Growth, characterization and modeling of new semiconductors and nanomaterials for electronic and optoelectronic applications
Date
2011
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Publisher
University of Delaware
Abstract
Electronic and optoelectronic materials are continually transforming to meet the ever-changing demands of the industry. Modification of material properties to meet these demands occurs through a number of processes: alloying, doping and the introduction of nanomaterials are all amongst them. The work presented in this thesis focuses on these three methods as ways to enhance material properties, and physical understanding, through the exploration of a new alloyed material system (InGaBiAs) and also the introduction of a thermodynamic model for nanoparticle doping. Bismuth is the largest and heaviest Group V atom with a local energy level that resides in the valence band of most other III-V alloys. As a result, bismuthide materials exhibit many interesting optical and electronic properties. InGaBiAs films were grown on InP under varying growth conditions and then characterized structurally, optically and electronically. This work shows that small amounts of bismuth cause an anomalously large reduction in the primary bandgap of these materials, most likely due to the effects of anti-crossing between host matrix valence bands and the local bismuth states. The large reduction in bandgap of InGaBiAs can push peak wavelengths of these materials into the mid-infrared region. The possibility of InGaBiAs lattice-matched to InP is also shown to exist, which would result in fewer potentially harmful defects in stacked devices. In addition, because most of the band-bending occurs in the valence band of these materials, the high electron mobilities associated with InGaAs can be preserved. This thesis encompasses the optical and electronic properties of InGaBiAs with Bi concentrations up to 3.6%. A bandgap of 0.496 eV is achieved for a Bi concentration of 3.1%, which corresponds to a peak wavelength of 2.5 μm. The combination of small bandgaps, high electron mobilities and the possibility for lattice-matching make InGaBiAs a promising optoelectronic material to add to already well-established InP based systems. The doping of nanoparticles is a relatively new, hotly debated topic. Since bulk semiconductor doping is a well-established technique that is used to modify material properties, doping may be used to further enhance the already unique properties associated with nanomaterials. Successfully incorporating dopants into the core of nanostructures has proven difficult experimentally, with inconsistent results. To understand doping on the nanoscale, impurity incorporation into nanoparticles is generally modeled using one of two methods, kinetics or thermodynamics. The model presented in this work is based on a full thermodynamic treatment, including Gibbs free energy, enthalpy and entropy. For small particles, entropically-driven impurity incorporation is reduced, rendering doping difficult. It is shown that the free energy of surface impurities in small nanoparticles is lower than core impurities, and surface doping therefore occurs preferentially to core doping. A critical size for core doping, below which it is energetically unfavorable, is identified. All cases presented in this thesis show core impurity concentration is reduced as particle size decreases. The predictions of this model are in excellent qualitative and quantitative agreement with various experimental results.