LO-phonon replicas

DAP and Shallow Acceptor-Bound Exciton Transitions and LO-Phonon Replicas in PL... 

The spectral region containing the donor-acceptor pair transition and LO-phonon replicas of the main transitions has not been studied widely for single crystal ZnO. 

The relatively broad peak around 3.280 eV in Figure 3.12 is the first LO-phonon replica associated with the bound exciton line at 3.3564eV. Finally, the first-, the second-, and the third-order LO-phonon replicas of the TES lines are also clearly observed in the PL spectra. These peaks are labeled as ILO, 2LO, and 3LO (TES) and they are positioned at 3.252, 3.182, and 3.112 eV, respectively. 

In ZnO doped with As, which is another potential acceptor in ZnO, a line at about 3.23-3.24 eV, followed by two LO-phonon replicas has been observed in the PL spectrum with an intensity exceeding the exciton emission intensity [83]. This emission has been attributed to the shallow As acceptor, the activation energy of which was estimated at 180 lOmeV from the temperature dependence of the PL spectrum. Concurrently with the DAP emission line, a sharp line at 3.325 eV appeared in these samples, tentatively attributed to the As acceptor-bound exd-ton [83]. In yet another study [84], the 3.322 and 3.273 eV lines greatly increased with... 

Figure 3.14 Temperature-dependent PL spectrum for a forming gas-annealed ZnO substrate. The inset shows the PL in the DAP and LO-phonon replicas region up to 160 K. The spectrum for each temperature is displaced vertically for clarity. The room-temperature PL data are also included at the bottom. The lines drawn on some peaks are for guidance. (After Ref [50].)...

LO-phonon replica coupled with the line broadening of each of these peaks, hampers an accurate determination of the peak positions above 160 K. Therefore, the room-temperature peak should be considered a combination of these multiple... 

Figure 3.15 Temperature-dependent peak positions of the A-free exciton, FX (r5) and its ILO- and 2LO-phonon replicas. Also shown are the temperature evolutions ofthe peak positions oftwo major neutral donor-bound exciton transitions at 3.3606 and 3.3650eV. The FX (r5) data were fit using Varshni s equation and LO-phonon replicas were fit to the equation shown in the figure. (After Ref [50].)...

Figure4.4 (a) Low-temperature PL spectra of (1) ZnO reference layer and N-implanted samples with (2) 1 X 10 cm" and (3) 5 x 10 cm N concentrations after annealing at 800°C. DAP transition and its phonon replicas are visible for all samples. The emission at 3.286eV is the LO phonon replica of the donor-bound exciton.

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...

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.

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... 

Bulk silicon is a semiconductor with an indirect band structure, as schematically shown in Fig. 7.12 c. The top of the VB is located at the center of the Brillouin zone, while the CB has six minima at the equivalent (100) directions. The only allowed optical transition is a vertical transition of a photon with a subsequent electron-phonon scattering process which is needed to conserve the crystal momentum, as indicated by arrows in Fig. 7.12 c. The relevant phonon modes include transverse optical phonons (TO 56 meV), longitudinal optical phonons (LO 53.5 meV) and transverse acoustic phonons (TA 18.7 meV). At very low temperature a splitting (2.5 meV) of the main free exciton line in TO and LO replicas can be observed [Kol5]. 

Typical luminescence spectra of as-grown crystals show ultra-violet emission due to free excitons (including their longitudinal optical phonons-EO replicas) as well as t5 ical bands of green and yellow-orange luminescence (the temperature is 77-80 K). When the temperature is 4.2 K, the luminescence of bound excitons (including their LO - replicas) dominates. ... 

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