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Understanding the electrical contact characteristics in amorphous-In2Ga2ZnO7 Thin Film Transistor and its vertical device applications

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융합과학기술대학원 나노융합학과
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서울대학교 대학원
학위논문 (박사)-- 서울대학교 융합과학기술대학원 : 나노융합학과, 2013. 2. 황철성.
Amorphous oxide semiconductors (AOSs) have been widely researched for thin-film transistors (TFTs) in high performance display devices due to its transparency and superior carrier transport properties. Since the AOSs are composed of post-transition-metal cations which have a large wave function overlap with neighboring atoms in the s-orbitals, the electrical properties are not significantly altered from the atomically regular crystalline material even when in the amorphous structure. As a result, a high carrier density and mobility can be simultaneously obtained. Recently, it was reported that AOSs can be potentially utilized in low-voltage logic devices, as well as memory applications.
In this regard, the amorphous nature and high carrier mobility of In2Ga2ZnO7 (a-IGZO) thin films, which can be achieved by simple sputtering processes at room temperature, attract a great deal of attention for this approach. In addition, the high performance of a-IGZO TFTs also allows them to be easily integrated with Si-based devices even at low processing temperature, which is another forte of this concept. However, despite these promising factors, research on applications for a-IGZO has mostly been focused on the development of thin-film transistors for display or planar-type devices. Furthermore, while there have been many efforts to improve the electrical performance of a-IGZO, only a few results have been reported about the device scaling or novel device applications. Although the issues related with scaling have not been highlighted for display devices, a proper understanding of the overall conduction mechanism is very important for new applications such as logic and memory devices. Therefore, it is very important to establish a concrete understanding of the device performance variation in terms of device scaling and contact characteristics. Furthermore, to be implemented in novel devices applications, various device structures should be suggested and their electrical characteristics should be investigated.
In this dissertation, the operation characteristics of a-IGZO TFTs were investigated in detail. In addition, based on the understanding the operation characteristics, a vertically integrated TFT device (V-TFT) structure was suggested and a-IGZO V-TFTs with submicron channel length were fabricated using a sputtering-based low temperature process.
Firstly, to clarify the overall operation characteristics of amorphous In2Ga2ZnO7 (a-IGZO) amorphous oxide semiconductor thin film transistors (TFTs), a numerical simulation model was suggested. The localized states were considered in the simulation model and the influences of its energy-density profile within the channel layer were analyzed in terms of the transfer characteristics of a-IGZO TFTs. The origins of the device parameters such as threshold voltage, sub-threshold swing and mobility were investigated using this simulation model and its variation according to changes in material parameters were estimated. The simulation results were verified with experimental results and the transfer characteristics for various operation regimes were well reproduced by the simulation. From these results, the overall operation characteristics of a-IGZO TFTs were explained in detail.
In addition, using the transmission line method (TLM), the effects of device scaling are investigated by analysis of TFT electrical performance using an a-IGZO channel. Using the TLM, the channel characteristics independent of contact resistance were extracted for two different contact metals, Ti and Mo. Based on these results, the mobility characteristics were compared in terms of devices scaling and contact structure in the source/drain overlap region. In addition, the transport characteristics according to the contact structure of the source/drain metal electrode were investigated in detail and the results were quantitatively evaluated by comparison with the simulation results. Furthermore, Asymmetric Schottky contact thin-film-transistors (ASC-TFTs) with an a-IGZO channel were fabricated, and their operation characteristics were examined. Ti, Ni and Pt were evaluated as source/drain metal, and the variations in device performance were analyzed in terms of energy level and bias polarity, which were carefully simulated to understand the influence of the contact properties on the device performance. Based on these results, by applying different metal for each source and drain metal, ASC-TFTs integrating TFTs and Schottky diodes were fabricated, which showed a rectification ratio of drain current higher than 108 according to the bias direction. In addition, the transfer and output characteristics of ASC-TFTs were evaluated for various operation regimes, and the roles of the Schottky junction in device operation were studied in detail.
Finally, based on the understanding the operation characteristics of a-IGZO, a vertically integrated TFT device (V-TFT) structure was suggested and a-IGZO V-TFTs with submicron channel length were fabricated using a sputtering-based low temperature process. Furthermore, the effect of device geometry on device performance was examined in detail. The fabricated V-TFTs show well behaved transfer characteristics with an Ion/Ioff current ratio greater than 104 and a threshold voltage of 1.7V. The influence of the vertical structure on device performance was analyzed in detail. In addition, current polarity characteristics that arise from different metal/a-IGZO contacts were also examined.
In this dissertation, fundamental operation characteristics of a-IGZO TFTs were investigated and based on the understanding of operation characteristics, the novel devices structures were proposed and fabricated. By analyzing the operation characteristics of fabricated devices, novel devices application of AOSs TFTs were suggested.
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