Luminescence spectra band edge

We believe that the luminescence at 1.0 eV is due to a structural damage induced by ion implantation rather than to a chemical doping effect, since the spectrum does not depend on the chemical species of the ion. These centers may be similar to the vacancies induced by 3-MeV electron-beam irradiation, as reported by Troxell and Watkins (1979), who find donorlike and acceptorlike levels —0.1 eV from the band edges. 

Figure 1.7 Near-band-edge photoluminescence spectrum of 4H-SIC showing the bound exciton-related luminescence denoted and (where h is the energy in meV of the phonon involved in the transition) and free exciton-related luminescence denoted /. (Data provided by Docent Anne Henry at Linkoping University.)...

Donor-acceptor absorption can also be observed in semiconductors, but this process is weak because of the small overlap of the wavefunctions (like an n -> n transition). Donor-acceptor absorption is best monitored through the emission, that is by excitation spectra. In the normal situation, the donor-acceptor absorption can be observed but the valence band-to-donor and the acceptor-to-conduction band transitions can also be seen, as they also contribute to the luminescence. All three of these transitions are weak but of similar strengths [6]. In undoped AgCl and AgBr, only a very weak excitation spectrum is seen, which consists of a relatively sharp line near the band edge. In Cd2 + doped AgBr both the sharper line, whose onset is about... 

Similar behavior has been observed in CdSe clusters [60], Using laser excitation near the red edge of the absorption band, sharp luminescence with well-defined vibronic structures can be observed. The decay kinetics shows two components—a temperature-insensitive 100-ps component and a microsecond, temperature-sensitive component. The luminescence spectrum develops a 70-cm-1 red shift as the fast component decays. The three-level thermal equilibration model again has to be invoked to explain these kinetic data. Based on the polarization measurement, the authors suggest that it is the hole, instead of the electron, that is shallowly trapped. The trap depth is estimated to be 9 meV. The authors further propose that strong resonant mixing exists between the internal MOs and surface MOs. 

Relatively sharp luminescence lines observed near the band edge arise from the recombination of electron-hole pairs that form bound excitons (BEs) at impurity sites or free-to-bound (FB) transitions that involve the recombination of free electrons (holes) with holes (electrons) bound at neutral acceptors (donors). Figure 12 shows the PL spectrum at 1.96 K for an undoped 3C-SiC epilayer grown on Si by CVD (63). In the energy range 2.4-2.2 eV, five sharp lines are seen. Choyke et al. (64) have observed five similar sharp luminescence peaks for a 3C-S1C crystal grown by the Lely method. These peaks have been assigned as the zero phonon line (ZPL, 2.379 eV) and its phonon replicas TA, LA, TO, and LO. They attributed these lines to... 

Dependencies of luminescence bands (both fluorescence and phosphorescence), anisotropy of emission, and its lifetime on a frequency of excitation, when fluorescence is excited at the red edge of absorption spectrum. Panel a of Fig. 5 shows the fluorescence spectra at different excitations for the solutes with the 0-0 transitions close to vI vn, and vra frequencies. Spectral location of all shown fluorescence bands is different and stable in time of experiment and during lifetime of fluorescence (panel b)... 

Each of the mentioned PL bands has its own individual excitation spectrum shown in Figure 9.8 the UV emission has an excitation band peaking at 245 nm (curve 1), the Blue emission is excited at 280 nm (curve 2), and the Red emission has wide excitation bands at 270 and 400 nm (curve 3). Besides, all emission bands can be excited in the near-edge absorption region 200-220 nm. Individual character of the excitation spectra of the emission bands proves their origin from different luminescence centers. 

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