Auger recombination processes

Equation (3) can be explained in simple terms as follows. The rate of change of the carrier density (i.e., dNc/dt) is equal to the rate at which new carriers are injected, minus the rate at which carriers are removed via nonradiative, radiative, and Auger recombination processes, minus the rate at which carriers are removed via stimulated recombination. Note that the rate of stimulated recombination is proportional to the differential gain given by Eq. (2). This is because the creation of a new photon by stimulated emission (i.e., the gain process) corresponds precisely to the loss of one carrier through stimulated recombination. 

The constant Tj is known as the spontaneous carrier lifetime and may be interpreted as the average time for which a carrier lives in the conduction band before spontaneously recombining via any one non-radiative, radiative or Auger recombination processes. The spontaneous carrier lifetime is typically on the order of 1 ns. 

The decay on a picosecond time-scale, the so-called fast band, is understood as a quasi-direct recombination process in the silicon crystallites or as an oxide-related effect [Tr2, Mgl]. This fast part of the luminescence requires an intense excitation to become sizable it then competes with non-radiative channels like Auger recombination. The observed time dependence of the slow band is explained by carrier recombination through localized states that are distributed in energy, and dimensionally disordered [Gr7]. 

Femtosecond flash photolysis studies on Q-state CdS [107] indicate that reaction (4a) proceeds via two recombination processes a 50 ps decay at low excitation intensities, postulated to correspond to geminate e h+ recombination, and a faster 2 ps decay at higher flash fluences, corresponding to non-geminate or possibly three body Auger charge carrier recombination. Other studies by Nosaka and Fox [118] indicate that the second order rate coefficient for electron-hole recombination within CdS particles is of the order 9 x 10 t7 m3 s l. 

If the equilibrium of a semiconductor is disturbed by excitation of an electron from the valence to the conduction band, the system tends to return to its equilibrium state. Various recombination processes are illustrated in Fig. 1.16. For example, the electron may directly recombine with a hole. The excess energy may be transmitted by emission of a photon (radiative process) or the recombination may occur in a radiationless fashion. TTie energy may also be transferred to another free electron or hole (Auger process). Radiative processes associated with direct electron-hole recombination occur mainly in semiconductors with a direct bandgap, because the momentum is conserved (see also Section 1.2). In this case, the corresponding emission occurs at a high quantum yield. The recombination rate is given by... 

Considered from a very simple mechanistic viewpoint, the various photoemission yield techniques can be rationalized as follows. A core state (occupied state) is excited by photoadsorption and de-excited by one or more of three possible processes, viz. Auger transitions, direct recombination of surface excitations or direct recombination processes of excitons involving the valence bands. The electrons resulting from these processes are inelastically scattered and so generate secondary electrons. Therefore, by separating the optical excitations from Auger de-excitation... 

Auger processes encompass Auger recombination and its opposite impact ionization. Beattie defined ten basic Auger processes in material with a single conduction band and with a heavy holes and light holes band. In the processes of nonradiative interband recombination phonon states, localized states and impurity levels may take part. Landsberg described 70 such secondary Auger processes [30]. 

In order to present some basic expressions for the Auger lifetime let us denote Auger generation (impact ionization) rate as Ga and Auger recombination rate as Ra- We consider first the Auger 1 process. Since two electrons and one hole take part in it, the recombination rate will be proportional to n p... 

The net recombination rate of Auger 1 process is obtained by subtracting... 

For Auger 7 processes recombination rate is proportional to p n, thus... 

The photon management methods also represent a pathway toward a decrease of generation-recombination processes and the related noise phenomena and pose a viable way to overcome obstacles that until recently appeared insurmountable. The mechanism of photon recycling by cavity enhancement ensures direct suppression of radiative noise, while the possibility to localize fields at a subwavelength level ensures vastly smaller photodetector volumes, thus helping overcome problems with Auger and Shockley-Read phenomena as well. 

Schematic depiction of impact ionization of a high-energy electron-hole pair. (1) Electron-hole pair (ei and hi) is generated, for example, by a photon. (2) e2 gets excited from a VB state to a CB state via Coulomb interaction with e2, leaving hole h2 behind. The reverse process is referred to be Auger recombination...

For each photoelectron that leaves the surface, an atom with a core hole is left behind in a highly excited state, which relaxes both by radiative and nonradiative processes. In a radiative recombination process, the core hole is filled in an electronic transition from a core level of lower binding energy or a valence level. The surplus energy is released by the emission of an X-ray photon, in a so-caUed X-ray fluorescence process. In this process, the emitted photon has a lower energy than the exciting photon and dipole selection rules apply for both, excitation and de-excitation. Conversely, Auger processes are nonradiative de-excitation channels... 

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