Surface Adsorption Kinetics of Ga Wire Arrays on Si(112)
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is available in this system, there exists no real time information about the changes in surface structure and hence details of the kinetic processes that are responsible for the formation of these
Si( 1 2) (6x1 )-Ga surface
,A
aFig.
1 Modelfor Ga adatom self assembly along step
0
edges of a facet Si (112)
surface. From ref 3.
..........
chains. Thus, we have used an optical technique, reflectance difference anisotropy (RDA), to monitor the behavior of this surface in real time, as a function of Ga atom deposition and desorption [5]. We show that we can easily observe submonolayer coverage of the Si surface by Ga and a change from a 5x1 to a 6x1 periodicity, as well as observe kinetic regimes for Ga line formation ranging from diffusion to evaporation limited regimes, depending on substrate temperatures and Ga atom deposition rates. EXPERIMENTAL All experiments were performed in situ in a UHV chamber having a base pressure of 3xl O01°torr and which included Auger and LEED spectrometers, and optical access to the RDA set up. The Ga deposition was performed using a Knudsen evaporation cell and the Ga depositions were performed at substrate temperatures between 300'C and 550'C, at a pressure of 3 x 10.9 torr. The samples were initially cleaned using a modified Shiroki etch, involving alternate dips in hydrofluoric acid (HF), boiling Nitric acid, followed by boiling in a solution of peroxide and ammonium hydroxide. The RDA apparatus used in this work [5] included two HeNe lasers operating at either 632.8nm or 543.Onm, a polarizer and a Hinds International photoelastic modulator. The modulated light was reflected at near normal incidence from the sample, which was inside the vacuum chamber and located at the focal point of the Ga and Al K-cells. The reflected light was detected with a Si photodiode and the modulated component, AR, was detected at twice the modulation frequency with a PAR 5210 lock-in amplifier. RESULTS In Fig. 2, the change in surface reflectivity as measured by RDA is shown as a function of Ga atom deposition time at a substrate temperature of 402TC. The time at which the shutter was opened and closed is marked in the figure. The reflectivity consists of a very steep initial rise,
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followed by two distinct peaks in the signal. Since the RDA signal remains stable upon the closure of the deposition shutter, it was possible to examine the surface structure at the maxima of the two peaks using LEED. In this way, it was determined that the first signal maximum corresponds to a 5x1 surface reconstruction and the second maximum corresponds to a 6x1 reconstruction. The drop of the RDA intensity between these two maxima corresponds to a local rearrangement of the Ga chains from a four atom to a five atom average length [3,5].
CO
5000
6X
T=42305x T
"C
S/
Ga off
aafunction of Ga deposition time at a substrate temperature of 423 °C. The two maxima shown to the 5x] and 6x] recontructions.
Cts F 3000-
Fig. 2 RDA signal of Ga wire formation as
oncorrespond
0 C 2000
6
0 "1o0
150
200
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