TL and TSC studies on TlGaSe2 layered single crystals

Journal of Luminescence 144 (2013) 163–168

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TL and TSC studies on TlGaSe2 layered single crystals M. Isik a,n, E. Bulur b, N.M. Gasanly b a b

Department of Electrical and Electronics Engineering, Atilim University, 06836 Ankara, Turkey Department of Physics, 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 7 February 2013 Received in revised form 10 May 2013 Accepted 9 July 2013 Available online 16 July 2013

Defects in – as grown – TlGaSe2 layered single crystals were investigated using Thermoluminescence (TL) and Thermally Stimulated Currents (TSC) techniques in the temperature range 10–300 K. TL and TSC curves of samples illuminated using a light with energy greater than the band gap of the material, i.e. blue light (∼470 nm) at 10 K, exhibited peaks around 27 and 28 K, respectively, when measured by heating up the samples at a rate of 1 K/s. TL and TSC curves were analyzed to characterize the defects responsible for the peaks. Both TL and TSC peaks were observed to be obeying first order kinetics. Thermal activation energies of the peaks were determined using various methods: curve fitting, initial rise, peak shape and different heating rates. For both TL and TSC peaks, thermal activation energy was determined as around 8 meV, implying that they may originate from similar kinds of trapping centers. A distribution of traps (in terms of energy) was experimentally verified by illuminating the sample at different temperatures and measuring the TL curves. As a result of this, the apparent thermal energies were observed to be shifted from ∼8 to ∼17 meV by increasing the illumination temperature from 10 to 16 K. & 2013 Elsevier B.V. All rights reserved.

Keywords: Thermoluminescence Thermally stimulated currents Chalcogenides Defects Impurities

1. Introduction Semiconductor chalcogenides formulated with TlBX2 (where B ¼Ga or In; X¼S or Se) have attracted much attention due to their structural characteristics and potential optoelectronics applications [1–4]. These layered semiconducting compounds belong to the monoclinic system with space group of C2/c at room temperature. The lattice of these crystals consists of alternating twodimensional layers arranged perpendicular to the [0 0 1] direction. Each successive layer makes a 901 angle with the previous layer. Structural, optical and electrical properties of TlGaSe2 single crystals have been investigated by many authors. Raman, Brillouin and infrared spectra [5–7], optical and dielectric properties [8], absorption spectra [9], dark electrical conductivity and Hall measurements [10] are some of these characterization studies that have been carried out on the crystal. Earlier, a photovoltaic effect was revealed in In/TlGaSe2 and InSe/TlGaSe2 barrier structures at room temperature [11]. Optical indirect and direct band gap energies were found to be 1.97 and 2.26 eV, respectively [12]. Previously, we have performed photoluminescence (PL) measurements on undoped TlGaSe2 single crystals [13]. Analysis of the PL spectra showed that a shallow acceptor level and a moderately

n

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0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.07.010

deep level exist at 0.012 eV above the top of the valence band and at 0.317 eV below the bottom of the conduction band, respectively. It is known that, optical and electrical properties of the materials are affected by the presence of defects and impurities. Thermoluminescence (TL) and Thermally Stimulated Current (TSC) are two basic experimental methods which have been used for long times to determine the properties of the energy levels created due to the presence of defects and/or impurities in semiconductors and insulators. The most widely known application area of the thermoluminescence technique is in the field of radiation dosimetry; where interaction of ionizing radiation creates free charges in the solid (generally an insulator or a wide band gap semiconductor) some of which are captured by defects in the structure. Upon warming up the sample (generally a linear heating profile is employed) a weak luminescence emission is observed as a result of recombination of thermally liberated electrons and holes. The plot of luminescence emission against temperature called a glow curve and may contain more than one glow peaks with peak temperatures corresponding to the charge trapping states. Intensity of a glow peak is dependent on the number of trapped charges and can be used for radiation dose measurement with the help of a calibration curve that relates the peak intensity to radiation dose [14]. However, the technique has also been employed in the investigation of defects in semiconductor samples. As the band gap of the semiconductors are smaller, free electrons and holes can be created using visible or near ultraviolet light. The traps that may capture and hold these charges are only stable at low

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temperatures. Thus the experiments should be carried out at low temperatures. Analysis of the thermoluminecence properties of a semiconductor enables one to obtain information about generally shallow defects (and/or trapping states) in the material and the thermoluminescence is very sensitive to small defect concentrations. The literature related to the topic is extensive, a recent study carried out on Cu or Tb implanted ZnO can be given as an example [15]. Another recent report was focused on the low temperature thermoluminescence from shallow traps in Ce doped yttrium aluminum garnet (YAG) which is considered as one of the most promising material for fast scintillator [16]. Another application area is in the biological sciences, which concerns the study of photosynthesis processes in photosynthetic bacteria, cyanobacteria and plants [17]. In the present study, we give the results of the low temperature (10–300 K) TL experiments on the TlGaSe2 layered single crystals. Experimental data were analyzed for characterizing the energy levels in the undoped crystals. In addition to TL measurements, we have also carried out TSC experiments on the same sample to figure out the consistencies and differences between the TL and TSC glow curves and results of their analysis. Applying both techniques on the same crystal and investigating the differences in the low temperature range may provide data for a better understanding of defects in TlGaSe2 crystals.

2. Experimental details TlGaSe2 polycrystal was synthesized from high-purity elements taken in stoichiometric proportions. Gallium (Aldrich cat. no. 263273) and selenium (Aldrich cat. no. 204307) were of 99.999% purity, and thallium (Fluka cat. no. 88202) was of 99.99% purity. The melting point of TlGaSe2 was estimated as 817 1C. Single crystal of TlGaSe2 was grown by the Bridgman method 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 from 830 to 460 1C at a rate of 1.0 mm h  1. The resulting ingot with 10 mm in diameter and about 25 cm in length was air/moister stable. The ingot appears dark red in color and the freshly cleaved surfaces were mirror like. Since freshly cleaved platelet (along the layer plane (0 0 1)) was mirror-like, no further polishing and cleaning treatments were required. The dimensions of the sample used for TL measurements were 8  5  1 mm3. TL and TSC experiments were carried out using a home-made setup. The system was built around a closed cycle helium gas cryostat (Advanced Research Systems, Model CSW202) which operates in the temperature range of 10–300 K. Sample temperature was controlled using a LakeShore Model 331 temperature controller which is connected to a personal computer via the GPIB bus. Temperature was measured using a semiconductor diode detector. The temperature controller is able to control the sample temperature between 10 and 300 K and can ramp the temperature linearly at a maximum rate of 1.2 K/s. Since at low enough temperatures, the probability of thermal release of trapped carriers is negligible, we have irradiated the sample at 10 K. The temperature was controlled within an accuracy of 0.5 K. A light tight measurement chamber which carries the detector (a photomultiplier tube, PMT), light source (Light Emitting Diode, LED) and the optics was connected to the optical access port of the cryostat (quartz window) where the sample lies on the focal plane of the optics for both measurement and illumination. Luminescence emitted from the crystal was focused by lenses on the photomultiplier tube (Hamamatsu R928; spectral response: 185–900 nm) working in photon counting regime. Pulses from the photomultiplier were converted into transistor–transistor logic (TTL) pulses (0 5 V) using a fast amplifier/discriminator (Hamamatsu Photon Counting Unit

C3866) and counted by the counter of a data acquisition module (National Instruments, NI-USB 6211). A high power (3 W) blue light emitting diode (LED) generating the light at a peak of 2.6 eV which is bigger than band gap energy was used to illuminate the sample. The flux at the sample position was about a few mW/cm2. Whole measurement system was controlled by a computer using software written in LabViewTM (National Instruments) graphical development environment. For TL measurement, the sample was irradiated at 10 K for 100 s which is enough to fill the traps completely. Then, after 120 s of waiting, the sample was warmed up linearly at a predetermined heating rate and the emitted luminescence is recorded as a function of temperature. The details of our system are given in Ref. [18], where we have reported the results of lowtemperature thermoluminescence study on TlGaS2 layered crystals. TSC experimental procedure applied on the crystals as follows; at low temperatures, when the probability of thermal release of the trapped charges is negligible, the sample was illuminated with the same type of LED to excite the charge carriers. Then the sample was heated under the voltage applied across the contacts connected to the opposite surfaces of the sample. While heating the sample with a constant heating rate, the transient electric currents in the sample were measured (using a Keithley Model 6485 Picoammeter) as a function of the temperature. The trap filling was performed under bias voltage of V1 ¼1 V at the initial temperature T0 ¼10 K for about 100 s. When the excitation was turned off and an expectation time (120 s) has elapsed, the bias voltage of V2 ¼50 V was applied to the sample.

3. Results and discussion 3.1. Crystal characterization The crystal structure properties of the TlGaSe2 were identified using X-ray diffraction experiments. Measurements were performed using “Rigaku miniflex” diffractometer with CuKα radiation (λ ¼0.154049 nm). The scanning speed of the diffractometer was 0.02 deg/s. Fig. 1 shows X-ray diffractogram of TlGaSe2 crystal. Least-squares computer program “TREOR 90” was used for indexing the registered diffractogram. The calculated Miller indices (h k l) are displayed in Fig. 1. The observed and calculated interplanar spacings of the diffraction lines are found to be consistent. The determined lattice parameters of the monoclinic unit cell a¼ 1.0756, b¼ 1.0730, c ¼1.5596 nm and β¼99.921 are well correlated with the results reported in Ref. [5].

Fig. 1. X-ray powder diffraction pattern of TlGaSe2 crystal.

M. Isik et al. / Journal of Luminescence 144 (2013) 163–168

Fig. 2. Energy dispersive spectroscopic analysis of TlGaSe2 crystal.

is detected above 50 K, we have carried out the experiments in the 10–50 K range. As shown in Fig. 3, a single peak around 25 K arises in the TL glow curve. The theoretical approach for the analysis of the TL curve differs for the first order (slow retrapping) and second order (fast retrapping) kinetics. Therefore, the order of kinetics must be determined before starting to analyze the curve. For this purpose, the excitation time dependence (which corresponds to a change in the trapped charge population) of the peak maximum position (Tm) was investigated (see Fig. 3). In the slow retrapping case, the shape of the curve and Tm value are not affected by the trapped charge concentration of the traps. However, in the fast retrapping case, the concentration of the carriers in the trap levels affects Tm value since retrapped charges will take role in the TL process more than one time. As it can be seen from the Fig. 3, peak position and the shape of the TL peak do not change when the carrier concentration is varied. This indicates that slow retrapping is the dominant case for the TL processes in TlGaSe2. Inset in Fig. 3 represents the variation of the peak height with the excitation time. As observed, trapping centers are fully filled for an excitation time of ∼50 s. Therefore, in the present work, the sample was illuminated for 100 s for TL measurements. 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 employs the least square fitting of the glow curve based on a model equation using a software program were reported in our previous study on the thermally stimulated current measurements [19]. TL intensity for slow retrapping case is given as [20]  I TL ¼ n0 νexp

Fig. 3. TL curves of TlGaSe2 crystals for different illumination times at a constant heating rate β ¼1.0 K/s. Inset: maximum value of TL intensity as a function of illumination time. The dash-dotted line is only guide for the eye.

165



Et  kT

Z

T T0

ν expðEt =kTÞ dT β

 ð1Þ

where Et is the activation energy, ν is the attempt-to-escape frequency, β is the heating rate and n0 is the initial concentration of the charge carriers in the trap level(s). Fig. 4 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 the first order process as the fitted line successfully describes the experimental data. Thermal activation energy of the trapping level responsible for the TL peak was found to be ∼8 meV as an outcome of the curve fitting. Attempt-to-escape frequency of the revealed trap was also obtained from the fitting program (Table 1). Then

The determination of the chemical composition of TlGaSe2 single crystals was accomplished using the energy dispersive spectroscopy experiments. The experiments were performed using JSM-6400 scanning electron microscope having two equipments called as “Noran System6 X-ray microanalysis system” and “Semafore Digitizer” which take part in the analysis of experimental data. Fig. 2 shows the resulting spectrum obtained from the measurements carried out in 0–9 keV energy range. Since every element has distinctly unique energy levels, each element produces characteristic X-rays which make it possible to determine the elemental composition of the sample by analyzing the spectra. The amount of each element present in the material can also be determined from the relative counts of the detected X-rays. The atomic composition ratio of constituent elements ( Tl:Ga:Se) in the crystal was found out as 25.4:25.2:49.4, respectively. 3.2. TL and TSC measurements Fig. 3 shows examples of TL glow curves in the temperature range of 10–50 K measured with a constant heating rate of 1.0 K/s. At the beginning of the experiments, we have performed the measurements in the 10–300 K range. However, since no TL peak

Fig. 4. Experimental TL spectrum of TlGaSe2 crystal with heating rate of 1.0 K/s. Open circles are experimental data. Solid curve shows the fit to the experimental data. Inset: TL intensity vs. 1000/T. The circles present the experimental data and the line represents the theoretical fit using the initial rise method.

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Table 1 Activation energy (Et), capture cross section (St) and attempt-to-escape frequency (ν) of trap obtained from TL and TSC experiments in TlGaSe2 crystals. Method Tm (K) Curve fit method TL TSC

26.7 28.0

St (cm2)

Et (meV)

8 8

ν (s  1)

Initial rise Peak shape method method 7 7

10 9

2.4  10  24 2.8 1.1  10  24 1.5

Fig. 5. Experimental TSC spectrum of TlGaSe2 crystal with heating rate of 1.0 K/s. Open circles are experimental data. Solid curve shows the fit to the experimental data. Inset 1: typical experimental TSC curves of TlGaSe2 crystal under opposite bias voltage. Circles and stars represent the experimental data obtained when the polarity of illuminated sample surface was positive and negative, respectively. Inset 2: TSC vs. 1000/T. The circles present the experimental data and the line represents the theoretical fit using the initial rise method.

capture cross section (St) of the trap can be calculated using the expression St ¼

ν N v υth 2

where N v ¼ 2ð2πmnh kT=h Þ3=2 is the effective density of states in the valence band and υth is thermal velocity of a free hole. The capture cross section of the trap level was calculated as 2.4  10  24 cm2 using the effective mass mnh ¼ 0:52m0 reported for TlGaSe2 [10]. In addition to the TL experiments, we have also performed TSC experiments to expand our research on the luminescence properties of the sample and to get important and valuable information on the consistency of the both experimental techniques used for trap level characterization. Fig. 5 shows the TSC curve obtained in the temperature range of 10–50 K and constant heating rate of 1.0 K/s. Illumination time dependence of the TSC curve was investigated in a similar way mentioned above for TL measurements (not shown). A similar behavior (only current magnitude variation) was also observed for TSC curve: the peak position of the TSC curve was observed to be not affected by the initial trap population. This indicates that slow retrapping is also the dominant case for the TSC processes. Curve fitting method was applied for TSC experimental data. Thermally stimulated current is given for slow retrapping case as [20]     Z T V Et ν I TSC ¼ n0 τνeμ Aexp   expðEt =kTÞ dT ð2Þ L kT T0 β where τ is the carrier lifetime, μ is the mobility, e is the elementary charge, V is the applied voltage, A and L are cross sectional area

and length of the sample. The comparison of Eq. (1) and (2) denoted that TL intensity and TSC behave in a same manner according to variation of temperature. Only the amplitudes which affect the magnitude of ITL and ITSC differ for these equations. Anyway, thermal activation energy does not depend on the magnitude of ITL and ITSC. Solid line in Fig. 5 is the fitted line according to Eq. (2). The analysis revealed the presence of one trapping center located at ∼8 meV. The corresponding St and ν values of this trap level were also evaluated (see Table 1). This result coincides with that of TL measurement. Therefore, this trapping center can be thought as active for both techniques. However, it was observed that Tm values of the TL and TSC curves differ nearly as an amount of 1.3 K. In the TL process, excited charge carriers spend virtually no time in the conduction and/or valence bands and recombine immediately with opposite sign carriers. However, in the TSC process, excited carriers may spend a non-negligible length of time in the bands before recombining. Up to recombination, excited carriers contribute to the conductivity of the sample [20]. Therefore, a shift of Tm value for TSC to higher values is an expected result. In the TSC measurements, there is an opportunity to determine the type of the trapping center(s). TSC curves are obtained for illumination of the front surface of the crystal connected to positive and negative terminals of the supply voltage alternately. When the front surface of the sample is illuminated, both types of carriers are created in this region. Only one type of carriers will be driven along the whole field zone, while the second type is collected very quickly depending on the bias voltage. Only the former can be trapped. As shown from the inset 1 of Fig. 5, TSC curve has the highest current when the polarity of the illuminated surface is positive. It indicates that the holes are distributed in the crystal and then trapped. Therefore, the peak appearing in the TSC spectra can be assigned to a hole trap. In the TL measurements, there is no way to get information about the type of trapping centers. As a second analysis method, we have performed the initial rise method to determine the activation energy. TL and TSC intensities are proportional to exp(–Et/kT) when the trapped carriers start to be excited to the non-localized states [20]. Correspondingly, when the initial portion of the curve was analyzed, ln(ITL,TSC) vs. 1/T graph gives a straight line with a slope of (  Et/k). Inset of Fig. 4 and inset 2 of Fig. 5 represent the mentioned plots (open circles) and their linear fits (solid line). Activation energies of the traps were found as ∼7 meV for both experimental techniques from the slope of the fitted lines (Table 1). These results are in good agreement with those obtained from the curve fitting method. The peak shape method is another technique to find the energies of the trapping center(s) [20]. In this method, the activation energy is evaluated using parameters: τ¼Tm  Tl, δ¼Th  Tm, w¼ Th  Tl and μg ¼ δ/w, where Tl and Th are low and high half-intensity temperatures, respectively. The activation energies for the observed TL and TSC peaks via the peak shape method were found as ∼10 and ∼9 meV, respectively (Table 1). These results are also in agreement with those of the two techniques mentioned above. We have also studied the dependence of the TL curve on heating rate and excitation temperature. Since the main aim of this work is to study the termoluminescence measurements and compare the TL and TSC curves, we have carried out the dependency measurements only for TL curve. The dependence of TL curve on heating rate (β) was studied for rates between 0.4 and 1.0 K/s (see Fig. 6). According to Chen and McKeever [14], peak maximum temperature shifts to higher temperatures with increasing heating rate. Moreover, increase of β also leads to a decrease in the peak height and an increase in the width keeping the area of the peak (i.e. concentration n0) constant [21]. These behaviors are observed to be realized in our TL measurements

M. Isik et al. / Journal of Luminescence 144 (2013) 163–168

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properties belonging to the remaining distributed traps. The new calculated activation energies correspond to the distributed traps located at deeper energy levels. Fig. 7 shows the TL curves obtained at different illuminated temperatures (Texc ¼10, 12, 13, 14 and 16 K). The inset of Fig. 7 shows how the activation energies of the TL peak change with excitation temperatures. These activation energies were determined using the curve fitting method on the glow curves. As seen from this inset, traps show a distribution in the forbidden band gap. The activation energy increases from ∼8 to ∼17 meV with increasing the excitation temperature from 10 to 16 K. The increase of the activation energy values when Texc increases is consistent with the gradual emptying of shallowest trapping levels during each preheating treatment [14,23]. Similar results on traps distribution were also reported previously in the Refs. [24–28].

4. Conclusion Fig. 6. Experimental TL curves of TlGaSe2 crystal with different heating rates. Inset: The plot of ln(β) vs. 1000/Tm. Open circles and solid line represent the experimental data and its linear fit, respectively.

In the present work, two basic techniques, TL and TSC were applied on undoped TlGaSe2 single crystals to characterize the trapping center (s). Experiments accomplished in the temperature range of 10–300 K and at constant heating rate of 1.0 K/s revealed the presence of TL peak arising in the 10–50 K region. The results obtained from both techniques showed a very good agreement in respect to determination of activation energy of the observed trapping center. The curve fitting method applied under the light of slow retrapping process revealed a center at around 8 meV. Trap parameters determined using initial rise and peak shape methods were observed to be consistent with those of curve fitting method. Moreover, behavior of the TL curve for different heating rates (0.4–1.0 K/s) has been studied. The shift of peak maximum to higher temperatures with increasing heating rate was observed. Distribution of the traps has also been revealed by exciting the sample at different temperatures. It was established that the activation energy increases from ∼8 to ∼17 meV with increasing the excitation temperature from 10 to 16 K. References

Fig. 7. The glow curves of TlGaSe2 crystals at different excitation Texc temperatures at heating rate β ¼1.0 K/s. Inset: The variation of activation energies on Texc values.

carried out on TlGaSe2 crystals. In the literature, there are several methods to determine the activation energy from the heating rate dependence of peak maximum temperature (Tm). In the thermally stimulated phenomena, the dependence of heating rate on Tm is given as [14] β ¼ ðνk=Et ÞT 2m expðEt =kT m Þ

ð3Þ

In the right hand side of this equation, exponential term is the dominant Tm dependent factor rather than the Tm2 term. Consequently, the plot of ln(β) vs. 1/Tm results in a line with a slope of Et/ k. Inset of Fig. 6 gives this plot (open circles) and its linear fit (solid line). The activation energy value was obtained from the slope as ∼7 meV. This result is in consistency with those of above given analysis methods. In an attempt to see and understand the distribution of traps, the sample is irradiated at different temperatures (Texc) which correspond to a lower temperature than the peak maximum temperature and higher than T0 [14,22]. By this way, traps close to conduction and/or valence band are emptied, since they have high probability to be excited at Texc temperature. The TL curve obtained for this type measurement includes all the luminescence

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