- Email: [email protected]
Thermally stimulated current measurements in undoped Ga3InSe4 single crystals
Journal of Physics and Chemistry of Solids 72 (2011) 768–772
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Thermally stimulated current measurements in undoped Ga3InSe4 single crystals M. Isik a,n, N.M. Gasanly b a b
Department of Electrical and Electronics Engineering, Atilim University, 06836 Ankara, Turkey Department of Physics, Middle East Technical University, 06531 Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 February 2011 Received in revised form 5 March 2011 Accepted 16 March 2011 Available online 23 March 2011
The trap levels in nominally undoped Ga3InSe4 crystals were investigated in the temperature range of 10–300 K using the thermally stimulated currents technique. The study of trap levels was accomplished by the measurements of current flowing along the c-axis of the crystal. During the experiments we utilized a constant heating rate of 0.8 K/s. Experimental evidence is found for one hole trapping center in the crystal with activation energy of 62 meV. The analysis of the experimental TSC curve gave reasonable results under the model that assumes slow retrapping. The capture cross-section of the trap was determined as 1.0 10 25 cm2 with concentration of 1.4 1017 cm 3. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Semiconductors A. Chalcogenides D. Defects D. Electrical properties
1. Introduction AIIIBVI-type semiconducting compounds, GaSe and InSe, are members of the group of layered crystals. They have become attractive in the fields of both research and technical applications due to their interesting structural, optical and electrical properties. GaSe and InSe layered compounds form a restricted series of mixed crystals GaxIn1 xSe. The results of optical, structural and electrical characterization in the two narrow ranges of composition (0 rxr0.2 and 0.9 rx r1) were published in Ref. [1]. The energy band gap variation versus composition was determined at room temperature from the registration of absorption and photoconductivity spectra. It was established that the gap variation takes place between 1.2 and 2.0 eV in the visible range, which makes it interesting for solar energy conversion. The energy band gap for the composition x¼0.75 was estimated as 1.8 eV. Moreover, Raman scattering [2], photoluminescence [3,4] and transmission spectra for 0.8 rxr 1 [4] and absorption spectra for 0.6 rxr1 [5] ranges were also studied in the mixed crystals GaxIn1 xSe. The results of infrared investigation in GaxIn1 xSe crystals were reported in Ref. [6]. Reflectivity measurements in the far infrared region (50–450 cm 1) have shown that these mixed crystals have a two-mode behavior. A Kramer–Kronig analysis of infrared spectra enabled the authors to determine
the transverse and longitudinal optical mode frequencies, damping constants and low- and high-frequency dielectric constants. One of the factors affecting the performance of the semiconductor devices is the presence of impurity or defect centers in the crystal. Therefore it is very important to get detailed information about the trapping centers. One of the experimental methods for determining the properties of trapping centers in semiconductors is the thermally stimulated currents (TSC) technique. Recently, we have carried out the TSC measurements on GaSe crystal, which is one of the constituents of the GaxIn1 xSe mixed crystals [7]. The experiments carried out in the wide temperature range of 10–300 K showed that there are three trapping centers located at 0.02, 0.10 and 0.26 eV in the energy band gap. The purpose of the present work is to obtain detailed information about the trapping centers in undoped Ga3InSe4 single crystals in the temperature range 10–300 K by using TSC measurements. The activation energy, capture cross-section, attemptto-escape frequency and concentration of observed traps of Ga3InSe4 crystals were determined. Furthermore, the chemical composition of Ga3InSe4 crystals was determined from the energy dispersive spectroscopic analysis. The parameters of orthorhombic unit cell were revealed by the X-ray powder diffraction technique.
2. Experimental details n
Corresponding author. Tel.: þ90 312 586 87 15; fax: þ 90 312 586 80 91. E-mail address: [email protected] (M. Isik).
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.03.008
Ga3InSe4 polycrystals were synthesized from high-purity elements (at least 99.999%) prepared in stoichiometric proportions.
M. Isik, N.M. Gasanly / Journal of Physics and Chemistry of Solids 72 (2011) 768–772
The melting point of the Ga3InSe4 crystals was estimated as 860 1C. Single crystals of Ga3InSe4 were grown by the Bridgman method. The resulting ingot appears dark red in color. The samples were prepared by easy cleavage of an ingot parallel to the crystal layer (perpendicular to the c-axis). The freshly cleaved surfaces are mirror-like. The crystals obtained were not subjected to additional annealing. The dimensions of the crystals used for TSC measurements were 7 6.5 1 mm3. The connection of the crystal to the circuit was achieved by applying a sandwich geometry configuration to the crystal using silver paste. Hot probe method application on the Ga3InSe4 crystal showed that the electrical conductivity of the sample was p-type. Thermally stimulated current measurements procedure was applied on the Ga3InSe4 crystals as follows: at low temperatures, when the probability of thermal release is negligible, a lightemitting diode (LED) generating the light at a maximum peak of 2.6 eV was used to excite the charge carriers. Then the sample was heated under the voltage applied across the contacts connected to the sample. While heating the sample with a constant heating rate, the transient electric currents in the sample were measured as a function of the temperature. Constant heating rate of b ¼0.8 K/s achieved by the Lake-shore 331 temperature controller has been obtained by applying dc current to a 37.5 O NiCr heater filament wound around the sample holder. The TSC measurements were performed in the temperature range of 10–300 K using an Advanced Research Systems closed-cycle helium cryostat. A Keithley 228A voltage/current source and a Keithley 6485 picoammeter were used for the TSC measurements. The nominal instrumental sensitivities of temperature and current measurement devices were about 10 mK and 2 pA, respectively. The trap filling is performed under bias voltage of V1 ¼1 V at the initial temperature T0 ¼10 K for about 600 s. When the excitation was turned off and an expectation time (300 s) has elapsed, the bias voltage of V2 ¼ 50 V was applied to the sample. The carrier lifetime was determined by the help of photoconductivity decay experiments, which include decay of photocurrent from steady state after a sharp termination of illumination. For lifetime measurements, a NI-6211 USB interface was used both to generate a pulsed light, which was switched off after around 1 ms, and to measure the variation of the resulting current with respect to time. The illumination of the sample was achieved using a high-efficiency LED, which had the ability to respond to a very fast switching off signal from the NI-6211 USB interface. The signal was transmitted to the computer and analyzed to find the decay time of the photocurrent.
3. Results and discussion 3.1. Crystal characterization The characterization of Ga3InSe4 crystal was accomplished by the energy dispersive spectroscopic analysis (EDSA) and X-ray diffraction experiments. Fig. 1 shows the EDSA results used to determine the chemical composition of Ga3InSe4 crystal. EDSA measurements carried out with a JSM-6400 scanning electron microscope showed that the atomic composition ratio of the studied sample Ga:In:Se was found to be 39.8:11.3:48.9. Moreover, EDSA indicates that carbon and/or oxygen impurities are present in Ga3InSe4 crystal. X-ray diffraction technique used to obtain the structural parameters of the sample was accomplished using a ‘‘Rigaku miniflex’’ diffractometer with CuKa radiation (l ¼0.154049 nm) working at a scanning speed of 0.021/s. The crystal system, the Miller indices of the diffraction peaks and the lattice parameters were evaluated using a least-squares computer program ‘‘TREOR
769
Fig. 1. Energy dispersive spectroscopic analysis of the Ga3InSe4 crystal.
Fig. 2. X-ray powder diffraction pattern of Ga3InSe4 crystal.
90’’. Fig. 2 shows the X-ray diffractogram of Ga3InSe4 crystal. Miller indices (h k l), the observed and calculated interplanar spacings (d) and the relative intensities (I/I0) of the diffraction lines are given in Table 1. The lattice parameters of the orthorhombic unit cell were found to be a ¼0.62365 nm, b¼0.52206 nm and c ¼1.59194 nm. The result of the X-ray diffraction experiment indicates that the addition of In atoms to GaSe compound (a ¼0.3755 nm, c ¼1.5946 nm [8]) changes the structure of mixed crystal from hexagonal to orthorhombic with abovementioned lattice parameters. Probably, this fact may be associated with the difference between the atomic radii of indium and gallium atoms (about 14%) leading to the local distortions of the crystal structure [5]. 3.2. Determination of the type of the charge carriers and minimum excitation time TSC measurements carried out on the Ga3InSe4 crystals in the temperature range of 10–300 K showed that there is one peak in the TSC curve starting to exist nearly at 50 K and ending nearly at 245 K. Therefore the figures related to TSC measurements in this
770
M. Isik, N.M. Gasanly / Journal of Physics and Chemistry of Solids 72 (2011) 768–772
Table 1 X-ray powder diffraction data for Ga3InSe4 crystals. Number
hkl
dobs (nm)
dcalc (nm)
I/I0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
200 021 040 520 241 202 721 641 801 830 920 642 562 623 852
0.79647 0.40011 0.31186 0.28344 0.25371 0.24794 0.19775 0.18828 0.18593 0.17942 0.17026 0.15977 0.14486 0.14172 0.13363
0.79597 0.40032 0.31182 0.28357 0.25374 0.24804 0.19774 0.18845 0.18594 0.17949 0.17017 0.15979 0.14482 0.14171 0.13363
7.1 100 27.0 43.7 17.7 7.4 16.5 22.8 19.8 28.9 9.4 9.8 8.9 7.8 16.1
after nearly 600 s. Therefore, the illumination time for TSC experiments was taken as 600 s. Inset 2 of Fig. 3 shows the TSC curves of Ga3InSe4 crystal measured for forward and reverse bias conditions for the observed trapping center in the 50–245 K temperature range. The contact on the front surface of the crystal was connected to the positive or negative terminals of the supply voltage and TSC measurements were taken for both cases. 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. It was revealed, that if the polarity of the illuminated surface is positive, the intensity of the TSC curve was the highest. It means that the holes are distributed in the crystal and then trapped. Therefore, the peak appearing in the TSC spectra of Ga3InSe4 crystal can be assigned to hole traps. 3.3. Determination of activation energy Among the several analysis methods, trapping parameters are evaluated using three of them: curve fitting, initial rise and peak shape methods. The theoretical form of the TSC curve of a discrete set of traps with trapping level Et is described under the monomolecular conditions (i.e., slow retrapping) by the equation [10] Z T Et n Et dT , ð1Þ IðTÞ ¼ C exp exp kT kT T0 b
Fig. 3. TSC spectra of Ga3InSe4 crystal for various illumination times. Inset 1: the dependence of peak maximum current on illumination time. The dash-dotted lines are only guides for the eye. Inset 2: typical experimental TSC curves of Ga3InSe4 crystal under opposite bias voltage. Circles and stars represent the experimental data obtained at illumination of positive and negative contacts, respectively.
study were plotted in the 50–245 K temperature range. For the analysis of the TSC spectra, it is important to reveal the type of trapping kinetics [9]. The comparison of the relative magnitudes of capture cross-sections St and Sr of the trapping and recombination centers determines the type of the trapping kinetics. For monomolecular process St)Sr, the slow retrapping occurs. In the case of slow retrapping, holes thermally excited from traps are assumed to have much greater probability of recombining with electrons than of being retrapped. In this case, the concentration of filled traps has no effect either on the shape of the TSC curve or on the position of the peak. For the case St*Sr, fast retrapping occurs. For fast retrapping process, holes are retrapped large number of times before recombination occurs. Thus, the peak position and shape of the TSC curve depend on the concentration of filled traps. To change the initial density of traps, the TSC spectra of Ga3InSe4 crystal were recorded for different illumination times (0–600 s) at constant heating rate of b ¼0.8 K/s (Fig. 3). The shape of the TSC spectra and Tmax values remain almost invariable for different values of illumination time. This result indicates that the observed trap may be considered under the monomolecular (slow retrapping) conditions. Inset 1 of Fig. 3 shows the dependence of peak maximum currents on illumination period for observed peak. It is possible to reveal that trap is filled completely
where C is a constant, which depends on the experimental conditions and properties of the crystal, n is the attempt-toescape frequency, k is the Boltzmann constant, b is the heating rate and T0 is the temperature where heating begins after filling of the traps. When the curve fit analysis method [11] was used to fit the experimental data under the theoretical study of slow retrapping process, data was fitted successfully with one peak (solid line in Fig. 4) having activation energy of Et ¼62 meV (Table 2). Initial rise method, independent of the recombination kinetics, is based on the assumption that the TSC is proportional to exp( Et/kT) when the traps begin to empty with temperature [10]. Therefore,
Fig. 4. Experimental TSC 0.8 K/s. Open circles are experimental data. Inset: present the experimental initial rise method.
spectrum of Ga3InSe4 crystal with a heating rate of experimental data. Solid curve shows the fit to the thermally stimulated current vs. 1000/T. The circles data and the line represents the theoretical fit using
M. Isik, N.M. Gasanly / Journal of Physics and Chemistry of Solids 72 (2011) 768–772
771
Table 2 The activation energy (Et), capture cross-section (St), attempt-to-escape frequency (v) and concentration of observed trap of Ga3InSe4 crystal. Tm (K)
156.8
Et (meV) Curve fitting Method
Initial rise Method
Peak shape Method
62
61
65
St (cm2)
n (s 1)
Nt (cm 3)
1.0 10 25
2.30
1.4 1017
when the initial portion of the TSC curve is analyzed, the plot of ln(I) as a function of 1/T gives a straight line with a slope of ( Et/k). The activation energy of the trap calculated from this slope (inset of Fig. 4) was found as 61 meV (Table 2). Peak shape method [10] depends on the analysis of the low and high half-intensity temperatures of the TSC curve. The activation energy of the trap calculated using this method was found as 65 meV (Table 2). The value of characteristic parameter mg were predicted by Chen and Kirsh [10] as 0.42 for slow retrapping process (first order kinetics) and 0.52 for fast retrapping process (second order kinetics). The mg value for the trap in Ga3InSe4 crystal was obtained as 0.45, which is closer to the value suggested for the slow retrapping process. This is also another indication of the monomolecular conditions for the trap level of the Ga3InSe4 crystal. 3.4. Differential analysis of TSC curve The derivative of the natural logarithm of the thermally stimulated current (Eq. (1)), under the assumption of that v is independent of T, is obtained as, dðln IÞ Et nt ¼ expðEt =kTÞ: dT kT 2 b
Fig. 5. Derivative of the thermally stimulated current. Open circles are experimental data and the line is the tangent at Tm ¼156.7 K.
ð2Þ
Since the current is maximum at T¼Tm, Eq. (2) yields Et nt ¼ expðEt =kTm Þ: 2 b kTm
ð3Þ
Using Eq. (3), the second derivative of the ln (I) can be written at the peak maximum temperature as dðln IÞ Et Et ¼ 2 þ ð4Þ ¼ am : 3 kTm dT 2 m kTm Activation energy of the observed trap can be calculated from the slope of the tangent (am) at T¼Tm of the first derivative of the TSC curve. When the graph of the first derivative of the current is plotted (Fig. 5), it was revealed that the derivative is equal to zero at Tm ¼156.7 K with a tangential slope of am ¼ 1.26 10 3. The activation energy of the trap corresponding to this am value is found from Eq. (4) as Et ¼ 63 meV. 3.5. Determination of capture cross-section and concentration of the traps The capture cross-sections St and attempt-to-escape frequencies n of the observed traps were calculated using the values of Et and Tm determined from the curve fit analysis. Table 1 shows the results of the calculations in which effective mass was taken as mh ¼ 0:20 m0 [12]. The concentration of the traps was estimated using the relation [13] Nt ¼
Q , ALeG
ð5Þ
where Q is the quantity of charge released during the TSC measurement that can be calculated from the area of the TSC
Fig. 6. Photoconductivity decay curve for the Ga3InSe4 crystal. Open circles are experimental data. Solid line shows the fit to the experimental data.
peak and G is the photoconductivity gain calculated from [14] G¼
t ttr
¼
tmV3 L2
:
ð6Þ
where t is the carrier lifetime and ttr is the carrier transit time between the electrodes. We used the photoconductivity decay experiments to obtain the carrier lifetime and then to calculate the photoconductivity gain G. The photocurrent decay is nearly exponential after termination of light pulse at t ¼to. The carrier
772
M. Isik, N.M. Gasanly / Journal of Physics and Chemistry of Solids 72 (2011) 768–772
lifetime t was determined from the corresponding output voltage expressed as [15]: t ð7Þ V ¼ V0 þ D exp ,
calculated to be 1.0 10 25 cm2. Also the concentration of the trap is estimated to be 1.4 1017 cm 3.
where V0 is the voltage at t ¼ 1 and D is a constant. Fig. 6 shows the theoretical fit to the experimental data using Eq. (7) for Ga3InSe4 crystals. The carrier lifetime was obtained as 0.11 ms from the decay of the photocurrent. The corresponding photoconductivity gain was found to be 6.9 from Eq. (6), using V3 ¼3 V and m ¼210 cm2/V s [12]. Then the value of Nt obtained for trap is evaluated as Nt ¼1.4 1017 cm 3 (Table 2).
References
t
4. Conclusions TSC measurements on the Ga3InSe4 single crystals revealed that there exists one hole trapping center at 62 meV energy level. The results of the various activation energy determination methods agreed with each other. Since the analysis of the experimental TSC curve gives reasonable results under the model that assumes slow retrapping, the retrapping process is negligible for the observed trap level. As the crystals studied are not intentionally doped, the observed levels are thought to exist due to the presence of defects created during the growth of crystals and/or unintentional impurities. The capture cross-section of the trap is
[1] L. Gouskov, A. Gouskov, M. Hajjar, L. Soonckindt, C. Llinares, Physica B 99 (1980) 291. [2] E.A. Vinogradov, N.M. Gasanly, A.F. Goncharov, B.M. Dzhavadov, N.N. Melnik, Sov. Phys. Solid State 21 (1979) 906. [3] V.I. Tagirov, N.M. Gasanly, B.M. Dzhavadov, M.A. Sobeikh, V.M. Salmanov, Phys. Status Solidi (a) 47 (1978) K157. [4] G.L. Belenkii, M.O. Godzhaev, N.T. Mamedov, E.Yu. Salaev, R.A. Suleimanov, Phys. Status Solidi (a) 53 (1979) 137. [5] A.I. Malik, Z.D. Kovalyuk, N.T. Savitskii, V.B. Baranyuk, Sov. Phys. Semicond. 12 (1978) 1433. [6] G. Saintonge, J.L. Brebner, Can. J. Phys. 62 (1984) 730. [7] N.M. Gasanly, A. Aydinli, O. Salihoglu, Cryst. Res. Technol. 36 (2001) 295. [8] A. Kuhn, A. Chevy, R. Chevalier, Phys. Status Solidi (a) 31 (1975) 469. [9] S. Ozdemir, N.M. Gasanly, M. Bucurgat, Phys. Status Solidi (a) 196 (2003) 422. [10] R. Chen, Y. Kirsh, Analysis of Thermally Stimulated Processes, Pergamon Press, Oxford, 1981. [11] N.S. Yuksek, N.M. Gasanly, H. Ozkan, Semicond. Sci. Technol. 18 (2003) 834. [12] G. Ottaviani, C. Canali, F. Nava, Ph. Schmid, E. Mooser, R. Minder, I. Zschokke, Solid State Commun. 14 (1974) 933. [13] C. Manfredotti, R. Murri, A. Quirini, L. Vasanelli, Phys. Status Solidi (a) 38 (1976) 685. [14] R. Bube, Photoelectronic Properties of Semiconductors, Cambridge University Press, Cambridge, 1992. [15] T. Pisarkiewicz, Opto-Electron. Rev. 12 (2004) 33.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Thermally stimulated current measurements in undoped Ga3InSe4 single crystals M. Isik a,n, N.M. Gasanly b a b
Department of Electrical and Electronics Engineering, Atilim University, 06836 Ankara, Turkey Department of Physics, Middle East Technical University, 06531 Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 February 2011 Received in revised form 5 March 2011 Accepted 16 March 2011 Available online 23 March 2011
The trap levels in nominally undoped Ga3InSe4 crystals were investigated in the temperature range of 10–300 K using the thermally stimulated currents technique. The study of trap levels was accomplished by the measurements of current flowing along the c-axis of the crystal. During the experiments we utilized a constant heating rate of 0.8 K/s. Experimental evidence is found for one hole trapping center in the crystal with activation energy of 62 meV. The analysis of the experimental TSC curve gave reasonable results under the model that assumes slow retrapping. The capture cross-section of the trap was determined as 1.0 10 25 cm2 with concentration of 1.4 1017 cm 3. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Semiconductors A. Chalcogenides D. Defects D. Electrical properties
1. Introduction AIIIBVI-type semiconducting compounds, GaSe and InSe, are members of the group of layered crystals. They have become attractive in the fields of both research and technical applications due to their interesting structural, optical and electrical properties. GaSe and InSe layered compounds form a restricted series of mixed crystals GaxIn1 xSe. The results of optical, structural and electrical characterization in the two narrow ranges of composition (0 rxr0.2 and 0.9 rx r1) were published in Ref. [1]. The energy band gap variation versus composition was determined at room temperature from the registration of absorption and photoconductivity spectra. It was established that the gap variation takes place between 1.2 and 2.0 eV in the visible range, which makes it interesting for solar energy conversion. The energy band gap for the composition x¼0.75 was estimated as 1.8 eV. Moreover, Raman scattering [2], photoluminescence [3,4] and transmission spectra for 0.8 rxr 1 [4] and absorption spectra for 0.6 rxr1 [5] ranges were also studied in the mixed crystals GaxIn1 xSe. The results of infrared investigation in GaxIn1 xSe crystals were reported in Ref. [6]. Reflectivity measurements in the far infrared region (50–450 cm 1) have shown that these mixed crystals have a two-mode behavior. A Kramer–Kronig analysis of infrared spectra enabled the authors to determine
the transverse and longitudinal optical mode frequencies, damping constants and low- and high-frequency dielectric constants. One of the factors affecting the performance of the semiconductor devices is the presence of impurity or defect centers in the crystal. Therefore it is very important to get detailed information about the trapping centers. One of the experimental methods for determining the properties of trapping centers in semiconductors is the thermally stimulated currents (TSC) technique. Recently, we have carried out the TSC measurements on GaSe crystal, which is one of the constituents of the GaxIn1 xSe mixed crystals [7]. The experiments carried out in the wide temperature range of 10–300 K showed that there are three trapping centers located at 0.02, 0.10 and 0.26 eV in the energy band gap. The purpose of the present work is to obtain detailed information about the trapping centers in undoped Ga3InSe4 single crystals in the temperature range 10–300 K by using TSC measurements. The activation energy, capture cross-section, attemptto-escape frequency and concentration of observed traps of Ga3InSe4 crystals were determined. Furthermore, the chemical composition of Ga3InSe4 crystals was determined from the energy dispersive spectroscopic analysis. The parameters of orthorhombic unit cell were revealed by the X-ray powder diffraction technique.
2. Experimental details n
Corresponding author. Tel.: þ90 312 586 87 15; fax: þ 90 312 586 80 91. E-mail address: [email protected] (M. Isik).
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.03.008
Ga3InSe4 polycrystals were synthesized from high-purity elements (at least 99.999%) prepared in stoichiometric proportions.
M. Isik, N.M. Gasanly / Journal of Physics and Chemistry of Solids 72 (2011) 768–772
The melting point of the Ga3InSe4 crystals was estimated as 860 1C. Single crystals of Ga3InSe4 were grown by the Bridgman method. The resulting ingot appears dark red in color. The samples were prepared by easy cleavage of an ingot parallel to the crystal layer (perpendicular to the c-axis). The freshly cleaved surfaces are mirror-like. The crystals obtained were not subjected to additional annealing. The dimensions of the crystals used for TSC measurements were 7 6.5 1 mm3. The connection of the crystal to the circuit was achieved by applying a sandwich geometry configuration to the crystal using silver paste. Hot probe method application on the Ga3InSe4 crystal showed that the electrical conductivity of the sample was p-type. Thermally stimulated current measurements procedure was applied on the Ga3InSe4 crystals as follows: at low temperatures, when the probability of thermal release is negligible, a lightemitting diode (LED) generating the light at a maximum peak of 2.6 eV was used to excite the charge carriers. Then the sample was heated under the voltage applied across the contacts connected to the sample. While heating the sample with a constant heating rate, the transient electric currents in the sample were measured as a function of the temperature. Constant heating rate of b ¼0.8 K/s achieved by the Lake-shore 331 temperature controller has been obtained by applying dc current to a 37.5 O NiCr heater filament wound around the sample holder. The TSC measurements were performed in the temperature range of 10–300 K using an Advanced Research Systems closed-cycle helium cryostat. A Keithley 228A voltage/current source and a Keithley 6485 picoammeter were used for the TSC measurements. The nominal instrumental sensitivities of temperature and current measurement devices were about 10 mK and 2 pA, respectively. The trap filling is performed under bias voltage of V1 ¼1 V at the initial temperature T0 ¼10 K for about 600 s. When the excitation was turned off and an expectation time (300 s) has elapsed, the bias voltage of V2 ¼ 50 V was applied to the sample. The carrier lifetime was determined by the help of photoconductivity decay experiments, which include decay of photocurrent from steady state after a sharp termination of illumination. For lifetime measurements, a NI-6211 USB interface was used both to generate a pulsed light, which was switched off after around 1 ms, and to measure the variation of the resulting current with respect to time. The illumination of the sample was achieved using a high-efficiency LED, which had the ability to respond to a very fast switching off signal from the NI-6211 USB interface. The signal was transmitted to the computer and analyzed to find the decay time of the photocurrent.
3. Results and discussion 3.1. Crystal characterization The characterization of Ga3InSe4 crystal was accomplished by the energy dispersive spectroscopic analysis (EDSA) and X-ray diffraction experiments. Fig. 1 shows the EDSA results used to determine the chemical composition of Ga3InSe4 crystal. EDSA measurements carried out with a JSM-6400 scanning electron microscope showed that the atomic composition ratio of the studied sample Ga:In:Se was found to be 39.8:11.3:48.9. Moreover, EDSA indicates that carbon and/or oxygen impurities are present in Ga3InSe4 crystal. X-ray diffraction technique used to obtain the structural parameters of the sample was accomplished using a ‘‘Rigaku miniflex’’ diffractometer with CuKa radiation (l ¼0.154049 nm) working at a scanning speed of 0.021/s. The crystal system, the Miller indices of the diffraction peaks and the lattice parameters were evaluated using a least-squares computer program ‘‘TREOR
769
Fig. 1. Energy dispersive spectroscopic analysis of the Ga3InSe4 crystal.
Fig. 2. X-ray powder diffraction pattern of Ga3InSe4 crystal.
90’’. Fig. 2 shows the X-ray diffractogram of Ga3InSe4 crystal. Miller indices (h k l), the observed and calculated interplanar spacings (d) and the relative intensities (I/I0) of the diffraction lines are given in Table 1. The lattice parameters of the orthorhombic unit cell were found to be a ¼0.62365 nm, b¼0.52206 nm and c ¼1.59194 nm. The result of the X-ray diffraction experiment indicates that the addition of In atoms to GaSe compound (a ¼0.3755 nm, c ¼1.5946 nm [8]) changes the structure of mixed crystal from hexagonal to orthorhombic with abovementioned lattice parameters. Probably, this fact may be associated with the difference between the atomic radii of indium and gallium atoms (about 14%) leading to the local distortions of the crystal structure [5]. 3.2. Determination of the type of the charge carriers and minimum excitation time TSC measurements carried out on the Ga3InSe4 crystals in the temperature range of 10–300 K showed that there is one peak in the TSC curve starting to exist nearly at 50 K and ending nearly at 245 K. Therefore the figures related to TSC measurements in this
770
M. Isik, N.M. Gasanly / Journal of Physics and Chemistry of Solids 72 (2011) 768–772
Table 1 X-ray powder diffraction data for Ga3InSe4 crystals. Number
hkl
dobs (nm)
dcalc (nm)
I/I0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
200 021 040 520 241 202 721 641 801 830 920 642 562 623 852
0.79647 0.40011 0.31186 0.28344 0.25371 0.24794 0.19775 0.18828 0.18593 0.17942 0.17026 0.15977 0.14486 0.14172 0.13363
0.79597 0.40032 0.31182 0.28357 0.25374 0.24804 0.19774 0.18845 0.18594 0.17949 0.17017 0.15979 0.14482 0.14171 0.13363
7.1 100 27.0 43.7 17.7 7.4 16.5 22.8 19.8 28.9 9.4 9.8 8.9 7.8 16.1
after nearly 600 s. Therefore, the illumination time for TSC experiments was taken as 600 s. Inset 2 of Fig. 3 shows the TSC curves of Ga3InSe4 crystal measured for forward and reverse bias conditions for the observed trapping center in the 50–245 K temperature range. The contact on the front surface of the crystal was connected to the positive or negative terminals of the supply voltage and TSC measurements were taken for both cases. 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. It was revealed, that if the polarity of the illuminated surface is positive, the intensity of the TSC curve was the highest. It means that the holes are distributed in the crystal and then trapped. Therefore, the peak appearing in the TSC spectra of Ga3InSe4 crystal can be assigned to hole traps. 3.3. Determination of activation energy Among the several analysis methods, trapping parameters are evaluated using three of them: curve fitting, initial rise and peak shape methods. The theoretical form of the TSC curve of a discrete set of traps with trapping level Et is described under the monomolecular conditions (i.e., slow retrapping) by the equation [10] Z T Et n Et dT , ð1Þ IðTÞ ¼ C exp exp kT kT T0 b
Fig. 3. TSC spectra of Ga3InSe4 crystal for various illumination times. Inset 1: the dependence of peak maximum current on illumination time. The dash-dotted lines are only guides for the eye. Inset 2: typical experimental TSC curves of Ga3InSe4 crystal under opposite bias voltage. Circles and stars represent the experimental data obtained at illumination of positive and negative contacts, respectively.
study were plotted in the 50–245 K temperature range. For the analysis of the TSC spectra, it is important to reveal the type of trapping kinetics [9]. The comparison of the relative magnitudes of capture cross-sections St and Sr of the trapping and recombination centers determines the type of the trapping kinetics. For monomolecular process St)Sr, the slow retrapping occurs. In the case of slow retrapping, holes thermally excited from traps are assumed to have much greater probability of recombining with electrons than of being retrapped. In this case, the concentration of filled traps has no effect either on the shape of the TSC curve or on the position of the peak. For the case St*Sr, fast retrapping occurs. For fast retrapping process, holes are retrapped large number of times before recombination occurs. Thus, the peak position and shape of the TSC curve depend on the concentration of filled traps. To change the initial density of traps, the TSC spectra of Ga3InSe4 crystal were recorded for different illumination times (0–600 s) at constant heating rate of b ¼0.8 K/s (Fig. 3). The shape of the TSC spectra and Tmax values remain almost invariable for different values of illumination time. This result indicates that the observed trap may be considered under the monomolecular (slow retrapping) conditions. Inset 1 of Fig. 3 shows the dependence of peak maximum currents on illumination period for observed peak. It is possible to reveal that trap is filled completely
where C is a constant, which depends on the experimental conditions and properties of the crystal, n is the attempt-toescape frequency, k is the Boltzmann constant, b is the heating rate and T0 is the temperature where heating begins after filling of the traps. When the curve fit analysis method [11] was used to fit the experimental data under the theoretical study of slow retrapping process, data was fitted successfully with one peak (solid line in Fig. 4) having activation energy of Et ¼62 meV (Table 2). Initial rise method, independent of the recombination kinetics, is based on the assumption that the TSC is proportional to exp( Et/kT) when the traps begin to empty with temperature [10]. Therefore,
Fig. 4. Experimental TSC 0.8 K/s. Open circles are experimental data. Inset: present the experimental initial rise method.
spectrum of Ga3InSe4 crystal with a heating rate of experimental data. Solid curve shows the fit to the thermally stimulated current vs. 1000/T. The circles data and the line represents the theoretical fit using
M. Isik, N.M. Gasanly / Journal of Physics and Chemistry of Solids 72 (2011) 768–772
771
Table 2 The activation energy (Et), capture cross-section (St), attempt-to-escape frequency (v) and concentration of observed trap of Ga3InSe4 crystal. Tm (K)
156.8
Et (meV) Curve fitting Method
Initial rise Method
Peak shape Method
62
61
65
St (cm2)
n (s 1)
Nt (cm 3)
1.0 10 25
2.30
1.4 1017
when the initial portion of the TSC curve is analyzed, the plot of ln(I) as a function of 1/T gives a straight line with a slope of ( Et/k). The activation energy of the trap calculated from this slope (inset of Fig. 4) was found as 61 meV (Table 2). Peak shape method [10] depends on the analysis of the low and high half-intensity temperatures of the TSC curve. The activation energy of the trap calculated using this method was found as 65 meV (Table 2). The value of characteristic parameter mg were predicted by Chen and Kirsh [10] as 0.42 for slow retrapping process (first order kinetics) and 0.52 for fast retrapping process (second order kinetics). The mg value for the trap in Ga3InSe4 crystal was obtained as 0.45, which is closer to the value suggested for the slow retrapping process. This is also another indication of the monomolecular conditions for the trap level of the Ga3InSe4 crystal. 3.4. Differential analysis of TSC curve The derivative of the natural logarithm of the thermally stimulated current (Eq. (1)), under the assumption of that v is independent of T, is obtained as, dðln IÞ Et nt ¼ expðEt =kTÞ: dT kT 2 b
Fig. 5. Derivative of the thermally stimulated current. Open circles are experimental data and the line is the tangent at Tm ¼156.7 K.
ð2Þ
Since the current is maximum at T¼Tm, Eq. (2) yields Et nt ¼ expðEt =kTm Þ: 2 b kTm
ð3Þ
Using Eq. (3), the second derivative of the ln (I) can be written at the peak maximum temperature as dðln IÞ Et Et ¼ 2 þ ð4Þ ¼ am : 3 kTm dT 2 m kTm Activation energy of the observed trap can be calculated from the slope of the tangent (am) at T¼Tm of the first derivative of the TSC curve. When the graph of the first derivative of the current is plotted (Fig. 5), it was revealed that the derivative is equal to zero at Tm ¼156.7 K with a tangential slope of am ¼ 1.26 10 3. The activation energy of the trap corresponding to this am value is found from Eq. (4) as Et ¼ 63 meV. 3.5. Determination of capture cross-section and concentration of the traps The capture cross-sections St and attempt-to-escape frequencies n of the observed traps were calculated using the values of Et and Tm determined from the curve fit analysis. Table 1 shows the results of the calculations in which effective mass was taken as mh ¼ 0:20 m0 [12]. The concentration of the traps was estimated using the relation [13] Nt ¼
Q , ALeG
ð5Þ
where Q is the quantity of charge released during the TSC measurement that can be calculated from the area of the TSC
Fig. 6. Photoconductivity decay curve for the Ga3InSe4 crystal. Open circles are experimental data. Solid line shows the fit to the experimental data.
peak and G is the photoconductivity gain calculated from [14] G¼
t ttr
¼
tmV3 L2
:
ð6Þ
where t is the carrier lifetime and ttr is the carrier transit time between the electrodes. We used the photoconductivity decay experiments to obtain the carrier lifetime and then to calculate the photoconductivity gain G. The photocurrent decay is nearly exponential after termination of light pulse at t ¼to. The carrier
772
M. Isik, N.M. Gasanly / Journal of Physics and Chemistry of Solids 72 (2011) 768–772
lifetime t was determined from the corresponding output voltage expressed as [15]: t ð7Þ V ¼ V0 þ D exp ,
calculated to be 1.0 10 25 cm2. Also the concentration of the trap is estimated to be 1.4 1017 cm 3.
where V0 is the voltage at t ¼ 1 and D is a constant. Fig. 6 shows the theoretical fit to the experimental data using Eq. (7) for Ga3InSe4 crystals. The carrier lifetime was obtained as 0.11 ms from the decay of the photocurrent. The corresponding photoconductivity gain was found to be 6.9 from Eq. (6), using V3 ¼3 V and m ¼210 cm2/V s [12]. Then the value of Nt obtained for trap is evaluated as Nt ¼1.4 1017 cm 3 (Table 2).
References
t
4. Conclusions TSC measurements on the Ga3InSe4 single crystals revealed that there exists one hole trapping center at 62 meV energy level. The results of the various activation energy determination methods agreed with each other. Since the analysis of the experimental TSC curve gives reasonable results under the model that assumes slow retrapping, the retrapping process is negligible for the observed trap level. As the crystals studied are not intentionally doped, the observed levels are thought to exist due to the presence of defects created during the growth of crystals and/or unintentional impurities. The capture cross-section of the trap is
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