Three - University of Arkansas Physics Department

APPLIED PllYSICS LETTERS VOLUME 75. NUMBER 3
Morphology of lnAs self-organized islands on AlAs surfaces
P. ~allet,~) J. B. Smathers, and G. J. Salamob)
Department of Physics, Uriiversi@ oj'drkansas, Fnyelteville, Arknrtsas 72701
(Received 19 April 1999; accepted for publication 21 May 1999)
We report an in situ n~olecular-bean1 epitaxy-scanning tunneling microscopy study of
three-dimensional (3D) sclf-organizcd lnAs islands on AlAs surfaces. The evolution of thc density
and n~orphology of these islands is investigated as a function of the InAs coverage and substrate
temperature. It is shown that the 2D island density is already high just prior to 3D island formation
and remains constant for 3D structures as the InAs coverage is increased. This observation contrasts
with the InAsJGaAs systein and makes possible the growth of very high densities of sinall quantum
dots. O 1999 Anlerican Institute of Physics. [S0003-6951(99)00329-01
The use of the heteroepitaxy of highly nlisn~atched inaterials
to produce coherent self-assembled three-diinensional
(3D) islands lying on a two-dimensional (2D) wetting layer
has lead the way to a new generation of quantum functional
devices.'-3 These devices are based on the extraordinary bchavior
that results from carrier confinenlent in all three dimensions
and are called quantum dots. Quantuin dot lasers,
for example, have lower thresholds, increased modulation
speed, and larger differential gain, while quanhlm dot detectors
can be used at normal incidence.
While the quantum dot concept sounds like there are
many exciting applications, in practice, there arc some serious
probleins that limit its possibilities. Onc challenge is to
achieve fabrication of a large nuinber of dot structures without
significant size fluctuations. Size fluctuation leads to inhon~ogeneous
broadening or a shifted energy level structure
for each dot, preventing formation of states that are concentrated
in k space. Applications will clearly depend on the
development of fabrication techniques that can control the
quality, size, and uniformity of dots. For this reason, the
structural characterization of self-assembled InAsIGaAs islands
has been extensively pursued in the past few years.4"
These investigations indicate a growth inode that is strongly
depcndent on deposition conditions and interface inte~niixing.
Despite the intense investigation of this system, however,
many questions remain about the nature of the 2D to
3D transition.
While the growth of InAs islands inside a GaAs matrix
has been the center of attention, careful studies have also
been made in 1n~b/CJa~s,~ I~G~AS/G~AS,'~ I~G~PII~P,"
~i~e/~i,'~ and 1n~sJ~l~s.l~ The InAsIAlAs quantum dots
provide an especially attractive system for optical and electronic
devices due to their significantly higher barrier energy.
For example, the higher barrier makes possible greater tunability
of the intersubband infrared transitions as well as enhanced
stability at high temperatures.14
In this letter, we investigate the 2D to 3D transition during
the molecular-bean1 epitaxy (MBE) of InAs on AIAs. We
also investigated the density and morphology of the 3D islands
as a function of InAs coverage and substrate tempera-
ture. We carried out our investigation using a MBE facility
with an in sit11 scanning tunneling microscope (STM). Substrate
ternpcratures during the growth wcre measured using
an optical technique based on the wavelength dependence of
the band edge as a hnction of temperature. Details about the
experimental apparatus can be found in Ref. 15. Samples
were grown on n-type GaAs wafers. After the growth of a
0.5 pm GaAs buffer layer at 580 "C, a flat GaAs surface is
prcpared by annealing under an As4 flux. Thc growth of
AlAs is initiated at 580 OC and the substrate temperature is
ramped and stabilized at 500 OC while depositing AIAs. InAs
is then deposited with an As4 flux of 6.OX Torr. The
growth rate is chosen to be 0.1 nionolayer per second (MLI
s). We exposed the surface to both In and As fluxes during 3
s following by 17 s of As only. This sequence is repeated
until the desired coverage is reached. The sample is then
cooled to room temperature while an As4 flux is maintained
until the temperature falls below 350°C. After the sample
reaches 200°C, it is transferred under vacuum to the STM
chamber. Interestingly, our investigation of InAsIAlAs 3D
structures shows behavior that is remarkably different from
that of InAsIGaAs 3D structures, despite the fact that both
AlAs and GaAs have the same lattice mismatch of 7% with
InAs. This difference begins with the nature of the wetting
layer. Figure l(a) shows STM images of the InAs wetting
layer on GaAs after the deposition of 2.1 MLs at 500 OC. The
"meandering chains" indicate a highly cormgatcd surfacc
due to intennixing between indium and galliurn atoms.'"
Meanwhile, under the exact same conditions, STM images
")~lcch.onic mail: pbalIct@comp.uark.edu
b'~lectronic ruarl: sala~no(~comp.uaik.edu
FIG. I. High-resolution 35X 35 nm STM imagcs of 2.1 ML of I ds dcpositcd
on (a) GaAs and (b) A1As. Thc whitc rcgions on both imagcs arc onc
~nionolayer high, 2D features.
0003-6951/99/75(3)/337/3/$15.00 337 0 1999 American Institute of Physics
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