Doping uniformity

When deposited-thin films must be doped uniformly (as in the fabrication of many optoelectronic devices), it is common practice to use a CVD process wherein small amounts of a precursor gas containing the dopant are added to the reacting mixture. For example, Si-doped GaAs can be made using (C2Hs)3Ga and AsHs with a little SiH4 added. 

The result is the formation of a dense and uniform metal oxide layer in which the deposition rate is controlled by the diffusion rate of ionic species and the concentration of electronic charge carriers. This procedure is used to fabricate the thin layer of soHd electrolyte (yttria-stabilized 2irconia) and the interconnection (Mg-doped lanthanum chromite). 

Autodopiag occurs whea dopants are unintentionally released from a substrate through diffusion and evaporation, and subsequently reiacorporated during the deposition layer. Epitaxial layers are typically doped at concentrations of lO " -10 atoms/cm. The higher levels of doping are used in bipolar technology where the epilayer forms the transistor base. The epitaxial layer can be up to several hundred micrometers, and as thin as 0.05—0.5 p.m. Uniformities of 5% are common. 

Dielectric Film Deposition. Dielectric films are found in all VLSI circuits to provide insulation between conducting layers, as diffusion and ion implantation (qv) masks, for diffusion from doped oxides, to cap doped films to prevent outdiffusion, and for passivating devices as a measure of protection against external contamination, moisture, and scratches. Properties that define the nature and function of dielectric films are the dielectric constant, the process temperature, and specific fabrication characteristics such as step coverage, gap-filling capabihties, density stress, contamination, thickness uniformity, deposition rate, and moisture resistance (2). Several processes are used to deposit dielectric films including atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), or plasma-enhanced CVD (PECVD) (see Plasma technology). 

This reaction is occasionally used for doping crystals uniformly after they have been grown. The process is called transmutation doping (37). 

Nonmolecular species, including radiant quanta, electrons, holes, and phonons, may interact with the molecular environment. In some cases, the electronic environment (3), in a film for example, may be improved by doping with impurities (4). Contamination by undesirable species must at the same time be limited. In general, depending primarily on temperature, molecular transport occurs in and between phases (5), but it is unlikely that the concentration ratios of molecular species is uniform from one phase to another or that, within one phase, all partial concentrations or their ratios are uniform. Molecular concentrations and species that are anathema in one appHcation may be tolerable or even desirable in another. Toxic and other types of dangerous gases are handled or generated in vacuum systems. Safety procedures have been discussed (6,7). 

Velapoldl et al. (64) used a similar approach but prepared fibers of uniform diameter (5-45 pm) from Inorganic Ion-doped glasses. The fluorescence parameters of these materials can be changed by substituting various Ions, such as Tb , Sm , Eu , Mn, UOj, Cu, and Sn. They show excellent stability under Irradiation using Incident excitation (measurement Imprecision of 1% under continuous Irradiation In the microscope for 24 h) and have a fluorescence flux density proportional to the fiber length. 

Such yes/no decisions are of great importance in foodstuffs control and environmental analysis. They also play an important role in pharmacy in the form of content uniformity tests. Without suitable screening methods for rapid detection of positive samples it would scarcely be possible to carry out economic doping controls and toxicological investigations or to recognize medicament abuse. 

Ru(dpp)3]2+) sequestered within the xerogels. The results of SEM and luminescence measurements shown that certain ([Ru(dpp)3]2) doped Octyl-triEOS/TEOS composites form uniform, crack-free xerogel films that can be used to construct high-sensitivity O2 sensors that have linear calibration curves and excellent long-term stability (over a period of 11 months). 

CombiCHEM System (Fig. 3.9) For small-scale combinatorial chemistry applications, this barrel-type rotor is available. It can hold two 24- to 96-well microtiter plates utilizing glass vials (0.5-4 mL) at up to 4 bar at 150 °C. The plates are made of Weflon (graphite-doped Teflon) to ensure uniform heating and are sealed by an inert membrane sheet. Axial rotation of the rotor tumbles the microwell plates to admix the individual samples. Temperature measurement is achieved by means of a fiber-optic probe immersed in the center of the rotor. 

The neutron beam is larger than the dimensions of the sample to be doped hence the neutron flux impinging onto the crystal is uniform. 

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