Oxide native

The surface of silicon in air is always covered with a very thin oxide film. Table 2.10 shows that the thickness of native oxide formed on the surface of silicon after several days in air varies from 5 to 20 A depending on preparation conditions. Such a large variation in the thickness of native oxide indicates the great sensitivity of the surface reactivity to minute variations of environmental and material conditions. 

The rate of formation of native oxide in air depends on the initial condition of the surface and the cleanness of the air. The formation of oxide on a perfectly clean silicon surface, e.g., a cleaved surface, is relatively fast, reaching 5-7 A within a few minutes after exposure to air. The formation of native oxide on the surface after being 

TABLE 2.10. Native Oxide Films Formed in Air and Solutions 

There is very little growth up to about 200 min after which the growth rate increases significantly. The S-shaped curve in Fig. 2.17 is attributed to a layer-by-layer growth of the oxide with 5.4 to 7.6 A corresponding to two layers of native oxide. 

FIGURE 2.17. Oxide thickness data plotted as a function of the logarithm of the exposure time of wafers to air at room temperature. The H2O concentration in air is 1.2% on the average (42% relative humidity). (Reprinted with permission from Morita et al 1989, American Institute of Physics.) 

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For some materials, the most notable being silicon, heating alone sufiBces to clean the surface. Commercial Si wafers are produced with a thin layer of silicon dioxide covering the surface. This native oxide is inert to reaction with the atmosphere, and therefore keeps the underlying Si material clean. The native oxide layer is desorbed, i.e. removed into the gas phase, by heating the wafer in UHV to a temperature above approximately 1100 °C. This procedure directly fonus a clean, well ordered Si surface. 

Figure Bl.5.12 SH and SF spectra (frill dots) for the CaF2/Si(l 11) interface (a) SH intensity as a fiinction of the photon energy of the tunable laser (b) SF intensity obtamed by mixmg the tunable laser with radiation at a fixed photon energy of 1.17 eV. For comparison, the open circles in (a) are signals obtained for a native-oxide covered Si(l 11). The fiill line is a fit to the theory as discussed in [79].

This is demonstrated by the XPS spectra in figure B 1.25.5(a) which show the Si 2p spectra of a silicon crystal with a thin (native) oxide layer, measured under take-off angles of 0° and 60° [12]. When the take-off angle is... 

Aronoff Y G, Chen B, Lu G, Seto C, Schwartz J and Bernasek S L 1997 Stabilization of self-assembled monolayers of carboxylic acids on native oxides of metals J. Am. Chem. Soc. 119 259-62... 

Folkers J P, Gorman C B, Laibinis P E, Buchholz S and Whitesides G M 1995 Self-assembled monolayers of long-chain hydroxamic acids on the native oxides of metals Langmuir 813-24... 

With gallium arsenide, additional elements, such as Si, S, and Cl, are of interest because of their doping character. Impurity levels on the order of lO cm are encountered with commercial substrates, which can be readily assessed using direct TXRF." VPD-TXRF is not possible in this case because of the lack of a native oxide layer on gallium arsenide. 

Vapor-phase decomposition and collection (Figs 4.16 to 4.18) is a standardized method of silicon wafer surface analysis [4.11]. The native oxide on wafer surfaces readily reacts with isothermally distilled HF vapor and forms small droplets on the hydrophobic wafer surface at room temperature [4.66]. These small droplets can be collected with a scanning droplet. The scanned, accumulated droplets finally contain all dissolved contamination in the scanning droplet. It must be dried on a concentrated spot (diameter approximately 150 pm) and measured against the blank droplet residue of the scanning solution [4.67-4.69]. VPD-TXRF has been carefully evaluated against standardized surface analytical methods. The user is advised to use reliable reference materials [4.70-4.72]. 

In the UV most of the materials of interest, e.g. Si, polysilicon, SiGe, GaAs, and other semiconductor materials, are strongly absorbing this enables surface-sensitive measurements. Surface roughness, native oxide covering, material composition, and structural properties can be analyzed. 

An example of interaction stiffness and force curves for a Si surface with a native oxide at 60% relative humidity (RH) is shown in Fig. 12 [104]. The stiffness and force data show an adhesive interaction between the tip and substrate. The hysteresis on retraction is due to a real change in contact area from surface oxide deformation and is not an experimental artifact. The adhesive force observed during retraction was consistent with capillary condensation and the surface energy measured from the adhesive force was close to that of water. 

Figure 13. Schematic presentation of a small segment of polyheteromicrophase SEI (a) and its equivalent circuit (b) A, native oxide film B, LiF or LiCl C, non conducting polymer D, Li2CO, or LiCO, R GB, grain boundary. RA,/ B,RD, ionic resistance of microphase A, B, D. Rc >Rqb charge-transfer resistances at the grain boundary of A to B or A to D, respectively. CA, CB, CD SEI capacitance for each of the particles A to D.

The major differences between polymer and liquid electrolytes result from the physical stiffness of the PE. PEs are either hard-to-soft solids, or a combination of solid and molten in phases equilibrium. As a result, wetting and contact problems are to be expected at the Li/PE interface. In addition, the replacement of the native oxide layer covering the lithium, under the... 

Figure 15. Schematic presentation of the Li/Pc interphase r5, 6] A, native oxide film B, freshly formed SEI C, void D, PE (solid)...

HfCl2[N(SiMe3)2]2 was synthesized with the reaction of anhydrous HflCU and Na[N(SiMe3)] in toluene [5]. The films were grown in a cold-wall flow-type ALD reactor on (100) oriented p-Si substrates in the temperature range of 150-400 °C. Prior to deposition. Si substrate was etched in dilute HF solution to remove the native oxide and then rinsed in deionized water. The pressure in the reactor was fixed at about 0.5 torr. Argon (99.99995%) was used as a... 

Figure 5. Morphology and particle size distribution of an island silver thin film deposited on native oxide covered silicon (a) before ion bombardment and after (b) 0.5 keV Ar sputtering with 1.1 X 10, (c) 2.5 X 10, and (d) 3.9 x 10 ion/cm dose. Sputtering speed for silver was around 3-4ML/min. Total elapsed sputtering time is indicated on each size distribution graphs. (Reprinted from Ref [123], 2003, with permission from Springer.)...

Figure 8. Valence band XPS (a) and UPS (b) spectra of silver islands on native oxide covered Si(l 0 0) during bombardment with 1 keV Ar" ions. Substrate related contributions are removed. Numbers at each spectra stand for the Ag/Si ratio determined from the appropriate XPS core level spectra. The uppermost curve is the spectrum of polycrystalline bulk Ag. (Reprinted from Ref [146], 1998, with permission from Elsevier.)...

In a subsequent publication [68] the influence of the substrate on the self-assembly process was elucidated. The comparison of a polyimide substrate with a native oxide (Si( )A) covered wafer revealed that differences in interac-... 

The CBD CdS on CIGS or CIS devices produce superior solar cells relative to solar cells made by dry chemical vapor deposition (CVD) or physical vapor deposition (PVD) of CdS. Several groups15-20 put forward plausible reasons for the superior performance, including selective etching or removal of native oxides by ammonia and also that the CBD process does not cause any physical damage, which could occur during CVD or PVD processes. 

Under ambient atmospheric conditions a native oxide is formed on cleaved Si surfaces. The properties of native and chemical oxides are discussed in Section 5.2. The well-defined surface conditions produced by wet processes like rinsing and cleaning procedures will be discussed below. 

Note that a native oxide film also forms under dry conditions in ambient air the oxidation rate of this process can be enhanced by ultraviolet (UV)-ozone photooxidation (Tal, Vil]. Oxide-covered Si surfaces exhibit low contact angles. Only if the oxide surface is contaminated, for example by a monolayer of absorbed hydrocarbons, may larger contact angles be observed. 

Silicon is stable in acidic solutions that do not contain fluoride because the silicon surface is passivated by a native oxide. If only H F is present in an aqueous solution the etch rate remains low, showing values below 0.1 rim miri 1 on single crystalline silicon depending on the OFT concentration [Hu2]. This low etch rate... 

An oxide layer of about 1 nm thickness is present on a silicon wafer as received from the supplier. This oxide is called a native oxide and forms on every bare silicon surface exposed to ambient air. A bare silicon surface can be generated, for example, by cleaving a silicon crystal in high vacuum. 

The formation of a chemical oxide in pure DI water was found to depend critically on the DOC of the water [Gr3, Mol, Li9]. In DI water of very low DOC (<0.004 ppm), no native oxide forms. Furthermore the reverse reaction is observed for elevated temperatures (80 °C) and long etching times (60 min) thin native or chemical oxides are removed and a hydrogen-terminated surface is established [Wa2]. 

Hydrogen, which covers the internal surface of PS, can also be used to estimate its structural dimensions. IR measurements indicated a stoichiometry of roughly SiH for electrochemically prepared micro PS [Be2]. If dihydride groups are assumed to cover the internal surface, every second atom must be a surface atom. This is the case for a cube of about 1000 atoms that has a diameter of approximately 2 nm. A stoichiometry of SiH04 obtained by thermodesorption measurements points to a crystallite diameter in the order of 4nm [Pe2]. The chemical composition for a hydride coverage surface and for a 0.5 nm thick native oxide layer are given in Table 6.1. 

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