Folded acoustical phonons

The primitive cells of the n H and 3n R SiC poly types contain n formula (Si-C) units, and the unit cell of the polytypes along the c-axis is n times larger than that of the basic 3 C SiC polytype. The BZ of the corresponding polytype is thus reduced in the T — L direction by a factor f/n [70]. One then speaks of folded BZ and some of the folded acoustical phonons with non-zero frequencies at the zone centre are IR- and Raman-active. Their absorptions, with lines as sharp as 0.03 cm-1 at LHeT have been reported for the 6H and 15R SiC polytypes ([77], and references therein). 

Figure 1. Raman spectra of the SisGes superlattice, as grown and laser annealed with the energy densities indicated. The peaks at 499, 410 and 305 cm" belong to the Si-Si, Si-Ge and Ge-Ge modes, respectively. The band at 150 cm" is due to the folded acoustical phonons (FAP). The first three modes also appear in the Raman spectra of a SiGe alloy, though at slightly different frequencies [4]. The FAP band is a fingerprint of the SL. A weak peak at 511cm is observed (Si-Si vibrations in the capping layer).

It has been shown that the binding energy of a Be acceptor can be varied from its bulk value of 28 meV to a maximum of around 55 meV in a narrow GaAs/AlAs quantum well. T he c orresponding s eparation o f t he i mpurities i nternal 1 s a nd 2 p levels is around 21 to 42 meV, which is equivalent to a wavelength of between 60 and 30 mm. Pump probe spectroscopy of the same samples has shown that the lifetime of the upper (2p) level varies from around 350 ps in bulk material to 80 ps in the narrowest quantum well. This variation in lifetime is thought to be due to non-radiative scattering due to zone-folded acoustic phonon modes, which arise from the symmetry of the multiple quantum well potentials. 

Colvard C, Merlin R, Klein MV, Gossard AC (1980) Observation of folded acoustic phonons in a semiconductor superlattice. Phys Rev Lett 45 298-301 Esfaqani K, Chen G, Stokes HT (2011) Heat transport in silieon from first principle calculations. Phys Rev B 84 085204... 

Figure 10 Low-frequency Raman spectra of ZnSo.iSeo.g/ZnSe superlattices with different period lengths. The spectra show the redshift of the peak frequency of the first folded acoustical phonon doublet with increasing period length. (Reproduced from Ref. 149.)...

Colvard C, Gant TA, Klein MV, Merlin R, Fischer R, Morkoc FI, Gossard AC (1985) Folded acoustic and quantized optic phonons in (GaAl)As superlattices. Phys Rev B 31 2080... 

It can be seen from Eqs. (11) and (12), that the acoustical phonon branch is folded into the new minizone and minigaps open at the zone center and boundary for 5 0 (see... 

Figure 9 Scheme of the folding of the acoustical phonon branch into the new artificial Brillouin zone of a superlattice with period length D = irlSa. The dots mark the intersections of the folded phonon branch and the wave vector of the incident light. 

A Bartels, X Dekorsy, H Kurz, K Kohler. Coherent zone-folded longitudinal acoustic phonons in semiconductor superlattices—Excitation and detection. Phys Rev Lett 82 1044-1047, 1999. 

Doublets of folded longitudinal acoustic (LA) phonons due to the superlattice periodicity [143] can also be seen in the Raman spectra of the SLs (indicated by arrows in Fig. 21.2). The positions of the doubled peaks agree well with the first doublet frequencies calculated within the elastic continuum model [144]. The observation of the LA phonon folding suggests that these superlattices possess the requisite structural quality for acoustic Bragg mirrors and cavities to be used for potential coherent phonon generation applications [145-147]. 

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