Neutral bound exciton

Figure 3. Level sehemes and selection rules for neutral bound exciton transitions.

Indeed, the fact that the onset of photoconductivity occurs at a higher energy than the absorption edge in the polydiacetylenes, both in single crystal samples and in thin films cast from solution is a clear indication that the photoexcitations generated below 2.3 eV are neutral bound excitons [200]. 

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. 

In the UV range, the PL spectrum of oxygen-enriched ZnO (p-type) shows a strong line at 369.5 nm with a fiill width at half maximum (FWHM) of 14 meV. In the visible range, we observe a weak band centered at 400 nm. Comparison of the luminescence spectra of n- and / -t5 e ZnO indicates that the 369.0 and 369.5-nm lines are due to bound excitons. As shown by Butkhuzi et alf the 369.0 and 369.5-nm emissions arise from excitons bound to neutral donors and acceptors, respectively. Increasing the annealing time at 710 K increases the intensity of the 400-nm band. The spectra of samples annealed for 4 h show only the 400-nm band, which spans the entire... 

The values of gh derived for the U and I9 lines are very close to the gh = -1.24 obtained for the hole involved in the exciton bound to ionized donor and to the g factor of the hole in li free exciton state. " On the other hand, the expected g/ values of the holes involved in the acceptor bound exciton transitions differ significantly from the g values of the holes involved into excitons bound to ionized or neutral donors. This is similar to the situation found in CdS. Therefore we conclude that both U and I9 transitions should be assigned to the (Do,2Ci(r7)) complex rather than to the (y4o(T7) A(T7)). 

The inverted Vj A), TeCil), Vj C) ordering of the valence subbands in bulk ZnO was confirmed by the detailed analysis of the Zeeman splitting of the free and bound excitons. The polarization properties and the angular dependence of the transition energies from excitons bound to ionized and neutral impurity centers indicated the T7 character of the upper A valence band. The obtained Tv effective g values are in good with theoretical calculations. We observed no low temperature PL transitions involving the Tg hole states from the B valence subband. 

Identification of residual and doped donors have been identified in expitaxial GaAs using the photolumines-cence technique in the presence of applied magnetic fields. Transitions occur between excited initial and final states of the neutral-donor-bound-exciton complexes. The magnetic field compresses the wave function which sharpens the optical transitions. The magnetic field also separates the different donors when viewed from the neutral-donor-bound-exciton transitions. These two effects make possible the identification of donors when the donor concentration is in the mid lOlScm" range. 

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... 

It was mentioned in Sect. 1.3.2 that in semiconductors, isoelectronic impurity centres could present a relatively strong attracting potential for electrons or holes. Excitons can be trapped by or created at these isoelectronic centres to form an isoelectronic bound exciton (IBE). The electron (hole) of this exci-ton is also more strongly bound to the isoelectronic centre than in classical excitons and the second constituent of the exciton, hole (electron) can be considered to be bound to a compound negative or positive ion. These structures are similar to those of neutral donors or acceptors and they are called isoelectronic donors or acceptors [104]. When formed by near band-gap or above band-gap laser illumination, the long lifetimes of these IBEs result in sharp PL lines, and this has for some time aroused interest for these centres as potential near IR radiation emitters. 

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,... 

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