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(GaAl) As hetero-junctions
Journal of Luminescence 40&41 (1988) 763 764 North-Holland, Amsterdam
763
PHOTOLUMINESCENCE RELATED TO THE INTERFACE OF GaAs/(GaA1)As HETERO—JUNCTIONS
W. OSSAU, E. BANGERT, C. LANDWEHR Physikalisches
Institut
der UniversitBt WUrzburg, Ri5ntgenring 8, D—87 Wiirzburg, ERG
W. Forschungsinstitut
der Deutschen Bundespost beim FTZ, D—61 Darmstadt, FRG
We have investigated an unusual new line observed in CaAs/(AlCa)As hetero—junctions, referred to as the H—band, by low—temperature photoluminescence experiments in magnetic fields up to 9.5 T. ‘Ihe direction of the magnetic field was turned from perpendicular to parallel to the interface. From the energetic shift, the splitting—behaviour, the lineshape and the temperature dependence of the luminescence line, we conclude that the H—band is emitted by the recombination of a flat—band electron with a hole confined to an excited subband.
1. Introduction
________________________________________
In the low temperature photolumirsescence spectrum of GaAs/(A1Ga)As heterostructures
an
unusual new line has been observed, referred to 1 which seems to be related to as the H—band, the interface. Up to now, however, there is no
“
convincing experimental evidence that the line
.-
is connected with the interface. 2. Experimental Results
-
E
The p—type GaAs/(AlGa)As—heterojunctions used
—
-
— -
*
,r~
-
.4”.
~..
thicknesses (9.5 nm < d < 53 mm), and aluminium concentrations of the (GaA1)As layers (0.5 < x
+5
-‘-
-
,_
in this study were grown by MBE. We investigated samples with different doping levels, space r—
-
+
z
-‘
W
<0.65) in order to observe the dependence of the photoluminescence spectrum on these parameters. The energy difference
B II z
E of the H—band to the
fundamental gap varies between 7 and 15 meV. From our study of 8 p—type samples we conclude that the
E increases with increasing aluminium
0
concentration and decreasing spacer—thickness. In a magnetic field perpendicular to the interface the H—band splits into four components with different polarization. The photon energies of the peak positions are plotted as a function of magnetic field in Fig. 1. We observe a linear
0022 2313/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
1
2
3
4
5
6
MAGNETIC FIELD
7
8
9
10
CT)
FIGURE 1 Peak positions of the H—band and the acceptor bound exciton as a represent function of—, magnetic strength. Crosses dots + field polari— zation of the H—band. The squares indicate the shift and splitting of the acceptor bound exci— ton
W~Ossau et al
764
/
Interface of GaAs
(GaA1)As heterojunctions
shift and splitting with magnetic field for the
the lowest electron Landau—level, we conclude
three low energy lines. The fourth component
that the H—band luminescence is caused by the
seems to have another origin. That it behaves
recombination of a free conduction band electron
qualitatively different can be seen by extrapo—
in the flat—band region with a hole confined in
lating the non—linear curve in Fig. To prove the conjecture
1 to B
=
0 T.
that the H—band is
subbands of a two—dimensional hole gas in a
correlated to the interface we have varied the angle
one of the aubbands near the interface. Electric magnetic field have been calculated in the self— 2 If we compare consistent Hartree—approximation.
between magnetic field and sample nor—
mal. The splitting of the H—band is a linear
these
function of the cosine of the angle between
small shift of the Fermi—energy towards the
interface and magnetic field. This means that
valence band, caused by the photoexcitation, we
the splitting is a function of the effective
find that the final state of the H—band recombi—
magnetic field strength normal to the inter—
nation fits well with the second heavy hole
face. When the magnetic fields is parallel to
subband. The three components shifting linearly
the interface the H—band shows no splitting. For
are attributed to the first three Landau—levels
this field direction the shift of the remaining
of this subband. The fourth component visible in
component is still linear with magnetic field
high magnetic fields (Fig. 1) coincides with the
and about 8.4 meV at 10 T. This energy shift is
first
gleater than that of bound exciton lines
and
calculations with the data and consider a
light hole subband only 1 meV apart.
The proposed transition is indirect in real
nearly identical with that of the first Landau—
space. Because the recombination between conduc—
level of GaAs conduction band electrons (8.6
tion band electron and subband hole depends on
meV).
the overlap of the wave functions,
Furthermore, the line shape of the H—band
a transition
can described very well by the product of the
to the highest subband is not probable. The
density of states function of conduction band
calculations show that for the sample discussed
electrons and a Boltzman—factor.
in this paper the wavefunction connected with
For a lattice
temperature of 10 K we obtain from the line
the first excited subband vanishes only at dis—
shape and effective
tances from the interface as large as 50 nm.
electron temperature of
about 24 K. This shows unambiguously that the
Consequently, the assumption of sufficient
the photoexcited
lap of the electron and hole wavefunctions is
carriers
have lost their excess
energy and are cooled via carrier—phonon
not unreasonable.
scattering.
Acknowledgements:
3. Discussion
over—
The work was supported by the Deutsche
From the energy shift in a magnetic field it
Forschungsgemeinschaft.
is obvious that the electron and the hole causing the luminesce are not bound by Coulomb interaction. Excitonic binding would result in a non—linear — diamagnetic — shift, much smaller than the one observed. The angular dependence the line in a magnetic field clearly indicates
two—dimensional behaviour. As the shift in pa— rallel fields corresponds to that expected for
of
References: 1. Y.R. Yuan, M.A.A. Pudensi, G.A. Vawter and J.L. Merz, J. Appl. Phys. 58, 397, 1985
2. E. Bangert and G. Landwehr, Superlattices and Microstructures, 1, 363, 1985.
763
PHOTOLUMINESCENCE RELATED TO THE INTERFACE OF GaAs/(GaA1)As HETERO—JUNCTIONS
W. OSSAU, E. BANGERT, C. LANDWEHR Physikalisches
Institut
der UniversitBt WUrzburg, Ri5ntgenring 8, D—87 Wiirzburg, ERG
W. Forschungsinstitut
der Deutschen Bundespost beim FTZ, D—61 Darmstadt, FRG
We have investigated an unusual new line observed in CaAs/(AlCa)As hetero—junctions, referred to as the H—band, by low—temperature photoluminescence experiments in magnetic fields up to 9.5 T. ‘Ihe direction of the magnetic field was turned from perpendicular to parallel to the interface. From the energetic shift, the splitting—behaviour, the lineshape and the temperature dependence of the luminescence line, we conclude that the H—band is emitted by the recombination of a flat—band electron with a hole confined to an excited subband.
1. Introduction
________________________________________
In the low temperature photolumirsescence spectrum of GaAs/(A1Ga)As heterostructures
an
unusual new line has been observed, referred to 1 which seems to be related to as the H—band, the interface. Up to now, however, there is no
“
convincing experimental evidence that the line
.-
is connected with the interface. 2. Experimental Results
-
E
The p—type GaAs/(AlGa)As—heterojunctions used
—
-
— -
*
,r~
-
.4”.
~..
thicknesses (9.5 nm < d < 53 mm), and aluminium concentrations of the (GaA1)As layers (0.5 < x
+5
-‘-
-
,_
in this study were grown by MBE. We investigated samples with different doping levels, space r—
-
+
z
-‘
W
<0.65) in order to observe the dependence of the photoluminescence spectrum on these parameters. The energy difference
B II z
E of the H—band to the
fundamental gap varies between 7 and 15 meV. From our study of 8 p—type samples we conclude that the
E increases with increasing aluminium
0
concentration and decreasing spacer—thickness. In a magnetic field perpendicular to the interface the H—band splits into four components with different polarization. The photon energies of the peak positions are plotted as a function of magnetic field in Fig. 1. We observe a linear
0022 2313/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
1
2
3
4
5
6
MAGNETIC FIELD
7
8
9
10
CT)
FIGURE 1 Peak positions of the H—band and the acceptor bound exciton as a represent function of—, magnetic strength. Crosses dots + field polari— zation of the H—band. The squares indicate the shift and splitting of the acceptor bound exci— ton
W~Ossau et al
764
/
Interface of GaAs
(GaA1)As heterojunctions
shift and splitting with magnetic field for the
the lowest electron Landau—level, we conclude
three low energy lines. The fourth component
that the H—band luminescence is caused by the
seems to have another origin. That it behaves
recombination of a free conduction band electron
qualitatively different can be seen by extrapo—
in the flat—band region with a hole confined in
lating the non—linear curve in Fig. To prove the conjecture
1 to B
=
0 T.
that the H—band is
subbands of a two—dimensional hole gas in a
correlated to the interface we have varied the angle
one of the aubbands near the interface. Electric magnetic field have been calculated in the self— 2 If we compare consistent Hartree—approximation.
between magnetic field and sample nor—
mal. The splitting of the H—band is a linear
these
function of the cosine of the angle between
small shift of the Fermi—energy towards the
interface and magnetic field. This means that
valence band, caused by the photoexcitation, we
the splitting is a function of the effective
find that the final state of the H—band recombi—
magnetic field strength normal to the inter—
nation fits well with the second heavy hole
face. When the magnetic fields is parallel to
subband. The three components shifting linearly
the interface the H—band shows no splitting. For
are attributed to the first three Landau—levels
this field direction the shift of the remaining
of this subband. The fourth component visible in
component is still linear with magnetic field
high magnetic fields (Fig. 1) coincides with the
and about 8.4 meV at 10 T. This energy shift is
first
gleater than that of bound exciton lines
and
calculations with the data and consider a
light hole subband only 1 meV apart.
The proposed transition is indirect in real
nearly identical with that of the first Landau—
space. Because the recombination between conduc—
level of GaAs conduction band electrons (8.6
tion band electron and subband hole depends on
meV).
the overlap of the wave functions,
Furthermore, the line shape of the H—band
a transition
can described very well by the product of the
to the highest subband is not probable. The
density of states function of conduction band
calculations show that for the sample discussed
electrons and a Boltzman—factor.
in this paper the wavefunction connected with
For a lattice
temperature of 10 K we obtain from the line
the first excited subband vanishes only at dis—
shape and effective
tances from the interface as large as 50 nm.
electron temperature of
about 24 K. This shows unambiguously that the
Consequently, the assumption of sufficient
the photoexcited
lap of the electron and hole wavefunctions is
carriers
have lost their excess
energy and are cooled via carrier—phonon
not unreasonable.
scattering.
Acknowledgements:
3. Discussion
over—
The work was supported by the Deutsche
From the energy shift in a magnetic field it
Forschungsgemeinschaft.
is obvious that the electron and the hole causing the luminesce are not bound by Coulomb interaction. Excitonic binding would result in a non—linear — diamagnetic — shift, much smaller than the one observed. The angular dependence the line in a magnetic field clearly indicates
two—dimensional behaviour. As the shift in pa— rallel fields corresponds to that expected for
of
References: 1. Y.R. Yuan, M.A.A. Pudensi, G.A. Vawter and J.L. Merz, J. Appl. Phys. 58, 397, 1985
2. E. Bangert and G. Landwehr, Superlattices and Microstructures, 1, 363, 1985.