Pulsed laser deposited nanostructured InN thin films as field emitters

Article abstract of DOI:10.1016/j.ssc.2006.12.027

InN thin films were deposited on c-cut Al₂O₃ substrates using the DC plasma assisted pulsed laser deposition technique. X-ray diffraction (XRD) studies showed the single phase, polycrystalline nature of the InN thin films with wurtzite structure. Surface morphology of the films, as seen by atomic force microscopy (AFM), consisted of densely packed nanocrystalline grains of InN. The rms value of surface roughness was found to be ～35?nm. AFM images also revealed the hexagonal features with sharp edges and protrusions of InN. The field emission characteristics of InN/Al₂O₃ were investigated in ultra high vacuum (1×10^-8?Torr) using the diode configuration. The turn-on field, required to draw an emission current density of 10?μA/cm², was observed to be ～3.5?V/μm. The maximum emission current density obtained was 230?μA/cm² when the applied electric field strength was ～4?V/μm. The Fowler-Nordheim (FN) plot obtained from the current-voltage characteristic was found to be linear in accordance with the quantum mechanical tunneling phenomenon. The field enhancement factor, β, estimated from the slope of the FN plot was 21,167?cm^-1. To the best of our knowledge this is the first report of field emission studies of InN/Al₂O₃ with such high values of β, which is comparable to that obtained for GaN or IrO₂.

Full text of DOI:10.1016/j.ssc.2006.12.027

Solid State Communications 142 (2007) 110–113
www.elsevier.com/locate/ssc
Pulsed laser deposited nanostructured InN thin films as field emitters
K.P. Adhia,∗, Sanjay Harchirkara, Suhas M. Jejurikara, P.M. Koinkara, M.A. Morea, D.S. Joaga,
L.M. Kukrejab
a
Centre for Advanced Studies in Materials Science and Solid State Physics, Advanced Materials Processing Laboratory, Department of Physics,
University of Pune, Pune – 411 007, India
Thin Film Laboratory, Raja Ramanna Centre for Advance Technology (RRCAT), Indore – 452 013, India
b
Received 24 November 2006; accepted 29 December 2006 by H. Ohno
Available online 17 January 2007
Abstract
InN thin films were deposited on c-cut Al O substrates using the DC plasma assisted pulsed laser deposition technique. X-ray diffraction
2
3
(XRD) studies showed the single phase, polycrystalline nature of the InN thin films with wurtzite structure. Surface morphology of the films, as
seen by atomic force microscopy (AFM), consisted of densely packed nanocrystalline grains of InN. The rms value of surface roughness was found
to be ∼35 nm. AFM images also revealed the hexagonal features with sharp edges and protrusions of InN. The field emission characteristics of
−8
InN/Al O were investigated in ultra high vacuum (1×10 Torr) using the diode configuration. The turn-on field, required to draw an emission
2
3
2
2
current density of 10 µA/cm , was observed to be ∼3.5 V/µm. The maximum emission current density obtained was 230 µA/cm when the
applied electric field strength was ∼4 V/µm. The Fowler–Nordheim (FN) plot obtained from the current–voltage characteristic was found to be
linear in accordance with the quantum mechanical tunneling phenomenon. The field enhancement factor, β, estimated from the slope of the FN
−1
plot was 21,167 cm . To the best of our knowledge this is the first report of field emission studies of InN/Al O with such high values of β,
2
3
which is comparable to that obtained for GaN or IrO .
2
c
ꢀ 2007 Elsevier Ltd. All rights reserved.
PACS: 81.15.Fg; 81.05.Ea; 68.55.Jk; 81.70.Fy; 61.10-i
Keywords: A. Thin films; A. Semiconductors; B. Laser processing; C. Field emission
1. Introduction
doped diamond like carbon films [6], GaN on molybdenum
tips [7], carbon nano-tubes (CNT) [8] etc. have been studied
for some time. It is only recently that wide band gap
semiconductors such as ZnO nano-wires [9], AlN nano-
rods [10], nanostructured GaN thin films [11] etc. have been
the focus of field emission studies.
Among the wide band gap semiconductors, ZnO, AlN and
GaN have stimulated extensive research interest due to their
potential applications as field emitters. InN, however, is yet
to be examined to that extent, which is probably due to its
low dissociation temperature, making the synthesis of InN
very difficult. InN is unstable above 550 ◦C. However, in
the last few years, the interest in InN has been on the rise,
both for fundamental understanding as well as its potential
applications [12,13]. From the point of view of basic studies,
some of the issues being addressed are (a) accuracy and
variation in the determination of band gap of InN. Earlier the
In the recent past, there has been growing interest in the area
of field electron emission [1–3]. This interest stems from the
fact that along with the scientific importance of enhancing basic
understanding, it is also technologically essential in realizing
various devices such as flat panel displays, electron sources in
vacuum microelectronics etc. based on field emission. While
new materials/compounds with better field emission properties
are being explored [4–7], attempts are also being made to
comprehend the influence of surface morphology, in terms
of shape, structure viz. nano structures, nano-rods, pyramidal
patterns etc., on field electron emission characteristics [8–11].
A variety of materials such as Gd2O3:Eu3+ [4], In2O3 [5], Si
∗
Corresponding author. Tel.: +91 20 2569 2678; fax: +91 20 2569 1684.
E-mail address: kpa@physics.unipune.ernet.in (K.P. Adhi).
c
0038-1098/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2006.12.027

K.P. Adhi et al. / Solid State Communications 142 (2007) 110–113
111
value obtained was 1.89 eV [13,14] but recent studies suggest
the band gap to vary between 0.7 and 0.9 eV [15]. Several
mechanisms such as the oxygen contamination, quantum size
effects, Mie resonance and the Burstein–Moss effects have been
used to explain the deviation in case of InN thin films, and (b)
the lattice parameters of wurtzite InN, reported in the literature
are observed to be different leading to uncertainty [16]. In terms
of practical use, InN and its alloys with GaN and AlN make
it possible to extend the emission range of nitrides from the
ultraviolet to near infra-red region [17]. Recent study reveals
that InN has promising transport and optical properties and a
large drift velocity at room temperature even higher than that
of GaN. Hence, InN has a high potential as a material for
the fabrication of high-speed, high-performance heterojunction
FETs [17] etc.
Study of InN as a field emitter has not yet received
significant attention. To the best of our knowledge, there
are only a couple of reports of InN nano-rods/structures as
field emitters [18,19]. Here we report a systematic study
of field emission characteristics from pulsed laser deposited
polycrystalline InN thin films on c-axis oriented Al2O3.
Fig. 1. X-ray diffractogram of polycrystalline InN thin films deposited on
c-axis Al O .
2
3
morphology of the sample was investigated using atomic force
microscopy (AFM-Jeol, JSPM 5200) in contact mode. The field
emission studies were carried out in an ultra high vacuum
chamber evacuated to a base vacuum of 1×10−8 Torr. A typical
‘diode’ configuration, consisting of a phosphor coated tin-oxide
glass plate anode (circular disk of diameter 50 mm) and the
deposited thin film cathode was used. The sample was mounted
on a stainless steel stub connected to a linear motion drive,
which helps in adjusting the distance between the anode and
the cathode. In the present study, the distance between the two
was adjusted to 5 mm. The details of the field emission set up
are mentioned elsewhere [11].
2. Experimentation
Radiation from an excimer laser (gas KrF, wavelength
λ = 248 nm, pulse duration tp = 20 ns, repetition rate
= 5 Hz) was focused on an indium target (purity 99.99%)
mounted in the deposition chamber. The polar angle between
the incident radiation and the normal to the target surface
was 45◦. The fluence incident on the targets surface was
0.5 J/cm2. Prior to deposition, the c-cut Al2O3 substrates were
ultrasonically degreased, sequentially, in trichloroethylene,
acetone and methanol for five minutes each and dried under
a nitrogen jet. The cleaned substrate was mounted on a
heater and placed parallel to the target at a distance of
5 cm in the deposition chamber. A copper ring was mounted
in between the indium target and the Al2O3 substrate at a
distance of 0.5 cm from the target surface. The chamber
was evacuated to a base vacuum of 1 × 10−5 Torr. High
purity (99.999%) nitrogen was introduced into the chamber
and maintained at a pressure of 25 mTorr throughout the
deposition process. Before starting the deposition, nitrogen
plasma was initiated by applying 900 V (DC) to the copper
ring with respect to the indium target. Fundamental to the
DC plasma assisted PLD process is the generation of reactive
nitrogen species, which reacts with the indium plasma plume
to form InN. The reacted species impinge, inelastically, onto
the substrate, which was maintained at 500 ◦C throughout
the deposition. During deposition the target was continuously
rotated at 10 rpm to avoid texturing/pitting of the target surface.
After the deposition, the samples were slowly cooled to room
temperature in nitrogen ambient at atmospheric pressure.
3. Results and discussions
The thin films deposited on c-cut Al2O3 had uniform
thickness of 400 ± 10 nm.
Fig. 1 shows the XRD (θ–2θ) pattern of the deposited
thin film on the c-axis oriented Al2O3 substrate. The peaks
located at 2θ values of 31.4◦ and 33.2◦ correspond to wurtzite
¯
InN (0002) and (1011) planes respectively, indicating the
polycrystalline nature of the thin films. The third peak located
at 2θ = 42◦ corresponds to the (0004) plane of the Al2O3
substrate. Hence, the single-phase, polycrystalline InN thin
films with wurtzite structure were successfully deposited on c-
axis oriented Al2O3 substrates.
The surface morphology of the InN thin film is as shown
in Fig. 2(a) and (b). The following observations can be made
from Fig. 2(a); (i) the surface morphology of the deposited
film shows densely packed, elongated nanocrystalline grains of
InN (ii) the surface of the thin film is rough i.e. the variation
in height (rms surface roughness) across the grains over the
complete film is estimated to be ∼3.5 nm. Similar results were
obtained from different regions of the InN thin films suggesting
uniformity in the deposition and growth process. Fig. 2(b)
shows the 3D image of the InN thin film of the same region as
shown in Fig. 2(a). It is observed that (i) the elongated grains,
seen in Fig. 2(a), actually show multiple hexagonal structures
with sharp edges and protrusions and (ii) each grain shows a
rough surface. The multiple hexagonal structures with sharp
edges and protrusions are uniformly distributed over the entire
The thickness of the deposited thin films was measured using
a Tally step instrument at four or five different positions on
the films. X-ray diffraction (XRD) was used to study the phase
formation, crystallographic orientation and structural quality of
the deposited thin films. The Cu Kα radiation at wavelength
˚
of 1.542 A was used for the diffraction studies. The surface

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K.P. Adhi et al. / Solid State Communications 142 (2007) 110–113
2
Fig. 3. Plot of emission current density, J (µA/cm ) as a function of applied
electric field E, (V/µm).
Fig. 2. AFM images of InN thin films deposited on c-axis Al O showing the
2
3
surface morphology in (a) two dimensions and (b) three dimensions—The sharp
edges of the hexagonal structures of InN thin films are clearly visible.
2
4
Fig. 4. F–N plot of log (I/V ) versus (10 /V ).
is probably due to the nanometric features of the hexagonal
structures of InN thin films, observed in Fig. 2(b).
The field emission current density–electric field (J–E)
characteristics were analyzed using the Fowler–Nordheim (FN)
equation.
surface and are observed to be of the order of a few 10’s of
nanometers.
The field emission pattern consisted of bright spots with very
stable intensity. Fig. 3 shows the plot of measured field emission
current density (J) as a function of applied electric field
strength (E) for the polycrystalline InN thin films deposited
on c-cut Al2O3. An emission current density of ∼5.0 µA/cm2
was drawn for an applied electric field strength of ∼1.5 V/µm
reproducibly. The current density was observed to double
(10 µA/cm2) on enhancing the applied electric field from
1.5 to ∼3.5 V/µm. The turn on field (Eto) is defined as the
applied electric field required drawing a current density of
10 µA/cm2, which is needed for typical operation of flat panel
displays [10]. Hence, in the case of InN thin films deposited on
c-axis Al2O3, Eto is 3.5 V/µm, which is comparable with that
of carbon nanotubes [8], AlN nano-needle arrays [20], and a-
GaN films [11]. With further increase in the field strength, the
emission current density is observed to increase exponentially
and a maximum current density of ∼230 µA/cm2 was obtained
for an applied electric field strength of 4 V/µm. The reason for
the low turn on field and high field emission current density
J = (Aβ2 E2/φ) exp(−Bφ3/2/βE),
where φ is the work function of InN, β is the field enhancement
factor, A and B are the constants having values 1.56 ×
10−10 (A V−2 eV) and 6.83 × 103 (V eV−3/2 µm−1),
respectively. Fig. 4 shows the FN plot of log (I/V 2) as
a function of 104/V . Within the measurement range, the
plot shows a linear relationship, which follows the quantum
mechanical tunneling process.
Field enhancement factor β is associated with the magnitude
of the local electric field existing at the emission sites in the InN
thin films and is defined as,
V
Elocal = βE = β
d
where Elocal is the local electric field close to the emission
sites, d is the average distance between the electrodes and
V is the applied voltage. The field enhancement factor β is

K.P. Adhi et al. / Solid State Communications 142 (2007) 110–113
113
estimated from the slope ‘m’ of the FN plot, using the relation
References
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The authors, KPA, LMK and SMJ, thank DAE-BRNS for
the financial support extended for carrying out this work under
the project sanction No. (2002/34/21/BRNS).