Date on Master's Thesis/Doctoral Dissertation

5-2018

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Electrical and Computer Engineering

Degree Program

Electrical Engineering, PhD

Committee Chair

Alphenaar, Bruce

Committee Co-Chair (if applicable)

Sunkara, Mahendra

Committee Member

Sunkara, Mahendra

Committee Member

McNamara, Shamus

Committee Member

Walsh, Kevin

Author's Keywords

GaN schottky diodes; power switching; field termination structure; surface passivation

Abstract

Gallium nitride (GaN) has enormous potential for use in devices operating at high power, frequency and temperature. Its wide band gap, high critical electric field and favourable carrier properties lead to lower switching losses and conduction losses in power electronic devices. However, most GaN rectifiers reported to date exhibit an ON-resistance (Ron) versus breakdown voltage much below theoretical predictions. Heteroepitaxial growth of GaN on substrates such as SiC, Si, and sapphire suffer from a high density of threading dislocations defects due to the mismatch in lattice constants and thermal expansion coefficients. Vertical devices, in which a bulk GaN substrate is used, have much lower defect densities. However, field crowding at the periphery of the rectifying contact remains a problem and results in avalanche break down at much lower voltages than the theoretical maximum. This work will describe the design, simulation and fabrication of a novel wraparound field plate termination structure for high voltage Schottky diodes. Simulations show that the wrap around structure has an improved electric field distribution leading to higher breakdown voltages than conventional diode designs. The fabrication process was first developed using low-cost commercially grown HVPE GaN on sapphire substrates. This is the first work in the field of GaN based devices at the University of Louisville, so all fabrication processes, including ICP/RIE based dry etch, ohmic metal contact deposition and dielectric deposition steps, were developed and optimized. Current-voltage (I-V) measurements were used to extract on-resistance and break down voltage and these results were compared to simulation. Experimentally found breakdown values differed from the theoretical predictions. Device failure analysis based on I-V characterization showed the presence of additional current conduction paths along the SiNx and the defective HVPE films. To prevent these leakage currents a less defective MOCVD film grown on Ammono bulk GaN was used to fabricate the wrap-around diode. Planar GaN diodes, and diodes with standard field plate and our novel wraparound field plate were built and tested. Interestingly, planar diodes showed higher performance compared to standard field plate and wraparound field plate designs, contradicting to simulation results. Also, the diodes with a standard and a wraparound field plate structures showed higher leakage currents in both forward and reverse bias. To trace out the source of leakage currents, device failure analysis based on I-V measurements were carried out after each fabrication step of the diode. In this process, initially planar diodes were tested with a Schottky and ohmic contacts formed on the top and on the back side of the wafer. Then, diodes with mesa are built and tested. The diodes with mesa showed an improvement in breakdown values, with the highest breakdown voltage of 421V and on resistance of 3 mOhms-cm2. Also, the experimentally determined breakdown voltages in mesa diodes were found to match with simulation results. Proving that modification in device geometry results in uniform field distribution at the edges and improving the breakdown. Then, a thick SiNx was deposited on mesa diodes using PECVD. The I-V after dielectric deposition showed almost 3 orders higher currents in both forward and reverse bias currents. A similar increase in leakage currents was observed in earlier diodes made on HVPE films. This indicates that PECVD deposited SiNx is modifying the GaN surface and is resulting in additional currents along the GaN and SiNx interface. To overcome these passive currents, a higher K dielectric material was deposited using ALD prior to SiNx. The new bilayer passivation was successful in preventing the leakage currents and resulted in improved breakdown voltages. However, the breakdown values were still below the theoretical predictions and also lower than the diode with a standalone mesa and no additional dielectric layer. Indicating that improvement produced by the device geometry modification is negated by dielectric deposition. Further, we compared some of our diodes with best breakdown characteristics to the literature. We found that with the given material quality and drift layer thickness, we were able to achieve higher breakdown compared to most of the devices reported in the literature. However, there are few diodes with better on resistance and breakdown values compared to ours. As these diodes used almost 60 to 100 times thicker films compared to ours. We were able to make Schottky diodes with relatively high breakdown voltages. However, to utilize the effect of wraparound field plate to its fullest potential, there is a need to develop an alternative dielectric material and deposition technique in future.

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