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In vitro stimulation of cell death in the moulting glands of Oncopeltus fasciatus by 20-hydroxyecdysone
Journal of Physics and Chemistry of Solids 64 (2003) 1941–1947 www.elsevier.com/locate/jpcs
Photoconductive properties of HgGa2S4 P.C. Riccia,*, A. Aneddaa, R. Corpinoa, I.M. Tiginyanub, V.V. Ursakic a
Dipartimento di Fisica, Universita` di Cagliari, Cittadella Universitaria SP Monserrato-Sestu Km 0,700, INFM, unita` di Cagliari, 09042 Monserrato (Ca), Italy b Laboratory of Low-Dimensional Semiconductor Structures, Technical University of Moldova, 2004 Chisinau, Moldova c Institute of Applied Physics, Academy of Sciences of Moldova, 2028 Chisinau, Moldova
Abstract Mercury thiogallate, HgGa2S4 is a defect chalcopyrite semiconductor with the space group S24 which offers a combination of attractive properties for applications. In order to obtain information about the electron states in the energy gap, photoconductivity measurements are performed in the 80 – 300 K range. Photoconductivity spectra show two peaks related to intrinsic and extrinsic excitation at about 410 and 500 nm, respectively; these maxima show a temperature dependence similar to the linear coefficient of the energy gap. Thermally stimulated currents have been studied by exciting the samples with intrinsic light at different temperatures. For all excitation temperatures a single TSC peaks were obtained. The analysis of TSC curves allowed one to estimate the kinetics of the trap emptying, trap energy distribution and thermal activation energy. A model for the level distribution in the semiconductor energy gap is suggested which in good agreement with the results of a previous photoluminescence study. q 2003 Elsevier Ltd. All rights reserved. Keywords: A. Semiconductors; D. Diffusion; A. Chalcogenides; D. Luminescence; A. Surfaces
1. Introduction Mercury thiogallate has widely used in different applications for its interesting physical proprieties, the wide transparency range (from 0.55 to 13 mm), high nonlinear susceptibility coefficients, and for the large birefringence [1 – 4]. For mid-IR Optical Parametric Oscillator with Nd:YAG-laser pumping the crystals of mercury thiogallate is optimal because as the value of effective square-low non-linear susceptibility, the transparent region and the value of absorption coefficient, the type of variance of principle values of refractive indexes, potential optical quality and resistance to laser radiation [5]. The conversion efficiency is more than two times more than the results obtained with single crystal of argentum thiogallate [5]. According to the literature, the creation of powerful tunable mid- and far IR sources of light pulses is most real on the basis of chalcogenide crystals of argentum thiogallate, mercury thiogallate and cadmium – mercury thiogallate [5]. It should be pointed out * Corresponding author. E-mail address: [email protected] (P.C. Ricci).
that in HgGa2S4 and Hg12xCdxGa2S4 crystals the parametric oscillation with pumping by the radiation of the second harmonic of the Nd:YAG-laser is possible because of specificity of refractive indexes variance in visible region [4]. Single crystal of HgGa2S4 is currently used for detect the radiation in the atmospheric transmission window, for the visualisation of the spectra in the fast process from 8 to 12 mm and for a novel travelling wave type optical parametric generator [6,7]. Photoluminescence measurements, performed at room temperature with 488 nm line excitation at a power density of 6 W/cm2, show different behaviour for different quenching temperature. The PL maximum shift from 1.6 eV for a Tq ¼ 500 8C to 2.4 eV for Tq ¼ 0: The PL maximum depends on the temperature and the excitation power density and in our sample can be shift from 2.2 eV at room temperature at power density of 6 W/cm2 to 1.9 eV at 20 K and power density 0.2 W/cm2 [1]. In this work we analyse the photoconductivity properties of HgGa2S4 single crystal with the aim to study the nonradiative process and for a better understanding of the levels distribution in the band gap of the semiconductor.
0022-3697/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0022-3697(03)00206-3
1942
P.C. Ricci et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1941–1947
2. Experimental HgGa2S4 single crystals in the form of yellow needles with dimensions of 10 £ 1 £ 1 mm3 were grown by vapour transport method, iodine being used as a transport agent. The crystals were grown at 620 8C temperature. The temperature was decreased in different regimes after the sample growth. It was slowly decreased down to a certain Tq temperature, and then the samples were quenched at 350 8C. Photocurrent (PC) spectra were measured using a tungsten lamp as excitation light coupled to a Spectra-Pro 275 monochromator with a grating 1200 grooves/mm to select the excitation wavelength. A 9 DC voltage source was used to supply the crystal and a Keithley digital electrometer was connect in series as ammeter, interfaced to a personal computer. The PC spectra were taken for fixed point when the current has reached a stable value. In order to obtain the thermally stimulated current curves, the sample was excited by light at suitable temperature in order to trap the photoproduced electrons.
The sample was successively heated and thus the thermal rise of trapped electrons to the conduction band is obtained. The current peak can be analysed to obtain information about the kinetics of trap emptying, energy distribution and thermal activation. The sample was excited by intrinsic light at different temperature ranging from 80 to 150 K. When the excitation source was switched off, the sample was kept in darkness and cooled to 77 K. The temperature rise was linear with a scan rate of , 5 K/min. The excitation power density on the sample was varied during the lux-ampere characteristic curve using neutral filters from 0.1 to 3 optical density. The PC spectra were measured in a temperature range between 77 and 300 K.
3. Results and discussion Fig. 1 presents the PC spectra at room temperature and at 83 K normalised by comparison with a black body detector.
Fig. 1. Photoconductivity spectra of HgGa2S4 single crystal at 300 and 83 K.
P.C. Ricci et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1941–1947
The spectra taken at 300 K present two maxima, at 2.82 and 2.38 eV, respectively. Because the band gap of the mercury thiogallate is , 2.83 eV at room temperature, the first peak is related to an intrinsic transition while the second to an extrinsic one. The spectra collected at the liquid nitrogen temperature presents two maxima, too; at 2.95 and 2.48 eV. The behaviour of the two spectra appear similar but the two peaks are rigidly shift towards the high energy. This variation is in agreement with the temperature dependence of the band gap of HgGa2S4 [1]. Actually, it rise of about 0.13 eV decreasing the temperature from 300 to 100 K. In order to explain the high value of the extrinsic peak in comparison with the intrinsic maximum, we divide the PC spectra in three region [8]: –
–
–
Low absorption region ðad , 1Þ in which the carriers are generated all over the thickness of the sample and the generation rate is proportional to the absorption coefficient a. Increasing the value of a the photons are absorbed in thinner and thinner layer below the surface (medium absorption region); the PC response can be no proportional to the absorption coefficient but the peak position depends on the carrier diffusion length. In the high absorption region, the light is absorbed in a very thin layer below the surface, and the recombination rate mostly depends on the surface carriers lifetime.
The indirect peak at 500– 520 nm belong to the very low absorption coefficient and the value of the excitation can be
1943
regarded as a volume effect. On the other hand the peak in the intrinsic region is related to an higher value of the absorption coefficient and the carrier excited belong only at the first layers of the sample. While the maximum at higher energy in Fig. 1 is related to a band to band transition, the second peak is generated by electron transition from impurity levels to the conduction band. Its relative enhancement at low temperature and the observation of a maximum in luminescence excitation spectra, is not in agreement with the attribution to the indirect band gap and involves the contribution of a localised state [9]. As it has been shown in Fig. 2, the relaxation time of the PC after intrinsic and extrinsic excitation is in the order of tens of minutes and is not strongly dependent on temperature. The temperature dependence of the PC excited at 2.47 eV is shown in Fig. 3. The extrinsic PC displays two different behaviours for different temperature ranges. The thermal quenching of PC, with an activation energy of about 30 meV is present between 80 and 200 K. Above 200 K, the PC is thermally enhanced, following the behaviour of the dark current. The characteristic features of trap distribution in side the semiconductor band gap can be obtained from the analysis of TSC [10]. In Fig. 4 are shown the four different TSC curves, obtained illuminating the sample with intrinsic light at different temperatures. For all the excitation temperatures a single peak of thermocurrent was detect. The value of the maximum is higher than the temperature at which the excitation source
Fig. 2. Relaxation time of the photocurrent after intrinsic and extrinsic excitation at 300 and 83 K. Curve (a) excited at 3 eV at 80 K; (b) 2.47 eV at 80 K; (c) 3 eV at 300 K; (d) 2.47 eV at 300 K.
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P.C. Ricci et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1941–1947
Fig. 3. Dark current and photocurrent generated by extrinsic excitation dependence with temperature. From the slope of the current is possible to estimate the activation energy.
was switched off. This fact supports the idea of a quasicontinuous trap distribution [11]. We have illuminate the sample also with 2.47 eV light energy at 84 K; the curve (Fig. 5) present a very similar behaviour to the respective one illuminate at 2.95 eV, it presents a weakly shift to the lower temperature. On the contrary illuminating the sample with energy 2.37 eV, we do not observe any maximum in the TSC curve. We
suppose that the excitation with 2.47 eV is directly from an acceptor level to the conduction band, while the illumination with 2.37 eV does not give the necessary energy to excite the electrons directly in the conduction band. Because the band gap of the semiconductor at the low temperature is 2.95 eV, it is possible to estimate the position of the acceptor level between 0.47 and 0.57 eV from the valence band.
Fig. 4. Thermally stimulated current curves excited at different temperature. The temperature rise was linear with a rate of 5 K/min.
P.C. Ricci et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1941–1947
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Fig. 5. Thermally stimulated current excited by extrinsic radiation (curve A, 2.47 eV) and intrinsic (curve B, 3 eV) at 80 K. v is the full-width at half maximum and d is the half-width toward the high temperature.
According to the literature [12], the order of kinetics in a thermoluminescence process is related to the symmetry factor m ¼ d=v; where d and v are, respectively, the halfwidth at half maximum toward high temperature and the full-width of the glow curve. These consideration can be extended also to the TSC process as well [13]. From Fig. 5 we obtain m ¼ 0:55 for the curve illuminate with 2.47 eV and m ¼ 0:57 for the curve illuminate with 2.95 eV. These values are very similar to 0.52 which is characteristic for a pure second order process in thermoluminescence [13]. Fig. 6 shows the lux-ampere characteristic at room temperature and at 80 K with 500 and 400 nm incident light. The PC signal versus intensity is described by the relation, too IPC / ðIL Þa the value of a could be an indication of the kinetics of the recombination process. Values of a around 0.5 implies a bimolecular kinetics while values around 1 indicate a monomolecular kinetics. In particular a power dependence between 0.5 and 1 is commonly found for the lux-ampere curves of n-type materials characterised in electron traps exponentially distributed below the bottom of the conduction band [14]. In our experiments we have a constant value of a < 0:50 for the measure taken illuminating the sample with intrinsic excitation and for measure with extrinsic light at low temperature. These values relative to a second order kinetics are in agreement with the analysis of the TSC curves at low temperature. On the contrary, exciting the sample at room
temperature with intrinsic light we have observed a quasilinear dependence of the PC with the intensity. However, all the values of a calculated are always between 0.5 and 1, in agreement with a exponential distribution of the electron trap below the conduction band. We suppose that the difference in the power dependence is given by different kinetics involved in the recombination process. The kinetics of recombination process for extrinsic excitation change from room temperature to 80 K while it is preserved for intrinsic excitation light. Taking in account the experimental results and previous work based on photoluminescence measure [1], we propose a model of the energy level in the band gap of the semiconductor (Fig. 7). The extrinsic excitation (transition a) is generated from the ionised acceptor level directly to the conduction band, in agreement with previous work this is possible also at lower temperatures [1]. At high temperature the recombination process is more probable through the exponential distribution of electron traps under the conduction band, giving rise to a kinetics of the first order. As the temperature decreases the electron trap do not more contribute to the recombination process. The electron will recombine directly to the holes freed in the valence band. The same recombination process will be still valid for intrinsic excitation (transition b), in which the mechanism will remain statistically bimolecular also at high temperatures for the high number of holes freed in the valence band during the illumination. The exponential distribution of electrons traps is explained by the behaviour
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P.C. Ricci et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1941–1947
Fig. 6. Lux-ampere characteristics curves at 80 K and at room temperature with extrinsic (2.47 eV) and intrinsic excitation (3 eV).
of the TSC curves at different excitation temperature and by the long relaxation time after that the light switched off. Moreover, as has been pointed out from different authors [9, VI 11,15 –19], a common feature of AII BIII 2 C4 compound is the presence of a quasi-continuously distributed electrons trap near the bottom of the conduction band. It is generated by the high degree of intrinsic disorder caused by cationic A– B exchange. The energy gap between the top of this distribution and the conduction band strongly depends upon the technological parameters of crystal growth.
We have estimate the mean activation energy for our sample from the shape of the TSC using the Lushchik’s formula [20]: Ea ¼ 2kðTm Þ2 =d where Tm is the temperature of the peak maximum and k is the Boltzmann’s constant. The values that we have obtained are 72 and 90 meV for the curve illuminated with extrinsic and intrinsic light, respectively.
Fig. 7. Scheme for electron state in HgGa2S4 single crystal. Refer to the text for explanations.
P.C. Ricci et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1941–1947
4. Conclusions The photoconductivity proprieties of HgGa2S4 has been studied. The long relaxation time after excitation is typical for an high exponential distribution of traps in the energy gap. The study of thermally stimulates current show, as far as the lux-ampere characteristics, some basic physical proprieties of the levels engaged in this process. The kinetics of the recombination process involved depend from the temperature range and from the excitation light. The model of the energy levels in the gap of the semiconductor agree very well with previous measure of photoluminescence and complete the study of the excitation process in mercury thiogallate. We propose this material as candidate for the fabrication of high efficiency light emitting devices based on HgGa2S4 single crystal.
References [1] V.V. Ursaki, P.C. Ricci, I. Tiginyanu, A. Anedda, N.N. Syrbu, V.E. Tezlevan, J. Phys. Chem. Solids 63 (2002) 1823. [2] H. Hahn, G. Frank, W. Klinger, A. Sto¨rger, G. Sto¨rger, Z. Anorg. Allg. Chem. 279 (1955) 241. [3] H. Schwer, V. Kra¨mer, Z. Kristallogr. 190 (1990) 103.
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[4] B.F. Levine, C.G. Bethea, H.M. Kasper, F.A. Thiel, IEEE J. Quantum Electron. QE-12 (1976) 367. [5] A. Komarow, V.S. Solomatin, Sov. Tech. Phys. Lett. 8 (1982) 375. [6] S.A. Komarov, A.N. Meleshko, A.N. Pleshanov, V.S. Solomatin, Sov. Tech. Phys. Lett. 6 (1980) 375. [7] F. Rotermund, V. Petrov, Opt. Lett. 25 (2000) 746. [8] H. Bube, Electronic Properties of Crystalline Solids, Academic press, London, 1974, p. 455. [9] A. Anedda, L. Garbato, F. Raga, A. Serpi, Phys. Status Solidi 50 (1978) 643. [10] H. Bube, Photoconductivity of Solids, Wiley, New York, 1960. [11] A. Serpi, J. Phys. D 9 (1976) 1881. [12] A. Halperin, Braner 117 (1960) 408. [13] R. Chen, Y. Kirsh, Analysis of Thermally Stimulated Process, Pergamon press Ltd, Oxford, 1981. [14] A. Rose, Concepts in Photoconductivity and Allied Problems, Wiley, New York, 1964. [15] L. Baldassarre, V. Cappozzi, G. Maggipinto, A. Minafra, Phys. Status Solidi A 46 (1978) 589. [16] D. Crandles, E. Fortin, Phys. Status Solidi A 95 (1986) K47. [17] V.L. Panyutin, B.E. Ponedelnikov, A.E. Rozenson, V.I. Chizhikov, Sov. Phys. Semicond. 13 (1979) 711. [18] S.I. Radautsan, I.M. Tiginyanu, V.N. Fulga, Yu.O. Derid, Phys. Status Solidi A 114 (1989) 259. [19] I.M. Tiginyanu, Sov. Phys. Semicond. 23 (1989) 974. [20] C.B. Luschchik, Sov. Phys. JETP 3 (1956) 390.
Photoconductive properties of HgGa2S4 P.C. Riccia,*, A. Aneddaa, R. Corpinoa, I.M. Tiginyanub, V.V. Ursakic a
Dipartimento di Fisica, Universita` di Cagliari, Cittadella Universitaria SP Monserrato-Sestu Km 0,700, INFM, unita` di Cagliari, 09042 Monserrato (Ca), Italy b Laboratory of Low-Dimensional Semiconductor Structures, Technical University of Moldova, 2004 Chisinau, Moldova c Institute of Applied Physics, Academy of Sciences of Moldova, 2028 Chisinau, Moldova
Abstract Mercury thiogallate, HgGa2S4 is a defect chalcopyrite semiconductor with the space group S24 which offers a combination of attractive properties for applications. In order to obtain information about the electron states in the energy gap, photoconductivity measurements are performed in the 80 – 300 K range. Photoconductivity spectra show two peaks related to intrinsic and extrinsic excitation at about 410 and 500 nm, respectively; these maxima show a temperature dependence similar to the linear coefficient of the energy gap. Thermally stimulated currents have been studied by exciting the samples with intrinsic light at different temperatures. For all excitation temperatures a single TSC peaks were obtained. The analysis of TSC curves allowed one to estimate the kinetics of the trap emptying, trap energy distribution and thermal activation energy. A model for the level distribution in the semiconductor energy gap is suggested which in good agreement with the results of a previous photoluminescence study. q 2003 Elsevier Ltd. All rights reserved. Keywords: A. Semiconductors; D. Diffusion; A. Chalcogenides; D. Luminescence; A. Surfaces
1. Introduction Mercury thiogallate has widely used in different applications for its interesting physical proprieties, the wide transparency range (from 0.55 to 13 mm), high nonlinear susceptibility coefficients, and for the large birefringence [1 – 4]. For mid-IR Optical Parametric Oscillator with Nd:YAG-laser pumping the crystals of mercury thiogallate is optimal because as the value of effective square-low non-linear susceptibility, the transparent region and the value of absorption coefficient, the type of variance of principle values of refractive indexes, potential optical quality and resistance to laser radiation [5]. The conversion efficiency is more than two times more than the results obtained with single crystal of argentum thiogallate [5]. According to the literature, the creation of powerful tunable mid- and far IR sources of light pulses is most real on the basis of chalcogenide crystals of argentum thiogallate, mercury thiogallate and cadmium – mercury thiogallate [5]. It should be pointed out * Corresponding author. E-mail address: [email protected] (P.C. Ricci).
that in HgGa2S4 and Hg12xCdxGa2S4 crystals the parametric oscillation with pumping by the radiation of the second harmonic of the Nd:YAG-laser is possible because of specificity of refractive indexes variance in visible region [4]. Single crystal of HgGa2S4 is currently used for detect the radiation in the atmospheric transmission window, for the visualisation of the spectra in the fast process from 8 to 12 mm and for a novel travelling wave type optical parametric generator [6,7]. Photoluminescence measurements, performed at room temperature with 488 nm line excitation at a power density of 6 W/cm2, show different behaviour for different quenching temperature. The PL maximum shift from 1.6 eV for a Tq ¼ 500 8C to 2.4 eV for Tq ¼ 0: The PL maximum depends on the temperature and the excitation power density and in our sample can be shift from 2.2 eV at room temperature at power density of 6 W/cm2 to 1.9 eV at 20 K and power density 0.2 W/cm2 [1]. In this work we analyse the photoconductivity properties of HgGa2S4 single crystal with the aim to study the nonradiative process and for a better understanding of the levels distribution in the band gap of the semiconductor.
0022-3697/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0022-3697(03)00206-3
1942
P.C. Ricci et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1941–1947
2. Experimental HgGa2S4 single crystals in the form of yellow needles with dimensions of 10 £ 1 £ 1 mm3 were grown by vapour transport method, iodine being used as a transport agent. The crystals were grown at 620 8C temperature. The temperature was decreased in different regimes after the sample growth. It was slowly decreased down to a certain Tq temperature, and then the samples were quenched at 350 8C. Photocurrent (PC) spectra were measured using a tungsten lamp as excitation light coupled to a Spectra-Pro 275 monochromator with a grating 1200 grooves/mm to select the excitation wavelength. A 9 DC voltage source was used to supply the crystal and a Keithley digital electrometer was connect in series as ammeter, interfaced to a personal computer. The PC spectra were taken for fixed point when the current has reached a stable value. In order to obtain the thermally stimulated current curves, the sample was excited by light at suitable temperature in order to trap the photoproduced electrons.
The sample was successively heated and thus the thermal rise of trapped electrons to the conduction band is obtained. The current peak can be analysed to obtain information about the kinetics of trap emptying, energy distribution and thermal activation. The sample was excited by intrinsic light at different temperature ranging from 80 to 150 K. When the excitation source was switched off, the sample was kept in darkness and cooled to 77 K. The temperature rise was linear with a scan rate of , 5 K/min. The excitation power density on the sample was varied during the lux-ampere characteristic curve using neutral filters from 0.1 to 3 optical density. The PC spectra were measured in a temperature range between 77 and 300 K.
3. Results and discussion Fig. 1 presents the PC spectra at room temperature and at 83 K normalised by comparison with a black body detector.
Fig. 1. Photoconductivity spectra of HgGa2S4 single crystal at 300 and 83 K.
P.C. Ricci et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1941–1947
The spectra taken at 300 K present two maxima, at 2.82 and 2.38 eV, respectively. Because the band gap of the mercury thiogallate is , 2.83 eV at room temperature, the first peak is related to an intrinsic transition while the second to an extrinsic one. The spectra collected at the liquid nitrogen temperature presents two maxima, too; at 2.95 and 2.48 eV. The behaviour of the two spectra appear similar but the two peaks are rigidly shift towards the high energy. This variation is in agreement with the temperature dependence of the band gap of HgGa2S4 [1]. Actually, it rise of about 0.13 eV decreasing the temperature from 300 to 100 K. In order to explain the high value of the extrinsic peak in comparison with the intrinsic maximum, we divide the PC spectra in three region [8]: –
–
–
Low absorption region ðad , 1Þ in which the carriers are generated all over the thickness of the sample and the generation rate is proportional to the absorption coefficient a. Increasing the value of a the photons are absorbed in thinner and thinner layer below the surface (medium absorption region); the PC response can be no proportional to the absorption coefficient but the peak position depends on the carrier diffusion length. In the high absorption region, the light is absorbed in a very thin layer below the surface, and the recombination rate mostly depends on the surface carriers lifetime.
The indirect peak at 500– 520 nm belong to the very low absorption coefficient and the value of the excitation can be
1943
regarded as a volume effect. On the other hand the peak in the intrinsic region is related to an higher value of the absorption coefficient and the carrier excited belong only at the first layers of the sample. While the maximum at higher energy in Fig. 1 is related to a band to band transition, the second peak is generated by electron transition from impurity levels to the conduction band. Its relative enhancement at low temperature and the observation of a maximum in luminescence excitation spectra, is not in agreement with the attribution to the indirect band gap and involves the contribution of a localised state [9]. As it has been shown in Fig. 2, the relaxation time of the PC after intrinsic and extrinsic excitation is in the order of tens of minutes and is not strongly dependent on temperature. The temperature dependence of the PC excited at 2.47 eV is shown in Fig. 3. The extrinsic PC displays two different behaviours for different temperature ranges. The thermal quenching of PC, with an activation energy of about 30 meV is present between 80 and 200 K. Above 200 K, the PC is thermally enhanced, following the behaviour of the dark current. The characteristic features of trap distribution in side the semiconductor band gap can be obtained from the analysis of TSC [10]. In Fig. 4 are shown the four different TSC curves, obtained illuminating the sample with intrinsic light at different temperatures. For all the excitation temperatures a single peak of thermocurrent was detect. The value of the maximum is higher than the temperature at which the excitation source
Fig. 2. Relaxation time of the photocurrent after intrinsic and extrinsic excitation at 300 and 83 K. Curve (a) excited at 3 eV at 80 K; (b) 2.47 eV at 80 K; (c) 3 eV at 300 K; (d) 2.47 eV at 300 K.
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P.C. Ricci et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1941–1947
Fig. 3. Dark current and photocurrent generated by extrinsic excitation dependence with temperature. From the slope of the current is possible to estimate the activation energy.
was switched off. This fact supports the idea of a quasicontinuous trap distribution [11]. We have illuminate the sample also with 2.47 eV light energy at 84 K; the curve (Fig. 5) present a very similar behaviour to the respective one illuminate at 2.95 eV, it presents a weakly shift to the lower temperature. On the contrary illuminating the sample with energy 2.37 eV, we do not observe any maximum in the TSC curve. We
suppose that the excitation with 2.47 eV is directly from an acceptor level to the conduction band, while the illumination with 2.37 eV does not give the necessary energy to excite the electrons directly in the conduction band. Because the band gap of the semiconductor at the low temperature is 2.95 eV, it is possible to estimate the position of the acceptor level between 0.47 and 0.57 eV from the valence band.
Fig. 4. Thermally stimulated current curves excited at different temperature. The temperature rise was linear with a rate of 5 K/min.
P.C. Ricci et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1941–1947
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Fig. 5. Thermally stimulated current excited by extrinsic radiation (curve A, 2.47 eV) and intrinsic (curve B, 3 eV) at 80 K. v is the full-width at half maximum and d is the half-width toward the high temperature.
According to the literature [12], the order of kinetics in a thermoluminescence process is related to the symmetry factor m ¼ d=v; where d and v are, respectively, the halfwidth at half maximum toward high temperature and the full-width of the glow curve. These consideration can be extended also to the TSC process as well [13]. From Fig. 5 we obtain m ¼ 0:55 for the curve illuminate with 2.47 eV and m ¼ 0:57 for the curve illuminate with 2.95 eV. These values are very similar to 0.52 which is characteristic for a pure second order process in thermoluminescence [13]. Fig. 6 shows the lux-ampere characteristic at room temperature and at 80 K with 500 and 400 nm incident light. The PC signal versus intensity is described by the relation, too IPC / ðIL Þa the value of a could be an indication of the kinetics of the recombination process. Values of a around 0.5 implies a bimolecular kinetics while values around 1 indicate a monomolecular kinetics. In particular a power dependence between 0.5 and 1 is commonly found for the lux-ampere curves of n-type materials characterised in electron traps exponentially distributed below the bottom of the conduction band [14]. In our experiments we have a constant value of a < 0:50 for the measure taken illuminating the sample with intrinsic excitation and for measure with extrinsic light at low temperature. These values relative to a second order kinetics are in agreement with the analysis of the TSC curves at low temperature. On the contrary, exciting the sample at room
temperature with intrinsic light we have observed a quasilinear dependence of the PC with the intensity. However, all the values of a calculated are always between 0.5 and 1, in agreement with a exponential distribution of the electron trap below the conduction band. We suppose that the difference in the power dependence is given by different kinetics involved in the recombination process. The kinetics of recombination process for extrinsic excitation change from room temperature to 80 K while it is preserved for intrinsic excitation light. Taking in account the experimental results and previous work based on photoluminescence measure [1], we propose a model of the energy level in the band gap of the semiconductor (Fig. 7). The extrinsic excitation (transition a) is generated from the ionised acceptor level directly to the conduction band, in agreement with previous work this is possible also at lower temperatures [1]. At high temperature the recombination process is more probable through the exponential distribution of electron traps under the conduction band, giving rise to a kinetics of the first order. As the temperature decreases the electron trap do not more contribute to the recombination process. The electron will recombine directly to the holes freed in the valence band. The same recombination process will be still valid for intrinsic excitation (transition b), in which the mechanism will remain statistically bimolecular also at high temperatures for the high number of holes freed in the valence band during the illumination. The exponential distribution of electrons traps is explained by the behaviour
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P.C. Ricci et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1941–1947
Fig. 6. Lux-ampere characteristics curves at 80 K and at room temperature with extrinsic (2.47 eV) and intrinsic excitation (3 eV).
of the TSC curves at different excitation temperature and by the long relaxation time after that the light switched off. Moreover, as has been pointed out from different authors [9, VI 11,15 –19], a common feature of AII BIII 2 C4 compound is the presence of a quasi-continuously distributed electrons trap near the bottom of the conduction band. It is generated by the high degree of intrinsic disorder caused by cationic A– B exchange. The energy gap between the top of this distribution and the conduction band strongly depends upon the technological parameters of crystal growth.
We have estimate the mean activation energy for our sample from the shape of the TSC using the Lushchik’s formula [20]: Ea ¼ 2kðTm Þ2 =d where Tm is the temperature of the peak maximum and k is the Boltzmann’s constant. The values that we have obtained are 72 and 90 meV for the curve illuminated with extrinsic and intrinsic light, respectively.
Fig. 7. Scheme for electron state in HgGa2S4 single crystal. Refer to the text for explanations.
P.C. Ricci et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1941–1947
4. Conclusions The photoconductivity proprieties of HgGa2S4 has been studied. The long relaxation time after excitation is typical for an high exponential distribution of traps in the energy gap. The study of thermally stimulates current show, as far as the lux-ampere characteristics, some basic physical proprieties of the levels engaged in this process. The kinetics of the recombination process involved depend from the temperature range and from the excitation light. The model of the energy levels in the gap of the semiconductor agree very well with previous measure of photoluminescence and complete the study of the excitation process in mercury thiogallate. We propose this material as candidate for the fabrication of high efficiency light emitting devices based on HgGa2S4 single crystal.
References [1] V.V. Ursaki, P.C. Ricci, I. Tiginyanu, A. Anedda, N.N. Syrbu, V.E. Tezlevan, J. Phys. Chem. Solids 63 (2002) 1823. [2] H. Hahn, G. Frank, W. Klinger, A. Sto¨rger, G. Sto¨rger, Z. Anorg. Allg. Chem. 279 (1955) 241. [3] H. Schwer, V. Kra¨mer, Z. Kristallogr. 190 (1990) 103.
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