P.K. Baumann

Electron affinity and Schottky barrier height of metal–diamond (100), (111), and (110) interfaces

The electron emission properties of metal–diamond (100), (111), and (110) interfaces were characterized by means of UV photoemission spectroscopy (UPS) and field-emission measurements. Different surface cleaning procedures including annealing in ultrahigh vacuum (UHV) and rf plasma treatments were used before metal deposition. This resulted in diamond surfaces terminated by oxygen, hydrogen, or free of adsorbates. The electron affinity and Schottky barrier height of Zr or Co thin films were correlated by means of UPS. A negative electron affinity (NEA) was observed for Zr on any diamond surface. Co on diamond resulted in NEA characteristics except for oxygen-terminated surfaces. The lowest Schottky barrier heights were obtained for the clean diamond surfaces. Higher values were measured for H termination, and the highest values were obtained for O on diamond. For Zr, the Schottky barrier height ranged from 0.70 eV for the clean to 0.90 eV for the O-terminated diamond (100) surface. Values for Co ranged fr...

Negative electron affinity surfaces of aluminum nitride and diamond

Abstract The electron affinity of diamond and AlGaN surfaces are studied by UV photoemission spectroscopy. It is shown that H terminated diamond surfaces exhibit a negative electron affinity while oxide terminated surfaces exhibit a positive electron affinity. In addition, thin metal layers can also induce a NEA on both (100) and (111) surfaces of diamond. Photoemission results of AlGaN alloy films grown on 6HSiC indicate a negative electron affinity for as-prepared and air exposed surfaces with high Al concentrations.

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.

Negative electron affinity effects on H plasma exposed diamond (100) surfaces

Abstract The effects of annealing and a H plasma exposure on natural type IIb diamond (100) were investigated by means of ultraviolet photoemission spectroscopy (UPS). The diamond (100) surface was found to exhibit a negative electron affinity (NEA) following a 900 °C anneal in ultrahigh vacuum. After a H plasma exposure the NEA peak in the UPS spectra had doubled in height. An anneal to 1100 °C resulted in the removal of the sharp NEA feature. A second H plasma treatment resulted in the reappearance of the NEA peak like after the first H plasma exposure. A 2 × 1 reconstructed low-energy electron diffraction (LEED) pattern was observed subsequent to the anneals as well as the H plasma treatments. The fact that a NEA can be induced or removed repeatedly by means of a H plasma exposure or a 1100 °C anneal respectively provides evidence to correlate the appearance of a NEA with the presence of a monohydride terminated surface.

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.

Electron emission from metal-diamond (100), (111) and (110) interfaces

Electron emission characteristics of Cu, Co or Zr films on diamond (100), (111) and (110) surfaces were measured by employing ultraviolet photoemission spectroscopy (UPS) and field emission measurements. Prior to metal deposition, the diamond substrates were terminated with oxygen, hydrogen or were free of adsorbates. Deposition of thin Cu or Co films induced a NEA on clean and H-terminated surfaces. A positive electron affinity was observed for Cu or Co on oxygenated surfaces, and depositing thin Zr films resulted in a NEA on all surfaces considered. UPS can be used to correlate the electron affinity and Schottky barrier height. Schottky barriers of metals on clean surfaces were the lowest, whereas they were the highest on oxygen-covered surfaces. Values for the Schottky barrier height ranged from 0.70 eV to 1.60 eV for Cu, 0.35 eV to 1.40 eV for Co and 0.70 eV to 0.95 eV for Zr. A field emission threshold of 79 V μm−1 was measured for oxygenated (100) surfaces. The lowest value of 20 V μm−1 was observed for Zr on the clean (100) surface. For all the metals studied, it was found that a lower Schottky barrier height results in a lower electron affinity, and a lower electron affinity results in a lower field emission threshold.

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.

Epitaxial Cu contacts on semiconducting diamond

Abstract In this study Cu films of 30 nm and 200 nm thickness have been grown on natural type IIb semiconducting diamond C(001) substrates by electron-beam evaporation at 500 °C in UHV. As evidenced by Rutherford backscattering/channeling techniques and in situ low-energy electron diffraction, the as-deposited layers were shown to be epitaxial, with χCu = 49%. In addition, the technique of atomic force microscopy has demonstrated island morphology, indicative of three-dimensional growth. Moreover, the Cu films displayed excellent adhesion properties with the underlying diamond substrate. Corresponding current-voltage (I–V) measurements conducted at room temperature have shown rectifying characteristics. In addition, a Schottky barrier height of ΦB ≈ 1.1eV has been determined from ultraviolet photoemission spectroscopy.

Characterization of cobalt-diamond (100) interfaces : electron affinity and Schottky barrier

Abstract UV photoemission measurements were used to relate the electron affinity and Schottky barrier of thin Co layers on diamond (100) surfaces. Cobalt films of 2 A thickness were deposited on natural single crystal diamond (100) substrates by hot filament evaporation in ultra-high vacuum (UHV). The surfaces were characterized with auger electron spectroscopy and atomic force microscopy. The study explores the properties of the cobalt-diamond interface as a function of different surface cleaning procedures. Prior to deposition the diamond samples have been cleaned by UHV anneals at either 500°C or 1150°C. Following either of these anneals a positive electron affinity was deduced from the ultraviolet photoemission measurements. The measurements indicate that the surface annealed at 500°C is terminated with oxygen while the surface annealed at high temperature is free of adsorbates. Upon deposition of Co on the surface heated to 1150°C, a negative electron affinity (NEA) was detected, and a Schottky barrier height of 0.35 eV was measured. However, for Co films deposited on substrates annealed to 500°C a positive electron affinity and a Schottky barrier height of 1.45 eV were observed. The results are discussed in terms of a model that relates the electron affinity to the metal workfunction and the Schottky barrier.

Negative Electron Affinity Effects And Schottky Barrier Height Measurements Of Metals On Diamond (100) Surfaces

The effects of growing thin cobalt films on natural type IIb diamond (100) substrates were investigated by means of ultraviolet photoemission spectroscopy (UPS). Prior to deposition the diamond samples were annealed to 1150{degrees}C in UHV resulting in a positive electron affinity surface. Upon deposition of 2{Angstrom} of Cobalt a negative electron affinity (NEA) was observed and a Schottky barrier height of 0.35 eV was measured by means of UPS. The presence of a cobalt layer was confirmed employing in-situ Auger electron spectroscopy (AES). As evidenced by atomic force microscopy (AFM) uniform Co films were deposited replicating the underlying diamond substrates.