氧化鎵/氮化鎵奈米線金氧半場效電晶體之製作與特性研究

Abstract

本論文提出利用水平選擇性成長氮化鎵奈米線技術與光致電化學氧化法將氮化鎵致換成氧化鎵技術，製作出氧化鎵/氮化鎵奈米線金氧半場效電晶體，並應用氮化鎵極化特性，在氮化鎵/氧化鎵表面產生二維電子氣，氮化鎵/藍寶石基板上產生負的空間電荷，解決了當閘極長度小於50nm時，碰到的短通道效應。且元件在直流與高頻特性上，獲得了相當優異的結果。 本文以固液氣三相反應機制，開發N-face氮化鎵水平定向成長技術，並以光致電氧化技術，研製氧化鎵/氮化鎵奈米線金氧半場效電晶體。並發現由於極化不連續，在氮化鎵與氧化鋁基板上會產生負的空間電荷，抑止了短通道效應。在50nm 線徑與 Lg=50nm下，獲得電流/功率截止頻 150/180GHz。 本文氧化鎵/氮化鎵奈米線寬約為50nm，且為等腰三角形結構，當閘極長度為50nm時，此元件具有120uA 飽和電流、轉導值為 77uS、電流開關比為10000、次臨界擺幅為90mV/dec且電流/功率截止頻150/180GHz。其物理機制可歸納於以下，利用奈米線水平成長以最低自由能與近無缺陷之單晶生長模式，在藍寶石基板與氮化鎵形成陡峭介面。由於極化場量不連續，在氮化鎵/藍寶石介面，提供負空間電荷；而在氧化鎵/氮化鎵介面，提供正空間電荷， 並在氧化鎵/氮化鎵表面形成二維電子氣以維持電中性。並估算此二維電子氣之空間侷限位能井，具備文獻上以複雜超晶格磊晶技術所欲達成之背向位能屏障功能；同時也因負空間電荷相斥效應，抑制短通道電流向基板之洩漏，而使電晶體有較優之直流與高頻特性。

This research aims to provide experimental solutions to resolve a long-standing issue, i.e., short channel effect, which hinders scaling-down of nano-electronics at gate length below 50nm. Taking advantage of our recent development of (i) horizontal and selective site growth of - gallium nitride (GaN) nanowires (NWs) on sapphire, (ii) photo-enhanced oxidation to transform GaN to crystalline gallium oxide (Ga2O3), and (iii) polarization engineering to provide 2D electron gas confined at the Ga2O3/GaN interfaces and negative space charge at the GaN/sapphire back interface, I realize a new structure design of GaN NW-MOSFETs that possess superior DC and RF characteristics to its planar III-Nitride HEMT using complicated material design. This thesis outlines further strategic development to reach GaN high gain NW-MOSFET with cut-off frequency exceeding >150GHz at 50nm gate length. I demonstrated a top-gate Ga2O3/GaN NW-MOSFET by combining a Vapor-Liquid-Solid mechanism to grow high crystalline GaN NWs on sapphire and the photo-enhanced chemical oxidation process to transform the out-shell GaN to a thin Ga2O3 passivation layer. The short channel effect can be suppressed due to formation of negative space charge confined at the GaN/sapphire interface due to the polarization-discontinuity effect. I achieved high-speed modulation of the Ga2O3/GaN NW-MOSFET with a cut-off frequency =150GHz at a gate length of 50nm. The latter opens a route utilizing semiconductor NWs for high speed electronics applications. The transport measurement from a 50nm gate length Ga2O3/GaN NW-MOSFET with 50nm size of isosceles triangular cross-section revealed the following characteristics: saturation current of 120µA, transconductance of 77µS, current on/off ratio of 10000, subthreshold swing of 90mV/dec, and unity current/power gain bandwidth fT/fmax at 150/180GHz. Using a 3D diffusion and drift model analysis, we reconstructed the I-V characteristics which perfectly agree with the experimental observations. My analysis suggests that for the Ga2O3/GaN NW-MOSFET with 50nm gate length and 6nm-thick Ga2O3. The superior DC/RF characteristics can be ascribed the polarization-induced 2D electron gas confined at the abrupt and atomic smooth semi-polar Ga2O3/GaN interfaces, where the high crystallinity NW channel provides field effect mobility approaching the bulk value of GaN.