The field emission of indium-doped ZnO films fabricated by room temperature DC magnetron sputtering

Article abstract of DOI:10.1016/j.physb.2011.09.120

Indium doped ZnO film was fabricated at room temperature by co-sputtering a zinc target and an indium plate under the flow of oxygen and argon. The film was then characterized and the field emission of the film was studied. The indium composition x in the film (Zn_1-xIn_xO) is 5%. The film is hexagonal without any secondary phases or precipitates. The film has two major emission peaks, one related to the band edge emission and another possibly related to the electron acceptor transition. The possible acceptor is nitrogen occupying oxygen site in ZnO. The film is n-type and very resistive. The turn on field of the film at an emission current density of 10 μA/cm ² is 17.5 V/μm. The relatively weak field emission property is due to the unintentional incorporation of acceptors such as substitutional nitrogen and oxygen vacancies, which increase the work function of ZnO by reducing the electron density and lowering the Fermi level position of the ZnO:In film.

Full text of DOI:10.1016/j.physb.2011.09.120

Physica B 407 (2012) 64–67
Contents lists available at SciVerse ScienceDirect
Physica B
journal homepage: www.elsevier.com/locate/physb
The field emission of indium-doped ZnO films fabricated by room
temperature DC magnetron sputtering
a
a,n
b
a
a
a
Fan Ye , Xing-Min Cai , Fu-Ping Dai , Dong-Ping Zhang , Ping Fan , Li-Jun Liu
a
School of Physical Science and Technology and Shenzhen Key Laboratory of Sensor Technology, Shenzhen University, Shenzhen 518060, China
Department of Applied Physics, Northwestern Polytechnic University, Xian 710072, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Indium doped ZnO film was fabricated at room temperature by co-sputtering a zinc target and an
indium plate under the flow of oxygen and argon. The film was then characterized and the field
Received 25 March 2011
Received in revised form
x
emission of the film was studied. The indium composition x in the film (Zn1ꢀxIn O) is 5%. The film is
hexagonal without any secondary phases or precipitates. The film has two major emission peaks, one
5
September 2011
Accepted 24 September 2011
Available online 29 September 2011
related to the band edge emission and another possibly related to the electron acceptor transition. The
possible acceptor is nitrogen occupying oxygen site in ZnO. The film is n-type and very resistive. The
Keywords:
2
turn on field of the film at an emission current density of 10
m
A/cm is 17.5 V/
m
m. The relatively weak
ZnO
field emission property is due to the unintentional incorporation of acceptors such as substitutional
nitrogen and oxygen vacancies, which increase the work function of ZnO by reducing the electron
density and lowering the Fermi level position of the ZnO:In film.
Indium doping
Field emission
DC sputtering
&
2011 Elsevier B.V. All rights reserved.
1
. Introduction
also fabricated under the same condition. A circular Zn (99.999%)
target was used as the Zn source. When depositing ZnO:In, a thin
fan-shaped In plate (99.999%) was placed on the Zn target to act
as the In source. The In plate has the same radius with that of the
Zn target. The area ratio of In to Zn was 1 to 11. Ar (99.999%) and
ZnO has many outstanding physical and chemical properties,
such as a direct band gap of 3.37 eV at room temperature, a large
exciton binding energy of 60 meV, and a high thermal stability,
etc. [1]. Due to these outstanding properties, ZnO can be widely
used in solar cells [2], Schottky diodes [3], light emitting diodes
O
2
(99.999%) were the gases. Ar flow rate was 75 standard cubic
centimeters per minute (sccm) and O flow rate was 100 sccm.
The base pressure of the vacuum chamber was 4.5 ꢁ 10
2
ꢀ
3
[
4], lasers [5] and field emitters [6]. To further widen its applica-
Pa. The
ꢀ
1
tion, ZnO has been doped with IIIB elements such as Al, Ga and In
to modify its electrical, optical and magnetic properties [7–9].
ZnO films [10] and nanostructures [11] have shown very nice field
emission properties. The field emission of ZnO:In nanostructures
sputtering pressure was set to be 3.0 ꢁ 10 Pa and was auto-
matically fixed by an electromagnetic valve. N-type Si (1 0 0) was
used as the substrate. The substrates were ultrasonically cleaned
in acetone, ethanol and de-ionized water. The cleaning in each
liquid lasted for 15 min. During sputtering, the substrates were
not heated. The substrate holder rotated at 60 rounds per minute.
The sputtering current was 0.14 A and the voltage was 370 V. The
sputtering deposition lasted for 30 min.
[
12–14] has been reported. For example, Huang et al. studied the
field emission of ZnO:In nanowires [12]. Zhang et al. studied the
field emission of ZnO:In nanotips and nanowires [13]. Jung et al.
investigated the field emission of ZnO:In tetrapods [14]. However,
as far as the authors know, there is no report about the field
emission of ZnO:In thin films. In this paper, In doped ZnO thin
films were deposited by reactive direct current (DC) magnetron
sputtering at room temperature. The samples were then char-
acterized and the field emission of the ZnO:In films was studied.
The compositions of the samples were obtained with energy
dispersive X-ray spectroscopy (EDX, Oxford INCA X-MAX-50). The
structures were obtained with X-ray diffraction (XRD, D/MAX 2500-
18 kW, Cu Ka radiation). Raman scattering (JY-HR800 micro-Raman
spectrometer in backscattering geometry, 325 nm He–Cd laser) and
room temperature photoluminescence (PL, RF-540 spectrofluorophot-
ometer, 325 nm He–Cd laser) were used to study the optical proper-
ties of the samples. The surface morphology of the samples was
obtained with atomic force microscopy (AFM, Nano Scope IIIa). The
samples were also measured with Seebeck effect and four probe
instrument. The field emission property of the film was measured
with the equipment used for measuring that of GaN film [15] and the
2
. Experiments
Reactive direct current (DC) magnetron sputtering was used to
deposit indium doped ZnO (ZnO:In). For comparison, ZnO was
n
only difference was that the cathode–anode distance was 100
when measuring the field emission of ZnO:In films here.
mm
Corresponding author. Tel.:þ86 755 2653 6219; fax:þ86 755 2653 8735.
E-mail address: caixm@szu.edu.cn (X.-M. Cai).
0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2011.09.120

F. Ye et al. / Physica B 407 (2012) 64–67
65
3
. Results and discussion
EDX shows that the samples contain Zn, In and O and the In
composition x in the sample (Zn1ꢀxIn O) is 0.05, as shown in Fig. 1.
x
The XRD pattern of the sample is shown in Fig. 2. For comparison,
ZnO prepared under the same conditions is also shown. The samples
are polycrystalline, though deposited at room temperature. Jung
et al. reported that ZnO:In deposited by DC magnetron sputtering
was amorphous when the substrate temperature was below 350 1C
[16]. Kim et al. reported that ZnO:In deposited by DC magnetron
sputtering at room temperature was amorphous [17]. Our result is
different from theirs. For both the samples, three peaks are
observed. The first peak is from the Si substrates. The second and
third peaks are due to the (0 0 2) and (1 0 3) reflection of hexagonal
ZnO [18]. No peaks related to secondary phase or precipitates are
found, which implies that In is successfully incorporated into ZnO
lattices though the substrates were not intentionally heated during
deposition. The (0 0 2) diffraction peak of ZnO powder is located at
34.401 [18]. Compared with this, our sample is blue shifted. This
agrees with others [19]. The blue shift is mainly due to the fact that
3
þ
2þ
˚
In ions (with the radii of 0.76 A) replaced Z ions (with the radii
˚
Fig. 3. Raman scattering spectra of the ZnO and ZnO:In films.
of 0.74 A) [20], which will lead to the lattice expansion. The grain
size of ZnO:In is calculated to be 2.9 nm according to Sherra’s
formula [21].
Fig. 3 shows the Raman scattering spectra of ZnO and ZnO:In
ꢀ
1
films. Two peaks located at about 580 and 1160 cm
are
Fig. 1. EDX spectroscopy of the ZnO:In film.
Fig. 4. Photoluminescence spectrum of the ZnO:In film.
observed. The zone center Raman active optical phonons of ZnO
are predicted to be þE þ2E , where both A and E modes
¼A
are polar and split into transverse optical (TO) and longitudinal
optical (LO) phonons [22,23]. The theoretical values of A (LO) and
(LO) are 559 and 577 cm , respectively [24], while the
G
1
1
2
1
1
1
ꢀ
1
E
1
ꢀ
1
experimental values of both are often around 580 cm
The peak around 580 cm in ZnO has been assigned to B
mode [25], A (LO) [1,26–28]or E (LO) [29] mode. Compared with
1
[22].
ꢀ
1
1
(high)
1
that of ZnO, the first peak of ZnO:In shifted to smaller wave
numbers. This agrees with Ref. 29. The first peak is therefore
1
assigned to the E (LO) phonon mode and the second peak is the
corresponding two-phonon mode. No peaks related to secondary
phases were observed, which is in agreement with the XRD data
in Fig. 2. The Raman scattering data further confirms that the
films are hexagonal and In is successfully incorporated into ZnO
lattices without causing any secondary phases [28].
The room temperature photoluminescence (PL) spectrum of
the ZnO:In film is present in Fig. 4. Two peaks located at 368.1 nm
Fig. 2. X-Ray diffraction patterns of the ZnO and ZnO:In films.

6
6
F. Ye et al. / Physica B 407 (2012) 64–67
(3.37 eV) and 390.2 nm (3.18 eV) can be observed. The peak at
3
68.1 nm is from the band edge emission of ZnO. The band gap of
our ZnO:In film is larger than ZnO:In nanostructures [14]. The
peak at 390.2 nm is possibly due to the electron acceptor transi-
tion and the possible acceptor is nitrogen, which substitutes
oxygen [30–32]. The source of nitrogen might come from the
residual gases in the chamber.
Seebeck effect shows that the film is n-type and this agrees
with the result of Jung et al. [16]. Four probe instrument
measurement shows that the resistivity of the film is very high.
In doping is expected to reduce the resistivity of ZnO [33] since In
will be donor and will increase electron density in ZnO. The high
resistivity of the film very likely results from the unintentional
incorporation of acceptors such as substitutional nitrogen (N
O
) as
well as oxygen vacancies (V ). Both are acceptors in the film as
O
shown by the PL spectrum in Fig. 4 and will compensate the
electrons generated by indium doping [34]. Fig. 5 shows the AFM
image of the film. As can be seen from Fig.5, protrusions can be
observed on the surface of the film and the image is different from
that of ZnO [3].The root mean square (rms) roughness of the film
is 6.252 nm, which is larger than those deposited by pulsed DC
magnetron sputtering [19].
The curve of the emission current density J versus the
electrical field E is present in Fig. 6(a). If the electric field
2
E at an emission current density J of 10
m
A/cm is defined
as the turn on field, the turn on field of the film will be around
1
7.5 V/
mm. The maximum emission current density J is around
2
1
10 A/cm at the electric field of 24 V/mm. Fig. 6(b) shows
m
the Fowler–Nordheim (FN) plots of the film. As can be seen
from Fig. 6(b), the curve is nonlinear. Compared with other ZnO
samples, the field emission properties of our ZnO:In film
is very poor. This is very likely due to the unintentional incor-
poration of acceptors in the film. Substitutional nitrogen
(N
O
) atoms are possible acceptors in the film and will produce
holes in the film. Additionally, oxygen vacancies (V ) might also
exist in the ZnO:In films. Oxygen vacancies (V ) are also acceptors
O
O
and self-compensation can take place [34]. The produced
holes will compensate electrons generated by donors such as
In doping and will lower the electron density, which is equivalent
to lowering the Fermi level position. The lowering of the Fermi
level position implies the increase in the work function. The
increase in the work function implies poorer field emission,
though the surface morphology as shown in Fig. 5 favors field
emission,
2
Fig. 6. (a) The current density J (A/cm ) versus the electric field E (V/
mm) curve of
the ZnO:In film; (b) The Fowler–Nordheim (FN) plot of the ZnO:In film.
4. Conclusion
In doped ZnO film was deposited by reactive DC magnetron
sputtering at room temperature. The film was characterized
with EDX, XRD, Raman scattering, PL and AFM. The field emission
property of the film was also studied. EDX shows that the atomic
x
percentage x of In is 5% in the film (Zn1ꢀxIn O). XRD demonstrates
that the film is hexagonal ZnO and In is successfully doped
into ZnO lattice. These are further confirmed by Raman scattering.
PL reveals a band edge emission and an electron acceptor
transition. The acceptor is nitrogen occupying oxygen sites.
The film is n-type and it is very resistive. The field emission of
the film is very poor due to the unintentional incorporation of
acceptors, which possibly includes substitutional nitrogen atoms
O O
(N ) and oxygen vacancies (V ). The acceptors will lower the
electron density and the Fermi level position but improve the
work function of the films. This work shows that In can be
successfully doped into ZnO by reactive DC magnetron sputtering
at room temperature and unintentional incorporation of accep-
tors has to be avoided to achieve a better field emission of
the films.
Fig. 5. AFM image of the ZnO:In film.

F. Ye et al. / Physica B 407 (2012) 64–67
67
Acknowledgement
[15] F. Ye, E.Q. Xie, X.J. Pan, H. Li, H.G. Duan, C.W. Jia, J. Vac. Sci. Technol. B 24
2006) 1358.
(
[
[
16] Y.S. Jung, J.Y. Seo, D.W. Lee, D.Y. Jeon, Thin Solid Films 445 (2003) 63.
17] D.G. Kim, S. Lee, D.H. Kim, G.H. Lee, M. Isshiki, Thin Solid Films 516 (2008)
2045.
18] Joint Committee on Powder Diffraction, Standards-International Center for
Diffraction Data, Card nos. 80-0075 and 80-0018.
This work is supported by Shenzhen Key Laboratory of Sensor
Technology Open Projects (Project no. SST201105), and the
Shenzhen City R&D Funding for Science and Technologies-Basic
Research Program (Project no. JC201005280405A).
[
[
19] Y.R. Park, D. Jung, K.-C. Kim, S.J. Suh, T.S. Park, Y.S. Kim, J. Electroceram. 23
(2009) 536.
[
[
20] R.D. Shannon, Acta Crystallogr. Sect. A: Found. Crystallogr. 32 (1976) 751.
21] M.B. Rahmani, S.H. Keshmiri, M. Shafiei, K. Latham, W. Wlodarski, J. du
Plessis, K. Kalantar-Zadeh, Sens. Lett. 7 (2009) 1.
References
[
[
1] J. Zhao, X. Yan, Y. Yang, Y. Huang, Y. Zhang, Mater. Lett. 64 (2010) 569.
2] L. Shen, Z.Q. Ma, C. Shen, F. Li, Bo.He and F. Xu, Superlattices Microstruct. 48
[22] C.A. Arguello, D.L. Rousseau, S..P.S. Porto, Phys. Rev. 181 (1969) 1351.
[23] Y. Jin, Q. Cui, G. Wen, Q. Wang, J. Hao, S. Wang, J. Zhang, J. Phys. D: Appl. Phys.
42 (2009) 215007-1.
[24] F. Decremps, J. Pellicer-Porres, A.M. Saitta, J.C. Chervin, A. Polian, Phys. Rev. B
65 (2002) 092101-1.
(
2010) 426.
3] B.T. Lai, C.T. Lee, J.D. Hong, S.L. Yao, D.S. Liu, Jpn. J. Appl. Phys. 49 (2010)
85501-1.
[
0
[
[
[
[
[
4] C.S. Hong, H.H. Park, J. Moon, H.H. Park, Thin Solid Films 515 (2006) 957.
5] K.J. Kim, Y.R. Park, Appl. Phys. Lett. 78 (2001) 475.
6] D. Banerjee, S.H. Jo, Z.F. Ren, Adv. Mater. 16 (2004) 2028.
7] H.-J. Jin, M.-J. Song, C.-B. Park, Physica B 404 (2009) 1097.
8] C.E. Kim, P. Moon, I. Yun, S. Kim, J.-M. Myoung, H.W. Jang, J. Bang, Phys. Status
Solidi A 207 (2010) 1572.
[25] F.J. Manj o´ na, B. Mar ı´ , J. Serrano, A.H. Romero, J. Appl. Phys. 97 (2005) 053516-1.
[26] Y. Huang, M. Liu, Z. Li, Y. Zeng, S. Liu, Mater. Sci. Eng. B 97 (2003) 111.
[27] D.P. Li, G.Z. Wang, X.H. Han, J. Phys. D: Appl. Phys. 42 (2009) 175308-1.
[28] Y.W. Chen, Y.C. Liu, S.X. Lu, C.S. Xu, C.L. Shao, C. Wang, J.Y. Zhang, Y.M. Lu,
D.Z. Shen, X.F. Fan, J. Chem. Phys. 123 (2005) 134701-1.
[29] H. Liu, J. Yang, Z. Hua, Y. Zhang, L. Yang, L. Xiao, Z. Xie, Appl. Surf. Sci. 256
(2010) 4162.
[
9] K.W. Liu, M. Sakurai, M. Aono, J. Appl. Phys. 108 (2010) 043516-1.
[
[
[
[
[
10] J.B. You, X.W. Zhang, P.F. Cai, J.J. Dong, Y. Gao, Z.G. Yin, N.F. Chen, R.Z. Wang,
H. Yan, Appl. Phys. Lett. 94 (2009) 262105-1.
11] Q. Zhao, X.Y. Xu, X.F. Song, X.Z. Zhang, D.P. Yu, C.P. Li, L. Guo, Appl. Phys. Lett.
[30] F.Y. Ran, M. Imaoka, M. Tanemura, Y. Hayashi, T.S. Herng, S.P. Lau, Phys.
Status Solidi B 246 (2009) 1243.
[31] G. Xiong, K.B. Ucer, R.T. Williams, J. Lee, D. Bhattacharyya, J. Metson, P. Evans,
J. Appl. Phys. 97 (2005) 043528-1.
[32] C.X. Xu, X.W. Sun, X.H. Zhang, L. Ke, S.J. Chua, Nanotech. 15 (2004) 856.
[33] M.P. Taylor, D.W. Readey, C.W. Teplin, M.F.A.M. van Hest, J.L. Alleman,
M.S. Dabney, L.M. Gedvilas, B.M. Keyes, B. To, J.D. Perkins, D.S. Ginley, Meas.
Sci. Technol. 16 (2005) 90.
[34] A. Gonz a´ lez-Carrazco, M. Herrera-Zald ı´ var, U. Pal, J. Nanosci. Nanotech. 8
(2008) 6598.
8
8 (2006) 033102-1.
12] Y. Huang, Y. Zhang, Y. Gu, X. Bai, J. Qi, Q. Liao, J. Liu, J. Phys. Chem. C 111
2007) 9039.
13] Z. Zhang, J. Huang, H. He, S. Lin, H. Tang, H. Lu, Z. Ye, Solid-State Electron. 53
2009) 578.
(
(
14] M.N. Jung, S.H. Ha, S.J. Oh, J.E. Koo, Y.R. Cho, H.C. Lee, S.T. Lee, T.-I. Jeon,
H. Makino, J.H. Chang, Curr. Appl. Phys. 9 (2009) e169.