Diamond vacuum field emission devices

This article reports the development of (a) vertical and (b) lateral diamond vacuum field emission devices with excellent field emission characteristics. These diamond field emission devices, diode and triode, were fabricated using a self-aligning gate formation technique from silicon-on-insulator wafers using conventional silicon micropatterning and etching techniques. High emission current >0.1 A was achieved from the vertical diamond field emission diode with an indented anode design. The gated diamond triode in vertical configuration displayed excellent transistor characteristics with high DC gain of 800 and large AC output voltage of 100 V p–p. Lateral diamond field emission diodes with cathode–anode spacing less than 2 μm were fabricated. The lateral diamond emitter exhibited a low turn-on voltage of 5 V and a high emission current of 6 μA. The low turn-on voltage (field 3 V/μm) and high emission characteristics are the best of reported lateral field emitter structures.     

Fabrication and emission properties of diode- and triode-type diamond field emitter array based on the transfer mold technique

Diode- and triode-type diamond field emitter arrays are fabricated using the transfer mold technique and self-align process. A size-inverted pyramidal mold of micron size is formed on (100) silicon substrate using conventional photolithography and anisotropic etching. A diamond film thicker than 100 μm is grown on the mold, which remains as a free-standing film of a diamond tip array after etching the mold. Silicon dioxide and molybdenum (Mo) layers are deposited conformally on the diamond tip array as an insulating and a gate layer, respectively. A flat photo-resist (PR) layer, whose thickness is thinner on the tip than the surrounding area, is then coated. The uniform removal of the PR layer exposes the apex of the tips while the rest of the surface is still covered with PR. The remaining PR layer around the tips acts as a mask for the etching of the Mo and silicon oxide layers over the diamond tips. As a result, a diamond emitter array with a Mo gate is fabricated. We also find that a proper treatment of the diamond tips with hydrogen can substantially improve the emission.     

Field emission devices for advanced electronics comprised of lateral nanodiamond or carbon nanotube emitters

Nanocarbon-derived electron emission devices, specifically, nanodiamond lateral field emission diodes and gated carbon nanotube triodes are new configurations for robust nanoelectronic devices. These novel micro/nanostructures provide an alternative and efficient means of accomplishing electronics that are impervious to temperature and radiation. Nitrogen-incorporated nanocrystalline diamond has been lithographically micropatterned to utilize the material as an electron field emitter. Arrays of laterally arranged “finger-like” nanodiamond emitters constitute the cathode in a versatile diode configuration with small interelectrode separation. Nanodiamond lateral tip conditioning techniques are employed to improve emission and the subsequent device performance discussed. A low diode turn-on voltage of 7 V and a high emission current of 90 μA at an anode voltage of 70 V (electric field of ∼ 7 V/μm) is reported for the nanodiamond lateral device. Also, the development of a field emission triode amplifier based on aligned carbon nanotubes (CNTs) with low turn-on voltage and small gate leakage current, utilizing a dual-mask microfabrication process is reported. The2 × 20 μm CNT triode array displays a gate turn-on voltage of ∼ 44 V, and low gate currents less than 3% of the anode currents. The low gate leakage currents observed confirmed the effectiveness of the convex-shaped gated CNT emitter in alleviating the cathode-gate leakage problem that compromises the operation of a field emission triode.     

Field emission characteristics of a single free standing carbon nanotube with gate electrode

In this paper, we have constructed and analyzed the field emission behavior of a single vertically aligned free standing carbon nanotube (CNT) with a gate electrode in order to verify the feasibility of using a single CNT as the low-voltage field emission electron source. The single vertically aligned CNT with gate electrode was fabricated by combining optical lithography, electron beam lithography (EBL) and inductively coupled plasma chemical vapor deposition (ICP-CVD) processes. A self-aligned process with a single mask was utilized to define the gated structure and the nano-size catalyst for CNT growth. A single vertically aligned CNT was then grown within the gate hole by ICP-CVD. The length-to-diameter ratio of CNT could be varied by adjusting the e-beam exposure time, and the CNT height was controlled to equal to the gate-to-cathode spacing (800) nm in one gated device and less than the spacing (530 nm) in another device. The field emission characteristics of the integrated gate electrode devices were then measured under a scanning electron microscopy (SEM) with a three-axis nano-positioning device. The turn-on field of the gated devices with 800 and 530 nm height CNT were 2.77 and 3.57 V/μm, respectively, with applying − 10 V gate voltage, and 0 V anode voltage.     

Nanodiamond lateral field emitter devices on thick insulator substrates for reliable high power applications

Nanocrystalline diamond field emitter array devices on a thick insulator substrate are being developed for high power applications. These monolithic lateral emitter diodes in comb array configurations demonstrate potential for high emission current applications. A 640 μm-thick aluminium nitride insulating substrate has been integrated with nanodiamond for device electrode isolation. The fabrication process and preliminary field emission characterization results are discussed. The nanodiamond lateral vacuum device may be a superior way to achieve reliable high-speed and high-power electronics.     

Numerical calculations of field enhancement and field amplification factors for a vertical carbon nanotube in parallel-plate geometry

Three types of models, with various emitter shapes in planar electrodes, have been employed to investigate field electron emission characteristics by finite element method. With the model of “hemisphere on a post”, two definitions of field enhancement and field amplification factors and their relation have been discussed systemically at different anode locations. The field distribution in the gap, between emitter apex and anode, is quantified by a field saturation factor. Simulation indicates that field distribution is an important aspect for explaining the nonlinear variation of field amplification factor at different anode locations. Furthermore, the field saturation factor can reflect the saturation degree of field amplification factor. The findings can be used not only in the model of “hemisphere on a post” but also approximately in the models of “two-stage hemisphere on a post” and “hemi-ellipsoid on plane”. Therefore, the fact can be approximately applied to every protruding shape of emitter that is supposed. Comparisons with previously reported experimental and theoretical results are also given.     

Array geometry, size and spacing effects on field emission characteristics of aligned carbon nanotubes

Microwave plasma-enhanced chemical vapor deposition (MPECVD) has been shown capable of producing vertically aligned mutli-walled CNTs as a result of self-bias of the microwave plasma. These CNTs are relevant to field emission applications. However, it is also known that closely packed or mat-like CNTs are not effective field emitters due to field screening effects among neighboring tubes. In this study, an approach whereby “micro-” patterning of CNT arrays, adjusting their geometry, size and array spacing by conventional photolithography, rather than “nano-” patterning a single CNT by electron-beam lithography, is employed to fabricate efficient emitters with enhanced field emission characteristics. MPECVD with catalysts are used on Si substrate to fabricate micropatterned vertically aligned CNT arrays with various geometries, sizes and spacing. The field emission results show that a circular array with 20 μm spacing has the lowest turn-on field of 2 V/μm at 1 μA/cm² and achieves the highest current density of 100 μA/cm² at 3 V/μm. Investigation on the array spacing effect shows that 10 × 10 μm CNT square array with an array spacing of 20 μm displays the lowest turn-on field of 9 V/μm and achieved a very high current density of 100 mA/cm² at 20 V/μm. Furthermore, the results suggest that the array spacing of the 10 × 10 μm CNT square array can be reduced to at least 20 μm without affecting the field enhancement factor of the emitter. The results clearly indicate further optimization of spacing in the arrays of CNT emitters could result in lower turn-on field and higher current density.     

Low temperature time of flight mobility measurements on synthetic single crystal diamond

We study the temperature dependence of low charge injection drift mobility in single crystal (SC) diamond using an alpha particle source. We present time of flight (ToF) mobility measurements to investigate the charge carrier scattering mechanisms in SC synthetic diamond in the temperature range 200 K–300 K. We have used a gold contacted pad detector, with a “sandwich” contact structure, fabricated using a SC chemical vapour deposition (CVD) diamond synthesised by Element Six Ltd. ToF analysis of alpha particle induced current pulses shows a strong increase in hole mobility at reduced temperatures, consistent with acoustic phonon scattering processes dominating the charge carrier transport. On the other hand, electron mobility values appear to remain relatively constant with lower temperatures suggesting different mechanisms than optical or acoustic phonon scattering limiting the charge transport.     

Carbon nanotubes field emission devices grown by thermal CVD with palladium as catalysts

The effects of palladium (Pd) catalyst film thickness and ammonia (NH₃) in thermal chemical vapor deposition (CVD) growth of carbon nanotubes (CNTs) are systematically compared per the resulting morphologies, Raman spectra and field emission characteristics. The CNT field emitters were tested under identical experimental configurations. Field emission characteristics were described with Fowler-Nordheim field emission theory. Experimental results demonstrate that thermally grown CVD CNTs configured as diode field emitters exhibit low turn-on fields and high emission current density. The work is extended to include the study of gated field emitters or field emission triode, important to achieving high-resolution, full gray-scale imaging for field emission, flat-panel displays. The gated device was fabricated utilizing single-mask, self-aligned gate electrode with conventional integrated-circuit (IC) fabrication process. The CNT-triode showed gate-controlled modulation of emission current where higher gate voltage gives rise to higher anode currents. The triode fabrication process using silicon-on-insulator (SOI) wafers is discussed.     

Nanodiamond lateral comb array field emission diode for high current applications

Nanodiamond comb-shaped lateral field emitter arrays in diode configuration were fabricated and characterized for high current field emission. Nitrogen-incorporated nanocrystalline diamond with grain size of 5–10 nm was micropatterned using RIE to realize interconnected arrays of comb structures equipped with uniformly spaced high aspect ratio lateral emitter fingers. A 9000-fingered nanodiamond lateral comb array diode with an inter-electrode spacing of 8 μm demonstrated a high emission current of  25 mA at an anode voltage of 260 V (electric field  32 V/μm) in 10^− 7 Torr vacuum. The lateral emitter configuration shows potential for higher power with no emission current saturation observed. These vacuum micro/nanoelectronic devices comprised of nanodiamond lateral field emission diodes are attractive for low-voltage operating high current electron sources, high-power and high-speed switches, and other extreme demand/extreme environment electronics.     

Controlled synthesis and enhanced field emission characteristics of conical carbon nanotubular arrays

Conical carbon nanotubes (CCNTs) with unique structural characteristics arising from their tapered morphologies compared to uniform diameter carbon nanotubes, have been shown to exhibit enhanced field emission properties and support high current densities. Specifically, several CCNT arrays with different morphological characteristics (tip radius, aspect ratio, density and wall structure) were synthesized by variations in the process parameters using a microwave plasma chemical vapor deposition (MWCVD) reactor. The field emission characteristics for a CCNT array sample with a tip radius of 5 nm, density of 10⁸/cm² and having the highest aspect ratio exhibited a low turn-on electric field (< 0.7 V/μm) and a high field enhancement factor (β > 7500). Other samples with lower emission characteristics were attributed either to the presence of field screening effect resulting from higher CCNT density or due to the corresponding tip and wall characteristics.     

Homoepitaxial growth on fine columns of single crystal diamond for a field emitter

We have fabricated a pyramidal emitter array on single crystal diamond using an etching technique and a homoepitaxial growth technique. We carried out homoepitaxial growth on fine columns of a single crystal diamond in an NIRIM-type microwave-plasma-assisted chemical vapor deposition reactor in the undoped condition. It was found that temperature and methane concentration had a great influence on the oriented growth. We have found that there is a substantial tendency for a lower methane concentration to result in 〈111〉-oriented growth and for a higher methane concentration to give 〈100〉-oriented growth for a polycrystalline diamond film on Si. By controlling the growth conditions, we have obtained various shapes of diamond particle (cubic, cuboctahedral, octahedral) and have also fabricated a pyramidal sharp tip on single crystal diamond (100) and (110) surfaces. For a (100) substrate, we found that a fine pyramid tip was surrounded by not only four {111} planes but also four slightly slopes faces at the base. We surmise that the crystalline face agreed with the slightly sloped plane and grew selectively because it has the highest lateral growth rate among the other oriented crystalline faces. For a (110) substrate, a fine pyramidal tip was surrounded by two {100} planes and two {111} planes and the top of the tip had a ridge between two {111} planes because of the less than optimum conditions. The field emission from the (100) and (110) substrates was also measured. The emission current of the (110) substrate was comparatively large.     

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

A practical field emission enhancement technique for diamond tips with sp² content utilizing boron doping and surface treatment, achieving a very low turn-on electric field of 1 V/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 sp² 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 sp² 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 sp²–diamond–sp² embedded microstructures and field forming process with enhanced hole accumulation after surface treatment.     

Structural studies of diamond films and ultrahard materials by Raman and micro-Raman spectroscopies

Correlations between phonon Raman spectra and structure were established and illustrated with natural diamond, graphite, carbon, novel brilliants and novel ultrahard substances. Diamond-like film is characterized by a Raman band at 1540 ± 20 cm⁻¹ which differs from those of graphite and amorphous carbon. A new structure named “bridged graphite” or “diamite”, with interlayer bonds is proposed for diamond-like material. A mechanism for the destruction of ultrahard coatings is also suggested.     

Radio-frequency plasma system to selectively grow vertical field-aligned carbon nanofibers from a solid carbon source

Vertical field-aligned carbon nanofibers (CNFs), exhibiting a “herring-bone” and a “bamboo-like” structure, were grown at 560 °C using nickel (Ni) as a catalyst and an innovative radio-frequency (RF) plasma-enhanced chemical vapor deposition system. To limit the carbon supply, thereby providing a highly selective growth process with no detrimental parasitic carbon layer formation, a solid graphite sample-holder, RF-polarized, was used as a single carbon source in combination with a pure H₂ feed gas. The morphology and the dimensions of the obtained CNFs are investigated with respect to the growth duration. High-resolution transmission electron microscopy analyses typically display a Ni particle at the fiber tip, but this particle is not encapsulated by graphene layers, allowing its easy removal with a chemical acid treatment. Moreover, the particle’s upper surface consists of a peculiar polycrystalline area, assumed to be essential for the growth mechanisms and possibly made of nickel carbide. The crucial role played by the average vertical electric field, naturally created in the plasma sheath and responsible for sample-holder and substrate bombardment by cationic species, is highlighted to understand the growth mechanisms of these as-grown oriented CNFs and their progressive base destruction by etching phenomena.     

Extreme UV single crystal diamond photodetectors by chemical vapor deposition

The detection properties of a UV photodetector realized on a 150 μm thick CVD single crystal diamond film, grown at Roma “Tor Vergata” University on a low cost HPHT diamond substrate, are reported. The device was tested in the 210–2400 nm spectral range using pulsed laser irradiation and in the 20–250 nm range in continuous mode by both a deuterium lamp and a helium DC gas source irradiation.The detector shows more than five orders of magnitude of visible/UV rejection ratio, a very sharp signal drop of about 10⁴ being observed in correspondence of the diamond energy gap. In the extreme UV range, the He II 25.6 and 30.4 nm as well as the He I 58.4 nm emission lines are clearly detected. The diamond time response is demonstrated to be considerably lower than 5 ns and 0.2 s in pulse and continuous mode, respectively.The extremely good signal to noise ratio, stability and reproducibility of the device response obtained, indicate that no persistent photoconductivity nor undesirable pumping effects are present, which represented so far the main problems preventing the use of diamond based detectors for UV applications.     

Simple fabrication process of a screen-printed triode-CNT field emitter array

We introduced a simple fabrication process for field emission devices based on carbon nanotubes (CNTs) emitters. Instead of using the ITO material as a transparent electrode, a metal (Au) with thickness of 5–20 nm was used. Moreover, the ITO patterning process was eliminated by depositing metal layer, before the CNT printing process. In addition, the thin metal layer on a photoresist (PR) layer was used as UV block. We fabricated the CNT field emission arrays of triode structure with a simple process. And I–V characteristics of field emission arrays were measured. The maximum current density of 254 μA/cm² was achieved when the gate and the anode voltages were kept 150 and 3000 V, respectively. The distance between anode and cathode was kept constant.     

Precisely-controlled fabrication of single ZnO nanoemitter arrays and their possible application in low energy parallel electron beam exposure

Precisely-controlled fabrication of single ZnO nanoemitter arrays and their possible application in low energy parallel electron beam exposure are reported. A well defined polymethyl methacrylate (PMMA) nanohole template was employed for local solution-phase growth of single ZnO nanoemitter arrays. Chlorine plasma etching for surface smoothing and pulsed-laser illumination in nitrogen for nitrogen doping were performed, which can significantly enhance the electron emission and improve the emitter-to-emitter uniformity in performance. Mechanisms responsible for the field emission enhancing effect are proposed. Low voltage (368 V) e-beam exposure was performed by using a ZnO nanoemitter array and a periodical hole pattern (0.72-1.26 μm in diameter) was produced on a thin (25 nm) PMMA. The work demonstrates the feasibility of utilizing single ZnO nano-field emitter arrays for low voltage parallel electron beam lithography.     

Memory effects of carbon nanotube-based field effect transistors

In this paper, we report the study on the non-volatile memory effects of carbon nanotube-based field effect transistors (CNTFETs), in which semiconducting single-wall carbon nanotubes (SWNTs) bridge the gold electrodes and the doped silicon substrate acts as the back gate. We find that our CNTFETs exhibit good performance with on/off ratio of more than 10⁴ and they also show strong memory effects. Hysteretic behaviors of the drain current as a function of the gate voltage are clearly observed at room temperature. The threshold voltage shift increases with increasing the sweeping range of the gate voltage. The CNTFET memory effects show good charge retention capability with the data storage time of around 7 days at ambient condition. Besides, the threshold voltage shift of the as-prepared CNTFETs is found to decrease with time and saturate after around 3 days. Water and alcohol molecules adsorbed on the carbon nanotube are suggested to be the origin of the phenomena. It is also observed that the threshold voltage shift in “top-contact” structures is larger than those in “bottom-contact” structures at the same gate voltage sweeping range.     

Mechanical Properties and Residual Stress Measurements in Anodic Aluminium Oxide Structures Using Nanoindentation

Anodic alumina has been used widely for corrosion protection of aluminum surfaces or as a dielectric material in microelectronics applications. It exhibits a homogeneous morphology of parallel pores that can easily be controlled between 10 and 400 nm. It has been applied as a template for fabrication of the nanometer-scale composite. In this study, the mechanical properties of anodic aluminum oxide (AAO) structures are measured by the nanoindentation method. The nanoindentation technique is one of the most effective methods to measure the mechanical properties of nanostructures. Using the nanoindentation method, we investigated the residual stress and mechanical properties such as the indentation modulus and hardness of the AAO structure with different-sized nanoholes. As a result, we find that the “hole effect” that changes the mechanical properties is the hole size.Original English Text Copyright © 2005 by Fizika i Khimiya Stekla, Ko, D. Lee, Jee, Park, K. Lee, Hwang.This article was submitted by the authors in English.