Impurity bound exciton recombination

At low temperatures, donors and acceptors remain neutral when they trap an electron hole pair, forming a bound exciton. Bound exciton recombination emits a characteristic luminescence peak, the energy of which is so specific that it can be used to identify the impurities present. Thewalt et al. (1985) measured the luminescence spectrum of Si samples doped by implantation with B, P, In, and T1 before and after hydrogenation. Ion implantation places the acceptors in a well-controlled thin layer that can be rapidly permeated by atomic hydrogen. In contrast, to observe acceptor neutralization by luminescence in bulk-doped Si would require long Hj treatment, since photoluminescence probes deeply below the surface due to the long diffusion length of electrons, holes, and free excitons. 

The 3.3598 eV Is) neutral donor-bound exciton line was observed to be prominent in Ga-doped epitaxial ZnO films and in ZnO epitaxial films that were grown on GaN templates, which resulted in Ga interdiffusion into ZnO, as verified by SIMS experiments [65]. The attribution of the Ig line to the Ga impurity was also supported by the findings of Ko et al. [74], who also reported Ga-donor-bound exciton recombination at 3.359 eV. 

The FEs can bind to neutral shallow impurities and become bound excitons (BEs), with a value of Eex slightly larger than the one of the FE. The difference is called the localization energy E oc of the BE. For the P donor, it is 4 meV in silicon, but 75 meV in diamond. E oc is given approximately by Haynes empirical rule [20] as 0.1 A, where A is the ionization energy of the impurity. BEs are created by laser illumination of a semiconductor sample at an energy larger than Eg and the study of their radiative recombination by PL... 

Photoluminescence could be due to the radiative annihilation (or recombination) of excitons to produce a free exciton peak or due to recombination of an exciton bound to a donor or acceptor impurity (neutral or charged) in the semiconductor. The free exciton spectrum generally represents the product of the polariton distribution function and the transmission coefficient of polaritons at the sample surface. Bound exciton emission involves interaction between bound charges and phonons, leading to the appearance of phonon side bands. The above-mentioned electronic properties exhibit quantum size effect in the nanometric size regime when the crystallite size becomes comparable to the Bohr radius, qb- The basic physics of this effect is contained in the equation for confinement energy,... 

Edge emission is due to exciton recombination (Sect. 3.3.1). Usually this emission is due to bound excitons, i.e. an exciton of which either the electron or the hole is trapped at an imperfection in the lattice. The elucidation of the nature of this imperfection is often a difficult task. As an example of such an emission we can mention the exciton emission of GaP N. Nitrogen is an isoelectronic dopant (on phosphorous sites). The exciton is bound to this nitrogen impurity before it decays. The emission is situated at about 0.02 eV below Ej,. Another semiconductor for which exciton emission has been thoroughly studied is CdS. 

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... 

The study of the radiative recombination of excitons makes it possible to investigate the influence of the radiation-stimulated destruction of C60 fullerenes (partially of C70) on changes of the singlet states within the energy gap. It is known that the emission of excitons in this case is the result of the presence of own dimeric traps [11] and X-centers, caused by the chemically bound with fullerenes and intercalated impurities [8], and also of taking into account the corresponding phonon states. 

Radiative recombination of an exciton bound to a shallow impurity generally leaves this impurity in the electronic ground state, resulting in the principal BE (PBE) line, but weaker PL lines can also be observed at lower energies, where the impurity is left in an electronic excited state. These so-called two-electron or two-hole PL spectra are usually observed in their phonon-assisted form, and they mainly involve s-like excited states whose detection escapes the absorption experiments. These PL experiments are, therefore, valuable complements to absorption spectroscopy, which involves mainly the p-like excited states, and examples will be given when appropriate. 

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