P-layer

Fig. 4.7. A semiconductor detector operated as a pin diode with a reverse voltage or bias. An incident X-ray photon ultimately produces a series of electron-hole pairs. They are "swept out" by the bias field of-500 V- electrons in the direction ofthe n-layer holes in the direction ofthe p-layer. Thus, a small charge pulse is produced after [4.21],...

Note that lines of 0.13 p, i.e.- 0.13 microns, in width are being formed routinely in the manufacture of 1C dice today (Just 3 years ago, the number was 0.18 p. It is expected to go to 0.08 p by 2010). Once the basic circuit is formed, then subsequent steps on each individual die form both n- and p-layers, electrically isolated from each other by a dielectric such as Si02. Both resistors and condensers are routinely formed in place as well. Note that formation of some of the layers is accomplished by use of gases such as AsHs, BBr3, or PH3. 

Several differences from that of an integrated circuit can be noted. First of all, two (2) electrlced contacts must be established across the bulk of the silicon wafer. When light strikes the surface of the solcU cell, its absorption within the silicon bulk releases electrons which are collected as a current. Also shown is the p-n junction. However, the actual silicon disc is only about 350 pm. in thickness. Diffusion processes are used, as a matter of practicality, to form both the p-layer and the n-layer. Thus, the... 

Once the silicon disc is cleaned, the first step is diffuse ions into either side of the silieon disc to first form either the p-layer or the n-layer. Some manufacturers like to have the n-layer closer to the light source, as shown in the above diagram, while others prefer the opposite. At any rate, ions like and are generally used to form the active electrical layers. A number of differing processes have been developed to do this, the exact nature of which depending upon the speeific manufacturer of solar cells. Sputtering, vapor-phase and evaporation are used. The most common process uses a volatile boron or phosphorous compound to contact the surface. 

Absorption in the p-layer can be reduced by using an a-SiC H alloy with a bandgap of about 2 eV [584]. Carbon profiling within the p-layer further improves the window properties [585]. An intentionally graded p-i interface (buffer layer) 10 nm in thickness enhances the spectral response in the blue [125, 494, 586], which can be attributed to a reduced interface recombination. 

Popik P., Layer R., Skolnik P. The putative anti-addictive drug ibogaine is a competitive inhibitor of pH]MK-801 binding to the NMDA receptor complex. Psychopharmacology. 114 672, 1994. 

The enzymatic activities of intercalated GOx-AM P layered nanocomposites at various pH values and temperatures were compared with the native enzyme in aqueous solution. In both cases, characteristic linear plots consistent with Michalis-Menton kinetics were obtained. The Lineweaver-Burk plots indicated that the reaction rates (Vmax) for free and intercalated GOx (3.3 and 4.0 pM min 1 respectively), were comparable, suggesting that the turnover rate at substrate saturation was only marginally influenced by entrapment between the re-assembled organoclay sheets. However, the dissociation constant (Km) associated with the activity of the enzyme was higher for intercalated GOx (6.63 mM) compared to native GOx (2.94 mM), suggesting... 

Fig. 11(a). Variation of deposition rate and Ge content of Ni-Ge-P layers with aspartic acid concentration. Adapted from ref. 112. 

In the ionic implantation, a beam of trivalent ions (e.g. B+ ions), is used to produce a p+ layer. Pentavalent ions (e.g. P ions), instead, create an n+ layer. The main advantage of this technique rely in the fact that the ions penetrate the crystal only for a short distance from the surface ( 200nm). The penetration depth depends on the beam energy (25 100keV). Typical doping level is 1018 1019ions/cm3. With such a high dopant concentration, the layer becomes practically a metal. 

Photodiodes make use of the unique properties of semiconductors, such as silicon. Silicon can be doped with impurities to make it either electron rich (an n-type semiconductor) or electron poor (a p-type semiconductor). When an n-type semiconductor is in contact with a p-type semiconductor, electronic changes occur at the boundary, or junction. A photodiode is a p-n junction constructed with the top p layer so thin that it is transparent to fight. Light shining through the p layer creates additional free electrons in the n layer that can diffuse to the p layer, thus creating an electrical current that depends on the intensity of the fight. This small current is easily amplified and measured. 

Typical photodiode detectors consist of a p layer which is made of an electron deficient material an n layer which is electron abundant and a depletion region, the p-n junction, located between the two layers. At equilibrium, when no light or current is applied to the system, the p-n junction is in electrostatic equilibrium and the alignment of electrons and electron holes on the two sides of thejunction region creates a contact potential voltage. As incident light strikes the surface of the diode, the... 

Popik P, Layer RT, Fossom LH, Benveniste M, Geter-Douglass B, Witkin JM, Skolnick P. (1995). NMDA antagonist properties of the putative antiaddictive drug, ibogaine. J Pharmacol Exp Then 275(2) 753-60. 

The 3D potential map was examined section by section perpendicular to the c-axis. There are totally 6 layers stacked along the c axis in each unit cell. Only two of these 6 layers are unique, one flat layer occurring twice (at z = 0.25 and 0.75) and one puckered layer occurring four times (at z 0.10, 0.40, 0.60 and 0.90). Sections corresponding to the flat (F) and puckered (P) layers are shown in Figs. 6a and b, respectively. The flat layers coincide with mirror planes. The stacking sequence is PFP (PFP" ) , where P relates to P via a mirror reflection on the flat layer, and the (PFP ) block is related to the PFP block by a 63 operation along the c axis. 

The difference between a photoconductive detector and a photodiode detector lies in the presence of a thin p-doped layer at the surface of the detector element, above the bulk n-type semiconductor. Holes accumulate in the p-layer, and electrons in the n-type bulk, so between the two there is a region with a reduced number density of carriers, known as the depletion layer. The important effect of this is that electron-hole pairs, generated by photon absorption within this depletion layer, are subjected to an internal electric field (without the application of an external bias voltage) and are automatically swept to the p and n regions, and... 

Diffusion barriers are coatings that serve in that role specifically, protection against undesirable diffusion. One of the best examples is that of a 100- tm-thick electrode-posited copper layer that serves as an effective barrier against the diffusion of carbon. Another example is that of nickel and nickel alloys (notably, electrolessly deposited Ni-P) that block diffusion of copper into and through gold overplate. This is achieved by the deposition of a relatively thin Ni-P layer (less than 1 /mm) between the copper and its overlayer. Naturally, the effectiveness of the diffusion barrier increases with its thickness. Other factors in the effectiveness of a diffusion barrier... 

To illustrate the point, we present Figure 20.3, a schematic cross section of a thin-fihn hard disk. The Ni-P layer, whose purpose is to render a nearly perfectly smooth, rigid. 

FIGURE 1.3 (a) Three Ac/p layers showing the ABAB... stacking sequence (b) three Aqo layers showing the narrow channels through the layers. 

We have seen examples of the 1 1 stoichiometry corresponding to NaCl (3-2PO) and ZnS (3-2PT or 2-2PT). Rutile (Ti02) has P layers filled by O2 with Ti4+ in O sites. The stoichiometry is 1 2 and this is clear from the notation 2-2POi/2. Only 1/2 of the sites in the O layer are filled and the P layers are hep. Later we will examine a more systematic treatment of applications of the PTOT notation. It applies to thousands of inorganic crystal structures including more complex structures. It is not limited to metals and MX, MX2 and M2X com-... 

It is important to note that each sphere is part of two tetrahedra, so there are twice as many T sites as P sites or between any two close-packed P layers there are two T layers, T+ and T. The positions (A, B, or C) of the T sites correspond to the apex P positions. For hep, these are A and B. In the sequence from the Pa layer, for the first T+ layer each tetrahedron has its apex in the second P layer (Pg), and in the second T layer, each tetrahedron has its apex in the first layer (Pa) giving PaTbTaPbTaTb Pa— Figure 3.2c shows three P layers and the tetrahedra between these with apices in the Pg layer. The bases are staggered (Pa and Pc). 

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