Phonon replica

The PL spectrum and onset of the absorption spectrum of poly(2,5-dioctyloxy-para-phenylene vinylene) (DOO-PPV) are shown in Figure 7-8b. The PL spectrum exhibits several phonon replica at 1.8, 1.98, and 2.15 eV. The PL spectrum is not corrected for the system spectral response or self-absorption. These corrections would affect the relative intensities of the peaks, but not their positions. The highest energy peak is taken as the zero-phonon (0-0) transition and the two lower peaks correspond to one- and two-phonon transitions (1-0 and 2-0, respectively). The 2-0 transition is significantly broader than the 0-0 transition. This could be explained by the existence of several unresolved phonon modes which couple to electronic transitions. In this section we concentrate on films and dilute solutions of DOO-PPV, though similar measurements have been carried out on MEH-PPV [23]. Fresh DOO-PPV thin films were cast from chloroform solutions of 5% molar concentration onto quartz substrates the films were kept under constant vacuum. 

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).

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. 

Figure 7.19 shows PL spectra recorded at 2K for 2.2, 0.7, and 1.5 pm thick PLD ZnO films on a-plane, c-plane, and r-plane sapphire, respectively [63], The full widths at half maximum (FWHM) of the most intense bound exciton peaks are 1.4, 1.7, and 2.6 meV for a-, c-, and r-sapphire, respectively. The film on a-plane sapphire shows the narrowest FWHM among the films under investigation and the free A-exciton (Xa) is most clearly resolved, thus indicating best structural properties of ZnO on a-plane sapphire. The ZnO films on a- and c-plane sapphire grow c-axis textured, whereas films on r-plane sapphire grow a-axis oriented with the ZnO c-axis being in-plane, as demonstrated already in Fig. 7.4. The PL spectrum of the film on r-plane sapphire shows no phonon replica, probably due to the changed ZnO orientation.

Another significant and not yet understood difference in Mg doping in MBE and MOVPE is the different photoluminescence behaviour. Whereas MBE-material [27] exhibits the well known donor-acceptor pair transition at 380 nm, generally followed by phonon replicas, MOVPE-material shows a dominant feature at about 430 nm, whose origin is not yet known. 

The in-phase and quadrature PA spectra of a-6T measured at 80 K with a modulation frequency of 200 Hz are shown in Figure 7-26 [44]. The spectrum exhibits three bands at 0.80,1.1, and 1.54 eV, respectively each of which is accompanied by a high-energy phonon replica. The bleaching of the in-phase PA above 1.7 eV is due to thermal modulation of the PL due to heating of the sample by the probe. The relative positions of the PA bands correspond to the DIA bands shown in Figure... 

This figure shows no phonon replicas in the Au spectrum. A p3/2 transmission spectrum of Pt at LHeT showing the I (0) splitting is displayed in Fig. 7.18. The measured FWHM of the components of I (0) is lcm-1 ( 124 j.eV) and the true width should be somewhat smaller, but excited shallower levels are broader. 

The widths of the phonon replicas I (1) and /2 (1) are only 2-4 times larger than the no-phonon lines, and this implies a relatively small coupling with the electronic transitions, which has been discussed by Kleverman et al. [100] in terms of a pseudolocalized phonon in the vicinity of the acceptor atom. The positions of the no-phonon acceptor lines of Au and Pt and of the phonon replicas of Pt in silicon are given in Table 7.18. 

Fig. 7.17. Comparison between the Au and Pt p3/2 spectra in silicon obtained from photothermal ionization measurements at LHeT. To facilitate the assignment of the Pt phonon replicas, markers indicating 0, 1, and 2 phonon energies (hu> = 57 cm-1 or 7.1 meV) are included. The Au spectrum extends from about 600 to 650 meV and the Pt one from 910 to 960 meV [100]. Copyright 1988 by the American Physical Society...

Fig. 7.25. Absorption spectrum of Li in CdTe at 1.5 K. The inset shows the attribution of the main electronic lines. The final state of line 1 should be 3P5/2 (rs) and lines 2, 3, and 4 should be attributed to other transitions to nP states with n > 2. G, D, and C are LO (T) phonon replicas of lines G, D, and C. The lines denoted 2LO and 3LO are attributed to local phonon modes coupled with the Li acceptors (after [61]). Reproduced with permission from the Institute of Physics...

There are 12 atoms per unit cell (consequently 36 phonon branches) and three inequivalent carbon sites in the 6H polytype (two cubic and one hexagonal). Since the nitrogen replaces carbon equally on each of the carbon sites, one may observe three series of lines, each comprised of a zero-phonon line (ZPL) and its phonon replicas, totalling three ZPLs and 108 phonon replicas, disregarding phonon degeneracy. 

Each pair (A and B) is considered as a no-phonon (NP) transition and its TO phonon replica. The pairs are supposed to originate from two subsets of dots. 

Figure 1. PL of the as-grown (a) and passivated (b) samples and conesponding fittings with Gaussian components, (c) PL of the samples prior to and after irradiation with the maximum and the minimum doses. The indices TO and LO label corresponding phonon replica. FE are free excitons, BE are excitons bound to shallow impurities in the Si substrate. WL is the wetting layer luminescence.

Fig. 3 compares the low temperature (20 K) PL spectra obtained for capped Ge dots on patterned and non-patterned areas. Both spectra show an equally strong PL line at 1.1 eV attributed to the TO-phonon replica of the substrate. The spectrum taken at the non-patterned area of the sample shows the typical wetting layer luminescence located around 1 eV and a rather broad PL line at 800 meV... 

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