Processing of vacuum microelectronic devices by focused ion and electron beams

Localized physical and chemical reactions induced by focused ion and electron beams, i.e. dual beams, have been used to fabricate field emitters (FEs) and their arrays, field-emitter arrays (FEAs), without masking and annealing processes. Issues arising from beam processing such as beam-induced damage and contamination were eliminated to provide FEAs with low leakage current. Quick prototyping and repairing processes of FEs and FEAs using dual-beam processing have been demonstrated. Nb- or Au-gated Pt FEAs have been fabricated using dual beams. The fabricated FEAs showed a turn-on voltage of 40 V for field emission with a typical emission current of about 1 μA/tip. Received: 21 August 2002 / Accepted: 21 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +81-6/6850-6662, E-mail: takai@rcem.osaka-u.ac.jp     

Pulsed-laser-deposited epitaxial aluminum nitride films on (111) Si for surface acoustic-wave applications

Epitaxial (001) aluminum nitride (AlN) thin films on (111) Si substrates are prepared using pulsed-laser deposition. The epitaxial structure of the as-prepared thin films is characterized by checking the X-ray-diffraction θ-2 θ scan and pole-figure, using scanning electron microscopy, infrared radiation (IR) spectroscopy and Raman spectroscopy. The surface acoustic-wave resonance at 345 MHz for a 1.5 μm thick AlN film on a (111) Si substrate is observed using an inter-digital electrode. Received: 18 September 2001 / Accepted: 29 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: liujm@nju.edu.cn     

Structure,transport and field-emission properties of compound nanotubes: CN<Subscript>x</Subscript> vs. BNC<Subscript>x</Subscript> (<Emphasis Type="Italic">x</Emphasis><0.1)

Transport and field-emission properties of as-synthesized CN_x and BNC_x (x<0.1) multi-walled nanotubes were compared in detail. Individual ropes made of these nanotubes and macrofilms of those were tested. Before measurements, the nanotubes were thoroughly characterized using high-resolution and energy-filtered electron microscopy, electron diffraction and electron-energy-loss spectroscopy. Individual ropes composed of dozens of CN_x nanotubes displayed well-defined metallic behavior and low resistivities of ∼10–100 kΩ or less at room temperature, whereas those made of BNC_x nanotubes exhibited semiconducting properties and high resistivities of ∼50–300 MΩ. Both types of ropes revealed good field-emission properties with emitting currents per rope reaching ∼4 μA(CN_x) and ∼2 μA (BNC_x), albeit the latter ropes se- verely deteriorated during the field emission. Macrofilms made of randomly oriented CN_x or BNC_x nanotubes displayed low and similar turn-on fields of ∼2–3 V/μm. 3 mA/cm² (BNC_x) and 5.5 mA/cm² (CN_x) current densities were reached at 5.5 V/μm macroscopic fields. At a current density of 0.2–0.4 mA/cm² both types of compound nanotubes exhibited equally good emission stability over tens of minutes; by contrast, on increasing the current density to 0.2–0.4 A/cm², only CN_x films continued to emit steadily, while the field emission from BNC_x nanotube films was prone to fast degradation within several tens of seconds, likely due to arcing and/or resistive heating. Received: 29 October 2002 / Accepted: 1 November 2002 / Published online: 10 March 2003 RID="*" ID="*"Corresponding author. Fax: +81-298/51-6280, E-mail: golberg.dmitri@nims.go.jp     

Synthesis and photoluminescence of aligned SiO<Subscript>x</Subscript> nanowire arrays

Aligned SiO_x nanowire arrays standing on a Si substrate were successfully synthesized using a simple method by heating a single-crystalline Si slice covered with SiO₂ nanoparticles at 1000 °C in a flowing Ar atmosphere. The SiO_x nanowire arrays were characterized by scanning electron microscopy and transmission electron microscopy. The SiO_x nanowires become progressively thinner from bottom to top. The formation process of the SiO_x nanowire arrays is closely related to a vapor–solid mechanism. Room-temperature photoluminescence measurements under excitation at 260 nm showed that the SiO_x nanowire arrays had a strong blue–green emission at 500 nm (about 2.5 eV), which may be related to oxygen defects. Received: 29 April 2002 / Accepted: 30 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-551-559-1434, E-mail: gwmeng@mail.issp.ac.cn     

Field emission from single-walled carbon nanotubes aligned on a gold plate using a self-assembly monolayer

Field emission from single-walled carbon nanotubes (SWNTs) aligned on a patterned gold surface is reported. The SWNT emitters were prepared at room temperature by a self-assembly monolayer technique. SWNTs were cut into sub-micron lengths by sonication in an acidic solution. Cut SWNTs were attached to the gold surface by the reaction between the thiol groups and the gold surface. The field-emission measurements showed that the turn-on field was 4.8 V/μm at an emission current density of 10 μA/cm². The current density was 0.5 mA/cm² at 6.6 V/μm. This approach provides a novel route for fabricating CNT-based field-emission displays. Received: 3 May 2002 / Accepted: 6 May 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +82-54/279-8298, E-mail: ce20047@postech.ac.kr     

Field electron emission of carbon-based nanocone films

Diamond nanocone, graphitic nanocone, and mixed diamond and graphitic nanocone films have been synthesized through plasma enhanced hot filament chemical vapor deposition (HFCVD). The field emission properties of these films have been experimentally investigated. The studies have revealed that all three kinds of nanocone films have excellent field electron emission (FEE) properties including low turn-on electric field and large emission current at low electric field. Compared with the diamond nanocone films (emission current of 86 μA at 26 V/μm with the turn-on field of 10 V/μm), the graphitic nanocone films exhibit higher FEE current of 1.8×10² μA at 13 V/μm and a lower turn-on filed of 4 V/μm. The mixed diamond and graphitic nanocone films have been found to posses FEE properties similar to graphitic nanocone films (emission current of 1.7×10² μA at 20 V/μm with the turn-on field of 5 V/μm), but have much better FEE stability than the graphitic nanocone films. PACS 81.07.Bc; 81.05.Uw; 79.70.+q     

Structural and optical properties of Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ferroelectric thin films prepared by chemical solution methods

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films have been grown on Pt/Ti/SiO₂/Si substrates by chemical solution methods. X-ray diffraction analysis shows that BLT thin films are polycrystalline with (171)-preferential orientation. Atomic force microscopy investigation shows that they have large grains about 120 nm in size. A Pt/BLT/Pt capacitor has been fabricated and showed excellent ferroelectricity, with a remnant polarization and coercive field of 24 μC/cm² and 116 kV/cm, respectively. The capacitor shows no polarization fatigue up to 10⁹ switching cycles. The optical constants (n,k) of the BLT thin films in the wavelength range 0.35–1.7 μm were obtained by spectroscopic ellipsometry measurements, and the band-gap energy was found to be about 3.25 eV. Received: 16 October 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/65830-734, E-mail: gswang@mail.sitp.ac.cn     

The synthesis of highly oriented GaN nanowire arrays

Highly oriented GaN nanowire arrays have been achieved by the catalytic reaction of gallium with ammonium. The resulting materials were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). SEM images show that the resulting materials are nanowire arrays with a uniform length of about 10 μm. XRD, EDS, TEM and SAED indicate that the nanowire arrays are single-crystal hexagonal GaN with a wurtzite structure. They have diameters of 10 to 20 nm. Received: 2 October 2002 / Accepted: 7 October 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. E-mail: wwwangjc@sina.com     

Growth of a textured quasicrystalline phase in Ti-Ni-Zr films prepared by pulsed laser deposition

The preparation in thin film form of the known icosahedral phase in Ti-Ni-Zr bulk alloys has been investigated as a function of substrate temperature. Films were deposited by pulsed laser deposition on sapphire substrates at temperatures ranging from room temperature to 350 °C. Morphological and structural modifications have been followed by grazing-incidence and θ–2θ X-ray diffraction, transmission electron diffraction and imaging. Chemical composition has been analyzed by electron probe microanalysis. The in-depth variation of composition has been studied by secondary neutral mass spectroscopy. We show that pulsed laser deposition at 275 °C makes the formation of a 1-μm-thick film of Ti-Ni-Zr quasicrystalline textured nanocrystallites possible. Received: 7 June 2001 / Accepted: 18 February 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +33-3/8357-6300, E-mail: brien@mines.u-nancy.fr     

Group-IV nanocluster formation by ion-beam synthesis

A short review of our investigations devoted to the use of ion-beam-synthesized nanoclusters for silicon-based light emission and nonvolatile memory effects is presented. Blue-violet light emission is demonstrated based on Ge-implanted silicon dioxide layers thermally grown on silicon substrates. This version of silicon-based light emission relies on Ge-related defects in the amorphous ≡Si–O–Si≡ network. The photoluminescence and electroluminescence are excited by a singlet S₀–S₁ transition of a neutral oxygen vacancy and by electron injection from the silicon substrate into the silicon dioxide layer, respectively. Whereas the photoluminescence excitation is a well-known mechanism, for the case of electroluminescence an interpretation was performed for the first time in the course of our studies. It was found that the most probable way to excite luminescence centers is the impact excitation by hot electrons. Whereas the injection is explained by trap-assisted tunneling of electrons from the substrate into the oxide, the electrons will be transported via traps or in the SiO₂ conduction band. The application of the silicon-based light-emitting devices for an integrated optocoupler arrangement is described. Another application of nanoclusters is based on the investigation of thin Si-implanted silicon dioxide layers for nonvolatile memory devices. First promising results demonstrate that the observed programming window can reach several volts and the devices exhibit excellent retention behavior. A 256 K-nv-SRAM is demonstrated showing a programming window of >1 V for write pulses of 12 V/8 ms. Received: 21 August 2002 / Accepted: 21 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-351/260-3411, E-mail: w.skorupa@fz-rossendorf.de     

GaN nanotweezers

A new form of GaN nanomaterial (nanotweezers) has been obtained by chemical vapor deposition on an etched cubic MgO (100) plane. The nanotweezers consist of a bottom rod and two arms. The bottom rods have diameters of about 100–150 nm and lengths of about 200–500 nm, on which two arms grow out. The bottoms of the arms are about 40–70 nm and the tops are about 15–30 nm in diameter, and 0.8–1.5 μm in length. X-ray and electron diffractions indicate the nanotweezers are zinc blende gallium nitride. We infer that the fabrication of the GaN nanotweezers is associated with small convex hillocks on the surface of the etched cubic MgO (100) single-crystal substrates and that the nanotweezers grow by a growth mechanism that is similar to vapor-phase heteroepitaxy. Received: 23 April 2002 / Accepted: 25 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-10/8264-9531, E-mail: xlchen@aphy.iphy.ac.cn     

Wavelength-agile fiber laser using group-velocity dispersion of pulsed super-continua and application to broadband absorption spectroscopy

A swept-wavelength source is created by connecting four elements in series: a femtosecond fiber laser at 1.56 μm, a non-linear fiber, a dispersive fiber and a tunable spectral bandpass filter. The 1.56-μm pulses are converted to super-continuum (1.1–2.2 μm) pulses by the non-linear fiber, and these broadband pulses are stretched and arranged into wavelength scans by the dispersive fiber. The tunable bandpass filter is used to select a portion of the super-continuum as a scan-wavelength output. A variety of scan characteristics are possible using this approach. As an example, an output with an effective linewidth of approximately 1 cm^-1 is scanned from 1350–1550 nm every 20 ns. Compared to previous scanning benchmarks of approximately 1 nm/μs, such broad, rapid scans offer new capabilities: a gas sensing application is demonstrated by monitoring absorption bands of H₂O, CO₂, C₂H₂ and C₂H₆O at a pressure of 10 bar. Received: 5 August 2002 / Revised version: 23 September 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +1-608/265-2316, E-mail: ssanders@engr.wisc.edu     

High-speed vertical-cavity surface-emitting laser (VCSEL) absorption spectroscopy of ammonia (NH<Subscript>3</Subscript>) near 1.54 μm

A single-frequency VCSEL has been used for the first time for high-resolution spectroscopy near 1.5 μm. The incorporated buried-tunnel-junction technology enabled the realization of a long-wavelength InGaAlAs/InP VCSEL with low threshold current (0.925 mA), high output powers (0.576 mW) and low series resistance (60 Ω). The high-speed tuning capability of the long-wavelength VCSEL was investigated and used to conduct high-speed absorption spectroscopy. The peak tuning speed was measured to be 3.4 cm^-1/μs and a 4.5-cm^-1-wide NH₃ spectrum was recorded in 2 μs. The VCSEL was used to measure highly resolved low-pressure spectra for pressures ranging from 9.6 mbar to 1 bar. The measured Doppler-broadened linewidth of 0.02 cm^-1 agrees within 3% with the theoretical calculations. The availability and various advantages of 1.3–2-μm single-frequency VCSELs as compared to edge-emitting diode lasers, such as a large current tuning range even at very high tuning frequencies, and low production costs, should significantly expand the application fields for near-infrared laser gas sensors. Received: 17 July 2002 / Revised version: 4 December 2002 / Published online: 12 May 2003 RID="*" ID="*"Corresponding author. Fax: +43-1/58801-15999, E-mail: Gerhard@Totschnig.com     

Sub-ps laser microstructuring of soft X-ray Mo/Si multilayer gratings

The sub-picosecond laser microstructuring of multilayer gratings is presented in this paper. A micromachining system operating with a 0.5 ps KrF laser at 248 nm was used to etch grating structures with a groove width of 1–2 μm in Mo/Si and Si/Mo multilayers. Atomic force microscopy, scanning electron microscopy and X-ray reflectivity were used to characterize the microetched patterns. The ω-scans around the 1st Bragg maximum show symmetric satellites up to 3rd order, with positions corresponding to the grating period. The use of sub-picosecond laser pulses minimizes the thermally affected zone and enhances the quality of the etched features. Short pulse laser processing is advantageous for the fabrication of high spatial resolution microstructures required in X-ray optics. Received: 21 May 2002 / Accepted: 19 August 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Email: dpapa@iesl.forth.gr     

Application of mid-infrared cavity-ringdown spectroscopy to trace explosives vapor detection using a broadly tunable (6–8 μm) optical parametric oscillator

A novel instrument, based on cavity-ringdown spectroscopy (CRDS), has been developed for trace gas detection. The new instrument utilizes a widely tunable optical parametric oscillator (OPO), which incorporates a zinc–germanium–phosphide (ZGP) crystal that is pumped at 2.8 μm by a 25-Hz Er,Cr:YSGG laser. The resultant mid-IR beam profile is nearly Gaussian, with energies exceeding 200 μJ/pulse between 6 and 8 μm, corresponding to a quantum conversion efficiency of approximately 35%. Vapor-phase mid-infrared spectra of common explosives (TNT, TATP, RDX, PETN and Tetryl) were acquired using the CRDS technique. Parts-per-billion concentration levels were readily detected with no sample preconcentration. A collection/flash-heating sequence was implemented in order to enhance detection limits for ambient air sampling. Detection limits as low as 75 ppt for TNT are expected, with similar concentration levels for the other explosives. Received: 1 April 2002 / Revised version: 13 June 2002 / Published online: 12 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-408/524-0551, E-mail: mtodd@picarro.com     

13CO2/12CO2 isotopic ratio measurements using a difference frequency-based sensor operating at 4.35 micrometers

A portable modular gas sensor for measuring the ¹³C/¹²C isotopic ratio in CO₂ with a precision of 0.8‰(±1σ) was developed for volcanic gas emission studies. This sensor employed a difference frequency generation (DFG)-based spectroscopic source operating at 4.35 μm (∼2300 cm^-1) in combination with a dual-chamber gas absorption cell. Direct absorption spectroscopy using this specially designed cell permitted rapid comparisons of isotopic ratios of a gas sample and a reference standard for appropriately selected CO₂ absorption lines. Special attention was given to minimizing undesirable precision degrading effects, in particular temperature and pressure fluctuations. Received: 16 April 2002 / Revised version: 28 May 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-713/5245237, E-mail: fkt@rice.edu     

Low-macroscopic field emission from nanocrystalline Al doped SnO<Subscript>2</Subscript> thin films synthesized by sol–gel technique

We have observed low-macroscopic field electron emission from wide bandgap nanocrystalline Al doped SnO₂ thin films deposited on glass substrates. The emission properties have been studied for different anode-sample spacings and for different Al concentrations in the films. The turn-on field and approximate work function were calculated and we have tried to explain the emission mechanism from this. The turn-on field was found to vary in the range 5.6–7.5 V/μm for a variation of anode sample spacing from 80–120 μm. The turn-on field was also found to vary from 4.6–5.68 V/μm for a fixed anode-sample separation of 80 μm with a variation of Al concentration in the films 8.16–2.31%. The Al concentrations in the films have been measured by energy dispersive X-ray analysis. Optical transmittance measurement of the films showed a high transparency with a direct bandgap ∼3.98 eV. Due to the wide bandgap, the electron affinity of the film decreased. This, along with the nanocrystalline nature of the films, enhanced the field emission properties. PACS 81.20.Fw; 61.10.-i; 79.70.+q     

Photoacoustic gas sensing employing fundamental and frequency-doubled radiation of a continuously tunable high-pressure CO2 laser

We present the first photoacoustic spectrometer for gas sensing employing both the fundamental and the frequency-doubled radiation of a continuously tunable high-pressure CO₂ laser with room temperature operation. A quasi-phase-matched diffusion-bonded GaAs crystal is used in the system for second-harmonic generation. A pulsed photoacoustic detection scheme with a non-resonant cell, equipped with an 80-microphone array, is employed. The wide continuous tuning range in the fundamental (9.2–10.7 μm) and the frequency-doubled (4.6–5.35 μm) regimes, together with the narrow linewidth of 540 MHz (0.018 cm^-1) for the 10-μm region and of 1050 MHz (0.0315 cm^-1) for the 5-μm region, allow the measurement of gas mixtures, individual species and isotope discrimination. This is illustrated with measurements on NO and CO₂. The measured isotope ratio ¹⁵ NO/¹⁴ NO=(3.58±0.55)×10^-3 agrees well with the literature (3.700×10^-3) and demonstrates the good selectivity of the system. Received: 30 April 2002 / Revised version: 10 June 2002 / Published online: 2 September 2002 RID="*" ID="*"Corresponding author. Fax: +41-1/633-1077, E-mail: sigrist@iqe.phys.ethz.ch     

Copper nanowire arrays for infrared polarizer

A micropolarizer of copper nanowire arrays within anodic alumina membrane (AAM) was fabricated by anodization of pure Al foil and electrodeposition of Cu, respectively. X-ray diffraction, scanning electron microscopy and transmission electron microscopy investigations reveal that the ordered Cu nanowires are essentially single crystal, and have an average diameter of 90 nm. Spectrophotometer measurements show that the copper nanowire arrays embedded in AAM can only transmit polarized light vertical to the wires. An extinction ratio of 24 to 32 dB and an average insertion loss of 0.5 dB in the wavelength range of 1 to 2.2 μm were obtained, respectively. Therefore Cu nanowire/AAM can be used as a wire grid type micropolarizer. Received: 28 January 2002 / Accepted:17 May 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +86-551/559-1434, E-mail: ytpang@263.net     

Silicon nanowires grown from Au-coated Si substrate

Amorphous Si nanowires were grown on an Au-coated Si substrate by heat treatment at 1000 °C under an H₂ atmosphere. The nanowires have a length of several tens of a micron and a diameter of 10–20 nm. The growth mechanism of the nanowires was investigated and explained with a solid–liquid–solid model. Received: 11 July 2002 / Accepted: 7 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86/10-62751615, E-mail: yudp@pku.edu.cn