Wide Bandgap Semiconductors Based Energy-Efficient Optoelectronics and Power Electronics

abstract: Wide bandgap (WBG) semiconductors GaN (3.4 eV), Ga2O3 (4.8 eV) and AlN (6.2 eV), have gained considerable interests for energy-efficient optoelectronic and electronic applications in solid-state lighting, photovoltaics, power conversion, and so on. They can offer unique device performance compared with traditional semiconductors such as Si. Efficient GaN based light-emitting diodes (LEDs) have increasingly displaced incandescent and fluorescent bulbs as the new major light sources for lighting and display. In addition, due to their large bandgap and high critical electrical field, WBG semiconductors are also ideal candidates for efficient power conversion. In this dissertation, two types of devices are demonstrated: optoelectronic and electronic devices. Commercial polar c-plane LEDs suffer from reduced efficiency with increasing current densities, knowns as “efficiency droop”, while nonpolar/semipolar LEDs exhibit a very low efficiency droop. A modified ABC model with weak phase space filling effects is proposed to explain the low droop performance, providing insights for designing droop-free LEDs. The other emerging optoelectronics is nonpolar/semipolar III-nitride intersubband transition (ISBT) based photodetectors in terahertz and far infrared regime due to the large optical phonon energy and band offset, and the potential of room-temperature operation. ISBT properties are systematically studied for devices with different structures parameters. In terms of electronic devices, vertical GaN p-n diodes and Schottky barrier diodes (SBDs) with high breakdown voltages are homoepitaxially grown on GaN bulk substrates with much reduced defect densities and improved device performance. The advantages of the vertical structure over the lateral structure are multifold: smaller chip area, larger current, less sensitivity to surface states, better scalability, and smaller current dispersion. Three methods are proposed to boost the device performances: thick buffer layer design, hydrogen-plasma based edge termination technique, and multiple drift layer design. In addition, newly emerged Ga2O3 and AlN power electronics may outperform GaN devices. Because of the highly anisotropic crystal structure of Ga2O3, anisotropic electrical properties have been observed in Ga2O3 electronics. The first 1-kV-class AlN SBDs are demonstrated on cost-effective sapphire substrates. Several future topics are also proposed including selective-area doping in GaN power devices, vertical AlN power devices, and (Al,Ga,In)2O3 materials and devices.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Thick GaN film stress-induced self-separation

Cracking of thick GaN films on sapphire substrates during the cooling down after the growth was studied. The cracking was suppressed by increasing the film-to-substrate thickness ratio and by using an intermediate carbon buffer layer, that reduced the binding energy between the GaN film and the substrate. Wafer-scale self-separation of thick GaN films has been demonstrated.Comment: Published in Proceedings of the 2019 IEEE Conference of Russian Young Researchers in Electrical and Electronic Engineering (EIConRus

Fabrication technology for high light-extraction ultraviolet thin-film flip-chip (UV TFFC) LEDs grown on SiC

The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to their poor light extraction efficiency (LEE). To improve the LEE of AlGaN LEDs, we developed a fabrication technology to process AlGaN LEDs grown on SiC into thin-film flip-chip LEDs (TFFC LEDs) with high LEE. This process transfers the AlGaN LED epi onto a new substrate by wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a highly selective SF6 plasma etch that stops at the AlN buffer layer. We optimized the inductively coupled plasma (ICP) SF6 etch parameters to develop a substrate-removal process with high reliability and precise epitaxial control, without creating micromasking defects or degrading the health of the plasma etching system. The SiC etch rate by SF6 plasma was ~46 \mu m/hr at a high RF bias (400 W), and ~7 \mu m/hr at a low RF bias (49 W) with very high etch selectivity between SiC and AlN. The high SF6 etch selectivity between SiC and AlN was essential for removing the SiC substrate and exposing a pristine, smooth AlN surface. We demonstrated the epi-transfer process by fabricating high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further enhance the light extraction, the exposed N-face AlN was anisotropically etched in dilute KOH. The LEE of the AlGaN LED improved by ~3X after KOH roughening at room temperature. This AlGaN TFFC LED process establishes a viable path to high external quantum efficiency (EQE) and power conversion efficiency (PCE) UV-C LEDs.Comment: 22 pages, 6 figures. (accepted in Semiconductor Science and Technology, SST-105156.R1 2018

Origin and Control of OFF-State Leakage Current in GaN-on-Si Vertical Diodes

Conventional GaN vertical devices, though promising for high-power applications, need expensive GaN substrates. Recently, low-cost GaN-on-Si vertical diodes have been demonstrated for the first time. This paper presents a systematic study to understand and control the OFF-state leakage current in the GaN-on-Si vertical diodes. Various leakage sources were investigated and separated, including leakage through the bulk drift region, passivation layer, etch sidewall, and transition layers. To suppress the leakage along the etch sidewall, an advanced edge termination technology has been developed by combining plasma treatment, tetramethylammonium hydroxide wet etching, and ion implantation. With this advanced edge termination technology, an OFF-state leakage current similar to Si, SiC, and GaN lateral devices has been achieved in the GaN-on-Si vertical diodes with over 300 V breakdown voltage and 2.9-MV/cm peak electric field. The origin of the remaining OFF-state leakage current can be explained by a combination of electron tunneling at the p-GaN/drift-layer interface and carrier hopping between dislocation traps. The low leakage current achieved in these devices demonstrates the great potential of the GaN-on-Si vertical device as a new low-cost candidate for high-performance power electronics

Vertical gallium nitride schottky diodes for power switching applications.

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

Barrier Inhomogeneity of Schottky Diode on Nonpolar AlN Grown by Physical Vapor Transport

An aluminum nitride (AlN) Schottky barrier diode (SBD) was fabricated on a nonpolar AlN crystal grown on tungsten substrate by physical vapor transport. The Ni/Au-AlN SBD features a low ideality factor n of 3.3 and an effective Schottky barrier height (SBH) of 1.05 eV at room temperature. The ideality factor n decreases and the effective SBH increases at high temperatures. The temperature dependences of n and SBH were explained using an inhomogeneous model. A mean SBH of 2.105 eV was obtained for the Ni-AlN Schottky junction from the inhomogeneity analysis of the current-voltage characteristics. An equation in which the parameters have explicit physical meanings in thermionic emission theory is proposed to describe the current-voltage characteristics of inhomogeneous SBDs.Comment: 6 pages, 6 figure