Radiative recombination 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. 

In the study of any radiative recombination process, one tries to answer a number of fairly well defined questions, mostly related to potential curves. From what electronic states is emission observed With what atomic states do these molecular states correlate Does the recombination take place on a single potential curve, or is a transition between two curves involved Is a potential curve with a significant maximum involved Is a third body necessary, either to stabilize the atom pair on a single curve, or to induce a transition to another curve In the case of a transition between two electronic states, is there an approximate equilibrium What is the vibrational and rotational distribution of newly formed molecules What is the recombination rate coefficient as a function of temperature or cross section as a function of energy In principle these questions can be answered either theoretically or experimentally. In fact, they have been answered experimentally in most cases, but the answers are seldom as certain or as numerous as one would wish. This becomes clear in the following discussion of particular cases. 

In direct gap GaAs, an excited electron at the bottom of the conduction band can relax spontaneously back into a hole in the valence band by emitting a photon at the band gap energy. This electron-hole radiative recombination process can only occur in Si if momentum is conserved, i.e., the excited electron wave vector must be reduced to zero. This, in pure Si, occurs via the transfer of momentum to a phonon that is created with equal and opposite wave vector to that of the initial state in the conduction band. Such a three-body process is quite inefficient compared with direct gap recombination.1218 This is why Si is such a poor light emitter. 

Trapped charge carriers (e-f, At) can further participate in radiative and non-radiative recombination processes ... 

The ultrafast charge transfer process was subsequently time resolved [175,176] the data directly confirm that charge transfer occurs within a few hundred femtoseconds. Since the charge transfer rate is more than two orders of magnitude faster than competing radiative and non-radiative recombination processes, the quantum efficiency for charge transfer must be close to unity. 

The obtained results demonstrate good perspectives of nc-Si/SiO2 Er for applications in light emitting devices. However, it is necessary to improve the structural, electronic and optical properties of the samples to suppress the non-radiative recombination processes and energy back-transfer from Er3+ to nc-Si. The contributions of the stimulated optical transitions in Er3+ ions can be obviously enhanced by optimizing the sample properties. 

Niki, M., Vedda, A., Fasoli, M. etal. 2007b. Shallow traps and radiative recombination processes in Lu3Al50,2 Ce single crystal scintillator. Physical Review B 76 195121. 

Electron-hole recombination A radiative (or nonradia-tive) process which an electron in the conduction band recombines with a hole in the valence band. In the radiative recombination process, photons are generated, while in the nonradiative recombination process, phonons are generated. 

CdSe QDs have been used as an optical probe for the analysis of vitamin Bi concentration (Sun et al. 2008). The technique comprises the fluorescence quenching of light emitted by the CdSe QDs in presence of vitamin Bi under alkaline conditions. Vitamin Bi is an efficient quencher and can interrupt the radiative recombination process by adsorbing on the surface of CdSe QDs. The QDs produced an emission at 591 nm with excitation at 380 nm. Thus, vitamin Bi was determined from the linear relationship between concentration of vitamin Bi and fluorescence quenching of QDs. The obtained LOD was achieved at nanogram scale. The concentration of vitamin Bi in tablet and injection samples was determined by this method. 

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... 

Defect states can enhance non-radiative recombination processes. 

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