Interest in wide bandgap semiconductors such as silicon carbide (SiC), gallium nitride (GaN), gallium oxide (Ga 2 O 3 ) and diamond has increased due to their ability to deliver high power, high switching frequency and low loss electronic devices for power conversion applications. To meet these requirements, semiconductor material defects, introduced during growth and fabrication, must be minimized. Otherwise, theoretical limits of operation cannot be achieved. In this dissertation, the non-ideal current- voltage (IV) behavior of GaN-based Schottky diodes is discussed first. Here, a new model is developed to explain better the temperature dependent performance typically associated with a multi-Gaussian distribution of barrier heights at the metal-semiconductor interface [Section 3.1]. Application of this model gives researches a means of understanding not only the effective barrier distribution at the MS interface but also its voltage dependence. With this information, the consequence that material growth and device fabrication methods have on the electrical characteristics can be better understood. To show its applicability, the new model is applied to Ru/GaN Schottky diodes annealed at increasing temperature under normal laboratory air, revealing that the origin of excess reverse leakage current is attributed to the low-side inhomogeneous barrier distribution tail [Section 3.2]. Secondly, challenges encountered during MOCVD growth of low-doped GaN drift layers for high-voltage operation are discussed with focus given to ongoing research characterizing deep-level defect incorporation by deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) [Section 3.3 and 3.4]. It is shown that simply increasing TMGa so that high growth rates (>4 µm/hr) can be achieved will cause the free carrier concentration and the electron mobilities in grown drift layers to decrease. Upon examination of the deep-level defect concentrations, it is found that this is likely caused by an increase in 4 deep level defects states located at E C - 2.30, 2.70, 2.90 and 3.20 eV. Finally, samples where the ammonia molar flow rate is increased while ensuring growth rate is kept at 2 µm/hr, the concentrations of the deep levels located at 0.62, 2.60, and 2.82 eV below the conduction band can be effectively lowered. This accomplishment marks an exciting new means by which the intrinsic impurity concentration in MOCVD-grown GaN films can be reduced so that >20 kV capable devices could be achieved. / Doctor of Philosophy / We constantly rely on electronics to help assist us in our everyday lives. However, to ensure functionality we require that they minimize the amount of energy lost through heat during operation. One contribution to this inefficiency is incurred during electrical power conversion. Examples of power conversion include converting from the 120 V wall outlet to the 5 V charging voltage used by cellphones or converting the fluctuating voltage from a solar panel (due to varying sun exposure) to the 120 V AC power found in a typical household. Electrical circuits can be simply designed to accomplish these conversions; however, consideration to every component must be given to ensure high efficiency.
A popular example of an electrical power conversion circuit is one that switches the input voltage on and off at high rates and smooths the output with an inductor/capacitor network. A good analogy of this process is trying to create a small stream of water from a fire hydrant which can either be off or on at full power. Here we can use a small cup but turning the fire hydrant on and trying to fill the cup will destroy it. However, if the fire hydrant is pulsed on and off at very short intervals (1 µs), its possible to fill the cup without damaging it or having it overflow. Now, under ideal circumstances if a small hole is poked in the bottom of the cup and the interval of the fire hydrant is timed correctly, a small low power stream of water is created without overflowing the cup and wasting water. In this analogy, a devices capable of switching the stream of water on and off very fast would need to be implemented. In electrical power conversion circuits this device is typically a transistor and diode network created from a semiconducting material. Here, similar to the fire hydrant analogy, a switch would need to be capable of holding off the immense power when in the off position and not impeding the powerful flow when in the on position. The theoretical limit of these two characteristics is dependent on the material properties of the switch where typically used semiconductors include silicon (Si), silicon carbide (SiC), or gallium nitride (GaN).
Currently, GaN is considered to be a superior option over Si or SiC to make the power semiconductor switching device, however research is still required to remove non-ideal behavior that ultimately effects power conversion efficiency. In this work, we first examine the spurious behavior in GaN-based Schottky diodes and effectively create a new model to describe the observed behavior. Next, we fabricated Ru/GaN Schottky diodes annealed at different temperatures and applied the model to explain the room-temperature electrical characteristics. Finally, we grew GaN under different conditions (varying TMGa and ammonia) so that quantum characteristics, which have been shown to affect the overall ability of the device, could be measured.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/102924 |
| Date | 09 October 2019 |
| Creators | Allen, Noah P. |
| Contributors | Electrical Engineering, Guido, Louis J., Ngo, Khai D., Lester, Luke F., De La Ree, Jaime, Lu, Guo Quan |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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