M. T. McClure

The origin of the broadband luminescence and the effect of nitrogen doping on the optical properties of diamond films

Raman and various photoluminescence (PL) techniques were employed to investigate the role of nitrogen doping on the optical spectra of chemical‐vapor‐deposited (CVD) diamond films and to determine the origin of the characteristic broadband luminescence which is observed from approximately 1.5 to 2.5 eV and centered at ∼2 eV. The PL transitions attributed to the zero‐phonon lines (ZPL) of nitrogen centers are observed at 1.945 and 2.154 eV. A new possible nitrogen center at 1.967 eV is also observed as well as the band A luminescence centered at ∼2.46 eV. The experimental results preclude the possibility of the broadband PL being due to electron‐lattice interaction of the nitrogen ZPL centers. We establish the presence of an in‐gap state distribution in CVD diamond films attributed to the sp2 disordered phase and show that its optical transitions are the likely cause of the broadband luminescence. A model of the in‐gap state distribution is presented which is similar to models previously developed for amor...

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.

Energy distribution of field emitted electrons from diamond coated molybdenum tips

Field emission energy distribution (FEED) measurements were performed on Mo and diamond coated Mo tips under ultrahigh vacuum conditions to investigate the origin of field emitted electrons. Mo emitters were prepared by electrochemical etching and were subsequently coated with diamond powder by a dielectrophoretic procedure. Field emission energy spectra were taken on the same samples before and after diamond coating. In vacuo thermal annealing of coated samples was essential to obtain stable field emission. FEED data indicated that the field emission current originated from the diamond/vacuum interface, and that electrons were emitted from the conduction band minimum of diamond.

Bias voltage dependent field-emission energy distribution analysis of wide band-gap field emitters

We have studied the origin of field emission from wide band-gap semiconductors by a combination of voltage dependent field-emission energy distribution and I–V measurements. For this purpose, tip-shaped molybdenum emitters were coated with 100–1000 nm thick layers of nominally undoped diamond and cubic boron nitride (c-BN) powders. Electron energy spectra revealed that significant band bending occurred due to field penetration into wide band-gap materials. Voltage drops on the order of several volts were measured across the coatings, for applied voltages on the order of 1 kV, and a cathode–gate distance of 500 μm. These voltage drops showed a linear dependence with the applied bias voltage for well-annealed diamond coatings and a strongly nonlinear behavior for unannealed diamond and c-BN coatings. In general, annealing of diamond coated Mo tips led to improved emission current stability and lower “turn-on” voltages due to the removal of oxide and the formation of conductive carbide layers between the met...

Field emission from diamond coated molybdenum field emitters

Diamond deposition onto single Mo field emitters was accomplished by two methods: microwave plasma chemical vapor deposition and a dielectrophoresis of diamond powder. Observation by transmission electron microscopy and scanning electron microscopy revealed a significant amount of deposition at the tips. The field emission characteristics were measured before and after diamond deposition on the same emitters. Field emission from diamond coated emitters yielded significant increases in emission current and lower Fowler–Nordheim slopes. We discuss a possible mechanism to explain current enhancement that depends primarily upon the Mo‐diamond interface.

Bias‐enhanced nucleation of highly oriented diamond on titanium carbide (111) substrates

The bias‐enhanced nucleation (BEN) technique has been applied to TiC(111) substrates and resulted in deposition of oriented diamond particles. The orientation was observed via scanning electron microscopy. A dense region of oriented particles was not observed on the samples, presumably due to the excessive twinning of the diamond. However, micrographs taken throughout the substrate showed diamond particles having similar orientation with respect to each other. Some of the diamond particles showed evidence of azimuthal twist and tilting, resulting most likely from the ∼21% lattice mismatch. Raman spectra of the diamond crystals show a strong feature at 1332 cm−1, which is indicative of diamond, and smaller features at 1480 and 1602 cm−1 due to sp2‐bonded carbon.

Mechanisms of field emission from diamond coated Mo emitters

Abstract A combination of field emission energy distribution (FEED) and I–V measurements was used to study the field emission mechanisms of tip-shaped molybdenum emitters electrophoretically coated with nominally intrinsic diamond powders. Field-induced band bending was studied as a function of applied voltage and was interpreted in terms of a two-barrier model. Field emitted electrons originated from the conduction band minimum of diamond. Electron injection at the Mo/diamond interface was identified as the dominant field emission current limiting factor. It was concluded that potential negative electron affinity (NEA) properties of diamond did not contribute to a current enhancement. The latter statement was confirmed by the observation that graphite coatings enhanced emission currents in a similar way to diamond coatings.

Electron emission mechanism from cubic boron nitride-coated molybdenum emitters

The energy distribution of field-emitted electrons from Mo tips coated with intrinsic cubic boron nitride (c-BN) was studied in an effort to determine the origin of the emitted electrons. Voltage-dependent field-emission energy distribution (V-FEED) spectra were collected from the Mo emitters under ultra-high-vacuum conditions both before and after being coated. Emission current at a given voltage increased by as much as two orders of magnitude for the c-BN-coated emitters relative to bare emitters. The energy of field-emitted electrons from the c-BN-coated emitters was linearly dependent upon the applied voltage. Extrapolation of V-FEED data from c-BN-coated emitters to the flatband condition evidenced that the electrons were emitted from the conduction-band minimum of the c-BN coating at the c-BN/vacuum interface.

Highly oriented diamond deposited using a low pressure flat flame

Abstract A multi-step process for the achievement of highly oriented, 〈100〉 textured diamond films on silicon using flat flame deposition has been developed. First, a bias-enhanced technique was used to achieve oriented nuclei on a Si 〈100〉 substrate in a microwave plasma reactor. Substrates were then transferred to the combustion system and rapidly grown into coalesced 〈100〉 films at a growth rate of 4–5 μm/h. X-ray texture analysis was used to characterize the films. It showed a 12 ° misalignment of the crystallites with respect to the surface normal, while the azimuthal misalignment was measured to be 20 °.