Electron Field Emission Process Research Articles

Structural modification of nanocrystalline diamond films via positive/negative bias enhanced nucleation and growth processes for improving their electron field emission properties

Electron field emission (EFE) properties of nanocrystalline diamond (NCD) films synthesized by the bias-enhanced growth (beg) process under different bias voltages were investigated. The induction of the nanographitic phases is presumed to be the prime factor in enhancing the EFE properties of negative biased NCD films. Transmission electron microscopic investigations reveal that a negative bias voltage of −300 V increases the rate of growth for NCD films with the size of the grains changing from nano to ultranano size. This effect also is accompanied by the induction of nanographitic filaments in the grain boundaries of the films. The turn-on field (E0) for the EFE process then effectively gets reduced. The EFE process of the beg-NCD−300V films can be turned on at E0 = 3.86 V/μm, and the EFE current density achieved is 1.49 mA/cm2 at an applied field of 7.85 V/μm. On the other hand, though a positive-bias beg process (+200 V) results in the reduction of grain size, it does not induce sufficient nanographitic phases to lower the E0 value of the EFE process. Moreover, the optical emission spectroscopic investigation indicates that one of the primary causes that changes the granular structure of the NCD films is the increase in the proportion of C2 and CH species induced in the growing plasma. The polarity of the bias voltage is of less importance in the microstructural evolution of the films.

Direct observation and mechanism for enhanced electron emission in hydrogen plasma-treated diamond nanowire films.

The effect of hydrogen plasma treatment on the electrical conductivity and electron field emission (EFE) properties for diamond nanowire (DNW) films were systematically investigated. The DNW films were deposited on silicon substrate by N2-based microwave plasma-enhanced chemical vapor deposition process. Transmission electron microscopy depicted that DNW films mainly consist of wirelike diamond nanocrystals encased in a nanographitic sheath, which formed conduction channels for efficient electron transport and hence lead to excellent electrical conductivity and EFE properties for these films. Hydrogen plasma treatment initially enhanced the electrical conductivity and EFE properties of DNW films and then degraded with an increase in treatment time. Scanning tunneling spectroscopy in current imaging tunneling spectroscopy mode clearly shows significant increase in local emission sites in 10 min hydrogen plasma treated diamond nanowire (DNW10) films as compared to the pristine films that is ascribed to the formation of graphitic phase around the DNWs due to the hydrogen plasma treatment process. The degradation in EFE properties of extended (15 min) hydrogen plasma-treated DNW films was explained by the removal of nanographitic phase surrounding the DNWs. The EFE process of DNW10 films can be turned on at a low field of 4.2 V/μm and achieved a high EFE current density of 5.1 mA/cm(2) at an applied field of 8.5 V/μm. Moreover, DNW10 films with high electrical conductivity of 216 (Ω cm)(-1) overwhelm that of other kinds of UNCD films and will create a remarkable impact to diamond-based electronics.

Improvement in plasma illumination properties of ultrananocrystalline diamond films by grain boundary engineering

Microstructural evolution of ultrananocrystalline diamond (UNCD) films as a function of substrate temperature (TS) and/or by introducing H2 in Ar/CH4 plasma is investigated. Variation of the sp2 and sp3 carbon content is analyzed using UV-Raman and near-edge X-ray absorption fine structure spectra. Morphological and microstructural studies confirm that films deposited using Ar/CH4 plasma at low TS consist of a random distribution of spherically shaped ultra-nano diamond grains with distinct sp2-bonded grain boundaries, which are attributed to the adherence of CH radicals to the nano-sized diamond clusters. By increasing TS, adhering efficiency of CH radicals to the diamond lattice drops and trans-polyacetylene (t-PA) encapsulating the nano-sized diamond grains break, whereas the addition of 1.5% H2 in Ar/CH4 plasma at low TS induces atomic hydrogen that preferentially etches out the t-PA attached to ultra-nano diamond grains. Both cases make the sp3-diamond phase less passivated. This leads to C2 radicals attaching to the diamond lattice promoting elongated clustered grains along with a complicated defect structure. Such a grain growth model is highly correlated to explain the technologically important functional property, namely, plasma illumination (PI) of UNCD films. Superior PI properties, viz. low threshold field of 0.21 V/μm with a high PI current density of 4.10 mA/cm2 (at an applied field of 0.25 V/μm) and high γ-coefficient (0.2604) are observed for the UNCD films possessing ultra-nano grains with a large fraction of grain boundary phases. The grain boundary component consists of a large amount of sp2-carbon phases that possibly form interconnected paths for facilitating the transport of electrons and the electron field emission process that markedly enhance PI properties.

The role of nano-graphite phase on enhancing the plasma illumination characteristics of the diamond-coated inverted pyramidal cavities

The enhancement on the plasma illumination characteristics of inverted pyramidal cavity (IPC) devices due to the utilization of diamond films as cathodes was illustrated. The performance of the plasma illumination characteristics of the IPC devices was closely related to the electron field emission (EFE) properties of the cathode materials. The duplex-structured microcrystalline/ultrananocrystalline (MCD/UNCD) diamond films, which possessed lower turn-on field for inducing the EFE process (E0=10.9V/μm), resulted in the better plasma illumination performance for the IPC devices. The plasma can be triggered at the lower threshold field of Eth=2300V/mm. Transmission electron microscopic studies revealed that the MCD/UNCD films contain nano-graphite clusters, which facilitated the transport of electrons and were presumed to be the prime factor, resulting in superior EFE properties for the MCD/UNCD films and the better plasma illumination characteristics for the IPC devices.

N-ion implantation of micro‐nanocrystalline duplex structured diamond films for enhancing their electron field emission properties

The improvement on the electron field emission (EFE) properties of duplex-structured diamond films by N-ion implantation/post-annealing processes was investigated. The duplex-structured diamond films were synthesized by a two-step microwave plasma enhanced CVD process. Transmission electron microscopy (TEM) examinations reveal that all the as-prepared, N-ion implanted and post-annealed diamond films contained large microcrystalline-diamond (MCD) aggregates sparsely distributed among the matrix of ultra-small diamond grains. While the granular structure of the MCD aggregates was insignificantly modified due to the N-ion implantation/post-annealing processes, that of the UNCD regions was markedly altered. The EFE process for the MCD/UNCD films can be turned on at (E0)MCD/UNCD=8.21V/μm, which is even smaller than the E0-field for the UNCD films ((E0)UNCD=13.34V/μm). These N-ion implanted/post-annealed diamond films attained an EFE current density of (Je)MCD/UNCD=0.4mA/cm2 at an applied field of 20.0V/μm that is even larger than the Je-value for the UNCD films ((Je)UNCD<0.05mA/cm2 at the same applied field). Presumably, the enhanced EFE properties are resulted from the presence of nano-graphites in the small-grain region of MCD/UNCD films that form an interconnected path, facilitating the transport of electrons.

Bias-enhanced nucleation and growth processes for improving the electron field emission properties of diamond films

The evolution of diamond films in bias-enhanced-nucleation (BEN) and bias-enhanced-growth (BEG) processes was systematically investigated. While the BEN process can efficiently form diamond nuclei on the Si substrates, BEG with large enough applied field (&amp;gt; –400 V) and for sufficiently long periods (&amp;gt;60 min) was needed to develop proper granular structure for the diamond films so as to enhance the electron field emission (EFE) properties of the films. For the films BEG under −400 V for 60 min (after BEN for 10 min), the EFE process can be turned on at a field as small as 3.6 V/μm, attaining a EFE current density as large as 325 μA/cm2 at an applied field of 15 V/μm. Such an EFE behavior is even better than that of the ultrananocrystalline diamond films grown in CH4/Ar plasma. Transmission electron microscopic examination reveals that the prime factor enhancing the EFE properties of these films is the induction of the nano-graphite filaments along the thickness of the films that facilitates the transport of electrons through the films.

The induction of a graphite-like phase by Fe-coating/post-annealing process to improve the electron field emission properties of ultrananocrystalline diamond films

The electron field emission (EFE) process for ultrananocrystalline diamond (UNCD) films was tremendously enhanced by Fe-coating and post-annealing processes. The extent of enhancement changes with the granular structure of the UNCD films and the post-annealing conditions (temperature and atmosphere). The best EFE properties are obtained by post-annealing the films at 900°C in an H2 environment for 5min. The EFE behavior of the films can be turned on at E0=1.28V/μm, attaining a large EFE current density of 772μA/cm2 at an applied field of 8.8V/μm. Microstructural analysis indicates that the mechanism for the improvement in the EFE process is the formation of graphene-like phase (a-few-layer graphite) with good crystallinity, surrounding the Fe (or Fe3C) nanoclusters. Presumably, the nanographites were formed via the reaction of Fe-clusters with diamond films, viz. the Fe-clusters dissolved the carbons in the diamond grains and the re-precipitated them on the surface of the other side of clusters, a process similar to the growth of carbon nanotubes via Fe clusters as catalyst.

The role of nano-graphite phase on the enhancement of electron field emission properties of ultrananocrystalline diamond films

Here, we demonstrated with examples that the presence of nano-graphite phase markedly enhanced the electron field emission (EFE) properties of the ultrananocrystaline diamond films. The nano-graphite phase was induced by different mechanism. In the two-step microwave plasma enhanced CVD (MPECVD) process, the nano-graphite phase was induced at the interfaces of large-grain and ultra-small-grains regions, which was clearly illustrated by transmission electron microscopy (TEM). The EFE process was thus markedly enhanced, viz. low turn-on field of 9.2V/μm with large emission current density of 800μA/cm2 at 27.5V/μm was achieved. In the Au-ion irradiation of UNCD films, the presence of nano-graphite phase was not clearly resolved by TEM but was strongly inferred by the Raman spectroscopy. The induction of graphitic phase was also believed to be the factor, resulting in marvelous EFE properties for these UNCD films. In the two-step MPECVD and Au-ion irradiation processes, the nano-graphite phase was presumably induced by the grain growth phenomenon. However, the most marked enhancement on the EFE properties for the UNCD films is resulted from the utilization of CH4/H2 plasma to replace the CH4/Ar plasma in MPECVD process. The nano-graphite phase was induced, forming an encapsulation layer surrounding the needle-like diamond grains. The EFE process of the nitrogen-doped UNCD films can be turned on at 8.3V/μm, achieving 2160.0μA/cm2 EFE capacities at an applied field of 18.0V/μm.

Atomic layer coating of hafnium oxide on carbon nanotubes for high-performance field emitters

Carbon nanotubes coated with hafnium oxide exhibit excellent electron emission characteristics, including a low turn-on voltage, a high field enhancement factor, and exceptional current stability. Their enhanced emission performance was attributed to a decrease in the work function and an increase in the electron density of states at the carbon nanotube Fermi level closest to the conduction band minimum of hafnium oxide. In addition, the enhanced current stability was attributed to the ability of hafnium oxide to protect the carbon nanotubes against ions and free radicals created in the electron field emission process.

The induction of a graphite-like phase on diamond films by a Fe-coating/post-annealing process to improve their electron field emission properties

The electron field emission (EFE) process for diamond films was tremendously enhanced by Fe-coating and post-annealing processes. Microstructural analysis indicates that the mechanism for the improvement in the EFE process is the formation of nanographites with good crystallinity that surround the Fe (or Fe3C) nanoclusters. Presumably the nanographites were formed via the reaction of Fe clusters with diamond films, viz. by the dissolution of carbons into Fe (or Fe3C) clusters and the reprecipitation of carbon species to the surface of the clusters, a process similar to the growth of carbon nanotubes via Fe clusters as catalyst. Not only is a sufficiently high post-annealing temperature (900°C) required but also a highly active reducing atmosphere (NH3) is needed to give a proper microstructure for enhancing the EFE process. The best EFE properties are obtained by post-annealing the Fe-coated diamond films at 900°C in an NH3 environment for 5 min. The EFE behavior of the films can be turned on at E0 = 1.9 V/μm, attaining a large EFE current density of 315 μA/cm2 at an applied field of 8.8 V/μm (extrapolation using the Fowler–Nordheim model leads to Je = 40.7 mA/cm2 at a 20 V/μm applied field).

Investigation in the role of hydrogen on the properties of diamond films grown using Ar/H 2/CH 4 microwave plasma

The transition of diamond grain sizes from micron- to nano- and then to ultranano-size could be observed when hydrogen concentration is being decreased in the Ar/CH 4 plasma. When grown in H 2-rich plasma (H 2 = 99% or 50%), well faceted microcrystalline diamond (MCD) surface with grain sizes of less than 0.1 μm are observed. The surface structure of the diamond film changes to a cauliflower-like geometry with a grain size of around 20 nm for the films grown in 25% H 2-plasma. In the Ar/CH 4 plasma, ultrananocrystalline diamond (UNCD) films are produced with equi-axed geometry with a grain size of 5–10 nm. The H 2-content imposes a more striking effect on the granular structure of diamond films than the substrate temperature. The induction of the grain growth process, either by using H 2-rich plasma or a higher substrate temperature increases the turn-on field in the electron field emission process, which is ascribed to the reduction in the proportion of grain boundaries.

Stacked Silicon Nanowires with Improved Field Enhancement Factor

This work describes newly structured stacked silicon nanowires (s-SiNWs), consisting of nanosized silicon wires on top of silicon microrods (SiMRs) and exhibiting pronouncedly superior electron field emission (EFE) characteristics to the conventional SiNWs, by using a two-step electroless metal deposition process. Experimental results indicate that for these s-SiNWs, the electrostatic "screen effect" is markedly suppressed and the field enhancement factor (beta-value) is significantly increased ((beta)(s-SiNWs) = 2533). Additionally, the turn-on field (E(0)) for triggering the EFE process is reduced to a level comparable with that of carbon nanotubes, viz. (E(0))(s-SiNWs) = 2.0 V/mum. This simple and robust modified electroless metal deposition approach does not require either a high temperature or an expensive photolithographic process and possesses great potential for applications.

Effect of N 2 addition in Ar plasma on the development of microstructure of ultra-nanocrystalline diamond films

The effect of the N 2 and H 2 addition in Ar plasma on the characteristics of the UNCD films was systematically investigated. It is found that, while the N 2/Ar plasma results in UNCD films with ultra-small grains (~ 5 nm), incorporation of H 2 into the N 2/Ar plasma increased monotonously the size of the grains. Moreover, the diamond grains synthesized in H 2 free plasma are of equi-axed geometry and those grown in H 2-containing plasma are of plate-like one. The optical emission spectroscopic investigation indicated that the increase in electron temperature due to the addition of H 2 into Ar plasma is the main cause, altering the microstructure of the UNCD films. As the H 2 content increases, the spherical diamond grains first agglomerated to form elongated grains, which coalesce to form dendrite clusters. The proportion of grain boundaries is thus decreased that increased the turn-on field necessary for inducing the electron field emission process.

Growth behavior of nanocrystalline diamond films on ultrananocrystalline diamond nuclei: The transmission electron microscopy studies

Micron-crystalline diamond (MCD) films with a unique microstructure were synthesized using a modified nucleation and growth process, in which a thin layer of ultrananocrystalline diamond (UNCD) was used as nucleation layer for growing diamond films in H2-plasma. Thus obtained (MCD)UNCD diamond films consist of nanosized diamond clusters (∼10 nm in size) surrounding the large diamond grains (∼300 nm in size), exhibiting better electron field emission (EFE) properties than the conventional diamond materials with faceted grains. The EFE of these (MCD)UNCD films can be turned on at E0=11.1 V/μm, achieving EFE current density as large as (Je)=0.7 mA/cm2 at 25 V/μm applied field, which can be attributed to the presence of large proportion of UNCD grains lying in between the MCD grains, forming an electron conduction path and thus facilitating the EFE process. Transmission electron microscopy examinations reveal that such a unique microstructure was formed by agglomeration and coalescence of the nanosized UNCD grains.

Modeling of the electron field emission process in polycrystalline diamond and diamond-like carbon thin films

Electron field emission has been observed from carbon thin films at relatively low electric fields. These films range from amorphous carbon to polycrystalline diamond films. There are many models that attempt to account for the electron field emission process observed in these films. The initial models that were based on the emission due purely to a negative electron affinity have now been modified. The emission from diamond like carbon (DLC) films, although following a Fowler–Nordheim type curve, do not give realistic values for the emission areas or barriers purely based on a tunneling mechanism. Therefore, a model based on space charge band bending at the back junction is proposed to account for the electron emission at low electric fields from DLC. In this “space charge interlayer” model the real cathode is the substrate, from which hot electrons are created due to the fully depleted DLC film the electrons encounter before reaching the front surface of the film. In this article we extend the model to incorporate the emission of electrons from polycrystalline diamond thin films.

Self-texturing of nitrogenated amorphous carbon thin films for electron field emission

The electron field-emission process for nitrogenated amorphous carbon (a-C:H:N) thin films deposited using a magnetically confined hydrocarbon plasma is examined. The morphology of the films obtained using an atomic force microscope is compared to the field-emission properties. Beyond a chemical composition of 14 at. % nitrogen, the mirror smooth a-C:H:N films become self-texturing, and multiple “domelike” cathodes of nanometer scale are observed. The dimensions of these “domelike” cathodes varies with time, and after a 15 min deposition have dimensions of approximately 50 nm base diameter and 20 nm in height. When the electronic field emission of these textured films (N content 15 at. %) are measured, there is an enhancement in the emitted current density of ∼2 orders of magnitude at an electric field of 20 V/μm, in comparison to the untextured films with a nitrogen content of 11 at. %.

Evidence for surface-state-enhanced field emission in rf superconducting cavities

Measurements of the x radiation and the electron loading in a niobium cavity resonant in the TM010 mode at 1208 MHz have been made in order to study enhanced electron field emission in superconducting cavities. Measurements have also been made of the low-temperature dc electron field emission from bismuth, niobium, copper, and tungsten utilizing a configuration which approximates the point-to-plane geometry. In the rf cavity measurements, it is found that the x-radiation count rate data follow a modified Fowler-Nordheim equation, which takes into account the time-dependent rf fields, the electron dynamics in the rf fields, the emission of secondary electrons, and the production and absorption of x radiation. Further, it is found that the x radiation from the cavity can be reduced by a factor of 104−106 by helium-ion sputter processing at low temperature. From x-radiation data and electron loading data interpreted with the modified Fowler-Nordheim equation, it has been shown that, after helium-ion sputter processing at low temperature, the electron field emission process can be consistently described by ordinary Fowler-Nordheim electron field emission from geometrical projections at which the electric field is enhanced. Before processing, the electron field emission process cannot be consistently described by ordinary Fowler-Nordheim electron field emission. A point of view which is consistent with the data is that before helium-ion sputter processing the electron field emission is enhanced by resonant tunneling through surface states due to adsorbed atoms, and that as a result of processing, the adsorbed atoms are removed. This point of view is supported by calculated sputtering rates during the processing. In the dc measurements, it is found that the low-temperature electron field emission is consistent with the Fowler-Nordheim theory for an emission tip that has been cleaned by reverse field desorption. However, it is found that the dc electron field emission increases slowly with time, presumably as a result of adsorption of material ejected by electron bombardment of the anode. The resulting changes in the dc electron field emission characteristics are of just the type expected for resonant tunneling through adsorbate surface states, and are remarkably similar to the differences in x-radiation count rate data observed for a superconducting cavity before and after helium-ion sputter processing.