- Email: [email protected]
Si diode structures
ARTICLE IN PRESS Physica E 41 (2009) 899–901
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Features of erbium nonradiative deexcitation and electroluminescence temperature quenching in sublimation MBE-grown Si:Er/Si diode structures K.E. Kudryavtsev a,, V.P. Kuznetsov b, D.V. Shengurov a, V.B. Shmagin a, Z.F. Krasilnik a a b
Institute for Physics of Microstructures, Russian Academy of Sciences, Russian Federation Physical–Technical Research Institute, University of Nizhny Novgorod, Russian Federation
a r t i c l e in fo
abstract
Available online 14 August 2008
Temperature dependencies of electroluminescence intensity and decay kinetics from n-Si:Er:O/p-Si diodes, grown by sublimation molecular-beam epitaxy, have been studied. Radiative lifetime of excited Er3+ ion (1.1 ms) and activation energy of the nonradiative deexcitation process (70 meV) have been measured. Contribution of nonradiative deexcitation of erbium ions to the temperature quenching of electroluminescence has been determined. It is revealed that considerable part of luminescence temperature quenching is due to the decrease in erbium excitation efficiency with an increase in temperature. & 2008 Elsevier B.V. All rights reserved.
PACS: 78.60.Fi 81.15.Hi Keywords: Erbium doped silicon Electroluminescence Sublimation MBE
1. Introduction Erbium-doped silicon still represents the area of interest to research groups over the world as a possible pathway to the silicon optoelectronics [1]. While there are a number of different approaches to silicon-based light-emitting diodes, such as dislocation luminescence [2] or GeSi/Si quantum-confined structures [3], Si:Er seems to be far more promising material for laser applications. It also counts in favor of Si:Er that optically active centers with extremely narrow emission lines can be formed using the sublimation molecular-beam epitaxy (SMBE) technique [4]. In spite of relatively low precipitation limit of erbium in silicon (in order of 1018 cm3) and very long spontaneous emission time (1 ms), such a narrow emission band may result in acceptable value of possible optical gain (up to 30 cm1) [5]. In this work, we present the study of nonradiative deexcitation in SMBE-grown Si:Er/Si diodes. Experiment has been aimed to suppress Auger deexcitation processes and to focus on other mechanisms, causing temperature quenching of luminescence signal above 100 K. The spontaneous emission time and activation energy of nonradiative deexcitation are determined.
2. Experiment The structures under study were fabricated by SMBE on the boron-doped Cz–Si (10 0) wafers (2–10 O cm). At first 200 nm p+-layer was deposited to achieve better pump current spreading. The next was 1-mm-thick Si:Er layer. Different growth temperatures Corresponding author.
E-mail address: [email protected] (K.E. Kudryavtsev). 1386-9477/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2008.08.018
were used to produce certain types of optically active centers. The structure was capped by 400 nm n+-Si layer. Solid Al contact was sputtered onto the back (substrate) side of structure, while Ti/Au 500 mm diameter contact spots were applied to the front side. Measurements were performed in the closed-cycle helium cryostat at temperatures up to 160 K. Electroluminescence (EL) spectra were analyzed by 0.6 m grating monochromator and recorded by liquid nitrogen-cooled InGaAs detector. Measurements were taken in pulse mode using the standard lock-in technique. However, the optical efficiency of monochromator was insufficient for time-resolved measurements. In order to obtain better signal-to-noise ratio, we used interference filter (transmission band 1510–1550 nm) instead of monochromator. EL signal was detected by the same InGaAs detector and digitized by BORDO110 oscilloscope board. System response time was 70 ms.
3. Results and discussion The EL spectra for structures with predominance of SiOx-like and Er-O1 optically active centers are shown in Fig. 1. According to Ref. [6], high temperatures (520–580 1C) lead to the formation of precipitate-like centers with relatively broad emission spectrum, while lower temperatures (400–440 1C) lead to dominance of Er-O1 centers with sharp line spectrum. EL of Si:Er/Si diode structures seems to be more informative than photoluminescence, as it is possible to separate different deexcitation pathways [7]. In diode structure, we have neutral region and space charge (depletion) region. Due to the presence of free charge carriers in the first region and the absence in the second, prevalent deexcitation processes should be completely different in these regions. Auger deexcitation should dominate in
ARTICLE IN PRESS 900
K.E. Kudryavtsev et al. / Physica E 41 (2009) 899–901
3.2 (b) 2.8 Tgr= 400 °C, (as grown)
2.4 1/τ, ms-1
EL signal
(b)
2.0
(a)
1.6 Tgr = 520 °C, (as grown)
1.2 (a) 0.8 8
7 6300
6400
6500
9
10
11
1000/T, K-1
6600
Wavenumber, cm-1 Fig. 1. EL spectra for Si:Er/Si diodes with dominance of SiOx-like (a) and Er-O1 (b) optically active centers.
Fig. 3. Electroluminescence decay time versus temperature for SiOx-like (a) and Er-O1 (b) centers.
0.9
1
1
dn-Si:Er
5
0.6
0.5
0
τ/τ0
IEL (norm.)
0.7 Pump bias, V
Depleted region width, μm
0.8
-5 Vrev. max~ 17 V
-10
0.1
0.1
-15 -20 0
0.4
2
4 6 Time, ms
8
10
80
90
100
110
120
130
140
150
T, K
0
2
4
6
8 10 12 Reverse bias, V
14
16
18
Fig. 2. Depleted region width versus reverse bias and (in the inset) waveform of pump voltage, used for time-resolved measurements.
case of neutral region, while so-called ‘‘back transfer’’ should prevail in depleted region. In the present work, we focus on the depleted region to investigate corresponding deexcitation processes. We apply strong reverse bias to the diode after the pump pulse to extend depletion region to the whole Si:Er layer. The dependence of depletion region width on the reverse bias has been derived from C–V profiling. Fig. 2 shows the one obtained for impurity concentration of about 1016 cm3, typical for diodes with SiOx-like centers. At reverse bias of about 17 V the depletion region completely fills active Si:Er layer, and free charge carriers are forced out of it. In addition, low excitation current (o0.5 A/cm2) is used to prevent erbium deexcitation on the excessive injected carriers. Measured time-dependent EL signal is well fitted by the exponential decay curve after the pump turns off. Fig. 3 displays EL decay time versus temperature for samples with SiOx-like (a) and Er-O1 (b) centers. In both cases the temperature dependence of decay time is given by the equation 1 1 E ¼ þ W 0 exp A , t t0 kT where t0 is the spontaneous emission time and EA is the activation
Fig. 4. Temperature dependencies of electroluminescence intensity and decay time for diode with SiOx-like centers.
energy of nonradiative deexcitation. Fitting experimental data points with this model equation gives t0 ¼ 0.9–1.1 ms and EA ¼ 70 meV for both SiOx-like and Er-O1 centers. The excited state lifetime is close to well established value [1,7,8], while the activation energy of deexcitation process is rather small and nearly two times less than commonly reported for ion-implanted structures 150 meV [7,8]. In order to explain this activation energy, thermally stimulated capacitance and deep-level transient spectroscopy (DLTS) measurements have been performed. However, no corresponding gap states were found, and so no reliable interpretation of observed deexcitation energy can be derived at the present moment. Notice that for ion-implanted structures temperature behavior of EL intensity coincides with the temperature behavior of decay time [8]. While donor level at (Ec—150 meV) is considered to be responsible for both excitation and deexcitation of erbium in ionimplanted structures, the value of 70 meV, obtained for SMBEgrown structures, represents only deexcitation processes. In Fig. 4 we can compare temperature dependencies of EL intensity and decay time for SMBE-grown diode with SiOx-like radiative centers. Obviously, the temperature quenching of luminescence is much stronger than intensification of nonradiative deexcitation. It means that not only nonradiative deexcitation is responsible for EL temperature quenching, but also a decrease in excitation
ARTICLE IN PRESS K.E. Kudryavtsev et al. / Physica E 41 (2009) 899–901
reduced excitation efficiency at elevated temperatures. However, due to complexity of excitation process no experimentally proved model can be proposed by now. Further investigations should clarify this point.
10 Excitation cross-section, 10-16 cm-2
901
9 4. Conclusion
8
7
6 90
100
110 120 Temperature, K
130
Fig. 5. Effective excitation cross-section of Er3+ ion in SiOx-like centers versus temperature.
efficiency should be taken into account. Within the limits of two level excitation model of erbium in silicon, dependence of EL intensity on the pump current is given by the equation IEL ðjÞ ¼ Isat
stj=e , 1 þ stj=e
where s is the excitation cross-section, t the excited state lifetime, j/e the pump carrier density. We have measured dependencies of luminescence intensity on pump current at a number of temperatures. All experimental curves are well fitted by this model equation, so we can derive temperature dependence of (st). Taking into account the temperature dependence of excited state lifetime, obtained earlier in this work, it is possible to plot temperature dependence of effective excitation cross-section of erbium ions in our structures as well. Fig. 5 demonstrates effective excitation cross-section of Er3+ ions versus temperature for structures with SiOx-like optically active centers, and the descending behavior of this curve is the direct evidence of
We have showed that temperature quenching of EL signal in SMBE-grown structures is caused not only by thermally activated nonradiative deexcitation, but also by the decrease in excitation efficiency of Er3+ ions. The relative contribution of these two factors into the temperature quenching seems to be nearly equal concerning SiOx-like erbium-related radiative centers. The excited state radiative lifetime is determined to be about 1.1 ms, and this value is in good agreement with that commonly reported for Si:Er structures. At the same time, activation energy of nonradiative deexcitation is found to be only 70 meV, independently of the prevalent radiative centers, and much less than typical 150 meV for ion-implanted structures.
Acknowledgements This work has been supported by RFBR (Russian Foundation for Basic Research) Projects ] 06-02-16563, 07-02-01304 and RFBRNWO 047.011.2005.003. References [1] A.J. Kenyon, Semicond. Sci. Technol. 20 (2005) R65. [2] V. Kveder, M. Badylevich, E. Steinman, A. Izotov, M. Seibt, W. Schro¨ter, Appl. Phys. Lett. 84 (2004) 2106. [3] W.-H. Chang, et al., Appl. Phys. Lett. 83 (2003) 2958. [4] N.Q. Vinh, H. Przybylinska, Z.F. Krasil’nik, T. Gregorkiewicz, Phys. Rev. B 70 (2004) 115332. [5] Z.F. Krasilnik, V.Y. Aleshkin, B.A. Andreev, et al., in: L. Pavesi, S. Gaponenko, L.D. Negro (Eds.), NATO Science Series: II: Mathematics, Physics and Chemistry, vol. 93, Kluwer Academic Publishers, Dordrecht, 2003, p. 445. [6] A.Yu. Andreev, B.A. Andreev, M.N. Drozdov, Z.F. Krasil’nik, et al., Semiconductors 41 (3) (1999). [7] J. Palm, F. Gan, B. Zheng, J. Michel, L.C. Kimerling, Phys. Rev. B 54 (1996) 17603. [8] F. Priolo, G. Franz’o, S. Coffa, A. Carnera, Phys. Rev. B 57 (1998) 4443.
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Features of erbium nonradiative deexcitation and electroluminescence temperature quenching in sublimation MBE-grown Si:Er/Si diode structures K.E. Kudryavtsev a,, V.P. Kuznetsov b, D.V. Shengurov a, V.B. Shmagin a, Z.F. Krasilnik a a b
Institute for Physics of Microstructures, Russian Academy of Sciences, Russian Federation Physical–Technical Research Institute, University of Nizhny Novgorod, Russian Federation
a r t i c l e in fo
abstract
Available online 14 August 2008
Temperature dependencies of electroluminescence intensity and decay kinetics from n-Si:Er:O/p-Si diodes, grown by sublimation molecular-beam epitaxy, have been studied. Radiative lifetime of excited Er3+ ion (1.1 ms) and activation energy of the nonradiative deexcitation process (70 meV) have been measured. Contribution of nonradiative deexcitation of erbium ions to the temperature quenching of electroluminescence has been determined. It is revealed that considerable part of luminescence temperature quenching is due to the decrease in erbium excitation efficiency with an increase in temperature. & 2008 Elsevier B.V. All rights reserved.
PACS: 78.60.Fi 81.15.Hi Keywords: Erbium doped silicon Electroluminescence Sublimation MBE
1. Introduction Erbium-doped silicon still represents the area of interest to research groups over the world as a possible pathway to the silicon optoelectronics [1]. While there are a number of different approaches to silicon-based light-emitting diodes, such as dislocation luminescence [2] or GeSi/Si quantum-confined structures [3], Si:Er seems to be far more promising material for laser applications. It also counts in favor of Si:Er that optically active centers with extremely narrow emission lines can be formed using the sublimation molecular-beam epitaxy (SMBE) technique [4]. In spite of relatively low precipitation limit of erbium in silicon (in order of 1018 cm3) and very long spontaneous emission time (1 ms), such a narrow emission band may result in acceptable value of possible optical gain (up to 30 cm1) [5]. In this work, we present the study of nonradiative deexcitation in SMBE-grown Si:Er/Si diodes. Experiment has been aimed to suppress Auger deexcitation processes and to focus on other mechanisms, causing temperature quenching of luminescence signal above 100 K. The spontaneous emission time and activation energy of nonradiative deexcitation are determined.
2. Experiment The structures under study were fabricated by SMBE on the boron-doped Cz–Si (10 0) wafers (2–10 O cm). At first 200 nm p+-layer was deposited to achieve better pump current spreading. The next was 1-mm-thick Si:Er layer. Different growth temperatures Corresponding author.
E-mail address: [email protected] (K.E. Kudryavtsev). 1386-9477/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2008.08.018
were used to produce certain types of optically active centers. The structure was capped by 400 nm n+-Si layer. Solid Al contact was sputtered onto the back (substrate) side of structure, while Ti/Au 500 mm diameter contact spots were applied to the front side. Measurements were performed in the closed-cycle helium cryostat at temperatures up to 160 K. Electroluminescence (EL) spectra were analyzed by 0.6 m grating monochromator and recorded by liquid nitrogen-cooled InGaAs detector. Measurements were taken in pulse mode using the standard lock-in technique. However, the optical efficiency of monochromator was insufficient for time-resolved measurements. In order to obtain better signal-to-noise ratio, we used interference filter (transmission band 1510–1550 nm) instead of monochromator. EL signal was detected by the same InGaAs detector and digitized by BORDO110 oscilloscope board. System response time was 70 ms.
3. Results and discussion The EL spectra for structures with predominance of SiOx-like and Er-O1 optically active centers are shown in Fig. 1. According to Ref. [6], high temperatures (520–580 1C) lead to the formation of precipitate-like centers with relatively broad emission spectrum, while lower temperatures (400–440 1C) lead to dominance of Er-O1 centers with sharp line spectrum. EL of Si:Er/Si diode structures seems to be more informative than photoluminescence, as it is possible to separate different deexcitation pathways [7]. In diode structure, we have neutral region and space charge (depletion) region. Due to the presence of free charge carriers in the first region and the absence in the second, prevalent deexcitation processes should be completely different in these regions. Auger deexcitation should dominate in
ARTICLE IN PRESS 900
K.E. Kudryavtsev et al. / Physica E 41 (2009) 899–901
3.2 (b) 2.8 Tgr= 400 °C, (as grown)
2.4 1/τ, ms-1
EL signal
(b)
2.0
(a)
1.6 Tgr = 520 °C, (as grown)
1.2 (a) 0.8 8
7 6300
6400
6500
9
10
11
1000/T, K-1
6600
Wavenumber, cm-1 Fig. 1. EL spectra for Si:Er/Si diodes with dominance of SiOx-like (a) and Er-O1 (b) optically active centers.
Fig. 3. Electroluminescence decay time versus temperature for SiOx-like (a) and Er-O1 (b) centers.
0.9
1
1
dn-Si:Er
5
0.6
0.5
0
τ/τ0
IEL (norm.)
0.7 Pump bias, V
Depleted region width, μm
0.8
-5 Vrev. max~ 17 V
-10
0.1
0.1
-15 -20 0
0.4
2
4 6 Time, ms
8
10
80
90
100
110
120
130
140
150
T, K
0
2
4
6
8 10 12 Reverse bias, V
14
16
18
Fig. 2. Depleted region width versus reverse bias and (in the inset) waveform of pump voltage, used for time-resolved measurements.
case of neutral region, while so-called ‘‘back transfer’’ should prevail in depleted region. In the present work, we focus on the depleted region to investigate corresponding deexcitation processes. We apply strong reverse bias to the diode after the pump pulse to extend depletion region to the whole Si:Er layer. The dependence of depletion region width on the reverse bias has been derived from C–V profiling. Fig. 2 shows the one obtained for impurity concentration of about 1016 cm3, typical for diodes with SiOx-like centers. At reverse bias of about 17 V the depletion region completely fills active Si:Er layer, and free charge carriers are forced out of it. In addition, low excitation current (o0.5 A/cm2) is used to prevent erbium deexcitation on the excessive injected carriers. Measured time-dependent EL signal is well fitted by the exponential decay curve after the pump turns off. Fig. 3 displays EL decay time versus temperature for samples with SiOx-like (a) and Er-O1 (b) centers. In both cases the temperature dependence of decay time is given by the equation 1 1 E ¼ þ W 0 exp A , t t0 kT where t0 is the spontaneous emission time and EA is the activation
Fig. 4. Temperature dependencies of electroluminescence intensity and decay time for diode with SiOx-like centers.
energy of nonradiative deexcitation. Fitting experimental data points with this model equation gives t0 ¼ 0.9–1.1 ms and EA ¼ 70 meV for both SiOx-like and Er-O1 centers. The excited state lifetime is close to well established value [1,7,8], while the activation energy of deexcitation process is rather small and nearly two times less than commonly reported for ion-implanted structures 150 meV [7,8]. In order to explain this activation energy, thermally stimulated capacitance and deep-level transient spectroscopy (DLTS) measurements have been performed. However, no corresponding gap states were found, and so no reliable interpretation of observed deexcitation energy can be derived at the present moment. Notice that for ion-implanted structures temperature behavior of EL intensity coincides with the temperature behavior of decay time [8]. While donor level at (Ec—150 meV) is considered to be responsible for both excitation and deexcitation of erbium in ionimplanted structures, the value of 70 meV, obtained for SMBEgrown structures, represents only deexcitation processes. In Fig. 4 we can compare temperature dependencies of EL intensity and decay time for SMBE-grown diode with SiOx-like radiative centers. Obviously, the temperature quenching of luminescence is much stronger than intensification of nonradiative deexcitation. It means that not only nonradiative deexcitation is responsible for EL temperature quenching, but also a decrease in excitation
ARTICLE IN PRESS K.E. Kudryavtsev et al. / Physica E 41 (2009) 899–901
reduced excitation efficiency at elevated temperatures. However, due to complexity of excitation process no experimentally proved model can be proposed by now. Further investigations should clarify this point.
10 Excitation cross-section, 10-16 cm-2
901
9 4. Conclusion
8
7
6 90
100
110 120 Temperature, K
130
Fig. 5. Effective excitation cross-section of Er3+ ion in SiOx-like centers versus temperature.
efficiency should be taken into account. Within the limits of two level excitation model of erbium in silicon, dependence of EL intensity on the pump current is given by the equation IEL ðjÞ ¼ Isat
stj=e , 1 þ stj=e
where s is the excitation cross-section, t the excited state lifetime, j/e the pump carrier density. We have measured dependencies of luminescence intensity on pump current at a number of temperatures. All experimental curves are well fitted by this model equation, so we can derive temperature dependence of (st). Taking into account the temperature dependence of excited state lifetime, obtained earlier in this work, it is possible to plot temperature dependence of effective excitation cross-section of erbium ions in our structures as well. Fig. 5 demonstrates effective excitation cross-section of Er3+ ions versus temperature for structures with SiOx-like optically active centers, and the descending behavior of this curve is the direct evidence of
We have showed that temperature quenching of EL signal in SMBE-grown structures is caused not only by thermally activated nonradiative deexcitation, but also by the decrease in excitation efficiency of Er3+ ions. The relative contribution of these two factors into the temperature quenching seems to be nearly equal concerning SiOx-like erbium-related radiative centers. The excited state radiative lifetime is determined to be about 1.1 ms, and this value is in good agreement with that commonly reported for Si:Er structures. At the same time, activation energy of nonradiative deexcitation is found to be only 70 meV, independently of the prevalent radiative centers, and much less than typical 150 meV for ion-implanted structures.
Acknowledgements This work has been supported by RFBR (Russian Foundation for Basic Research) Projects ] 06-02-16563, 07-02-01304 and RFBRNWO 047.011.2005.003. References [1] A.J. Kenyon, Semicond. Sci. Technol. 20 (2005) R65. [2] V. Kveder, M. Badylevich, E. Steinman, A. Izotov, M. Seibt, W. Schro¨ter, Appl. Phys. Lett. 84 (2004) 2106. [3] W.-H. Chang, et al., Appl. Phys. Lett. 83 (2003) 2958. [4] N.Q. Vinh, H. Przybylinska, Z.F. Krasil’nik, T. Gregorkiewicz, Phys. Rev. B 70 (2004) 115332. [5] Z.F. Krasilnik, V.Y. Aleshkin, B.A. Andreev, et al., in: L. Pavesi, S. Gaponenko, L.D. Negro (Eds.), NATO Science Series: II: Mathematics, Physics and Chemistry, vol. 93, Kluwer Academic Publishers, Dordrecht, 2003, p. 445. [6] A.Yu. Andreev, B.A. Andreev, M.N. Drozdov, Z.F. Krasil’nik, et al., Semiconductors 41 (3) (1999). [7] J. Palm, F. Gan, B. Zheng, J. Michel, L.C. Kimerling, Phys. Rev. B 54 (1996) 17603. [8] F. Priolo, G. Franz’o, S. Coffa, A. Carnera, Phys. Rev. B 57 (1998) 4443.