Exciton bound

Another consequence of acceptor neutralization is the disappearance of excitons bound to acceptors. Their characteristic luminescence can be restored by the thermal release of the hydrogen. 

VI. Effect of Hydrogenation on the Luminescence of Excitons Bound to Acceptors... 

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. 

Defects with large central cell correction have very localized wave functions. The larger the correction, the more localized the wave function and the higher the probability of interaction between the core (or central cell) and the electron and/or the exciton bound to the defect. Hence the reason why the line is so much smaller than the Q line in the spectrum, as well as the reason why the phonon replicas to the Q-line, is simply a matter of probability, since the central cell correction is so much larger for a nitrogen defect on a cubic site than a hexagonal site. 

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... 

An exciton bound to a shallow neutral donor of interstitial zinc (Fig 1 a) and of interstitial lithium (Fig. lb) is presented, for example, in our spectra. In some instances the radiative recombination of an exciton bound to a neutral defect may not lead to the ground state of the respective defect but to an excited state of the carrier at this occupied center (2 - electron transition). In a hydrogenic model we can calculate an ionization energy of the neutral donor state of interstitial zinc to 0.05 eV and of interstitial lithium to 0.033 eV. 

Heat treatment of as-grown crystals in Li2C03 at 400 to 700°C for 2 to 7 days causes a partial substitution of lithium for Zn sites causing considerable growth of resistivity to p 10 -10 ° Qcm because of the LTi = Li zn and noticeable lowering of the Fermi level. The luminescence spectra of such samples are characterized by enhanced yellow-orange emission noticeable quenching of emission due to excitons bound to neutral shallow donors. 

The Zeeman behavior of the emission lines from U to R is different from that of the Wh lines. Their linear splitting in B c indicates transitions originating from excitons bound to neutral impurities. We discuss the emission lines R and h in detail. These lines represent the dominant... 

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. 

Figure 1. Schematic representation of radiative recombination of an exciton bound to a neutral donor where the final state is the donor in the ground or in the excited configuration. The inset shows the initial state of the neutral-donor-bound exclton in the ground and several excited rotational states. (Reproduced with permission from Ref. 24. Copyright 1983 American Physical Society.)...

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