Three - University of Arkansas Physics Department

APPLIED P1IYSIC:S LETTERS VOLUME 77, NUMBER 21 20 NOVEMBER 2000
Scanning tunneling microscopy investigation of truncated InPIGalnP,
self-assernbled islands
P. Ballet, J. B. Smathers, H. Yang, C. L. Workman, and G. J. salamoa)
Physics Departmett, University ojArkonsas, E'nyetteville, Arkansas 72701
(Received 19 July 2000; accepted for publication 19 September 2000)
We present an investigation of the morphology of InPIGaInP three-dimensional (3D) islands
obtained by molecular beam epitaxy. This material system should represent the counterpart of the
InGaAsIGaAs systenl for thc visible range. The islands are found to be truncatcd pyramids with
observable phosphorous-rich surface reconstruction on top. The investigation of the effect of P
overprcssurc rcvcals a path to achicvc extrcmely hornogencous 3D islands through an island shapc
transition. These results help us understand the emerging issue of 3D island shape trans~tion.
Q 2000 American Institute of Physics. [S0003-695 1(00)00147-91
Self-assenlbly of semicond.uctor nanostructures has demonstrated
the potential of highly strained layer epitaxy to
produce quanhlm objects with reduced dimensionality, such
as quantum Highly rnismatchcd systems can bc easily
found in 11-VI and 111-V compounds as well as GeISi.
This great variety of materials, having different band gaps
and lattice parameters, allows for strain engineering as well
as band structure engineering. As a result there have been
numerous investigations on quantum dots using the GeISi
system3*' and the As-based 111-V ~~stcrns,'~~~~-~
thc most
striking example being (In, Ga)As/GaAs for which extensive
literature exists. Surprisingly, however, the structural properties
of phosphorous-based systems, especially InP selfassembled
quantum dots, have been left comparatively unexplored.
Motivation for study, however, is now strong, given
that InPIGaInP quantum dot devices have bcen demonstrated,
and that they exhibit strong emission of light in the
longer wavelength region of the visible spectrum.9310 Both
molecular beam epitaxy (MBE)-grown and metalorganic
chemical vapor deposition (M0CVD)-grown structures have
been developed. The forn~er technique produces higher densities
of smaller islands and a rather disordered phase for the
GaInP alloy "-I3.
In this letter we present a detailed investigation of the
morphology of InP/GaInP1 self-assembled islands. Our findings
clearly show that the InP island structure exhibits striking
differences with respect to InAs 3D islands. We also
report on the role of a phosphorous overpressure in obtaining
extremely narrow 3D island size distributions.
Our sanlples have been grown using a Riber solid source
MBE. The GaInP, layers of 0.2 pm thickness have been
grown at 500°C on GaAs nt wafers. The co~nposition of
these layers, leading to the latticc match with GaAs, has been
checked using high resolution x-ray diffraction and monitored
from growth to growth using in situ rcflcction high
energy electron diffraction (RHEED) oscillations. The InP
0.33 monolayer per second (MLls) growth rate remained
constant between the growth of GaInP2 and the InP deposition,
leading to thc formation of 3D islands. InP is ~rovided
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"~uthor to whom corrcspondence should be addrcsscd; clectl-onic mail:
sala~no(~comp.~~ark.rdu
by sequences of 0.66 ML followed by exposing the surface
to only P for a period of 30 s. The sequences are repeated
until the desired InP coverage of about 3.3 ML s is reached.
The P2 overpressure is monitored using an ion gauge and is
varied between samples froin 7.5X to I .5X lo-' Torr
corresponding to 1:25 and 1:47 1II:V ratios, respectively.
The presence of 3D islands above I .4 ML deposition is identified
by RHEED. After conlpletion of the growth, the
samples are immediately transferred to the attached scanning
tunneling nlicroscopy (STM) chamber. Room temperature
filled state images using 3.0 V sample bias are acquired during
the few hours following the transfer.
Figure l(a) shows a RHEED pattern obtained with the
electron beam in the [- 1101 direction and exhibits strong
transmission spots with chevrons attributable to the presence
of facetted 3D islands. Also distinguishable is a streaky pattem
superimposed on the transmission pattern. The 4X periodicity
of the streaks corresponds to that of the InP 2 X4
surface reconstruction. However this pattern does not originate
from the InP wetting layer surface but rather from the
top of the islands. Using high resolution STM, we found the
islands to be truncated. A STM picture at the top of one of
those islands is displayed in Fig. l(b). Clearly, the top surface
is perfectly flat and shows evidence of the presence of
the P din~er rows responsible for the 2 X4 surfacc reconstruction,
and therefore explains the presence of streaks in
the RHEED pattcm. Truncated islands have been predicted
FIG. 1. Rl-IEED pattern taken in the [- 1101 azimut (a) and STM Ilnage of
the top of ,an InP 3D island (b). The z scalc in thc STM imagc has bcen
adjustcd to rcvcal thc phosphorous di~ncrows of thc 2 X4 surface reconstruction.
0003-69~1/2000/~~(21)13406131$17.00 3406 0 2000 American lnst~tute of Physics
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