Kangho Lee

High performance ZnO nanowire field effect transistors with organic gate nanodielectrics: effects of metal contacts and ozone treatment

High performance ZnO nanowire field effect transistors (NW-FETs) were fabricated using a nanoscopic self-assembled organic gate insulator and characterized in terms of conventional device performance metrics. To optimize device performance and understand the effects of interface properties, devices were fabricated with both Al and Au/Ti source/drain contacts, and device electrical properties were characterized following annealing and ozone treatment. Ozone-treated single ZnO NW-FETs with Al contacts exhibited an on-current (Ion) of ~4 µA at 0.9 Vgs and 1.0 Vds, a threshold voltage (Vth) of 0.2 V, a subthreshold slope (S) of ~130 mV/decade, an on–off current ratio (Ion:Ioff) of ~107, and a field effect mobility (μeff) of ~1175 cm2 V−1 s−1. In addition, ozone-treated ZnO NW-FETs consistently retained the enhanced device performance metrics after SiO2 passivation. A 2D device simulation was performed to explain the enhanced device performance in terms of changes in interfacial trap and fixed charge densities.

Device considerations for development of conductance-based biosensors

Design and fabrication of electronic biosensors based on field-effect-transistor (FET) devices require understanding of interactions between semiconductor surfaces and organic biomolecules. From this perspective, we review practical considerations for electronic biosensors with emphasis on molecular passivation effects on FET device characteristics upon immobilization of organic molecules and an electrostatic model for FET-based biosensors.

Comparative passivation effects of self-assembled mono- and multilayers on GaAs junction field effect transistors

Control of semiconductor interface state density with molecular passivation is essential for developing conduction-based biosensors. In this study, GaAs junction field effect transistors (JFETs) are fabricated and characterized before and after passivation of the GaAs surface with self-assembled mono- and multilayers. The JFETs functionalized with 1-octadecanethiol monolayers and two types of self-assembled organic nanodielectric (SAND) multilayers exhibit significantly different threshold voltage (Vth) and subthreshold slope (Ssub) characteristics versus the unpassivated devices and provide useful information on the quality of the passivation. Two-dimensional device simulations quantify the effective density of fixed surface charges and interfacial traps and argue for the importance of the type-III SAND ionic charges in enhancing GaAs JFET response characteristics.

Proton radiation hardness of single-nanowire transistors using robust organic gate nanodielectrics

In this contribution, the radiation tolerance of single ZnO nanowire field-effect transistors (NW-FETs) fabricated with a self-assembled superlattice (SAS) gate insulator is investigated and compared with that of ZnO NW-FETs fabricated with a 60nm SiO2 gate insulator. A total-radiation dose study was performed using 10MeV protons at doses of 5.71 and 285krad(Si). The threshold voltage (Vth) of the SAS-based ZnO NW-FETs is not shifted significantly following irradiation at these doses. In contrast, Vth parameters of the SiO2-based ZnO NW-FETs display average shifts of ∼−4.0 and ∼−10.9V for 5.71 and 285krad(Si) H+ irradiation, respectively. In addition, little change is observed in the subthreshold characteristics (off current, subthreshold slope) of the SAS-based ZnO NW-FETs following H+ irradiation. These results strongly argue that the bulk oxide trap density and interface trap density formed within the SAS and/or at the SAS-ZnO NW interface during H+ irradiation are significantly lower than those for th...

Mixed adlayer of alkanethiol and peptide on GaAs(100): Quantitative characterization by X-ray photoelectron spectroscopy

Homogeneous and mixed adlayers composed of an alkanethiol (1-octadecanethiol, ODT) and a peptide (CGISYGRKKRRQRRR) on GaAs(100) were formed in two different solvent systems: phosphate-buffered saline (PBS) and N,N-dimethylformamide (DMF). The chemical composition of each adlayer was characterized by X-ray photoelectron spectroscopy (XPS). The data showed that the makeup of the adlayer and its stability largely depends on the solvent used. Angle-resolved XPS also revealed that the adlayer thickness and tilt angles were different from values obtained from ellipsometry measurements and vastly varied between the two solvents used. The coverage data extracted from the XPS measurements indicated that homogeneous adlayers of peptide in PBS buffer form a multilayered film. Homogeneous alkanethiol adlayers exhibited monolayer coverage under all solvent treatments. Coadsorbed layers containing both alkanethiol and peptide have fractional monolayer coverage in both solvents.

Measurement of I-V characteristic of organic molecules using step junction

We investigate devices fabricated using a step junction technique, which is a simple fabrication method to build electrodes with average gap width of 10/spl sim/20 nm using only micro-fabrication. 1,4-benzenedimethane-thiols(xylyl-dithiol; XYL) are self-assembled onto this nanoscale junction (sacrificial step junction) and connected in series by 20 nm gold clusters (GC). Since the gap size in some devices is believed to be locally as small as a single molecule dimension due to variations in the step junction fabrication, another sample (titanium step junction) was prepared using only XYL molecules to bridge the gap. The current-voltage characteristic of XYL was measured using these structures, and compared with the tunneling current before molecular deposition. The conductance of sacrificial step junction devices increased with a yield of 30% after forming a molecule/cluster/molecule bridge, while a lower yield (/spl sim/5%) was observed for the molecule-only devices. Temperature dependent measurement performed on titanium step junction devices, indicate that the molecular conduction is via tunneling.

ZnO Nanowire Field-Effect Transistors: Ozone-Induced Threshold Voltage Shift and Multiple Nanowire Effects

ZnO nanowire field-effect transistors (NW-FETs) employing single nanowires were fabricated, using a self-assembled superlattice (SAS) as the gate insulator. Both depletion-mode and enhancement-mode ZnO NW-FETs were fabricated and characterized. An electrostatic model is proposed to describe observed threshold voltage shift upon optimum ozone treatment. Temperature-dependent current-voltage characteristics of depletion-mode ZnO NW-FETs verify this model, indicating the existence of body current through ZnO nanowires with low activation energy. In addition, NW-FETs that use multiple ZnO nanowires and a SiO2gate insulator were fabricated to achieve higher on-current without significant degradation in on-off current ratio, threshold voltage shift, and subthreshold slopes.

Hopping transport through self-assembled molecules: transport mechanism for conduction-based chemical sensors

We have fabricated a testbed for hopping transport through a self-assembled monolayer (SAM), using a back-to-back Schottky barrier structure on semi-insulating GaAs, and various molecules were self-assembled on the testbed. The leakage current of the testbed is less than 1 nA. However, it was found that some of deposited molecules induce significant increase in conductivity, compared to predeposition current. There exist two possible conduction paths that might cause this phenomenon: 1) hopping conduction through SAM 2) surface potential change due to SAM. To verify that the hopping transport is a dominant conduction mechanism, I-V characteristics of thiophenol and octadecanethiol have been compared to investigate the effects of molecular dipole moment and /spl pi/-electrons on conductivity enhancement. In addition, the application of the testbed to a chemical sensor has been demonstrated by depositing redox molecules on the testbed.

Performance Enhancement of ZnO Nanowire Field-effect Transistors with Self-Assembled Organic Nanodielectrics

Conventional display circuits are built using poly-silicon thin-film transistors (poly-TFTs). However, poly-TFTs are not transparent, causing inefficiency in the aperture ratio on active matrix arrays and, correspondingly, increased power consumption. They also lack flexibility and compatibility with plastic substrates, which are two important requirements for future flexible display devices. In this sense, the development of display devices has been focused on enhancing transparency and flexibility while maintaining or enhancing other device metrics such as on-current, on-off ratio, and subthreshold slope. One promising candidate that satisfies these requirements is ZnO nanowire field-effect transistors (ZnO NW-FETs) because ZnO is a transparent material with a wide bandgap (3.37 eV) and nanowires are known to have inherent flexibility. However, the previously reported device metrics of ZnO NW-FETs were not good enough to replace poly-TFTs, especially in terms of mobility. [1,2] In this study, we report high mobility ZnO NW-FETs using a self-assembled organic superlattice (SAS) as a gate insulator, and investigations of the dependence of current-voltage characteristics of devices using SiO2 insulators on ozone and oxygen plasma treatments. FETs containing single ZnO-NWs were fabricated using a device structure (Fig. 1) in a typical backgate configuration using a heavily doped n-type Si substrate as a common gate. The 15nm SAS film used in this study consists of four interlinked layer-by-layer self-assembled organic monolayers. Fig. 2 illustrates the excellent insulating properties of SAS with a large specific capacitance, 180 nF/cm, and a low leakage current density, 1×10 A/cm. SAS-based ZnO NW-FETs exhibits excellent drain current saturation at Vds = 0.5V, threshold voltage (Vth) of -0.4V, a channel mobility of ~ 196 cm/V-sec, an on/off ratio of ~10, and a subthreshold slope of 400 mV/dec, as shown in Fig. 3. The mobility of SAS-based ZnO NW-FETs is far greater than recently reported values (8~18 cm2/V-sec) of SiO2-based ZnO NWFETs [1,2] and even comparable to that of poly-TFTs. In addition, the inherent flexibility of SAS might be an enabling technology for low-power flexible display devices. An appropriate annealing treatment is also expected to reduce interfacial trap density in the SAS, improving subthreshold slope and threshold voltage shift. [3] ZnO NW-FETs with SiO2 gate dielectrics were also fabricated, and exhibited device performance comparable to that reported in the literature. To optimize the device performance of ZnO NW-FETs, investigations on several annealing and surface treatments are in progress. It has been observed that the oncurrent of SiO2-based ZnO NW-FETs was increased by an order of magnitude, with steeper subthreshold slope, by using ozone or oxygen plasma treatments.