Optical properties of strain-engineered multilayer Si/SiGe nanostructures

OPTICAL PROPERTIES OF STRAIN-ENGINEERED MULTILAYER Si/SiGe NANOSTRUCTURES by Selina Akter Mala The long carrier radiative lifetimes in indirect band gap semiconductors such as crystalline Si (c-Si) and Ge impede the development of efficient light-emitting devices and lasers. Multilayer Si/SiGe nanostructures are considered to be the strong candidates for efficient and high-speed optoelectronic devices integrated into CMOS platforms. Since c-Si and Ge have a considerable lattice mismatch of ~ 4.2%, Si/Si1-xGex (x < 0.5) nanostructures in the form of nano-layers (NLs) or cluster multilayers (CMs) modify the band structure and create non-uniform strain distribution. Engineering of Si/Si1-xGex nanostructures with the predicted composition and interface abruptness, which controls spatial separation between electrons and holes and carrier radiative recombination rate, is critical in producing the desired fast and efficient photoluminescence (PL) peaked around 0.8-0.9 eV. This study investigates the structural, optical, and thermal properties of Si/Si1xGex nanostructures with different layer thicknesses, Ge compositions, and SiGe heterointerface abruptness. A comprehensive experimental and theoretical analysis of Raman scattering in various Si/Si1-xGex multilayered nanostructures with well-defined Ge composition (x) and layer thicknesses is presented. Using Raman and transmission electron microscopy data, Si/SiGe intermixing and strain are discussed and modeled. The studied samples exhibit significant dependence of the Raman scattering intensity on the excitation light penetration depth. Local temperature and thermal conductivity are calculated by analyzing the measured Stokes and anti-Stokes Raman spectra, and the developed model of heat dissipation in the samples under an intense laser illumination is in a good agreement with the experiment. A correlation is found between the SiGe/Si volume fraction ratio and thermal conductivity, which is explained and suggestions are made of applications of the developed model in the field of thermoelectric, electronic, and optoelectronic devices. In this thesis, PL measurements are focused on specifically designed Si/Si1-xGex nanostructures with a single 3-5 nm thick Si1-xGex layer with x ≈ 8% incorporated into Si/Si0.6Ge0. 4 CMs. Under pulsed laser excitation, the PL decay associated with the Si0.92Ge0. 08 NL is found to be nearly a 1000 times faster compared to that in Si/Si0.6Ge0. 4 CMs, and the SiGe NL PL intensity does not saturate as a function of excitation energy density up to 50 mJ/cm 2 . These dramatic differences in the observed PL properties are attributed to the difference in the structures of the Si/SiGe NL and CM heterointerfaces. A model considering Si/SiGe heterointerface composition and explaining the fast and slow time-dependent recombination rates is proposed and found to be in excellent agreement with the experimental data. OPTICAL PROPERTIES OF STRAIN-ENGINEERED MULTILAYER Si/SiGe NANOSTRUCTURES by Selina Akter Mala A Dissertation Submitted to the Faculty of New Jersey Institute of Technology In Partial Fulfillment of the requirements for the degree of Doctor of Philosophy in Electrical Engineering Helen and John C. Hartmann Department of Electrical and Computer Engineering