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Room-temperature electroluminescence of Er-doped hydrogenated amorphous silicon
Journal of Non-Crystalline Solids 227–230 Ž1998. 1164–1167
Room-temperature electroluminescence of Er-doped hydrogenated amorphous silicon Oleg Gusev a,) , Mikhail Bresler a , Alexey Kuznetsov a , Vera Kudoyarova a , Petr Pak a , Evgenii Terukov a , Konstantin Tsendin a , Irina Yassievich a , Walther Fuhs b, Gerhard Weiser c a
A F Ioffe Physico-Technical Institute, Politekhnicheskaya 26, 194021 St. Petersburg, Russian Federation b Hahn-Meitner Institut, Abteilung PhotoÕoltaik, Rudower Chaussee 5, D-12489 Berlin, Germany c Phillips-UniÕersitat Marburg, Fachbereich Physik, D-35032 Marburg, Germany
Abstract We have observed room-temperature erbium-ion electroluminescence in erbium-doped amorphous silicon. Electrical conduction through the structure is controlled by thermally activated ionization of deep Dy defects in an electric field and the reverse process of capture of mobile electrons by D 0 states. Defect-related Auger excitation ŽDRAE. is responsible for excitation of erbium ions located close to dangling-bond defects. Our experimental data are consistent with the mechanisms proposed. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Erbium-ion electroluminescence; Amorphous silicon; Dangling-bond defects; Defect-related Auger excitation; Ionization
1. Introduction The idea to fabricate light emitting diodes ŽLEDs. integrable into silicon electronics and emitting at the wavelength of 1.5 m m corresponding to the absorption minimum of optical fibers attracted attention to the luminescent properties of erbium-doped silicon w1a,1bx. Recently, we have demonstrated that the films of erbium-doped amorphous hydrogenated silicon Ža-Si:HŽEr.. prepared by cosputtering have room-temperature photoluminescence and the lifetime of erbium ions in this material is considerably shortened by the effect of amorphous-matrix disorder
)
Corresponding author.
w2x. We have reported also on the first observation of Er-luminescence in a diode structure based on aSi:HŽEr. w3x. Here we present more detailed results of these electroluminescence studies.
2. Experimental procedures Electroluminescent structures on the basis of erbium-doped amorphous hydrogenated silicon were prepared by cosputtering, applying the magnetron-assisted silane-decomposition Ž MASD. technique where mixtures of Ar and SiH 4 are used as the sputtering gas w4x. The films of 1 m m thickness were deposited on n-type silicon substrate with a donor concentration of 5 = 10 17 cmy3 . The erbium concen-
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 2 8 3 - X
O. GuseÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1164–1167
tration measured by Rutherford back scattering was about 10 20 cmy3 . The diameter of aluminium contacts to the amorphous film was approximately 1 mm. A typical I–V characteristic at 300 K had conventional diode shape with rectifying properties. Electroluminescence was measured in the temperature interval 77 to 300 K with square-shaped current pulses at 25 to 100 Hz and with a duty cycle of 1:2, both in forward and reverse bias. The emission was observed through the crystalline silicon substrate. The measurements were done in the constant-current regime. The forward direction corresponds to negative voltage at the n-type substrate.
1165
Fig. 2. Electroluminescence intensity as a function of electric field Žexperiment.. Dashed line indicates the asymptotic behavior of this dependence.
3. Experimental results Erbium luminescence corresponding to a line at 1.54 m m Žthe energy of 805 meV. is observed only for reverse bias. A typical spectrum measured at room temperature for reverse bias is shown in Fig. 1. Besides the line corresponding to the 4 I 13r2 ™4 I 15r2 transition in the 4f-shell of erbium ions a band with greater width at 0.8–0.9 eV can be seen. This band corresponds to defect-related luminescence of the a-Si:H matrix w5x. Erbium luminescence is nearly linear in excitation current. The dependence of erbium luminescence intensity on electric field is presented in Fig. 2. At larger electric fields it approaches an E 2-dependence. ŽDue to contact potential and resistance of the amorphous
material, the voltage applied to the structure decreases practically only across the amorphous layer.. This behaviour is unusual for amorphous silicon and indicates the occurrence of thermally activated ionization of deep defect Žor impurity. states well known in crystalline semiconductors. The temperature dependence of the erbium luminescence intensity for different currents through the structure is shown in Fig. 3. This result is also completely unexpected: whereas the intensity of photo- and electroluminescence usually decreases at higher temperatures Ži.e., suffers temperature quenching., in our case, it is much smaller at liquid nitrogen temperatures but increases while approaching room temperature.
Fig. 1. Electroluminescence spectrum for reverse bias at 300 K Žexperiment..
Fig. 3. Temperature dependence of erbium electroluminescence at constant current Žexperiment..
1166
O. GuseÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1164–1167
4. Discussion The introduction of erbium ions into the amorphous matrix is accompanied by the formation of a large number of dangling-bond defects with concentration ; 10 18 cmy3 as determined in our measurements of optical absorption w2x. Therefore, we assume that dangling-bond defects are associated with the erbium ions. According to our dark-conductivity measurements w3x, the Fermi level in erbium-doped amorphous silicon at room temperature is displaced upward from the middle of the gap, i.e., the doped samples are slightly n-type. In this case, defects connected with the dangling bonds occur in D 0 or Dy states. These levels are situated near the midgap and are separated by ; 0.2 eV. In the case of reverse bias, electrons are excited into the conduction band due to thermally activated ionization of Dy-defects in an electric field. While moving in the amorphous layer, these electrons excite erbium and defect-related luminescence by capture of electrons from the conduction band by D 0-defects w3x. The transition e q D 0 ™ Dy can occur by radiative or nonradiative Žmulti-phonon. process. In the case when dangling-bond states are correlated with erbium ions, a third recombination channel is opened, i.e., nonradiative transition accompanied by erbiumion excitation due to Coulomb interaction between the electron being captured to the D 0 center and the f-electron of the erbium-ion. This defect-related Auger excitation ŽDRAE. process is specially effective because of its nearly resonant character: the energy difference of radiative transition between e q D 0 and Dy states is close to the energy difference of 4 I 13r2 and 4 I 15r2 states of the 4f-shell. The excess energy of the transition is transferred to local phonons. In the steady state, we can write a balance equation for thermally activated ionization of Dy states Žthe dependence on electric field is given explicitly., and capture of electrons from the conduction band by D 0 defects 0 b i exp Ž E 2rEc2 . Ny D s cnND ,
Ž 1.
where b i and c are the ionization and capture coeffi0 y cients, Ny D , ND , and n are concentrations of D -de-
fects, D 0-defects, and electrons in the conduction band, E is the electric field applied to the structure, Ec is a characteristic electric field. Using the detailed balance condition to establish the correspondence between b i and c we arrive at the result n s n 0 exp Ž E 2rEc2 . ,
Ž 2.
which indicates that the concentration of electrons in the conduction band rises exponentially in high electric field Ž n 0 is the equilibrium concentration.. ŽThe increase of the concentration of the free electrons was detected directly from the measurements of the voltage drop across the structure. w3x. Now the intensity of erbium luminescence is I L s cA nND0
t tR
s cA n 0 exp Ž E 2rEc2 . ND0
t tR
,
Ž 3.
in agreement with the electric field dependence of erbium electroluminescence shown in Fig. 2. Ž cA is the contribution to the capture coefficient from the DRAE process, t and t R are total and radiative lifetimes of erbium ions in the excited state.. The characteristic field determined from the plot of Fig. 2 is Ec s 1.8 = 10 5 Vrcm. The temperature dependence of the intensity of erbium luminescence in the constant-current regime is I L s cA ND exp
ž
´yy z kT
/
j0
t
q m Ej t R
,
Ž 4.
where the electric field, Ej , applied to the structure at current, j0 , is a weaker function of temperature than the exponent, q is the electron charge, m is the mobility of electrons. From Fig. 3, we estimate the energy of the Fermi level counted from the level of the Dy-defect: ´yy z f 110 meV in reasonable agreement with the n-type property of the amorphous silicon layer. In this simple model, we have neglected the Žpower. temperature dependence of the electric field, Ej , and of the mobility of electrons above the mobility edge. Then, the temperature dependence of the erbium luminescence intensity is determined by population of D 0 states.
O. GuseÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1164–1167
5. Conclusions In conclusion, we have first observed room-temperature erbium-ion electroluminescence in amorphous-silicon-based semiconductor structure. The properties of the structure seem promising for further development of LEDs emitting at 1.5 m m and integrable into silicon electronics. The mechanism of electrical current conduction through the structure is described; it is controlled by thermally activated ionization of deep Dy-defects in high electric fields and the reverse process of capture of mobile electrons by D 0 states. Defect-related Auger excitation ŽDRAE. is responsible for excitation of erbium ions located close to dangling-bond defects. The whole set of our experimental data is consistent with the mechanisms proposed.
Acknowledgements This work was partially supported by Volkswagen Stiftung under the project Ir71 646, by INTAS Žthe
1167
project number 93-1816., and Russian Foundation for Basic Research ŽGrants No. 95-02-04163a and No. 96-02-16931a..
References w1ax G.S. Pomrenke, P.B. Klein, D.W. Langer ŽEds.., Rare Earth Doped Semiconductors, Vol. 301, Mat. Res. Soc. Symp., 1993. w1bx S. Coffa, A. Polman, R.N. Schwartz ŽEds.., Rare Earth Doped Semiconductors II, Vol. 422, Mat. Res. Soc. Symp., 1996. w2x M.S. Bresler, O.B. Gusev, V.Kh. Kudoyarova, A.N. Kuznetsov, P.E. Pak, E.I. Terukov, I.N. Yassievich, B.P. Zakharchenya, W. Fuhs, A. Sturm, Appl. Phys. Lett. 67 Ž1995. 3599. w3x O.B. Gusev, A.N. Kuznetsov, E.I. Terukov, M.S. Bresler, V.Kh. Kudoyarova, I.N. Yassievich, B.P. Zakharchenya, W. Fuhs, Appl. Phys. Lett. 70 Ž1997. 240. w4x V. Marakhonov, N. Rogachev, J. Ishkalov, J. Marakhonov, E. Terukov, V. Chelnokov, J. Non-Cryst. Solids 137–138 Ž1991. 817. w5x I. Ulber, R. Saleh, W. Fuhs, H. Mell, J. Non-Cryst. Solids 190 Ž1995. 9.
Room-temperature electroluminescence of Er-doped hydrogenated amorphous silicon Oleg Gusev a,) , Mikhail Bresler a , Alexey Kuznetsov a , Vera Kudoyarova a , Petr Pak a , Evgenii Terukov a , Konstantin Tsendin a , Irina Yassievich a , Walther Fuhs b, Gerhard Weiser c a
A F Ioffe Physico-Technical Institute, Politekhnicheskaya 26, 194021 St. Petersburg, Russian Federation b Hahn-Meitner Institut, Abteilung PhotoÕoltaik, Rudower Chaussee 5, D-12489 Berlin, Germany c Phillips-UniÕersitat Marburg, Fachbereich Physik, D-35032 Marburg, Germany
Abstract We have observed room-temperature erbium-ion electroluminescence in erbium-doped amorphous silicon. Electrical conduction through the structure is controlled by thermally activated ionization of deep Dy defects in an electric field and the reverse process of capture of mobile electrons by D 0 states. Defect-related Auger excitation ŽDRAE. is responsible for excitation of erbium ions located close to dangling-bond defects. Our experimental data are consistent with the mechanisms proposed. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Erbium-ion electroluminescence; Amorphous silicon; Dangling-bond defects; Defect-related Auger excitation; Ionization
1. Introduction The idea to fabricate light emitting diodes ŽLEDs. integrable into silicon electronics and emitting at the wavelength of 1.5 m m corresponding to the absorption minimum of optical fibers attracted attention to the luminescent properties of erbium-doped silicon w1a,1bx. Recently, we have demonstrated that the films of erbium-doped amorphous hydrogenated silicon Ža-Si:HŽEr.. prepared by cosputtering have room-temperature photoluminescence and the lifetime of erbium ions in this material is considerably shortened by the effect of amorphous-matrix disorder
)
Corresponding author.
w2x. We have reported also on the first observation of Er-luminescence in a diode structure based on aSi:HŽEr. w3x. Here we present more detailed results of these electroluminescence studies.
2. Experimental procedures Electroluminescent structures on the basis of erbium-doped amorphous hydrogenated silicon were prepared by cosputtering, applying the magnetron-assisted silane-decomposition Ž MASD. technique where mixtures of Ar and SiH 4 are used as the sputtering gas w4x. The films of 1 m m thickness were deposited on n-type silicon substrate with a donor concentration of 5 = 10 17 cmy3 . The erbium concen-
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 2 8 3 - X
O. GuseÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1164–1167
tration measured by Rutherford back scattering was about 10 20 cmy3 . The diameter of aluminium contacts to the amorphous film was approximately 1 mm. A typical I–V characteristic at 300 K had conventional diode shape with rectifying properties. Electroluminescence was measured in the temperature interval 77 to 300 K with square-shaped current pulses at 25 to 100 Hz and with a duty cycle of 1:2, both in forward and reverse bias. The emission was observed through the crystalline silicon substrate. The measurements were done in the constant-current regime. The forward direction corresponds to negative voltage at the n-type substrate.
1165
Fig. 2. Electroluminescence intensity as a function of electric field Žexperiment.. Dashed line indicates the asymptotic behavior of this dependence.
3. Experimental results Erbium luminescence corresponding to a line at 1.54 m m Žthe energy of 805 meV. is observed only for reverse bias. A typical spectrum measured at room temperature for reverse bias is shown in Fig. 1. Besides the line corresponding to the 4 I 13r2 ™4 I 15r2 transition in the 4f-shell of erbium ions a band with greater width at 0.8–0.9 eV can be seen. This band corresponds to defect-related luminescence of the a-Si:H matrix w5x. Erbium luminescence is nearly linear in excitation current. The dependence of erbium luminescence intensity on electric field is presented in Fig. 2. At larger electric fields it approaches an E 2-dependence. ŽDue to contact potential and resistance of the amorphous
material, the voltage applied to the structure decreases practically only across the amorphous layer.. This behaviour is unusual for amorphous silicon and indicates the occurrence of thermally activated ionization of deep defect Žor impurity. states well known in crystalline semiconductors. The temperature dependence of the erbium luminescence intensity for different currents through the structure is shown in Fig. 3. This result is also completely unexpected: whereas the intensity of photo- and electroluminescence usually decreases at higher temperatures Ži.e., suffers temperature quenching., in our case, it is much smaller at liquid nitrogen temperatures but increases while approaching room temperature.
Fig. 1. Electroluminescence spectrum for reverse bias at 300 K Žexperiment..
Fig. 3. Temperature dependence of erbium electroluminescence at constant current Žexperiment..
1166
O. GuseÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1164–1167
4. Discussion The introduction of erbium ions into the amorphous matrix is accompanied by the formation of a large number of dangling-bond defects with concentration ; 10 18 cmy3 as determined in our measurements of optical absorption w2x. Therefore, we assume that dangling-bond defects are associated with the erbium ions. According to our dark-conductivity measurements w3x, the Fermi level in erbium-doped amorphous silicon at room temperature is displaced upward from the middle of the gap, i.e., the doped samples are slightly n-type. In this case, defects connected with the dangling bonds occur in D 0 or Dy states. These levels are situated near the midgap and are separated by ; 0.2 eV. In the case of reverse bias, electrons are excited into the conduction band due to thermally activated ionization of Dy-defects in an electric field. While moving in the amorphous layer, these electrons excite erbium and defect-related luminescence by capture of electrons from the conduction band by D 0-defects w3x. The transition e q D 0 ™ Dy can occur by radiative or nonradiative Žmulti-phonon. process. In the case when dangling-bond states are correlated with erbium ions, a third recombination channel is opened, i.e., nonradiative transition accompanied by erbiumion excitation due to Coulomb interaction between the electron being captured to the D 0 center and the f-electron of the erbium-ion. This defect-related Auger excitation ŽDRAE. process is specially effective because of its nearly resonant character: the energy difference of radiative transition between e q D 0 and Dy states is close to the energy difference of 4 I 13r2 and 4 I 15r2 states of the 4f-shell. The excess energy of the transition is transferred to local phonons. In the steady state, we can write a balance equation for thermally activated ionization of Dy states Žthe dependence on electric field is given explicitly., and capture of electrons from the conduction band by D 0 defects 0 b i exp Ž E 2rEc2 . Ny D s cnND ,
Ž 1.
where b i and c are the ionization and capture coeffi0 y cients, Ny D , ND , and n are concentrations of D -de-
fects, D 0-defects, and electrons in the conduction band, E is the electric field applied to the structure, Ec is a characteristic electric field. Using the detailed balance condition to establish the correspondence between b i and c we arrive at the result n s n 0 exp Ž E 2rEc2 . ,
Ž 2.
which indicates that the concentration of electrons in the conduction band rises exponentially in high electric field Ž n 0 is the equilibrium concentration.. ŽThe increase of the concentration of the free electrons was detected directly from the measurements of the voltage drop across the structure. w3x. Now the intensity of erbium luminescence is I L s cA nND0
t tR
s cA n 0 exp Ž E 2rEc2 . ND0
t tR
,
Ž 3.
in agreement with the electric field dependence of erbium electroluminescence shown in Fig. 2. Ž cA is the contribution to the capture coefficient from the DRAE process, t and t R are total and radiative lifetimes of erbium ions in the excited state.. The characteristic field determined from the plot of Fig. 2 is Ec s 1.8 = 10 5 Vrcm. The temperature dependence of the intensity of erbium luminescence in the constant-current regime is I L s cA ND exp
ž
´yy z kT
/
j0
t
q m Ej t R
,
Ž 4.
where the electric field, Ej , applied to the structure at current, j0 , is a weaker function of temperature than the exponent, q is the electron charge, m is the mobility of electrons. From Fig. 3, we estimate the energy of the Fermi level counted from the level of the Dy-defect: ´yy z f 110 meV in reasonable agreement with the n-type property of the amorphous silicon layer. In this simple model, we have neglected the Žpower. temperature dependence of the electric field, Ej , and of the mobility of electrons above the mobility edge. Then, the temperature dependence of the erbium luminescence intensity is determined by population of D 0 states.
O. GuseÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 1164–1167
5. Conclusions In conclusion, we have first observed room-temperature erbium-ion electroluminescence in amorphous-silicon-based semiconductor structure. The properties of the structure seem promising for further development of LEDs emitting at 1.5 m m and integrable into silicon electronics. The mechanism of electrical current conduction through the structure is described; it is controlled by thermally activated ionization of deep Dy-defects in high electric fields and the reverse process of capture of mobile electrons by D 0 states. Defect-related Auger excitation ŽDRAE. is responsible for excitation of erbium ions located close to dangling-bond defects. The whole set of our experimental data is consistent with the mechanisms proposed.
Acknowledgements This work was partially supported by Volkswagen Stiftung under the project Ir71 646, by INTAS Žthe
1167
project number 93-1816., and Russian Foundation for Basic Research ŽGrants No. 95-02-04163a and No. 96-02-16931a..
References w1ax G.S. Pomrenke, P.B. Klein, D.W. Langer ŽEds.., Rare Earth Doped Semiconductors, Vol. 301, Mat. Res. Soc. Symp., 1993. w1bx S. Coffa, A. Polman, R.N. Schwartz ŽEds.., Rare Earth Doped Semiconductors II, Vol. 422, Mat. Res. Soc. Symp., 1996. w2x M.S. Bresler, O.B. Gusev, V.Kh. Kudoyarova, A.N. Kuznetsov, P.E. Pak, E.I. Terukov, I.N. Yassievich, B.P. Zakharchenya, W. Fuhs, A. Sturm, Appl. Phys. Lett. 67 Ž1995. 3599. w3x O.B. Gusev, A.N. Kuznetsov, E.I. Terukov, M.S. Bresler, V.Kh. Kudoyarova, I.N. Yassievich, B.P. Zakharchenya, W. Fuhs, Appl. Phys. Lett. 70 Ž1997. 240. w4x V. Marakhonov, N. Rogachev, J. Ishkalov, J. Marakhonov, E. Terukov, V. Chelnokov, J. Non-Cryst. Solids 137–138 Ž1991. 817. w5x I. Ulber, R. Saleh, W. Fuhs, H. Mell, J. Non-Cryst. Solids 190 Ž1995. 9.