Abstract
Phonon spectra of ultrathin (GaAs${)}_{\mathrm{n}}$(AlAs${)}_{\mathrm{n}}$ (001) superlattices are studied theoretically using linear-response density-functional techniques. Results are presented for n=1,2,3 superlattices, along with prototype supercell calculations aimed at simulating a completely disordered (alloy) as well as some partially disordered superlattices. Besides interfacial disorder, which modifies the effective confinement length of low-order longitudinal-optic phonons, we find that---in the ultrathin regime---some degree of cationic mixing must also affect inner planes in order to explain experimental findings.
Highlights
Phonon spectra of ultrathin (GaAs), (A1As)„(001) superlattices are studied theoretically using linear-response density-functional techniques
In ultrathin SL's, the interpretation of Raman measurements, in the longitudinal-optical (LO) region, is still controversial. Both in the GaA1As2 chalcopyrite structure. The (GaAs)- and A1As-like energy ranges, the frequencies of the highest LO modes coLO, almost coincide with the LO modes in the corresponding alloy for the monolayer SL, whereas they smoothly tend to the bulk limit toLo(I ) for increasing thickness of the slab
In GaAs, on the other hand, the bulk dispersion is more isotropic' and much larger than the LO-TO splitting: For SL GaAs-like modes it is dificult to distinguish the effects of disorder from those of confinement, because they both lead to a frequency shift within the bulk-allowed range. %'e conclude that the A1As-like eLp, frequency would be most suited for characterization purposes, as it is almost unaffected by confinement effects, and its departure from the value of BARONI, GIANNOZZI, AND MOLINARI
Summary
Phonon spectra of ultrathin (GaAs), (A1As)„(001) superlattices are studied theoretically using linear-response density-functional techniques. The agreement between our calculations and experiments both for the individual bulks and for the alloy makes us confident that the present first-principles results are accurate and reliable enough to allow predictive statements on the SL spectra.
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