- Email: [email protected]
AlGaAs quantum wells. Energy structure of the isolated centers, and their collective behavior
ARTICLE IN PRESS Physica B 404 (2009) 5148–5149
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Optical studies of A þ -centers in GaAs/AlGaAs quantum wells. Energy structure of the isolated centers, and their collective behavior P.V. Petrov , Yu.L. Ivanov, V.E. Sedov, N.I. Sablina, N.S. Averkiev Ioffe Physico-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia
a r t i c l e in f o
Keywords: GaAs Acceptors Quantum wells
a b s t r a c t We report measurements of photoluminescence (PL) spectra due to A þ -centers in GaAs/AlGaAs quantum well (QW). We observe temperature and pump dependence of the PL spectra as well as polarized PL spectra taken under an applied magnetic field and uniaxial stress. It was shown that hole– hole exchange interaction brings the A þ -centers into a state characterized by J ¼ 2 split by QW potential in two states: ground mJ ¼ 0 and excited mJ ¼ 7 1; 7 2 ones. Coulomb interaction changes the density of states (DOS) in the valence band thus a Coulomb gap appears. Our Monte-Carlo calculations of DOS demonstrate good agreement with the measured temperature PL dependence. & 2009 Elsevier B.V. All rights reserved.
The A þ -centers i.e. double charged acceptors [1] plays an important role in a metal–insulator transition problem, especially in 2D. It was experimentally shown [2]that in GaAs/AlGaAs QWs doped both in wells and barriers equilibrium A þ -centers appear. In Ref. [2] the presence of A þ -centers was observed through transport in upper Hubbard band and in PL spectra as radiative transition from conduction band. We attempted [3] to describe A þ -centers by zero-radius potential formalism, and a qualitative agreement of the calculated binding energy and wave function radius with the results of Ref. [2] was obtained. In present work we experimentally show that the exchange hole–hole interaction in description of A þ -centers plays an important role. Our study is conducted on two samples with GaAs=Al0:35 Ga0:65 As QWs, well widths are 13 and 15 nm. Details of the samples growth are given in Ref. [3], the sheet concentrations of A þ -centers are 3 1010 and 5 1010 cm2 , respectively. We carry out the PL measurements in 1:8255 K temperature range with varying pump intensities. Polarized PL spectra were measured at liquid helium bath. To improve accuracy we use two-channel photon counter and modulate PL polarization with Pockels cell. The both samples show qualitatively same results with weak difference due to samples geometry. Two peaks can be resolved in PL spectra: the higher-energy peak increases in relation to the lower-energy one upon rising pump temperature (Fig. 1) or intensity (Fig. 2). We identify that peaks as ground mJ ¼ 0 (A) and excited mJ ¼ 7 1; 72 (B) states of A þ -centers. Upon increasing pump intensities or temperatures, lines of band-to-band radiative transitions for heavy (C) and light (D) holes appears in PL in addition to transitions of the A þ -centers. Electrostatic interaction of charged
Corresponding author.
E-mail address: [email protected] (P.V. Petrov). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.08.294
acceptors changes DOS, thus Coulomb gap appears near Fermi energy. Therefore, the band-to-band line overlay the line related to excited state of A þ -center. We perform Monte-Carlo calculations of DOS in order to account for both Coulomb interaction and temperature activation of A þ -centers. Calculated DOS is in a good agreement with experimental spectra in 6–55 K range (Fig. 1). Suggestion of the band-to-band origin of the lines that appears at the high temperatures is supported by measurements of the linear PL polarization under uniaxial stress at T ¼ 77 and 300 K taken on samples with the same geometry [4]. We carry out measurements of linear PL polarization under uniaxial stress at T ¼ 1:8 K when only A þ -centers contribute to PL spectra in the spectral range under consideration. Increasing stress causes similar high energy shift of both peaks. It proves our suggestion that the both peaks originate from the degenerate J ¼ 2 hole state split into two by the exchange interaction. The polarization ratio gradually decrease from the ground-state peak to the excited-state peak which is because the most probable transition from the excited state is the transition in the final state of the acceptor with mJ ¼ 7 12. The PL circular polarization spectra under applied magnetic field in Faraday geometry at T ¼ 4:2 K support our previous interpretation of the results. At moderate magnetic field the ground state stays unchanged, thus circular PL polarization of the peak depends only on the splitting of final hole state of the acceptor. Consequently, the spectral dependence of polarization within mJ ¼ 0 ground state’s line should be equal to zero near the PL maximum, exactly as we measured (Fig. 2). The mJ ¼ 71; 7 2 excited states are positively polarized due to the large positive g-factor of the heavy hole. Therefore the polarization degree increases on the high energy side of spectra. In conclusion, we carry out optical studies of A þ -centers in GaAs/AlGaAs QWs. The fine structure of the isolated A þ -centers was investigated and its collective behavior explanation was suggested.
ARTICLE IN PRESS P.V. Petrov et al. / Physica B 404 (2009) 5148–5149
simulated density of states, arb. units
B
A
Temperature: 1 -6 K 2 -9 K 3 -1 5K 4 -2 5K 5 -3 0K 6 -4 0K 7 -5 5K
B
PL intensity, arb. units
Temperature: 1 -6 K 2 -9 K 3 -1 5K 4 -2 5K 5 -3 0K 6 -4 0K 7 -5 5K
A
5149
1 2 3 C
C
4
D -0.015
-0.010
-0.005
0.000 energy, eV
0.005
5 6 7
D 0.010
1.515
0.015
1.520
1.525
1.530 1.535 energy, eV
1.540
1.545
Fig. 1. Simulated DOS (left) and PL spectra (right) at different temperatures.
A
8
B
polarization degree, %
PL intensity, arb. units.
6 103
102
A
7
4
Magneticf ield 1 - 1.3 T 7 - 3.3 T
2 B 0 -2 -4
PL intensity, arb.units
104
101 -6
1.520
1.525
1.530
1.535
1.540
Energy, eV
-8 1.524
1 1.526
1.528
1.530 1.532 energy, eV
1.534
1.536
Fig. 2. PL spectra at different pump intensity (left) and magnetic fields (right).
The study was supported by the RF Presidential Foundation (2951.2008.2), Russian Foundation for Basic Research (09-0200904-a), and Russian Academy of Sciences. References [1] M.A. Lampert, Phys. Rev. Lett. 1 (1958) 450.
[2] N.V. Agrinskaya, Y.L. Ivanov, P.A. Petrov, V.M. Ustinov, Solid State Commun. 126 (2003) 369. [3] N.S. Averkiev, A.E. Zhukov, Y.L. Ivanov, P.V. Petrov, K.S. Romanov, A.A. Tonkikh, V.M. Ustinov, G.E. Tsyrlin, Semiconductors 38 (2004) 217. [4] N.S. Averkiev, Yu.L. Ivanov, A.A. Krasivichev, P.V. Petrov, N.I. Sablina, V.E. Sedov, Semiconductors 42 (2008) 316.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Optical studies of A þ -centers in GaAs/AlGaAs quantum wells. Energy structure of the isolated centers, and their collective behavior P.V. Petrov , Yu.L. Ivanov, V.E. Sedov, N.I. Sablina, N.S. Averkiev Ioffe Physico-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia
a r t i c l e in f o
Keywords: GaAs Acceptors Quantum wells
a b s t r a c t We report measurements of photoluminescence (PL) spectra due to A þ -centers in GaAs/AlGaAs quantum well (QW). We observe temperature and pump dependence of the PL spectra as well as polarized PL spectra taken under an applied magnetic field and uniaxial stress. It was shown that hole– hole exchange interaction brings the A þ -centers into a state characterized by J ¼ 2 split by QW potential in two states: ground mJ ¼ 0 and excited mJ ¼ 7 1; 7 2 ones. Coulomb interaction changes the density of states (DOS) in the valence band thus a Coulomb gap appears. Our Monte-Carlo calculations of DOS demonstrate good agreement with the measured temperature PL dependence. & 2009 Elsevier B.V. All rights reserved.
The A þ -centers i.e. double charged acceptors [1] plays an important role in a metal–insulator transition problem, especially in 2D. It was experimentally shown [2]that in GaAs/AlGaAs QWs doped both in wells and barriers equilibrium A þ -centers appear. In Ref. [2] the presence of A þ -centers was observed through transport in upper Hubbard band and in PL spectra as radiative transition from conduction band. We attempted [3] to describe A þ -centers by zero-radius potential formalism, and a qualitative agreement of the calculated binding energy and wave function radius with the results of Ref. [2] was obtained. In present work we experimentally show that the exchange hole–hole interaction in description of A þ -centers plays an important role. Our study is conducted on two samples with GaAs=Al0:35 Ga0:65 As QWs, well widths are 13 and 15 nm. Details of the samples growth are given in Ref. [3], the sheet concentrations of A þ -centers are 3 1010 and 5 1010 cm2 , respectively. We carry out the PL measurements in 1:8255 K temperature range with varying pump intensities. Polarized PL spectra were measured at liquid helium bath. To improve accuracy we use two-channel photon counter and modulate PL polarization with Pockels cell. The both samples show qualitatively same results with weak difference due to samples geometry. Two peaks can be resolved in PL spectra: the higher-energy peak increases in relation to the lower-energy one upon rising pump temperature (Fig. 1) or intensity (Fig. 2). We identify that peaks as ground mJ ¼ 0 (A) and excited mJ ¼ 7 1; 72 (B) states of A þ -centers. Upon increasing pump intensities or temperatures, lines of band-to-band radiative transitions for heavy (C) and light (D) holes appears in PL in addition to transitions of the A þ -centers. Electrostatic interaction of charged
Corresponding author.
E-mail address: [email protected] (P.V. Petrov). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.08.294
acceptors changes DOS, thus Coulomb gap appears near Fermi energy. Therefore, the band-to-band line overlay the line related to excited state of A þ -center. We perform Monte-Carlo calculations of DOS in order to account for both Coulomb interaction and temperature activation of A þ -centers. Calculated DOS is in a good agreement with experimental spectra in 6–55 K range (Fig. 1). Suggestion of the band-to-band origin of the lines that appears at the high temperatures is supported by measurements of the linear PL polarization under uniaxial stress at T ¼ 77 and 300 K taken on samples with the same geometry [4]. We carry out measurements of linear PL polarization under uniaxial stress at T ¼ 1:8 K when only A þ -centers contribute to PL spectra in the spectral range under consideration. Increasing stress causes similar high energy shift of both peaks. It proves our suggestion that the both peaks originate from the degenerate J ¼ 2 hole state split into two by the exchange interaction. The polarization ratio gradually decrease from the ground-state peak to the excited-state peak which is because the most probable transition from the excited state is the transition in the final state of the acceptor with mJ ¼ 7 12. The PL circular polarization spectra under applied magnetic field in Faraday geometry at T ¼ 4:2 K support our previous interpretation of the results. At moderate magnetic field the ground state stays unchanged, thus circular PL polarization of the peak depends only on the splitting of final hole state of the acceptor. Consequently, the spectral dependence of polarization within mJ ¼ 0 ground state’s line should be equal to zero near the PL maximum, exactly as we measured (Fig. 2). The mJ ¼ 71; 7 2 excited states are positively polarized due to the large positive g-factor of the heavy hole. Therefore the polarization degree increases on the high energy side of spectra. In conclusion, we carry out optical studies of A þ -centers in GaAs/AlGaAs QWs. The fine structure of the isolated A þ -centers was investigated and its collective behavior explanation was suggested.
ARTICLE IN PRESS P.V. Petrov et al. / Physica B 404 (2009) 5148–5149
simulated density of states, arb. units
B
A
Temperature: 1 -6 K 2 -9 K 3 -1 5K 4 -2 5K 5 -3 0K 6 -4 0K 7 -5 5K
B
PL intensity, arb. units
Temperature: 1 -6 K 2 -9 K 3 -1 5K 4 -2 5K 5 -3 0K 6 -4 0K 7 -5 5K
A
5149
1 2 3 C
C
4
D -0.015
-0.010
-0.005
0.000 energy, eV
0.005
5 6 7
D 0.010
1.515
0.015
1.520
1.525
1.530 1.535 energy, eV
1.540
1.545
Fig. 1. Simulated DOS (left) and PL spectra (right) at different temperatures.
A
8
B
polarization degree, %
PL intensity, arb. units.
6 103
102
A
7
4
Magneticf ield 1 - 1.3 T 7 - 3.3 T
2 B 0 -2 -4
PL intensity, arb.units
104
101 -6
1.520
1.525
1.530
1.535
1.540
Energy, eV
-8 1.524
1 1.526
1.528
1.530 1.532 energy, eV
1.534
1.536
Fig. 2. PL spectra at different pump intensity (left) and magnetic fields (right).
The study was supported by the RF Presidential Foundation (2951.2008.2), Russian Foundation for Basic Research (09-0200904-a), and Russian Academy of Sciences. References [1] M.A. Lampert, Phys. Rev. Lett. 1 (1958) 450.
[2] N.V. Agrinskaya, Y.L. Ivanov, P.A. Petrov, V.M. Ustinov, Solid State Commun. 126 (2003) 369. [3] N.S. Averkiev, A.E. Zhukov, Y.L. Ivanov, P.V. Petrov, K.S. Romanov, A.A. Tonkikh, V.M. Ustinov, G.E. Tsyrlin, Semiconductors 38 (2004) 217. [4] N.S. Averkiev, Yu.L. Ivanov, A.A. Krasivichev, P.V. Petrov, N.I. Sablina, V.E. Sedov, Semiconductors 42 (2008) 316.