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Optical and electrical spin injection in spin-LED
Available online at www.sciencedirect.com
Physica E 17 (2003) 358 – 360 www.elsevier.com/locate/physe
Optical and electrical spin injection in spin-LED B.L. Liua;∗ , M. S+en,esa , S. Couderca , J.F. Boboa , X. Mariea , T. Amanda , C. Fontaineb , A. Arnoultb a Laboratoire
de Physique de la Matiere Condensee, UMR 5830 CNRS-INSA-UPS, 135 Avenue de Rangueil, 31077 Toulouse cedex 4, France b LAAS, UMR 8001 CNRS, Avenue du Colonel Roche, 4 31077 Toulouse cedex, France
Abstract We have fabricated and characterized a spin-LED with a tunnel barrier made of a ferromagnetic metal (Co)/semiconductor (AlGaAs) Schottky junction [Phys. Rev. B 62 (2000) R16267]. Under a longitudinal magnetic >eld of B=0:8 T, we measured a circular polarization degree of the electro-luminescence of ∼4%. Using the electron spin relaxation and recombination times measured by time-resolved photoluminescence, the yield of spin injection through Schottky contact of the spin-LED has been determined. We >nd ∼30%. ? 2002 Elsevier Science B.V. All rights reserved. PACS: 72.25.Pn; 85.35.Be; 78.67.De; 78.47.+p Keywords: Spin injection; Quantum well device; Circular polarization; Time-resolved spectroscopy
1. Introduction EGcient spin injection from the ferromagnetic metals and dilute magnetic semiconductors into the semiconductors is the fundamental requirement of the semiconductor-based spintronic devices. Spin injection using spin-polarized light-emitting diode has been recently demonstrated [1,2]. It is generally considered that the circular polarization degree of the time-integrated luminescence is identical to the spin polarization of the electrically injected carriers in quantum well (QW) structures according to the quantum selection rules. In fact, this description is oversimpli>ed because the measured cw electro-luminescence (EL) polarization includes the electrical spin
∗
Corresponding author. E-mail address: [email protected] (B.L. Liu).
injection eGciency () at the Co/AlGaAs interface and all the spin relaxation mechanisms which may occur in the AlGaAs barrier and in the GaAs/AlGaAs QW. In this paper, we have measured the spin relaxation time (s ), the energy relaxation time (en ) and the radiative decay time (r ) by time-resolved photoluminescence (TRPL) experiments. This allows us to determine the electrical spin injection eGciency in the spin-LED. 2. Experiments The spin-LED device structure, as shown in Fig. 1, was grown by molecular beam epitaxy (MBE) on a p-doped GaAs(0 0 1) substrate with a 500-nm-thick p-doped Al0:08 Ga0:92 As buLer layer. The active layer consists of a 10-nm-thick GaAs QW separated by 50-nm-thick (bottom) and 40-nm-thick (top) Al0:08 Ga0:92 As barriers. On top of this intrinsic
1386-9477/03/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-9477(02)00809-3
B.L. Liu et al. / Physica E 17 (2003) 358 – 360
359
4 +
Au
EL Intensity (a.u.)
B
Au Co
-
n- AlGaAs i-AlGaAs
I I
Pc(%)
σ
+
2
0.0
0.2
0.4
0.6
0.8
B (T)
T=22K
0.8 T
V
GaAs QW
+
1.50
1.51
1.52
p- AlGaAs
p-GaAs Substrate
1.53
1.54
1.55
1.56
Energy (eV) Fig. 2. EL spectra at a >eld of 0.8 T. Inset: circular polarization degree as a function of external magnetic >eld.
Fig. 1. Spin-LED structures and EL experiment set-up.
0.6
0.2 0.0
T=17K +
I I
0
200
400
600
800
Delay Times (ps)
(a)
τs
Excitation
region, a 40-nm-thick n-doped Al0:08 Ga0:92 As layer was grown. Then, 15-nm-thick Co >lm was deposited by sputtering on the semiconductor structure before capping by a 2-nm-thick Pt protection layer. For the EL measurement, the spin-LED was placed into a magnetic >eld and EL signals were detected in the Faraday geometry by a classical PL set-up. For TRPL experiments, a mode-locked Ti:sapphire laser (1:5 ps pulse width) was used for the non-resonant circularly polarized excitation at ∼1:642 eV (i.e. in the barriers). The PL signals were detected by a 2D synchroscan streak camera, which provides an overall temporal resolution better than 5 ps. The circular polarization was analysed by passing EL and PL through a =4 wave plate and a linear polarizer.
PL Intensity (a.u.)
0.4
Polarization degree
i-AlGaAs
τs
τen
τr
“hot exciton” “cold exciton”
hν
ground state
3. Results and discussion EL spectra of the spin-LED at 22 K are shown in Fig. 2. Only one peak at 1:53 eV can be observed in agreement with the design of the QW active region. The circular polarization is de>ned as PC = (I + − I − )=(I + + I − ), where I + (I − ) is the right (left) circularly polarized EL component peak intensity. When a magnetic >eld is applied, PCExp increases with increas-
(b) Fig. 3. (a) TRPL intensity of + and − . The time evolution PC is also shown. (b) Phenomenological model.
ing the magnetic >eld as shown in the inset of Fig. 2. For B = 0:8 T, we measured PCExp = 4:0%. Fig. 3(a) presents the polarization- and TRPL spectra at 17 K and B = 0. Using the simple model
360
B.L. Liu et al. / Physica E 17 (2003) 358 – 360
presented in Fig. 3(b), we obtain the r = 100 ps and en = 50 ps by >tting the experimental kinetics I = I + + I − . The circular polarization as a function of time delay shows a monoexponential decay, s ∼410 ps. The relationship between PC measured in cw experiment and initial spin polarization P(0) is written as P(0) : (1) PC = (1 + (r =s )) (1 + (en =s )) We know that, according to the optical selection rules, the initial polarization of the QW emission is P(0) = 50% when excited with circularly polarized light in the barriers. The spin polarization of the time-integrated intensity can then be calculated using Eq. (1). We found PC (Calc:) = 35:8%, in agreement with the experimental result, which is ∼36% (the spectrum is not shown here). We can conclude that our phenomenological model is valid. Now, we apply this model to calculate the spin injection eGciency at Co/AlGaAs interface. Here, we ignore the spin loss
of carriers when the carriers drift from Co/AlGaAs interface to QW under the applied electrical >eld. The initial spin polarization P(0) = 5:6% can be obtained when we put PCExp = 4:0% into Eq. (1). The Co magnetic moment is saturated out-of-plane for an applied magnetic >eld of B = 1:8 T. The electron spin polarization near the Fermi energy is then about PCo = 42% [3] (35% [4]). Below this value, the magnetization of Co increases linearly with external magnetic >eld. We thus deduce that the spin polarization of the Co contact is about PCo ∼18:7% (15.5%) at 0:8 T. The spin injection eGciency can be calculated as follows: = P(0)=PCo . We obtain ≈ 30% (36%).
References [1] [2] [3] [4]
H.J. Zhu, Phys. Rev. Lett. 87 (2001) 016601-1. A.T. Hanbicki, et al., Appl. Rev. Lett. 80 (2002) 1240. R.J. Soulen, et al., Science 282 (1998) 85. P.M. Tedrow, et al., Phys. Rep. 238 (1994) 173.
Physica E 17 (2003) 358 – 360 www.elsevier.com/locate/physe
Optical and electrical spin injection in spin-LED B.L. Liua;∗ , M. S+en,esa , S. Couderca , J.F. Boboa , X. Mariea , T. Amanda , C. Fontaineb , A. Arnoultb a Laboratoire
de Physique de la Matiere Condensee, UMR 5830 CNRS-INSA-UPS, 135 Avenue de Rangueil, 31077 Toulouse cedex 4, France b LAAS, UMR 8001 CNRS, Avenue du Colonel Roche, 4 31077 Toulouse cedex, France
Abstract We have fabricated and characterized a spin-LED with a tunnel barrier made of a ferromagnetic metal (Co)/semiconductor (AlGaAs) Schottky junction [Phys. Rev. B 62 (2000) R16267]. Under a longitudinal magnetic >eld of B=0:8 T, we measured a circular polarization degree of the electro-luminescence of ∼4%. Using the electron spin relaxation and recombination times measured by time-resolved photoluminescence, the yield of spin injection through Schottky contact of the spin-LED has been determined. We >nd ∼30%. ? 2002 Elsevier Science B.V. All rights reserved. PACS: 72.25.Pn; 85.35.Be; 78.67.De; 78.47.+p Keywords: Spin injection; Quantum well device; Circular polarization; Time-resolved spectroscopy
1. Introduction EGcient spin injection from the ferromagnetic metals and dilute magnetic semiconductors into the semiconductors is the fundamental requirement of the semiconductor-based spintronic devices. Spin injection using spin-polarized light-emitting diode has been recently demonstrated [1,2]. It is generally considered that the circular polarization degree of the time-integrated luminescence is identical to the spin polarization of the electrically injected carriers in quantum well (QW) structures according to the quantum selection rules. In fact, this description is oversimpli>ed because the measured cw electro-luminescence (EL) polarization includes the electrical spin
∗
Corresponding author. E-mail address: [email protected] (B.L. Liu).
injection eGciency () at the Co/AlGaAs interface and all the spin relaxation mechanisms which may occur in the AlGaAs barrier and in the GaAs/AlGaAs QW. In this paper, we have measured the spin relaxation time (s ), the energy relaxation time (en ) and the radiative decay time (r ) by time-resolved photoluminescence (TRPL) experiments. This allows us to determine the electrical spin injection eGciency in the spin-LED. 2. Experiments The spin-LED device structure, as shown in Fig. 1, was grown by molecular beam epitaxy (MBE) on a p-doped GaAs(0 0 1) substrate with a 500-nm-thick p-doped Al0:08 Ga0:92 As buLer layer. The active layer consists of a 10-nm-thick GaAs QW separated by 50-nm-thick (bottom) and 40-nm-thick (top) Al0:08 Ga0:92 As barriers. On top of this intrinsic
1386-9477/03/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-9477(02)00809-3
B.L. Liu et al. / Physica E 17 (2003) 358 – 360
359
4 +
Au
EL Intensity (a.u.)
B
Au Co
-
n- AlGaAs i-AlGaAs
I I
Pc(%)
σ
+
2
0.0
0.2
0.4
0.6
0.8
B (T)
T=22K
0.8 T
V
GaAs QW
+
1.50
1.51
1.52
p- AlGaAs
p-GaAs Substrate
1.53
1.54
1.55
1.56
Energy (eV) Fig. 2. EL spectra at a >eld of 0.8 T. Inset: circular polarization degree as a function of external magnetic >eld.
Fig. 1. Spin-LED structures and EL experiment set-up.
0.6
0.2 0.0
T=17K +
I I
0
200
400
600
800
Delay Times (ps)
(a)
τs
Excitation
region, a 40-nm-thick n-doped Al0:08 Ga0:92 As layer was grown. Then, 15-nm-thick Co >lm was deposited by sputtering on the semiconductor structure before capping by a 2-nm-thick Pt protection layer. For the EL measurement, the spin-LED was placed into a magnetic >eld and EL signals were detected in the Faraday geometry by a classical PL set-up. For TRPL experiments, a mode-locked Ti:sapphire laser (1:5 ps pulse width) was used for the non-resonant circularly polarized excitation at ∼1:642 eV (i.e. in the barriers). The PL signals were detected by a 2D synchroscan streak camera, which provides an overall temporal resolution better than 5 ps. The circular polarization was analysed by passing EL and PL through a =4 wave plate and a linear polarizer.
PL Intensity (a.u.)
0.4
Polarization degree
i-AlGaAs
τs
τen
τr
“hot exciton” “cold exciton”
hν
ground state
3. Results and discussion EL spectra of the spin-LED at 22 K are shown in Fig. 2. Only one peak at 1:53 eV can be observed in agreement with the design of the QW active region. The circular polarization is de>ned as PC = (I + − I − )=(I + + I − ), where I + (I − ) is the right (left) circularly polarized EL component peak intensity. When a magnetic >eld is applied, PCExp increases with increas-
(b) Fig. 3. (a) TRPL intensity of + and − . The time evolution PC is also shown. (b) Phenomenological model.
ing the magnetic >eld as shown in the inset of Fig. 2. For B = 0:8 T, we measured PCExp = 4:0%. Fig. 3(a) presents the polarization- and TRPL spectra at 17 K and B = 0. Using the simple model
360
B.L. Liu et al. / Physica E 17 (2003) 358 – 360
presented in Fig. 3(b), we obtain the r = 100 ps and en = 50 ps by >tting the experimental kinetics I = I + + I − . The circular polarization as a function of time delay shows a monoexponential decay, s ∼410 ps. The relationship between PC measured in cw experiment and initial spin polarization P(0) is written as P(0) : (1) PC = (1 + (r =s )) (1 + (en =s )) We know that, according to the optical selection rules, the initial polarization of the QW emission is P(0) = 50% when excited with circularly polarized light in the barriers. The spin polarization of the time-integrated intensity can then be calculated using Eq. (1). We found PC (Calc:) = 35:8%, in agreement with the experimental result, which is ∼36% (the spectrum is not shown here). We can conclude that our phenomenological model is valid. Now, we apply this model to calculate the spin injection eGciency at Co/AlGaAs interface. Here, we ignore the spin loss
of carriers when the carriers drift from Co/AlGaAs interface to QW under the applied electrical >eld. The initial spin polarization P(0) = 5:6% can be obtained when we put PCExp = 4:0% into Eq. (1). The Co magnetic moment is saturated out-of-plane for an applied magnetic >eld of B = 1:8 T. The electron spin polarization near the Fermi energy is then about PCo = 42% [3] (35% [4]). Below this value, the magnetization of Co increases linearly with external magnetic >eld. We thus deduce that the spin polarization of the Co contact is about PCo ∼18:7% (15.5%) at 0:8 T. The spin injection eGciency can be calculated as follows: = P(0)=PCo . We obtain ≈ 30% (36%).
References [1] [2] [3] [4]
H.J. Zhu, Phys. Rev. Lett. 87 (2001) 016601-1. A.T. Hanbicki, et al., Appl. Rev. Lett. 80 (2002) 1240. R.J. Soulen, et al., Science 282 (1998) 85. P.M. Tedrow, et al., Phys. Rep. 238 (1994) 173.