Dilute bismuthides on InP platform: growth, characterization, modeling and application

Zhong, Yujun
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University of Delaware
Conventional III-V compounds (GaAs/ InGaAs/ InAlAs) containing a small amount of bismuth are called dilute bismuthides (a.k.a. dilute bismides). They are a relatively new class of materials and have interesting optical and electrical properties that lead to a large number of novel applications in mid-infrared(mid-IR) optoelectronics, IR transparent contact materials, photovoltaics and thermoelectrics. This dissertation focuses on the growth and characterization of dilute bismuthides with potential use in the first three applications. Incorporating Bi into conventional III-V compounds will cause a unique phenomenon called valence band anticrossing(VBAC). The interaction between the bismuth atom and the matrix material will make the valence band split into two bands: E+ and E-; E+ is closer to the conduction band than the original valence band of the matrix material. Using this effect, we can adjust the band gap and the valence band position of dilute bismuthides by controlling the bismuth concentration. The growth of bismuth-containing materials using molecular beam epitaxy (MBE) requires low growth temperature and strict stoichiometric III-V ratio. This dissertation will discuss in detail the optimum growth condition of InGaBiAs, the challenge of increasing the bismuth concentration, and the possible solution to produce high bismuth concentration samples. Accordingly, composition, strain and relaxation, surface morphology, optical properties and electrical properties of InGaBiAs thin films are characterized to study these materials. The first application of InGaBiAs is mid-IR optoelectronic materials. The band gap of InGaBiAs can be tuned within the mid-IR range, and the film can be produced being lattice-matched to the InP substrate. In addition, degenerately doped InGaBiAs:Si is an ideal choice for the transparent contact material in the infrared range due to its high transmittance and conductivity in this wavelength range. We next proposed a new upconversion solar cell design with the incorporation of dilute bismuthides, which is expected to enable very high solar cell efficiency. Finally, this dissertation discussed some future directions in this field: high bismuth concentration films, a measurement to fully understand the band structure of InGaBiAs and a proposal of temperature-insensitive application. As a conclusion, dilute bismuthides remain promising as optoelectronic materials.