Three - University of Arkansas Physics Department

JOURPU'AL OF APPLIED PHYSICS VOLUME 90. NUMBER 1 I JULY 2001
Control of size and density of InAs/(AI,Ga)As self-organized islands
P. Ballet, J. 6. Smathers, H. Yang, C. L. Workman, and G. J. salamoa)
Department of Physics, University of'Arkansas, Fayetteville, Arkansas 72701
(Received 19 July 2000; accepted for publication 29 January 2001)
We report on the influence of the chemical composition of the (Al, Ga)As surface on the forniation
of strain induced three-dimensional (3D) InAs islands. Thc expcriments havc bcen carried out using
a molecular beam epitaxy facility combined with a scanning tunneling ~nicroscope enabling in situ
surface characterization. The evolution of the dcnsity and nlorphology of these islands is
investigated as a function of the Al composition. The InAs deposition, substrate temperature, and
annealing time cffccts on the island foln~ation and nlorphology are studied. The morphologies of the
(Al, Ga)As surface as well as that of the reconstructed InAs "wetting layer" are also described.
Rcsults indicate that there are major differences between the InAsIGaAs and the lnAs/AIAs systems
despite the same lattice mismatch. We observe these differences varying the aluminum content in
the starting (Al, Ga)As surface. We show that control of the A1 fraction leads to control of the size
and density of the 3D islands. The control of island density and size as well as the growth mode of
these islands is explained by considering the difference in surface mobility and cation intermixing
between these two systems. Our observation is that strain energy is not the only parameter
goveining the fonnation of 3D islands but the chemical nature of the different layers involved is
proved to significantly affect island properties. 0 2001 An~erican Institute oJ'Physics.
[DOI: 10.1063/1.1357784]
I. INTRODUCTION
Strain induced self-assembled three-dimensional (3D) islands
have been studied extensively during the past few
years. Both the electronic and optical properties of quantum
dots obtaincd by capping thesc islai~ds with a higher bandgap
material have also been the subject of numerous experimental
and theoretical works.'--"his intense interest has
been driven by the quantum dot unique properties, such as, a
discrete atomic-like energy level structure, due to carrier
confinement. For cxample, the atomic-like structure, and its
consequence in the form of a "phonon bottleneck" effect,
has the potential to significantly impact optoelectronic devices
through the lengthening of carrier ~ifetiines."~ The experimental
report of Guyot-Sionllest et nl. on the intraband
relaxation time in CdSe colloidal quantum dots shows what
is evidence of this effect with decays on the order of 200 ps.9
Longer lifetimes can result in lower laser thresholds and
highly sensitive detectors. Despite many exciting possibilities,
howevcr, the inhoinogeneous broadening resulting froin
size fluctuations can be seen as a serious obstacle. The size
inhomogeneity causes the emission linewidth of an ensemble
of quantum dots to be typically 2 orders-of-magnitude larger
than that of a single dot.",' While relatively narrow photoluminescence
linewidths, on the order of 20-30 meV, have
recently been reported, these results have not been achieved
by reducing the size inhomogeneity. Rather, the narrow linewidths
have been cleverly achieved by either growing large
dots, leading to decreased sensitivity of the emission energy
to the size of the dots, or by growing layers of dots and
enhancing carrier tunneling to the biggest neighbor prior tc)
recombination. '"I3
Even with the difficulty of the size inhomogeneity, significant
applications have been demonstrated using quantum
dots as active layers and recombination centers. These include
light emitting diodes (LEDS)" and room teinperaturc
lasers oscillating at 1.3 pnl using either thc classical cdgc
emitting design1' or the VCSEL configuration.'%ore recently,
lasing from excited states has been evidenced."
Other demonstrated applications use the confinement enhanced
optical nonlinearity for inidinfrared second and thirdharmonic
enhanced carrier trapping efficiency
for photorefraction20 and the mid- and far-infrared
~onductivity''.~~ for the fabrication of quantum dot infrared
photodetectors.23-25 In some cases some very tantalizing
suggestions to make use of the size inhomogeneity may even
lead to applications.2h
During the last several years research carried out on
quantum dot structures in 111-V seniiconductors has focused
on Stranski-Krastanov (SK) growth inodcs. The SK growth
Inode takes place during the growth of lattice-mismatched
systems and has been shown to produce narrow size distributions
of 3D isla~lds. For this growth mode, i.e., small interface
energy but large lattice mismatch, initial growth is
layer by layer. As a result, deposited materials first fosnl a
two-dimensional (7D) wctting laycr on the substrate. As material
continues to be deposited it is energetically favorable
for material to add to the step edges of the 2D islands as
opposed to adding to the top to begin 3D island formation. In
this way, the 2D islands grow laterally in size. However, this
is done at energy cost as the strain energy continues to increase
due to the lattice n~isn~atch. Soon it bccomcs cnergeti-
@ 2001 American Institute of Physics
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