Far-infrared magneto-optical study of two-dimensional electrons and holes
in InAs/Al
x
Ga
1 x
Sb quantum wells
J. Kono* and B. D. McCombe
†
Department of Physics, State University of New York at Buffalo, Buffalo, New York 14260
J.-P. Cheng
‡
Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
I. Lo,
§
W. C. Mitchel, and C. E. Stutz
Wright Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio 45433-6533
Received 6 May 1996; revised manuscript received 15 July 1996
We present results of a detailed far-infrared magneto-optical study on a series of high-mobility
InAs/Al
x
Ga
1 -x
Sb x =1.0, 0.8, 0.5, 0.4, 0.2, and 0.1 type-II single quantum wells. A wide range of phenom-
ena arising from the unusual properties of two-dimensional 2D electrons and holes and their Coulomb
interaction in high magnetic fields has been revealed. Semiconducting samples x 0.4, in which only 2D
electrons exist in the InAs wells, exhibit cyclotron-resonance CR splittings due to large conduction-band
nonparabolicity. Semimetallic samples x =0.1 and 0.2, in which both 2D electrons in InAs and 2D holes in
Al
x
Ga
1 -x
Sb are present, show two additional lines e and h - X lines as well as electron and hole CR. The
X -lines increase in intensity at the expense of CR with increasing electron-hole ( e - h ) pair density, decreasing
temperature, or increasing magnetic field at low field, suggesting that they are associated with e - h binding
which is increased by the magnetic field. The electron CR shows strongly oscillatory linewidth, amplitude, and
mass, part of which are interpreted in the light of the unusual ‘‘antinonparabolic’’ band structure resulting from
band overlap and coupling between conduction-band states in InAs and valence-band states in Al
x
Ga
1 -x
Sb;
part of these results are qualitatively consistent with the predictions of Altarelli and co-workers. The X lines are
attributed to internal transitions of correlated electron e - h pairs excitons in high magnetic fields mediated by
the excess electron density. S0163-18299704403-2
I. INTRODUCTION
Semiconductor heterostructures made from the combina-
tion of InAs and Al
x
Ga
1 -x
Sb have been of considerable in-
terest for the past two decades because of their unusual band-
edge alignment,
1
due to which electrons and holes are
spatially separated and confined in different layers. Interest
in this material system has also been stimulated by the large
nonparabolicity, small mass, and large g factor of the elec-
trons contained in the InAs layers, compared to commonly
used GaAs/Al
x
Ga
1 -x
As systems, as well as possible appli-
cations as infrared detectors and sources. Until recently,
however, systematic experimental studies of intrinsic proper-
ties of this system had been limited to the end member
InAs/GaSb,
2
primarily due to the difficulty in growing high-
mobility, impurity-free samples. The past several years have
seen remarkable progress in the growth of InAs/Al
x
Ga
1 -x
Sb
quantum-well QW structures with high-mobility 10
5
cm
2
/V s two-dimensional 2D carriers.
3–5
This has led to
such experimental observations as zero-field spin splitting
due to asymmetric potential wells,
6
magnetic-field-induced
semimetal-semiconductor transitions,
7–9
cyclotron-resonance
CR splittings due to nonparabolicity,
10,11
strong CR
oscillations,
12
far-infrared FIR transitions attributed to
intra-excitonic resonance,
12,13
and magnetic-field-induced
spin-conserving and spin-flip intersubband transitions.
14
The unusual band lineups of the Al
x
Ga
1 -x
Sb/
InAs/Al
x
Ga
1 -x
Sb type-II single QW’s under ideal condi-
tions with no band bending are schematically depicted in
Fig. 1 for three different Al compositions x =0, 0.3, and
1.0. The effective band gap E
G
* , defined by the energy dif-
ference between the lowest electron subband and the highest
hole subband, can be varied over a wide range ( -0.15 eV
E
G
* +0.3 eV) by varying x and/or the well width. The
end member, GaSb/InAs/GaSb x =0, is a misaligned or
broken-gap type-II ‘‘semimetallic’’ QW. When the well
FIG. 1. Schematic conduction solid lines and valence dashed
lines band lineups for InAs-Al
x
Ga
1 -x
Sb x =0, 0.3, and 1.0 single
quantum wells under ideal conditions charge transfer and band
bending not included. The ideal system is ‘‘semimetallic’’ when
x 0.3 neglecting confinement.
PHYSICAL REVIEW B 15 JANUARY 1997-I VOLUME 55, NUMBER 3
55 0163-1829/97/553/161720/$10.00 1617 © 1997 The American Physical Society