Synthesis and properties of light-emitting Si-based nanostructures

Citation

Min, Kyu Sung (2000) Synthesis and properties of light-emitting Si-based nanostructures. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/tjep-v350. https://resolver.caltech.edu/CaltechTHESIS:10122010-100418753

Abstract

The concept of silicon-based optoelectronics has attracted much scientific and technological interests over the past decade. The vision of Si-based optoelectronics is based on integration of Si-based photonic components, in which light can be generated, waveguided, modulated, amplified, and detected, with the advanced Si electronics onto the same Si substrate to make monolithically integrated Si-based optoelectronic circuits. The main driving force for development of Si-based optical components comes from unsurpassed qualities of Si as the substrate material on which the electronic components rest: superior native oxide as well as excellent thermal, mechanical, and economic properties. Despite superior substrate properties, the field still remains a frontier at large. The main technological limitation comes from the lack of materials for efficient Si-based light sources such as Si-based lasers and light-emitting devices. Two novel Si-based nanostructures are studied for potential application as visible and infrared light sources: ion-beam synthesized Ge and Si nanocrystals in SiO_2 and coherently strained quantum well and quantum dots based on the Si-Sn system grown by molecular beam epitaxy. The study of Ge and Si nanocrystals is motivated by the prediction that quantum confinement of carriers leads to efficient luminescence despite the indirect nature of the energy gaps. Ge and Si nanocrystals in thermal SiO_2 films are synthesized via precipitation from a supersaturated solid solution of Ge and Si in SiO_2 made by Ge^+ and Si^+ ion implantation. The precipitation of nanocrystals occurs upon thermal annealing in vacuum. It is demonstrated that the SiO_2films containing Ge nanocrystals only exhibit defect-related luminescence and that the Ge nanocrystals do not exhibit luminescence from quantum-confined excitons due to the poor nanocrystal/SiO_2 interface. The visible luminescence from SiO_2 films containing Si nanocrystals, on the other hand, is unambiguously demonstrated to be originating from quantum-confined excitons in Si nanocrystals, based on systematic photoluminescence and photoluminescence decay rate measurements. In agreement with the predictions of the theory of quantum confinement, the peak energy of visible photoluminescence from Si nanocrystals can be continuously tuned throughout most of the visible spectrum by controlling the size distribution of the nanocrystals. The growth of nanostructures based on the Si-Sn system by molecular beam epitaxy is motivated by the fact that diamond cubic α-Sn is a zero band gap semiconductor and that band structure calculations predict a direct and tunable energy gap for Sn-rich Sn_(x)Si_(1-x) alloy system. However, the large lattice mismatch (19%) and severe segregation of Sn to the surface during growth prevent growth of Sn-rich Sn_(x)Si_(1-x) films by ordinary thermal molecular beam epitaxy. The growth of pseudomorphic Sn/Si and Sn_(x)Si_(1-x)/Si heterostructures is demonstrated via a modified molecular beam epitaxy technique employing temperature and growth rate modulations. The growth of pseudomorphic single quantum well structures as well as superlatttice structures is demonstrated. In addition, a novel route for synthesis of coherent Sn-rich Sn_(x)Si_(1-x) quantum dots in Si matrix is presented. Due to chemical instability of the Si-Sn mixture, Stranski-Krastonow growth of coherently strained Sn-rich Sn_(x)Si_(1-x) quantum dot structures using conventional molecular beam epitaxy techniques is very difficult. The novel technique involves phase separation of Sn-rich Sn_(x)Si_(1-x) quantum dots at elevated temperatures from an epitaxially stabilized homogeneous Sn_(x)Si_(1-x)/Si metastable solid solution grown by low temperature molecular beam epitaxy. The dots have been verified to be completely coherent with the surrounding Si matrix by high-resolution transmission electron microscopy.

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