Surface properties sapphire

Some of the surface properties of the c-plane are described. The surface is not very flat in the atomic scale typically, as received from the vendors, sapphire substrates have a root-mean-square (RMS) roughness of 2 - 3 ML (monolayer) (0.4 - 0.6 nm) [26-28], The surface atoms are probably a mixture of... 

Occurs in nature in abundance the principal forms are bauxites and lat-erites. The mineral corundum is used to produce precious gems, such as ruhy and sapphire. Activated aluminas are used extensively as adsorbents because of their affinity for water and other polar molecules and as catalysts because of their large surface area and appropriate pore sturcture. As adsorbents, they are used for drying gases and liquids and in adsorption chromatography. Catalytic properties may be attributed to the presence of surface active sites (primarily OFT, 02, and AF+ ions). Such catalytic applications include sulfur recovery from H2S (Clauss catalysis) dehydration of alcohols, isomerization of olefins and as a catalyst support in petroleum refining. 

Figure 13.5(d) presents experimental stiffness measurements using differential UFM for three high modulus surfaces sapphire, Si(100) and LiF(lOO) (Dinelli et al. 2000b). The samples were probed with the same silicon tip on a V-shaped cantilever (nominally cantilever stiffness was kc - 2.8 nN nm 1,and radius of curvature R = 10 nm). The surface RMS roughness of the surfaces was less than 0.2 nm over a few square micrometres for all three samples. The relative difference between the three sets of data reveals that the elastic properties of these three materials can be distinguished by differential UFM the relative independence of the applied force may indicate the fact that the tip had been flattened by extended contact with such hard samples. 

Two major improvements in the fabrication of an ion-sensitive FET that avoid most of the tedious polymer encapsulauon process have been reported. Matsuo and his coworkers (4, 37) fabricated a probe-type FET with a three-dimensional silicon nitride passivation layer around most of its surface, as shown in Fig. 2. The probe-type FET has one disadvantage Its fabrication requires a three-dimensional process that is uncommon for semiconductor construction facilities. An alternative approach utilizes a silicon-on-sapphire (SOS) wafer for FET fabrication (38, 39). The structure of a SOS-FET is depicted in Fig. 3. It has an island-like silicon layer on a sapphire substrate, in which an ion-sensitive FET is fabricated. The bare lateral sides do not need encapsulation because of the high insulation property of sapphire. 

The optical properties of the wall also contribute to the emitted spectra. The usual arc tube wall material, polycrystalline alumina (PCA), has rather low "in-line" transmission and scatters most of the incident radiation. That portion which is scattered back into the discharge medium and that is also in the spectral region inside or near the peaks of the self-reversed resonance lines, is likely to be absorbed. Thus we have found greater Na reversal for PCA arc tube lamps than for sapphire lamps with the same diameters, Na-Hg alloy composition and reservoir temperatures. Indeed, various authors have reported 3-8% higher efficacies for sapphire vs PCA lamps depending upon the quality of the PCA and its surface treatment 3). 

The (0001) surface of sapphire (a-alumina, corundum) is one of the most widely used substrate for the growth of metal, semi-conductor or high-temperature superconductor thin films. It is also used as a substrate in silicon on sapphire (SOS) technology. Moreover, its initial state is known to play a role on the overlayer properties [50]. 

The surfaces of mtile Ti02 have been the subject of intense research because of their photo-catalytic properties for the dissociation of water. The hydroxylation rate on the surface and the kinetics of the reaction were shown to depend strongly on the surface stoichiometry and detailed atomic structure. In addition, like the two above surfaces of sapphire and magnesium oxide, rutile titanium dioxide surfaces stand as model metal oxide surfaces. Their atomic structure is thus of fundamental interest. 

The above studies have benefited from a fertile exchange with experimental groups using the Surface Force Apparatus (SFA). The SFA allows the mechanical properties of fluid Aims to be studied as a function of thickness over a range from hundreds of nanometers down to contact. The fluid is confined between two atomically flat surfaces. The most commonly used surfaces are mica, but silica, polymers, and mica coated with amorphous carbon, sapphire, or aluminum oxide have also been used. The surfaces are pressed together with a constant normal load, and the separation between them is measured using optical interferometry. The fluid can then be sheared by translating one surface... 

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