B. L. Ward

Growth of GaN and Al0.2Ga0.8N on Patterened Substrates via Organometallic Vapor Phase Epitaxy

The selective growth of GaN and Al0.2Ga0.8N has been achieved on stripe and circular patterned GaN/AlN/6H-SiC(0001) multilayer substrates. Growth morphologies on the stripe patterns were a function of the widths of the stripes and the flow rate of triethylgallium. No ridge growth was observed along the top edges of the truncated stripe patterns. Smooth (0001) top facets formed on stripes ≥5 µ m wide. Uniform hexagonal pyramid arrays of undoped GaN and Si-doped GaN were successfully grown on 5 µ m circular patterns. Field emission measurements of a Si-doped GaN hexagonal pyramid array exhibited a turn-on field of 25 V/µ m for an emission current of 10.8 nA at an anode-to-sample distance of 27 µ m.

Field emission properties of nitrogen-doped diamond films

This study explores the field emission properties of nitrogen-doped diamond grown by microwave plasma chemical vapor deposition. Over 70 nitrogen-doped diamond samples were grown on silicon and molybdenum under varying process conditions. Under certain conditions, films can be grown which exhibit photoluminescence bands at 1.945 and 2.154 eV that are attributed to single substitutional nitrogen. Photoelectron emission microscopy with UV free electron laser excitation indicated a 0 or negative electron affinity. Field emission characteristics were measured in an ultrahigh vacuum with a variable distance anode technique. For samples grown with gas phase [N]/[C] ratios less than 10, damage from microarcs occurred during the field emission measurements. Samples grown at higher [N]/[C] content could be measured prior to an arcing event. Contrary to other reports on nitrogen-doped diamond, these measurements indicate relatively high threshold fields (>100 V/μm) for electron emission. We suggest that the nitroge...

Thin films of aluminum nitride and aluminum gallium nitride for cold cathode applications

Cold cathode structures have been fabricated using AlN and graded AlGaN structures (deposited on n-type 6H-SiC) as the thin film emitting layer. The cathodes consist of an aluminum grid layer separated from the nitride layer by a SiO2 layer and etched to form arrays of either 1, 3, or 5 μm holes through which the emitting nitride surface is exposed. After fabrication, a hydrogen plasma exposure was employed to activate the cathodes. Cathode devices with 5 μm holes displayed emission for up to 30 min before failing. Maximum emission currents ranged from 10–100 nA and required grid voltages ranging from 20–110 V. The grid currents were typically 1 to 104 times the collector currents.

Electron emission characteristics of GaN pyramid arrays grown via organometallic vapor phase epitaxy

Selective growth of arrays of silicon-doped GaN (Si:GaN) pyramids for field emitter applications has been achieved. The electron emission characteristics of these arrays has been measured using techniques such as field emission, field emission energy distribution analysis (FEED), photoemission electron microscopy (PEEM), and field emission electron microscopy (FEEM). The field emission current–voltage (I–V) results indicate an average threshold field as low as 7 V/μm for an emission current of 10 nA. It is suggested that the low threshold field value is a consequence of both the low work function of Si:GaN and the field enhancement of the pyramids. The results of the FEEM and FEED measurements indicate agreement with the field emission I–V characteristics. The FEED results indicate that the Si:GaN pyramids are conducting, and that no significant ohmic losses are present between the top contact to the array and the field emitting pyramids. The PEEM and FEEM images show that the emission from the arrays is ...

Electron emission measurements from CVD diamond surfaces

Abstract Electron emission measurements on diamond films synthesized by chemical vapor deposition are reported. UV photoemission spectroscopy indicates that the samples exhibit a negative electron affinity after exposure to hydrogen plasma. Secondary electron emission yields vary from 2.2 to 9.2. Field emission current-voltage measurements indicate threshold voltages ranging from 28 to 84 V μm −1 . The film with the highest secondary yield also exhibits the lowest emission threshold.

Structural and electronic properties of boron nitride thin films containing silicon

The incorporation of silicon into boron nitride films (BN:Si) has been achieved during ion beam assisted deposition growth. A gradual change from cubic boron nitride (c-BN) to hexagonal boron nitride (h-BN) was observed with increasing silicon concentration. Ultraviolet photoelectron spectroscopy, field emission, and field emission electron energy distribution experiments indicated that the observed electron transport and emission were due to hopping conduction between localized states in a band at the Fermi level for the undoped c-BN films and at the band tails of the valence band maximum for the BN:Si films. A negative electron affinity was observed for undoped c-BN films; this phenomenon disappeared upon silicon doping due to the transformation to h-BN. No shift of the Fermi level was observed in any BN:Si film; thus, n-type doping can be excluded.

Electron emission properties of crystalline diamond and III-nitride surfaces

Abstract Wide bandgap semiconductors have the possibility of exhibiting a negative electron affinity (NEA) meaning that electrons in the conduction band are not bound by the surface. The surface conditions are shown to be of critical importance in obtaining a negative electron affinity. UV-photoelectron spectroscopy can be used to distinguish and explore the effect. Surface terminations of molecular adsorbates and metals are shown to induce an NEA on diamond. Furthermore, a NEA has been established for epitaxial AlN and AlGaN on 6H–SiC. Field emission measurements from flat surfaces of p-type diamond and AlN are similar, but it is shown that the mechanisms may be quite different. The measurements support the recent suggestions that field emission from p-type diamond originates from the valence band while for AlN on SiC, the field emission results indicate emission from the AlN conduction band. We also report PEEM (photo-electron emission microscopy) and FEEM (field electron emission microscopy) images of an array of nitride emitters.

Negative) electron affinity of AlN and AlGaN alloys

The electron affinity of a semiconductor defines the relationship of the vacuum level and the semiconductor band structure. It is dependent on the atomic orbitals of the material and the surface termination. We report experimental and theoretical results that support the presence of a negative electron affinity on AlN and the Al rich AlGaN alloys. The GaN surface is found to exhibit a (positive) electron affinity of 3.3eV. The experimental measurements employ UV-photoemission spectroscopy on in situ gas-source MBE samples and on CVD samples. Theoretical results indicate that the (negative) electron affinity of AlN depends sensitively on the surface reconstruction and adatom termination. The experimental dependence of the electron affinity on alloy concentration is presented. The results indicate that AlGaN alloys with band gap similar or greater than that of diamond will exhibit a negative electron affinity. Field emission results are reported, and the characteristics are similar to those obtained from a diamond film. Issues related to cold cathode electronic devices based on NEA surfaces are noted.

THERMOCHEMICAL STABILITY OF SILICON-OXYGEN-CARBON ALLOY THIN FILMS : A MODEL SYSTEM FOR CHEMICAL AND STRUCTURAL RELAXATION AT SIC-SIO2 INTERFACES

Alloy thin films of hydrogenated silicon–oxygen–carbon (Si,C)Ox x<2, were deposited and analyzed in terms of changes in structure and bonding as a function of rapid thermal annealing between 600 and 1100 °C using a combination of Fourier transform infrared spectroscopy, Raman scattering and high-resolution transmission electron microscopy. Results showed that three structural/chemical transformations took place upon annealing. The initial reaction (600–800 °C) involved the loss of hydrogen bonded to both silicon and carbon. At intermediate temperatures (900–1000 °C) a Si–O–C type bond was observed to form, and subsequently disappear after annealing to 1050 °C. The formation of ordered amorphous-SiC regions, nanocrystalline-Si regions, and stoichiometric, thermally relaxed SiO2 accompanied the disappearance of the Si–O–C bond at the 1050 °C annealing temperature. Using this alloy as a model system, important information is obtained for optimized processing of SiC–SiO2 interfaces for device applications.

Characterization of metal-diamond interfaces: Electron affinity and Schottky barrier height

Abstract In this study, the electron affinity and Schottky barrier height of thin Cu and Zr films on diamond (100) substrates were correlated by means of UV photoemission spectroscopy (UPS) measurements. Prior to metal deposition the diamond crystals were cleaned by a 1150°C or 500°C anneal in UHV, and the surfaces were characterized by AES and AFM. This resulted in surfaces terminated with oxygen or free of chemisorbed species. By means of UPS it was found that whether a metal did induce a negative electron affinity (NEA) on a diamond surface was dependent on the surface preparation before metal deposition and on the metal work function. In particular, the Schottky barrier height for clean surfaces was lower than for surfaces terminated by oxygen. Metal-diamond interfaces exhibiting a NEA had a lower Schottky barrier height than those exhibiting a positive electron affinity. These effects were attributed to different interfacial layers. Field emission measurements were performed before and after metal deposition. For all cases a reduction in the threshold electric field was observed upon metal overgrowth.