Direct Energy Bandgap Group IV Alloys and Nanostructures
<p>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<sub>x</sub>Ge<sub>1-x</sub> alloy system was studied due to the previous...
| Summary: | <p>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<sub>x</sub>Ge<sub>1-x</sub> alloy system was studied due to the
previous demonstration of an indirect to direct energy bandgap transition for
strain-relieved Sn<sub>x</sub>Ge<sub>1-x</sub> 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.</p>
<p>Coherently strained, epitaxial Sn<sub>x</sub>Ge<sub>1-x</sub> films on Ge(001) substrates were
synthesized with film thickness exceeding 100 nm for the first time. The
demonstration of dislocation-free Sn<sub>x</sub>Ge<sub>1-x</sub> films is a step toward the fabrication
of silicon-based integrated infrared optoelectronic devices. The optical
properties of coherently strained Sn<sub>x</sub>Ge<sub>1-x</sub>/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<sub>x</sub>Ge<sub>1-x</sub>/Ge(001) alloys was measured via Fourier transform infrared
spectroscopy. Coherency strain did not change the Sn<sub>x</sub>Ge<sub>1-x</sub> 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].</p>
<p>In addition to being the only group IV alloy exhibiting a direct energy
bandgap, when grown beyond a critical thickness, Sn<sub>x</sub>Ge<sub>1-x</sub>/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<sub>x</sub>Ge<sub>1-x</sub> rods oriented along [001]. The Sn<sub>x</sub>Ge<sub>1-x</sub> alloy system was
used as a model system to gain insight to the physical mechanisms governing
self-assembly and ordering during molecular beam epitaxy.</p>
<p>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 10<sup>11</sup> cm<sup>-2</sup>. The
achievement of single crystal Sn nanowires fabricated using pressure injection in
porous alumina templates was demonstrated.</p>
<p>The fabrication of α-Sn quantum dots embedded in Ge was achieved by
annealing 1 μm thick Sn<sub>x</sub>Ge<sub>1-x</sub> 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.</p> |
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