument NI-USB 2011).
Fig. 1. The X-ray diffraction pattern for Tl4Ga3InSe8 crystal.
3
Fig. 2. Principles of TL experiment for Tl4Ga3InSe8 crystals: (1) time period of applied illumination; (2) temperature variation with time; (3) TL signal for five linear heating rates: (a) 0.2, (b) 0.4, (c) 0.5, (d) 0.8, and (e) 1.0 K s 1.
Number of photon counts was obtained as a function of temperature using a software program written in LABVIEW (National Instruments) graphical development environment. The details of illumination and heating parameters for the optimum TL conditions in Tl4Ga3InSe8 crystal are shown in Fig. 2.
3. Results and discussion Fig. 3 shows a typical TL curve for Tl4Ga3InSe8 crystal (cooled and illuminated at T0 ¼10 K) in the temperature range 10–200 K with heating rate of β¼ 1.0 K s 1. At the beginning of the experiments, we have performed the measurements in the 10–300 K range. However, since no TL peak is detected above 200 K, we have carried out the experiments in the 10–200 K range. Two overlapping peaks (A and B) were observed with maximum temperatures (Tmax) nearly at 46 and 123 K, respectively. To isolate the overlapping TL peaks, we used thermal cleaning technique which was applied as follows [7]: The sample was cooled and irradiated at the temperature (Till) less than TmaxA. Then, the light source was switched off and the sample was cooled until T0 ¼10 K. Further, the sample was heated in the dark at a constant rate β¼ 1.0 K s 1. The shallower trap level corresponding to peak A was substantially emptied by this way as the illumination temperature value Till ¼ 40 K was experienced. This allowed the observation of a distinctive peak B due to carriers released from the remaining filled trap. The resultant TL curve after thermal cleaning is shown in Fig. 4 (circles). The theoretical approach for the analysis of the TL curve differs for the first order (slow retrapping) and second order (fast retrapping) kinetics. Therefore, one should preliminary reveal the order of kinetics. The illumination time dependence (which corresponds to a change in the trapped charge population) of the peak maximum position (Tmax) was studied to get information on
S. Delice, N.M. Gasanly / Physica B 441 (2014) 37–41
Fig. 3. Experimental TL curve of Tl4Ga3InSe8 crystal with heating rate β ¼ 1.0 K s 1 and decomposition of this curve into two separate peaks ((A) and (B)). Open circles are experimental data. Dashed curves represent decomposed peaks. Solid curve shows total fit to the experimental data. Inset: TL intensity vs. 1000/T for observed peaks in TL spectra of Tl4Ga3InSe8 crystal. Open circles are experimental data and solid lines are the fitted straight lines.
39
where ν is the attempt-to-escape frequency, β is the heating rate, n0 is the initial concentration of the charge carriers in the trap level(s) and T0 is the starting temperature of heating process. Fig. 3 shows the measured TL curve (open circles) together with the fitted curve (solid line) obtained via curve fitting method accomplished for slow retrapping process according to Eq. (1). As seen from the figure, the measured TL peak can be approximated using first order process as the fitted line successfully describes the experimental data. The dashed–dot curves in Fig. 3 under the main curve represent decomposed peaks related with each defect center. Thermal activation energies of the trapping levels responsible for the TL peaks were found to be 7 and 41 meV as an outcome of the curve fitting. Initial rise method is one of the effective analysis methods valid for all types of kinetics. In this useful technique, initial part of the glow curve (T oTmax) is taken account the consideration since the number of trapped charge carriers can be supposed to be constant at the initial tail of peak. Under the light of this assumption, the initial part of the TL peak should be proportional to exp( Et/kT) when the trapped carriers start to be excited to the non-localized states [7]. When the initial portion of the curve was analyzed, ln (ITL) versus 1/T graph gives a straight line with a slope of ( Et/ k). Inset of Fig. 3 represents the corresponding plot of experimental data (open circles) and their linear fits (solid lines). Activation energies of the traps were found as 6 and 45 meV (Table 1). These results show a good consistency with those obtained from curve fitting method. The peak shape method is another technique in which the analysis is based on the geometric shape of the single TL peak. In this method, peak maximum temperature (Tmax), low temperature (Tl) and high temperature (Th) sides at half maximum TL intensity of the glow curve is taken under consideration. Then, the activation energy of the charge carrier released from trap(s) can be calculated using the parameters: τ ¼Tmax Tl, δ¼Th Tmax, w¼Th Tl and μg ¼δ/w in the following expressions [7] Eτ ¼ ½1:51 þ 3:0ðμg 0:42ÞkT 2max =τ ½1:58 þ4:2ðμg 0:422kT max Eδ ¼ ½0:976 þ 7:3ðμg 0:42ÞkT 2max =δ Ew ¼ ½2:52 þ 10:2ðμg 0:42kT 2max =w 2kT max
Fig. 4. Experimental TL curves of Tl4Ga3InSe8 crystal with heating rate of β ¼1.0 K s 1. Stars and circles are data before and after thermal cleaning, respectively.
the order of kinetics. In the case of first order of kinetics (slow retrapping), the shape of the curve and Tmax value are not influenced by the trapped charge concentration [8,9]. However, in the non-first order of kinetics case, the concentration of the carriers in the trap levels affects Tmax value since retrapped charges will participate repeatedly in the TL process. The experiments carried out showed that the shape of the curve and Tmax value do not change with illumination time. This is a vigorous indication of the first order kinetics. The results of the curve fitting analysis (mentioned in the following part) also support the validity of this case. In the analysis of the experimental data, we have used curve fitting, initial rise and peak shape methods. The details of the curve fitting method which is based on the least square fitting of the glow curve in the light of theoretical equation using a software program were reported in our previous study on the thermally stimulated current measurements [10]. TL intensity for slow retrapping case is given as [11] I TL ¼ n0 v exp
Et kT
Z
T T0
ν expð Et =kTÞdT β
ð1Þ
The averaged values of the energies Eτ, Eδ and Ew of revealed trapping centers were found as 9 and 49 meV, respectively (Table 1). These results are also in agreement with those of the two techniques mentioned above. The heating rate is a significant parameter influencing the TL glow curves associated with traps. For this reason, the heating rate dependence of revealed traps can give illustrative knowledge about the studied materials. The TL curve in Fig. 3 carries the properties of overlapped trap levels (peaks A and B). Since the characteristics of peak B can be obtained after thermal cleaning method, the detailed analysis of the heating rate dependence was performed only for the isolated peak B. Fig. 5 presents TL spectra of the Tl4Ga3InSe8 crystals measured at five heating rates of β ¼0.2 1.0 K s 1 with step 0.2 K s 1 in the temperature range of 40 200 K. In accordance with the TL theory, variation of the Table 1 The activation energies (Et) of revealed traps of the Tl4Ga3InSe8 single crystal. Peak Tm (K) Et (meV) Curve fitting method Initial rise method Peak shape method A B
46 123
7 41
6 45
9 49
40
S. Delice, N.M. Gasanly / Physica B 441 (2014) 37–41
Fig. 5. Experimental TL curves of Tl4Ga3InSe8 crystal (peak B) with different heating rates. Inset: Heating rate dependencies of peak maximum temperature (Tmax), full-width-half-maximum (FWHM) and area of TL curves (peak B). The dash-dotted lines are only guides for the eye.
Fig. 6. Experimental TL curves of Tl4Ga3InSe8 crystal (peak B) at different excitation temperatures. Inset 1: The variation of activation energy versus illumination temperature. The dash-dotted line is only guide for the eye. Inset 2: ln(S0) plot as a function of activation energy. Circles are the experimental data and solid line is the fitted straight line.
Table 2 TL parameters for Tl4Ga3InSe8 crystal (peak B) at different illumination temperatures. Curve Illumination temperature (K) Maximum temperature (K) Curve area (a.u.) Activation energy (meV)
1 40 126.7 3020 45
2 50 128.8 2520 59
3 60 135.0 1690 87
4
5
70 142.8 725 127
80 153.8 317 147
heating rates results with change in the shape and position of the TL peak [7]. As observed from Fig. 5, TL intensities of the curves decrease and their maximum temperature values shift to higher temperatures with increasing the heating rates. Many authors explained the decrease of peak height versus heating rate through the possibility of thermal quenching whose efficiency increases as the temperature increases [12–14]. Inset of Fig. 5 demonstrates the
heating rate dependencies of Tmax, full-width-half-maximum (FWHM) and area values of the TL curves. The FWHM grows from 39 to 55 K as heating rate changes from 0.2 to 1.0 K s 1. The area under the TL curve is proportional to the total number of carriers released from the traps. Since the heating rate dependence was performed for the case of completely filled traps, the area obtained at each measurement should be same. The areas of the curves were found nearly equal to each other (5% variation between the smallest and biggest areas). To reveal the traps distribution responsible for the isolated (thermally cleaned) peak B in Tl4Ga3InSe8 crystals, the sample was irradiated by light for 600 s at various illumination temperatures (T0i) ranging from 40 to 80 K. Then, the light source was switched off and the sample was cooled down to 10 K. Thereafter, the sample was heated at a constant rate β¼1.0 K s 1 to excite the trapped charge carriers to the conduction band. The experimental TL spectra of Tl4Ga3InSe8 crystal at different illumination temperatures (T0i ¼40, 50, 60, 70, and 80 K) are shown in Fig. 6. The intensity of the TL spectra decreased and the maximum values of peak B shifted towards higher temperatures with increasing illumination temperatures. This fact supports the validity of a quasi-continuous traps distribution [15–18]. The area enclosed under the curve obtained with illumination at temperature T0i is proportional to the number of carriers released from the traps during the heating process. The activation energies of the revealed trapping centers were observed to be shifted from 45 to 147 meV by increasing the illumination temperature from 40 to 80 K (inset 1 of Fig. 6 and Table 2). The increase of the activation energy values when T0i increases is consistent with the gradual emptying of shallowest trapping levels during each preheating treatment [7,11,19]. Similar results on traps distribution were also reported previously in Refs. [20–22]. Inset 2 of Fig. 6 presents the plot of ln(S0) (where S0 is the area under the curve) as a function of the activation energies for peak B. This dependence is well approximated by the exponent, which suggests the presence of an exponential distribution of the traps. The exponential distribution is consistent with the presence of fluctuations in the crystal potential, caused by structural defects, which give rise to localized states acting as traps for the carriers [16]. The following expression can be written for the traps filled at the illumination temperature T0i by assuming an exponential traps distribution, whose density at energy Eti will be given by Nti ¼Aexp ( α Eti) [15]. This relation implies that S0 aAexpð αEti Þ Here, α is the energy parameter that characterizes the traps distribution. From the analysis of the graph ln(S0) versus activation energy for peak B, the value of α was found to be 0.0213 meV 1, which corresponds to the variation of one order of magnitude in the traps density for every 108 meV.
4. Conclusion Tl4Ga3InSe8 layered single crystals have been studied in the temperature range of 10–200 K using thermally stimulated luminescence technique. The TL curve demonstrated the presence of two overlapping peaks with maximum temperatures of 46 and 123 K. The observed TL curve have been analyzed using curve fit, initial rise and peak shape methods. As a result of the analysis, two trapping centers with activation energies of 7 and 41 meV have been revealed. Since the studied crystals were not intentionally doped, these centers are thought to originate from stacking faults, which are quite possible in Tl4Ga3InSe8 due to the weakness of the van der Waals forces between the layers. Moreover,
S. Delice, N.M. Gasanly / Physica B 441 (2014) 37–41
the behavior of high-temperature peak for different heating rates (0.2–1.0 K s 1) has been studied. TL intensities of the curves decrease and their maximum temperature values shift to higher temperatures with increasing the heating rates. And finally, analysis of experimental TL curves registered at different light illumination temperatures revealed the exponential distribution of the traps in studied crystals. The activation energies of the distributed trapping centers were found to be increasing from 45 to 147 meV with increasing the illumination temperature from 40 to 80 K. The variation of one order of magnitude in the traps density for every 108 meV was estimated. References [1] K.A. Yee, A. Albright, J. Am. Chem. Soc. 113 (1991) 6474. [2] A.M. Panich, J. Phys.: Condens. Matter 20 (2008) 293202. [3] M. Hanias, A. Anagnostopoulos, K. Kambas, J. Spyridelis, Mater. Res. Bull 27 (1992) 25. [4] K. Goksen, N.M. Gasanly, Physica B 400 (2007) 266. [5] K. Goksen, N.M. Gasanly, R. Turan, Cryst. Res. Technol 41 (2006) 822. [6] N.M. Gasanly, N.A.P. Mogaddam, H. Ozkan, Cryst. Res. Technol 41 (2006) 1100.
41
[7] R. Chen, S.W.S. McKeever, Theory of Thermoluminescence and Related Phenomena, World Scientific, Singapore, 1997. [8] A.J.J. Bos, Radiat. Meas. 41 (2007) S45. [9] V.M. Skorikov, V.I. Chmyrev, V.V. Zuev, E.V. Larina, Inorg. Mater. 38 (2002) 751. [10] N.S. Yuksek, N.M. Gasanly, H. Ozkan, Semicond. Sci. Technol. 18 (2003) 834. [11] R. Chen, Y. Kirsh, Analysis of Thermally Stimulated Processes, Pergamon Press, Oxford, 1981. [12] G. Kitis, C. Furetta, M. Prokic, V. Prokic, J. Phys. D: Appl. Phys. 33 (2000) 1252. [13] G. Kitis, M. Spiropulu, J. Papadopoulos, S. Charalambous, Nucl. Instrum. Methods Phys. Res. 73 (1993) 367. [14] A. Ege, E. Ekdal, T. Karali, N. Can, M. Prokic, Meas. Sci. Technol. 18 (2007) 889. [15] A. Serpi, J. Phys. D: Appl. Phys. 9 (1976) 1881. [16] A. Bosacchi, B. Bosacchi, S. Franchi, L. Hernande, Solid State Commun. 13 (1973) 1805. [17] A. Anedda, M.B. Casu, A. Serpi, I.I. Burlakov, I.M. Tiginyanu, V.V. Ursaki, J. Phys. Chem. Solids 58 (1997) 325. [18] P.C. Ricci PC, A. Anedda, R. Corpino, I.M. Tiginyanu, V.V. Ursaki, J. Phys. Chem. Solids 64 (2003) 1941. [19] R.H. Bube, Photoelectronic Properties of Semiconductors, Cambridge University Press, Cambridge, 1992. [20] V. Correcher, J.M. Gomez-Ros, J. Garcia-Guinea, M. Lis, L. Sanchez-Munoz, Radiat. Meas. 43 (2008) 269. [21] J.M. Gomez-Ros, V. Correcher, J. Garcia-Guinea, A. Delgado, Radiat. Prot. Dosim. 119 (2006) 93. [22] J.M. Gomez-Ros, C. Furetta, E. Cruz-Zaragoza, M. Lis, A. Torres, G. Monsivais, Nucl. Instr. Meth. Res. A 566 (2006) 727.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Determination of trapping parameters in Tl4Ga3InSe8 single crystals by thermally stimulated luminescence S. Delice n, N.M. Gasanly Physics Department, Middle East Technical University, 06800 Ankara, Turkey
art ic l e i nf o
a b s t r a c t
Article history: Received 28 December 2013 Accepted 6 February 2014 Available online 14 February 2014
Thermoluminescence (TL) measurements were performed on Tl4Ga3InSe8 layered single crystals grown by Bridgman method in the temperature range of 10–200 K. After illuminating the sample with blue light at 10 K and heating at a rate of 1.0 K s 1 in dark, TL curve exhibited peaks around 46 and 123 K. Thermal activation energies of the trap levels corresponding to the observed peaks were determined using curve fitting, initial rise and peak shape methods. Analyses have revealed the presence of two defect centers with activation energies of 7 and 41 meV. The consistency between the theoretical predictions for slow retrapping and experimental results showed that the retrapping process for the observed centers was negligible. Measurements at different heating rates and illumination temperatures were also carried out for the high-temperature peak. Distribution of traps has been established as a result of experiments. An increase of activation energy from 45 to 147 meV was revealed with the change of illumination temperature from 40 to 80 K. & 2014 Elsevier B.V. All rights reserved.
Keywords: Thermoluminescence Chalcogenides Defects Impurities
1. Introduction A wide variety of ternary and quaternary layered-structured thallium-based semiconductors attract much interest due to possible optoelectronic applications from ultraviolet to the infrared [1,2]. For the most part, optoelectronic properties of these materials are dominated by defects of various types and the interactions between them. In the recent years, a growing interest has been given to the physical properties of the layer- and chainlike structured materials such as TlGaSe2 and TlInSe2 crystals. There are large number of TlGaSe2 applications as memory switching elements, emission modulators and nonlinear optical transducers in nonlinear optics and photoelectronics [2,3]. The TlInSe2 compound exhibits, in its electrical behavior, many nonlinear effects, such as S-type characteristics with voltage oscillations in the negative differential resistance region, and switching and memory effects [3]. The quaternary compound Tl4Ga3InSe8 is a structural analog of TlGaSe2 [1] in which one quarter of gallium atoms are replaced by indium ones. Previously, we studied the optical properties of Tl4Ga3InSe8 crystals through the transmission measurements in the wavelength range of 500–1100 nm [4]. The analysis of the absorption
n
Corresponding author. Tel.: þ 90 312 2105059; fax: þ90 312 2105099. E-mail addresses: [email protected], [email protected] (S. Delice).
http://dx.doi.org/10.1016/j.physb.2014.02.007 0921-4526 & 2014 Elsevier B.V. All rights reserved.
data revealed the presence of both optical indirect and direct transitions with band gap energies of 1.94 and 2.20 eV, respectively. Transmission measurements carried out in the temperature range of 10–300 K established that the rate of change of the indirect band gap with temperature is γ¼ 4.1 10 4 eV K 1. Goksen et al. [5] investigated the low-temperature photoluminescence (PL) of Tl4Ga3InSe8 in the wavelength region of 600–750 nm. Radiative transitions from donor level (190 meV) to shallow acceptor level (30 meV) were suggested to be responsible for the observed PL band. The results of thermally stimulated current studies on Tl4Ga3InSe8 crystal in the temperature range 10–160 K were reported in Ref. [6]. Experimental evidence was found for two trapping centers with activation energies of 17 and 27 meV. Defects are crucial phenomena in order to understand the electrical and optical properties of semiconductor materials. Their influence can be decisive on the performance of the devices produced in the semiconductor industrial areas such as optoelectronic. For instance, in LEDs or lasers, defects may display behaviors as though they are tunneling and nonradiative recombination channels lowering the internal quantum efficiency, depending on defect density. In the case of electronic devices, defects introduce scattering centers lowering carrier mobility. Therefore, it is useful to get information on parameters of trapping centers in semiconductors in order to obtain high-quality devices. Among the several experimental methods to determine the properties of trapping centers, thermally stimulated
38
S. Delice, N.M. Gasanly / Physica B 441 (2014) 37–41
luminescence (TL) measurements provide valuable information on trap states [7]. The purpose of the present work is to get the detailed information about the trapping centers and their distribution in undoped Tl4Ga3InSe8 single crystals by TL experiments in the low-temperature range of 10–200 K.
2. Experimental details Single crystals of Tl4Ga3InSe8 were grown by the Bridgman method from the stoichiometric melt 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 30 1C cm 1 at a rate of 1.0 mm h 1. The resulting ingots (dark-red in color) showed good optical quality and were easily cleaved along the planes, which are perpendicular to the c-axis of the crystal. The chemical composition of Tl4Ga3InSe8 crystals was obtained by energy dispersive spectroscopic analysis using a JSM-6400 electron microscope. The composition of the studied samples (Tl:Ga:In:Se) was found to be 25.6:19.3:6.4:48.7, respectively. The crystal structure of the Tl4Ga3InSe8 was identified using X-ray diffraction experiments. Measurements were performed using “Rigaku miniflex” diffractometer with CuKα radiation (λ¼0.154049 nm). The scanning speed of the diffractometer was 0.021 s 1. Fig. 1 shows X-ray diffractogram of Tl4Ga3InSe8 crystal. Least-squares computer program “DICVOL 04” was used for indexing the registered diffractogram. The calculated Miller indices (hkl) are displayed in Fig. 1. The lattice parameters of the monoclinic unit cell were found as a¼0.7659, b¼0.7698, c¼1.0308 nm, and β¼94.251. TL measurements, performed in a temperature range of 10–300 K by means of “Advanced Research Systems” closed cycle helium gas cooling cryostat, were accomplished by illuminating the sample at low temperatures using LED creating light at a maximum peak of 2.6 eV. The sample with dimensions of 7 4 0.5 mm3 was illuminated for 600 s, which is experimentally determined time to completely fill the traps. Illuminated sample was left in dark for an expectation time (E 120 s) after the illumination was turned off and Lake-Shore 331 temperature controller was adjusted to increase the temperature of the sample with a constant heating rate. The temperature sensitivity of the system was about 10 mK. An optical system comprising lenses and mirrors was used to gather emitted photons which were focused on the photomultiplier tube (Hamamatsu R928; spectral response: 185–900 nm). Pulses from the photomultiplier were converted into TTL pulses using a photon-counting unit (Hamamatsu C3866) and counted by a counter (National instrument NI-USB 2011).
Fig. 1. The X-ray diffraction pattern for Tl4Ga3InSe8 crystal.
3
Fig. 2. Principles of TL experiment for Tl4Ga3InSe8 crystals: (1) time period of applied illumination; (2) temperature variation with time; (3) TL signal for five linear heating rates: (a) 0.2, (b) 0.4, (c) 0.5, (d) 0.8, and (e) 1.0 K s 1.
Number of photon counts was obtained as a function of temperature using a software program written in LABVIEW (National Instruments) graphical development environment. The details of illumination and heating parameters for the optimum TL conditions in Tl4Ga3InSe8 crystal are shown in Fig. 2.
3. Results and discussion Fig. 3 shows a typical TL curve for Tl4Ga3InSe8 crystal (cooled and illuminated at T0 ¼10 K) in the temperature range 10–200 K with heating rate of β¼ 1.0 K s 1. At the beginning of the experiments, we have performed the measurements in the 10–300 K range. However, since no TL peak is detected above 200 K, we have carried out the experiments in the 10–200 K range. Two overlapping peaks (A and B) were observed with maximum temperatures (Tmax) nearly at 46 and 123 K, respectively. To isolate the overlapping TL peaks, we used thermal cleaning technique which was applied as follows [7]: The sample was cooled and irradiated at the temperature (Till) less than TmaxA. Then, the light source was switched off and the sample was cooled until T0 ¼10 K. Further, the sample was heated in the dark at a constant rate β¼ 1.0 K s 1. The shallower trap level corresponding to peak A was substantially emptied by this way as the illumination temperature value Till ¼ 40 K was experienced. This allowed the observation of a distinctive peak B due to carriers released from the remaining filled trap. The resultant TL curve after thermal cleaning is shown in Fig. 4 (circles). The theoretical approach for the analysis of the TL curve differs for the first order (slow retrapping) and second order (fast retrapping) kinetics. Therefore, one should preliminary reveal the order of kinetics. The illumination time dependence (which corresponds to a change in the trapped charge population) of the peak maximum position (Tmax) was studied to get information on
S. Delice, N.M. Gasanly / Physica B 441 (2014) 37–41
Fig. 3. Experimental TL curve of Tl4Ga3InSe8 crystal with heating rate β ¼ 1.0 K s 1 and decomposition of this curve into two separate peaks ((A) and (B)). Open circles are experimental data. Dashed curves represent decomposed peaks. Solid curve shows total fit to the experimental data. Inset: TL intensity vs. 1000/T for observed peaks in TL spectra of Tl4Ga3InSe8 crystal. Open circles are experimental data and solid lines are the fitted straight lines.
39
where ν is the attempt-to-escape frequency, β is the heating rate, n0 is the initial concentration of the charge carriers in the trap level(s) and T0 is the starting temperature of heating process. Fig. 3 shows the measured TL curve (open circles) together with the fitted curve (solid line) obtained via curve fitting method accomplished for slow retrapping process according to Eq. (1). As seen from the figure, the measured TL peak can be approximated using first order process as the fitted line successfully describes the experimental data. The dashed–dot curves in Fig. 3 under the main curve represent decomposed peaks related with each defect center. Thermal activation energies of the trapping levels responsible for the TL peaks were found to be 7 and 41 meV as an outcome of the curve fitting. Initial rise method is one of the effective analysis methods valid for all types of kinetics. In this useful technique, initial part of the glow curve (T oTmax) is taken account the consideration since the number of trapped charge carriers can be supposed to be constant at the initial tail of peak. Under the light of this assumption, the initial part of the TL peak should be proportional to exp( Et/kT) when the trapped carriers start to be excited to the non-localized states [7]. When the initial portion of the curve was analyzed, ln (ITL) versus 1/T graph gives a straight line with a slope of ( Et/ k). Inset of Fig. 3 represents the corresponding plot of experimental data (open circles) and their linear fits (solid lines). Activation energies of the traps were found as 6 and 45 meV (Table 1). These results show a good consistency with those obtained from curve fitting method. The peak shape method is another technique in which the analysis is based on the geometric shape of the single TL peak. In this method, peak maximum temperature (Tmax), low temperature (Tl) and high temperature (Th) sides at half maximum TL intensity of the glow curve is taken under consideration. Then, the activation energy of the charge carrier released from trap(s) can be calculated using the parameters: τ ¼Tmax Tl, δ¼Th Tmax, w¼Th Tl and μg ¼δ/w in the following expressions [7] Eτ ¼ ½1:51 þ 3:0ðμg 0:42ÞkT 2max =τ ½1:58 þ4:2ðμg 0:422kT max Eδ ¼ ½0:976 þ 7:3ðμg 0:42ÞkT 2max =δ Ew ¼ ½2:52 þ 10:2ðμg 0:42kT 2max =w 2kT max
Fig. 4. Experimental TL curves of Tl4Ga3InSe8 crystal with heating rate of β ¼1.0 K s 1. Stars and circles are data before and after thermal cleaning, respectively.
the order of kinetics. In the case of first order of kinetics (slow retrapping), the shape of the curve and Tmax value are not influenced by the trapped charge concentration [8,9]. However, in the non-first order of kinetics case, the concentration of the carriers in the trap levels affects Tmax value since retrapped charges will participate repeatedly in the TL process. The experiments carried out showed that the shape of the curve and Tmax value do not change with illumination time. This is a vigorous indication of the first order kinetics. The results of the curve fitting analysis (mentioned in the following part) also support the validity of this case. In the analysis of the experimental data, we have used curve fitting, initial rise and peak shape methods. The details of the curve fitting method which is based on the least square fitting of the glow curve in the light of theoretical equation using a software program were reported in our previous study on the thermally stimulated current measurements [10]. TL intensity for slow retrapping case is given as [11] I TL ¼ n0 v exp
Et kT
Z
T T0
ν expð Et =kTÞdT β
ð1Þ
The averaged values of the energies Eτ, Eδ and Ew of revealed trapping centers were found as 9 and 49 meV, respectively (Table 1). These results are also in agreement with those of the two techniques mentioned above. The heating rate is a significant parameter influencing the TL glow curves associated with traps. For this reason, the heating rate dependence of revealed traps can give illustrative knowledge about the studied materials. The TL curve in Fig. 3 carries the properties of overlapped trap levels (peaks A and B). Since the characteristics of peak B can be obtained after thermal cleaning method, the detailed analysis of the heating rate dependence was performed only for the isolated peak B. Fig. 5 presents TL spectra of the Tl4Ga3InSe8 crystals measured at five heating rates of β ¼0.2 1.0 K s 1 with step 0.2 K s 1 in the temperature range of 40 200 K. In accordance with the TL theory, variation of the Table 1 The activation energies (Et) of revealed traps of the Tl4Ga3InSe8 single crystal. Peak Tm (K) Et (meV) Curve fitting method Initial rise method Peak shape method A B
46 123
7 41
6 45
9 49
40
S. Delice, N.M. Gasanly / Physica B 441 (2014) 37–41
Fig. 5. Experimental TL curves of Tl4Ga3InSe8 crystal (peak B) with different heating rates. Inset: Heating rate dependencies of peak maximum temperature (Tmax), full-width-half-maximum (FWHM) and area of TL curves (peak B). The dash-dotted lines are only guides for the eye.
Fig. 6. Experimental TL curves of Tl4Ga3InSe8 crystal (peak B) at different excitation temperatures. Inset 1: The variation of activation energy versus illumination temperature. The dash-dotted line is only guide for the eye. Inset 2: ln(S0) plot as a function of activation energy. Circles are the experimental data and solid line is the fitted straight line.
Table 2 TL parameters for Tl4Ga3InSe8 crystal (peak B) at different illumination temperatures. Curve Illumination temperature (K) Maximum temperature (K) Curve area (a.u.) Activation energy (meV)
1 40 126.7 3020 45
2 50 128.8 2520 59
3 60 135.0 1690 87
4
5
70 142.8 725 127
80 153.8 317 147
heating rates results with change in the shape and position of the TL peak [7]. As observed from Fig. 5, TL intensities of the curves decrease and their maximum temperature values shift to higher temperatures with increasing the heating rates. Many authors explained the decrease of peak height versus heating rate through the possibility of thermal quenching whose efficiency increases as the temperature increases [12–14]. Inset of Fig. 5 demonstrates the
heating rate dependencies of Tmax, full-width-half-maximum (FWHM) and area values of the TL curves. The FWHM grows from 39 to 55 K as heating rate changes from 0.2 to 1.0 K s 1. The area under the TL curve is proportional to the total number of carriers released from the traps. Since the heating rate dependence was performed for the case of completely filled traps, the area obtained at each measurement should be same. The areas of the curves were found nearly equal to each other (5% variation between the smallest and biggest areas). To reveal the traps distribution responsible for the isolated (thermally cleaned) peak B in Tl4Ga3InSe8 crystals, the sample was irradiated by light for 600 s at various illumination temperatures (T0i) ranging from 40 to 80 K. Then, the light source was switched off and the sample was cooled down to 10 K. Thereafter, the sample was heated at a constant rate β¼1.0 K s 1 to excite the trapped charge carriers to the conduction band. The experimental TL spectra of Tl4Ga3InSe8 crystal at different illumination temperatures (T0i ¼40, 50, 60, 70, and 80 K) are shown in Fig. 6. The intensity of the TL spectra decreased and the maximum values of peak B shifted towards higher temperatures with increasing illumination temperatures. This fact supports the validity of a quasi-continuous traps distribution [15–18]. The area enclosed under the curve obtained with illumination at temperature T0i is proportional to the number of carriers released from the traps during the heating process. The activation energies of the revealed trapping centers were observed to be shifted from 45 to 147 meV by increasing the illumination temperature from 40 to 80 K (inset 1 of Fig. 6 and Table 2). The increase of the activation energy values when T0i increases is consistent with the gradual emptying of shallowest trapping levels during each preheating treatment [7,11,19]. Similar results on traps distribution were also reported previously in Refs. [20–22]. Inset 2 of Fig. 6 presents the plot of ln(S0) (where S0 is the area under the curve) as a function of the activation energies for peak B. This dependence is well approximated by the exponent, which suggests the presence of an exponential distribution of the traps. The exponential distribution is consistent with the presence of fluctuations in the crystal potential, caused by structural defects, which give rise to localized states acting as traps for the carriers [16]. The following expression can be written for the traps filled at the illumination temperature T0i by assuming an exponential traps distribution, whose density at energy Eti will be given by Nti ¼Aexp ( α Eti) [15]. This relation implies that S0 aAexpð αEti Þ Here, α is the energy parameter that characterizes the traps distribution. From the analysis of the graph ln(S0) versus activation energy for peak B, the value of α was found to be 0.0213 meV 1, which corresponds to the variation of one order of magnitude in the traps density for every 108 meV.
4. Conclusion Tl4Ga3InSe8 layered single crystals have been studied in the temperature range of 10–200 K using thermally stimulated luminescence technique. The TL curve demonstrated the presence of two overlapping peaks with maximum temperatures of 46 and 123 K. The observed TL curve have been analyzed using curve fit, initial rise and peak shape methods. As a result of the analysis, two trapping centers with activation energies of 7 and 41 meV have been revealed. Since the studied crystals were not intentionally doped, these centers are thought to originate from stacking faults, which are quite possible in Tl4Ga3InSe8 due to the weakness of the van der Waals forces between the layers. Moreover,
S. Delice, N.M. Gasanly / Physica B 441 (2014) 37–41
the behavior of high-temperature peak for different heating rates (0.2–1.0 K s 1) has been studied. TL intensities of the curves decrease and their maximum temperature values shift to higher temperatures with increasing the heating rates. And finally, analysis of experimental TL curves registered at different light illumination temperatures revealed the exponential distribution of the traps in studied crystals. The activation energies of the distributed trapping centers were found to be increasing from 45 to 147 meV with increasing the illumination temperature from 40 to 80 K. The variation of one order of magnitude in the traps density for every 108 meV was estimated. References [1] K.A. Yee, A. Albright, J. Am. Chem. Soc. 113 (1991) 6474. [2] A.M. Panich, J. Phys.: Condens. Matter 20 (2008) 293202. [3] M. Hanias, A. Anagnostopoulos, K. Kambas, J. Spyridelis, Mater. Res. Bull 27 (1992) 25. [4] K. Goksen, N.M. Gasanly, Physica B 400 (2007) 266. [5] K. Goksen, N.M. Gasanly, R. Turan, Cryst. Res. Technol 41 (2006) 822. [6] N.M. Gasanly, N.A.P. Mogaddam, H. Ozkan, Cryst. Res. Technol 41 (2006) 1100.
41
[7] R. Chen, S.W.S. McKeever, Theory of Thermoluminescence and Related Phenomena, World Scientific, Singapore, 1997. [8] A.J.J. Bos, Radiat. Meas. 41 (2007) S45. [9] V.M. Skorikov, V.I. Chmyrev, V.V. Zuev, E.V. Larina, Inorg. Mater. 38 (2002) 751. [10] N.S. Yuksek, N.M. Gasanly, H. Ozkan, Semicond. Sci. Technol. 18 (2003) 834. [11] R. Chen, Y. Kirsh, Analysis of Thermally Stimulated Processes, Pergamon Press, Oxford, 1981. [12] G. Kitis, C. Furetta, M. Prokic, V. Prokic, J. Phys. D: Appl. Phys. 33 (2000) 1252. [13] G. Kitis, M. Spiropulu, J. Papadopoulos, S. Charalambous, Nucl. Instrum. Methods Phys. Res. 73 (1993) 367. [14] A. Ege, E. Ekdal, T. Karali, N. Can, M. Prokic, Meas. Sci. Technol. 18 (2007) 889. [15] A. Serpi, J. Phys. D: Appl. Phys. 9 (1976) 1881. [16] A. Bosacchi, B. Bosacchi, S. Franchi, L. Hernande, Solid State Commun. 13 (1973) 1805. [17] A. Anedda, M.B. Casu, A. Serpi, I.I. Burlakov, I.M. Tiginyanu, V.V. Ursaki, J. Phys. Chem. Solids 58 (1997) 325. [18] P.C. Ricci PC, A. Anedda, R. Corpino, I.M. Tiginyanu, V.V. Ursaki, J. Phys. Chem. Solids 64 (2003) 1941. [19] R.H. Bube, Photoelectronic Properties of Semiconductors, Cambridge University Press, Cambridge, 1992. [20] V. Correcher, J.M. Gomez-Ros, J. Garcia-Guinea, M. Lis, L. Sanchez-Munoz, Radiat. Meas. 43 (2008) 269. [21] J.M. Gomez-Ros, V. Correcher, J. Garcia-Guinea, A. Delgado, Radiat. Prot. Dosim. 119 (2006) 93. [22] J.M. Gomez-Ros, C. Furetta, E. Cruz-Zaragoza, M. Lis, A. Torres, G. Monsivais, Nucl. Instr. Meth. Res. A 566 (2006) 727.