Substrate Resistivity

For most SBSCs the use of silicon substrates with resistivities within the range 1 - 10 Q cm (impurity concentration between 10 m and 10 m ) provides a satisfactory match between spectral response and the solar spectrum. 

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The formation of two-layer PS on p-Si involves two different physical layers in which the potential-current relations are sensitive to the radius of curvature. The space charge layer of p-Si under an anodic potential is thin, which is responsible for the formation of the micro PS. The non-linear resistive effect of the highly resistive substrate is responsible for that of macro PS. The effect of high substrate resistivity should also occur for lowly doped n-Si. However, under normal conditions, the thickness of the space charge layer under an anodic potential, at which macro PS is formed, is on the same order of magnitude as the dimension... 

Chloramphenicol (Chloromycetin) is a nitrobenzene derivative that affects protein synthesis by binding to the 50S ribosomal subunit and preventing peptide bond formation. It prevents the attachment of the amino acid end of aminoacyl-tRNA to the A site, hence the association of peptidyltransferase with the amino acid substrate. Resistance due to changes in the ribosomebinding site results in a decreased affinity for the drug, decreased permeability, and plasmids that code for enzymes that degrade the antibiotic. 

A very convenient hydroxymethylation process has been developed based on the Sml2-mediated Bar-bier-type reaction. Treatment of aldehydes or ketones with benzyl chloromethyl ether in the presence of Smh provides the alkoxymethylated products in good to excellent yields. Subsequent reductive cleavage of the benzyl ether provides hydroxymethylated products. Even ketones with a high propensity for enolization can be alkylated by this process in reasonable yields. The method was utilized by White and Soners as a key step in the synthesis of ( )-deoxystemodinone (equation 27). This particular ketone substrate resisted attack by many other nucleophilic reagents (such as methyllithium) owing to conpeti-tive enolate formation. 

FIGURE 8.25. The relationship between pore diameter and n-type doping density of the silicon substrate for stable formation of macropore arrays. The upper and lower limits of stable pore formation are shown as a function of substrate resistivity (dashed lines). (Reprinted from Lehmann and Griining. 1997, with permission from Elsevier Science.)... 

FIGURE 8.40. Dependence of thickness of the dense surface layer and PS on substrate resistivity [/ (111), 50% HF, lOOmAycm ]. After Unagami. (Reproduced by permission of The Electrochemical Society, Inc.)... 

FIGURE 8.42. Etched layer thickness as a function of the substrate resistivity. (Reprinted from Al Rifai et 2000, with permission from Elsevier Science.)... 

The types ofPS can be categorized into three groups according to this model (1) Space charge layer controlled this includes all PS except for the macro PS formed on p-Si The diameter of the pores in this group is comparable to tbe width of the space charge layer. (2) Substrate resistance controlled this includes the macro PS formed on lowly doped/i-Si and possibly on lowly doped n-Si (a prediction). (3) Photocarrier controlled this includes two-layer PS (micro PS for > SCL and macro PS for a < SCL) and the micro PS structures resulting from photocorrosion. 

T. Smedes, N.P. van der Mejis and A.J. van Gendered. Boundary Element methods for 3D capacitance and substrate resistance calculations in inhomogeneous media in a VLSI layout verification package. Advances in Engineering Software 20. pp 19-27. 1994. 

Among many key parameters which affect the pore morphology (such as current density, voltage, time of anodization, composition of electrolyte, etc.) substrate doping concentration (or resistivity) plays a particularly significant role. However, not much data related to the effect of SiC substrate resistivity on the formation of a porous layer has been published so far. In this work, we discuss surface and pore morphology of SiC and explore its correlation with substrate doping concentration. 

The parameters that control the macroscopic current distribution (in the absence of substrate resistance) can be represented in terms of the Wagner number, defined by the ratio of the activation resistance of the surface reaction, (Ra), to the electrolyte ohmic resistance, (Rn) ... 

Fig. 4 Computer simulation (Cell-Design ) of copper deposition on a resistive wafer. An axi-symetric cross-section through a 200 mm wafer is shown, with the wafer center on the left and the electrical contact on the right. Current density 35 mA/cm2. Five growth steps, 20 sec. each, are simulated. The darker region is proportional to the deposit thickness (for clarity, the vertical axis has been magnified). Copper kinetics (no additives) are assumed io = 1 mA/cm2 ac = 0.5 aA = 1.5 T = 25°C. Initial seed thickness is 1000A. Substrate resistivity is updated with deposit build-up. (Left) 0.24 M CuSC>4 + 1.8 M H2SO4. Deposit thickness range 1.08 - 1.52 p. (34% variation). (Right) 0.85 M CuSC>4. Deposit thickness range 1.28 - 1.41 p. (9.6% variation).

Fig. 5 Deposit thickness profile affected by the substrate resistance, as function of the electrolyte conductivity. Simulated by Cell-Design . All parameters are identical to those of Fig. 4, except that here i=20 mA/cm2, and a shorter deposition time was applied (simulations were stopped when center thickness reached 1 p).

Hansen (57) pointed out that evaporation of a solvent from a polymer solution faced two barriers when cast on an impermeable substrate resistance to solvent loss at the air-liquid interface and diffusion from within the film to the air interface. Evaporation of neat solvents as well as moderately dilute solutions is limited by resistance at the air interface, but as solvent concentration becomes low (5-10-15%), the rate-controlling step is diffusion through the film. Hansen pointed out that at the point when solvent loss changes to a diffusion-limited process, the concentration of solvent is sufficient to reduce the glass transition temperature, Tg, of the polymer to the film temperature. 

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