Paper EM-TuM9
Control of Electrical Properties of Atomic Layer Deposition ZnO Channel Layer for Thin Film Transistor: In-Situ Nitrogen Doping and Post-deposition Ultra-violet Treatments

Tuesday, October 21, 2008, 10:40 am, Room 210

For emerging transparent flexible display, ZnO is considered as a promising material for channel layer of thin film transistors (TFTs). For the application, the control of key electrical parameters for the ZnO thin films are required to improve off current (IOFF), on-off current ratio, mobility, and threshold voltage (VTH) etc for ZnO TFT. Atomic layer deposition is one of the promising deposition techniques for the ZnO active layer due to its low growth temperature and good uniformity over large area. However, the control of electrical properties of ALD ZnO for the application of TFT active layer has not been widely studied. In this study, we fabricated the ALD ZnO based TFTs with controlled electrical properties by two ways with low process temperature. First, the in situ nitrogen doping using NH4OH as a reactant was employed to control the electrical properties of thermal ALD ZnO. We effectively reduced the high carrier concentration in ALD ZnO thin films (as high as 1018 cm-3) by nitrogen doping, and obtained low carrier concentration down to 1013 cm-3. High performance inverted staggered type TFTs was fabricated using these nitrogen doped ZnO thin films at low growth temperature (<150 °C), with saturation mobility (µsat) = 6.7 cm2/Vs, on-off current ratio (ION/OFF) = 9.46 × 107, IOFF = 2.03×10-12 A, and subthreshold swing = 0.67 V/dec. In addition, VTH values were controlled by changing the amount of nitrogen incorporation. Second, for plasma-enhanced ALD (PE-ALD) ZnO films has too small carrier concentration in contrary to thermal ALD using oxygen plasma as a reactant. As a result PEALD ZnO TFT does not turn on within voltage sweep range, thus postdeposition ultra-violet (UV) treatments were employed to reduce the carrier concentration. As a result, we obtained proper device properties after UV treatment in vacuum. Additionally VTH of PEALD ZnO TFTs decreased with increasing UV exposure time due to the increment of carrier concentration. We will discuss the effects of nitrogen incorporation and UV treatments on ZnO film properties. Especially, we will focused on demonstrate of the production of ZnO TFTs either depletion mode or enhancement mode controllably.