Overcoming Longstanding Synthesis Challenges Toward Realizing the Full Device Potential of III-Nitride Semiconductors

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Matthews, Chris
Doolittle, William Alan
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III-nitrides have the potential to address a large number of fundamental needs for semiconductor device applications, including full-spectrum/tandem-with-silicon solar cells (InGaN), RGB LEDs (InGaN), UV LEDs and lasers (AlGaN), and high-power diodes and transistors (AlGaN). However, many of these applications remain unrealized due to challenges in growing high-quality material by traditional growth techniques like metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Through control of growth kinetics, metal-modulated epitaxy (MME) has been shown to have success in growing III-nitrides, especially highly doped films and ternary alloys with compositions in the miscibility gaps unreachable by other techniques. This control of growth kinetics is expected to lead to the realization of devices that have driven interest in this material system but have thus far been unachievable. This dissertation focuses on progressing the understanding of material synthesis and properties at the extreme ends of the III-nitride material range, with a particular emphasis on InGaN, AlGaN, and AlN. The history of InGaN synthesis as it relates to phase separation is reviewed, and a revised definition of phase separation is proposed. The prior assumption of spinodal decomposition in III-nitrides is re-examined and found to be unlikely due to the density and packing of these materials. Phase separation is reconsidered as a function of surface processes, especially for epitaxy by physical vapor deposition methods such as MBE and MME. A set of surface processes (thermal decomposition, lateral cation separation, vertical cation segregation, and preferential incorporation) are proposed to contribute to phase separation in InGaN, AlInN, and even AlGaN. This revised definition of phase separation in III-nitrides is discussed as needing further examination experimentally, theoretically, or both. The range of growth conditions for MME is much larger than for MOCVD or MBE, and the throughput of all of these techniques is low, so it is necessary to develop a model that can quantitatively describe the growth kinetics under any growth condition in order to simplify the task of quantifying the revised definition of phase separation and eventually realizing devices of interest. Such a model is described, implemented, and evaluated against experimental data from phase-separated AlGaN. This model is simplified to only include vertical cation segregation and preferential incorporation due to the high thermal stability of AlGaN and high growth rates used in MME. Both mechanisms are found to be important in modeling phase separation in AlGaN that is similar to the experimental data. Previously, a major impediment to AlN semiconductor device progress was achieving high, bulk carrier concentrations through impurity doping. Low-temperature epitaxial methods are investigated and found to play a key role in enabling the doping of AlN and the eventual realization of AlN-based semiconductor devices. Both silicon and beryllium doping of AlN are hindered by temperature-dependent processes during epitaxy, such as lattice expansion, dopant desorption, and generation of compensating impurities. Using metal-modulated epitaxy to grow AlN at low temperatures, p- and n-type AlN films with carrier concentrations of 4.4 × 1018 cm-3 and 6 × 1018 cm-3 and resistivities of 0.045 Ω-cm and 0.02 Ω-cm, respectively, are achieved. Doping and defect states in doped aluminum nitride films are examined via cathodoluminescence (CL) spectroscopy. Energy levels within the band gap are observed and potential associated defects are proposed. Fermi-Dirac statistics are used to identify three effective donor states in Si-doped AlN and a single effective acceptor energy in Be-doped AlN. CL investigation reveals near-band-edge and defect luminescence for both n- and p-type AlN films. AlN is found to be a promising optoelectronic material, but requires significant further study on contaminant and defect mitigation before high-quality devices can be realized.
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