Theoretical Studies On Raman Scattering In Wurtzite Low-dimensional Semiconductors

Nitride semiconductors, which are characterized by a wide band-gap and strongatomic bonding, have the potential to outperform GaAs-based counterparts. Nitridesemiconductors have both hexagonal （wurtzite） and cubic （zinc-blende） structures. Sincethe wurtzite crystals have a diferent unit-cell structure and lower symmetry comparedto the zinc-blende counterparts, the phonon dynamics and electron-phonon interactionare substantially diferent from those with cubic symmetry. Due to the anisotropyof wurtzite low-dimensional semiconductors, more distinct phonon modes exist in thewurtzite semiconductors. As is well known, Fr¨ohlich-induced electron-phonon interactionplays an important role in determining the physical properties. Hence, it is urgentto understand various physical properties of these structures. Raman scattering is astandard optical characterization technique for studying various aspects of solids such aslattice properties, electronic properties, and magnetic properties. Raman scattering hasmany advantages when compared with other spectroscopic techniques: it is in principlenon-destructive, contactless, and requires no special sample preparation technique. Inthe case of Raman scattering by phonons, the scattering efciency is higher in covalentcrystals than in ionic crystals, because the valence electrons are less localized andlarger fuctuation of the polarizability can be induced by lattice vibration. From thisviewpoint, nitride semiconductors are suitable for Raman scattering studies, since thechemical bonding is a mixture of covalent and ionic bonding. In the present dissertation,a theoretical study of the electron Raman scattering process has been presented in thewurtzite semiconductor quantum structures. The thesis is divided in to the following5chapters.In the frst chapter, the general characters and applications of wurtzite nitride low-dimensional semiconductor structures are summarized briefy and a simple introductionto the current research of Raman scattering in the wurtzite low-dimensional structures isgiven. The diference between the wurtzite and cubic semiconductors is also presented.In the second chapter, electron Raman scattering （ERS） in wurtzite coupled quantumwells （CQWs） is investigated by efective-mass approximation and second-perturbationapproach, including a strong built-in electric feld （BEF） efect due to the piezoelectricityand spontaneous polarization. The dependence of diferential cross-section （DCS） on structural parameters of CQWs is studied. Our results show that the strong BEF givesrise to a remarkable reduction of the DCS, which is around three orders smaller than thatof the CQWs without BEF. With the presence of the BEF, the emitted photon energydecreases about10times as a consequence of quantum-confned Stark efect. Hence,the BEF is a factor of high importance for the optical properties of the optoelectronicdevices and must be considered seriously in the design of GaN-based devices.In the third chapter, We present a theoretical study on electron resonant Ramanscattering （ERRS） in a free-standing wurtzite nanowire （NW）, including the bulklongitudinal optical （LO）, surface optical （SO） and quasi-confned （QC） phonons. Withinthe framework of the dielectric continuum model and Loudon,uniaxial crystal model,the Fr¨ohlich electron-phonon interaction is considered. Numerical results and discussionsare presented for various radii. The selective rules are also given. The results reveal thatthe positions of four outgoing resonant peaks change with the phonon frequency. Thebulk LO and QC phonons are the main factors contributing to the DCS when the wire isthick, which is around4orders larger than that of the SO phonon. Hence, the efect ofSO phonons can be negligible and the QC phonon is a factor of high importance for theoptical properties and must be considered seriously in Raman scattering process.In the fourth chapter, We report the electron resonant Raman scattering （ERRS）process related to the longitudinal optical （LO）, interface optical （IO） and quasi-confned（QC） phonons in a cylindrical GaN-AlN quantum well wire （QWW）. We present therelation between the diferential cross-section（DCS） and secondary radiation photonenergy, and discuss the selective rules in the scattering process. The Dispersivefrequencies ωpas a function of the radius r is also discussed. The results reveal that theIO, QC, and LO₁phonons give main contribution to the DCS in small radius, which isabout108～109than that of the LO₂. Whereas for large radius, the contribution relatedto the LO₁and QC phonons to the DCS are dominant. Both the IO and the QC opticalphonon modes are obvious only when the radius is small. Moreover, when the radius islarge, the frequencies of the IO and QC phonons converge to two limit values of103.15meV and90.97meV, respectively. The emitted photon energy decreases with increasingthe radius because of the Raman size-selective efect and the quantum confned Starkefect.In the last chapter, summary of the paper and the main results are given, further research are also mentioned.