A.c properties of tin selenide thin films prepared by an encapsulated selenization method are investigated. The measurements obtained from al/snse/al sandwich structures showed strong indication of frequency and temperature dependence of capacitance, dielectric loss and conductance over the ranges of 5-200 khz and 228-373 k, respectively. Dielectric behaviour was expected to be due to space charge polarization which contributed to a.c conduction. This was generally explained in terms of hopping of the charge carriers between localized states with activation energies 0.03-0.08 ev. Parameters such as trap binding energy (0.94 ev) and minimum hopping distance (1.01 nm) were also predicte

Deraman, Karim

Othaman, Zulkafli

Sakrani, Samsudi

Wahab, Yussof

English

Universiti Teknologi Malaysia Institutional Repository

J. Fiz. UTM. Vol. 3. (2008) 99-108 0128-8644/2008  Jabatan Fizik UTM. All right reserved 99 A STUDY ON A.C PROPERTIES OF TIN SELENIDE THIN FILMS  Samsudi Sakrani, Zulkafli Othaman, Karim Deraman and Yussof Wahab  Thin Film Laboratory, Physics Department Faculty of Science, University Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia   ABSTRACT  A.c properties of tin selenide thin films prepared by an encapsulated selenization method are investigated. The measurements obtained from Al/SnSe/Al sandwich structures showed strong indication of frequency and temperature dependence of capacitance, dielectric loss and conductance over the ranges of 5-200 kHz and 228-373 K, respectively. Dielectric behaviour was expected to be due to space charge polarization which contributed to a.c conduction. This was generally explained in terms of hopping of the charge carriers between localized states with activation energies 0.03-0.08 eV. Parameters such as trap binding energy (0.94 eV) and minimum hopping distance (1.01 nm) were also predicted.  Keywords: selenide, thin film capacitance, dielectric loss, conductance, polarization  INTRODUCTION  Recent interest in tin selenide (SnSe) thin films has resulted, mostly from characterization measurements of electro-optic properties and applications of electronic devices, such as memory switching, photodiodes and heterojunctions [1-3]. SnSe is one of the IV-VI group semiconducting material in view of its covalent bonding [4] and positive electro-negativities existing in of both elements. Studies on d.c properties have been extensively covered, with the measured resistivities, 0.07-300 cm and activation energies, 0.01-0.28 eV [6-11]. The different conduction phenomena in SnSe thin films was also observed, apparently due to hopping [12] and Poole-Frenkel [13] conduction at low and high temperatures, respectively. Prominent results in a.c properties of SnSe thin films was reported recently by Pejova and Grozdanov [14] using chemical synthesis. Attention on this matter is given in the present work in order to acquire some information obtained from the a.c measurements, in particular the extent of dielectric behaviour and conduction mechanisms. These studies are apparently similar to the works of Suguna et al. [15] and Sharma et al. [16] but different in the methods of thin film preparation.    EXPERIMENTAL PROCEDURE  Samples was prepared using a E306 vacuum coater with masking system and deposition parameters set for fabricating eight Al-SnSe-Al sandwich structures on glass substrates, each of active area, 5.89x10-6 m2. The Al electrodes, of thickness 150 nm, were evaporated from tungsten filaments at the rates 1 nms-1, while SnSe was prepared using an encapsulated selenization of the stacked layers [17]. In the latter method granular tin and selenium of purity 5N were evaporated from molybdenum boats at pressures less than 10-5 and rates 0.8 nms-1 (Sn) and 0.2 nms-1 (Se) mbar in order to form Sn/Se stacked layers of equal thickness ratio and placed them in a high grade carbon block. Annealing was performed in an evacuated chamber under the running argon gas at 240 C for three hours to form stoichiometric composition of SnSe after room temperature cooling. These procedures were aimed at preventing material losses during heating and ensuring the optimum crystallization process; thus total film thicknesses of SnSe were typically 100-J. Fiz. UTM. Vol. 3. (2008) 99-108 0128-8644/2008  Jabatan Fizik UTM. All right reserved 100 500 nm. During each deposition, film thicknesses were monitored using a FTM 5 quartz crystal and the resulting films were later confirmed using a stylus. It was earlier confirmed that the contacts between Al-SnSe at both interfaces were ohmic, even after annealing in vacuum condition.  The capacitance of 100, 200, 300, 400 and 500 nm (thickness of SnSe) samples were measured using a Hewlett-Packard 4192A LCZ impedance analyzer (13.6 MHz) at a fixed frequency 100 kHz and room temperature in an evacuated container for the determination of dielectric constant. A.c measurements were also carried out on the 200 nm samples using a similar equipment at varying frequencies 5-200 kHz and temperatures 228-373 K; these were purposely for the establishment of graphs of capacitance, dielectric loss and conductivity as functions of frequency and temperature. All the measurements were carried out on the samples placed in a liquid nitrogen cooled cryostat at a pressure 10-2 mbar.    RESULTS AND DISCUSSION  Dielectric constant  The capacitance, Cd of two parallel metallic plates filled with a dielectric material of thickness, d is given by  C = dAos      (1)  where s is a relative permittivity of a dielectric, o permittivity in vacuum space and A an effective area. The dielectric constant is obtained by simply plotting a graph of C against 1/d (Figure 1) and equating the gradient of a straight line with soA, in order to deduce s. Nevertheless, the straight line is not passing the origin and it rather intersecting the C-axis at 5.73 nF. According to Gould et al. [18] there exist a parallel capacitance, C with respect to C which can give rise to an equivalent capacitance,   Ce = C + C =  s o Ad + C    (2)  The dielectric constant, obtained from Figure 1 is s = 14.52, which is comparable to those of bulk SnSe, 13.38 [14, 15].  Capacitance   The graphs of capacitance as functions of frequency and temperature are shown in Figure 2 and Figure 3, respectively. Noticeably, the fall in capacitance is very rapid in the initial frequency, starting between 15-22 nF and then gradually attaining 11 nF at a frequency f = 100 kHz for all the temperatures (Figure 2).  J. Fiz. UTM. Vol. 3. (2008) 99-108 0128-8644/2008  Jabatan Fizik UTM. All right reserved 101 681012141 3 5 7 9 111/d  (106 m-1)C (10-9  F) Figure 1: Capacitance plotted as a function reciprocal film thickness   In contrast, the temperature dependency of capacitance appears to be much significant at higher temperatures greater than 280 C and lower frequencies (Figure 3). A marked 30% increase in capacitance is observed for a 5 kHz curve. These results give the approximate dielectric constants which vary with temperature in an increasing manner to that shown in Fig. 3 according to Equation (1).   101214161820221 10 100 1000Frequency, f (kHz)Capacitance, C (nF)228 K248 K268 K313 K343 K353 K373 K  Figure 2: Variation of capacitance with frequency at different temperatures.  Dielectric loss   Dielectric loss or tan was obtained for different ranges of frequency 5-120 kHz and temperature 228-373 K. It is seen that, from the graphs shown in Fig. 4, the modes of variation are associated with the presence of loss minimum for all the temperatures. These values are observed to displace slightly to the right (for 228-313 K) and left sides (for 333-373 K) of the 100 kHz mark, with overall changes in tan between 0.116-0.180 over the range of frequencies.    J. Fiz. UTM. Vol. 3. (2008) 99-108 0128-8644/2008  Jabatan Fizik UTM. All right reserved 102 812162024200 250 300 350 400T  (K)C (nF)5 kHz21 kHz45 kHz85 kHz125 kHz165 kHz197 kHz  Figure 3: Variation of capacitance with temperature at different frequencies.  Using a Goswami and Goswamis model [19] the relationship between dielectric loss and frequency is given by     tan = )1)(()]1()[1(22222222222ppppCRCRCRrRCR           = ppprCCRrRC 21 = prCRrD  1              (3)  where D = 1/RCp,  = 2f is the angle frequency, R and r being the dielectric resistance and resistance due to electrode and connecting wires, respectively. Equation (3) gives the interpretation of tan in terms of the parameters , R, Cp and r. Since R  r, then rCp = 1/2RCp and the minimum dielectric loss occur at a frequency,   min= 21prRC       (4)  If R2Cp  r or r/R  1, then Equation (3) is simplified to  tan = )(1 ppprCDrCRC          (5)  In case of smaller , 1/RCprCp, thus the second term of Equation (5) becomes negligibly small, giving an equation of the first term,  tan = DRCp1       (6)  J. Fiz. UTM. Vol. 3. (2008) 99-108 0128-8644/2008  Jabatan Fizik UTM. All right reserved 103 0.111 10 100 1000 f (kHz) tan  228 K248 K268 K313 K333 K353 K373 K Figure 4: Dielectric loss versus frequency for different temperatures   It is also possible that  is considerably large, so that 1/RCp  rCp, and elimination of the first term of equation (5) results in another expression,  tan = rCp        (7)  The treatments given in Equations (6) and (7) can be correlated to the curves plotted in Figure 4 where the lowering mode implies to the former equation, whilst the increasing mode refers to the later one. These results are only valid for a capacitor system comprising of capacitive element Cp in parallel with temperature-dependent resistive element R due to dielectric film, both in series with a constant low value resistance r. In addition, the metal-dielectric interfaces must be ohmic in order to satisfy Equation (2). The effect of temperature can be explained indirectly using Equation (4), i.e. R decreases with increasing T, and the minimum angular frequency min follows accor-dingly if r remains unchanged. This is consistent with slight shifts of minimum fre-quencies to the right-hand side of the curves for temperatures between 228-313 K. However, the trend is no longer applied to the graphs measured at higher temperatures (313-373 K) due to opposing shifts of min, i.e. toward left-hand side. This contradiction is somewhat misunderstood and no quantitative explanation could be given at this stage of investigation. A possible reason may intuitively be predicted from the effect of contaminants, such as thin oxidized layers, oil grease, etc. which may give rise to the series and parallel resistance that eventually change equation (4) to a rather complex expression. In review of the results obtained from Fig. 3 and 4 the lower values of s and tan at lower range of frequencies may be attributed to space charge polarization. It has been pointed out [20] that the influence of temperature on the dielectric constant is strongly attributed to the increase of lattice defects of the materials at high temperature, making space charge polarization dominant. Defects in SnSe thin films may be attributed from the polycrystalline nature such as grain boundaries, voids, etc. The dispersion in capacitance and dielectric loss were observed to be due to the metal-dielectric interface and crystal behavior of the material [14, 20].  J. Fiz. UTM. Vol. 3. (2008) 99-108 0128-8644/2008  Jabatan Fizik UTM. All right reserved 104 A.c conductivity  The variation of a.c conductivity with frequency and temperature are shown in Figure 5 and Figure 6, respectively. It is observed that the frequency variations of conductivity in the former graph is strongly related to a power law,  = As, where the index, s is decreasing marginally between 0.46-0.40 for frequency f = 10-70 kHz and increasing between 0.91-0.99 for f = 70-120 kHz. These values are smaller than that of the majority of dielectric materials with s = 2 to 16. The temperature variations of conductivity shown in Figs. 6 and 7 is apparently temperature sensitive and becoming more pronounced at a higher range of T  280 K. A general expression, a.c = A(T)s(T) was proposed by Anwar and Hogarth [21] where A is a complex parameter weakly temperature dependent. There is a possible electronic hopping which capable of inducing a.c conduction at low temperature, a similar phenomena occurring in the d.c conductivity of SnSe thin films [12]. The associated activation energy, E is related to Arrhenius equation,    kTExoac pe          (8)  where o is a pre-exponential factor and k is Boltzmann constant. Fig. 7 shows linear graphs of ln ac - 1/T which reasonably fit equation (8), and thus yielding E. These results are tabulated in Table 1 as follows:   Table 1: Thermal activation energies at different frequencies and temperatures.  f (kHz) 5 21 45 85 125 165 197 E (eV) 0.033 0.079 0.124 0.200 0.293 0.406 0.529   Sakrani and Jabar [12] have studied the low temperature hopping conduction in SnSe thin films using current-voltage measurements and found activation energies 0.040-0.048 eV and 0.066-0.069 eV at temperature ranges 123-188 K and 188-270 K, respectively and these values are consistent with 0.04 eV on d.c measurements at temperatures 193-303 K [17]. The present results, i.e. 0.033-0.079 eV in the first two column of Table 1 seem to agree with the d.c conductivity measurements, and therefore, supporting the evidence of hopping conduction mechanism occurring in SnSe thin films at temperatures less than 270 C. Hutagalung [17] has indicated that SnSe thin film prepared by an encapsulated selenization method is of polycrystal type, and it is possible that imperfection of SnSe structure during the crystallization process leads to the establishment of defects and traps within the energy gap which may effect the nature of carrier transport. A.c conduction in SnSe thin films is best explained using Elliotts model in association with the trapping parameters, such as A.c angular frequency ù, effective relaxation time, ô, dielectric constant, ås, defect density of states, N and trap binding energy, Eb , and is given by [19]   ó(ù) =  sbssEqN6222 8    (9) J. Fiz. UTM. Vol. 3. (2008) 99-108 0128-8644/2008  Jabatan Fizik UTM. All right reserved 105 where q is the electronic charge, s = 1-â is the gradient of the linear portion of the ln ac-ln f graphs shown in Fig. 5 and only valid for low temperature measurements. A set of s and thus â is obtained for frequency ranges f  70 kHz. Eb is calculated using an expression,       â = bEkT6      (10) 0.11101001 10 100 1000Frequency, f (kHz) AC Conductivity,  ac (10-5  S m-1 ) 358 K338 K323 K308 K263 K243 K  Figure 5:A.c conductivity versus frequency for different temperatures.  05101520230 250 270 290 310 330 350 370T  (K) ac (10-5  S m-1 )5 kHz21 kHz45 kHz85 kHz125 kHz165 kHz197 kHz  Figure 6: Variation of a.c conductivity with temperature at different frequencies.    J. Fiz. UTM. Vol. 3. (2008) 99-108 0128-8644/2008  Jabatan Fizik UTM. All right reserved 106 0.11101002.7 3.0 3.3 3.6 3.9 4.21/T  (103 K-1) ac (10-5  S m-1 )5 kHz 21 kHz 45 kHz85 kHz 125 kHz 165 kHz197 kHz  Figure 7: A.c conductivity plotted as a function of reciprocal temperature at different frequencies.   Thus, by plotting â against T the trap binding energy Eb can be deduced from the linear relationship. The plot shown in Fig. 8 seems to be in a good agreement with equation (10), resulting in a gradient 5.50 x 10-4 and the trap binding energy of Eb = 0.94 eV for frequencies f  70 kHz. It is also seen that the experimentally measured values of s = 0.46 at 50 kHz and s = 0.79 at 80 kHz are comparable to the predicted s = 0.72 when Eb = 0.80 eV [23] is substituted into equation (10). Furthermore, the parameter Eb = 0.94 eV lies within the range of optical band gaps E = 0.79-0.95 eV [24] measured on SnSe thin films prepared by encapsulated selenization and 0.95 eV [29], but still lower than 1.21 eV [25] and 1.30 eV [10] due to different in preparation techniques used. This suggests the validity of the Elliotts model at low temperatures less than 270 C. There exists a minimal condition between subsequent hopping via a distance, rmin that can be predicted using an equation,   rmin = bos Eq22       (11)  From Equation (11) the calculated value is rmin = 1.01 nm. It is a distance between pairs or trap levels encountered by hopping the carriers in order to overcome an energy barrier of Eb = 0.94.eV within the energy gap of SnSe.   J. Fiz. UTM. Vol. 3. (2008) 99-108 0128-8644/2008  Jabatan Fizik UTM. All right reserved 107 0.520.560.60.64200 250 300 350 400T (K) Figure 8: Temperature dependence of â =1-s which satisfies equation (10).    CONCLUSION  SnSe thin films exhibited some dielectric properties under a.c measurements. The computed dielectric constant of 12.13 was found to be consistent with those of bulk material. There were strong indication of both frequency and temperature dependency of capacitance, dielectric dissipation and conductivity within the measurable ranges 5-200 kHz and temperatures 228-373 K, respectively. The variation of capacitance with both frequency and temperature were found to follow the Goswami and Goswamis model. From the frequency variation of dielectric loss, there exist dielectric behaviour due to space charge polarization which occurred at low frequencies. A.c conductivity was observed to obey the Elliotts model and some of the computed trapping parameters showed a reasonable agreement with those measured earlier. Activation energies 0.03-0.08 eV were almost identical to d.c conductivity, and the low values were indicative of electronic hopping conduction at low frequencies and temperatures. Trap binding energy 0.94 eV was approximately equal to the band gap of SnSe thin film, with minimum hopping distance 1.01 nm.    ACKNOWLEDGEMENTS  We are gratefully appreciated the financial support from Universiti Teknologi Malaysia under the short term grant. Thanks are due to colleagues and members of the Thin Film Research Group for various assistance and discussion.         J. Fiz. UTM. Vol. 3. (2008) 99-108 0128-8644/2008  Jabatan Fizik UTM. All right reserved 108  REFERENCES  [1] D. Chun, R. M. Walser, R. N. Bene and T. H. Courtney, J. Appl. Phys. Lett., 24 (1974) 479. [2] H. Holloway, Thin Film IV-VI Semiconductor Photodiodes (Academic Press, N. York, 1980). [3] V. P Bhatt,  K. Gireesan and C. F. Desai, Indian J. Pure & Appl. Phys., 29 (1991) 27. [4] S. Asanabe, J. Phys. Soc. 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Study on A.C properties of tin selenidethin films

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Study on A.C properties of tin selenidethin films

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