Direct Energy Bandgap Group IV Alloys and Nanostructures

Bandstructure engineering;growth instabilities;monolithic integration;nanostructures;nanowire arrays;phase separation;quantum dots;self assembly

Ragan, Regina ; Atwater, Harry Albert

University:California Institute of Technology

Department:Engineering and Applied Science

关键词: Bandstructure engineering;    growth instabilities;    monolithic integration;    nanostructures;    nanowire arrays;    phase separation;    quantum dots;    self assembly;   

Others  :  https://thesis.library.caltech.edu/632/4/ragan-r-2002.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

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

Coherently strained, epitaxial Sn_xGe_1-x films on Ge(001) substrates weresynthesized with film thickness exceeding 100 nm for the first time. Thedemonstration of dislocation-free Sn_xGe_1-x films is a step toward the fabricationof silicon-based integrated infrared optoelectronic devices. The opticalproperties of coherently strained Sn_xGe_1-x/Ge(001) alloys were investigated boththeoretically and experimentally. Deformation potential theory calculationswere performed to predict the effect of coherency strain on the extrema points ofthe conduction band and the valence band. The energy bandgap ofSn_xGe_1-x/Ge(001) alloys was measured via Fourier transform infraredspectroscopy. Coherency strain did not change the Sn_xGe_1-x energy bandgapwhen the strain axis was along [001] but deformation potential theory predicted the absence of an indirect to direct energy bandgap transition when the strainaxis was along [111].

In addition to being the only group IV alloy exhibiting a direct energybandgap, when grown beyond a critical thickness, Sn_xGe_1-x/Ge(001) exhibits aninteresting phenomenon during MBE growth. Sn segregates via surfacediffusion to the crest of a surface undulation during growth and forms orderedSn-enriched Sn_xGe_1-x rods oriented along [001]. The Sn_xGe_1-x alloy system wasused as a model system to gain insight to the physical mechanisms governingself-assembly and ordering during molecular beam epitaxy.

Sn nanowires were fabricated in anodic alumina templates with lengthsexceeding 1 μm and diameters on the order of 40 nm. Anodic alumina templatescan be fabricated non-lithographically with ordered domains of hexagonallypacked pores greater than 1 μm and pore densities on the order of 10¹¹ cm^-2. Theachievement of single crystal Sn nanowires fabricated using pressure injection inporous alumina templates was demonstrated.

The fabrication of α-Sn quantum dots embedded in Ge was achieved byannealing 1 μm thick Sn_xGe_1-x films at 750°C. The measured diameter of thequantum dots was 32 nm and a 10% size variation was observed. Quantum sizeeffects were observed in α-Sn quantum dots. Optical transmittancemeasurements yield a value of 0.45 eV for the direct energy bandgap as a resultof quantum confinement. A high degree of tunability of the bandgap energywith the quantum dot radius is expected for α-Sn. Thus quantum-confinedstructures of α-Sn are promising for optoelectronic device applications.