S. J. Lim

High performance thin film transistor with low temperature atomic layer deposition nitrogen-doped ZnO

High performance thin film transistor (TFT) with atomic layer deposition (ALD) nitrogen doped ZnO (ZnO:N) as an active layer is demonstrated. The electrical properties of ZnO thin films were effectively controlled by in situ nitrogen doping using NH4OH as a source for reactants. Especially, the electron concentration in ZnO was lowered to below 1015cm−3. Good device characteristics were obtained from the inverted staggered type TFTs with ZnO:N channel and ALD Al2O3 gate insulator; μsat=6.7cm2∕Vs, Ioff=2.03×10−12A, Ion∕off=9.46×107, and subthreshold swing=0.67V∕decade. The entire TFT fabrication processes were carried out at below 150°C, which is a favorable process for plastic based flexible display.

Synthesis of horizontally aligned ZnO nanowires localized at terrace edges and application for high sensitivity gas sensor

We developed step edge decoration method for the fabrication of semiconductor ZnO nanodots and nanowires using pulsed laser deposition. We synthesized high quality ZnO nanowires with the small diameter of about 20nm and the uniform interval of about 80nm between each nanowire, which has a simple structure for the formation of contact electrodes. The ZnO nanowire-based sensor was prepared only with the simple process of a gold electrode formation. The ZnO nanowire-based sensor exhibited the high surface-to-volume ratio of 58.6μm−1 and the significantly high sensitivity of about 10 even for the low ethanol concentration of 0.2ppm.

Atomic Layer Deposition ZnO:N Thin Film Transistor: The Effects of N Concentration on the Device Properties

The electrical properties of atomic layer deposition (ALD) nitrogen-doped ZnO (ZnO:N) thin films were investigated as a function of incorporated nitrogen concentration. The nitrogen concentrations in the films were controlled by using different concentrations of NH 4 0H solution, which was used as a single source for the reactant and nitrogen doping for ALD ZnO:N. The carrier concentrations in ALD ZnO:N decreased down to 6.13 × 10 13 /cm 3 using 29% NH 4 OH solution. Thin film transistors (TFTs) were fabricated using ALD ZnO:N thin films with different N contents as active channel layers. The device properties were significantly changed by the amount of nitrogen incorporation due to the change in the electrical properties of ZnO:N films. Especially, threshold voltages were changed from 20.0 to 3.1 V by adjusting nitrogen doping. Additionally, dc bias stability was enhanced by the increment in nitrogen concentration, producing a robust TFT at high nitrogen incorporation. Finally, a high performance flexible TFT was fabricated using ALD ZnO:N as an active layer on poly(ethylene naphthalate) substrate.

Atomic layer deposition ZnO:N flexible thin film transistors and the effects of bending on device properties

ZnO:N flexible thin film transistors were fabricated by atomic layer deposition on polyethylene naphthalate substrates and the effects of bending on the device properties investigated. The threshold voltage and saturation mobility were observed to change with respect to the amount of substrate bending. These modulations can be explained in terms of piezoelectric nature of in ZnO. In comparison with the previously reported single crystal nanowires ZnO field effect transistors, the amount of the electrical property modulation under bent condition is significantly reduced and our report shows a much improved stability for ZnO:N as a flexible device material.

The Effects of Ultraviolet Exposure on the Device Characteristics of Atomic Layer Deposited-ZnO:N Thin Film Transistors

We investigated the effects of ultraviolet (UV) light illumination on nitrogen-doped atomic layer deposited (ALD)-ZnO:N thin film transistors (TFTs). ALD ZnO:N thin films grown at 125°C were used as active layers for back-gate TFT devices. As-fabricated ALD ZnO:N TFTs showed proper drain current modulation response to a gate voltage sweep with a 5.4 V threshold voltage and a clear pinch-off. However, the threshold voltage was significantly shifted in the negative direction by UV exposure due to an associated increase in carrier concentration, resulting in the loss of current modulation by gate voltage sweep. In addition, we observed a resistivity change in ALD ZnO:N thin films with time after UV exposure. The resistivity decreased by several orders of magnitude upon UV light exposure and recovered toward its original value after switching off the UV light. Accordingly, the transfer curves of TFT devices using a ZnO:N active layer also exhibited recovery characteristics. We formed a thin Al 2 O 3 passivation layer on top of the TFT surface in order to suppress the recovery effect.

Electrical property improvements of high-k gate oxide by in situ nitrogen incorporation during atomic layer deposition

Atomic layer deposition (ALD) process for oxynitrides of high-k gate dielectrics employing NH4OH as a single source for reactants, water and NH3, was studied. By this method, nitrogen was incorporated up to 1–3at.% for ALD Al2O3 and Ta2O5 films from metal organic precursors. A comparative study with water based ALD showed that the electrical properties were improved. The leakage current of oxide films from NH4OH based ALD had been reduced and, more importantly, the dielectric strength was found to be enhanced by more than two orders of magnitude from a time dependent dielectric breakdown measurement.

The Effects of UV Exposure on Plasma-Enhanced Atomic Layer Deposition ZnO Thin Film Transistor

We investigated the effects of UV treatment on the device properties of a plasma-enhanced atomic layer deposition (PE-ALD) ZnO thin film transistor (TFT). Remote PE-ALD ZnO thin films were used as active layers for bottom-gate TFT devices. The as-deposited PE-ALD ZnO film at a growth temperature of 200°C has an overly small carrier concentration, which produces a TFT device with no modulation by gate voltage sweep. However, after UV light exposure, the device shows proper TFT modulation by gate voltage. The threshold voltage (V th ) of the device can be controlled to the negative direction by increasing the UV exposure time. V th moves from 17.8 to -6.5 V with an increase in UV exposure time from 3 to 120 min.

Flatband voltage control in p-metal gate metal-oxide-semiconductor field effect transistor by insertion of TiO2 layer

Titanium oxide (TiO2) layer was used to control the flatband voltage (VFB) of p-type metal-oxide-semiconductor field effect transistors. TiO2 was deposited by plasma enhanced atomic layer deposition (PE-ALD) on hafnium oxide (HfO2) gate dielectrics. Comparative studies between TiO2 and Al2O3 as capping layer have shown that improved device properties with lower capacitance equivalent thickness (CET), interface state density (Dit), and flatband voltage (VFB) shift were achieved by PE-ALD TiO2 capping layer.