Electron field emission from nanostructured cubic boron nitride islands

Article abstract of DOI:10.1063/1.2830006

Nanocrystal-assembled cubic boron nitride (cBN) islands are formed by using low-energy (～20 eV) ion irradiation in an inductively coupled fluorine-containing plasma. The temporal evolution of surface morphology and roughness reveals three-dimensional island growth for initial s p2 -bonded BN and subsequent cBN, accompanied by a high frequency of renucleation. The formation of cBN islands enhances the field emission and reduces the turn-on field down to around 9 Vμm due to an increase in the island-related field. The results demonstrate the high potential of cBN for field emitters, comparable to other wide band gap semiconductors.

Full text of DOI:10.1063/1.2830006

Electron field emission from nanostructured cubic boron nitride islands
Kungen Teii, Seiichiro Matsumoto, and John Robertson
Citation: Applied Physics Letters 92, 013115 (2008); doi: 10.1063/1.2830006
View online: http://dx.doi.org/10.1063/1.2830006
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APPLIED PHYSICS LETTERS 92, 013115 ͑2008͒
Electron field emission from nanostructured cubic boron nitride islands
Kungen Teiia͒
Department of Applied Science for Electronics and Materials, Kyushu University, Kasuga, Fukuoka 816-
8580, Japan and Department of Engineering, University of Cambridge, Cambridge CB3 0FA,
United Kingdom
Seiichiro Matsumoto
Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba,
Ibaraki 305-0044, Japan and Ceramic Forum Co. Ltd., 1-6-6 Taitoh, Taitoh-ku, Tokyo 110-0016, Japan
John Robertson
Department of Engineering, University of Cambridge, Cambridge CB3 0FA, United Kingdom
͑Received 1 August 2007; accepted 10 December 2007; published online 7 January 2008͒
Nanocrystal-assembled cubic boron nitride ͑cBN͒ islands are formed by using low-energy
͑ϳ20 eV͒ ion irradiation in an inductively coupled fluorine-containing plasma. The temporal
evolution of surface morphology and roughness reveals three-dimensional island growth for initial
sp2-bonded BN and subsequent cBN, accompanied by a high frequency of renucleation. The
formation of cBN islands enhances the field emission and reduces the turn-on field down to around
9 V/␮m due to an increase in the island-related field. The results demonstrate the high potential of
cBN for field emitters, comparable to other wide band gap semiconductors. © 2008 American
Institute of Physics. ͓DOI: 10.1063/1.2830006͔
Fabrication of field emitters for cold cathodes is a key
technology to develop vacuum microelectronic devices such
as compact x-ray sources and ultrathin flat panel displays.
The emitter surfaces must be chemically inert and mechani-
cally tough as well as have good emission characteristics
such as low turn-on field, high current density, and long-term
stability. There are mostly three types of promising materials,
tip-shaped Mo, Si, etc., nanotubes, and semiconductor films.
Among various film-type candidates, wide band gap semi-
ences quasistatic surface processes of radicals assisted by
soft ion impact. The surface of several hundreds of nanom-
eter thickness films is relatively smooth for ultralow ion en-
ergies of a few eV or less and low ion fluxes, while its
roughness increases for low ion energies of a few tens of eV
and high ion fluxes. This tendency is attributed to low-energy
ion effects such as enhanced migration of adsorbed species,
which may enable to vary the size and number density of
cBN islands. In this letter, we present temporal evolution of
phase, surface roughness, and field emission properties of
cBN films during deposition with low-energy ͑ϳ20 eV͒ ion
irradiation. The results show that three-dimensional islands
consisting of cBN nanocrystals enhance the field emission
and confirm the high potential of cBN for field emitters.
The deposition procedure has been presented in detail
previously.8 BN films were prepared in the source region of
an inductively coupled radio-frequency ͑rf͒ plasma at the rf
power of 1.5 kW to increase the ion flux. n-type Si ͑100͒
wafers were cleaned in ethanol, dipped in a HF solution, and
rinsed in ethanol. He, N2, H2, and 10% BF3 diluted in He at
flow rates of 80, 10, 10, and 18 SCCM ͑SCCM denotes cu-
bic centimeter per minute at STP͒ were fed into the reactor.
The total pressure and the substrate temperature were kept at
0.3 Torr and 1020 20 K. Sheath potential was defined by a
difference between the plasma potential and the positive
direct-current substrate bias. In this work, the sheath poten-
tial was kept at 22 1.5 V with +30 V of biasing. Field emis-
sion properties were measured in a parallel-plate configura-
tion, where the cathode consists of the films to be tested and
the anode consists of an indium-tin-oxide coated glass plate.
The current-voltage curves were obtained for an interelec-
trode spacing of 100 ␮m with a Teflon spacer and over a
cathode area of 0.24 cm2. No pretreatments were needed for
the films to initiate the emission, so no data from damaged
surfaces by micro arcs were included. But some samples
needed a higher turn-on-field in the first measurement. This
conductors have gained
a heightened interest since
diamond,1 aluminum nitride,2 and hexagonal and cubic bo-
ron nitride3 ͑hBN and cBN͒ can feature negative electron
affinity ͑NEA͒. cBN has a number of excellent properties
analog to diamond. However, the amount of work on field
emission from cBN is extraordinary smaller than diamond.
In particular, field emission properties of vapor-deposited
cBN films have rarely been reported and are still far from
awesome in the rare work.4,5 This is most likely due to the
fact that as-deposited cBN films have usually very smooth
surfaces with the root-mean-square ͑rms͒ roughness as small
as 1 nm.6 This allows us to predict a low efficiency of field
emission because a field enhancement factor is not high. The
smooth surface is attributed to the high threshold energy of
ion bombardment ͑ജ50 eV͒ for deposition. A multiscale
model has explained that ion impact-induced downhill cur-
rents are responsible for the smoothing phenomena.7 Be-
sides, the films with high cBN contents are readily delami-
nated due to high residual stress induced by strong ion
impact and, hence, cannot be measured.
The authors have recently demonstrated a way to deposit
cBN films with mean ion impact energies from 45 down to a
few eV or less.8 The great decrease in the threshold energy is
made possible by using specific chemical effects of fluorine
and high ion-to-boron flux ratios. The system mainly experi-
a͒
Author to whom correspondence should be addressed. Electronic mail:
teii@asem.kyushu-u.ac.jp.
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0003-6951/2008/92͑1͒/013115/3/$23.00 92, 013115-1 © 2008 American Institute of Physics
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013115-2
Teii, Matsumoto, and Robertson
Appl. Phys. Lett. 92, 013115 ͑2008͒
FIG. 1. ͑Color online͒ Rms roughness of the films derived from 10
ϫ10 ␮m2 AFM images as a function of td. Phase assignments are based on
the characterization by FTIR and GXRD.
FIG. 2. ͑Color online͒ SEM images of the films at td= ͑a͒ 15 min and ͑b͒
30 min. ͑b͒ AFM and ͑d͒ magnified SEM images corresponding to ͑a͒
͑td =15 min͒.
is attributed to the activation process for removal of adsor-
bates or electronic changes.9,10
The rms roughness measured by tapping-mode atomic
force microscopy ͑AFM͒ is shown in Fig. 1 as a function of
deposition time ͑td͒. Phase assignments in Fig. 1 are based
on the characterization by Fourier transform infrared spec-
troscopy ͑FTIR͒ and glancing angle x-ray diffraction
͑GXRD͒. For 0Ͻtdഛ5 min, the film consists only of
sp2-bonded BN such as h-, turbostratic-, and amorphous BN.
For tdജ8 min, cBN is formed. For tdജ15 min, cBN be-
comes dominant at the surface with a minor amount of
sp2-bonded BN. For 15Ͻtdഛ30 min, the film is partially
delaminated in the substrate center region. The present phase
evolution is in accordance with the conventional one using
high-energy ion impact. In contrast, the rms roughness in-
creases initially up to around 60 nm for 0Ͻtdഛ3 min, then
decreases drastically for further increase in td. It increases
again for tdജ8 min soon after cBN formation and reaches
the range of 90–180 nm for td=15–30 min. The present
roughness evolution differs from the conventional one
clearly for two aspects, i.e., the large maximum values and
the large increments for both sp2BN and cBN growth re-
gions. These aspects are attributed to evolution of three-
dimensional topography by island growth, referred to as
Volmer-Weber mode. This is possible since the growth is
dominated by quasistatic surface processes of radicals as-
sisted by low-energy ion impact. A drop in rms roughness
just before cBN formation may be meaningful but it is be-
yond the scope of this letter.
cBN ͕111͖ peak by GXRD. The coalesced structures are at-
tributed to a high frequency of renucleation. Ion bombard-
ment alone is not the cause for renucleation as it is caused
even for ultralow ion energies of a few eV or less. The cBN
islands may enhance a topography-related field as deduced
from the brighter images and provide a high density of emis-
sion sites because of the nanocrystal-assemblies.
The field emission characteristics for td=3, 8, and
15 min are shown in Fig. 3. The average in-depth cBN con-
tent derived from FTIR spectra was 0%, 12%, and 60% for
td=3, 8, and 15 min, respectively. An electric field at a cur-
rent density of 5 nA/cm2 was taken as a turn-on field. With-
out cBN, the turn-on field is high ͑ജ16 V/␮m͒ and the cur-
rent density achieved is low ͑Ͻ1 ␮A/cm2͒. The present
turn-on field and maximum current range are in agreement
with a typical sp2BN film.11 With a low cBN content
͑td=8 min͒, the turn-on field is slightly reduced to
14.4 0.6 V/␮m and the current density is increased to
ϳ10 ␮A/cm2 despite a decrease in rms roughness. With a
high cBN content ͑td=15 min͒, the turn-on field is largely
reduced to 9.4 0.6 V/␮m and the current density is further
increased to ϳ0.1 mA/cm2. The result confirms that the
emission is enhanced by the formation and evolution of cBN
islands. The Fowler-Nordheim ͑FN͒ plot for td=15 min in
the inset is composed of two straight lines, indicating that the
emission apparently features a metallic-type FN tunneling.
Scanning electron microscopy ͑SEM͒ images at td=15
and 30 min are shown in Figs. 2͑a͒ and 2͑b͒. For
td=15 min, a number of micron-sized islands consisting of
several spherical grains as small as 1 ␮m are observed as
brighter regions on top of the surface. The size and number
density of the islands increased with increasing td from
8 to 30 min. For td=30 min, the surface is entirely covered
by these islands, most of which have crystallographic mor-
phology. From the temporal evolution of morphology and
phase composition, all or the majority of the islands are as-
signed to cBN. An AFM image at td=15 min shows a topo-
graphic arrangement of the islands and the underlying grains,
confirming that the growing islands occupy the higher loca-
tions than the rest ͓Fig. 2͑c͔͒ A magnification of an island at
td=15 min shows that it consists of subgrains smaller than a
hundred of nanometer ͓Fig. 2͑d͔͒ The apparent crystal size
FIG. 3. ͑Color online͒ Field emission current-voltage characteristics of the
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was calculated to be around 5 nm from the broadening of
films at td =3, 8, and 15 min. The inset shows the FN plot for td =15 min.
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013115-3
Teii, Matsumoto, and Robertson
Appl. Phys. Lett. 92, 013115 ͑2008͒
duction band of cBN by tunneling, thermalization to the con-
duction band minimum ͑CBM͒, and subsequent emission
from the CBM to the vacuum by tunneling ͑ii͒. The origin of
emission from the CBM has explained the field-emission en-
ergy distribution from cBN powders coated on Mo tips.12
This process lowers ⌽ by at most ϳ3 eV in comparison to
the aforementioned ͑i͒ for a positive electron affinity surface
and no longer depends upon the surface roughness for a
NEA surface. The band bending over ϳ3 eV is required to
lower the CBM of cBN below the Fermi level of sp2BN and
enable electron injection into the conduction band of cBN.
An experimental study has confirmed this condition,12 but a
theoretical study has not clearly yet.13 If the film has a p-type
character, the larger band bending is required.
FIG. 4. ͑Color online͒ Energy-band diagram for the sp2BN/cBN layered
structure. CBM: conduction band minimum, VBM: valence band maximum,
EF: Fermi level, Evac: vacuum level, PEA: positive electron affinity, and
NEA: negative electron affinity.
Finally, a slope decrease in the high field region of the
FN plot is attributed to depletion of available trapped elec-
trons to be emitted. This degradation may be caused by the
high potential barrier height for electron injection into the
conduction band of cBN or the high resistance of the initial
sp2BN layer.
The relationship among the slope of the FN plot ͑⌬͒, the
effective potential barrier height ͑⌽͒, and the field enhance-
ment factor ͑␤͒ is expressed as ⌬=−6840⌽3/2/␤, where ⌬
and ⌽ are in V/␮m and eV, respectively. For td=15 min, ⌬
is about −230 in the low field region, which is steeper than
about −40 in the high field region. This transition is attrib-
uted to different emission mechanisms as argued later. Here,
we focus on the change of ⌬ in the low field region. For
td=3 and 8 min, ⌬ is about −390 and −330, respectively. A
decrease in the magnitude of ⌬ with td means a decrease in
the ⌽3/2/␤ ratio. For td from 3 to 8 min, ⌽ decreases since ␤
usually decreases with decreasing surface roughness. This
indicates some that ⌽ decreases fundamentally by the forma-
tion of cBN. For td from 8 to 15 min, ⌽ changes a little,
while ␤ increases with increasing surface roughness and the
emission site density increases with increasing island density.
The energy-band diagram for the sp2BN/cBN layered
structure is depicted in Fig. 4. Band bending is caused by
field penetration or space charge in the film. Band gaps
of 6.0 and 6.2 eV are adopted for sp2BN and cBN,
respectively.3 The Fermi level is assumed in the middle of
the band gap. The electron transport mechanism for undoped
sp2BN and cBN is most likely hopping conduction at defect
levels such as grain boundaries and dangling bonds near the
Fermi level. There are two possible emission processes from
cBN surfaces. One consists of hopping at defect levels from
sp2BN to cBN and subsequent emission from the defect lev-
els to the vacuum by tunneling ͑i͒. This process is believed to
be typical of the emission from sp2BN surfaces.11 The other
consists of hopping at defect levels, electron injection from
the defect levels near the sp2BN/cBN interface into the con-
One of the authors ͑K.T.͒ acknowledges the financial
support from the Murata Science Foundation.
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