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DX centers and persistent photoconductivity in CdTe–In films
PERGAMON
Solid State Communications 113 (2000) 621–625 www.elsevier.com/locate/ssc
DX centers and persistent photoconductivity in CdTe–In films Z. Rivera-Alvarez a, L. Herna´ndez 1,a, M. Becerril a, A. Picos-Vega a, O. Zelaya-Angel a,*, R. Ramı´rez-Bon b, J.R. Vargas-Garcı´a c a
b
Departamento de Fı´sica, CINVESTAV-IPN, Apdo Postal 14-740, 07000 DF Mexico, Mexico Centro de Investigacio´n en Fı´sica, Universidad de Sonora, Hermosillo, Sonora, Apdo Postal 5-88, 8319 Hermosillo, Son., Mexico c Departamento de Ingenierı´a Metalu´rgica, I.P.N., Apdo Postal 75-874, 07300 D.F. Mexico, Mexico Received 21 December 1998; received in revised form 2 July 1999; accepted 24 November 1999 by S.G. Louie
Abstract In this work, we study the nature and behavior of the persistent photoconductivity (PPC) in CdTe–In films grown by cosputtering of CdTe–In–Cd targets. It was found that only when In atoms are substantially incorporated into CdTe films, the persistent photoconductivity is observed with a quenching temperature of about 270 K. We have also investigated the trapping centers in the CdTe films by using the thermally stimulated conductivity technique. Two localized deep levels were determined. One of them, with an activation energy of 0.42 eV, has been ascribed as a direct evidence of DX centers that are formed by Cd vacancies and In donors complexes. By formulating the PPC build-up and decay kinetics, we have associated the PPC effect in our films to the photoionization of this deep level (DX like centers). Up to date, the existence of DX centers in CdTe–In polycrystalline films have not been previously reported. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Thin films; C. Impurities in semiconductors; D. Electronic states (localized); D. Photoconductivity and photovoltaics
1. Introduction Recently, the persistent photoconductivity (PPC), photoconductivity that persists after the removal of the photoexcitation, has been used to optically write metallic patterns in an insulating matrix with high contrast and it has been estimated that features can be written with around 100 nm of resolution [1]. The optically written patterns continue under the same circumstances for which the photoconductivity itself is persistent, and can be canceled entirely by heating above the quenching temperature (Ta) of the PPC. The PPC can be a very attractive phenomenon in the applications of submicron device fabrication, optical switching and high-density data-storage [2,3]. The physical origin of PPC in many semiconductors has been an important subject in the last decade and several mechanisms have been suggested to describe the cause
* Corresponding author. Tel.: 152-5-747-3800; fax: 152-5-7477096. E-mail address: [email protected] (O. Zelaya-Angel). 1 Also at Facultad de Fı´sica, Universidad de La Habana, 10400 La Habana, Cuba.
[4–8]. One of these mechanisms is connected with some deep donors, known as DX centers [4,5]. Chadi and Chang [9] have established that the DX center is negatively charged and a highly localized defect center resulting from a large lattice distortion. In the ground state, the DX center is a deep donor that is negatively charged and the photoexcitation changes the DX into a shallow state. This excited state is metastable, hence a barrier to recombination is originated by the structural relaxation required to return to the DX state. At low temperatures the photogenerated electrons remain in the conduction band indefinitely and the PPC is observed. There is now substantial evidence supporting that the conversion of the shallow hydrogenic donor into a deep relaxed one can be achieved by adding In to CdTe originating DX centers [10–12]. Lately, by first-principles pseudopotential calculations, Park and Chadi have shown that the metastable behavior of the In donors in CdTe and the creation of the DX centers can be explained by the broken-bond model [13]. The goal of this paper is to study the nature and behavior of the PPC in CdTe–In films grown by co-sputtering of CdTe–In–Cd targets. It was found that only when In atoms are substantially incorporated into the CdTe films, the PPC is observed with Ta , 250 K: At the same time,
0038-1098/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00545-1
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Table 1 The Cd, Te, and In content in each sample that were measured using an AES system. Also the thickness is displayed Sample
% In-atoms
% Te-atoms
% Cd-atoms
Thickness (nm)
M-0 M-1 M-2 M-3 B-8 I-10
0.0 1.0 3.5 6.5 0.0 2.5
51.8 50.6 51.2 51.4 49.8 48.8
48.2 48.4 45.3 42.1 50.2 48.7
111.0 85.9 88.3 99.7 80.0 95.4
we have investigated the trapping centers in the CdTe films by using the thermally stimulated conductivity (TSC) technique. Two localized deep levels were determined. One of them, with an activation energy of ET 0:75 eV; has been assigned to Cd-vacancies. The second trap, with ET 0:42 eV; has been ascribed as a direct evidence of the DX centers that are formed by Cd vacancies and In donors complexes. Also, by formulating the PPC buildup and decay kinetics, we can associate the PPC effect in our films to the photoionization of the deep level (DX like centers), localized at 0.42 eV. These centers are responsible for the PPC effect with an energy barrier of 0.23 eV for the capture of electrons. Up to date, the existence of the DX centers in the CdTe–In polycrystalline films have not been previously reported.
Fig. 1. The TSC spectra of the M-0 and M-1 samples. The inset shows the Arrhenius plots of log ITSC vs 1000/T.
2. Experimental details A series of around 100 nm thick, n-type In-doped CdTe polycrystalline films has been prepared by co-sputtering of a CdTe–In–Cd target. The CdTe–In films were grown on 7059 Corning glass slides at room temperature using a diode rf sputtering system with a water cooled cathode. The rf power was 30 W and the distance between the target and the substrate was 4 cm. The Cd and In atoms were introduced in the samples by co-sputtering from a CdTe target with pieces of elemental indium and cadmium glued onto it to produce n-type conductivity films. The Cd on the target was added in order to provide the excess of cadmium needed to prevent Cd vacancies during the growth of CdTe films [14]. The CdTe (99.999% pure) target had an area of 4.92 cm 2. The indium area on the target was varied to cover 0, 0.25, and 2.0 and 4% of the total target area. The samples with different indium concentrations are denoted by M-0 to M-3, respectively. In this case, the area of Cd on the CdTe target was the same for all samples and it covered 1% of the total target area. In a subsequent experiment, we changed the area of Cd to cover 4% of the total target area without indium on it (sample B-8). Table 1 shows the Cd, Te, and In contents in the films that were measured using an Auger electron spectroscopy (AES) system. The crystalline structure of the films was determined by an X-ray diffractometer SIEMENS D5000. All the spectra display only the characteristic diffraction peaks of the cubic phase of CdTe and we have not observed peaks relative to CdIn2Te4 or In–Te compounds [15]. The CdTe–In polycrystalline films were grown at low In concentrations because an increase in the In concentration produces disorder and deformation of the CdTe lattice yielding to a crystalline structure change from the cubic to the hexagonal CdTe phase [16]. The TSC measurements were made on samples having a distance of 1 cm between coplanar ohmic contacts. Additional details of the contact fabrication and thermal annealing can be found elsewhere [17]. A liquid-nitrogen cryostat was used to vary the sample temperature from 77 to 420 K. The PPC and TSC data were taken in such a way that the samples were always heated up to 420 K and relaxed to equilibrium before each measurement, then cooled down
Z. Rivera-Alvarez et al. / Solid State Communications 113 (2000) 621–625
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Fig. 2. The TSC spectra of the M-2 and M-3 samples. The inset shows the Arrhenius plots of log ITSC vs 1000/T.
in darkness to 77 K. This ensures that each set of data have the same initial conditions. The PPC and TSC data are obtained by illuminating the sample at low temperature (80 K) with a white light at exactly the same intensity. The contacts were shielded against light by masks. A constant bias voltage between 40 and 100 V was applied to the sample as a function of the film resistivity and the current was measured by a programmable electrometer KEITHLEY model 617. When the light was turned off, the temperature was raised at a constant heating rate of 0.15 K/s with a control temperature from a DLTS spectrometer DL-4600, BIO-RAD.
3. Results and discussion Fig. 1 shows the TSC data for the M-0 and M-1 samples. Only one peak is observed, centered at T 255 K in the data from each sample. The spectra shape is independent of the illumination intensity and the bias voltage. No apparent shifts were observed in the peak position indicating firstorder kinetics. For the evaluation of the trapping level activation energy, the well-known initial rise method was used. This analysis is independent with regard to the kinetics of process and it applies to the low-temperature tail of the TSC curve. In this region the number of trapped electrons change only a small amount with the temperature and can be regarded as constant. Then, the intensity of the TSC can be expressed by ITSC , exp 2ET =KT; where K is the Boltzmann’s constant. The plot of the log ITSC as a function
Fig. 3. PPC as a function of temperature for the M-2 and M-3 samples. Cooling in the dark (open circles), and warming in the dark after illuminated at T 80 K (solid circles). The semicircles depict the noise level.
of inverse absolute temperature is shown in the inset of Fig. 1, which displays slopes with an activation energy of the trap of 0.75 eV for samples M-0 and M-1. From the equal values of ET, we can conclude that the nature of the trap is the same for both the samples. In order to identify the origin of the trap, we have researched the TSC behavior in the sample B-8 (see Table 1), which was grown with the area of the Cd on the CdTe target to cover 4% of the total target area, without indium on the target. In this film, the TSC signal was not detected indicating that the nature of the trap has its origin in Cd-vacancies. This deep level has been reported in the literature for CdTe grown with different methods and dopants and has been observed both with electrical and optical techniques. This trap has been widely attributed to an acceptor complex involving the native VCd 22 defect and impurity [18,19]. When the number of indium atoms are substantially incorporated to CdTe films, i.e. the indium area on the target covers 2.0 or 4% of the total target area, the measured TSC spectra differ in the peak position and in the region of the low-temperature tail. Fig. 2 displays the TSC spectra for the M-2 and M-3 samples showing the above-mentioned properties. From the Arrhenius plot in the inset of Fig. 2, the activation energy of the trap is 0.42 eV for the two samples. The thermal cleaning method of Taylor and Lilley [20] was applied in both samples but the trap with ET , 0:75 eV was
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Fig. 4. Buildup and decay kinetics of the PPC related with the M-2 sample at T 230 K: The solid curve is the least squares fit of experimental data with Eq. (1) for the buildup part and with Eq. (2) for the decay part. The inset shows the Arrhenius plot of logt vs 1000/T.
not observed. From the AES measurement shown in Table 1, we can predict that the incorporated In atoms partially fill the Cd vacancies, leading to a reduction in their concentration and, therefore, the non-appearance of this trap. In order to clarify the nature of the traps in samples M-2 and M-3, we have also studied the TSC behavior in sample I-10. This sample was grown with the area of In on the CdTe target to cover 4% of the total target area and with an excess of cadmium in order to impede the Cd vacancies. The Cd, Te and In contents of this film, were measured using AES, and are shown in the Table 1. The resistivity of sample I-10 is 5 × 101 V-cm; three orders lower than sample M-3, making it evident that at high contents of In, without an excess of Cd, result in a formation of lattice defects involving vacancies. Also, if the defects are highly mobile, they may migrate to the impurity atoms and create deep compensating complexes, inducing a decrease of the crystalline quality and, change the type of electrical activity of the impurity. Again, the TSC signal was not detected in the I-10 films, this fact supports the microscopic nature of the traps, with E T 0:42 eV in samples M-2 and M-3, which is with great probability correlated with the Cd vacancies–In donors complexes. Fig. 3 shows the temperature-dependent current in the M2 and M-3 samples. The dark current is measured on cooling from T 400 K (open circles). At T 80 K the films are
illuminated then the current rises until saturation. When the light was eliminated, the current remained at that value indefinitely: the photoconductivity becomes persistent. Subsequently, the films were heated in the dark and the current was then measured (solid circle). The warming curves reach the cooling dark values around Ta ù 270 K: Despite the fact that the exact origin of the In center in CdTe is unknown, Suski et al [21] have supposed that it is a substitutional donor at a cation site which binds one or two (negative-U case) electrons creating the DX centers. The trapping level with an activation energy 0.45 eV in CdTe– In grown by molecular beam epitaxy has been recently assigned as a DX center [12] corroborating the Park and Chadi model [13], in which the DX centers are formed by transitions of isolated In atoms from substitutional sites in the CdTe lattice to the broken-bond configuration with the trigonal C3v symmetry. Park and Chadi have calculated that at ambient pressure the binding energy of the In-DX states is negative (20.04 eV). Thus, they are expected to be unstable with respect to the shallow states. With an increasing pressure the binding enlarges and the DX states become the ground state of the In donors [13]. Recently, Kakrzewski et al. have observed a defect E 0:45 eV at ambient pressure whose concentration was established to be proportional to the concentration of the In donors introduced into CdTe. Also, they found that the hydrostatic pressure significantly enhanced the DLTS signal. Both facts were explained as a very strong statement for attributing them to the isolated donor atoms which, as a result of electron capture, undergo a transition to the broken-bond configuration and, thus, form the DX states [12]. As a consequence of the manifestation in the CdTe–In films of the PPC effect and the detected trap at 0.42 eV (a close value to those recently published by Kakrzewski et al. [12]), we can suppose that this trap forms the DX centers and they are responsible for the PPC behavior in the CdTe– In films grown by co-sputtering of a CdTe–In–Cd target. We have ruled out that the possibility of the presence of grain boundaries in these polycrystalline samples may play an important role in the PPC effect observed here, because from all the polycrystalline films, only when the In atoms are substantially incorporated to the CdTe films in samples M-2 and M-3, the PPC is observed. The manifestation of DX at ambient pressure in CdTe–In, which from theoretical calculations should be unstable with respect to the shallow levels, implies a reconsideration out of the scope of this paper, but we suppose that the deviation from the ideal crystal in the samples used for this work may promote such divergence. By this reason, we consider that the results obtained by Park and Chadi for a CdTe–In ideal crystal, is not suitable for our studied samples and neither for those researched by Kakrzewski et al. In order to verify the PPC mechanism we have studied its build-up and decay kinetics. It was found that in the M-2 sample the PPC build-up and decay kinetics measured are
Z. Rivera-Alvarez et al. / Solid State Communications 113 (2000) 621–625
identical to those of the DX centers in AlGaAs. The build-up current of PPC caused by the DX centers in AlGaAs has been experimentally observed and theoretically formulated [22,23]: IPPC t 2 Id Imax 2 Id 1 2 exp 2at;
1
where Id is the initial dark current, Imax the saturation level, and a a constant. While the decay of PPC related with the DX centers in AlGaAs follows a stretched-exponential function [22,23] b
IPPC t 2 Id Io 2 Id exp2 t=t ;
b , 1;
2
where Io is the current build-up level at the instant when the light excitation has finished, t the PPC decay time constant, and b the decay exponent. Fig. 4 displays the build-up and decay kinetics of PPC in the sample M-2 measured at T 230 K: The solid curve is the least squares fit of data with Eq. (1) for the PPC build-up and with Eq. (2) for the PPC decay with the fitted value of b 0:31: The PPC decay time constants, t, are very long, especially at low temperatures. At T . 230 K; t is thermally activated as shown in the inset of Fig. 4, from which we obtain an energy barrier for the capture of electrons, Ec 0:23 eV: This large energy barrier impedes the decay of photoexcited electrons. So that, by formulating the PPC build-up and decay kinetics, we can associate the PPC effect in the M-2 and M-3 samples to the photoionization of the deep level (DX like centers) localized at ET 0:42 eV: 4. Conclusions In this work, we have studied the nature and behavior of the PPC in the CdTe–In films grown by co-sputtering of the CdTe–In–Cd targets. It was found that only when the In atoms are substantially incorporated to the CdTe films, the PPC is observed with a Ta , 270 K: At the same time, we have investigated the trapping centers in the CdTe films by using the TSC technique. Two localized deep levels were determined. One of them, with the activation energy (ET) of 0.75 eV, has been assigned to Cd vacancies. The second deep level, with ET 0:42 eV; has been ascribed as a direct evidence of the DX centers that are formed by the Cd vacancies and the In donors complexes. Also, by formulating the PPC build-up and decay kinetics, we can associate the PPC effect in the M-2 and M-3 samples to the photoionization of the deep level (DX like centers), localized at 0.42 eV. These centers are responsible for the PPC effect, with an energy barrier of 0.23 eV for the capture of electrons. To our knowledge, this is the first report of the existence of the DX centers in the CdTe–In polycrystalline films up to date.
625
Acknowledgements The author is pleased to acknowledge helpful criticism of the manuscript by A. Guille´n. This work was partially supported by CONACyT (Mexico) under the Grants 4416A. One of the authors (L.H.) gratefully acknowledges the financial support of TWAS.
References [1] T. Thio, R.A. Linke, G.E. Devlin, J.W. Bennett, D.J. Chadi, Appl. Phys. Lett. 65 (1994) 1802. [2] R.A. Linke, T. Thio, D.J. Chadi, G.E. Devlin, Appl. Phys. Lett. 65 (1994) 16. [3] T. Thio, J.W. Bennett, D.J. Chadi, R.A. Linke, P. Becla, J. Cryst. Growth 159 (1996) 345. [4] D.V. Lang, R.A. Logan, M. Joros, Phys. Rev. B 19 (1979) 1015. [5] P.M. Mooney, J. Appl. Phys. 67 (1990) R1. [6] H.J. Queisser, D.E. Theodorou, Phys. Rev. B 33 (1986) 4027. [7] J.Y. Lin, H.X. Jiang, Phys. Rev. B 41 (1990) 5178. [8] H.X. Jiang, A. Dissanayake, J.Y. Lin, Phys. Rev. B 45 (1992) 4520. [9] D.J. Chadi, K.J. Chang, Phys. Rev. Lett. 61 (1988) 873. [10] C.H. Park, D.J. Chadi, Appl. Phys. Lett. 66 (1995) 3167. [11] D. Wasik, J. Przybytek, M. Baj, G. Karczewski, T. Wojtowicz, J. Kossut, J. Cryst. Growth 159 (1996) 392. [12] A.K. Kakrzewski, L. Dobaczewski, T. Wojtowicz, J. Kossut, G. Karczewski, Proceedings of the 23rd ICPS, Berlin, July 1996, World Scientific, Singapore, 1997, p. 3005. [13] C.H. Park, D.J. Chadi, Phys. Rev. B 52 (1995) 11 884. [14] F. Bassani, S. Tatarenko, K. Saminadayar, J. Bleuse, N. Magnea, J.L. Pautrat, Appl. Phys. Lett. 58 (1991) 2651. [15] R. Ramı´rez-Bon, R. Nun˜ez-Lo´pez, F.J. Espinoza-Beltra´n, O. Zelaya-Angel, J. Gonza´lez-Herna´ndez, J. Phys. Chem. Solids 58 (1997) 807. [16] M. Becerril, O. Zelaya-Angel, R. Ramı´rez-Bon, F.J. EspinozaBeltra´n, J. Gonza´lez-Herna´ndez, Appl. Phys. Lett. 70 (1997) 452. [17] R. Ramı´rez-Bon, F.J. Espinoza-Beltra´n, O. Vigil, O. ZelayaAngel, F. Sa´nchez-Sinencio, J.G. Mendoza-Alvarez, D. Stolik, J. Appl. Phys. 78 (1995) 3908. [18] C.P. Ye, J. Chen, J. Appl. Phys. 67 (1990) 2547. [19] U. Pal, P. Fernandez, J. Piqueras, N.V. Suchinski, E. Dieguez, J. Appl. Phys. 78 (1995) 1992. [20] G.C. Taylor, E. Lilley, J. Phys. D. 11 (1978) 567. [21] T. Suski, P. Wisniewski, E. Litwin-Staszewska, D. Wasik, J. Przybytek, M. Baj, G. Karczewski, T. Wojtowicz, A. Zakrewski, J. Kossut, J. Cryst. Growth. 159 (1996) 380. [22] A. Dissanayake, M. Elahi, H.X. Jiang, Y.Y. Lin, Phys. Rev. B 45 (1992) 13996. [23] J.Y. Lin, A. Dissanayake, G. Brown, H.X. Jiang, Phys. Rev. B 42 (1990) 5855.
Solid State Communications 113 (2000) 621–625 www.elsevier.com/locate/ssc
DX centers and persistent photoconductivity in CdTe–In films Z. Rivera-Alvarez a, L. Herna´ndez 1,a, M. Becerril a, A. Picos-Vega a, O. Zelaya-Angel a,*, R. Ramı´rez-Bon b, J.R. Vargas-Garcı´a c a
b
Departamento de Fı´sica, CINVESTAV-IPN, Apdo Postal 14-740, 07000 DF Mexico, Mexico Centro de Investigacio´n en Fı´sica, Universidad de Sonora, Hermosillo, Sonora, Apdo Postal 5-88, 8319 Hermosillo, Son., Mexico c Departamento de Ingenierı´a Metalu´rgica, I.P.N., Apdo Postal 75-874, 07300 D.F. Mexico, Mexico Received 21 December 1998; received in revised form 2 July 1999; accepted 24 November 1999 by S.G. Louie
Abstract In this work, we study the nature and behavior of the persistent photoconductivity (PPC) in CdTe–In films grown by cosputtering of CdTe–In–Cd targets. It was found that only when In atoms are substantially incorporated into CdTe films, the persistent photoconductivity is observed with a quenching temperature of about 270 K. We have also investigated the trapping centers in the CdTe films by using the thermally stimulated conductivity technique. Two localized deep levels were determined. One of them, with an activation energy of 0.42 eV, has been ascribed as a direct evidence of DX centers that are formed by Cd vacancies and In donors complexes. By formulating the PPC build-up and decay kinetics, we have associated the PPC effect in our films to the photoionization of this deep level (DX like centers). Up to date, the existence of DX centers in CdTe–In polycrystalline films have not been previously reported. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Thin films; C. Impurities in semiconductors; D. Electronic states (localized); D. Photoconductivity and photovoltaics
1. Introduction Recently, the persistent photoconductivity (PPC), photoconductivity that persists after the removal of the photoexcitation, has been used to optically write metallic patterns in an insulating matrix with high contrast and it has been estimated that features can be written with around 100 nm of resolution [1]. The optically written patterns continue under the same circumstances for which the photoconductivity itself is persistent, and can be canceled entirely by heating above the quenching temperature (Ta) of the PPC. The PPC can be a very attractive phenomenon in the applications of submicron device fabrication, optical switching and high-density data-storage [2,3]. The physical origin of PPC in many semiconductors has been an important subject in the last decade and several mechanisms have been suggested to describe the cause
* Corresponding author. Tel.: 152-5-747-3800; fax: 152-5-7477096. E-mail address: [email protected] (O. Zelaya-Angel). 1 Also at Facultad de Fı´sica, Universidad de La Habana, 10400 La Habana, Cuba.
[4–8]. One of these mechanisms is connected with some deep donors, known as DX centers [4,5]. Chadi and Chang [9] have established that the DX center is negatively charged and a highly localized defect center resulting from a large lattice distortion. In the ground state, the DX center is a deep donor that is negatively charged and the photoexcitation changes the DX into a shallow state. This excited state is metastable, hence a barrier to recombination is originated by the structural relaxation required to return to the DX state. At low temperatures the photogenerated electrons remain in the conduction band indefinitely and the PPC is observed. There is now substantial evidence supporting that the conversion of the shallow hydrogenic donor into a deep relaxed one can be achieved by adding In to CdTe originating DX centers [10–12]. Lately, by first-principles pseudopotential calculations, Park and Chadi have shown that the metastable behavior of the In donors in CdTe and the creation of the DX centers can be explained by the broken-bond model [13]. The goal of this paper is to study the nature and behavior of the PPC in CdTe–In films grown by co-sputtering of CdTe–In–Cd targets. It was found that only when In atoms are substantially incorporated into the CdTe films, the PPC is observed with Ta , 250 K: At the same time,
0038-1098/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00545-1
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Z. Rivera-Alvarez et al. / Solid State Communications 113 (2000) 621–625
Table 1 The Cd, Te, and In content in each sample that were measured using an AES system. Also the thickness is displayed Sample
% In-atoms
% Te-atoms
% Cd-atoms
Thickness (nm)
M-0 M-1 M-2 M-3 B-8 I-10
0.0 1.0 3.5 6.5 0.0 2.5
51.8 50.6 51.2 51.4 49.8 48.8
48.2 48.4 45.3 42.1 50.2 48.7
111.0 85.9 88.3 99.7 80.0 95.4
we have investigated the trapping centers in the CdTe films by using the thermally stimulated conductivity (TSC) technique. Two localized deep levels were determined. One of them, with an activation energy of ET 0:75 eV; has been assigned to Cd-vacancies. The second trap, with ET 0:42 eV; has been ascribed as a direct evidence of the DX centers that are formed by Cd vacancies and In donors complexes. Also, by formulating the PPC buildup and decay kinetics, we can associate the PPC effect in our films to the photoionization of the deep level (DX like centers), localized at 0.42 eV. These centers are responsible for the PPC effect with an energy barrier of 0.23 eV for the capture of electrons. Up to date, the existence of the DX centers in the CdTe–In polycrystalline films have not been previously reported.
Fig. 1. The TSC spectra of the M-0 and M-1 samples. The inset shows the Arrhenius plots of log ITSC vs 1000/T.
2. Experimental details A series of around 100 nm thick, n-type In-doped CdTe polycrystalline films has been prepared by co-sputtering of a CdTe–In–Cd target. The CdTe–In films were grown on 7059 Corning glass slides at room temperature using a diode rf sputtering system with a water cooled cathode. The rf power was 30 W and the distance between the target and the substrate was 4 cm. The Cd and In atoms were introduced in the samples by co-sputtering from a CdTe target with pieces of elemental indium and cadmium glued onto it to produce n-type conductivity films. The Cd on the target was added in order to provide the excess of cadmium needed to prevent Cd vacancies during the growth of CdTe films [14]. The CdTe (99.999% pure) target had an area of 4.92 cm 2. The indium area on the target was varied to cover 0, 0.25, and 2.0 and 4% of the total target area. The samples with different indium concentrations are denoted by M-0 to M-3, respectively. In this case, the area of Cd on the CdTe target was the same for all samples and it covered 1% of the total target area. In a subsequent experiment, we changed the area of Cd to cover 4% of the total target area without indium on it (sample B-8). Table 1 shows the Cd, Te, and In contents in the films that were measured using an Auger electron spectroscopy (AES) system. The crystalline structure of the films was determined by an X-ray diffractometer SIEMENS D5000. All the spectra display only the characteristic diffraction peaks of the cubic phase of CdTe and we have not observed peaks relative to CdIn2Te4 or In–Te compounds [15]. The CdTe–In polycrystalline films were grown at low In concentrations because an increase in the In concentration produces disorder and deformation of the CdTe lattice yielding to a crystalline structure change from the cubic to the hexagonal CdTe phase [16]. The TSC measurements were made on samples having a distance of 1 cm between coplanar ohmic contacts. Additional details of the contact fabrication and thermal annealing can be found elsewhere [17]. A liquid-nitrogen cryostat was used to vary the sample temperature from 77 to 420 K. The PPC and TSC data were taken in such a way that the samples were always heated up to 420 K and relaxed to equilibrium before each measurement, then cooled down
Z. Rivera-Alvarez et al. / Solid State Communications 113 (2000) 621–625
623
Fig. 2. The TSC spectra of the M-2 and M-3 samples. The inset shows the Arrhenius plots of log ITSC vs 1000/T.
in darkness to 77 K. This ensures that each set of data have the same initial conditions. The PPC and TSC data are obtained by illuminating the sample at low temperature (80 K) with a white light at exactly the same intensity. The contacts were shielded against light by masks. A constant bias voltage between 40 and 100 V was applied to the sample as a function of the film resistivity and the current was measured by a programmable electrometer KEITHLEY model 617. When the light was turned off, the temperature was raised at a constant heating rate of 0.15 K/s with a control temperature from a DLTS spectrometer DL-4600, BIO-RAD.
3. Results and discussion Fig. 1 shows the TSC data for the M-0 and M-1 samples. Only one peak is observed, centered at T 255 K in the data from each sample. The spectra shape is independent of the illumination intensity and the bias voltage. No apparent shifts were observed in the peak position indicating firstorder kinetics. For the evaluation of the trapping level activation energy, the well-known initial rise method was used. This analysis is independent with regard to the kinetics of process and it applies to the low-temperature tail of the TSC curve. In this region the number of trapped electrons change only a small amount with the temperature and can be regarded as constant. Then, the intensity of the TSC can be expressed by ITSC , exp 2ET =KT; where K is the Boltzmann’s constant. The plot of the log ITSC as a function
Fig. 3. PPC as a function of temperature for the M-2 and M-3 samples. Cooling in the dark (open circles), and warming in the dark after illuminated at T 80 K (solid circles). The semicircles depict the noise level.
of inverse absolute temperature is shown in the inset of Fig. 1, which displays slopes with an activation energy of the trap of 0.75 eV for samples M-0 and M-1. From the equal values of ET, we can conclude that the nature of the trap is the same for both the samples. In order to identify the origin of the trap, we have researched the TSC behavior in the sample B-8 (see Table 1), which was grown with the area of the Cd on the CdTe target to cover 4% of the total target area, without indium on the target. In this film, the TSC signal was not detected indicating that the nature of the trap has its origin in Cd-vacancies. This deep level has been reported in the literature for CdTe grown with different methods and dopants and has been observed both with electrical and optical techniques. This trap has been widely attributed to an acceptor complex involving the native VCd 22 defect and impurity [18,19]. When the number of indium atoms are substantially incorporated to CdTe films, i.e. the indium area on the target covers 2.0 or 4% of the total target area, the measured TSC spectra differ in the peak position and in the region of the low-temperature tail. Fig. 2 displays the TSC spectra for the M-2 and M-3 samples showing the above-mentioned properties. From the Arrhenius plot in the inset of Fig. 2, the activation energy of the trap is 0.42 eV for the two samples. The thermal cleaning method of Taylor and Lilley [20] was applied in both samples but the trap with ET , 0:75 eV was
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Fig. 4. Buildup and decay kinetics of the PPC related with the M-2 sample at T 230 K: The solid curve is the least squares fit of experimental data with Eq. (1) for the buildup part and with Eq. (2) for the decay part. The inset shows the Arrhenius plot of logt vs 1000/T.
not observed. From the AES measurement shown in Table 1, we can predict that the incorporated In atoms partially fill the Cd vacancies, leading to a reduction in their concentration and, therefore, the non-appearance of this trap. In order to clarify the nature of the traps in samples M-2 and M-3, we have also studied the TSC behavior in sample I-10. This sample was grown with the area of In on the CdTe target to cover 4% of the total target area and with an excess of cadmium in order to impede the Cd vacancies. The Cd, Te and In contents of this film, were measured using AES, and are shown in the Table 1. The resistivity of sample I-10 is 5 × 101 V-cm; three orders lower than sample M-3, making it evident that at high contents of In, without an excess of Cd, result in a formation of lattice defects involving vacancies. Also, if the defects are highly mobile, they may migrate to the impurity atoms and create deep compensating complexes, inducing a decrease of the crystalline quality and, change the type of electrical activity of the impurity. Again, the TSC signal was not detected in the I-10 films, this fact supports the microscopic nature of the traps, with E T 0:42 eV in samples M-2 and M-3, which is with great probability correlated with the Cd vacancies–In donors complexes. Fig. 3 shows the temperature-dependent current in the M2 and M-3 samples. The dark current is measured on cooling from T 400 K (open circles). At T 80 K the films are
illuminated then the current rises until saturation. When the light was eliminated, the current remained at that value indefinitely: the photoconductivity becomes persistent. Subsequently, the films were heated in the dark and the current was then measured (solid circle). The warming curves reach the cooling dark values around Ta ù 270 K: Despite the fact that the exact origin of the In center in CdTe is unknown, Suski et al [21] have supposed that it is a substitutional donor at a cation site which binds one or two (negative-U case) electrons creating the DX centers. The trapping level with an activation energy 0.45 eV in CdTe– In grown by molecular beam epitaxy has been recently assigned as a DX center [12] corroborating the Park and Chadi model [13], in which the DX centers are formed by transitions of isolated In atoms from substitutional sites in the CdTe lattice to the broken-bond configuration with the trigonal C3v symmetry. Park and Chadi have calculated that at ambient pressure the binding energy of the In-DX states is negative (20.04 eV). Thus, they are expected to be unstable with respect to the shallow states. With an increasing pressure the binding enlarges and the DX states become the ground state of the In donors [13]. Recently, Kakrzewski et al. have observed a defect E 0:45 eV at ambient pressure whose concentration was established to be proportional to the concentration of the In donors introduced into CdTe. Also, they found that the hydrostatic pressure significantly enhanced the DLTS signal. Both facts were explained as a very strong statement for attributing them to the isolated donor atoms which, as a result of electron capture, undergo a transition to the broken-bond configuration and, thus, form the DX states [12]. As a consequence of the manifestation in the CdTe–In films of the PPC effect and the detected trap at 0.42 eV (a close value to those recently published by Kakrzewski et al. [12]), we can suppose that this trap forms the DX centers and they are responsible for the PPC behavior in the CdTe– In films grown by co-sputtering of a CdTe–In–Cd target. We have ruled out that the possibility of the presence of grain boundaries in these polycrystalline samples may play an important role in the PPC effect observed here, because from all the polycrystalline films, only when the In atoms are substantially incorporated to the CdTe films in samples M-2 and M-3, the PPC is observed. The manifestation of DX at ambient pressure in CdTe–In, which from theoretical calculations should be unstable with respect to the shallow levels, implies a reconsideration out of the scope of this paper, but we suppose that the deviation from the ideal crystal in the samples used for this work may promote such divergence. By this reason, we consider that the results obtained by Park and Chadi for a CdTe–In ideal crystal, is not suitable for our studied samples and neither for those researched by Kakrzewski et al. In order to verify the PPC mechanism we have studied its build-up and decay kinetics. It was found that in the M-2 sample the PPC build-up and decay kinetics measured are
Z. Rivera-Alvarez et al. / Solid State Communications 113 (2000) 621–625
identical to those of the DX centers in AlGaAs. The build-up current of PPC caused by the DX centers in AlGaAs has been experimentally observed and theoretically formulated [22,23]: IPPC t 2 Id Imax 2 Id 1 2 exp 2at;
1
where Id is the initial dark current, Imax the saturation level, and a a constant. While the decay of PPC related with the DX centers in AlGaAs follows a stretched-exponential function [22,23] b
IPPC t 2 Id Io 2 Id exp2 t=t ;
b , 1;
2
where Io is the current build-up level at the instant when the light excitation has finished, t the PPC decay time constant, and b the decay exponent. Fig. 4 displays the build-up and decay kinetics of PPC in the sample M-2 measured at T 230 K: The solid curve is the least squares fit of data with Eq. (1) for the PPC build-up and with Eq. (2) for the PPC decay with the fitted value of b 0:31: The PPC decay time constants, t, are very long, especially at low temperatures. At T . 230 K; t is thermally activated as shown in the inset of Fig. 4, from which we obtain an energy barrier for the capture of electrons, Ec 0:23 eV: This large energy barrier impedes the decay of photoexcited electrons. So that, by formulating the PPC build-up and decay kinetics, we can associate the PPC effect in the M-2 and M-3 samples to the photoionization of the deep level (DX like centers) localized at ET 0:42 eV: 4. Conclusions In this work, we have studied the nature and behavior of the PPC in the CdTe–In films grown by co-sputtering of the CdTe–In–Cd targets. It was found that only when the In atoms are substantially incorporated to the CdTe films, the PPC is observed with a Ta , 270 K: At the same time, we have investigated the trapping centers in the CdTe films by using the TSC technique. Two localized deep levels were determined. One of them, with the activation energy (ET) of 0.75 eV, has been assigned to Cd vacancies. The second deep level, with ET 0:42 eV; has been ascribed as a direct evidence of the DX centers that are formed by the Cd vacancies and the In donors complexes. Also, by formulating the PPC build-up and decay kinetics, we can associate the PPC effect in the M-2 and M-3 samples to the photoionization of the deep level (DX like centers), localized at 0.42 eV. These centers are responsible for the PPC effect, with an energy barrier of 0.23 eV for the capture of electrons. To our knowledge, this is the first report of the existence of the DX centers in the CdTe–In polycrystalline films up to date.
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Acknowledgements The author is pleased to acknowledge helpful criticism of the manuscript by A. Guille´n. This work was partially supported by CONACyT (Mexico) under the Grants 4416A. One of the authors (L.H.) gratefully acknowledges the financial support of TWAS.
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