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Si quantum wells
ELS EVI E R
Thin Solid Films 380 (2000) 237-239
www.elsevier.com/locate/tsf
Terahertz emission of SiGe/Si quantum wells M.S. Kagan”’”,I.V. Altukhov”, V.P. Sinis”, S.G. Thomasb, K.L. Wangb, K.A. Chao“, I.N. Yassievichd ahtitUte of Radioengineering and Electronics of RAS, 11, Mokhovaya, 103907Moscow, Russia bUniversityof California, 66-147KK Engineering I E Los Angeles, C A 90095, USA “De artment of Physics, Lund University, Helgonavagen 5, S-223 62 Lund, Sweden 6 . F . Ioffe Physico-Technical Institute of RAS, 194021 St. Petersburg, Russia
Abstract
THz emission of stimulated character was observed in Si/SiGe/Si quantum well (OW) structures doped with boron. The resonance cavity formed by extremely parallel-structure planes due to total internal reflection, is necessary for the emission. The mechanism for the possible population inversion of strain-split acceptor levels is proposed. 0 2000 Elsevier Science B.V. All rights reserved. Keywords: THz emission; SiGe; Quantum wells
1. Introduction
The stimulated THz emission of uniaxially stressed p-Ge [1-4] has been shown to be due to a population inversion of strain-split acceptor levels, provided that one of them is in the continuum, creating a so-called resonant state. The inversion mechanism consists of depopulation of the ground state of an acceptor (being in the gap) by a strong electric field, while the resonant state (being in the continuum) is filled to some degree because of carrier exchange with valence band states. The intra-center population inversion seems to be rather general, since the resonant states can arise for quite different reasons. In particular, they should exist in strained quantum wells (QW), where acceptor states are split with no external stress due to internal strain and/or size quantization. In this report we present the data of intense THz-emission of boron-doped Si/Ge,Si,-,/Si QWs and propose an inversion mecha-
* Corresponding author. Tel.: + 7-095-2034812; fax: 2038414. E-mail address: [email protected] (M.S. Kagan).
+ 7-095-
nism similar to that in bulk p-Ge. The evidence for the mechanism of the intra-center population inversion was obtained from measurements of hole transport along the SiGe QW. 2. Experimental
p-Type Si/SiGe/Si QWS were grown pseudomorphically by molecular beam epitaxy (MBE) on n-type Si substrate and selectively doped with boron. The SiGe layer of 20-nm thickness was sandwiched between an undoped Si buffer (130 nm wide) and cap (60 nm) layers. The Ge content in the SiGe alloy was 0.15. The %layer of boron at a concentration of 6 x 1011cm-2 was grown in the QW middle. Two boron %layers with B concentration of 4 x 1011-1012 cm-’ were positioned within the buffer and cap layers at a distance of 30 nm from each QW interface with the aim of diminishing carrier outflow from the QW into surface states (see [5]). The pulsed (0.3 ps) electric field was applied along the SiGe layer, parallel to the interfaces, to the ohmic contacts. The measurements were performed at liquid He temperature. The THz emission was regis-
0040-6090/00/$ - see front matter 0 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 ( 0 0 ) 0 1 5 3 0 - 3
M.S. Kugun et ul. /Thin Solid Films 380 (2000)237-239
238
1, mA Fig. 1. Photodetector signal vs. current.
tered by a cooled Ga-doped Ge photodetector sensitive above 10 meV. For the samples measured as grown, the luminescence intensity increases smoothly with the applied field (or the current). The narrow lateral planes, which are perpendicular to the QW interfaces, are then well polished, and opposite planes are kept extremely parallel to each other. As a result, together with the parallel planes of growth, they form an optical resonator, due to total internal reflection within the whole structure, including undoped (non-absorbing) substrate. (The resonance cavity is believed to be similar to that we used for bulk p-Ge [1,3].) Under these conditions, we observed a jump in radiation intensity at a certain threshold voltage. The intense emission could be orders of magnitude stronger than the intensity of spontaneous emission. Fig. 1 shows the radiation intensity (photodetector signal) dependent on the current along the quantum well. No jump in the emission was observed in the same samples before polishing. The radiation and current was registered at voltages above 300-400 V/cm, which is the threshold of an impact ionization of acceptors in the QW. The voltage range for the intense emission was different for different samples, and could be from 0.3 up to 2.5 kV/cm. The narrow voltage range of the emission for one of the samples is shown in Fig. 1 to illustrate the nonthermal origin of the emission. Indeed, the dependence of the same character outside of the emission region is evident. The intense radiation was mainly originating out from the narrow QW planes, which is natural for the resonator used. It was determined by means of cutting filters that the wavelength of the emission is in the range 50-100 pm.
threshold electric field for the jump in emission, point to a stimulated character of the intense THz emission. The study of hole transport along the QW has shown (see [5]) that hole conductivity was due to a two-stage process, consisting of the thermal activation of holes from the ground to split-off states of acceptors, followed by hole tunneling into the hole-free bands. The latter results from a charge redistribution between acceptor-doped layers and the cap-layer surface, which becomes charged. The charging of the surface results in a potential drop across the QW, inclining the valence band (Fig. 2). As a result, the strain-split state of the acceptor can become resonant. Calculation of the valence-band profile shows the strong dependence of the electric field in QW, EQw,on the %doping level in barriers, Nb, at a fixed level of %doping in QW. If Nb < 4 x 10l1 crn-’, then EQw> 5 x lo 4 V/cm, and the situation illustrated in Fig. 2 can be realized. At higher Nb, the electric fields in QW decrease considerably. However, since practically all acceptors in the &layer in QW are charged, the edge of the two-dimensional conduction channel in QW is shifted compared with the valence band top. This shift is due to an overlap of Coulomb potentials of neighboring charged boron ions. The value of the shift is approximately 20 meV for the samples used. At these conditions, the split-off levels could overlap with continuum states and form resonant states. The existence of the resonant states allows us to suggest the mechanism of a population inversion, similar to that in stressed bulk p-Ge [3,4]. The scheme of the inversion is shown in Fig. 2. With a strong external electric field applied parallel to the interfaces, the holes in the ground states are impact-ionized into the valence band, and then electrically pumped into the resonant states. The optical transition for the THz radiation is from the resonant states to the ground states. The photon energy of the observed THz radiation being in the range between 12 and 24 meV is
3. Discussion
The necessity of a resonator, and the existence of a
Fig. 2. Scheme of inversion.
M.S. Kugun et ul. /Thin Solid Films 380 (2000)237-239
consistent with the energy of split of acceptor states found to be 18 meV 151. 4. Conclusion
The intense THz emission of boron-doped strained SiGe QWs was observed. The stimulated character of the emission was confirmed by the existence of a threshold electric field and by the necessity of a resonator. The mechanism of population inversion giving rise to stimulated emission is supposed to be due to the existence of resonant acceptor states, resulting from an inclining valence band by transverse potential. The latter appears to be due to a charge redistribution between acceptor-doped layers and the cap-layer surface, which becomes charged. The data obtained show that the strained quantum-sized semiconductor structures doped with acceptors have potential for lasing at THz frequencies.
239
Acknowledgements
This work was supported in part by Grants 99-0216169, 00-02-16093, and 00-02-16066 from the Russian Foundation for Basic Research and 97-10-55 from the Russian Ministry of Science and Technology. References [l] I.V. Altukhov, M.S. Kagan, K.A. Korolev, V.P. Sinis, F.A. Smirnov, Sov. Phys. JETP 74 (1992) 404. [2] M.A. Odnoblyudov, V.M. Chistyakov, I.N. Yassievich, M.S. Kagan, Phys. Status Solidi (b) 210 (1998) 873. [3] I.V. Altukhov, M.S. Kagan, K.A. Korolev, V.P. Sinis, E.G. Chirkova, M.A. Odnoblyudov, I.N. Yassievich, Sov. Phys. JETP 88 (1999) 51. [4] M.A. Odnoblyudov, I.N. Yassievich, M.S. Kagan, Y.M. Galperin, K.A. Chao, Phys. Rev. Lett. 83 (1999) 644. [5] I.V. Altukhov, M.S. Kagan, V.P. Sinis, S.G. Thomas, K.L. Wang, A. Blom, M.O. Odnoblyudov, Thin Solid Films (in press).
Thin Solid Films 380 (2000) 237-239
www.elsevier.com/locate/tsf
Terahertz emission of SiGe/Si quantum wells M.S. Kagan”’”,I.V. Altukhov”, V.P. Sinis”, S.G. Thomasb, K.L. Wangb, K.A. Chao“, I.N. Yassievichd ahtitUte of Radioengineering and Electronics of RAS, 11, Mokhovaya, 103907Moscow, Russia bUniversityof California, 66-147KK Engineering I E Los Angeles, C A 90095, USA “De artment of Physics, Lund University, Helgonavagen 5, S-223 62 Lund, Sweden 6 . F . Ioffe Physico-Technical Institute of RAS, 194021 St. Petersburg, Russia
Abstract
THz emission of stimulated character was observed in Si/SiGe/Si quantum well (OW) structures doped with boron. The resonance cavity formed by extremely parallel-structure planes due to total internal reflection, is necessary for the emission. The mechanism for the possible population inversion of strain-split acceptor levels is proposed. 0 2000 Elsevier Science B.V. All rights reserved. Keywords: THz emission; SiGe; Quantum wells
1. Introduction
The stimulated THz emission of uniaxially stressed p-Ge [1-4] has been shown to be due to a population inversion of strain-split acceptor levels, provided that one of them is in the continuum, creating a so-called resonant state. The inversion mechanism consists of depopulation of the ground state of an acceptor (being in the gap) by a strong electric field, while the resonant state (being in the continuum) is filled to some degree because of carrier exchange with valence band states. The intra-center population inversion seems to be rather general, since the resonant states can arise for quite different reasons. In particular, they should exist in strained quantum wells (QW), where acceptor states are split with no external stress due to internal strain and/or size quantization. In this report we present the data of intense THz-emission of boron-doped Si/Ge,Si,-,/Si QWs and propose an inversion mecha-
* Corresponding author. Tel.: + 7-095-2034812; fax: 2038414. E-mail address: [email protected] (M.S. Kagan).
+ 7-095-
nism similar to that in bulk p-Ge. The evidence for the mechanism of the intra-center population inversion was obtained from measurements of hole transport along the SiGe QW. 2. Experimental
p-Type Si/SiGe/Si QWS were grown pseudomorphically by molecular beam epitaxy (MBE) on n-type Si substrate and selectively doped with boron. The SiGe layer of 20-nm thickness was sandwiched between an undoped Si buffer (130 nm wide) and cap (60 nm) layers. The Ge content in the SiGe alloy was 0.15. The %layer of boron at a concentration of 6 x 1011cm-2 was grown in the QW middle. Two boron %layers with B concentration of 4 x 1011-1012 cm-’ were positioned within the buffer and cap layers at a distance of 30 nm from each QW interface with the aim of diminishing carrier outflow from the QW into surface states (see [5]). The pulsed (0.3 ps) electric field was applied along the SiGe layer, parallel to the interfaces, to the ohmic contacts. The measurements were performed at liquid He temperature. The THz emission was regis-
0040-6090/00/$ - see front matter 0 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 ( 0 0 ) 0 1 5 3 0 - 3
M.S. Kugun et ul. /Thin Solid Films 380 (2000)237-239
238
1, mA Fig. 1. Photodetector signal vs. current.
tered by a cooled Ga-doped Ge photodetector sensitive above 10 meV. For the samples measured as grown, the luminescence intensity increases smoothly with the applied field (or the current). The narrow lateral planes, which are perpendicular to the QW interfaces, are then well polished, and opposite planes are kept extremely parallel to each other. As a result, together with the parallel planes of growth, they form an optical resonator, due to total internal reflection within the whole structure, including undoped (non-absorbing) substrate. (The resonance cavity is believed to be similar to that we used for bulk p-Ge [1,3].) Under these conditions, we observed a jump in radiation intensity at a certain threshold voltage. The intense emission could be orders of magnitude stronger than the intensity of spontaneous emission. Fig. 1 shows the radiation intensity (photodetector signal) dependent on the current along the quantum well. No jump in the emission was observed in the same samples before polishing. The radiation and current was registered at voltages above 300-400 V/cm, which is the threshold of an impact ionization of acceptors in the QW. The voltage range for the intense emission was different for different samples, and could be from 0.3 up to 2.5 kV/cm. The narrow voltage range of the emission for one of the samples is shown in Fig. 1 to illustrate the nonthermal origin of the emission. Indeed, the dependence of the same character outside of the emission region is evident. The intense radiation was mainly originating out from the narrow QW planes, which is natural for the resonator used. It was determined by means of cutting filters that the wavelength of the emission is in the range 50-100 pm.
threshold electric field for the jump in emission, point to a stimulated character of the intense THz emission. The study of hole transport along the QW has shown (see [5]) that hole conductivity was due to a two-stage process, consisting of the thermal activation of holes from the ground to split-off states of acceptors, followed by hole tunneling into the hole-free bands. The latter results from a charge redistribution between acceptor-doped layers and the cap-layer surface, which becomes charged. The charging of the surface results in a potential drop across the QW, inclining the valence band (Fig. 2). As a result, the strain-split state of the acceptor can become resonant. Calculation of the valence-band profile shows the strong dependence of the electric field in QW, EQw,on the %doping level in barriers, Nb, at a fixed level of %doping in QW. If Nb < 4 x 10l1 crn-’, then EQw> 5 x lo 4 V/cm, and the situation illustrated in Fig. 2 can be realized. At higher Nb, the electric fields in QW decrease considerably. However, since practically all acceptors in the &layer in QW are charged, the edge of the two-dimensional conduction channel in QW is shifted compared with the valence band top. This shift is due to an overlap of Coulomb potentials of neighboring charged boron ions. The value of the shift is approximately 20 meV for the samples used. At these conditions, the split-off levels could overlap with continuum states and form resonant states. The existence of the resonant states allows us to suggest the mechanism of a population inversion, similar to that in stressed bulk p-Ge [3,4]. The scheme of the inversion is shown in Fig. 2. With a strong external electric field applied parallel to the interfaces, the holes in the ground states are impact-ionized into the valence band, and then electrically pumped into the resonant states. The optical transition for the THz radiation is from the resonant states to the ground states. The photon energy of the observed THz radiation being in the range between 12 and 24 meV is
3. Discussion
The necessity of a resonator, and the existence of a
Fig. 2. Scheme of inversion.
M.S. Kugun et ul. /Thin Solid Films 380 (2000)237-239
consistent with the energy of split of acceptor states found to be 18 meV 151. 4. Conclusion
The intense THz emission of boron-doped strained SiGe QWs was observed. The stimulated character of the emission was confirmed by the existence of a threshold electric field and by the necessity of a resonator. The mechanism of population inversion giving rise to stimulated emission is supposed to be due to the existence of resonant acceptor states, resulting from an inclining valence band by transverse potential. The latter appears to be due to a charge redistribution between acceptor-doped layers and the cap-layer surface, which becomes charged. The data obtained show that the strained quantum-sized semiconductor structures doped with acceptors have potential for lasing at THz frequencies.
239
Acknowledgements
This work was supported in part by Grants 99-0216169, 00-02-16093, and 00-02-16066 from the Russian Foundation for Basic Research and 97-10-55 from the Russian Ministry of Science and Technology. References [l] I.V. Altukhov, M.S. Kagan, K.A. Korolev, V.P. Sinis, F.A. Smirnov, Sov. Phys. JETP 74 (1992) 404. [2] M.A. Odnoblyudov, V.M. Chistyakov, I.N. Yassievich, M.S. Kagan, Phys. Status Solidi (b) 210 (1998) 873. [3] I.V. Altukhov, M.S. Kagan, K.A. Korolev, V.P. Sinis, E.G. Chirkova, M.A. Odnoblyudov, I.N. Yassievich, Sov. Phys. JETP 88 (1999) 51. [4] M.A. Odnoblyudov, I.N. Yassievich, M.S. Kagan, Y.M. Galperin, K.A. Chao, Phys. Rev. Lett. 83 (1999) 644. [5] I.V. Altukhov, M.S. Kagan, V.P. Sinis, S.G. Thomas, K.L. Wang, A. Blom, M.O. Odnoblyudov, Thin Solid Films (in press).