A study on thin film transistors with atomic layer deposition ZnO channel layer for transparent thin film transistor

Seong Joon Lim
Thesis name in original language
투명 박막트랜지스터를 위한 원자층 증착법 산화아연 박막 채널층에 관한 연구
Abstract & Cover

Transparent electronics was interesting field for the application to the transparent display, solar cells, smart window, defroster, and etc. Until now conducting oxide was investigated the most enthusiastically among transparent electronics with high performance and functionality. However, recently, the application of ZnO thin films as an active channel layer of transparent thin film transistor (TFT) has become of great interests due to it’s transparency, low process temperature and high performance, such as high mobility, since ZnO is the interesting material since it correspond to the properties. Thus many groups report on the transparent TFT with ZnO active layer. First we deposited ZnO thin films by atomic layer deposition (ALD) from diethyl Zn (DEZ) as a metal precursor and water as a reactant at growth temperatures between 100 ~ 250 °C. The lowest carrier concentration was about 1017 cm-3 at the Ts = 100 °C. However, we expect that the carrier concentration was too high to apply the films to TFT, since negative VTH was observed when we applicate the ZnO to TFT active layer. Thus we introduced nitrogen doping in ZnO to reduce carrier concentration. For this purpose, we used NH4OH solution as a reactant. As a result, the carrier concentrations were decreased down to 6.13×1013 /cm3 by the nitrogen incorporation in ZnO. Additionally, we introduced oxygen plasma as a reactant to for reducing carrier concentration. We deposited ZnO thin films by plasmaenhanced atomic layer deposition (PE-ALD) technique, and investigated structural, electrical, and optical properties of the films. ALD mode growth was achieved at between 200 and 250 °C. The resistivity was too high to measure under 200 °C of the growth temperature. However, the resistivity decreased with increasing growth temperature. According to the photoluminescence spectra, at low growth temperature band to band and oxygen interstitial emissions, act as acceptor, were observed, however the oxygen interstitials decreased and oxygen vacancies increase with increasing growth temperature. This result implies that oxygen defect is strongly related to the electrical properties of PE-ALD ZnO films. Thin film transistors were fabricated using ALD ZnO:N thin films with different N contents as active channel layers. Due to the electrical properties changes in ZnO:N films, the device properties were significantly changed by amount of nitrogen incorporation. Especially, threshold voltages were changed from 20.0 to 3.1 V by adjusting nitrogen doping. And also, DC bias stability was increased by the increment of nitrogen concentration. And also, we fabricated ALD ZnO:N TFT on the flexible PEN substrate to study the bending effect on the device properties. The threshold voltage was changed to 14.8 V and 16.2 V from 15.5 V during upward and downward bending with 0.83 cm of radius of curvature respectively. And also, the saturation mobility was changed a little from the 3.3 cm2/Vs to 3.1 cm2/Vs and 3.6 cm2/Vs under upward and downward bending with 0.83 cm of radius of curvature respectively. The downward bending raises the threshold voltage and reduces saturation mobility. On the contrary, the upward bending reduces the threshold voltage and raises saturation mobility. From the C-V measurement of the MIM and MISM structure, the device properties shift is attributed not by the gate insulator but by the properties change in the ZnO:N depending on gate insulator. The properties change in the ZnO:N is originated from the piezoelectric effect of ZnO:N. And also, we fabricated plasma enhanced atomic layer deposition (PEALD) ZnO TFT. However the TFT did not modulate within the gate voltage sweep range, due to high resistance of our PEALD ZnO films. After we expose UV light to the PE-ALD ZnO active layer, the device show the TFT modulation by gate voltage, and we found the VTH moved to the negative gate voltage direction with increasing UV exposure time. In detail, VTH moves from 17.8 V to -6.5 V with increasing UV exposure time from 3 min to 120 min. The UV exposure is effective way of activating resistive ZnO layer and control of VTH under low temperature. Additionally, we fabricated back gate type ALD ZnO:N TFT with enhancement mode to study UV exposure effect. We exposed UV light to the TFT and measured the change of VTH in the device to developing easy way for changing the enhancement to depletion mode. We obtained depletion mode TFT after UV exposure, however the properties are not stabilized and the VTH return to the initial value. Thus we passivated ZnO:N with ALD Al2O3 after UV exposure. We prove that the passivation was effective way for stabilize the UV exposure TFT. 

Source of Information
Pohang University of Science and Technology (POSTECH)
Pohang University of Science and Technology (POSTECH)
(Pohang, Korea)
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