Optical properties of strain-engineered multilayer Si/SiGe nanostructures

Author

Selina Akter Mala, New Jersey Institute of Technology

Document Type

Dissertation

Date of Award

Fall 1-31-2015

Degree Name

Doctor of Philosophy in Electrical Engineering - (Ph.D.)

Department

Electrical and Computer Engineering

First Advisor

Leonid Tsybeskov

Second Advisor

Haim Grebel

Third Advisor

Durgamadhab Misra

Fourth Advisor

Marek Sosnowski

Fifth Advisor

Andrei Sirenko

Abstract

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/Si_1-xGe_x(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/Si_1-xGe_xnanostructures 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/Si_{1- x}Ge_xnanostructures with different layer thicknesses, Ge compositions, and SiGe heterointerface abruptness.

A comprehensive experimental and theoretical analysis of Raman scattering in various Si/Si_1-xGe_xmultilayered 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/Si_1-xGe_xnanostructures with a single 3-5 nm thick Si_1-xGe_x layer with x ≈ 8% incorporated into Si/Si_0.6Ge_0.4 CMs. Under pulsed laser excitation, the PL decay associated with the Si_0.92Ge_0.08 N L is found to be nearly a 1000 times faster compared to that in Si/Si_0.6Ge_0.4 CMs, and the SiGe NL PL intensity does not saturate as a function of excitation energy density up to 50 mJ/cm² . These dramatic differences in the observed PL properties are attributed to the difference in the structures of the Si/SiGe NL and CM heteroi nterfaces. 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.

Recommended Citation

Mala, Selina Akter, "Optical properties of strain-engineered multilayer Si/SiGe nanostructures" (2015). Dissertations. 103.
https://digitalcommons.njit.edu/dissertations/103