2003 Welch Scholar Report
My
B.Sc. in Physics at the University of Waterloo, Canada, was followed
by an M.Sc. in Professor Tom Tiedje's Molecular Beam Epitaxy (MBE)
group in the Department of Physics at the University of British Columbia.
MBE is a finely-controlled ultra-high-vacuum technique for
depositing very pure crystal films onto a substrate using atomic or
molecular sources. Professor
Tiedje's group has a history of innovation in in situ monitoring techniques for MBE and
a strength in the study of surface phenomena during III-V semiconductor
crystal growth. My own project ("Pattern
Formation in Lateral Oxidation of Aluminum-Rich AlxGa(1-x)As",
UBC (2000)) was centered on methods to form high-refractive-index-contrast
patterns in GaAs/Al(Ga)As heterostructures for use in optoelectronic
and photonic semiconductor devices.
I was not an MBE operator, but stimulating lunchtime discussions
of growth dynamics and technological possibilities, tempered by constant
exposure to the trials and tribulations of those working with UHV
technology, left me with an interest in becoming more involved.
The
Welch Scholarship presented an opportunity to take up a Ph.D. project
in Cambridge, UK, in the MBE program of the Cavendish Laboratory Semiconductor Physics (SP) group, headed
by Professor Michael Pepper. The
MBE subgroup of SP has an ongoing research program in the growth of
III-V heterostructures on GaAs wafers that have been patterned, usually
by lithography and etching outside of the vacuum environment. In this way, some three-dimensional control
is exerted over a process that is often regarded (not entirely accurately)
as one-dimensional: the growth of layers in the vertical direction. The technique is usually known as regrowth
or overgrowth, as it usually involves two growth steps: the first to
prepare epilayers to be patterned, and the second to finish the structure
over the top of the patterned material. My Ph.D. work, supervised by
Dr. David Ritchie, involves the patterning, overgrowth, post-processing,
and measurement of electronic device structures.
Various
interesting applications exist for MBE regrowth.
Buried electronic gates of doped semiconductor material can be
used to manipulate the carrier density and mobility of a two-dimensional
electron gas, or to allow the independent electrical contacting of two
two-dimensional electron gas layers [1]. The creation of "lateral" p-n junctions
by the manipulation of amphoteric doping properties of silicon on different
crystal facets in GaAs [2], and the growth on patterned substrates to
control nucleation locations of InAs quantum dots (see for example [3]
and [4]) are just two further examples out of many.
Such structuring ability allows more options in the design of
devices for the study of electronic transport phenomena in mesoscopic
systems, which are of interest to various researchers within the Semiconductor
Physics Group.
Only
with painstaking cleanliness is this technique a viable tool in the
fabrication of device structures requiring high-quality material, such
as those containing low-dimensional, high-carrier-mobility layers.
Great care is taken when processing and handling the wafer ex situ, but in situ hydrogen
radical cleaning makes a crucial improvement. The introduction of the
in situ treatment steps to the regrowth
program in this lab was shown to reduce the necessary buffer thickness
between the regrowth interface and a high-mobility two-dimensional electron
gas [5]. It reduces hydrocarbon
contamination, and removes surface oxides at a lower temperature than
that needed for thermal oxide desorption, which results in a smoother
surface. This treatment takes place after the wafer has been carefully
stripped and cleaned in the cleanroom, and has been introduced to the
UHV facility. In a dedicated chamber, preliminary Secondary
Ion Mass Spectrometry (SIMS) inspection checks for photoresist contamination.
Hydrogen radical treatment follows, and a final SIMS scan confirms
oxide removal before the wafer is transported under continuous UHV conditions
to the MBE preparation chamber to await overlayer growth.
My
work with the Cavendish Semiconductor Physics Group continues.
I have certainly become more involved with the vacuum-technological
side of semiconductor research, as I wished, and have gained experience
in the meticulous processes that lead to the growth of high-quality
MBE epilayers. My focus is currently on the study and optimization of
devices containing buried patterned gates to manipulate two-dimensional
electron and/or hole gases. As the devices improve their application has
potential to diversify into several branches of research within the
group.
Christine
Nicoll
November
2002
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[1]
N. K. Patel, M.P. Grimshaw, J.H. Burroughes, M.L. Leadbeater,
D.A. Ritchie, and G.A.C. Jones, Appl. Phys. Lett. 66(7), 848 (1995).
[2]
R.J. Evans, T.M. Burke, J.H. Burroughes, M.P. Grimshaw, D.A.
Ritchie, and M. Pepper, Appl. Phys.Lett 68(12),
1708 (1996).
[3]
T. Ishikawa, S. Kohmoto, S. Nishikawa, T. Nishimura, and K. Asakawa,
J. Vac. Sci & Tech. B
18(6), 2635 (2000).
[4]
H. Lee, J. A. Johnson, M. Y. He, J. S. Speck, and P. M. Petroff,
Appl. Phys. Lett. 78(1), 105 (2001).
[5] T. M. Burke,
D.A. Ritchie, E.H. Linfield, M.P. O’Sullivan, J.H. Burroughes, M.L.
Leadbeater, S.N. Holmes, C.E. Norman, A.J. Shields, and M. Pepper,
Mater. Sci. Eng B 51, 202 (1998).
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