Quantum Dots: Phonons in Self-Assembled Multiple Germanium Structures
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Self-assembled Ge quantum dots by the Stranski–Krastanov growth mode have attracted much attention for many years. Similar to the purpose of the research on short-period Si/Ge superlattices and Er-doped Si, self-assembled Ge/Si quantum dots may be exploited to fabricate Si-based on-chip light emitting sources for 1.55-µm fiberoptic communication applications. Takagahara and Takeda and Ren have theoretically predicted that an indirect-to-direct conversion of the optical transition of SiGe quantum dots would occur whenever the sizes of the quantum dots were small enough. In order to increase the component of quasi-direct transition of the Ge/Si quantum dot system and thus to enhance light emission intensity of the quantum dots for practical applications, the understanding of nonradiative recombination mechanisms (such as phonon-assisted process) is essential.
To probe phonons from self-assembled semiconductor quantum dots, Raman spectroscopy is an efficient and indispensable experimental tool. Up to date, several groups have already reported Raman scattering studies of self-assembled quantum dots of group III–V systems, such as (In, Ga, Al)Sb/GaAs, InSb/InP, In(Ga)As/GaAs, InAs/InP, (Al, Ga)As/InAs, InAs/AlAs, and GaN/(Si)AlGaN, group II–VI system, such as CdSe/ZnSe, and group IV system, i.e., Ge/Si. Optical phonon spectra of any of these systems were basically used to extract the chemical composition of the quantum dots as a result of interdiffusion between the dots and the surrounding media or the substrates. It should be pointed out that a few other techniques for the determination of composition of self-assembled quantum dots have been reported. These include scanning tunneling microscopy, transmission electron microscopy (TEM) with high-resolution imaging, electron energy loss spectrometry, X-ray energy disperse spectrometry, high-resolution X-ray diffraction, and scanning TEM. Most of these techniques are capable to show nonuniform dot material distribution in the dots. Optical phonon Raman scattering method is simple and direct to give an average concentration of the dots. In contrast to the tremendous research on optical phonons, the effort on the research of phonon process in the low-frequency acoustic spectral region, however, is much smaller. In self-assembled Ge quantum dot system, for example, one early work reported the observation of equal-distance acoustic peaks in 25-period Ge quantum dot superlattices. Afterwards, Milekhin et al. investigated folded longitudinal acoustic phonons in their Ge dot superlattices and explained the acoustic vibrations by the elastic continuum model. Recently, resonant Raman scattering by acoustic phonons in double- and multilayered Ge dot structures was reported and the observed equal-distance oscillation peaks were explained by interference and ordering effects. The origins of these observed low-frequency acoustic phonon spectra in Ge dot superlattices therefore remain unclear and debatable because there is a lack of systematic studies, such as the dependence of acoustic phonons on island sizes and other island-related parameters.
In this entry, we systematically study Raman scattering by optical and acoustic phonons in multiple Ge quantum dots. The analysis of GeGe and SiGe optical phonon features takes the phonon confinement effect, strain effect, and atomic intermixing into account. Acoustic phonons are found to originate from folded acoustic phonons associated with a superlattice and can be explained by elastic continuum model.