Byung Moo Moon

High sensitivity capacitive humidity sensor with a novel polyimide design fabricated by MEMS technology

High sensitivity capacitive humidity sensor based on novel designed PI with cavity structure fabricated by MEMS technology is presented in this paper. The humidity sensor consists of a substrate with a cavity, a bottom electrode, a sensing layer, and a comb-shaped top electrode with branches. The cavity structure of the substrate was formed to protect the sensing material and improve reliability. PI was employed for the sensing layer due to its low hysteresis, good linearity, high sensitivity and high resistance to most chemicals. The comb-shaped top electrode was designed to have 50% fill factor with branches and the coated PI was etched by using O2 plasma asher in accordance with the top electrode passivation. This structure could improve the sensitivity and the response time of the humidity sensor due to larger area of contact between the PI and vapor, and shorter pathway of vapor absorption. The humidity sensor was fabricated on a 4 inch silicon wafer by MEMS technology. The humidity sensor with the etched PI showed a high sensitivity of 350 fF/%RH and the response time of 40sec from room humidity condition to 90%RH. These are more improved results compared with values before PI etching, which are sensitivity of 303 fF/%RH and response time of 122 sec. Further characterizations were carried out to measure the hysteresis and the stability. The humidity sensor showed the hysteresis of 1.3% and maintained stable capacitance values with maximum 0.17% error rate, that are enough values to be used as a reliable humidity sensor in various applications.

Liquid Crystal Alignment Effects for Nematic Liquid Crystal on Homeotropic Polyimide Surface Using New Ion-Beam Source

We have studied the liquid crystal (LC) alignment and tilt angle generation of a nematic liquid crystal (NLC) on a homeotropic polyimide (PI) surface using a new ion beam source. The tilt angle of the NLC on the homeotropic PI surface for all incident angles is about 38° and this angle has a stabilization trend. A good LC alignment of the NLC on the homeotropic PI surface by to exposure ion beam of 45° incidence angle was observed. Also, the tilt angle of the NLC on the homeotropic PI surface by to exposure ion beam of 45° incidence angle had a tendency to decrease as ion beam energy density increased. Thus, we demonstrated that the tilt angle could be controlled from vertical to horizontal directions. Also, the alignment character of the NLC on the homeotropic PI surface with respect to ion beam energy was good at more than 1500 eV. Finally, a superior LC alignment thermal stability on the homeotropic PI surface subject to ion beam exposure was achieved.

Ga-doped ZnO transparent electrodes with TiO2 blocking layer/nanoparticles for dye-sensitized solar cells

Ga-doped ZnO [GZO] thin films were employed for the transparent electrodes in dye-sensitized solar cells [DSSCs]. The electrical property of the deposited GZO films was as good as that of commercially used fluorine-doped tin oxide [FTO]. In order to protect the GZO and enhance the photovoltaic properties, a TiO2 blocking layer was deposited on the GZO surface. Then, TiO2 nanoparticles were coated on the blocking layer, and dye was attached for the fabrication of DSSCs. The fabricated DSSCs with the GZO/TiO2 glasses showed an enhanced conversion efficiency of 4.02% compared to the devices with the normal GZO glasses (3.36%). Furthermore, they showed better characteristics even than those using the FTO glasses, which can be attributed to the reduced charge recombination and series resistance.

Enhanced Performance of ZnO Nanowire Field Effect Transistors by H2 Annealing

The electrical properties of ZnO nanowires are significantly dependent on their surface states. The surface trap charges degrade the device performance of field effect transistors. These trap charges are reduced by H2 annealing. In this work, a back-gate ZnO nanowire field effect transistor (FET) was fabricated by a photolithographic process, and its electrical properties were characterized. This back-gate FET was subsequently annealed under a flow of H2/Ar gas for 20 min. The back-gate FET annealed for 20 min exhibited remarkably enhanced electrical characteristics, as compared with the as-fabricated back-gate FET; the peak transconductance was increased from 40 to 448 nS, the field effect mobility from 27 to 302 cm2 V-1 s-1, and the Imax/Imin ratio from 1.5 to 105.

Photocurrent of undoped, n- and p-type Si nanowires synthesized by thermal chemical vapor deposition

Photocurrent of undoped, n- and p-type Si nanowires synthesized by thermal chemical vapor deposition is investigated in this study. For an undoped Si nanowire biased at 3 V, photocurrent excited by the 633-nm wavelength light is stronger in intensity than that excited by the 325-nm wavelength light, and photoresponses are rapid when the light is switched on and off. In contrast, for the n- and p-type Si nanowires, photocurrent excited by the 633-nm wavelength light is not measurable, although one excited by 325-nm wavelength light is still detectable. And photoresponses obtained for the doped Si nanowires are slower, compared with the undoped Si nanowire. Photocurrent phenomena observed in the undoped, n- and p-type Si nanowires are discussed in this paper.

Temperature-dependent gate effect of sintered HgTe nanoparticles

In this study, the electronic properties of sintered HgTe nanoparticles are characterized to determine the type of charge carrier within them, and to investigate their gate effects as a function of temperature. HgTe nanoparticles synthesized by the colloidal method were first deposited on thermally oxidized Si substrates by spin-coating, and then sintered at 150 °C. The sintered nanoparticles were determined to be p-type by analyzing the drain current and drain–source voltage (Id–Vds) relationship as a function of the gate voltage (Vg). The field-effect mobilities of the holes in the sintered HgTe nanoparticles are estimated to be 0.041, 0.036, and 0.022 cm2/(Vs) at 60, 180, and 300 K, respectively. The variation in the slope of the Id–Vds curve as a function of Vg becomes more distinctive as temperature decreases. At temperatures lower than 140 K, an inversion mode was observed for the channel of the sintered nanoparticles.

Homogeneous liquid crystal orientation on ion beam exposure TiO2 surfaces depending on an anisotropic dipole field

We studied homogeneous liquid crystal (LC) alignment properties on ion-beam (IB) irradiated TiO2 films deposited by the electron beam evaporation method. Stable homogeneous LC alignment on a TiO2 surface resulted from IB irradiation energy over 1800 eV. X-ray photoelectron spectroscopy analyses showed that Ti4+ 2p3/2 and Ti4+ 2p1/2 peaks were increased with increasing IB energy. Assuming that the increased peaks produced anisotropy dipole fields in the direction of the IB exposure process, we confirmed that the increasing IB energy induced strengthened the surface energy for entirely clear and stable LC molecule orientation. The voltage-transmittance characteristics of the twisted-nematic cell on the TiO2 surface indicate that the TiO2 film has potential for use as the LC alignment layer.

Vertically aligned liquid crystals on tantalum oxide thin films using ion beam irradiation processing

In this study, we explored the effects of ion beam (IB) bombardment on the orientation of liquid crystal (LC) molecules on Ta 2 O 5 surfaces. Increasing the IB exposure time resulted in improved LC alignment characteristics. Through X-ray photoelectron spectroscopy analysis, it was also confirmed that IB irradiation caused Ta 4f and O 1s peak shifts in the positive direction. The breaking phenomenon of the O 1s bonds may cause anisotropic dipole-dipole forces to stably align the vertical LC molecules. Finally, measuring the contact angles on the IB irradiated Ta 2 O 5 surfaces, we concluded that increasing the IB irradiation time strengthened the anisotropic surface energy on the Ta 2 O 5 surfaces, and stable vertical LC alignment was achieved via the transformed surface energies.

Epitaxial ZnO/4H-SiC heterojunction diodes

High quality n-ZnO/p-SiC heterojunction diodes have been fabricated and their photoresponse properties have been investigated. X-ray diffraction (XRD) θ-2θ patterns show that highly c-axis oriented ZnO films were epitaxially grown on 4H-SiC. The RMS roughness is observed as low as 2 nm by atomic force microscope (AFM). Current-voltage (I–V) characteristics of the fabricated heterojunction diodes have a good rectifying characteristics, and a leakage current less than 10⁻⁹ A at −10 V, with a forward current of ∼10⁻⁵ A at +10 V. The responsivity is measured for different UV wavwlengths. As the intensity of UV wavelength is decreased from 365 nm to 254 nm, the photocurrent increased 1.7×10⁻⁵ A to 3×10⁻⁵ A.

Influence of plasma-etch damage on the interface states in SOI structures investigated by capacitance?voltage measurements and simulations

Au/SiO2/n-Si metal-oxide-silicon-on-insulator (MOSOI) capacitors were fabricated to study the damage caused by reactive ion etching (RIE) on (1 1 0) oriented silicon-on-insulator (SOI) substrates. The MOSOI capacitors with an etch-damaged SOI layer were characterized by capacitance–voltage (C–V) measurements and compared to the sacrificial oxidation treated samples and the reference samples without etching treatment. The measurements revealed that C–V curves significantly change and a negative voltage shift occurs for plasma-damaged capacitors. The simulated band diagram profiles and potential distribution of the corresponding structures indicate that the C–V shift is mainly due to the removal of a parasitic depletion capacitance (Cp) in the substrate, when the interface charges (Qf) are present at the gate oxide/SOI interface. For etch-damaged MOSOI samples, the surface roughness and the interface charges (Qf) have been found to increase by ~1.94 × 1012 cm−2 with respect to the reference devices, whereas the increase was reduced for sacrificial-oxidation treated samples, which implies a recovery from the plasma-induced etch damage on SOI structures.