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

[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).
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).

Bulletin 152