M. W. Geis

Diamond cold cathode

Diamond cold cathodes have been formed by fabricating mesa-etched diodes using carbon ion implantation into p-type diamond substrates. When these diodes are forward biased, current is emitted into vacuum. The cathode efficiency (emitted current divided by diode current) varies from 2*10/sup -4/ to 1*10/sup -10/ and increases with the addition of 10/sup -2/-torr partial pressure of O/sub 2/ into the vacuum system. Current densities of 0.1 to 1 A-cm/sup -2/ are estimated for a diode current of 10 mA. This compares favorably with Si cold cathodes (not coated with Cs), which have efficiencies of approximately 2*10/sup -5/ and current densities of approximately 2*10/sup -2/ A-cm/sup -2/. It is believed that higher current densities and efficiencies can be obtained with more efficient cathode designs and an ultrahigh-vacuum environment.<<ETX>>

Diamond emitters fabrication and theory

The fabrication of gated diamond field‐emission cathodes is described and a theory of their operation is discussed. These cathodes are made using commercial diamond grit with the addition of Ni and Cs salts to enhance emission. The resulting structure resembles a field‐emission Spindt cathode with the internal metal cone replaced by a ∼100 nm layer of diamond grit. Emission from these cathodes occurs at the lowest reported gate voltage of any field emission device and is unaffected by operation at pressures of over 100 Pa of N2. Operation in oxygen and H2S at pressures of 6×10−4 Pa degrades emission, but the cathodes recover once the ambient pressure is reduced to below 1×10−4 Pa. The emission current noise is 2.5% rms over an 8 h period and 1% rms over 3 ms. These cathodes suffer from high gate current that varies from 0.2 to 1000 times the emitted current. The high gate current is known to be process dependent and not inherent to the cathodes. The emission performance is explained by the stable negative...

ELECTRON FIELD EMISSION FROM DIAMOND AND OTHER CARBON MATERIALS AFTER H2, O2 AND CS TREATMENT

This letter reports, diamond field emitters, Cs treated, air stable, that emit electrons at the lowest reported field, <0.2 V μm−1. Field emission from B‐, Li‐, P‐, and N‐doped diamonds and carbonized polymer was characterized as a function of surface treatment. A treated with an O2 plasma, coated with Cs, heated, and exposed to O2 exhibited increased emission for all samples except for B‐doped diamond. The best emission was obtained from N‐doped diamond samples, followed by carbonized polymer, the Li‐doped, and polycrystalline P‐doped diamond. Li‐ and N‐doped samples treated with Cs were stable in laboratory air for several days. This stability of the surface‐activated diamond is believed to be due to the formation of a diamond–O–Cs salt. If the sample is treated with a H2 plasma instead of an O2 plasma, the Cs‐enhanced emission degrades with heat and exposure to O2. Subbands formed by Li and N impurities are believed to be responsible for this enhanced emission. The surface treatment on N‐doped diamond ...

Crystallographic orientation of silicon on an amorphous substrate using an artificial surface‐relief grating and laser crystallization

Uniform crystallographic orientation of silicon films, 500 nm thick, has been achieved on amorphous fused‐silica substrates by laser crystallization of amorphous silicon deposited over surface‐relief gratings etched into the substrates. The gratings had a square‐wave cross section with a 3.8‐μm spatial period and a 100‐nm depth. The 〈100〉 directions in the silicon were parallel to the grating and perpendicular to the substrate plane. We propose that orientation of overlayer films induced by artificial surface patterns be called graphoepitaxy.

High-temperature point-contact transistors and Schottky diodes formed on synthetic boron-doped diamond

Point-contact transistors and Schottky diodes have been formed on synthetic boron-doped diamond. This is the first report of diamond transistors that have power gain. Further, the transistors exhibited power gain at 510°C and the Schottky diodes were operational at 700°C.

A new surface electron-emission mechanism in diamond cathodes

An electron-emission mechanism for cold cathodes is described based on the enhancement of electric fields at metal–diamond–vacuum triple junctions. Unlike conventional mechanisms, in which electrons tunnel from a metal or semiconductor directly into vacuum, the electrons here tunnel from a metal into diamond surface states, where they are accelerated to energies sufficient to be ejected into vacuum. Diamond cathodes designed to optimize this mechanism exhibit some of the lowest operational voltages achieved so far.

Photonic ADC: overcoming the bottleneck of electronic jitter

Accurate conversion of wideband multi-GHz analog signals into the digital domain has long been a target of analog-to-digital converter (ADC) developers, driven by applications in radar systems, software radio, medical imaging, and communication systems. Aperture jitter has been a major bottleneck on the way towards higher speeds and better accuracy. Photonic ADCs, which perform sampling using ultra-stable optical pulse trains generated by mode-locked lasers, have been investigated for many years as a promising approach to overcome the jitter problem and bring ADC performance to new levels. This work demonstrates that the photonic approach can deliver on its promise by digitizing a 41 GHz signal with 7.0 effective bits using a photonic ADC built from discrete components. This accuracy corresponds to a timing jitter of 15 fs - a 4-5 times improvement over the performance of the best electronic ADCs which exist today. On the way towards an integrated photonic ADC, a silicon photonic chip with core photonic components was fabricated and used to digitize a 10 GHz signal with 3.5 effective bits. In these experiments, two wavelength channels were implemented, providing the overall sampling rate of 2.1 GSa/s. To show that photonic ADCs with larger channel counts are possible, a dual 20-channel silicon filter bank has been demonstrated.

Comparison of electric field emission from nitrogen‐doped, type Ib diamond, and boron‐doped diamond

Field emission of electrons from boron‐ and nitrogen‐doped diamond is compared. Emission from boron‐doped diamond requires vacuum electric fields of 20–50 V μm−1, while nitrogen‐doped, type Ib diamond requires fields of 0–1 V μm−1. Since boron‐doped diamond is very conductive, very little voltage drop occurs in the diamond during emission. Nitrogen‐doped diamond is insulating, so during emission a potential of 1–10 kV appears in the diamond. This potential is a function of the back contact metal‐diamond interface. A roughened interface substantially reduces the potential in the diamond and increases emission. The electrons are often emitted from the nitrogen‐doped diamond as beamlets. These beamlets leave the surface of the diamond at angles up to 45° from the substrate normal. Although the vacuum field is small, these electrons have energies of several kV. It is unknown whether the electrons are accelerated to these energies in the bulk of the diamond, or at high electric fields near the emitting surface.

CMOS-Compatible All-Si High-Speed Waveguide Photodiodes With High Responsivity in Near-Infrared Communication Band

Submicrometer silicon photodiode waveguides, fabricated on silicon-on-insulator substrates, have photoresponse from <1270 to 1740 nm (0.8 AW-1 at 1550 nm) and a 3-dB bandwidth of 10 to 20 GHz. The p-i-n photodiode waveguide consists of an intrinsic waveguide 500times250 nm where the optical mode is confined and two thin, 50-nm-thick, doped Si wings that extend 5 mum out from either side of the waveguide. The Si wings, which are doped one p-type and the other n-type, make electric contact to the waveguide with minimal effect on the optical mode. The edges of the wings are metalized to increase electrical conductivity. Ion implantation of Si+ 1times10 13 cm-2 at 190 keV into the waveguide increases the optical absorption from 2-3 dBmiddotcm-1 to 200-100 dBmiddotcm-1 and causes the generation of a photocurrent when the waveguide is illuminated with subbandgap radiation. The diodes are not damaged by annealing to 450 degC for 15 s or 300 degC for 15 min. The photoresponse and thermal stability is believed due to an oxygen stabilized divacancy complex formed during ion implantation

Exceptionally high voltage Schottky diamond diodes and low boron doping

Exceptionally pure epitaxial diamond layers have been grown by microwave plasma chemical vapour deposition, which have low boron doping, from 5 × 1014 to 1 × 1016 cm−3, and the compensating n-type impurities are the lowest reported for any semiconducting diamond, 700 °C for ~1 s in air. Schottky diodes made on these epitaxial diamond films have breakdown voltages >6 kV, twelve times the highest breakdown voltage reported for any diamond diode and higher than any other semiconductor Schottky diode.