Resonant magnetotunneling through individual self-assembled InAs quantum dots
I. E. Itskevich,* T. Ihn, A. Thornton, M. Henini, T. J. Foster, P. Moriarty, A. Nogaret, P. H. Beton, L. Eaves, and
P. C. Main
Department of Physics, University of Nottingham, Nottingham, NG7 2RD, United Kingdom
Received 20 August 1996
Resonant peaks are observed in the low-temperature current-voltage I ( V ) characteristics of a single-barrier
GaAs/AlAs/GaAs diode with InAs quantum dots incorporated in the AlAs tunnel barrier. We argue that each
peak arises from single-electron tunneling through a discrete zero-dimensional state of an individual InAs dot
in the barrier. Each peak splits into sharp components for magnetic field B I ; the I ( V ) curve probes the density
of Landau-quantized states in the emitter-accumulation layer. A dot size of 10 nm was estimated from the
diamagnetic peak shift for B I . S0163-18299611848-8
An array of quantum dots QD’s produced by self-
organized Stranski-Krastanov heteroepitaxial growth is
formed when more than a critical layer thickness is grown on
certain surfaces of different chemical composition and lattice
constant. The system that has received the most attention to
date consists of InAs dots grown on a GaAs or AlGaAs
surface.
1–12
The electronic states of self-assembled dots
capped by lattice matched layers have been investigated
mainly by optical
2,3,5,7,10–12
and capacitance
5,6
spectroscopy.
Due to variations in size, shape, and strain, a dot ensemble
has a wide distribution of eigenenergies. Typically the opti-
cal spectra correspond to the dot ensemble;
5,10–12
however,
photoluminescence and cathodoluminescence spectra taken
on submicron areas reveal emission lines corresponding to
individual dots.
2,3,7
In this paper we report tunnel current investigations of the
electron states in InAs quantum dots embedded in a thin
AlAs layer of a single-barrier GaAs/AlAs/GaAs heterostruc-
ture. By tuning the applied voltage we can observe resonant
tunneling through an individual dot. We use magnetotunnel-
ing spectroscopy to probe the initial and final states in the
tunneling transition. We are also able to estimate the spatial
extent of the confined electron wave function in the dot. In
addition, the tunnel current through the localized state is also
a sensitive probe of the properties of the electrons in the
emitter contact.
Our device was prepared by first growing a 1- m-thick
GaAs buffer layer with graded Si doping on a 100
n
+
-GaAs substrate, followed by 100 nm of undoped GaAs
and 5 nm of AlAs. The QD’s were formed by growing 1.8
ML of InAs on the AlAs at a growth temperature of
520 °C. The dots were then nominally capped with a further
5 nm of AlAs, thus creating a 10-nm AlAs tunnel barrier.
This was followed by an undoped 100-nm GaAs layer and
capped by 1 m of n
+
-GaAs of graded doping. Since we
cannot exclude possible Al alloying, the dots should strictly
be referred to as In-based, but for simplicity we henceforth
refer to them as InAs QD’s. A control sample, lacking the
InAs layer but with other parameters identical, was also pre-
pared. Circular mesas of various diameters, from 30 m to
400 m, were produced using optical lithography. AuGe
was alloyed into the n
+
-GaAs layers to form Ohmic con-
tacts.
To characterize the device, scanning electron and tunnel-
ing microscopy SEM and STM and photoluminescence
PL spectroscopy were used. SEM and STM imaging was
performed on samples of the same design but with the
growth terminated after depositing the InAs layers. It
allowed us to estimate the density of dots as 2 10
11
cm
-2
, with a dot size (1010) nm
2
. A PL spectrum of
our tunnel structure, recorded with a Ge detector using
He-Ne laser excitation ( =6328 Å, is shown in the inset of
Fig. 1. The spectrum exhibits a broad line with a maximum a
few hundred meV below the GaAs band-gap energy. The
line corresponds to the emission from the dot ensemble and
is similar to that reported by other groups.
2,11,12
The expected conduction-band potential profile for our
device is shown in Fig. 1. When a voltage V is applied be-
tween collector and emitter, a two-dimensional electron gas
2DEG, degenerate at low temperatures, accumulates in the
undoped GaAs region adjacent to the tunnel barrier. Reso-
nant tunnelling occurs if an electronic state of a QD in the
barrier is resonant with a state in the 2DEG. Note that V is
the external voltage applied to the device while the voltage
drop V
1
between the 2DEG Fermi level and the states in the
middle of the barrier is only a small fraction of V . As V
1
depends nonlinearly on V because of charge redistribution in
the structure, we define the leverage factor f as
( dV
1
/ dV )
-1
.
The current-voltage characteristics I ( V ), recorded for a
FIG. 1. A schematic energy band diagram of the sample under
an applied voltage V . Inset: photoluminescence spectrum from the
sample at 4.2 K.
PHYSICAL REVIEW B 15 DECEMBER 1996-I VOLUME 54, NUMBER 23
54 0163-1829/96/5423/164014/$10.00 16 401 © 1996 The American Physical Society