S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Materials Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
Improving the interface characteristic of SiO2/4H-SiC using nitridation and reduced oxidation process
- 공과대학 재료공학부
- Issue Date
- 서울대학교 대학원
- 4H-SiC; atomic layer deposition; power device MOSFET; Interface trap density (Dit); nitridation; post-oxidation annealing
- 학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2015. 2. 김형준.
- Silicon carbide (SiC) is a promising material due to its superior electrical, chemical and thermal properties and due to the technology for producing high quality of bulk substrates and epitaxial films. The SiC is a wide band gap material with a high breakdown voltage, high thermal conductivity, and high saturation drift velocity. Because of these characteristics, it can be used in severe environments involving high power, high frequency, and high temperature. However, 4H-SiC metal-oxide-semiconductor (MOS) field effect transistors (FET) have drawbacks such as high interface trap density (Dit) and low channel mobility owing to the residual carbon, carbon clusters, and dangling bonds both in oxide layer and at the interface. The fundamental understanding and modification of this oxide/SiC interface problem is the subject of this dissertation.
Nitridation using nitric oxide (NO) post oxidation annealing (POA) was employed to overcome the interface problem. Thermally grown oxide with NO POA can reduce both Dit and near interface trap density (Nit). The optimization of NO POA was conducted by controlling the oxidation method, duration and pressure of NO POA condition. The optimized NO POA was found to significantly improve Dit, Nit, and effective oxide charge (Qeff) of thermally grown oxides.
However, thermally grown oxide with NO POA has a limit of inevitable carbon related defects. Therefore, deposited oxide was adopted to minimize carbon incorporation. Among the deposition methods, atomic-layer-deposition (ALD) has low growth temperature which enables high mobility in a channel in the high field region owing to the low degree of surface roughness. In contrast, the thermal oxidation at high temperature induces the formation of carbon clusters and/or silicon oxycarbides and also increases the surface roughness of SiC. Therefore, the electrical properties of MOS capacitors (MOSCAP) with ALD oxides with NO POA were characterized and compared with those of MOSCAPs based on thermally grown oxides. The NO POA treated ALD oxides showed extremely low Dit, less than 1011 eV-1cm-1. A MOSFET with ALD oxide showed high field effect mobility, especially in the high electric field region.
The ALD oxide with NO POA lowered carbon component effectively by reducing oxidation time, however it had a drawback of re-oxidation by oxygen source of NO gas. The oxygen source increased oxide layer and lowered nitrogen amount during NO POA, so this process generated carbon component by re-oxidation. Therefore, ALD oxide with ammonia (NH3) POA was conducted to nitridation without re-oxidation. Because the NH3 POA oxide had a drawback of low breakdown filed (Eb) attributed to the uniform distribution of nitrogen throughout the oxide layer during NH3 POA, a stacked structure of ALD SiO2 with NH3 POA and another deposited oxides was adopted to exclude oxidation and to improve Eb, respectively. Inert gas annealing (IGA) or ozone (O3) treatment were used to reduce the defects of upper as-deposited ALD SiO2. The IGA was not effective for curing upper oxide because the penetration of nitrogen into the defective upper ALD oxide from lower NH3 treated ALD oxide. The oxygen vacancies, a major defect of as-deposited oxide, were effectively reduced by the O3 treatment. The ALD SiO2 with NH3 POA reduces the Dit which caused by the suppression of re-oxidation. The O3 treatment for upper oxide increased Eb effectively, but it did not reached Eb of thermally grown oxide yet. The high density plasma chemical vapor deposition (HDPCVD) SiO2 and ALD Al2O3 were also used for upper oxide. They reduced leakage current effectively more than O3 treated ALD SiO2 by restricting nitrogen accumulation region.
This nitridation method forms a nitrided SiO2 layer while minimizing the oxidation process. It lowers Dit and surface roughness by restricting carbon component generation and thermal roughening which have been major problem of gate oxide for SiC MOSFETs.