A. Wisitsora-at

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.

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.

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...

Efficient electron emitter utilizing boron-doped diamond tips with sp2 content

Abstract A practical technique to enhance the electron emission of diamond tips by incorporation of boron dopant and sp 2 content is reported. The effects of boron doping on electron field emission from an array of micro-patterned polycrystalline pyramidal diamond microtips with varying sp 2 content have been systematically studied. The field emission characteristics of undoped and boron-doped diamond tips are significantly improved by increasing sp 2 content of diamond tips. By increasing sp 2 content, the turn-on electric field can be reduced more than 50% for both undoped and boron-doped diamond tips. Likewise, the turn-on electric field of the diamond tips with higher sp 2 content decreases substantially with boron doping. A new field emission mechanism, that is, an increase in the field enhancement factor due to hole accumulation via the formation of cascaded sp 2 -diamond-sp 2 embedded microstructures in diamond tips, is a possible explanation for the enhanced effects of boron doping and sp 2 content on the diamond field emission characteristics.

Diamond as an active sensor material

Abstract Diamond has attractive properties as an advanced electronic material. Its combination of high carrier mobility, electric breakdown, and thermal conductivity results in the largest calculated figures of merit for speed and power of any material. Previously (J.L. Davidson, W.P. Kang, Examples of diamond sensing applications, Proceedings 3rd International Symposium on Diamond Film (ISDF-3), Polytechnical Institute of Russian Academy of Science, St. Petersburg, Russia, 16–19 June 1996) we reported the discovery and development of useful ‘secondary’ effects in diamond and applying them to interesting sensor applications. For example, boron-doped diamond piezoresistors for strain micro-gauges on rugged MEMS (microelectromechanical structures) pressure and acceleration sensors. This paper will present some recent developments with chemically vapor-deposited diamond for microelectromechanical sensing applications such as a new design all diamond pressure microsensor that measures pressure at high temperatures and an accelerometer with over 45xa0kHz resonant frequency. Also, presented are recent results on layered diamond films that behave as chemical sensors measuring hydrogen, oxygen and many other chemicals’ concentration. For example, a diamond-based chemical gas sensor using Pt/SnO x /i-diamond/p + -diamond metal–insulator–semiconductor diode structure for oxygen sensing is described. In addition, the latest emission properties of fabricated diamond microtips for field emitters are reviewed.

Field emission enhancement of diamond tips utilizing boron doping and surface treatment

Abstract A practical field emission enhancement technique for diamond tips with sp 2 content utilizing boron doping and surface treatment, achieving a very low turn-on electric field of 1xa0V/μm, has been developed. The effects of surface treatment and boron doping on electron field emission from an array of micropatterned polycrystalline diamond microtips with sp 2 content have been systematically investigated. Regardless of doping, the field emission characteristics of diamond tips are significantly enhanced and the turn-on electric field is reduced more than 60% after surface treatment. Likewise, regardless of surface treatment, the turn-on electric field of the diamond tips with sp 2 content decreases substantially with boron doping. Possible mechanisms responsible for the field emission enhancement are an increase in the field enhancement factor due to hole accumulation via the formation of cascaded sp 2 –diamond–sp 2 embedded microstructures and field forming process with enhanced hole accumulation after surface treatment.

Effect of annealing temperature on the electron emission characteristics of silicon tips coated with Ba0.67Sr0.33TiO3 thin film

Electron emission from an array of silicon tips coated with Ba0.67Sr0.33TiO3 (BST) ferroelectric thin film was systematically investigated by varying the sol–gel BST annealing temperature from 650 to 800u200a°C. It was found that the turn-on electric field of BST-coated silicon tips tend to decrease as the BST annealing temperature increases, except for the 750u200a°C annealing temperature case that has a considerably lower turn-on field than that of 800u200a°C. Analysis of the emission data using Fowler–Nordheim plots suggested that a higher annealing temperature would result in lower effective work function. To better understand this effect, solid-state BST–silicon junction current–voltage measurements for different annealing temperatures were also studied. The analysis suggests that the conduction mechanism of BST–silicon junction conforms to a space-charge limited injection current model with shallow and deep traps. The higher BST annealing temperature, the shallower the trap was found to be and the quasi-Fermi l...

Modeling of the transistor characteristics of a monolithic diamond vacuum triode

Transistor emission characteristics from a monolithic diamond vacuum triode fabricated by a self-aligning gate technique have been studied and modeled. The anode emission current of diamond triodes has been modeled per the Fowler–Nordheim triode equation and an empirical model for the emission transport factor described. The model was applied to two different diamond field emission triodes with distinct emission characteristics. A procedure for modeling parameter extraction is developed and demonstrated. The modeling results agree well with the experimental data. The empirical model can be incorporated into programs for field emission device simulation.