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Electric field effect on the electron emission from Te-DX in AlxGa1−xAs
Materials Science and Engineering C 26 (2006) 580 – 582 www.elsevier.com/locate/msec
Short communication
Electric field effect on the electron emission from Te-DX in Alx Ga1x As L. Bouzrara a, R. Ajjel a,*, H. Mejri b, M.A. Zaidi a, H. Maaref a a
Laboratoire de Physique des Semiconducteurs et des Composants Electroniques, Faculte´ des Sciences, 5019 Monastir, Tunisia b Ecole Pre´paratoire aux Acade´mies Militaires, Avenue Mare´chal Tito 4029 Sousse, Tunisia Available online 14 November 2005
Abstract The present work is aimed to investigate the electron emission from DX centers in tellurium-doped Alx Ga1x As with aluminium composition x = 0.40 using deep-level transient spectroscopy. Evidence that Te-DX exhibits an alloy splitting is shown from the temperature-dependent emission at a given reverse bias. The Poole – Frenkel effect was evoked to explain the electric field-enhanced emission. A discrepancy to this model is observed from experimental results. An attempt to explain this discrepancy as due mainly to the short-range character of the Te-DX-related potential. D 2005 Elsevier B.V. All rights reserved.
1. Introduction Deep donor levels, the so-called DX centers, have been observed in many compound semiconductors. In Alx Ga1x As alloys, the relatively stability of DX centers strongly depends on aluminium composition [1,2]. For x larger than 0.22, they are electrically active with a density nearly equal to the donor impurity concentration [3]. Deep-level transient spectroscopy (DLTS) investigations [4,5] and Hall measurements [6 –9] have evidenced the multiconfigurate character of the DX center. In the DLTS technique, filled traps are placed in the space charge region of a p+n junction or Schottky barrier and the emission kinetics are monitored through the change of the space-charge capacitance. This emission can be sensitive to the electric field present in the space-charge region [10]. The effect of an electric field on the emission of carriers from deep defects in semiconductors has been extensively studied. It was revealed that the field effect on the emission can be used as a probe to distinguish between built-in potentials based on their long-range behavior [11]. Models have been proposed to explain the field-enhanced emission but are limited to Coulombic potentials. For a deep trap, there are three mechanisms of emission in an electric field: (i) the Poole – Frenkel effect proper to the Coulombic potential, (ii) pure tunnelling and (iii) phonon-assisted tunnelling through a higher energy level. For electric fields not high enough, the pure
tunnelling can be neglected, explaining why only the first and the third emission processes are in general considered [12,13]. In the present work, we report DLTS measurements performed on an liquid phase epitaxy (LPE) grown Al0.4Ga0.6As:Te. An alloy splitting of Te-DX has been observed at a given reverse bias. In this paper, a study of electric field effect on the electron emission from Te-DX centers is also presented. 2. Results and discussion 2.1. Overview of the analysis method The method used to analyze DLTS data is based on a graphical resolution of the equation: hð e n Þ ¼ g ð e n ; T Þ with en en 3en hðen Þ ¼ exp exp exp 4f 2f 4f en þ exp f g ð en ; T Þ ¼
Rmax ðT Þ en 2f
and * Corresponding author. E-mail address: [email protected] (R. Ajjel). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.10.002
Rmax ðT Þ ¼ 0:152DC0 ðT Þ
L. Bouzrara et al. / Materials Science and Engineering C 26 (2006) 580 – 582
2.2. Field dependence of the emission
0,20
∆ Εi (eV)
DC 0(T) is taken equal to the amplitude at the temperature T of a normalized DLTS spectrum, f and e n are the frequency and the emission rate, respectively . The graphical resolution represents h(e n) and g(e n,T) as a function of e n for each temperature T. The crossing of the two plots gives e n for selected T. The obtained data, e n and T, allows to determine the thermal ionization energy E i from an Arrhenius plot log(T 2/e n) vs. 1000/T. This method is based on the double lock-in detection. It has an advantage to use only one DLTS spectrum. This would reduce errors resulting from discrepancy in emission conditions. The described method has been tested with success on a large number of traps.
0,15
0,10
0,05 480
The epitaxial set-up used for this investigation consists of an LPE-deposited Alx Ga1x As layer with x = 0.40. The sample is doped with tellurium. The net donor concentration, as deduced from capacitance – voltage measurements, is in the order of 1017cm 3. The DLTS experiments have been performed in the temperature range 10 –300 K using a double lock-in amplifier and a PAR 410 capacitance meter. The relevant spectra were obtained for an emission rate e n = 426 s 1, a filling pulse duration t p = 20 As, a reversed bias V 0 = 4 V and under different pulse amplitudes DV ranging from 1 to 3 V. Fig. 1 shows the electron emission as a function of temperature for Te-DX at different DV. To get these signatures, we have used the analysis method described above. A multiplicity in emission rates is observed in all of the Arrhenius plots. As indicated in the figure, each emission rate is assigned to an appropriate alloy-splitting state associated with Te in a relaxed microscopical configuration. Note that the exchange of electrons between Te-DX and the conduction band is assisted
8
ln (T2/en)
7
6
0 Al-Te
5
4
1 Al-Te 2 Al-Te
3
581
3 Al-Te
500
520 1/2
F
540
560
1/2
(V
580
600
620
-1/2
cm
)
Fig. 2. The decrease in ionization energy as a function of F 1/2 as deduced for (g) 0 Al – Te, (>) 1 Al – Te, ( ) 2 Al – Te, (†) 3 Al – Te and (r) 4 Al – Te.
˝
per cascade via intra-bound transitions into the trap and directly from the 0 Al –Te DX level to the band. For a pulse amplitude DV = 3 V, the activation energies as deduced from the Arrhenius plot are 0.185, 0.275, 0.326, 0.410 and 0.442 eV for 0 Al –, 1 Al– , 2 Al – , 3 Al – and 4 Al –Te, respectively. In Fig. 1, also reported is the electric field effect on the emission from Te-DX. As can be seen, an increase in DV increases the emission rates of the Te-related DX traps. The Poole –Frenkel effect can be used as a possible mechanism to be at the origin of the field enhanced emission. For a Coulombic potential, the decrease in the ionization energy DE i due to1 an applied electric 3 1 field F is given by [14] DEi ¼ q 2 ðe0 er pÞ 2 F 2 where q is the electronic charge and ( r is the relative dielectric constant. For the Al0.4 Ga0.6 As alloy being studied, the pre-factor of F 1/2 is in the order of 2.22 10 4 eV V 1/2 cm1/2. Fig. 2 shows the variation of DE i vs. F 1/2 for the Te-DX levels. A linear behavior is observed for all the emission plots, meaning that the Coulombic character of the potential is predominant at the long range. From experimental results, we deduced the prefactors 7.16, 5.63, 2.49, 3.92 and 3.85 in 10 4 eV V 1/2 cm1/2 for the 0 Al – , 1 Al– , 2 Al –, 3 Al – and 4 Al – Te atom arrangements. Compared to the Poole –Frenkel model, these pre-factors exhibit a discrepancy relative to the theoretical value. Such a discrepancy can be due to the fact that other possible processes such as phonon-assisted tunneling are not considered in analyzing the field dependence of the emission. This does not, however, exclude the localized character of the potential around Te-DX, which could be at the origin of this disagreement.
4 Al-Te
3. Summary
2 5,0
5,5
6,0
6,5
7,0
7,5
8,0
8,5
9,0
1000/ T (K-1) Fig. 1. Arrhenius plots for Te-DX centers in the sample obtained at different filling pulses: ( ) DV = 1 V, (>) DV = 1.5 V, (r) DV = 2 V, (‚) DV = 2.5 V and (g) DV = 3 V.
˝
We have investigated the electric field effect on the emission rates of the Te-DX traps in Alx Ga1x As. Two peculiar features were revealed: (i) the decrease in the ionization energy shows a linear trend as a function of the square root of the electric field;
582
L. Bouzrara et al. / Materials Science and Engineering C 26 (2006) 580 – 582
(ii) the pre-factors as deduced from experimental data show, however, a discrepancy from the Poole – Frenkel model. The linear behavior indicates that the potential of Te-DX at the long range is of Coulombic character. The disagreement observed is due most probably to the localized character of the Te-DX potential in the short range. References [1] R.E. Peale, Y. Mochizuku, H. Sun, G.D. Watkins, Phys. Rev., B 45 (1992) 5933. [2] A. Murai, Y. Oyama, J.-I. Nishizawa, J. Appl. Phys. 87 (2000) 223. [3] M. Zazoui, S.L. Feng, J.C. Bourgoin, Phys. Rev., B. 41 (1990) 8485. [4] T. Baba, M. Mizuta, T. Fugisawa, J. Yoshino, H. Kukimoto, Jpn. J. Appl. Phys. 28 (1989) L891.
[5] E. Colleja, F. Garcia, A. Gomez, E. Munoz, P.M. Mooney, T.N. Morgan, S.L. Wright, Appl. Phys. Lett. 56 (1990) 934. [6] G. Brunthaler, K. Koˆhler, Appl. Phys. Lett. 57 (1990) 2225. [7] F.R. Ouaja, H. Mejri, A. Selmi, P. Gibart, J. Appl. Phys. 82 (1997) 5509. [8] A. Triki, H. Mejri, F.R. Ouaja, A. Selmi, J. Phys. Sol. (B) 227 (2001) 541. [9] A. Triki, F.R. Ouaja, H. Mejri, A. Selmi, J. Appl. Phys. 93 (2003) 9769. [10] M.A. Zaidi, J.C. Bourgoin, H. Maaref, Semicond. Sci. Technol. 4 (1989) 739. [11] F.D. Auret, S.A. Goodman, W.E. Meyer, Semicond. Sci. Technol. 10 (1995) 1376. [12] N. Baber, H. Scheffler, A. Ostmann, T. Wolf, D. Bimberg, Phys. Rev., B 45 (1992) 4043. [13] S.D. Ganichev, E. Ziemann, W. Prettl, I.N. Yassievich, A.A. Istrtov, E.R. Weber, Phys. Rev., B 61 (2000) 10361. [14] R. Ajjel, M.A. Zaidi, S. Alaya, G. Bre´mond, G. Guillot, J.C. Bourgoin, Appl. Phys. Lett. 72 (1998) 302.
Short communication
Electric field effect on the electron emission from Te-DX in Alx Ga1x As L. Bouzrara a, R. Ajjel a,*, H. Mejri b, M.A. Zaidi a, H. Maaref a a
Laboratoire de Physique des Semiconducteurs et des Composants Electroniques, Faculte´ des Sciences, 5019 Monastir, Tunisia b Ecole Pre´paratoire aux Acade´mies Militaires, Avenue Mare´chal Tito 4029 Sousse, Tunisia Available online 14 November 2005
Abstract The present work is aimed to investigate the electron emission from DX centers in tellurium-doped Alx Ga1x As with aluminium composition x = 0.40 using deep-level transient spectroscopy. Evidence that Te-DX exhibits an alloy splitting is shown from the temperature-dependent emission at a given reverse bias. The Poole – Frenkel effect was evoked to explain the electric field-enhanced emission. A discrepancy to this model is observed from experimental results. An attempt to explain this discrepancy as due mainly to the short-range character of the Te-DX-related potential. D 2005 Elsevier B.V. All rights reserved.
1. Introduction Deep donor levels, the so-called DX centers, have been observed in many compound semiconductors. In Alx Ga1x As alloys, the relatively stability of DX centers strongly depends on aluminium composition [1,2]. For x larger than 0.22, they are electrically active with a density nearly equal to the donor impurity concentration [3]. Deep-level transient spectroscopy (DLTS) investigations [4,5] and Hall measurements [6 –9] have evidenced the multiconfigurate character of the DX center. In the DLTS technique, filled traps are placed in the space charge region of a p+n junction or Schottky barrier and the emission kinetics are monitored through the change of the space-charge capacitance. This emission can be sensitive to the electric field present in the space-charge region [10]. The effect of an electric field on the emission of carriers from deep defects in semiconductors has been extensively studied. It was revealed that the field effect on the emission can be used as a probe to distinguish between built-in potentials based on their long-range behavior [11]. Models have been proposed to explain the field-enhanced emission but are limited to Coulombic potentials. For a deep trap, there are three mechanisms of emission in an electric field: (i) the Poole – Frenkel effect proper to the Coulombic potential, (ii) pure tunnelling and (iii) phonon-assisted tunnelling through a higher energy level. For electric fields not high enough, the pure
tunnelling can be neglected, explaining why only the first and the third emission processes are in general considered [12,13]. In the present work, we report DLTS measurements performed on an liquid phase epitaxy (LPE) grown Al0.4Ga0.6As:Te. An alloy splitting of Te-DX has been observed at a given reverse bias. In this paper, a study of electric field effect on the electron emission from Te-DX centers is also presented. 2. Results and discussion 2.1. Overview of the analysis method The method used to analyze DLTS data is based on a graphical resolution of the equation: hð e n Þ ¼ g ð e n ; T Þ with en en 3en hðen Þ ¼ exp exp exp 4f 2f 4f en þ exp f g ð en ; T Þ ¼
Rmax ðT Þ en 2f
and * Corresponding author. E-mail address: [email protected] (R. Ajjel). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.10.002
Rmax ðT Þ ¼ 0:152DC0 ðT Þ
L. Bouzrara et al. / Materials Science and Engineering C 26 (2006) 580 – 582
2.2. Field dependence of the emission
0,20
∆ Εi (eV)
DC 0(T) is taken equal to the amplitude at the temperature T of a normalized DLTS spectrum, f and e n are the frequency and the emission rate, respectively . The graphical resolution represents h(e n) and g(e n,T) as a function of e n for each temperature T. The crossing of the two plots gives e n for selected T. The obtained data, e n and T, allows to determine the thermal ionization energy E i from an Arrhenius plot log(T 2/e n) vs. 1000/T. This method is based on the double lock-in detection. It has an advantage to use only one DLTS spectrum. This would reduce errors resulting from discrepancy in emission conditions. The described method has been tested with success on a large number of traps.
0,15
0,10
0,05 480
The epitaxial set-up used for this investigation consists of an LPE-deposited Alx Ga1x As layer with x = 0.40. The sample is doped with tellurium. The net donor concentration, as deduced from capacitance – voltage measurements, is in the order of 1017cm 3. The DLTS experiments have been performed in the temperature range 10 –300 K using a double lock-in amplifier and a PAR 410 capacitance meter. The relevant spectra were obtained for an emission rate e n = 426 s 1, a filling pulse duration t p = 20 As, a reversed bias V 0 = 4 V and under different pulse amplitudes DV ranging from 1 to 3 V. Fig. 1 shows the electron emission as a function of temperature for Te-DX at different DV. To get these signatures, we have used the analysis method described above. A multiplicity in emission rates is observed in all of the Arrhenius plots. As indicated in the figure, each emission rate is assigned to an appropriate alloy-splitting state associated with Te in a relaxed microscopical configuration. Note that the exchange of electrons between Te-DX and the conduction band is assisted
8
ln (T2/en)
7
6
0 Al-Te
5
4
1 Al-Te 2 Al-Te
3
581
3 Al-Te
500
520 1/2
F
540
560
1/2
(V
580
600
620
-1/2
cm
)
Fig. 2. The decrease in ionization energy as a function of F 1/2 as deduced for (g) 0 Al – Te, (>) 1 Al – Te, ( ) 2 Al – Te, (†) 3 Al – Te and (r) 4 Al – Te.
˝
per cascade via intra-bound transitions into the trap and directly from the 0 Al –Te DX level to the band. For a pulse amplitude DV = 3 V, the activation energies as deduced from the Arrhenius plot are 0.185, 0.275, 0.326, 0.410 and 0.442 eV for 0 Al –, 1 Al– , 2 Al – , 3 Al – and 4 Al –Te, respectively. In Fig. 1, also reported is the electric field effect on the emission from Te-DX. As can be seen, an increase in DV increases the emission rates of the Te-related DX traps. The Poole –Frenkel effect can be used as a possible mechanism to be at the origin of the field enhanced emission. For a Coulombic potential, the decrease in the ionization energy DE i due to1 an applied electric 3 1 field F is given by [14] DEi ¼ q 2 ðe0 er pÞ 2 F 2 where q is the electronic charge and ( r is the relative dielectric constant. For the Al0.4 Ga0.6 As alloy being studied, the pre-factor of F 1/2 is in the order of 2.22 10 4 eV V 1/2 cm1/2. Fig. 2 shows the variation of DE i vs. F 1/2 for the Te-DX levels. A linear behavior is observed for all the emission plots, meaning that the Coulombic character of the potential is predominant at the long range. From experimental results, we deduced the prefactors 7.16, 5.63, 2.49, 3.92 and 3.85 in 10 4 eV V 1/2 cm1/2 for the 0 Al – , 1 Al– , 2 Al –, 3 Al – and 4 Al – Te atom arrangements. Compared to the Poole –Frenkel model, these pre-factors exhibit a discrepancy relative to the theoretical value. Such a discrepancy can be due to the fact that other possible processes such as phonon-assisted tunneling are not considered in analyzing the field dependence of the emission. This does not, however, exclude the localized character of the potential around Te-DX, which could be at the origin of this disagreement.
4 Al-Te
3. Summary
2 5,0
5,5
6,0
6,5
7,0
7,5
8,0
8,5
9,0
1000/ T (K-1) Fig. 1. Arrhenius plots for Te-DX centers in the sample obtained at different filling pulses: ( ) DV = 1 V, (>) DV = 1.5 V, (r) DV = 2 V, (‚) DV = 2.5 V and (g) DV = 3 V.
˝
We have investigated the electric field effect on the emission rates of the Te-DX traps in Alx Ga1x As. Two peculiar features were revealed: (i) the decrease in the ionization energy shows a linear trend as a function of the square root of the electric field;
582
L. Bouzrara et al. / Materials Science and Engineering C 26 (2006) 580 – 582
(ii) the pre-factors as deduced from experimental data show, however, a discrepancy from the Poole – Frenkel model. The linear behavior indicates that the potential of Te-DX at the long range is of Coulombic character. The disagreement observed is due most probably to the localized character of the Te-DX potential in the short range. References [1] R.E. Peale, Y. Mochizuku, H. Sun, G.D. Watkins, Phys. Rev., B 45 (1992) 5933. [2] A. Murai, Y. Oyama, J.-I. Nishizawa, J. Appl. Phys. 87 (2000) 223. [3] M. Zazoui, S.L. Feng, J.C. Bourgoin, Phys. Rev., B. 41 (1990) 8485. [4] T. Baba, M. Mizuta, T. Fugisawa, J. Yoshino, H. Kukimoto, Jpn. J. Appl. Phys. 28 (1989) L891.
[5] E. Colleja, F. Garcia, A. Gomez, E. Munoz, P.M. Mooney, T.N. Morgan, S.L. Wright, Appl. Phys. Lett. 56 (1990) 934. [6] G. Brunthaler, K. Koˆhler, Appl. Phys. Lett. 57 (1990) 2225. [7] F.R. Ouaja, H. Mejri, A. Selmi, P. Gibart, J. Appl. Phys. 82 (1997) 5509. [8] A. Triki, H. Mejri, F.R. Ouaja, A. Selmi, J. Phys. Sol. (B) 227 (2001) 541. [9] A. Triki, F.R. Ouaja, H. Mejri, A. Selmi, J. Appl. Phys. 93 (2003) 9769. [10] M.A. Zaidi, J.C. Bourgoin, H. Maaref, Semicond. Sci. Technol. 4 (1989) 739. [11] F.D. Auret, S.A. Goodman, W.E. Meyer, Semicond. Sci. Technol. 10 (1995) 1376. [12] N. Baber, H. Scheffler, A. Ostmann, T. Wolf, D. Bimberg, Phys. Rev., B 45 (1992) 4043. [13] S.D. Ganichev, E. Ziemann, W. Prettl, I.N. Yassievich, A.A. Istrtov, E.R. Weber, Phys. Rev., B 61 (2000) 10361. [14] R. Ajjel, M.A. Zaidi, S. Alaya, G. Bre´mond, G. Guillot, J.C. Bourgoin, Appl. Phys. Lett. 72 (1998) 302.