Direct Energy Bandgap Group IV Alloys and Nanostructures

Citation

Ragan, Regina (2002) Direct Energy Bandgap Group IV Alloys and Nanostructures. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1WKJ-RZ66. https://resolver.caltech.edu/CaltechETD:etd-02142002-211940

Abstract

Novel group IV nanostructures were fabricated and the optical properties of such nanostructures were investigated for monolithic integration of optically active materials with silicon. The Sn_xGe_1-x alloy system was studied due to the previous demonstration of an indirect to direct energy bandgap transition for strain-relieved Sn_xGe_1-x films on Si(001). In addition, quantum confined structures of Sn were fabricated and the optical properties were investigated. Due to the small electron effective mass of α-Sn, quantum confinement effects are expected at relatively large radii.

Coherently strained, epitaxial Sn_xGe_1-x films on Ge(001) substrates were synthesized with film thickness exceeding 100 nm for the first time. The demonstration of dislocation-free Sn_xGe_1-x films is a step toward the fabrication of silicon-based integrated infrared optoelectronic devices. The optical properties of coherently strained Sn_xGe_1-x/Ge(001) alloys were investigated both theoretically and experimentally. Deformation potential theory calculations were performed to predict the effect of coherency strain on the extrema points of the conduction band and the valence band. The energy bandgap of Sn_xGe_1-x/Ge(001) alloys was measured via Fourier transform infrared spectroscopy. Coherency strain did not change the Sn_xGe_1-x energy bandgap when the strain axis was along [001] but deformation potential theory predicted the absence of an indirect to direct energy bandgap transition when the strain axis was along [111].

In addition to being the only group IV alloy exhibiting a direct energy bandgap, when grown beyond a critical thickness, Sn_xGe_1-x/Ge(001) exhibits an interesting phenomenon during MBE growth. Sn segregates via surface diffusion to the crest of a surface undulation during growth and forms ordered Sn-enriched Sn_xGe_1-x rods oriented along [001]. The Sn_xGe_1-x alloy system was used as a model system to gain insight to the physical mechanisms governing self-assembly and ordering during molecular beam epitaxy.

Sn nanowires were fabricated in anodic alumina templates with lengths exceeding 1 μm and diameters on the order of 40 nm. Anodic alumina templates can be fabricated non-lithographically with ordered domains of hexagonally packed pores greater than 1 μm and pore densities on the order of 10¹¹ cm^-2. The achievement of single crystal Sn nanowires fabricated using pressure injection in porous alumina templates was demonstrated.

The fabrication of α-Sn quantum dots embedded in Ge was achieved by annealing 1 μm thick Sn_xGe_1-x films at 750°C. The measured diameter of the quantum dots was 32 nm and a 10% size variation was observed. Quantum size effects were observed in α-Sn quantum dots. Optical transmittance measurements yield a value of 0.45 eV for the direct energy bandgap as a result of quantum confinement. A high degree of tunability of the bandgap energy with the quantum dot radius is expected for α-Sn. Thus quantum-confined structures of α-Sn are promising for optoelectronic device applications.

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