Chemistry Reference
In-Depth Information
step-up direction, the formation of step bunches was observed on the gold-deposited
surface.
Recently the formation of new type of the step instability, called as a periodic
step density wave, was reported after gold deposition on a high misoriented Si(111)
surface [
30
-
32
]. After 0.3ML of gold deposition on Si(111) and in the case of the
step-up heating current direction, periodic array of extremely straight step bunches
formed on the surface, whereas it had not been observed on clean Si(111).
In order to clarify the influence of gold on the surface morphology, we have
investigated precisely atomic step distribution on Si(111) in wide temperature range
(830-1260
◦
C) at various gold coverage (0-0.7ML) and different directions of the
heating current (step-up and step-down). For surface instability experiments the gold
was chosen because (i) adsorption of gold on silicon surface is well studied at least
at temperatures below 800
◦
C, (ii) gold has high melting temperatures, (iii) gold
does not form gold silicide at enhance temperatures, and (iv) gold is a well-known
surfactant for silicon substrate.
11.2 UHV Reflection Electron Microscopy
In situ observations of silicon surface morphology were performed using a reflection
electron microscope (UHV REM) with an ultra high vacuum chamber. Detailed con-
struction of the electron microscope was described previously [
33
]. REM is highly
sensitive to surface structural elements and provides spatial resolution sufficient to
visualize individual monatomic steps (0.31 nm high on Si(111) and 0.14 nm on
Si(001)), two-dimensional islands, and superstructural domains even at high tem-
peratures (e.g., 1300
◦
C). The UHV REM system includes a differential cryogenic
pumping device that provides an ultra high vacuum in the sample region and allows
sample heating and cooling from liquid-nitrogen to melting temperatures; it is pos-
sible to deposit atoms of various elements from molecular beams onto the surface
under study [
34
].
The samples, 8
3mm
3
in size, were cut from a nominally flat (111) silicon
wafer with an average interstep distance of 100 nm. After cutting procedure, atomic
steps were oriented perpendicular to the long side of the sample and the direction
of the electron beam incidence was parallel to the step edge. Before installation into
the microscope, the specimen was cleaned and chemically etched by ordinary treat-
ments. After chemical treatments the sample was mounted to a holder and placed
into the sample chamber. Then, the sample was outgassed in a UHV microscope
chamber at the temperature about 900
◦
C for several minutes. As the most convenient
way the clean Si(111) surfaces were prepared by typical flashing to 1250
◦
Cinthe
UHV chamber of the microscope. The sample holder was designed for providing
electrical current heating and rotating of the sample in azimuth and Bragg's planes.
Before each experiment the sample was cleaned in the UHV chamber by several
flash heating up to 1300
◦
C for several minutes. After cleaning procedures, no any
pinning centers for the motion of the atomic steps during sublimation were visual-
ized. Moreover, the superstructural phase transition
×
1
×
0
.
(
7
×
7
)
⇔
(
1
×
1
)
was observed
