Recombinations processes

There are many ways of increasing tlie equilibrium carrier population of a semiconductor. Most often tliis is done by generating electron-hole pairs as, for instance, in tlie process of absorjition of a photon witli h E. Under reasonable levels of illumination and doping, tlie generation of electron-hole pairs affects primarily the minority carrier density. However, tlie excess population of minority carriers is not stable it gradually disappears tlirough a variety of recombination processes in which an electron in tlie CB fills a hole in a VB. The excess energy E is released as a photon or phonons. The foniier case corresponds to a radiative recombination process, tlie latter to a non-radiative one. The radiative processes only rarely involve direct recombination across tlie gap. Usually, tliis type of process is assisted by shallow defects (impurities). Non-radiative recombination involves a defect-related deep level at which a carrier is trapped first, and a second transition is needed to complete tlie process. 

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 light emitted in the spontaneous recombination process can leave tire semiconductor, be absorbed or cause additional transitions by stimulating electrons in tire CB to make a transition to tire VB. In tliis stimulated recombination process anotlier photon is emitted. The rate of stimulated emission is governed by a detailed balance between absorjDtion, and spontaneous and stimulated emission rates. Stimulated emission occurs when tire probability of a photon causing a transition of an electron from tire CB to VB witli tire emission of anotlier photon is greater tlian that for tire upward transition of an electron from tire VB to tire CB upon absorjDtion of tire photon. These rates are commonly described in tenns of Einstein s H and 5 coefficients [8, 43]. For semiconductors, tliere is a simple condition describing tire carrier density necessary for stimulated emission, or lasing. This carrier density is known as... 

In an electron scattering or recombination process, the free center of the incoming electron has the functions Wi = ui U u, and the initial state of the free elechon is some function v/ the width of which is chosen on the basis of the electron momentum and the time it takes the electron to aiTive at the target. Such choice is important in order to avoid nonphysical behavior due to the natural spreading of the wavepacket. 

At low background flux this gives the temperature dependence of the Z9 shown in Eigure 4. At high flux, the Z9 equation (eq. 37) reduces to equation 12 except for a factor of (2) which is a result of the random recombination process not present in diodes. The scene sensitivity of a scanning photoconductor array infrared camera is ca 0.15°C. 

The radicals and other reaction components are related by various equiUbria, and hence their decay by recombination reactions occurs in essence as one process on which the complete conversion of CO to CO2 depends. Therefore, the hot products of combustion of any lean hydrocarbon flame typically have a higher CO content than the equiUbrium value, slowly decreasing toward the equiUbrium concentration (CO afterburning) along with the radicals, so that the oxidation of CO is actually a radical recombination process. 

A well-known example of non-prototropic tautomerism is that of azolides (acylotropy). The acyl group migrates between the different heteroatoms and the most stable isomer (annular or functional) is obtained after equilibration. In indazoles both isomers are formed, but 2-acyl derivatives readily isomerize to the 1-substituted isomer. The first order kinetics of this isomerization have been studied by NMR spectroscopy (74TL4421). The same publication described an experiment (Scheme 8) that demonstrated the intermolecular character of the process, which has been called a dissociation-recombination process. 

The first, and to this writing still only case of a ketone a-cleavage-recombi-nation sequence in the steroid field was reported by Butenandt, who found that 17-ketones epimerize at C-13. Ultraviolet irradiation of either stereoisomer produces an equilibrium mixture in which the thermodynamically more favored 13a-compound cf. (15)] with cw-fusion of rings C and D predominates at room temperature. As ultraviolet absorption energies and intensities of the two isomeric ketones are practically identical, the equilibrium composition depends largely on the rate of the competing recombination process from (14). For further examples of the photoisomerization at C-13 of 17-ketosteroids, see ref. 8, 12, 15 and 43. 

Thus, in order to reproduce the effect of an experimentally existing activation barrier for the scission/recombination process, one may introduce into the MC simulation the notion of frequency , lo, with which, every so many MC steps, an attempt for scission and/or recombination is undertaken. Clearly, as uj is reduced to zero, the average lifetime of the chains, which is proportional by detailed balance to Tbreak) will grow to infinity until the limit of conventional dead polymers is reached. In a computer experiment Lo can be easily controlled and various transport properties such as mean-square displacements (MSQ) and diffusion constants, which essentially depend on Tbreak) can be studied. 

The rearranged dicationic species 4, which has been shown to be an intermediate, leads to the stable benzidine 2 upon deprotonation. It has been demonstrated by crossover experiments that the rearrangement does not proceed via a dissoci-ation/recombination process. From the reaction of hydrazobenzene the benzidine is obtained as the major product (up to 70% yield), together with products from side reactions—2,4 -diaminobiphenyl 5 (up to 30% yield) and small amounts of 2,2 -diaminobiphenyl 6 as well as o- and /j-semidine 7 and 8 ... 

U. Lemmer. R. F. Malm, Y. Wada, A. Greiner, H. Basslcv. E.O. Gobel, Time resolved luminescence study of recombination processes in electroluminescent polymers, Appl. Phys. Lett. 1993, 62, 2827. 

A slow rate of p-scission also means that the main cage recombination process will be cage return to reform the peroxydicarbonate. Dialkyl peroxides are typically not found amongst the products of peroxydicarbonate decomposition. In these circumstances, cage recombination is unlikely to be a factor in reducing initiator efficiency. 

Baechler and coworkers204, have also studied the kinetics of the thermal isomerization of allylic sulfoxides and suggested a dissociative free radical mechanism. This process, depicted in equation 58, would account for the positive activation entropy, dramatic rate acceleration upon substitution at the a-allylic position, and relative insensitivity to changes in solvent polarity. Such a homolytic dissociative recombination process is also compatible with a similar study by Kwart and Benko204b employing heavy-atom kinetic isotope effects. 

The combination of photocurrent measurements with photoinduced microwave conductivity measurements yields, as we have seen [Eqs. (11), (12), and (13)], the interfacial rate constants for minority carrier reactions (kn sr) as well as the surface concentration of photoinduced minority carriers (Aps) (and a series of solid-state parameters of the electrode material). Since light intensity modulation spectroscopy measurements give information on kinetic constants of electrode processes, a combination of this technique with light intensity-modulated microwave measurements should lead to information on kinetic mechanisms, especially very fast ones, which would not be accessible with conventional electrochemical techniques owing to RC restraints. Also, more specific kinetic information may become accessible for example, a distinction between different recombination processes. Potential-modulation MC techniques may, in parallel with potential-modulation electrochemical impedance measurements, provide more detailed information relevant for the interpretation and measurement of interfacial capacitance (see later discus-... 

Surface recombination processes of charge carriers are mechanisms that cannot easily be separated from real semiconductor interfaces. Only a few semiconductor surfaces can be passivated to such an extent as to permit suppression of surface recombination (e.g., Si with optimized oxide or nitride layers). A pronounced dip is typically seen between the potential-dependent PMC curve in the accumulation region and the photocurrent potential curve (e.g., Fig. 29). This dip may be partially caused by a surface... 

If no transfer of translational energy occurs, then the charge exchange process probably takes place when the distance between the ion and the molecule is large. This means, however, that the ion and the molecule can be considered as isolated from each other, and therefore, the recombination process of the ion and the ionization process of the molecule must obey the spectroscopic transition laws. On the other hand, if a large transfer of translational energy takes place, then the process probably takes place when the distance is small, and possibly then all selection rules break down. 

The low concentration and high energy of the electrons in this region reduces the probability of ion-electron recombination processes. 

In deriving the kinetic equation describing the arrival of various ionic species at the cathode, it is assumed that the primary species N2 + is formed at the central wire at a constant rate, and during its passage in the direction x perpendicular to the axis its concentration is modified by various reactions. In this treatment both ion diffusion and ion-ion or electron-ion recombination processes are neglected because the geometry of the discharge tube and the presence of an electric field would... 

An important aspect of semiconductor photochemistry is the retardation of the electron-hole recombination process through charge carrier trapping. Such phenomena are common in colloidal semiconductor particles and can greatly influence surface corrosion processes occurring particularly in small band gap materials, such... 

H2S adsorption on the (2x2)-S covered Pt(lll) surface at IlOK contrasts with adsorption on the clean surface. On the (2x2)-S surface no complete dissociation Is observed at low temperature Instead, H2S partially dissociates to form an adsorbed SH Intermediate with a characteristic bend vibration at 585 cm . Heating adsorbed SH on the (2x2)-S covered surface leads to a SH+H recombination reaction not observed on clean Ft. The recombination process removes the excess SH so that the stable, high coverage (/3 X /3)R30 -S lattice can be formed. 

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