Hydrogen gas sensors based on PLD grown NiO thin film structures

Article abstract of DOI:10.1002/pssa.200778914

NiO thin films were grown by pulsed laser deposition on (100)Si substrates at 200 °C temperature. The effect of the O₂ pressure during the deposition process on the morphological, electrical and sensing properties of the films has been investigated. AFM images showed that the surface morphology of NiO films can be modified by the oxygen pressure during deposition. Electrical measurements showed that the well-known native p-type conductivity exhibits a conversion from p-type to n-type when the O₂ pressure is reduced. Resistance responses of NiO-thin films towards hydrogen (H₂) flow in air ambient have been measured. NiO thin film p - n homojuctions were then fabricated to investigate the electrical properties of such structures. The p - n homojunctions exhibited the distinct rectifying current - voltage (I - V) characteristics.

Full text of DOI:10.1002/pssa.200778914

phys. stat. sol. (a) 205, No. 8, 2064–2068 (2008) / DOI 10.1002/pssa.200778914
a
p s s
www.pss-a.com
applications and materials science
Hydrogen gas sensors based
on PLD grown NiO thin film structures
*
*, 1, 2
*, 1
1
2, 3
2, 3
2
M. Stamataki
, D. Tsamakis , N. Brilis , I. Fasaki , A. Giannoudakos , and M. Kompitsas
1
School of Electrical Engineering and Computer Science, National Technical University of Athens, Iroon Polytechniou 9 Zografou,
57 73 Athens, Greece
1
2
3
Theoretical and Phys./Chem. Institute, National Hellenic Research Foundation, Vas. Konstantinou Ave. 48, 116 35 Athens, Greece
School of Chemical Engineering, National Technical University of Athens, Iroon Polytechniou 9 Zografou, 157 73 Athens, Greece
Received 2 July 2007, revised 2 June 2008, accepted 30 June 2008
Published online 21 July 2008
PACS 07.07.Df, 68.37.Ps, 68.55.J–, 73.40.Lq, 73.61.Le, 81.15.Fg
*
*
*
*
Corresponding author: e-mail dtsamak@central.ntua.gr, Phone: +30 210 7723846, Fax: +30 210 7722281
e-mail marstam8@yahoo.gr, Phone: +30 210 7722576
NiO thin films were grown by pulsed laser deposition on known native p-type conductivity exhibits a conversion from
(100)Si substrates at 200 °C temperature. The effect of the O2 p-type to n-type when the O pressure is reduced. Resistance
2
pressure during the deposition process on the morphological, responses of NiO-thin films towards hydrogen (H ) flow in
2
electrical and sensing properties of the films has been investi- air ambient have been measured. NiO thin film p–n homojuc-
gated. AFM images showed that the surface morphology of tions were then fabricated to investigate the electrical proper-
NiO films can be modified by the oxygen pressure during ties of such structures. The p–n homojunctions exhibited the
deposition. Electrical measurements showed that the well- distinct rectifying current–voltage (I–V) characteristics.
©
2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
Introduction Nowadays the development of solid- properties between the oxides [4, 5]. Nevertheless, reports
state gas sensors for the detection of inflammable and toxic on homo-junction sensors based on semiconducting MO
gases such as hydrogen, carbon monoxide, etc. is a major are rather rare.
concern. H especially, is a renewable, abundant, efficient Nickel oxide (NiO) is considered to be a model p-type
2
energy source, which provides zero emissions [1]. In the semiconductor with a wide band-gap energy ranging from
near future it could be used as a city gas or to power cars in 3.6 eV to 4.0 eV [6, 7]. It has an excellent chemical stabil-
the same way as natural gas is used. As H has a lower ex- ity as well as good optical, electrical and magnetic proper-
2
plosion limit (LEL) at 40000 ppm [1], a device which de- ties. It has been used as p-type transparent film [6, 8], in
tects its presence at lower concentrations becomes indis- electro-chromic display devices [9–11] and recently it
pensable.
proved to be a promising functional material for applica-
A promising approach in the field of solid-state sensing tions in resistive-type gas sensors [1, 12–14].
devices is to use electrochemical gas sensors based on
Thin NiO films have been fabricated by various physi-
semiconducting metal oxides (MO), fabricated by using cal and chemical vapour deposition techniques, including
novel gas sensing materials. Gas sensors based on n-type spray pyrolysis [15–17], sol–gel process [18], reactive
MO semiconductors such as ZnO, SnO , In O have been sputtering [19–21] and pulsed laser deposition [22, 23].
2
2
3
widely investigated as sensing materials for the detection The preparation method and the deposition mode are fun-
of toxic gases. On the contrary, p-type MO semiconductors damental in determining the microstructure and conse-
have not yet been extensively studied for gas sensing ap- quently the gas sensing properties of MO thin films, but
plications.
the effective dependence of the process parameters on the
The sensing properties of MO sensors can be enhanced film properties is not yet well demonstrated.
by adopting methods like surface modifications, develop-
In this work, we investigated the influence of O pres-
2
ing p–n homo- or hetero-junctions [2–4]. The hetero- sure on the properties of undoped NiO thin films, grown by
junction sensors reported are mainly based on the interface pulsed laser deposition (PLD) on Si substrates at 200 °C.
©
2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original
Paper
phys. stat. sol. (a) 205, No. 8 (2008)
2065
The conductivity type and the electrical properties of the (ρ = 736 Ω cm) and n-type (111)Si (ρ = 5.47 Ω cm) sub-
high quality NiO thin films were controlled by changing strates. First the p-NiO layer for 120 min was deposited on
the pressure of the reactive O gas during film growth. The the substrate, followed by the growth of the n-NiO layer
2
surface morphology and roughness of the films were ob- for approximately 60 min.
served by an AFM operating in air. The response of these
The effect of the deposition parameters on the electri-
NiO films towards H has been demonstrated at operating cal and sensing properties of the grown NiO films was in-
2
temperatures in the 80–125 °C range. Finally, the fabrica- vestigated: the electrical resistivity of the samples was
tion of p-NiO/n-NiO homojunction structures as potential measured using the Van der Pauw technique, while the
gas sensors has been achieved within a simple deposition type of conductivity and the carrier concentration were ob-
cycle for the first time.
tained by Hall voltage (V ) measurements in a magnetic
H
field of 0.7 T. For the determination of the NiO film thick-
Experimental Undoped NiO films were grown by ness, reflectance spectra were recorded by a Perkin–Elmer
PLD on Si substrates in a stainless steel vacuum chamber. λ19 spectrometer. Thickness values ranging from 100 nm
Prior to each irradiation the vacuum chamber was evacu- to 175 nm were estimated as proposed by Manifacier et al.
2
–4
ated down to a residual pressure of 7 × 10 Pa. A Ni foil [24]. Surface morphology was studied by atomic force mi-
purity 99.999%) was used as target. To avoid fast drilling, croscopy (AFM) using a Veeco CP-II digital instrument in
the target was placed on a movable vacuum-compatible contact mode under normal air conditions.
(
computer-controlled XY translator.
Dynamic sensor response measurements were per-
The NiO monolayer films were deposited by using a formed against H in air flow at working temperatures be-
2
Quantel Nd:YAG laser (λ = 355 nm, τ = 10 ns) at a repeti- tween 25 °C and 125 °C in a stainless steel tube set-up.
p
2
tion rate of 10 Hz, with an estimated fluence of 2–4 J/cm . The sample holder can be resistively heated up to 400 °C
The laser beam was focused on the target with an incident with an accuracy of ± 1 °C by an ITC-502 (Oxford Instru-
angle of about 45° relative to the normal of the target sur- ments) temperature controller. A platinum resistor meas-
face. The (100)Si substrates were positioned at 50 mm ures the temperature and two mechanically-held point con-
downstream. All films were grown at 200 °C temperature tacts measure the film resistance. Tests were made against
and under O pressures ranging from 5 Pa to 50 Pa. The 30000 ppm H mixed with air before reaching the tube
2
2
deposition time was set at 90 min.
inlet. The gas flow was controlled by two calibrated flow-
For the growth of the double-layer NiO structures meters and fed into an injection point located below the
(p-NiO/n-NiO), a Lumonics Mo. TE-861T excimer laser sample holder. After introducing the H gas into the tube,
2
(λ = 248 nm, τ = 10 ns) was used at a repetition rate the film resistance versus time was recorded by a digital
p
2
of 10 Hz, and a fluence of 2–4 J/cm . The double-layer multimeter (Agilent 34401A) for various operating tem-
NiO structures were grown on both p-type (111)Si peratures.
Figure 1 (online colour at: www.pss-a.com) AFM 3D images of two NiO thin films deposited on (100)Si substrate at 200 °C with O
pressures of a) 5 Pa and b) 50 Pa, respectively.
2
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© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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M. Stamataki et al.: Hydrogen gas sensors based on PLD grown NiO thin film structures
Table 1 Properties of undoped NiO thin films grown on (100)Si substrate.
P (Pa)
5
10
20
30
50
thickness (nm)
174
144
114
100
100
surface roughness (nm)
ρ (Ω cm)
0.70
2.85
n
0.87
1.10
n
0.97
0.73
n
1.54
0.30
n
2.93
1.15
p
dominant carrier type
–3
p or n (cm )
15
16
17
16
16
2.5 × 10
1.1 × 10
2.6 × 10
2.5 × 10
9.7 × 10
H
H
3
3
Results and discussion
lated with adsorption/desorption processes occurring at the
.1 Thin film surface characterization Typical gas-surface interface. It has been shown that for effective
AFM images of the NiO film surface deposited on silicon operation of chemical sensors, the gas-sensing materials
substrates are shown in Fig. 1. The quantitative analysis of should have a specific relation between adsorption/
the roughness deduced from the AFM images, shows that desorption parameters for oxygen and the detecting gas
the roughness depends on the pressure of the oxygen ambi- [27–29].
ent during the deposition. The NiO film grown under 5 Pa
When the H molecules come in contact with the sur-
2
O pressure showed a smoother surface with small grains face of a thin MO film, they react with the adsorbed oxy-
2
– –
and a surface roughness of 0.70 nm. On the contrary, the gen (O or O ) and water molecules evaporate. The previ-
2
film surface of the NiO film deposited under 50 Pa O2 ously localized electrons are injected back into the film
showed that the grains create relatively larger crystallites bulk and induce a conductivity change of the oxide. In the
and the roughness value increased to 2.93 nm.
case of a p-type semiconductor, the resistivity of the sensor
increases, as a result of the recombination of the injected
.2 Electrical properties and gas sensing results electrons with the holes (majority carriers) in the valence
of NiO thin films The electrical properties of nickel ox- band. On the contrary, for a n-type semiconductor the re-
ide films prepared by PLD were significantly depending on sistance decreases when the sensor comes in contact with a
the O pressure inside the chamber during growth. reducing gas such as H [30].
3
2
2
The conductivity type of the NiO thin films was deter-
The recording of the dynamic response of the resist-
mined by Hall coefficient (R ) measurements at room tem- ance against H (30000 ppm) in air ambience of two un-
H 2
perature. The carrier concentration (n or p ) and the Hall doped NiO thin films, deposited at T = 200 °C, is shown in
H H
mobility (µ ) were obtained from the combined Hall coef- Fig. 3(a) and (b). The difference in behaviour of the two
H
ficient and electrical resistivity (ρ) measurements using the sensors is due to the different values of O pressure during
2
well-known equations:
their growth. In the first case, the NiO thin film was grown
under low O pressure (5 Pa) and it exhibited a decrease in
2
1
RH
n =
H
and µ =
H
.
(1) resistance (Fig. 3(a)) when exposed to a reducing gas. This
suggests that the metal-oxide layer behaved like an n-type
semiconductor, in agreement with the electrical measure-
eRH
ρ
The carrier concentration is given by the relation n = rnH,
ments above. Moreover, the H sensing properties of the
2
where r (scattering factor) is usually assumed to be 1. Thin
NiO thin film were strongly depending on the working
temperature. In particular, the response time decreased
from 20 min to 10 min and the recovery time reduced from
films grown in higher O pressure (>30 Pa) exhibited
2
p-type conductivity, as expected. When the O pressure
2
during the deposition is kept under 30 Pa, NiO films exhib-
ited n-type conductivity (Table 1).
17 min to 9 min, when the working temperature increased.
Probably the increase of the working temperature acceler-
ates the adsorption/desorption processes on the surface of
The conduction mechanism of undoped NiO film is
strongly related to the nickel vacancies and interstitial
oxygen existing in the microstructure [25]. The electrical
properties of NiO films are connected therefore to their
microstructure and composition and, consequently, are di-
rectly related to the deposition conditions [19, 20, 26].
3,0
n-type
2
,5
Figure 2 shows the influence of the ambient O partial
2
2,0
pressure on the conduction type of the undoped NiO films.
The relative response (sensitivity) against gas flow is
determined by the formula:
1
1
0
0
,5
,0
,5
,0
p-type
n-type
n-type
R - R
0
g
n-type
S =
R
,
(2)
R0
0
20
40
60
Oxygen Pressure (Pa)
where R is the film resistance in air atmosphere and R in
0
g
the presence of the H gas. Gas sensitivity is strongly re- Figure 2 Films resistivity versus O ambient pressure.
2
2
©
2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Original
Paper
phys. stat. sol. (a) 205, No. 8 (2008)
2067
Air
Air
8
0°C
H2
80°C
125°C
0
0
,8
,6
1
0
0
0
,00
,95
,90
,85
H2
125°C
H2
0
,4
Air
H2
H2
Air
Air
Air
0,2
0,0
Air
0
20
04
t (min)
06
08
0
20
04
t (min)
06
08
a)
b)
Figure 3 (online colour at: www.pss-a.com) Dynamic response characteristics of NiO thin films exposed in 30000 ppm H in air mix-
2
ture. (a) NiO film deposited on (100)Si substrate under 5 Pa O pressure at working temperature of 80 °C (᭹) and 125 °C (᭿), (b) NiO
2
film deposited on (100)Si substrate under 50 Pa O pressure at working temperature of 80 °C (᭹) and 125 °C (᭿).
2
the NiO thin film. For this reason the maximum sensitivity for the p-NiO/n-NiO junction grown on a p-type Si sub-
of the NiO film increased from 12% to 14% with the in- strate. The ideality factor of the diode was found to be ~15.
crease of the working temperature.
Figure 4b shows the log I versus V characteristic obtained
In the second case, the NiO thin film was grown under at room temperature with the contact region exposed to air
higher O pressure (50 Pa) and exhibited an increase in re- ambience for the p-NiO/n-NiO junction deposited on
2
sistance (Fig. 3(b)) when exposed in a reducing environ- n-type Si substrate. The ideality factor of the diode was
ment. This is a typical behaviour of p-type semiconductor found to be ~13.
[31]. The maximum sensitivity of the sensor is about 76%
at working temperature of 125 °C with response and re-
4 Conclusion NiO thin films were grown on (100)Si
covery time of about 15 min and 10 min respectively. At substrates under various O pressures by PLD. The O2
2
the working temperature of 80 °C the maximum sensitivity pressure variation was shown to strongly affect the con-
of the sensor found to be around 10% with response and ductivity type of the NiO thin films. Very low O pressure
2
recovery time of 12 min and 2 min respectively. The sensi- during the growth of undoped NiO thin films enhanced the
tivities appear to be closely related to surface roughness as generation of large electron populations, inverting the
it has been measured with AFM: the sensitivity increases well-known p-type native character of NiO films and re-
with the increase of the surface roughness, because the sulting in n-type conductivity.
number of the active adsorption sites for oxygen or hydro-
gen molecules on the surface is also increased [32].
In addition to the electrical measurements, NiO thin
films have been tested as potential H sensors. The dy-
2
namic response measurements against 30000 ppm H in air
2
3
.3 Study of the p-NiO/n-NiO structures Rectify- flow confirmed the conversion of the conduction type,
ing log I–V characteristics were recorded with a forward since in n-type NiO films the resistance decreased when
threshold voltage of about 0.8 Volts. Figure 4a shows the exposed to H and in p-type NiO films increased. More-
2
log I (current density) versus V plot, obtained at room tem- over, the sensitivity of NiO gas sensors is strongly depend
perature with the contact region exposed to air ambience ing on the grain size and surface morphology of the oxide
-
-
6
8
1
1
x 10
x 10
-
-
6
8
1
0
1
1
0
-10
1
x 10
-10
0
-12
10
-
4
-2
0
2
4
- 4
- 2
0
2
4
V (Volts)
V (Volts)
a)
b)
Figure 4 (a) log I–V plot of p-NiO/n-NiO on p-Si substrate, (b) log I–V plot of p-NiO/n-NiO on n-Si substrate.
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M. Stamataki et al.: Hydrogen gas sensors based on PLD grown NiO thin film structures
layer, since these parameters influence the effective [15] P. Puspharajah, S. Radkakrishna, and A. K. Arof, J. Mater.
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33].
Finally, novel double layer structures based on NiO
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(2006).
Acknowledgements Financial support by the Greek Gen-
eral Secretariat for Research and Technology in the frame of a bi-
lateral Research Agreement with the Slovak Republic is kindly
acknowledged.
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