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InP quantum wells in high magnetic field
Surface Science 229 ( 1990) 5 12-5 I4 North-Holland
512
PHOTOLUMINESCENCE AND TRANSPORT WELLS IN HIGH MAGNETIC FIELD D.G. HAYESa,b, MS. SKOLNICK”, and K.J. NASH”
STUDIES
OF WIDE (InGa)As/InP
L. EAVESb, P.E. SIMMONDS”,b.c,
L.L. TAYLOR”,
QUANTUM
S.J. BASS”
“RoyalSignals and Radar Establishment, St. Andrew Road, Great Malvern, Worcs. U’R14 3PS, UK bDepartment of Physics, University of Nottingham, Nottingham NG7 2RD, UK ‘Department of Physics, University of Wollongong, Woilongong, NSW 2500, Australia
Received I 1 July 1989; accepted for publication 14 September 1989
A photoluminescence and transport study of wide (InGa)As-InP
We have used photoluminescence (PL) and transport experiments to study the electronic properties of wide [ 1 ] (InGa)As/InP quantum wells ( QW’s) in magnetic fields up to 20 T [ 21. The wells are modulation doped to give electron densities of - lOI* cm-* in the QW. Results from similar experiments on an asymmetrically doped 80 nm well and a symmetrically doped 70 nm well in magnetic fields up to 10 T have been published previously [ 3,4]. We concentrate here on a symmetrically doped 70 nm well with higher carrier density than ref. [ 3 1, and extend our previous work from 10 up to 20 T. A self-consistent calculation of the conduction band for the structure (in zero field) indicates that three electronic subbands are populated. Due to band-bending, electrons in the two lowest eigenstates, E, and E,, are predominantly confined at opposite interfaces of the QW and are approximately equal in energy. The electrons in the third, high-energy subband, Ez, occupy states characteristic of the wide well rather than the narrow heterojunction potential and are only weakly confined. This results in a relatively high-electron probability density in the centre of the QW for this subband. A PL spectrum taken at 2 K under low excitation intensity shows two broad features, one corresponding to radiative recombination from E2 and the other, to lower energy, from E, and E, (which are not resolved). The E2 related feature is stronger since the photocreated holes in the valence band thermalise to 0039-6028/90/$03.50 (North-Holland)
0 Elsevier Science Publishers B.V.
quantum wells in magnetic fields up to 20 T is reported.
their lowest energy state, near the QW centre, where recombination with E2 electrons has high oscillator strength [ 1,3,4]. In magnetic field (B) applied perpendicular to the plane of the QW, the PL features evolve into Landau level (LL) structure. With increasing magnetic field, as the LL degeneracy increases, depopulation of the E2 subband is observed accompanied by a strong anomaly in the LL energies of the lower subbands. Fig. 1 shows the PL peak energies as a function of magnetic field; the solid lines represent slopes corresponding to the LL energies, given by E,,, = (n + $ )fieB/m* for the nth LL of the mth subband, where we use an electron effective mass m* = 0.048m0 [ 41. From the B= 0 PL spectra we find that the high energy cut-off for transitions from the electron Fermi sea corresponds to a transition energy of 82 1 meV. By extrapolation of the B < 5 T LL peaks to zero field, we obtain Fermi energies (EF) of 26, 26 and 10 meV for I&,,, EF, and EF2 respectively, which give a QW electron density of 2 x (5.21 x 10” cm-*) + (2.0 x 10” cm-*) = 1.24 x lOI* cm-*. Above - 6 T, higher LL’s of each subband have depopulated leaving only the lowest LL’s E2,0, E,,, and Eo,” occupied. Between 8 and 10 T, the lower energy feature rapidly becomes stronger with an associated decrease in the energy separation between the two features [ I] as the E2,0 LL depopulates and weakens (at Bz 12 T, the tilling factor u=4 at 12.8 T). Between 13 and 16 T, the higher energy feature
D. G. Hayes et al./Photoluminescence and transport of(lnGu,Ms/InP Q W’s
513
InGaAs-InP 70nm quantum wsli _ doubie-sided modulation doped
790
0
I
I
I
I
I
I
I
I
I
2
4
6
6
10
12
14
16
16
20
MAGNETIC FIELD [TJ
Fig. 1. Photoluminescence transition energies as a function of magnetic field from 0 to 20 T for a symmetric 70 nm quantum well. The peaks from the three subbands observed at B=O (the lowest two are degenerate), split into a series of Landau levels labelled E,,,. In this notation M refers to the subband index, and n to the Landau level number. Between V= 5 and v--4 ( 10 to 12.5 T). I&0 loses intensity as the m = 2 subband depopulates. Concomitant with this, En.,, Eric,move up in energy as the potential profile renormalizes as charge passes from the m = 2 subband down to the m = 0, I subbands. A further renormalization of the potential occurs at high field (tilling factor Y< 4), leading to a lifting of the m=O, I subband degeneracy, as indicated by the observation of separate I&, E,,e peaks on the figure. ,For B> 14 T, spin splitting of the lower energy peak is observed (labelled cr+, o- ). Open symbols on the figure represent weak spectral features.
gains intensity and the energy separation between the two features increases, and for B> 15 T the lower energy feature splits into two peaks. Polarisation studies show that the higher energy feature and the lower of the two split peaks have opposite polarisation to the middle split peak. We attribute the two lowest peaks to the two spin-split states of E,,, and the higher feature to the lowest spin-split level of E,,o. Above 14 T the conduction band profile is renormalised due to electron charge redistribution such that the E, and E, states are split by _ 10 meV enabling the two subbands to be resolved in the high field. Simultaneous magnetotransport measurements are extremely important in analysing the optical data. A Fourier analysis of the low field (Bt2.5 T) Shubnikov-de Haas ‘oscillations shows two different series corresponding to carrier densities of 5.56 x 10’ ’ and 1.5 x 10' 'cmm2. At these low fields we cannot resolve the slightly different electron densities of the
E0 and E, states. The total carrier density is therefore 2X(5.%X10” cm-2)+(l.5X10’1 cm-*)= 1.26~ lOi cm-* in very good agreement with that obtained from PL. The tilling factors Y f = h/Rxye2 ) obtained from quantum Hall measurements are essential in interpreting the fan diagram (fig. 1). Between8T (v=6)and 1OT (~=5),andbetween 13 T (~=4) and 16.5 T (~=3), the explanation of the apparently anomalous behaviour of the PL peak energies (in terms of the depopulation of electrons from higher to lower subbands, and the consequent renormalisation of the potential profile due to spatial redistribution of charge) is confirmed by the high field transport measurements. We would like to express our thanks to J.C. Maan and other staff of the Max-Planck-Institute, Grenoble, where the high field experiments were carried out.
LXG. Hayes et ai/Phorolutnlnescence and transport a~~~nGaj~~s/lFiP QWs
514
[2] The present work will be discussed
References
fully in a forthcoming
publication.
[ I ] Magneto-optical
experiments were first reported on wide GaAs/(AIGa)As quantum wells by: F. Meseguer, J.C. Maan and K. Ploog, Phys. Rev. B 35 ( 1987) 2505;
[3] DC. Hayes, M.S. Skolnick, L. Eaves, L.L. Taylor and S.J. Bass, in: High Magnetic Fields in Semiconductor Physics 11, Ed. G. Landwehr, Springer Series in Solid-State Sciences, Vol. 87 (Springer, Berlin, 1989) p. 305.
T. Rotger, J.C. Maan, P. Wyder, F. Meseguer J. Phys. (Paris) 48 ( 1987) CS-389.
[4] P.E. Simmonds, MS. Skolnick, L.L. Taylor, S.J. Bass and K.J. Nash, Solid State Commun., 67 (1988) 1151.
and K. Pioog.
512
PHOTOLUMINESCENCE AND TRANSPORT WELLS IN HIGH MAGNETIC FIELD D.G. HAYESa,b, MS. SKOLNICK”, and K.J. NASH”
STUDIES
OF WIDE (InGa)As/InP
L. EAVESb, P.E. SIMMONDS”,b.c,
L.L. TAYLOR”,
QUANTUM
S.J. BASS”
“RoyalSignals and Radar Establishment, St. Andrew Road, Great Malvern, Worcs. U’R14 3PS, UK bDepartment of Physics, University of Nottingham, Nottingham NG7 2RD, UK ‘Department of Physics, University of Wollongong, Woilongong, NSW 2500, Australia
Received I 1 July 1989; accepted for publication 14 September 1989
A photoluminescence and transport study of wide (InGa)As-InP
We have used photoluminescence (PL) and transport experiments to study the electronic properties of wide [ 1 ] (InGa)As/InP quantum wells ( QW’s) in magnetic fields up to 20 T [ 21. The wells are modulation doped to give electron densities of - lOI* cm-* in the QW. Results from similar experiments on an asymmetrically doped 80 nm well and a symmetrically doped 70 nm well in magnetic fields up to 10 T have been published previously [ 3,4]. We concentrate here on a symmetrically doped 70 nm well with higher carrier density than ref. [ 3 1, and extend our previous work from 10 up to 20 T. A self-consistent calculation of the conduction band for the structure (in zero field) indicates that three electronic subbands are populated. Due to band-bending, electrons in the two lowest eigenstates, E, and E,, are predominantly confined at opposite interfaces of the QW and are approximately equal in energy. The electrons in the third, high-energy subband, Ez, occupy states characteristic of the wide well rather than the narrow heterojunction potential and are only weakly confined. This results in a relatively high-electron probability density in the centre of the QW for this subband. A PL spectrum taken at 2 K under low excitation intensity shows two broad features, one corresponding to radiative recombination from E2 and the other, to lower energy, from E, and E, (which are not resolved). The E2 related feature is stronger since the photocreated holes in the valence band thermalise to 0039-6028/90/$03.50 (North-Holland)
0 Elsevier Science Publishers B.V.
quantum wells in magnetic fields up to 20 T is reported.
their lowest energy state, near the QW centre, where recombination with E2 electrons has high oscillator strength [ 1,3,4]. In magnetic field (B) applied perpendicular to the plane of the QW, the PL features evolve into Landau level (LL) structure. With increasing magnetic field, as the LL degeneracy increases, depopulation of the E2 subband is observed accompanied by a strong anomaly in the LL energies of the lower subbands. Fig. 1 shows the PL peak energies as a function of magnetic field; the solid lines represent slopes corresponding to the LL energies, given by E,,, = (n + $ )fieB/m* for the nth LL of the mth subband, where we use an electron effective mass m* = 0.048m0 [ 41. From the B= 0 PL spectra we find that the high energy cut-off for transitions from the electron Fermi sea corresponds to a transition energy of 82 1 meV. By extrapolation of the B < 5 T LL peaks to zero field, we obtain Fermi energies (EF) of 26, 26 and 10 meV for I&,,, EF, and EF2 respectively, which give a QW electron density of 2 x (5.21 x 10” cm-*) + (2.0 x 10” cm-*) = 1.24 x lOI* cm-*. Above - 6 T, higher LL’s of each subband have depopulated leaving only the lowest LL’s E2,0, E,,, and Eo,” occupied. Between 8 and 10 T, the lower energy feature rapidly becomes stronger with an associated decrease in the energy separation between the two features [ I] as the E2,0 LL depopulates and weakens (at Bz 12 T, the tilling factor u=4 at 12.8 T). Between 13 and 16 T, the higher energy feature
D. G. Hayes et al./Photoluminescence and transport of(lnGu,Ms/InP Q W’s
513
InGaAs-InP 70nm quantum wsli _ doubie-sided modulation doped
790
0
I
I
I
I
I
I
I
I
I
2
4
6
6
10
12
14
16
16
20
MAGNETIC FIELD [TJ
Fig. 1. Photoluminescence transition energies as a function of magnetic field from 0 to 20 T for a symmetric 70 nm quantum well. The peaks from the three subbands observed at B=O (the lowest two are degenerate), split into a series of Landau levels labelled E,,,. In this notation M refers to the subband index, and n to the Landau level number. Between V= 5 and v--4 ( 10 to 12.5 T). I&0 loses intensity as the m = 2 subband depopulates. Concomitant with this, En.,, Eric,move up in energy as the potential profile renormalizes as charge passes from the m = 2 subband down to the m = 0, I subbands. A further renormalization of the potential occurs at high field (tilling factor Y< 4), leading to a lifting of the m=O, I subband degeneracy, as indicated by the observation of separate I&, E,,e peaks on the figure. ,For B> 14 T, spin splitting of the lower energy peak is observed (labelled cr+, o- ). Open symbols on the figure represent weak spectral features.
gains intensity and the energy separation between the two features increases, and for B> 15 T the lower energy feature splits into two peaks. Polarisation studies show that the higher energy feature and the lower of the two split peaks have opposite polarisation to the middle split peak. We attribute the two lowest peaks to the two spin-split states of E,,, and the higher feature to the lowest spin-split level of E,,o. Above 14 T the conduction band profile is renormalised due to electron charge redistribution such that the E, and E, states are split by _ 10 meV enabling the two subbands to be resolved in the high field. Simultaneous magnetotransport measurements are extremely important in analysing the optical data. A Fourier analysis of the low field (Bt2.5 T) Shubnikov-de Haas ‘oscillations shows two different series corresponding to carrier densities of 5.56 x 10’ ’ and 1.5 x 10' 'cmm2. At these low fields we cannot resolve the slightly different electron densities of the
E0 and E, states. The total carrier density is therefore 2X(5.%X10” cm-2)+(l.5X10’1 cm-*)= 1.26~ lOi cm-* in very good agreement with that obtained from PL. The tilling factors Y f = h/Rxye2 ) obtained from quantum Hall measurements are essential in interpreting the fan diagram (fig. 1). Between8T (v=6)and 1OT (~=5),andbetween 13 T (~=4) and 16.5 T (~=3), the explanation of the apparently anomalous behaviour of the PL peak energies (in terms of the depopulation of electrons from higher to lower subbands, and the consequent renormalisation of the potential profile due to spatial redistribution of charge) is confirmed by the high field transport measurements. We would like to express our thanks to J.C. Maan and other staff of the Max-Planck-Institute, Grenoble, where the high field experiments were carried out.
LXG. Hayes et ai/Phorolutnlnescence and transport a~~~nGaj~~s/lFiP QWs
514
[2] The present work will be discussed
References
fully in a forthcoming
publication.
[ I ] Magneto-optical
experiments were first reported on wide GaAs/(AIGa)As quantum wells by: F. Meseguer, J.C. Maan and K. Ploog, Phys. Rev. B 35 ( 1987) 2505;
[3] DC. Hayes, M.S. Skolnick, L. Eaves, L.L. Taylor and S.J. Bass, in: High Magnetic Fields in Semiconductor Physics 11, Ed. G. Landwehr, Springer Series in Solid-State Sciences, Vol. 87 (Springer, Berlin, 1989) p. 305.
T. Rotger, J.C. Maan, P. Wyder, F. Meseguer J. Phys. (Paris) 48 ( 1987) CS-389.
[4] P.E. Simmonds, MS. Skolnick, L.L. Taylor, S.J. Bass and K.J. Nash, Solid State Commun., 67 (1988) 1151.
and K. Pioog.