Ji Hong Kim

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.

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.

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.

The millimeter-wave detector using vanadium oxide with planar structure antenna

We have developed a new millimeter-wave detector. The detector consists of a micro bolometric detecting part, a micro heater and a micro antenna. Vanadium oxide thin film was deposited for thermal detecting materials. The micro log-periodic bow-tie antenna having resonance frequency of 140 GHz was designed and simulated. The antenna coupled detectors were fabricated by MEMS technology. The fabricated detector showed resonance frequency of 140 GHz which is the same as simulation result and large TCR value of 4%.

Effects of Substrate Temperature on the Electrical and the Optical Properties of N-Type ZnO/P-Type 4H-SiC

We investigated the effect of the substrate temperature on the electrical and the optical properties of ZnO/4H-SiC structures. The n-type ZnO layer was grown on p-type 4H-SiC substrate by pulsed laser deposition to form p-n hetero-junction diode structure. The n-type ZnO thin films were deposited by pulsed laser deposition at different temperatures of 200, 400, and 600 °C, respectively. It was shown from transmission line method (TLM) and auger electron spectroscopy (AES) data that the sheet resistance of ZnO on SiC was increases from ~760 Ω/square to ~4000 Ω/square as the deposition temperature increases and the oxygen outdiffusion decreases. The I-V characteristics with and without illumination has also been studied.

GaZnO as a Transparent Electrode to Silicon Carbide

The characteristics of Ga-doped zinc oxide (GaZnO) thin films deposited at different substrate temperatures (TS~250 to 550oC) on 4H-SiC have been investigated. Structural and electrical properties of GaZnO thin film on n-type 4H-SiC (100)were investigated by using x-ray diffraction, atomic force microscopy (AFM), Hall effect measurement, and Auger electron spectroscopy (AES). Hall mobility is found to increase as the substrate temperature increase from 250 to 550 oC, whereas the lowest resistivity (~3.3 x 10-4 Ωcm) and highest carrier concentration (~1.33x1021cm-3) values are observed for the GaZnO films deposited at 400 oC. It has been found that the c-axis oriented crystalline quality as well as the relative amount of activated Ga3+ Introduction ions may affect the electrical properties of GaZnO films on SiC.

Epitaxially grown GaZnO thin films as transparent electrodes for 4H-SiC

As a wide-bandgap material with most matured wafer technologies, SiC is expected to replace conventional semiconductor materials by improving the power rating, switching frequency, high temperature performances, and ultra-violet (UV) detectors. Transparent conductive oxide (TCO) films have received eminent attention due to their potential use for electrodes of optoelectronic devices such as light emitting diode, solar cells, and liquid crystal displays as well as flat panels. So far, most of the studies on TCO have been focused on indium tin oxide (ITO), however, due to high cost of indium and its limited availability, it is required to develop alternative TCO materials with low cost, high conductivity, and high transparency [1]. Group 3 elements such as gallium and aluminum are the suitable materials as dopants to ZnO for the transparent n-type ZnO. Electrodes are one of the important factors for the SiC UV detectors. Also, there have been many Al or Ga doped ZnO related reports which focus on post annealing effect, transparent [2,3].

Photosensitivity analysis of n-ZnO/p-SiC heterojunction structures

Among the wide band gap semiconductor materials that have drawn a lot of attention recently, ZnO and SiC are seriously considered as materials for emerging electronics applications. ZnO has received attention for its application for UV light-emitters, transparent high power electronics, sensors, piezo electric transducers, and solar cells, because of its high chemical stability, non-toxicity, low cost and high optical band gap of ∼3.37 eV. 4H-SiC has very a small lattice mismatch to ZnO (∼5%), a wide band gap (∼3.2 eV) with excellent thermal properties, and thus suitable for applications such as optoelectronic, high-power, high-frequency, and high-temperature devices. In addition, its useful properties include the existence of the availability of large area substrate, and a high electron saturation velocity.

C-V Characterization of Plasma Etch-damage Effect on (100) SOI

Metal-oxide-semiconductor (MOS) capacitors were fabricated to investigate the plasma damage caused by reactive ion etching (RIE) on (100) oriented silicon-on-insulator (SOI) substrates. The thickness of the top-gate oxide, SOI, and buried oxide layers were 10 nm, 50 nm, and 100 nm, respectively. The MOS/SOI 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. The measured C-V curves were compared to the numerical results from corresponding 2-dimensional (2-D) structures by using a Silvaco Atlas simulator.