Chapter II: Experimental techniques and data analysis
A. UV-Vis Spectroscopy
4.2 Experimental
4.2.1 Clean surfaces of silicon
It is extremely important to prepare an atomically clean surface of the silicon substrate on which an epilayer is to be grown. Common impurities like O and C on the surface should be brought down below the detection limit. This is usually checked by Auger electron spectroscopy (AES).
However, we have used reflection high energy electron diffraction (RHEED) to detect characteristic diffraction patterns of clean surfaces followed by in- situ STM measurements and ensure that the substrate surface is clean before the deposition begins. The sample is degassed at 600oC for about 12 hours in the MBE chamber. Following this it is flashed at ~1200oC for about 2-3 minutes by direct heating. In this process the native oxide on the Si surface desorbs and a clean silicon surface is produced. During this heating, the pressure in the chamber should not increase above 1×10-9 mbar. Clean silicon surfaces are reconstructed. Surface reconstruction is the rearrangement of the surface atoms due to the termination of the bulk structure at the solid-vacuum interface.
78 Chapter IV
………
4.2.2 Si(111) surface reconstruction
During the preparation of atomically clean surface in ultra high vacuum condition, the oxide layer on top of the Si substrate was removed. Due to the absence of neighbouring atom on one side, the force acting on the surface of Si atom is modified. As a result due to minimum energy configuration surface is changed with respect to bulk structure. Here in the Fig. 4.1 we show the Si(111)-(7×7) reconstructed surface according to dimer-adatom- stacking-fault (DAS) model. This model was proposed by Takayanagi et al.
in 1985.15 (7×7) is the more stable configuration among all (2n+1) × (2n+1) where n=1,2,3… reconstructions are observed in Si(111) surface.
Fig. 4.1: Schematic diagram of Si(111)-(7×7) reconstructed surface according to DAS model
In this figure we show the top view of the reconstructed surface unit cell where different layer atoms are defined by different sizes and different color ball. In the lower part we show the side view of the surface. This model consists of 12 adatoms on the top surface which are divided by two triangular shape sub unit cell. One of which is called faulted and another is called un-faulted half within which one has stacking fault. There are nine
Growth of flat top Ag mounds on Si(111)-(7x7) reconstructed surfaces 79
………
dimers per unit cell and one deep corner hole. Each adatom saturates three dangling bond of underlying atom. In the above picture the adatoms in the half unit cell with stacking fault are shown brighter than the adatoms in the other half without stacking fault.16 In Fig. 4.2 two STM images of reconstructed surface are shown in positive and negative bias voltage. In positive bias electron tunnel into the unoccupied states of the sample and in this case both of the half are similar. We can distinguish two parts of the unit cell in negative bias condition when electron tunnel from the filled state of the sample into the tip. In the Fig. 4.2 (a) STM image is in positive sample bias condition and (b) in negative bias condition. In the second case we can see clearly the two half unit cell which are marked in the image.
Fig. 4.2: STM image of Si(111)(7×7) reconstructed surface obtained at different bias voltage.(a) Sample bias: 1.7 Volt, Tunneling current: 0.5nA, (b) Sample bias -1.9 volt; tunneling current 0.2nA. One unit cell is marked in (a) and two unit cell are marked in (b). Two parts of the unit cell are different, one is called Faulted half and another half is un-faulted half which is very clear in the –ve bias image.
4.2.3 Growth of Ag on Si
In spite of large lattice mismatch (25%) Ag can grow epitaxially on silicon substrate.17-19 This is possible by coincident site lattice matching.
Because there is 0.42% lattice mismatch between 3aSi and 4aAg (aSi=5.43Å, aAg=4.09Å).17, 20, 21 Coincident site lattice matching alone does not give rise to epitaxial growth. A favorable bonding configuration of substrates and
80 Chapter IV
………
over-layer atoms are required which exist in case Ag on Si(111)-(7×7) surfaces.
The Ag growth and scanning tunneling microscopy (STM) measurements were performed in a custom made molecular beam epitaxy (MBE) chamber coupled with an ultra high vacuum (UHV) variable temperature scanning tunneling microscopy (VTSTM, Omicron). The base pressure in the growth chamber was 1×10-10 mbar. A n-type Si(111) wafer (orientation ± 0.5o) with resistivity of 10-20 Ω cm was used as a substrate material. Atomically clean reconstructed Si(111)-(7×7) surface were prepared by the method described earlier. Ag atom were then evaporated from Knudsen cell made of pyrolytic boron nitride (PBN) and deposited on Si(111)-(7×7) reconstructed surface at RT at deposition rate 2 monolayer/min. We have deposited 1, 1.4, 1.6, 1.8, 2, 4, 5,10, 30, 40, 80 ML Ag on Si(111)-(7×7) reconstructed surface. Here we define 1 monolayer (ML) of Ag is equivalent to the nominal surface atomic density of Ag(111), 1.5×1015 atoms/cm2. The chamber pressure increases to 5×10-10 mbar at the time of deposition. Following deposition the samples were transferred to VTSTM chamber for morphology characterization.