There have been several examples of the ability of chemical functionalization of the Si sin- gle crystal surface to reduce and in some cases halt the formation of active surface charge
Si LVV O KLL
100 nm 100 nm
Figure 6.7: SAM image of a freshly prepared C2H5-terminated Si(111) surface investigated in both the Si LVV (92 eV, left) and the O KLL energy regions (510 eV, right). Both images were collected in 128 x 128 point scans with an electron beam held at 20 kV and 10 nA with spatial resolution of 15 nm. The individual pixel scans were combined to form an image map.
iv) Si LVV + O KLL 100.0 nm
i) SEM ii) Si LVV
iii) O KLL
100.0 nm
100.0 nm 100.0 nm
a)
b)
490 500 510 520
O KLL
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Kinetic Energy (eV) Si LVV
Figure 6.8: Auger electron spectroscopy of a C2H5-terminated Si(111) surface exposed to air for 8 days. a) SAM images of a ∼1µm2 area of the surface: i) SEM image of the surface; ii) SAM image collected by scanning in the Si LVV energy region (92 eV). Lighter gray indicates greater Si signal; iii) SAM image collected by scanning in the O KLL energy region (510 eV). Lighter gray indicates greater O signal; iv) composite SAM image of the overlayed maps of the Si LVV (red) and O KLL (blue) energy regions. SAM images were collected in 128 x 128 point scans with an electron beam held at 20 kV and 10 nA with a spatial resolution of 15 nm. The individual pixels were then combined to form an image map. b) Auger electron spectroscopy of a point on an isolated white discolored region in the SEM image (solid line) and on the dark background in the SEM image (dashed line).
carrier recombination trap sites.2, 13 This remarkable electrical pssivation has been corre- lated experimentally to the absence of large amounts of deleterious native silicon oxides, which can provide numerous defect sites at the disordered Si/SiOx interface.2, 14 The rela- tionship between low rates of surface electron-hole recombination and lack of native silicon oxides appears to be a complicated one, however, as shown in detail in the data presented here. Alkylated surfaces of single crystal Si(111) were investigated by SXPS under high surface sensitivity conditions to measure both the amount and oxidation state of native ox- ides introduced when exposed to air. We found that although the rate of silicon oxidation was dramatically slowed, some amount of silicon oxide did in fact appear. The native oxide on an unpassivated Cl-terminated Si(111) surface covered all available oxidation states of Si, from Si+ to Si4+, with the majority of the oxide signal from Si4+ species. This is sim- ilar to what has been observed previously on unpassivated H-terminated Si surfaces.15–18 In contrast, the submonolayer of native oxide observed on the alkyl-terminated surfaces described here appeared to correspond principally to Si+ and Si3+, albeit with small con- tributions from Si2+ and Si4+oxidation states.
The difference in Si oxidation states on the passivated versus unpassivated surfaces could be useful in determining the structure of the native oxide developing on those sur- faces. The structure of the Si/SiO2 interface has been the subject of prolonged interest be- cause of its importance in integrated circuit fabrication.4, 16, 19–26 Although the structure of the Si/SiO2interface depends on crystal face and growth conditions, it is generally agreed that the junction is not abrupt, but contains an amorphous region≥3 ˚A thick where all five oxidation states of Si (Si0–Si4+) are found.19, 25 Previous core photoelectron spectroscopic studies of the Si(111)/SiO2 interface have shown that Si+and Si3+are more abundant than Si2+ because disrupting an atomically flat (111) surface through layer-by-layer oxidation will cleave either 1 or 3 tetrahedral Si bonds.4, 16, 19, 24, 25 To have periodic Si+, however, it is necessary also to have some concentration of Si4+, which our data suggests is present only in very small quantities. Two Si bonds will be cleaved only at step edges and etch defects, which are a minor component of Si(111) surfaces prepared in this manner (Chapter 5). The SXPS data alone, therefore, suggest that the slow oxidation of alkyl-terminated Si(111) surfaces creates discrete patches of amorphous oxide that are spatially isolated from each
other.
Another difference between the unpassivated and passivated surfaces studied here was the behavior of surface-bound species as the sample was exposed to oxidizing conditions.
Studies of native oxidation growth of H-Si(111) surfaces at room temperature have pre- viously shown that the first oxidation step is insertion of O into a Si–Si backbound, not attack of the stronger Si–H bond.22, 23, 26 The data presented here on alkylated Si surfaces show the same behavior, in that Si–C features remain largely unchanged during the first several days of surface oxidation. On the Cl-terminated surface, the monolayer coverage of Cl atoms dropped precipitously after the sample had been in air for only 12 hr during the initial oxidation events. On alkylated surfaces, however, features in the spectra of the freshly prepared sample that have been previously identified as surface Si atoms bonded to carbon10did not shift significantly either in binding energy or in monolayer coverage. This indicates that the surface Si–C bonds are not altered by the oxidizing environment.
It has been suggested that oxidation of the Si(111) surface proceeds fastest at step edges, rather than along the atomically flat terraces.12 One possible explanation for the remarkable surface passivation reported here for alkylated Si(111), even when protected by bulky alkyl groups, is that the role of the surface-bound moieties is to passivate step edges, thereby preventing the initial oxidation events that quickly destroy the unpassivated H- or Cl-terminated Si(111) surface. If the bulky alkyl group only must bond to step-edge Si atoms and not to every surface Si atom in order to achieve adequate passivation, even large functional groups would work reasonably well. One test of this hypothesis is to observe the rate of oxidation on identically functionalized Si surfaces with differing step-edge den- sities, which can be obtained easily by increasing the miscut of the sample away from the (111) surface. When CH3- and C2H5-terminated Si(111) surfaces of different miscut an- gles were exposed to air for long time periods, however, no difference in oxidation rate or total amount of oxide of these surfaces was observed, indicating that any step-edge effects of alkylation are negligible. SAM studies, however, do confirm that oxide is growing on the C2H5-Si(111) surface in a non-uniform manner that suggests that the alkyl-terminated surfaces does prevent a continuous, general oxide from growing on the surface. Further investigation is required to determine if this oxide is nucleating specifically at step-edge or
other defect sites, or if its growth is initiated by some other mechanism.