Bound excitons

Excitons are electron-hole pairs weakly coupled through the band gap by Coulomb interaction. When they are free to propagate in the crystal, they are logically called free excitons (FEs) and are characterized by a binding energy Eex. Their properties are described in Sect. 3.3.2. 

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 

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. 

PL evidence for the binding of more than one exciton to a shallow impurity exists, starting with the excitonic molecule was first reported in silicon [20]. A model for the bound multi-exciton complexes in silicon (the shell model) has been elaborated by Kirczenow [27] to explain the experimental results of these centres. For a review on these centres, see [49]. 

In doped uncompensated semiconductors, very weak absorption lines due to the direct creation of excitons bound to neutral donors or acceptors can be observed at low temperature (typically 2K) at energies close to Eg [11,14,21]. 

Reference A-exciton bound to neutral or ionized donors Excited rotator states B-exciton bound to neutral donor h 

Another relatively strong emission line at 3.3724 eV (lo) was attributed to the transition due to the B-free exciton bound to the same main neutral donor (D Xb) by Teke et al. [50]. The energy separation between this peak and the main peak at 

Based on the Al implantation studies of Schilling et al. [73], who observed an increase in the intensity of the 3.3605 eV (,) emission line with increased Al concentration in ZnO, the (, transition is attributed to the Al impurity. Secondary ion mass spectroscopy (SIMS) revealed that Al was the dominant impurity with a concentration of more than one order of magnitude higher than that for other group III and group VII elements for samples where only the 3.3605 eV neutral donor-bound exciton line was observed [65]. Al was suggested to be an omnipresent impurity in vapor-grown ZnO [67]. 

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. 

3564 L)) and 3.3530eV (fio) emission lines were initially thought to be related to Na and Li impurities, because they were typically observed in films doped with these elements. However, PL studies of Meyer et al. [65] on Na (Li)-diffused samples using different Na(Li)-containing salts did not support this thesis. On the contrary, difiusion experiments using different concentrations of indium sulfate solutions led to the appearance of the Ig line with In diffusion into bulk ZnO, which was confirmed by SIMS [65]. After the identification of In donor by optically detected magnetic resonance, it has also been stated that indium donor in Li-doped ZnO is an unintentional, residual impurity [67]. 

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The temperature dependence of luminescence from the sample irradiated at 1 x 1013 cm-2 with 28Si+ indicates, above —110 K, an activation energy of 90 meV for the competing nonradiative recombination process— this competing process may be the thermal dissociation of geminate pairs or bound excitons at donorlike or acceptorlike centers. The 0.09-eV value of activation energy is consistent with the results of Troxell and Watkins (1979). 

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. 

Fig. 17 Photoluminescence spectra covering the no-phonon and TA phonon-replica energy regions taken at 4.2 K. The spectra show the bound exciton luminescence of samples implanted with B, In, and T1 before (a, c, e) and after (b, d, f) treatment in atomic H. Bound exciton luminescence due to the implanted impurities has been shaded in to distinguish it from the substrate luminescence. From Thewalt et al. (1985).

The introduction of electronic deep levels is demonstrated in Fig. 9 with low-temperature photoluminescence spectra for n-type (P doped, 8 Cl cm) silicon before (control) and after hydrogenation (Johnson et al., 1987a). The spectrum for the control sample is dominated by luminescence peaks that arise from the well-documented annihilation of donor-bound excitons (Dean et al., 1967). After hydrogenation with a remote hydrogen plasma, the spectrum contains several new transitions with the most prominent peaks at approximately 0.95, 0.98, and 1.03 eV. These transitions identify... 

Within the simple Bohr model used for weakly bound excitons, the radius of the electron-hole orbit is given by... 

Hallermann M, Haneder S, Da Como E (2008) Charge-transfer states in conjugated polymer/ fullerene blends below-gap weakly bound excitons for polymer photovoltaics. Appl Phys Lett 93 053307... 

The photoluminescence (PL) spectrum in Figure 1.7 shows a number of lines related to nitrogen-bound excitons and free excitons. SiC has an indirect bandgap, thus the exciton-related luminescence is often assisted by a phonon. Bound exciton luminescence without phonon assistance can, however, occur because conservation in momentum can be accomplished with the help of the core or the nucleus of the nitrogen atom. That is why the zero phonon lines of the nitrogen atom are seen, denoted and Q , in the spectrum but not the zero phonon line of the free exciton. 

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.)...

Two energies in fig. 21 are important quantities for the luminescence behavior. First, the energy Et represents the binding energy of the rare earth bound exciton with respect to the conduction band and second, bt, the necessary energy for back-transfer, corresponds to the difference between the bound exciton recombination energy and the 2Fs/2 2F7/2 transition... 

Ishizumi and Kanemitsu (2005) have studied PL properties of Eu3+ doped ZnO nanorods fabricated by a microemulsion method. The PL of bound exciton recombination and ZnO defects was observed near 370 and 650 nm under 325-nm light excitation, but no emission of Eu3+ occurred. On the other hand, the sharp PL peaks due to the intra-4f transitions of Eu3+ ions appeared under nonresonant excitation below the band-gap energy of ZnO (454 and 457.9 nm) in addition to direct excitation to 5D2 (465.8 nm). Therefore the authors concluded that the energy transfer occurs from the ZnO nanorods to Eu3+ ions through ZnO-defect states. This energy transfer mechanism seems very different from the previous one and more spectroscopic evidence is required to confirm it. 

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