Field Emission and Band Bending Considerations from High-Quality NEA Diamond
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Mat. Res. Soc. Symp. Proc. Vol. 423 c 1996 Materials Research Society
Undoped
E
.6
Boron Doped
e---- Phosphorus Doped
5
FIG. 1
5.2
5.4 5.6 5.8 Excitation Photon Energy (eV)
6
6.2
Photoelectric yield from microwave CVD polycrystalline diamond for near band gap excitation photon energies.
EXPERIMENT In addition to three polycrystalline diamond films, a type Ilb (boron doped, p-type) and a type lb (nitrogen doped) single crystal diamonds were used in this study. The films were grown using microwave plasma enhanced CVD on silicon substrates. Undoped, boron doped, and phosphorous-doped diamond films were investigated. The experiments were performed in an ultra high vacuum system with the sample at room temperature. A Hg-Xe arc-lamp was employed as a source for near band-gap radiation. The photon energy was selected with the use of a 1 m normal incidence monochromator and refocused at the sample position into a 1mm x 2mm rectangular spot with an energy resolution of 50 meV or less. The excitation spectra are normalized with respect to the photo-yield of sodium salicylate [10]. The field emission I-V measurements were performed using a flat 1 mm in diameter tungsten anode, positioned at about 100 jim from the sample surface. RESULTS AND DISCUSSION In Fig. 1 we show photoelectric yield measurements of the three high quality microwave CVD diamond films as a function of the excitation photon energy (normalized to the incident light intensity). In Fig. 2 we show the photoelectric yield from single crystal diamond under the conditions of nearly flat bands, downwards band bending, and (we suspect) upwards band
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at taana Type lIb: Downwards Band Bending Type Ib: Upwards Band Bending ?
W-
,
LW
5
FIG. 2
5.2
5.4 5.6 5.8 Excitaton Photon Energy (eV)
6
6.2
Photoelectric yield from single crystal diamond for near band gap excitation photon energies.
bending. There is a strong correspondence between the form of the photoelectric yield from the three CVD polycrystalline diamond films and the three single crystal measurements. Single crystal studies [9, 11, 12], based upon the three-step model, have understood the photoelectric yield characteristics of single crystal type Ilb diamond (Fig. 2) in terms of the observed changes in band bending (built-in electric fields) at the surface. Furthermore, empirical examination has found [13] that the photoelectric yield from hydrogenated type lb diamond material (Fig. 2) varies as the excess energy above threshold to the fourth power (threshold energy =4.3 eV). Near the surface, the band alignment of a semiconductor to the Fermi level is usually determined by the density of surface (and near-surface) states rather than bulk doping. Hydrogenated diamond surfaces are characterized by valence band maximum (VBM) to Fermi level energy separations of from 0.5 to 1.0 eV (see Table II in Ref. [9]). Doping determines the Fermi level alignment in the bulk. Therefore, band bending at the surface can depend upon both bulk doping and surface treatment. With identi
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