Electron emission from polycrystalline diamond particles
formed by MWP-CVD
Kenji Nose, Masao Kamiko and Yoshitaka Mitsuda
Institute of Industrial Science, The University of Tokyo.
4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, JAPAN.
Abstract: We report on efficient electron emission from non-doped diamond particles.
Polycrystalline diamond particles (PDPs) were synthesized on silicon substrate by microwave
plasma chemical vapor deposition (MWP-CVD). Bias enhanced nucleation (BEN) method in a
relatively high methane concentration (7 %) successfully increased the number of small crystals
with fine facets on the PDP surfaces. Electron emission from the PDPs was observed at low
electric fields (< 2 V/μm), and the threshold was reduced to 0.8 V/μm by increasing the number of
facets on the PDPs. The emission was maintained for over 24 hours at a constant electric field of
3.25 V/μm. Raman spectroscopy and Auger electron spectroscopy indicated that sp2-bonded grain
boundaries accounted for the electron transport from the substrate to the surface of the PDPs.
These results suggest that the intrinsic diamond particles without dopants are applicable to
electron emitters by controlling the phase and the structures on the surface.
Keywords: field emission, microwave plasma CVD, diamond thin film, negative electron affinity
1. Introduction
Field emission (FE) of electron from solid state
materials is extensively studied to achieve flat panel
displays and light sources with high energy efficiencies.
Diamond is also an attractive material for FE because of
its negative electron affinity (NEA) and chemical and
physical stabilities. When the dangling bonds of the
diamond surface are terminated by hydrogen, electron
affinity at the vicinity of the surface is modified to be
negative, which enhances the FE efficiency.[1] In addition,
the strong covalent bond of diamond is preferable to keep
the fine structures of the emission tips. These natures of
diamond can realize an efficient FE with a long operating
life.
As a matter of course, FE devices are essentially
composed of electrically conductive materials. Therefore,
diamond is intentionally doped by impurities such as B, P
and N to enable the carrier transport from the cathode to
the top of the emitter tips. It has been reported that FE
occurs at a very low electric field of 0.4 V/μm from
nitrogen-doped diamond.[2] However, the electrical band
diagram at the surface and the mechanisms of the
emission is an open question at this stage.
Structures of the emission tips are also important
for the high-efficient FE. A strong field enhancement is
achieved by introducing sharp tips to the surface of the
cold cathode. Pyramid, whisker and spike structures have
been intentionally formed on the surface of diamond so
far.[3] However, these structures formed by etching and
other ex-situ processes do not realize ideal sharpness of
the tips, and high field enhancement factors (β) are often
difficult to be achieved. Additionally, chemical etching in
these process can modify the surface termination,
resulting in a degradation of the overall emission
properties.[4]
We have been paying attention to FE from intrinsic
crystals of diamond as a FE tip. In this study, bias
enhanced nucleation was utilized to increase the number
density of the fine facets on the surface of diamond
particles. FE properties of these polycrystalline diamond
particles (PDPs) were examined on the basis of their
unique structures.
2. Experimental
An NIRIM (National Institute for Research in Inorganic
Materials)-type microwave plasma-chemical vapor
deposition (MWP-CVD) chamber was used to synthesize
PDPs on silicon.[5] A schematic diagram of the deposition
chamber is shown in Fig. 1. (100)-oriented p-type silicon
(10 Ωcm) substrates with a size of 10×10 mm2 were
ultrasonically cleaned in acetone, ethanol, and deionized
water. The natural oxidized layer of the substrate was
removed by diluted-HF. The cleaned substrate was
mounted on a 17 mm-diameter quartz stage by a Mo
cover with a hole of 9 mm-diameter at the center. The
total flow of gas, methane diluted by hydrogen, was 100
sccm. No doping gas was introduced into the chamber.
The pressure and MW power were maintained to be 6 kPa
and 400 W during the deposition. The substrate
temperatures were 1200 to 1240 K, measured with a
pyrometer (MINOLTA, TR-630) through a quartz
window. Structure of the PDPs were controlled by the
substrate temperature.
A three-step deposition procedure was applied for
the fabrication of the PDPs. Methane concentration was
2 % in the first step for 60 min. Secondary, in BEN
process, a negative dc bias of 230 V was applied to the
substrate for enhancing nucleation in 7 %-methane for 15
min. In the final step, the nuclei were grown in 0.5 %
methane for 15 or 60 min. As a reference, polyhedral
single-crystalline diamond particles (SDPs) were
deposited without the second step. The emission
properties of the as-deposited PDPs and the SDPs were
compared to each other.
The shapes and number densities of the particles
were analyzed by a scanning electron microscope (SEM,
JEOL, JSM-6330F). Auger electron spectroscopy (AES,
JEOL, JAMP-7830F) was applied for analyzing the
surface termination of particles. Raman spectroscopy
(JOBIN YVON, T-64000) was performed by a green laser
(LEXCEL 95, λ=514.5 nm) with a spot size of 1 μm
using a 100× objective.
Emission properties were measured by currentvoltage (I-V) characteristic in a parallel-plate
configuration (Fig. 2). The measurement chamber was
evacuated by a turbo-molecular pump to pressures (P)
less than 3×10-6 Pa. A copper anode with 5 mm in
diameter was used as a counter electrode. I-V
measurements were performed by an electrometer (ADC,
R8252). The anode and cathode were spatially separated
by a polyimide film with a thickness of 80 μm. The
electric field, E, was calculated as E=V/t. The voltage was
scanned from 0 to 199 V and the reverse direction for 5
times, and the average I-V curve was determined.
A fluorescent material (OSRAM SYLVANIA, P15#137)-coated indium tin oxide (ITO)/glass plate was
used for a macroscopic observation of emission spots
through an optical view port. A dc power supply
(TAKASAGO, ZX-400H), digital multimeter (ADC,
7461A) and a series resistance of 100 KΩ were used to
evaluate the stability of the emission at E > 2.5 V/ μm for
over 24 hours. Macroscopic distribution of the emission
spots on the cathode surface was observed by a CCD
camera.
3. Results
Surface density of the PDPs formed by BEN on the
Si cathode was approximately 1010 cm-2, which was six
orders of magnitude higher than that on an untreated Si
surface. SEM images of the PDPs are shown in Fig. 3
(a,b,c,d). We found two different sizes of the PDPs on the
surface, i.e., .3 μm and 250 nm. All the PDPs were
covered by fine facets of diamond crystals as shown in the
Figure 1. Schematic diagram of the deposition
chamber. Substrate dc bias was applied to the substrate.
Figure 2. Measurement setup for the electron emission.
enlarged views of the surfaces. As a counterpart,
polyhedral SDPs were achieved without BEN process as
shown in Fig. 3-(e). The lower substrate temperature
resulted in the more simple structures of the SDPs, i.e.,
polyhedrons with fewer facets as shown later. The size of
the SDPs was similar to that of the larger PDPs. These
results indicate that small facets on the surface of the both
sizes of PDPs were grown in the 2nd step, i.e., the BEN
with the relatively high methane concentration. PDPs with
a little bit fewer facets on the surface were also achieved
without the BEN process when the substrate temperature
was reduced from 1240 to 1200 K.
Raman spectra of the PDPs and the SDPs indicated
that both of them are composed of diamond and graphite
phase (Fig. 4). These spectra were similar to those of
polycrystalline diamond films. The weaker diamond peak
and more prominent graphite peak in the PDPs than the
SDPs support that the initial SDPs were covered by small
crystals with fine facets by the BEN process, which
resulted in the unique shape of the PDPs with small
crystals. It was expected that the graphitic phase existed
mainly in grain boundaries of small crystals on the
surface of the PDPs. As a proof, a stronger graphite peak
was found when we measured the agglomerates of the
smaller PDPs in Raman spectroscopy (not shown).
Emission currents from the cathode coated with the
PDPs and the SDPs were shown in Fig. 5. It was
reproducibly observed in more than 20 samples that
current rapidly increased at electric fields less than 2.0
V/μm for the PDPs (a,b). The typical threshold E was 0.8
V/μm for the PDPs with the relatively larger numbers of
a)
b)
c)
d)
e)
Figure 3. SEM images of (a,b)
the smaller and (c,d) the
larger PDPs deposited by the
BEN. Both the PDPs were
covered by the crystals with
fine facets as shown in
enlarged views (b, d). SDP is
shown in (e).
Figure 4. Raman spectra of single PDP and SDP.
facets on the surface like (a). The threshold E was
increased when the number of facets on the surface of the
particles was decreased (b). Furthermore, no emission
was detected in the polyhedral SDPs without small facets
on the surface (c). The Fowler-Nordheim plot of the I-V
characteristics of the PDPs indicates that the emission was
ruled by an electron tunneling of a barrier at the surface as
shown in Fig. 6. The barrier height was evaluated to be
0.03 eV. If we assume a work function of 1 eV at the
surface of the PDPs, β was calculated to be 200.
The emission current was enough stable to be
maintained for over 24 hours as shown in Fig. 7. It is
worth noting that that emission current became more
stable when E was adjusted from a stronger value to a
weaker. Meanwhile, the emission was unstable after E
was increased. This phenomenon is typically observed in
the FE from sharp metal tips.
Figure 5. Structures of the diamond particles and the
emission current plotted against the applied electric
field.
Figure 6. Fowler-Nordheim plot of the emission current
( J/E2 – 1/E ).
Surface distributions of the FE spots are shown in
Fig. 8. Bright points on the fluorescent screen distributed
inhomogeneously, indicating the same distribution of the
electron emission points. The number of the emission
points and the intensity increased as the E increased. The
ratio of the emission spots to the number of the PDPs was
calculated to be very small (~1/106), but the distribution
of the emission spots was roughly consistent with that of
the number density of the PDPs on the surface.
It is interesting to note that thermal annealing of
the PDPs in the air did not result in the degradation of
emission properties, but the FE operation in PO2=0.1 Pa
was immediately terminated the FE. The AES clarified
that the surface of the PDPs were terminated by hydrogen
atoms even in the atmospheric pressure and after this
operation at the high PO2. Hydrogen termination of
diamond is known to be stable up to the temperature
around 1300 K, and these results suggests that the
degradation of the FE properties occurred not by the
change of the surface termination but by the modification
of the fine structures of the tips by the ion irradiation
under the FE operation.
4. Discussion
The FE at E < 2 V/μm from the PDPs and no
emission from SDPs with the simple polyhedral structures
suggest that the great number density of the potential
emission points on the surface effectively reduced the
threshold E. This phenomenon has been previously
observed in Mo cone arrays.[6] On the cathode made of the
PDPs, the number density of the protrusions composed of
the fin facets reached 1011 to 1012 cm-2, which was much
greater than those of the SDPs deposited without the BEN
process.
The exact barrier height for electron at the surface
is difficult to be determined, but the low threshold E for
the emission indicates an small electron affinity of the
surface and a strong field enhancement. High β is
achievable in FE tips with high aspect ratios. For example,
β=800 was reported in an ideal single CNT.[7] The PDPs
do not have any high aspect ratios as shown in the SEM
images (Figs. 3 and 5), but the crystals with fine facets in
the tips of the PDPs might have ideally large curvatures,
accounting for the high β. We speculate that the apexes of
diamond crystals are much sharper than artificial cone
tips[8] made by lithography and following chemical
etching because the modification of the chemical bonds
from sp3 to sp2 can easily take place in these ex-situ
processes.
It is normally impossible to cause the electron
emission from non-doped diamond particles because of its
dielectric property. However, many researchers reported
electron emission from non-doped diamond films. In the
comparison of the emission properties of the SDPs and
the PDPs, we suggest that sp2-bonded phase in grain
boundaries took part in the electron transport from the
substrate to the emission points. In a previous report, it
was suggested that the contamination of sp2-bonded grain
boundaries to diamond films reduced the potential barrier
of the FE.[9] On the basis of these results, it is speculated
that the unique structure of the PDPs and electrically
conductive grain boundaries can account for the high
efficient emission in this study.
5. Conclusion
Electron emission was observed in the PDPs from
0.8 to 2.0 V/μm. The structure of the particle was
characterized by the large number of the protrusions made
of the crystalline facets on its surface, which might
enhance the FE at low E. The sp2–bonded grain
boundaries could account for the carrier transport in the
intrinsic diamond particles from the substrates. The more
efficient diamond emitters would be achieved by
controlling the directions and the structures of the fine
facets on the PDPs.
Figure 7. Continuous electron emission from PDP
sample. Emission current greater than 6 μA was
maintained for over 24 hours.
Figure 8. Light emission from a device composed of a
commercial fluorescent screen and the PDP emitter.
Red circles depict the window of the anode. The light
from the fluorescent screen was enough strong to be
identified in a room lighting.
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