第96回先端計測オープンセミナー

2017年4月7日 (金) 15:00 ~ 16:00

国立研究開発法人 物質・材料研究機構 
千現地区 研究本館8階 中セミナー室
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KUMAR Ashutosh (技術開発・共用部門/窒化ガリウム評価基盤領域/最先端電顕グループ、NIMSポスドク研究員)

GaN based bulk and nanoscale Schottky diodes: Graphene as a contact material and in-situ electrical measurements

Semiconductor devices based on GaN and its heterostructures have shown a tremendous potential for high-power, high-frequency and solid state lighting applications on bulk as well as nanoscale dimensions. In spite of a large number of efforts made towards the development of these devices, there are still issues to be resolved to utilize the full potential of GaN technology. One such issue is fabrication of a Schottky contact with high barrier height, near-unity ideality factor, low leakage current and good thermal stability as electrical behavior of this contact is severely affected by interface states and barrier inhomogeneities at metal/GaN interfaces.
In this talk, I shall focus on the fabrication and electrical characterization of conventional metal/GaN, unconventional graphene/GaN and GaN based nanoscale Schottky diodes. The electrical properties of metal/GaN diodes are found to be significantly improved on subjecting them to rapid thermal annealing. Although metal/GaN diode in our work showed ideality factor close to unity, still barrier height is always limited by Schottky-Mott limit.[1,2] Keeping this limitation as the starting point, graphene/GaN Schottky diodes are fabricated by selective transfer of exfoliated graphene on GaN. The diodes exhibited enhanced thermionic emission and low flicker noise in comparison to conventional Ni/GaN diodes. The barrier height value obtained using thermionic emission theory is found to be higher than predicted barrier height as per the Schottky–Mott model. Enhanced thermionic emission current, lower level of inhomogeneities and reduced flicker noise suggests that graphene–GaN Schottky diodes may have the underlying trend for replacing metal–GaN Schottky diodes. Our study also suggests that electronic transport in 2D/3D (graphene/GaN) system is different from conventional 3D/3D (Ni/GaN) system and one can reach beyond the Schottky-Mott limit in such systems.[3]
As electrical behavior of Schottky diodes is significantly affected by interfaces states and barrier inhomogeneities, an effort has been made to understand their impact on electrical behavior. In order to realize a nanoscale Schottky diode based on GaN nanorods (NRs), a thorough understanding of the current transport across interface is required as models which are used for explaining electrical transport in epitaxial films are not fully applicable to nano-devices because of small dimensions. The electrical behaviour of Schottky barrier diodes realized on vertically standing individual GaN NRs and array of NRs is investigated using in-situ nanoscale electrical measurements. Schottky diodes on individual NR show highest barrier height in comparison with large area diodes on NRs array and epitaxial film which is in contrast with previously published work. The discrepancy between the electrical behaviour of nanoscale Schottky diodes and large area diodes is explained using in-situ cathodoluminescence measurements, surface potential analysis using Kelvin probe force microscopy and 1ow frequency noise measurements. These barrier inhomogeneities in large area diodes resulted in reduced barrier height whereas due to the limited role of barrier inhomogeneities in individual NR based Schottky diode, a higher barrier height is obtained. [4]
References
1.Ashutosh Kumar et al. Appl. Phys. Lett. 107, 093502 (2015).
2.Ashutosh Kumar et al. J. Phys. D: Appl. Phys. 49, 47LT01 (2016).
3.Ashutosh Kumar et al. ACS Appl. Mater. Interfaces 8, 8213 (2016).
4.Ashutosh Kumar et al. Sci. Rep. 6, 27553 (2016)