W.P. Kang

A novel microelectronic gas sensor utilizing carbon nanotubes for hydrogen gas detection

Abstract A novel microelectronic gas sensor utilizing carbon nanotubes (CNTs) in a thin-layered Pd/CNTs/n+-Si structure for hydrogen detection has been achieved. The sensor is fabricated on an n-type silicon wafer, which is needed as an ohmic supporting substrate. Multiwalled CNTs were grown selectively on the substrate via catalytic activation with microwave plasma enhanced chemical vapor deposition. The I–V characteristics of the sensor exhibit Schottky diode behavior at room temperature with marked sensitivity or current changes in the presence of hydrogen. Increasing detection sensitivity in hydrogen sensing was observed with increasing operating temperature. The results demonstrate that CNTs configured as a gas sensor has high sensitivity to hydrogen over a wide temperature range. Behaviors of the sensor in the presence of hydrogen and at elevated temperature were discussed. The successful utilization of CNTs in gas sensors may open a new door for the development of novel nanostructure gas-sensing devices.

Comparison and analysis of Pd‐ and Pt‐GaAs Schottky diodes for hydrogen detection

Hydrogen‐sensing behaviors of Pd‐ and Pt‐GaAs Schottky diodes, fabricated on the same GaAs substrate, have been systematically compared and analyzed as a function of hydrogen partial pressure and temperature by I‐V and ΔI‐t methods under steady‐state and transient conditions. The effects of hydrogen adsorption on the device parameters such as the barrier height and the ideality factor are investigated. The significant differences in their hydrogen sensing characteristics have been examined in terms of sensitivity limit, linearity of response, response rate, and response time. Adsorption activation energy of hydrogen and the heat of adsorption per hydrogen molecule on the surface of Pd and Pt are investigated and compared in both devices in a low‐temperature range (27–100 °C). For the temperature range investigated, Pd‐GaAs shows better performance for H2 detection than Pt‐GaAs under the same testing conditions. The physical and chemical mechanisms responsible for hydrogen detection are discussed. Analysis...

Micropatterned polycrystalline diamond field emitter vacuum diode arrays

Electron field emission from an array of patterned pyramids of polycrystalline diamond for vacuum diode applications has been investigated. High current emission from the patterned diamond microtip arrays was obtained at low electric fields. An emission current from the diamond microtips of 0.1 mA was observed for a field of <10 V/μm. Field emission for these diamond microtips exhibits significant enhancement in total emission current compared to silicon emitters. Moreover, field emission from patterned polycrystalline diamond pyramidal tip arrays is unique in that the applied field is found to be lower compared to that required for emission from Si, Ge, GaAs, and metal surfaces. The fabrication process utilizes selective deposition of diamond film in a silicon cavity mold and subsequent creation of free standing polycrystalline diamond diaphragm with diamond pyramidal microtip array. The processing techniques are compatible with integrated circuit fabrication technology.

Temperature dependence and effect of series resistance on the electrical characteristics of a polycrystalline diamond metal‐insulator‐ semiconductor diode

Temperature dependency and the series resistance effect on the electrical characteristics of a polycrystalline diamond‐based (Au/Ti)/undoped‐diamond/doped‐diamond metal‐insulator‐ semiconductor Schottky diode were investigated in a temperature range 25–300 °C. The current‐voltage (I‐V) characteristics of the device show rectifying behavior with the forward bias conduction limited by series resistance. Over the temperature range investigated, the I‐V data confirmed that the conduction mechanism of the diode is controlled by thermionic field emission. Modifying the thermionic field emission equation to include the series resistance model allows the ideality factor and barrier height of the Schottky diode to be calculated. Temperature dependence of the ideality factor and apparent barrier height was determined. By extrapolating the forward saturation current data, the evaluated ideality factor was observed to decrease from 2.4 to 1.1 while the apparent barrier increased linearly from 0.68 to 1.02 eV in the t...

Effect of sp2 content and tip treatment on the field emission of micropatterned pyramidal diamond tips

Electron field emission characteristics of uniformly constructed micro-pyramids of polycrystalline diamond with varying sp2 content have been systemically investigated. Concurrently, tip surface treatment was performed and emission characteristics of the post-treated tips were evaluated. The experimental results show that the field emission characteristics of the diamond can be enhanced by increasing the sp2 content and performing surface treatment. The emission current is significantly improved and the turn-on electric field is drastically reduced. Hypotheses are proposed for the effect of sp2 content and surface treatment on the field emission enhancement of diamond tips: (i) lowering of the work function due to sp2 defect induced band and impurity desorption, and (ii) increase in field enhancement factor due to sp2-diamond-sp2 microstructures and a field forming process. Analysis of the experimental results indicates that (ii) is the more probable explanation.

A study of diamond field emission using micro-patterned monolithic diamond tips with different sp2 contents

Electron field emission from an array of micro-patterned monolithic diamond tips with varying sp2 content has been systematically investigated. The experimental results show that the field emission characteristics can be improved and the turn-on electric field can be reduced more than 50% by increasing sp2 content. Two hypotheses are proposed as an explanation of the effect of sp2 content on the field emission characteristics of diamond tips: the lowering of the work function due to defect-induced band generated by sp2 content in the diamond lattice and an increase in the field enhancement factor due to embedded sp2–diamond–sp2 cascaded microstructures.

A new hydrogen sensor using a polycrystalline diamond-based Schottky diode

A new hydrogen sensor utilizing plasma-enhanced chemical vapor deposited diamond in conjunction with palladium (Pd) metal has been developed. The device is fabricated in a layered Pd/Undoped diamond/p-doped diamond Schottky diode configuration. Hydrogen sensing characteristics of the device have been examined in terms of sensitivity, linearity, response rate, and response time as a function of temperature and hydrogen partial pressure. Hydrogen adsorption activation energy is investigated in the temperature range from 27 to 85 C. Analysis of the steady-state reaction kinetics using the I-V method confirm that the hydrogen adsorption process is responsible for the barrier height change in the diamond Schottky diode. The ability to fabricate diamond-based hydrogen sensor on a variety of substrates makes the device very versatile for gas sensing.

CVD diamond anisotropic film as electrode for electrochemical sensing

Abstract The unique electrochemical properties of diamond such as a large working potential window, low background current and prolonged stability make it attractive for applications in electroanalysis. High quality and conductive diamond films are known to exhibit active voltammetric response without the need for surface pretreatment. This paper reports on the design, fabrication and characterization of CVD diamond film for electrochemical sensing. Two types of planar boron-doped diamond electrodes were achieved by plasma enhanced chemical vapor deposition (PECVD) using in situ gas phase doping method. The first utilizes the “as grown” diamond surface with randomly microstructured topology as a planar diamond electrode. The second utilizes a micropatterning technique to produce a well-defined pyramidal diamond tips array with good uniformity control. The fabrication process for the electrodes is described. The diamond microelectrodes were evaluated electrochemically for the detection of ferrocyanide, Fe(CN) 6 4− , using cyclic voltammetry. The results suggest that diamond electrodes can be used in electrochemical sensing application.

Diamond field emission devices

Electron field emitter two and three terminal devices can be a new generation of micro-vacuum devices with potential high-speed capability and high temperature and radiation tolerance. Diamond emitters have excellent emission properties. We have developed micro-patterned diamond microtips on diamond films by a mold technique. We will discuss the fabrication of a self-align gated diamond emitter triode and present the emission and device characteristics of the diode and triode with low turn-on voltage and high emission current. For example, a diamond triode with low gate turn-on voltage of 10 V and high gain factor of 250 is reported. The gated diamond triode was fabricated with a self-aligning gate technique on a silicon-on-insulator (SOI) wafer. Ia–Vg plot of emission characteristics with 4 tips shows a very low gate turn-on voltage of 10 V and high emission current of 6 μA at gate voltage of 20 V. Ia–Va plots of emission characteristics demonstrate the desired saturation behavior of field emission transistor with a high voltage gain of 250. The low turn-on gate voltage and high gain factor comparable to solid-state devices, confirming the diamond field emission triode has significant potential for IC-compatible vacuum microelectronic applications and beyond. Ia–Va plots of emission characteristics (Figure below) demonstrate the saturation behavior of a field emission transistor with a high voltage gain factor of 250. The diamond triode has a high transconductance of ∼2.5 μS (4 tips) at a low operating gate voltage of 20 V. Fowler–Nordhiem (F–N) plots confirm that diamond field emission triode conforms to emission behaviors.

Recent development of diamond microtip field emitter cathodes and devices

Recent development of diamond field emitter cathodes and devices fabricated from molding process is presented. Practical modifications involving the sp2 content, surface treatment, boron doping, and tip sharpening to further enhance diamond field emission are discussed. A new fabrication process for achieving ultrasharp diamond tips with a radius of curvature less than 5 nm has been achieved and shows significant improvement in emission characteristics. Discussion of this enhanced emission in diamond microtips is presented in accordance with analysis of emission behavior. The development of high site density of uniform diamond microtip arrays is presented. We also report the development of a new technique to fabricate self-aligned gate diamond emitter diodes, which achieve very high emission characteristics at extremely low applied voltage. The latest development aims to integrate diamond field emitters with silicon-based MEMS processing technology and achieve totally monolithic diamond field emitter devi...