Improving 4H-SiC Schottky Barrier Diode by Field Limiting Ring Edge Termination and Nitric Oxide Post Oxidation Annealing

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공과대학 재료공학부
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서울대학교 대학원
Silicon carbideSchottky barrier diodebreakdown voltageFermi level pinning
학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2014. 8. 김형준.
Silicon carbide (SiC) is one of the most promising wide-band-gap semiconductors for high-power, high-frequency, and high-temperature electronic applications. Schottky barrier diode (SBD), the least complex SiC power device, is of very interest because it is a majority carrier device and consequently has very fast switching performance. However, one of the biggest challenges in realizing the full potential of high power SiC SBDs is how to achieve a high breakdown voltage for a given voltage blocking epilayer. Accordingly, a suitable edge termination scheme and surface preparation for ideal Schottky contact is required.
In this dissertation, Schottky barrier diode (SBD), a fundamental element in power devices, was made from 4H-SiC. The unit process for making Schottky barrier diodes was established. The surface state density of Schottky contacts was confirmed by making SBDs from five different kinds of metals and excellent Schottky contact was observed. Ni-SBDs showed outstanding performance. Therefore Ni was selected as the Schottky metal. According to calculation the increase in specific on resistance would not become a major problem during production of large area SBDs.
In plain SBDs the breakdown voltage, the most important characteristic in SBDs, decreases due to electric field crowding at metal edge. To stop this, edge termination is required. For edge termination Field Limiting Ring (FLR) method was applied. FLR increases breakdown voltage by inhibiting electric field crowding, which is done by increasing the metal edge depletion region through formation of p-ring by ion implant. FLR is affected by the doping concentration, doping depth, ring width, the number of rings and spacing between the rings. The optimum FLR structure was empirically which resulted in 96% of ideal breakdown voltage.
To be used as power device the forward current should be over 1 A. For this large area SBDs applying the optimum FLR structure was made and it showed good Schottky characteristics identical to small area SBDs. Forward current of 1 A was found to flow at 1.5 V. The breakdown voltage however decreased to 1436 V. This is due to Schottky barrier lowering, as the number of defects within the Schottky contact increases with increasing Schottky contact area.
To decrease the number of defects in Schottky contact and form ideal Schottky contact Nitric Oxide Post-Oxidation Annealing (NO POA) was introduced. Sacrificial oxides were formed, the specimen was annealed by NO gas, then the oxides were removed and metal was deposited.
The as-sacrificial oxidized SBD showed non-uniform ideal factor and Schottky barrier height and had large reverse leakage current density. After sacrificial oxidation residual carbon components exist on the SiC surface, which act as Schottky barrier energy level and lower the Schottky barrier. Otherwise, NO POA SBDs showed very uniform ideality factor and Schottky barrier height and had low reverse leakage current density. Formation of SiN and CN after NO POA was observed by XPS and SIMS. Si dangling bonds were removed by formation of SiN bonds. The energy level of CN resides near the valence band, therefore does not affect the Schottky barrier. Surface density was found to decrease by NO POA. Because oxides grow during NO POA, NO annealing without pre-oxidation is better in terms of process simplification.
Edge terminated SBDs were fabricated to observe the effects of NO POA at high voltages. The NO POA SBDs experienced hard breakdown at a nearly ideal breakdown voltage eventhough non-optimum FLR structure. NO POA reduces reverse leakage and increases breakdown voltage at high voltages.
By applying NO POA to large area SBDs ideality factor and Schottky barrier height became uniform and reverse leakage current density decreased, as in small area SBDs. In as-sacrificial oxidized SBDs the reverse leakage current density increased with increasing surface area. In contrast, in NO POA SBDs the reverse leakage current density was in the same range (10-8 Acm-2) as in small area SBDs. Defects affecting Schottky barrier height ware removed by NO POA, forming uniform Schottky contacts. Reverse leakage current density due to defects is also decreased, increasing breakdown voltage. Because large area SBDs are more strongly affected by the presence of defects, NO POA is mandatory to achieve good performance.
Ideal Schottky barrier diodes can be made by applying the optimum FLR structure obtained in this work and NO POA to the manufacturing process of 4H-SiC Schottky barrier diodes.
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College of Engineering/Engineering Practice School (공과대학/대학원)Dept. of Materials Science and Engineering (재료공학부)Theses (Ph.D. / Sc.D._재료공학부)
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