Biasing

Yugo et al.P employed a negative bias and high CH4 concentrations during pretreatment to generate diamond nuclei on a Si mirror surface in PACVD. The several-minute pretreatment resulted in an enormous nucleation enhancement. Diamond nucleation densities as high as 10 cm were achieved. For the onset of diamond nucleation, a minimum voltage of -70 V and a minimum concentration of 5 vol.% CH4 in H2 were necessary. 

both positive and negative biasing are effective for enhancing diamond nucleation density  

lower pressures, higher CH4 concentrations, and/or larger absolute values of substrate bias voltages lead to higher nucleation densities  

at the same absolute value of substrate bias voltage and for CH4eoneentrations ranging from 10 to 40 vol.%, the nucleation densities on the negatively biased substrates are one to two orders of magnitude higher than those on the positively biased substrates. 

In an in-depth study of diamond nucleation on Si in MW PACVD,0 1 substrates were pretreated by negative biasing in a 2 vol.%CH4-H2 plasma. The biasing enhanced diamond nucleation density on unscratched Si wafers, up to 10 cm , as compared to 10 cm on scratched Si wafers and 10 -10 cm-2[279] Qjj untreated Si wafers (Fig. 6). 

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We will see that superseding the functional fi(p ) in the form of Gibbs measure (4) ensures the linearity of equation (1), simplifies the iteration procedure, and naturally provides the support of any expected feature in the image. The price for this is, that the a priori information is introduced in more biased, but quite natural form. 

It takes time to train a film interpreter. In addition, human interpretation of weld quality based on film radiography is very subjective, inconsistent, labour intensive, and sometimes biased. It is thus desirable to develop some forms of computer-aided systems. 

Figure Bl.6.1 Equipotential surfaces have the shape of lenses in tlie field between two cylinders biased at different voltages. The focusing properties of the electron optical lens are specified by focal points located at focal lengthsandy2, measured relative to the principal planes, The two principal rays emanating...

An electron prisin , known as an analyser or monochromator, is created by tlie field between the plates of a capacitor. The plates may be planar, simple curved, spherical, or toroidal as shown in Figure Bl.6.2. The trajectory of an electron entering the gap between the plates is curved as the electron is attracted to the positively biased (iimer) plate and... 

Katzenellenbogen N and Grischkowsky D 1991 Efficient generation of 380 fs pulses of THz radiation by ultrafast laser pulse excitation of a biased metal-semiconductor interface Appl. Phys. Lett. 58 222-4... 

It is usefiil to write down here the basic fomuilae for sampling with an additional weight fimction applied, sometimes called non-Boltzmaim or umbrella sampling, and for sampling when the selection of trial moves is done in a biased way, i.e., the a matrix is not syimnetrical. 

The biased-sampling approach may be considerably generalized, to allow the construction of MC moves step-by-step, with each step depending on the success or failure of the last. Such a procedure is biased, but it is then possible to correct for the bias (by considering the possible reverse moves). The technique has dramatically speeded up polymer simulations, and is capable of wider application. 

The idea may be illustrated by considering first a method for increasing the acceptance rate of moves (but at the expense of trying, and discarding, several other possible moves). Having picked an atom to move, calculate the new trial interaction energy for a range of trial positions t = 1.. . k. Pick the actual attempted move from this set, with a probability proportional to the Boltzmann factor. This biases the move selection. 

The expense is justified, however, when tackling polymer chains, where reconstruction of an entire chain is expressed as a succession of atomic moves of this kind [121]. The first atom is placed at random the second selected nearby (one bond length away), the third placed near the second, and so on. Each placement of an atom is given a greater chance of success by selecting from multiple locations, as just described. Biasing factors are calculated for the whole multi-atom move, forward and reverse, and used as before in the Metropolis prescription. For fiirther details see [122, 123. 124. 125]. A nice example of this teclmique is the study [126. 127] of the distribution of linear and branched chain alkanes in zeolites. 

For a multicomponent system, it is possible to simulate at constant pressure rather than constant volume, as separation into phases of different compositions is still allowed. The method allows one to study straightforwardly phase equilibria in confined systems such as pores [166]. Configuration-biased MC methods can be used in combination with the Gibbs ensemble. An impressive demonstration of this has been the detennination by Siepmaim et al [167] and Smit et al [168] of liquid-vapour coexistence curves for n-alkane chain molecules as long as 48 atoms. 

Osborne M A, Balasubramanian S, Furey W S and Klenerman D 1998 Optically biased diffusion of single molecules studied by confocal fluorescence microscopy J. Chem. Phys. B 102 3160-7... 

Chiu D T and Zare R N 1996 Biased diffusion, optical trapping and manipulation of single molecules in solution J. Am. Chem. Soc. 118 6512-13... 

In addition to their practical importance, colloidal suspensions have received much attention from chemists and physicists alike. This is an interesting research area in its own right, and it is an important aspect of what is referred to as soft condensed matter physics. This contribution is written from such a perspective, and although a balanced account is aimed for, it is inevitably biased by the author s research interests. References to the original literature are included, but within the scope of this contribution only a fraction of the vast amount of literature on colloidal suspensions can be mentioned. 

Other techniques to detennine the corrosion rate use instead of DC biasing, an AC approach (electrochemical impedance spectroscopy). From the impedance spectra, the polarization resistance (R ) of the system can be detennined. The polarization resistance is indirectly proportional to j. An advantage of an AC method is given by the fact that a small AC amplitude applied to a sample at the corrosion potential essentially does not remove the system from equilibrium. 

Light is generated in semiconductors in the process of radiative recombination. In a direct semiconductor, minority carrier population created by injection in a forward biased p-n junction can recombine radiatively, generating photons with energy about equal to E. The recombination process is spontaneous, individual electron-hole recombination events are random and not related to each other. This process is the basis of LEDs [36]. 

The bipolar junction transistor (BIT) consists of tliree layers doped n-p-n or p-n-p tliat constitute tire emitter, base and collector, respectively. This stmcture can be considered as two back-to-back p-n junctions. Under nonnal operation, tire emitter-base junction is forward biased to inject minority carriers into tire base region. For example, tire n type emitter injects electrons into a p type base. The electrons in tire base, now minority carriers, diffuse tlirough tire base layer. The base-collector junction is reverse biased and its electric field sweeps tire carriers diffusing tlirough tlie base into tlie collector. The BIT operates by transport of minority carriers, but botli electrons and holes contribute to tlie overall current. 

A band diagram of a biased n-p-n BIT is shown in figure C2.16.8. Under forward bias, electrons are injected from tlie n type emitter, giving rise to tlie current 7. flowing into tlie p type base. Some of tlie carriers injected into tlie base recombine in tlie base or at tlie surface. This results in a reduction of tlie base current by 7, tlie lost recombination current, and tlie base current becomes 7g = At tlie same time, holes are injected from tlie... 

Figure C2.16.9. Schematic cross-section and biasing of a metai-oxide-semiconductor transistor. A unifonn conducting channei is induced between source (S) and drain (D) for > V. Voitage is appiied between the gate (G) and the source. Part (A) shows the channei for - V the transistor acts as a triode. The source-...

The biasing function is applied to spread the range of configurations sampled such that the trajectory contains configurations appropriate to both the initial and final states. For the creation or deletion of atoms a softcore interaction function may be used. The standard Lennard-Jones (LJ) function used to model van der Waals interactions between atoms is strongly repulsive at short distances and contains a singularity at r = 0. This precludes two atoms from occupying the same position. A so-called softcore potential in contrast approaches a finite value at short distances. This removes the sin-... 

We then require that the exact trajectory will have a weight of at least i, which will make its sampling possible in a search biased by the weight. 

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