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Mechanism of electroluminescence in the amorphous silicon-based erbium-doped structures
Materials Science and Engineering B81 (2001) 182– 184 www.elsevier.com/locate/mseb
Mechanism of electroluminescence in the amorphous silicon-based erbium-doped structures I.N. Yassievich *, M.S. Bresler, O.B. Gusev, P.E. Pak, K.D. Tsendin, E.I. Terukov A.F. Ioffe Physico-Technical Institute, 194021, St.Petersburg, Russia
Abstract We have studied electroluminescence (EL) in amorphous silicon-based erbium-doped structures at reverse bias in the temperature range 77–300 K. The intensity of electroluminescence at the wavelength of 1.54 mm corresponding to a radiative transition 4I13/2 4I15/2 in the internal 4f-shell of the erbium ion Er3 + is low at 77 K but sharply increases starting from 220 K and exhibits a maximum near the room temperature. Theoretical analysis and comparison with the experiment have shown that the excitation of erbium ions occurs by an Auger process, which involves the capture of conduction electrons by neutral dangling bonds (D0) defects located close to erbium ions. It is demonstrated that the stationary concentration of free electrons in the conduction band is kept by a reverse process being multiphonon tunnel ionization of erbium-induced donors and D−-centers by the applied electric field. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Erbium-doped amorphous silicon; Auger excitation; Electroluminescence
1. Introduction Recently in several works, an efficient photoluminescence (PL) [1–3] and electroluminescence (EL) [4] from erbium ions in amorphous hydrogenated silicon (aSi:HEr) was reported. We have proposed [5,6] that excitation of erbium ions in a-Si:HEr occurs due to an Auger-process, in which an electron from the conduction band is captured by a neutral defect of the dangling bond type D0, leading to the formation of the D− state. The energy released at this transition is transferred to an electron of the internal 4f-shell of the erbium ion exciting it from the ground 4I15/2 to the first excited 4I13/2 state (defect-related Auger excitation (DRAE)). In the case of EL, the capture of conduction electrons by D0-centers leads to the disappearance of free charges transmitting the current, therefore, a reverse process of ionization of deep centers should exist for high electric field. In the present work, we demonstrate that excitation of erbium ions in EL structures is done by the DRAE mechanism and the role of the reverse * Corresponding author. Tel.: +7-812-2479974; fax: + 7-8122471017. E-mail address: [email protected] (I.N. Yassievich).
process supplying free electrons in the conduction band is played by multiphonon tunnel ionization of erbiuminduced donors and D−-centers in presence of an electric field.
2. Experimental The structures studied were films of a-Si:HEr with a thickness of : 1 mm and a diameter of 1 mm deposited on a substrate from n-type single crystalline silicon of 300 mm thickness. The aluminum electrical contacts were sputter deposited on the amorphous silicon film and the substrate. A typical I–V characteristics has a conventional rectifying shape. At forward bias, (‘+’ at the upper aluminum contact, ‘−’ at the n-type crystalline silicon substrate) only EL of free excitons (u: 1.16 mm) from the substrate was observed at room temperature. At reverse bias, both erbium (u:1.54 mm) and defect (u: 1.34 mm) luminescence were detected. The temperature dependences of the EL intensity for different currents through the structure are shown in Fig. 1. The EL intensity is very low at liquid nitrogen temperatures but increases significantly while approaching room temperature. The voltage drop on the struc-
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 7 3 3 - 9
I.N. Yassie6ich et al. / Materials Science and Engineering B81 (2001) 182–184
ture measured in parallel with the EL measurements revealed a maximum at the temperature for which EL intensity starts to increase, then decreased on the rise of temperature and had a slight minimum at the temperature of EL maximum. The following experimental results were also obtained: (i) the erbium EL intensity depends linearly on the current through the structure j in the entire range of currents studied; (ii) for sufficiently high electric fields applied to the structure both the EL intensity IL and the current j approach asymptotically the exponential dependence on the electric field squared (Fig. 1).
3. Discussion The electroluminescent structures studied are mainly the structures of Al/a-Si:HEr/n+-c-Si/Al type, i.e. they have a Schottky barrier at the aluminum contact and an a-Si/c-Si heterojunction at the contact with the substrate. Our experimental results indicate a monopolar conduction in the structure. The current is transferred by holes in the case of forward bias and by electrons at reverse bias. Due to the high resistance of the amorphous layer, the potential drop concentrates on the bulk of it. When forward bias is applied to the structure, holes travel through the amorphous layer to the crystalline silicon substrate and we can observe luminescence of a free exciton from the n-type substrate (n: 1016 cm − 3). At reverse bias erbium EL at the wavelength of 1.54 mm and defect EL as a broad band are observed. No erbium EL in the structure with the n-type substrate is seen under forward bias demonstrating the absence of electron current in this case with no exciton luminescence from the substrate at reverse bias. Our results show that erbium ions are excited only by electrons. The position of the Fermi level mF determined from the temperature dependence of electrical conductivity indicates that in a large range of erbium concentrations
183
(1018 –1020 cm − 3), it is nearly independent of the concentration of erbium ions (mF : 0.5 –0.45 eV below the mobility edge). This means that the Fermi level is pinned at a special position in the amorphous silicon bandgap. It is known also that erbium-doped amorphous silicon has always n-type conduction. The doping of amorphous silicon is accompanied by formation of dangling bond defects (D-defects), the concentration of which increases with the concentration of dopant at low doping levels but tends to saturation at the value around 1018 cm − 3 at higher dopant concentrations. The concentration of D-defects as estimated from the defect absorption coefficient is ND : 1018 cm − 3. Since the concentration of erbium in our structures was higher by one order of magnitude, nearly all the D-centers captured an additional electron from the donor levels and were in D−-state. We assume also that, (i) the mechanism of excitation of erbium ions in the case of EL is the same as that for photoluminescence [5], i.e. an Auger-process of capture of free electrons from the conduction band by neutral dangling bonds (D0-centers) while the energy released in this process is transferred by Coulomb interaction to a 4f-electron of a nearby Er3 + ion (DRAE-process); (ii) the electric field applied to the structure concentrates on the amorphous silicon layer, whereas the voltage drop on the contact will be negligibly small. The validity of the latter assumption is based on a smallness of the characteristic field in the contact at no bias and the large resistance of the amorphous layer. To satisfy the condition IL 8 j, we should note that the only quantity which depends markedly on electric field is the concentration of conduction electrons n. To explain the experimentally observed dependence of EL and current through the structure on electric field the concentration n should be described by the formula n8exp(E 2/E 2c )
(1)
where, Ec is a characteristic electric field. The formula (1) can be obtained from the balance of capture and ionization for deep (donor) centers with taking into
Fig. 1. Left; temperature dependence of the intensity of erbium EL at 1.54 mm under reverse bias. The current through the structure: (1) 5 mA; (2) 10 mA; (3) 15 mA; (4) 20 mA. Right; the current through the structure (1) and the intensity of erbium EL at 1.54 mm (2) versus electric field squared. The measurements are taken at T= 300 K.
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account the thermally activated tunneling of electrons from deep centers in electric field [7]. The characteristic electric field Ec =(3m*'/~ 32q 2)1/2, where m* is effective mass, q the electron charge, ~2 is the tunneling time of a defect [7]. Since the dependence of both the current and the intensity of erbium luminescence on electric field is controlled mainly by the exponent entering the expression (1) these dependences are practically similar and the luminescence intensity is linear in electric current in agreement with the experiment. In the lowest approximation, we take into account only the exponential type dependence on the electric field and ignore the effect of the electric field on the electron mobility m and concentration of D0-centers N0D. The pinning of the Fermi level to the position of the donor state makes it possible for the electric field to influence strongly the concentration of free electrons with nearly no effect on the concentration of D0 and D−-centers. However, these can be changed by the temperature; the rise of temperature will lead to a redistribution of electrons between D-levels and donors and it is this redistribution which controls the temperature dependences of the EL intensity and resistance of the structure.
4. Conclusions In conclusion, we have studied EL of erbium-doped amorphous hydrogenated silicon in the temperature range 77–300 K. The intensity of erbium luminescence increased with the temperature rise and exhibited a maximum near room temperature. Excitation of erbium ions is determined by an Auger-process involving capture of conduction electrons by D0-centers with the
.
energy transfer to 4f-electrons of the erbium ion due to Coulomb interaction. The stationary state is kept by multiphonon tunnel ionization of negatively charged dangling bonds (D−-centers) and donors induced by introduction of erbium ions into amorphous silicon. The theoretical model proposed describes quantitatively the entire set of experimental data.
Acknowledgements This work was partially supported by Russian Foundation of Basic Research (grants 98-02-18246 and 9902-18079), Intas grant 99-01872, Copernicus program 977048-SIER and NATO Linkage grant HTECH.LG 972032. Three of the authors (M.S.B., O.B.G., and I.N.Y) are grateful to T. Gregorkiewicz for useful discussion of the results.
References [1] M.S. Bresler, O.B. Gusev, V.Kh. Kudoyarova, Appl. Phys. Lett. 67 (1995) 3599. [2] J.H. Shin, R. Serna, G.N. van den Hoven, Appl. Phys. Lett. 68 (1996) 997. [3] A.R. Zanatta, Z.A. Nunes, L.R. Tessler, Appl. Phys. Lett. 70 (1997) 511. [4] O.B. Gusev, A.N. Kuznetsov, E.I. Terukov, M.S. Bresler, Appl. Phys. Lett. 70 (1997) 240. [5] W. Fuhs, I. Ulber, G. Weiser, M.S. Bresler, O.B. Gusev, Phys. Rev. B 56 (1997) 9545. [6] I.N. Yassievich, M.S. Bresler, O.B. Gusev, J. Phys. Condens. Matter 9 (1997) 9415. [7] S.D. Ganichev, I.N. Yassievich, W. Prettl, Semicond. Sci. Technol. 11 (1996) 679.
Mechanism of electroluminescence in the amorphous silicon-based erbium-doped structures I.N. Yassievich *, M.S. Bresler, O.B. Gusev, P.E. Pak, K.D. Tsendin, E.I. Terukov A.F. Ioffe Physico-Technical Institute, 194021, St.Petersburg, Russia
Abstract We have studied electroluminescence (EL) in amorphous silicon-based erbium-doped structures at reverse bias in the temperature range 77–300 K. The intensity of electroluminescence at the wavelength of 1.54 mm corresponding to a radiative transition 4I13/2 4I15/2 in the internal 4f-shell of the erbium ion Er3 + is low at 77 K but sharply increases starting from 220 K and exhibits a maximum near the room temperature. Theoretical analysis and comparison with the experiment have shown that the excitation of erbium ions occurs by an Auger process, which involves the capture of conduction electrons by neutral dangling bonds (D0) defects located close to erbium ions. It is demonstrated that the stationary concentration of free electrons in the conduction band is kept by a reverse process being multiphonon tunnel ionization of erbium-induced donors and D−-centers by the applied electric field. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Erbium-doped amorphous silicon; Auger excitation; Electroluminescence
1. Introduction Recently in several works, an efficient photoluminescence (PL) [1–3] and electroluminescence (EL) [4] from erbium ions in amorphous hydrogenated silicon (aSi:HEr) was reported. We have proposed [5,6] that excitation of erbium ions in a-Si:HEr occurs due to an Auger-process, in which an electron from the conduction band is captured by a neutral defect of the dangling bond type D0, leading to the formation of the D− state. The energy released at this transition is transferred to an electron of the internal 4f-shell of the erbium ion exciting it from the ground 4I15/2 to the first excited 4I13/2 state (defect-related Auger excitation (DRAE)). In the case of EL, the capture of conduction electrons by D0-centers leads to the disappearance of free charges transmitting the current, therefore, a reverse process of ionization of deep centers should exist for high electric field. In the present work, we demonstrate that excitation of erbium ions in EL structures is done by the DRAE mechanism and the role of the reverse * Corresponding author. Tel.: +7-812-2479974; fax: + 7-8122471017. E-mail address: [email protected] (I.N. Yassievich).
process supplying free electrons in the conduction band is played by multiphonon tunnel ionization of erbiuminduced donors and D−-centers in presence of an electric field.
2. Experimental The structures studied were films of a-Si:HEr with a thickness of : 1 mm and a diameter of 1 mm deposited on a substrate from n-type single crystalline silicon of 300 mm thickness. The aluminum electrical contacts were sputter deposited on the amorphous silicon film and the substrate. A typical I–V characteristics has a conventional rectifying shape. At forward bias, (‘+’ at the upper aluminum contact, ‘−’ at the n-type crystalline silicon substrate) only EL of free excitons (u: 1.16 mm) from the substrate was observed at room temperature. At reverse bias, both erbium (u:1.54 mm) and defect (u: 1.34 mm) luminescence were detected. The temperature dependences of the EL intensity for different currents through the structure are shown in Fig. 1. The EL intensity is very low at liquid nitrogen temperatures but increases significantly while approaching room temperature. The voltage drop on the struc-
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 7 3 3 - 9
I.N. Yassie6ich et al. / Materials Science and Engineering B81 (2001) 182–184
ture measured in parallel with the EL measurements revealed a maximum at the temperature for which EL intensity starts to increase, then decreased on the rise of temperature and had a slight minimum at the temperature of EL maximum. The following experimental results were also obtained: (i) the erbium EL intensity depends linearly on the current through the structure j in the entire range of currents studied; (ii) for sufficiently high electric fields applied to the structure both the EL intensity IL and the current j approach asymptotically the exponential dependence on the electric field squared (Fig. 1).
3. Discussion The electroluminescent structures studied are mainly the structures of Al/a-Si:HEr/n+-c-Si/Al type, i.e. they have a Schottky barrier at the aluminum contact and an a-Si/c-Si heterojunction at the contact with the substrate. Our experimental results indicate a monopolar conduction in the structure. The current is transferred by holes in the case of forward bias and by electrons at reverse bias. Due to the high resistance of the amorphous layer, the potential drop concentrates on the bulk of it. When forward bias is applied to the structure, holes travel through the amorphous layer to the crystalline silicon substrate and we can observe luminescence of a free exciton from the n-type substrate (n: 1016 cm − 3). At reverse bias erbium EL at the wavelength of 1.54 mm and defect EL as a broad band are observed. No erbium EL in the structure with the n-type substrate is seen under forward bias demonstrating the absence of electron current in this case with no exciton luminescence from the substrate at reverse bias. Our results show that erbium ions are excited only by electrons. The position of the Fermi level mF determined from the temperature dependence of electrical conductivity indicates that in a large range of erbium concentrations
183
(1018 –1020 cm − 3), it is nearly independent of the concentration of erbium ions (mF : 0.5 –0.45 eV below the mobility edge). This means that the Fermi level is pinned at a special position in the amorphous silicon bandgap. It is known also that erbium-doped amorphous silicon has always n-type conduction. The doping of amorphous silicon is accompanied by formation of dangling bond defects (D-defects), the concentration of which increases with the concentration of dopant at low doping levels but tends to saturation at the value around 1018 cm − 3 at higher dopant concentrations. The concentration of D-defects as estimated from the defect absorption coefficient is ND : 1018 cm − 3. Since the concentration of erbium in our structures was higher by one order of magnitude, nearly all the D-centers captured an additional electron from the donor levels and were in D−-state. We assume also that, (i) the mechanism of excitation of erbium ions in the case of EL is the same as that for photoluminescence [5], i.e. an Auger-process of capture of free electrons from the conduction band by neutral dangling bonds (D0-centers) while the energy released in this process is transferred by Coulomb interaction to a 4f-electron of a nearby Er3 + ion (DRAE-process); (ii) the electric field applied to the structure concentrates on the amorphous silicon layer, whereas the voltage drop on the contact will be negligibly small. The validity of the latter assumption is based on a smallness of the characteristic field in the contact at no bias and the large resistance of the amorphous layer. To satisfy the condition IL 8 j, we should note that the only quantity which depends markedly on electric field is the concentration of conduction electrons n. To explain the experimentally observed dependence of EL and current through the structure on electric field the concentration n should be described by the formula n8exp(E 2/E 2c )
(1)
where, Ec is a characteristic electric field. The formula (1) can be obtained from the balance of capture and ionization for deep (donor) centers with taking into
Fig. 1. Left; temperature dependence of the intensity of erbium EL at 1.54 mm under reverse bias. The current through the structure: (1) 5 mA; (2) 10 mA; (3) 15 mA; (4) 20 mA. Right; the current through the structure (1) and the intensity of erbium EL at 1.54 mm (2) versus electric field squared. The measurements are taken at T= 300 K.
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account the thermally activated tunneling of electrons from deep centers in electric field [7]. The characteristic electric field Ec =(3m*'/~ 32q 2)1/2, where m* is effective mass, q the electron charge, ~2 is the tunneling time of a defect [7]. Since the dependence of both the current and the intensity of erbium luminescence on electric field is controlled mainly by the exponent entering the expression (1) these dependences are practically similar and the luminescence intensity is linear in electric current in agreement with the experiment. In the lowest approximation, we take into account only the exponential type dependence on the electric field and ignore the effect of the electric field on the electron mobility m and concentration of D0-centers N0D. The pinning of the Fermi level to the position of the donor state makes it possible for the electric field to influence strongly the concentration of free electrons with nearly no effect on the concentration of D0 and D−-centers. However, these can be changed by the temperature; the rise of temperature will lead to a redistribution of electrons between D-levels and donors and it is this redistribution which controls the temperature dependences of the EL intensity and resistance of the structure.
4. Conclusions In conclusion, we have studied EL of erbium-doped amorphous hydrogenated silicon in the temperature range 77–300 K. The intensity of erbium luminescence increased with the temperature rise and exhibited a maximum near room temperature. Excitation of erbium ions is determined by an Auger-process involving capture of conduction electrons by D0-centers with the
.
energy transfer to 4f-electrons of the erbium ion due to Coulomb interaction. The stationary state is kept by multiphonon tunnel ionization of negatively charged dangling bonds (D−-centers) and donors induced by introduction of erbium ions into amorphous silicon. The theoretical model proposed describes quantitatively the entire set of experimental data.
Acknowledgements This work was partially supported by Russian Foundation of Basic Research (grants 98-02-18246 and 9902-18079), Intas grant 99-01872, Copernicus program 977048-SIER and NATO Linkage grant HTECH.LG 972032. Three of the authors (M.S.B., O.B.G., and I.N.Y) are grateful to T. Gregorkiewicz for useful discussion of the results.
References [1] M.S. Bresler, O.B. Gusev, V.Kh. Kudoyarova, Appl. Phys. Lett. 67 (1995) 3599. [2] J.H. Shin, R. Serna, G.N. van den Hoven, Appl. Phys. Lett. 68 (1996) 997. [3] A.R. Zanatta, Z.A. Nunes, L.R. Tessler, Appl. Phys. Lett. 70 (1997) 511. [4] O.B. Gusev, A.N. Kuznetsov, E.I. Terukov, M.S. Bresler, Appl. Phys. Lett. 70 (1997) 240. [5] W. Fuhs, I. Ulber, G. Weiser, M.S. Bresler, O.B. Gusev, Phys. Rev. B 56 (1997) 9545. [6] I.N. Yassievich, M.S. Bresler, O.B. Gusev, J. Phys. Condens. Matter 9 (1997) 9415. [7] S.D. Ganichev, I.N. Yassievich, W. Prettl, Semicond. Sci. Technol. 11 (1996) 679.