(A) study on the active and gate insulator processed by plasma-enhanced atomic layer deposition in oxide thin-film transistors for the High resolution display고해상도 디스플레이용 산화물 박막트랜지스터를 위한 플라즈마 원자층 증착법의 활성층과 절연층에 대한 연구

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As the interest for high resolution with large size displays increases, the performance of their driving devices becomes critical factor. The devices for the driving high-end display should have high performance with high mobility and low driving voltage. The thin-film transistors (TFTs) with amorphous oxide semiconductor (AOS) as active layer have been attracted much attention due to their stellar performances, easy fabrication and scalability. For the implementation of ultra-high resolution with large area display, the high mobility with high stability of TFTs are essential. Hence the materials for active layer, such as indium zinc oxide (IZO), Al-doped indium tin zinc oxide, indium gallium zinc oxide and zinc oxynitride have been actively researched. However, not only mobility but also reducing parasitic capacitance in device is also critical for high performance. Among the several TFTs structures, top-gate self-aligned (SA) TFT has been regarded as the best for the Super-High Vi-sion (SHV) displays due to the smallest parasitic capacitance between the source/drain and the gate electrode. In addition, recently, the unusual structure, vertical TFT (V-TFT) are proposed as good candidate for the high-end display, which channel is vertically aligned. The V-TFT has advantages to drive the device in low voltage condition. In addition, it has very small foot print, so it makes possible to realize much higher device-packing densities. However, there are several concerns in processing high mobility top-gate SA oxide TFT and V-TFT. For the high mobility top-gate SA TFT, the gate insulator process is critical in controlling threshold-voltage and securing high stability. The conventional plasma-enhanced chemical vapor deposition (PECVD) process should be per-formed at high temperature to get high quality gate insulator of $SiO_2$. High temperature process for $SiO_2$ deposi-tion during the top-gate oxide TFT fabrication, however, induces degradation of the channel layer to result in negative threshold-voltage shift of TFT due to the hydrogen (H) incorporation into the active layer. This phe-nomenon becomes more serious as the mobility of TFT increases. Obtaining high quality GI without any degra-dation of oxide TFT therefore is very difficult in the top-gate structured oxide TFTs with high mobility and good stability. For the fabricating V-TFT, it is hard to obtain the vertically aligned active and gate insulator with con-ventional deposition methods, such as PECVD and sputter due to not perfect step-coverage. Therefore, new strategies for the high-end display driving device are necessary. Plasma-enhanced atomic layer deposition (PEALD) process is a good methodology to result in high quality with outstanding uniformity at relatively low temperature and excellent step coverage. In hence we investigated feasibility of indium zinc oxide (IZO) and $SiO_2$ processed by PEALD on oxide TFT as active and gate insulator respectively. Here we successfully deposited the IZO by using PEALD method and applied to the bottom-gate bottom-contact structured TFTs. Actually, to deposit multi-components films by means of ALD, the layers with single-composition are deposited by alternate stack. However, it is not appropriate for the active layer because, each single-composition layers are poly-crystalline phase and it shows degraded mobility characteristics due to electron scattering at grain boundaries. Here, the new method of ALD process for the deposition of amorphous and homogeneous films was suggested. The two kinds of cation precursors were injected to reaction chamber and chemisorbed on substrate. After that, they are oxidized simultaneously by $O_2$ plasma and obtain one atom-ic-layer of IZO. By repeating this step, the desired thickness of IZO was deposited. Their chemical, physical and electrical characteristics were analyzed by using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and Hall measurement. From the XPS, the films processed by PEALD contains the indium and zinc simultaneously, so the IZO films were successfully grown by using PEALD. In addition, we find out that IZO films by means of PEALD are amorphous phase from the XRD and TEM. The devices with IZO films processed by PEALD show reasonable mobility of over $16 cm^{2}/V \cdot s$ with outstanding positive bias temperature stress (PBTS) and negative bias temperature stress (NBTS) stability characteristics. We also applied gate insulator processed by PEALD to the devices and confirmed the feasibility of PEALD for the gate insulator deposition process. We compared the TFTs with gate insulator processed by PEALD and conventional PECVD in top gate structure we high-mobility active layer, IZO. The TFTs with PEALD process shows outstanding transfer characteristics before and after thermal annealing, however, the TFT with PECVD process shows degraded turn-on voltage with significantly shifted to negative direction. The device with PEALD process also shows reasonable PBTS and NBTS stability characteristics even the device shows high mobility of over $32 cm^{2}/V \cdot s$. As a results, the feasibility of active and gate insulator processed by PEALD for the high-performance TFTs was confirmed.
Advisors
Park, Sang-Heeresearcher박상희researcher
Description
한국과학기술원 :신소재공학과,
Publisher
한국과학기술원
Issue Date
2016
Identifier
325007
Language
eng
Description

학위논문(석사) - 한국과학기술원 : 신소재공학과, 2016.2 ,[viii, 57 p. :]

Keywords

Plasma-enhanced atomic layer deposition; Oxide thin film transistor; High mobility; Indium zinc oxide; High resolution; 산화물 박막 트랜지스터; 플라즈마 원자층 증착법; 고이동도; 실리콘 옥사이드; 인듐 징크 옥사이드

URI
http://hdl.handle.net/10203/221576
Link
http://library.kaist.ac.kr/search/detail/view.do?bibCtrlNo=649524&flag=dissertation
Appears in Collection
MS-Theses_Master(석사논문)
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