In Situ Surface Studies of III-V Semiconductor Compounds

Since its advent in the early 1980s, Scanning Tunnelling Microscopy (STM) has been
used to advance the knowledge of semiconductor grow processes. Hybridisation of STM
with other analytical methods and the Molecular Beam Epitaxy (MBE) growth technique
allowed a flexible and diverse approach to growth front exploration. The first hybrid,
limited the applicability of STM to in vacuo operation whereby the sample is rapidly
cooled or “quenched” in an attempt to preserve the growing surface, before imaging can
commence. This technique suffers dually from the unknown effects of the quenching
procedure and the limiting ability to only capture frozen-in-time images of the surface.
The ultimate evolution of STM would be to allow concurrent or in situ MBE and
STM operation. The ability to perform concurrent MBE and STM requires three basic
criteria: accurate and stable control of the sample temperature, reliable and maintainable
STM tunnelling tip procedures and controlled, sustained emission from the MBE effusion
cells within the STM chamber.
Samples are slivers 8 x 1 mm2 to 12 x 4 mm2 of wafer mounted for either direct
current heating or radiative pyrolytic boron nitride heating within the STM chamber. No
direct temperature monitoring method is available and thus a myriad of techniques were
employed to map the current-temperature response for samples including Reflection High
Energy Electron Diffraction (RHEED), thermocouples and thermography, yielding a
reliable heating profile.
Tunnelling tip fabrication involves manufacturing an atomically sharp tip via a
two-step electrochemical etching and annealing procedure. An extensive and exhaustive
investigation sought to produce a quantitative method for tip identification and etching
parameterisation based on the available variables of differential sensitivity, etching
voltage, immersion depth and etchant concentration. An optimised tip type transfer
diagram of tip fabrication resulted, after which, an anneal algorithm was formulated
resulting in clean, sharp tips without the side effect of apex distortion and melting.
Quality of the initial growth layer depends strongly on the clean-up conditions. As
a prequel to growth, sample preparation methods are investigated via STM analysis to determine the best preparation conditions in order to achieve high quality MBE growth in
the STM chamber.
The final stage involves MBE source operation during STM. Initial investigation
focused on flux alteration of surface reconstructions and allowed the effects of As4 on the
STM stage to be investigated. This is the first documented case where an e-beam As4
source has been successfully operated within an STM system, during imaging.
The inclusion of group III elements in the evaporation flux proves unequivocally
that III-V Molecular Beam Scanning Tunnelling Microscopy (MBSTM) is a realisable
investigatory technique. Simultaneous deposition of In and As whilst imaging allowed
dynamic observation of the InAs/GaAs wetting layer evolution on GaAs(001)-(2 × 4).
The experiment followed initial heteroepitaxial growth through wetting layer evolution to
the onset of 3D growth.