Three - University of Arkansas Physics Department

JOURNAL OF APPLIED PHYSICS VOLUME 89, NUMBER I2 15 JUNE 2001
Formation of quantum wires and dots on lnP(001) by AsIP exchange
Haeyeon ~ang,~) P. Ballet, and G. J. Salamo
Depi~rlmenl of Physics, Universiy of Arkansas. Fflj~etteville, Arkclnsas 72701
(Received 18 September 2000; accepted for publication 27 March 2001)
We report on the use of in situ scanning tunneling microscopy to study ASP exchange on InP(001)
surfaces by inolecular beam cpitaxy. Results demonstrate that the exchange process can be
controlled to selectively produce either quantum wires or quantum dots. 15 nm wide self-assembled
nanowires are observed, and they are elongated along the dimer row direction of the lnP(001)-2
X 4 surface with a length of over 1 pin and flat top 2 X 4 surfaces. In addition, when the nanowires
arc annealcd with no arsenic ovelpressure, the surfacc reconstruction transforms from 2 X4 to 4
X 2 and the llanowires transfornl into dots with a rectangular base and flat top. O 2001 American
hntihite qf Physics. [DOI: 10.10631 1,13726221
INTRODUCTION
Driven by the promise for different devices,' there has
been great interest in the growth of nanostructures. The
growth of heteroepitaxial nanostructures involves several
key issues such as strain due to a lattice mismatch, or material
exchange due to diffusion between different layers. Both
of these effects, usually considered detrimental to quality
interfaces, can play significant roles in nanostructure formation.
For example, self-assenlbled three dimensional (3D)
nanostructures are realized by utilizing the strain between
materials with different lattice constants such as InAs quantum
dots on GaAs substrates. In addition, coupled with the
formation of nanostructures, it is well known2 that when an
11~4s layer is fornled on GaAs, the Ga atoms and In atoms
diffuse and exchange. This inteinlixing affects the composition
of the nanostructure and plays a significant role in the
nature of the confined electronic state^.^
Other quantum confined systems which are driven by a
lattice mismatch, such as, InAs dots on InP s~bstrates~,~ and
GaSb on GaAs substrates,%ave been studied to exploit their
electronic propcrties. Growths of these hetcrostructurcs also
involves active exchange of group V elements. The exchange
of group V elements has been studied in many systems such
as AS IS^,^ s~R,' and ASP.".'^' The focus of these studies
tcnds to be on the effccts of cxchange on interface quality for
heteroepitaxial growth. While such exchange may be viewed
as a problem, like strain, it can also be a useful tool. For
example, exchange of Asp has been used to remove the
oxide layer on InP s~rfaces,"~'~ where the resultant interface
leads to epilayers of InGaAs with excellent electrical properties.
In this article, we present results that dcmonstrate that
intertnixing can also be a useful tool in the formation of
self-assembled 3D nanostructures.
Previously reported studies of InAs nanostructures on
planar InP(001) surfaces are on quantum dots formed either
by exchange4 or by direct deposition.'3~'4 Although quantum
wire formation has also been reported recently by direct
deposition of InAs on InP(001) s~bstrates,'~,'~ there are no
"Electronic mail: hayan~(irjcomp.uark.cdu
0021 -8979/2001/89(12)/787114/$18.00
comesponding reports on nanowire formation by an cxchange
process. Here we have demonstrated the foimation of
nanowires on an In(001) surface by simple exchange of
As/P, cxtending the possibility of using an exchange process
to fabricate devices based on quantum wires in heteroepitaxial
growths. We also report that by simple control of the
As overpressure, and the corresponding surface reconstniction,
either nanowires or dots can be selected for growth.
The experiment is carried out in a molecular beam epitaxy
(MBE) chamber with solid sourccs of arscnic and phosphorous
which are equipped with valves to provide control
over fluxes. The substrate temperature is measured using optical
transmission thcrnlomctry for reproducibility and absolute
measurement to within 2"~.'~ After loading a conlmercial
n-type planar (miscut within 0.05") InP(001) wafer
into the MBE chamber, the oxide layer on the wafer is removed
by annealing the wafer above 480 "C under a cracked
(950"C) phosphorous (P2) beam equivalent pressure (BEP!
of lo-' Torr. The resultant surface yields 2 X4 patterns in
reflection high energy electroil diffraction (RHEED), similar
to those of 2 X4 patterns of a GaAs(001) surface.''
After oxide removal, a 0.3 pm thick buffer layer of InP
is grown on the substrate at 470°C. To have a smooth
InP(001)-2 X4 surface, the substrate is annealed at 500 "C
under a P, pressure of 1 X Torr for 15 min and then
cooled to 480 OC. At 480 "C, thc valvc for thc P2 flux is
closed and the residual phosphorous is pumped out for about
3 min. During this pcriod the RHEED continuously indicates
a stable 2 X 4 surface reconstruction that is independent of
the P2 ovcrpressurc.
After preparing an InP surface, the sample is rapidly
cooled down below 250 "C and transferred to the scanning
tunneling microscope (STM) chamber through the ultrahigh
vacuum modutrack. STM images are takcn for fillcd states
(-3 V on the sample) with tunneling current around 100 PA.
The STM images of the starting surface of InP(001), just
before deposition of InAs, are shown in Figs. l(a) and l(b).
The images show terraces, vacancy islands, monolayer (ML)
high steps and 2 X 4 surface reconstruction with dimer rows
[the inset in Fig. I (b)] which are along the [Mo] direction.
The STM images indicatc a beautiful lnP(00 1) surfacc even
'871 O 2002 American Institute of Physics
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