Seokhwan Bang

Characteristics of the ZnO thin film transistor by atomic layer deposition at various temperatures

ZnO thin films were deposited by atomic layer deposition (ALD) at various temperatures and the resulting electrical and chemical properties were examined. The fraction of O–H bonds in ZnO films decreased from 0.39 to 0.24 with increasing processing temperatures. The O/Zn ratio decreased from 0.90 at 70 °C to 0.78 at 130 °C. The carrier concentration and resistivity changed sharply with decreasing temperature. The ZnO thin film transistors (TFTs) were fabricated at processing temperatures of 70 to 130 °C and the electrical properties of the TFT were as follows: the field-effect mobility ranged from 8.82 × 10−3 to 6.11 × 10−3 cm2 V−1 s−1, the on/off current ratio ranged from 1.28 × 106 to 2.43 × 106, the threshold voltage ranged from −12.5 to 14.7 V and the subthreshold swing ranged from 1.21 to 24.1 V/decade. The electrical characteristics of the ZnO TFT were enhanced as the processing temperature decreased.

Photocurrent detection of chemically tuned hierarchical ZnO nanostructures grown on seed layers formed by atomic layer deposition.

We demonstrate the morphological control method of ZnO nanostructures by atomic layer deposition (ALD) on an Al2O3/ZnO seed layer surface and the application of a hierarchical ZnO nanostructure for a photodetector. Two layers of ZnO and Al2O3 prepared using ALD with different pH values in solution coexisted on the alloy film surface, leading to deactivation of the surface hydroxyl groups. This surface complex decreased the ZnO nucleation on the seed layer surface, and thereby effectively screened the inherent surface polarity of ZnO. As a result, a 2-D zinc hydroxyl compound nanosheet was produced. With increasing ALD cycles of ZnO in the seed layer, the nanostructure morphology changes from 2-D nanosheet to 1-D nanorod due to the recovery of the natural crystallinity and polarity of ZnO. The thin ALD ZnO seed layer conformally covers the complex nanosheet structure to produce a nanorod, then a 3-D, hierarchical ZnO nanostructure was synthesized using a combined hydrothermal and ALD method. During the deposition of the ALD ZnO seed layer, the zinc hydroxyl compound nanosheets underwent a self-annealing process at 150 °C, resulting in structural transformation to pure ZnO 3-D nanosheets without collapse of the intrinsic morphology. The investigation on band electronic properties of ZnO 2-D nanosheet and 3-D hierarchical structure revealed noticeable variations depending on the richness of Zn-OH in each morphology. The improved visible and ultraviolet photocurrent characteristics of a photodetector with the active region using 3-D hierarchical structure against those of 2-D nanosheet structure were achieved.

Structural and Electrical Properties of ZnO Thin Films Deposited by Atomic Layer Deposition at Low Temperatures

In this study, ZnO thin films were deposited by atomic layer deposition (ALD) at various process temperatures. The purpose of this paper was to investigate the controllability of the preferred orientations of ZnO thin films by varying the process temperature and to determine the effect of the preferred orientations on the electrical properties of the films. The process temperature was varied from 70 to 250°C at several increments while the other ALD process parameters were fixed. The deposition rates and uniformities, crystal structures, and electrical properties of these films were evaluated at the various process temperatures. At process temperatures of 70 and 90°C, ZnO thin films showed strong (002) preferred orientations with cylindrical, fine, columnar crystal structures, almost a 1:1 stoichiometric chemical ratio of Zn to O, and n-type carrier concentrations in the range of 10 16 cm -3 with resistivities of 0.1-1 Ω cm. ZnO thin films deposited at temperatures higher than 110°C had wedge-shaped crystal structures, high oxygen deficiencies, and higher n-type carrier concentrations up to 10 20 cm -3 than the films deposited at lower temperatures.

Investigation of the effects of interface carrier concentration on ZnO thin film transistors fabricated by atomic layer deposition

Thin films of ZnO were deposited by atomic layer deposition (ALD) at different process temperatures and the resulting chemical and electrical characteristics were investigated. As the process temperature increased, the ZnO film exhibited an increase in carrier concentration from 1.3 × 1015 to 2.1 × 1019 cm−3, and a decrease in resistivity from 6.7 × 103 to 8.2 × 10−3 Ω cm. We utilized this temperature dependence of the electrical properties and fabricated thin film transistors (TFTs) at different temperatures with both single and double channel layers. In the ZnO-TFT with a single channel layer, the overall device performance of the ZnO-TFT, such as the Ion/Ioff ratio and subthreshold swing (SS), was degraded as the entire channel resistance decreased. In contrast, the ZnO-TFT with a double channel layer could control the turn-on voltage and threshold voltage by suppressing the increase in the off-current. The Ion/Ioff ratios were 8.6 × 105, 2.2 × 106 and 4.7 × 105 and the subthreshold swings exhibited 0.60 V/decade, 0.71 V/decade and 0.68 V/decade for the TFT with interface channel layers deposited at 120 °C, 140 °C and 160 °C, respectively. The saturation mobility slightly increased from 1.267 to 1.912 cm V−1s−1 as the process temperature of the interface channel layer increased.

Al2O3 buffer in a ZnO thin film transistor with poly-4-vinylphenol dielectric

We compared the characteristics of bottom-gate ZnO-thin film transistors using poly-4-vinylphenol (PVP) and PVP/Al2O3 dielectrics. The PVP dielectric is more hydrophobic than the PVP/Al2O3 dielectric and is not useful for TFT devices because of its high leakage current density, but this leakage current density can be significantly reduced by inserting Al2O3. We deposited ZnO and Al2O3 films by atomic layer deposition (ALD) because it is a low-temperature process. The ZnO-TFTs with either a PVP or a PVP/Al2O3 dielectric exhibit typical field-effect transistor characteristics with n-channel properties. The ZnO-TFT containing PVP/Al2O3 exhibits clear pinch-off and excellent saturation with an enhanced mode operation. The on/off ratio of 7.9 × 104 for the device containing the hybrid dielectric is about three orders of magnitude higher than the ratio of 47 for the device containing PVP. The subthreshold gate swings are 12 V/decade for the TFT containing PVP and 1.2 V/decade for the TFT containing PVP/Al2O3. The density of the interface trap state is significantly lower in the device containing PVP/Al2O3 than in the ZnO-TFT containing PVP. The saturation mobility was 0.05 and 0.8 cm2 V−1 s−1, respectively, in the TFTs containing PVP and PVP/Al2O3.

Physical and Electrical Properties of Hafnium–Zirconium–Oxide Films Grown by Atomic Layer Deposition

We deposited HfO 2 , ZrO 2 , and Zr x Hf 1-x O 2 films having different ZrO 2 contents on Si substrates by atomic layer deposition at 300°C and investigated their physical and electrical characteristics. The HfO 2 and ZrO 2 films with thicknesses of about 20 nm exhibited crystalline structures composed of monoclinic and tetragonal phases, respectively. As the ZrO 2 content in the hafnium-zirconium-oxide was increased, the ratio of the tetragonal phase seen in the crystal increased. These changes in crystal phase led to changes in electrical properties. The crystalline phases and electrical properties of the hafnium-zirconium-oxide films exhibited a strong dependence on their Hf/Zr composition ratio.

Pt Nanocrystals Embedded in Remote Plasma Atomic-Layer-Deposited HfO2 for Nonvolatile Memory Devices

Pt nanocrystals using HfO 2 as tunneling and control layers were investigated for nonvolatile memory application. A Pt layer was deposited by electron-beam evaporation and transformed into well-separated nanocrystals by rapid thermal annealing at 700°C. The fabricated Pt nanocrystals had a density of 2.1 × 10 12 cm -2 and an average size of 3.7 nm. The capacitance-voltage measurements demonstrate that the nonvolatile memory with Pt nanocrystals had a memory effect with ∼ 0.9 V flatband voltage shift under a gate voltage of 5 V. The device showed a competitive retention characteristic with a charge loss rate of 20% after 10 4 s.

Dual optical functionality of local surface plasmon resonance for RuO2 nanoparticle–ZnO nanorod hybrids grown by atomic layer deposition

We demonstrate that a hybrid nanostructure consisting of a RuO2 nanoparticle (NP)–ZnO nanorod confers dual local surface plasmon resonance (LSPR) enhancement of ultraviolet (UV) light emission and increase of visible light absorption. A RuO2 NP–ZnO nanorod hybrid was fabricated by an atomic layer deposition method. The size and compositional control of the RuO2 NP allowed (i) visible light absorption increase by the LSPR effect and (ii) proper interfacial electronic alignment of the RuO2–ZnO nanojunction, leading to LSPR coupling with UV light emission enhancement. Based on the combined LSPR effect factor, the dual functionality of LSPR was maximized with 10–20 nm sized RuO2 NPs. These results suggest that a sophisticated design of the nanostructure material heterointerface enables LSPR enhancement of both light harvesting and emission.

Characteristics of Hafnium–Zirconium–Oxide Film Treated by Remote Plasma Nitridation

Characteristics of hafnium-zirconium-oxide films, with and without remote plasma nitridation, have been investigated. The films were created by atomic layer deposition. After deposition, remote plasma nitridation was performed. Nitrogen atoms were successfully incorporated into the hafnium-zirconium-oxide films. As-deposited hafnium-zirconium-oxide film showed a partially crystallized structure. Remote plasma treatment of the hafnium-zirconium-oxide film can effectively suppress the crystallization of the film during rapid thermal annealing. The annealed hafnium-zirconium-oxide film treated by remote plasma nitridation showed a lower equivalent oxide thickness (EOT) and a lower leakage current density than a non-nitrided sample with the same physical thickness. The EOTs of the mixed oxide films with and without nitridation were approximately 1.8 and 2.0 nm, respectively, and the leakage current densities of the films were 5.5 X 10 -8 and 9.2 X 10 -6 A/cm 2 , respectively.