G. J. Wojak

Electron emission from diamond coated silicon field emitters

Polycrystalline diamond thin films have been formed on single crystal silicon field emitters using bias‐enhanced nucleation in a microwave plasma chemical vapor deposition system. A diamond nucleation density greater than 1010/cm2 with small grain sizes (<25 nm) was achieved on the surfaces of silicon emitters with nanometer scale curvature. Field emission from these diamond coated silicon emitters exhibited significant enhancement compared to the pure Si emitters both in total emission current and stability. Using a Fowler–Nordheim analysis a very large effective emitting area of nearly 10−11 cm2 was obtained from the diamond coated Si emitters compared to that of uncoated Si emitters (10−16 cm2). This area was found to be comparable to the entire tip surface area.

Field emission characteristics of diamond coated silicon field emitters

Single crystal silicon field emitters have been modified by surface deposition of diamond using bias‐enhanced microwave plasma chemical vapor deposition. Polycrystalline diamond with a high nucleation density (1010/cm2) and small grain size (<20 nm) was achieved on silicon field emitters. Field emission from these diamond coated emitters exhibited significant enhancement both in total emission current and stability compared to pure silicon emitters. A large effective emitting area comparable to the tip surface area was obtained from a Fowler–Nordheim analysis. The effective work function of the polycrystalline diamond coated emitter surface was found to be larger than that of a pure silicon emitter surface.

“Standardization” of field emission measurements

Interest in field emission and field emission devices has been renewed in the last 5 yr. This increase has been due to work on several new materials systems, which have shown promising field emission (FE) behavior. In turn, this interest gives impetus to the search for new FE sources. In order to move the technology ahead at a faster pace, there is a need for common ground rules and a “standardization” of the data reported so that it can be compared directly in a meaningful way and thereby accelerate the development process. In this article key factors affecting the FE data will be discussed and several parameters are suggested to initiate the process of developing a set of “standardized” FE parameters. A correct, or at least consistent, determination of characteristics such as work function, emission area, and field enhancement form the basis for developing a framework to make meaningful comparisons between different sets of data.

Electron emission from a hydrogenated diamond surface

Electron emission from a polycrystalline diamond coated silicon field emitter surface was studied using in situ exposure to various gas species during its operation. Significant enhancement of the electron emission was found after the emitting surface was exposed to hydrogen at pressures in the range 5×10−4 to 10−3 Torr. Introducing other gases such as Ne and He only suppressed the emission current. A continuous emission current was measured from such a hydrogen‐exposed surface even after the electric field was reduced to below the initial threshold for electron emission. No similar result was found for pure silicon surface when identical conditions applied. This phenomenon was interpreted as the formation of a dynamically vacuum‐stable layer by polarized hydrogen and the diamond surface. Such a surface layer may significantly lower the surface barrier and exhibit the negative electron affinity property.

Environmental effect on the electron emission from diamond surfaces

Electron emission from silicon field emitters with a thin coating of diamond were studied during exposure to varying pressures of Ne, He, H2, and D2. Introduction of Ne and He at pressures >10−4 Torr suppressed the emission current. Conditioning of field emitters in a 10−5 Torr helium ambient improved the emissivity. After hydrogen and deuterium exposure, a continuous emission current was measured below the initial threshold voltages for electron emission. The effects of deuterium were significantly greater than for hydrogen. We believe this phenomenon is due to the formation of a surface dipole layer of hydrogen or deuterium, bonded to the surface carbon atoms, which lowers the electron affinity of the diamond surface.

Electron energy distribution of diamond-coated field emitters

The influence of metal/diamond interfacial nanostructure and diamond surface treatment on electron emission from diamond-coated Mo emitters is presented. Diamond coatings are known to enhance electron emissivity but may do so at the interfacial barrier and/or the surface barrier, since both influence the magnitude of the current. Prior to annealing the energy distribution of the emitted electrons exhibits a linear displacement in peak position with applied voltage. After annealing in a hydrogen plasma at 600 °C, emission is further enhanced and the magnitude of the peak shift with applied voltage is reduced. A further oxygen plasma treatment yields an intermediate dependence of peak shift with voltage. The effects of hydrogen plasma annealing appear closely related to changes in the nanostructure at the Mo/diamond interface. During annealing Mo2C particles are formed while both molybdenum oxides and an amorphous layer present on the original diamond particles are removed. The smaller voltage drop after th...

Investigation of thickness effects on AlN coated metal tips by in situ I–V measurement

The thickness of a deposited coating can be a critical factor in determining the emission characteristics of coated emitters. Several researchers have reported a thickness dependence of emission from emitters coated with wide band gap materials. Thus systematic I-V experiments are required, preferably using a single emitter with deposited layers of varying thickness. In the present study, the thickness effects of ultra thin AlN layers on Mo emitters was investigated using an in situ I-V measurement technique.

Optimizing high-current yields from diamond coated field emitters

The data for the maximum emission currents from needle-shaped emitters with differing diamond coatings were empirically analyzed. The coatings studied were chemical vapor deposition diamond, natural diamond, and nanodiamond. Two parameters were chosen to characterize the emissive properties: (1) the dependence of the maximum current (Imax) on the coating thickness (D), i.e., I(D)=ΔImax/ΔD, and (2) the dependence of the threshold voltage Vth on [(D);ΔVth/ΔD]. The dependence of Imax(D) and Imax/Vmax(D) were determined from the experimental data for the three different diamond coatings. The maximum current Imax is very different for these three different coatings and is also a function of the coating thickness, D. Both the maximum current and the transconductance of field emission tips can be increased significantly by diamond coatings. A strong, nearly linear, dependence of Imax on diamond thickness was found. An empirical estimate of the thermal conductivity of nanodiamond, based on the field emission data...

Field emission from aluminum nitride and cubic boron nitride coatings

Recent studies have shown that thin layers of wide band gap materials such as silicon carbide and diamond can improve electron emission from of sharp field emitters. Aluminum and boron nitrides possess characteristics similar to those of diamond, e.g. chemical and mechanical stability, a wide band gap (from 6-7 eV), a reported negative electron affinity, and the ability to be doped p-type. There is also the possibility of n-type doping. In this study we investigate the field emission properties of these III-V nitrides deposited onto silicon and molybdenum by both reactive magnetron sputtering and by dielectrophoresis. Emission properties for diamond, AlN, and c-BN are compared and are projected to other wide band gap materials.

The effects of the interface and surface treatment on the electron emission from diamond coated field emitters

To improve the performance of molybdenum and silicon field emitters, thin diamond layers were deposited on needles by dielectrophoresis. Field emission characteristics were investigated before and after diamond deposition. SEM and TEM observation demonstrated that a significant amount of diamond was deposited. The emissivity depended upon the thickness deposited, the thermal treatment of the diamond after deposition. The influence of the emitter/diamond interface and the surface treatment of the diamond, are reported here, along with a discussion of the possible mechanisms.