Properties of III-V digital alloys grown by molecular beam epitaxy

Rockwell, Ann Kathryn
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Avalanche photodiodes (APDs) are widely used in industry due to their internal gain, which arises from impact ionization. Over the past 40+ years, III-V materials have been intensively studied for avalanche photodetectors, driven by applications including optical communications, imaging, quantum information processing, and autonomous vehicle navigation. Below 1.1μm, Si APDs are the current state-of-the-art, while above 4μm, HgCdTe APDs are the best option. However, the difficulties associated with growth and fabrication of these materials have motivated the search for alternatives. Impact ionization is a stochastic process that introduces noise, thereby limiting sensitivity and achievable bandwidths. Intense effort is required to mitigate this noise through the identification of different materials and device structures. The search for new materials has yielded InAs and the Al [subscript x] In [subscript 1-x] As [subscript y] Sb [subscript 1-y] alloy family, both III-V materials, as alternatives to HgCdTe and Si with very low-noise. However, the lack of a consensus on the importance of different fundamental properties for these materials and structures led to a mostly ad hoc exploration of their properties that has yielded limited success in noise mitigation. This dissertation describes an exciting step toward deterministic design of low-noise avalanche photodetector materials by alternating the composition at the monolayer scale. This represents a dramatic departure from previous approaches, which have concentrated on either unconventional compounds/alloys or nanoscale band-engineering. This dissertation will expand upon previous work on AlInAsSb digital alloys and take well-established materials, such as InGaAs and InAlAs, and improve important performance metrics, such as cutoff wavelength and excess noise, by growing them as digital alloys. Several explanations for such properties will also be proposed. Finally, it will be shown that many digital alloys exhibit favorable temperature-stable bandgaps compared with typical III-V semiconductors, offering the possibility of enhancing the temperature response in APDs