Chemistry Reference
In-Depth Information
at 830
◦
C at RHEED pattern that indirectly argues to atomic cleanness of the sur-
face. To reduce a thermal drift, the specimen was kept at 900-1050
◦
C about 30min.
After final cleaning treatments, the atomic steps without any pinning centers asso-
ciated with the particles of contamination have been observed reproducibly in REM
images [
35
].
For providing gold deposition, a small-sized gold evaporator, a wolfram crucible,
was placed against the sample face. The amount of deposited gold measured in units
of monolayer (1ML
10
15
atoms/cm
2
) was monitored with time of applied
current calibrated previously by the Au/Si(111) surface phase diagram known from
other studies. The step transformations were analyzed with REM in imaging and
diffraction modes at high temperatures and in real time due to video recorder system
made by GATAN to picture quick surface reactions. It should be pointed out that
recorded REM images are foreshortened by a factor of
≈
1.6
×
1/50 in the direction of the
electron beam incidence due to a small angle between surface and electron beam.
∼
11.2.1 Monatomic Steps on Silicon Surface
Figure
11.2
represents REM image of atomically clean vicinal silicon (111) surface
at 900
◦
C after thermal annealing in a UHV chamber of a reflection electron micro-
scope. White arrow indicates the direction of the electron beam incidence. Thin
dark wavy lines at the image are atomic steps, 0.31 nm in height. The atomic step
contrast in REM images is a superposition of two contrast types, i.e., diffraction and
phase. The diffraction (Bragg) contrast is caused by deformed regions of the atomic
lattice near monatomic steps, which locally change the diffraction conditions on
steps. The phase (Fresnel) contrast is caused by the phase shift of electrons reflected
from adjacent atomic planes separated by a monatomic step.
Atomic steps move over the crystal surface during sublimation, which is caused
by atomic step generation into an adsorption layer from which atoms are detached
into the vapor phase. Evaporation of a single monolayer displaces each step for a
distance equal to the distance to the adjacent step [
36
]. The step motion is initiated
by consequent detachment from the step, diffusion along the step, diffusion on the
terraces, and subsequent evaporation of the silicon atoms during sublimation. The
step rate depends on the terrace width with the sublimation energy of
≈
4.2 eV [
35
,
37
].
Atomic step distribution on the surface was not unchangeable during crystal
evaporation. As it was described above, steps distribution was unstable upon fluc-
tuations of the terrace width in the case of electrical current heating of the crys-
tal [
15
]. In the case of the step-down DC direction, regular distribution of atomic
steps was broken and formation of the step-bunched morphology was observed at
900
◦
C (Fig.
11.2
b). The redistribution of regular steps to step bunches and vice
versa was observed after switching the electrical current direction. Increasing the
sample temperature caused enhancing step mobility and redistribution of the atomic
steps occurred faster. In the case of alternative electrical current (AC) heating the
system of regular steps was stable on clean silicon (111) surface at any temperature.
