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n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e8
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n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors Sumati Pati*, P. Banerji, S.B. Majumder Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India
article info
abstract
Article history:
Selectivity has been the central issue in the research area of semiconducting metal oxide
Received 18 April 2014
gas sensors and still remains a challenge for the researchers. In the present work, we
Received in revised form
report a promising hydrogen (reducing gas) sensing characteristics of nanocrystalline in-
9 July 2014
dium doped zinc oxide thin film sensing elements. At sensor operating temperature in the
Accepted 14 July 2014
range (200e300) C, ‘n’ to ‘p’ type carrier reversal is detected in the measured resistance
Available online xxx
transients. Furthermore, for a given operating temperature the type of carriers remain unaltered for each concentrations of gas. Thus, temperature plays a crucial role for this
Keywords:
transition irrespective of the gas concentration. However, such carrier reversal is not
Thin film
observed in NO2 (oxidizing gas) environment even with the variation of operating tem-
Indium doping
perature and gas concentration. This is attributed to the gas sensing mechanism related to
Carrier reversal
the surface conductivity and the underlying variations of the carrier type. It is argued that,
Gas sensor
it can pave the way for selective detection of hydrogen at lower operating temperatures.
Selectivity
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction For a variety of applications, selective and low temperature detection of flammable hydrogen gas (lower explosion limit is 4% in air) is highly desirable. Large number of studies are devoted to detect hydrogen in various industrial applications, as well as automobile, space and aeronautics sectors [1,2]. Factors such as lower cost, ease of production and operation, simplicity in use, make semiconducting metal oxides (especially ZnO) as attractive hydrogen gas sensors. Undoped ZnO exhibits n-type conductivity due to the presence of intrinsic 00 defects such as oxygen vacancies (VO ,) and zinc interstitials 00 (Zni ). Doping with higher valence impurities (viz. aluminum (Al), tin (Sn), iron (Fe), indium (In) etc.) further enhances its
conductivity [3e5]. Recently thin film type sensing elements are opted to make miniaturized integrated gas sensors. Since the gas sensing mechanism in these thin films involve surface oxidation, electron exchange and desorption of the reaction product; the measured resistance transients (during response and recovery) depends on a variety of factors interrelated to each other. For example, the response is related to the type (n or p) and concentration of intrinsic charge carriers. During gas detection, depending on the sensing material, the characteristics of the sensing layer is changed in various ways and the transducer subsequently transforms it to an electrical signal. When the chemi-resistive gas sensors are heated in ambient air oxygen is chemi-adsorbed on the sensor surface. When exposed to reducing gases at constant temperature the sensor resistance decreases for ‘n’- type semiconductors, whereas it
* Corresponding author. Tel. þ91 09438855121. E-mail addresses: [email protected], [email protected] (S. Pati). http://dx.doi.org/10.1016/j.ijhydene.2014.07.075 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Pati S, et al., n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.075
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e8
increases when the dominant carriers are of ‘p’- type [6]. In contrast, some researchers have reported the increase in resistance of n-type semiconducting metal oxides in presence of reducing gases under different operating conditions. Various mechanisms are proposed to explain the observed behaviour [7e10]. In one of our previous works [10] we have reported ‘n’ to ‘p’ type transition behavior of undoped ZnO thin films during butane detection at higher operating temperature. Reviewing all these research works we felt that the origin and the nature of this n to p transition behavior is poorly understood and needs further investigation. To the best of our knowledge, for hydrogen sensing there is no report on the study of n to p transition behavior in indium doped ZnO thin films grown by economic solegel deposition technique. In this work we have studied both the reducing (H2) and oxidizing (NO2) gas sensing characteristics of indium doped ZnO thin films.n to p carrier reversal was observed during H2 sensing (for various test gas concentration) at relatively lower operating temperature regimes. The plausible mechanism related to this n-p transition is discussed.
Experimental details Zinc acetate dihydrate (purity 98%) and indium nitrate dihydrate (purity 99.99%) powders were first mixed in 97: 3 molar ratios to prepare 3 wt. % indium doped ZnO precursor. The mixed powder was dissolved in a mixture of 2-methoxyethanol and MEA (monoethanolamine) solution at room temperature. The solution was heated to 60 c and continuously stirred for 2 h to yield clear and homogeneous solution. The solution (concentration ~0.4 M) was spin cast onto quartz substrates using a spin coater unit (SCU 2007, apex instruments co.). The spin speed and spin time was kept at 3000 rpm for 30 s. Prior to the film deposition, the quartz substrates were ultrasonically cleaned for five minutes each in trichloro ethylene, followed by acetone and methanol. The substrates were then rinsed in deionized water (for three minutes), subsequently dried in flowing N2 and used for film deposition [11]. After deposition, the films were heat treated at 300 C for 5 min to evaporate the solvent and to remove the organic residuals. The coating and firing cycle was repeated to yield films with desired thickness. The films were finally annealed at 600 C for 1 h in air. The phase formation behavior of the deposited thin films were studied by X-ray diffraction (Ultima III, Rigaku, Japan) analyses using Cu Ka radiation in 2q range 20e80 at a scanning rate 3 min1. For X-ray diffraction measurements, the accelerating voltage and current were maintained at 40 kV and 30 mA respectively. The micro structural characteristics of the films were investigated using field emission scanning electron microscope (FESEM) (SUPRA-40, Carl Zeiss, Germany). To obtain clear image, electron potential and working distance were maintained to be 5.00 kV and 2.0e5.0 mm respectively. The elemental analysis was carried out by the X-ray photoelectron spectroscopy (XPS) (PHI 5000 Versa probe II) at a pressure of <106 Pa; pass energy of 58.700 eV, electron take off angle 45 and overall resolution 0.51 eV. To measure the gas sensing characteristics of the synthesized films the surface of the films were sputter coated with gold
electrodes. The gas sensing performance was characterized using an automated dynamic flow gas sensing measurement set-up developed in our laboratory. The details of the measurement set up is reported, elsewhere [12].
Results and discussion Structural characterization Fig. 1 shows the X-ray diffraction pattern of solution synthesized indium doped ZnO thin films deposited on quartz substrate. The presence of a strong peak (002) at 2Ø ~34.8 indicates the textured growth of the film along c-axis (perpendicular to the substrate). As reported in the literature this predominant growth could be due to the highest packing density of Zn atoms along (002) planes [13,14]. No other impurity phase has been detected in these textured films. This confirms the substitutional doping of indium atoms into ZnO lattice. From the XRD pattern, the average crystallite size (D) was estimated using the DebyeeScherer relation [15]: D ¼ 0:9l =bcosq
(1)
where l (¼0.154 nm) is the wavelength of the X-ray radiation used, q is the Bragg diffraction angle of the XRD peak and b (measured in radian) is the broadening of the diffraction line at half maxima. Using this relation the crystallite size is estimated to be ~14.3 nm. The other relevant parameters estimated from the XRD pattern are given in the inset of Fig. 1.
Micro structural characterization Fig. 2 shows a typical surface morphology of the indium doped ZnO thin films deposited on quartz substrate. As observed the film exhibits uniform and granular surface morphology. In addition, it also reflects the presence of micro pores or small voids uniformly throughout the film. From the cross-sectional micrograph (Fig. 3), the film thickness is estimated ~310 nm. The thickness uniformity is also apparent in the cross
Fig. 1 e XRD pattern of solegel derived indium doped ZnO thin film deposited on fused quartz substrate. The relevant parameters estimated from XRD pattern is shown in the inset.
Please cite this article in press as: Pati S, et al., n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.075
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e8
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Fig. 2 e FESEM micrograph showing the surface morphology of indium doped ZnO thin film.
sectional micrograph. This uniformity in surface and cross sectional morphology is attributed to the orderly arrangement of indium atoms inside ZnO lattice.
X-ray photoelectron spectroscopic studies Further, X-ray photoelectron spectroscopy (XPS) measurement was performed to investigate the quality and composition of the film. Fig. 4 shows the XPS spectra of (a) zinc, (b) indium, and (c) oxygen. The elemental binding energies of these elements were corrected based on Carbon (C) 1S binding energy at 284.5 eV. Fig. 4(a) shows two peaks at 1021.37 eV and 1044.48 eV which are typical corresponding to Zn 2p3/2 and Zn 2p5/2 respectively [16]. Incorporation of indium in ZnO lattice is confirmed from the presence of two characteristic peaks of In 3d3/2 and In 3d5/2 at 444.39 eV and 451.89 eV respectively, (Fig. 4(b)). Fig. 4(c) shows O1s peak at binding energy ~530.24 eV which was deconvoluted into two peaks at binding
Fig. 4 e XPS spectrum of indium doped ZnO thin film for (a) zinc, (b) indium, and (c) oxygen.
energies of 530.24 eV and 530.52 eV. The atomic ratio of Zn:In:O matches well with the nominal film composition.
Gas sensing characteristics
Fig. 3 e FESEM micrograph showing the cross-sectional morphology and the thickness uniformity of the indium doped ZnO thin film.
The grown indium doped ZnO thin films are characterized in terms of their reducing (H2) and oxidizing (NO2) gas sensing characteristics. It is well known that ZnO is an n-type semiconductor and this n-type conductivity increases further by doping it with indium. The gas sensing mechanism for a typical n-type semiconducting metal oxide gas sensor is well explained in the literature [17e19]. In brief, when the sensor is kept at elevated temperature oxygen is adsorbed on it by
Please cite this article in press as: Pati S, et al., n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.075
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Sensing material
Gas type
Observed anomalous behavior
Underlying mechanism for this anomaly
Ref
a-Fe2O3
H2S
nep transition sensing behavior induced by the variation of working temperature and pen transition sensing behavior related to the increase of H2S concentration
[7]
In2O3
CO, H2
SnO2
NH3 , NO2
ZnO
C4H10
a-Fe2O3
O2
At Toper <250e300 C the decrease of In2O3 film conductivity took place Gas response of the SnO2 films to NH3 and NO2 gases shows an increase and decrease in resistance, respectively. At sensor operating temperature in the range 350e380 C, ‘n’ to ‘p’ type carrier reversal in ZnO thin films were observed during butane sensing. Switching from n- to p- type conductivity
ZnO
NO2
n-p transition for ZnO nanotubes on exposure of NO2 gas
TiO2/ Poly (vinyl acetate) composite ZnO
NO2
Unusual response (n- to p- inversion) patterns were observed at high NO2 concentrations (> 12.5 ppm)
Large density of unstable surface states resulting from high surface-to-volume ratio would be beneficial for the formation of a surface inversion layer and account for the nep transition The mechanism of CO and H2 interactions with chemisorbed molecular oxygen has been proposed to explain the observed effect. The observed p-type conductivity is attributed to the holes generated by these interstitial oxygen ions. The cracking of alkanes and reaction of the lattice oxygen of ZnO with the highly reactive species (formed after cracking), explained the observed carrier reversal The oxygen adsorption and formation of a surface inversion layer and, therefore, to the inversion of the surface conduction type The changes of majority carrier density can lead to the inversion of the type of mobile carriers at the surface This is attributed to the surface-trap limited conduction facilitated by the high surface-to-volume ratio of this material
H2
The conductivity type of both the unannealed and annealed ZnO films converted from p-type to n-type while increasing the operating temperature
TeO2
ethanol
n- to p- type conductivity is induced by the variation of the ethanol concentration
SnO2
H2
p- and ndoped ZnO
O2 or H2
Fe-doped SnO2
O2
The change of work function in the same concentration of hydrogen shows different trends according to the variation of the film temperature Exposure of p-type ZnO films to relatively high H2 concentrations (>1000 ppm) inverts the majority carriers from holes to electrons in a reversible way It responded as a p-type semiconductor to oxygen concentrations of up to 10% at 300 C. As the temperature increased to 400 C, the material responded as an n-type semiconductor.
The origin of the p-type conductivity in the unannealed and annealed ZnO films should be attributed to oxygen related defects and zinc vacancies related defects, respectively. For the n-type semiconductor, the conductance increases due to the increase of electron concentration in the semiconductor and for the p- type semiconductor, the conductance decreases because of the combination of holes with electrons released from the surface reaction. The conductivity change of the sensors depends on the change of carrier concentration which results from the adsorption or desorption of oxidizing or reducing gases on the surface The sensor response to O2 or H2 was found to be profoundly dependent on the carrier type and concentration As the operating temperature increases, a greater number of electrons jump into the conduction band from the valence band, thus switches from p- (hole dominated) to n-type (electron dominated).
[8] [9] [10]
[23] [24] [25]
[26]
[6]
[27]
[28]
[29]
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e8
Please cite this article in press as: Pati S, et al., n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.075
Table 1 e Summary of the anomalous behavior in gas sensing reported in recent literature.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e8
trapping electrons from the conduction band of metal oxide, thus leading to an increase in resistance. Now the resistance (Ra) attains saturation with the saturation of oxygen adsorption. Reaction of reducing gas with this adsorbed oxygen brings back the trapped electrons to the conduction band which leads to a decrease in resistance. When all the trapped electrons are returned back, the resistance (Rg) get saturated. The sensor response (S) is estimated from the following relation [19]: S¼
Ra Rg 100 for n type sensorÞ Ra
(2)
S¼
Rg Ra 100 for p type sensorÞ Rg
(3)
During test gas detection, the response and recovery time of these n-type sensors were estimated as the time required for 63% change of their base resistance, resistance measured in air (response time (tres)) or the resistance measured in test gas (recovery time (trec)). For p-type response the above is reversed [20]. From this gas sensing mechanism of n-type semiconducting metal oxides, it is known that the resistance of the sensor decreases on exposure of reducing gases (e.g. H2, CO) whereas it increases in presence of oxidizing gases (e.g. O3, NO2) [21,22]. However in numerous literature reports anomalous behavior have been reported in various sensor materials [6e10,23e29]. Table 1 summarizes some of these reported results. As outlined in Table 1 the origin of such anomaly is widely varied. Thus, Korotcenkov et al. [8] observed the acceptor-like effect in In2O3 thin films and suggested the participation of surface molecular oxygen and chemisorbed water (OHgroups) in conversion of reducing gases, for the unusual behavior. They considered the conversion of chemisorbed oxygen from molecular form to atomic one during the low temperature interaction with reducing gases. Such process results in the increase of total amount of surface charge and, therefore, leads to the decrease of the In2O3 film conductivity. Wang et al. [24] reported n-p transition for ZnO nanotubes on exposure to NO2 gas. They explained that since the surface conduction of semiconducting oxides arises from the contributions of both electrons and holes, the changes of majority carrier density can lead to the inversion of the type of mobile carriers at the surface. The n to p transition occurs when the electron concentration is smaller than the hole concentration. B Yea et al. [27] explained this change in conductivity using work function measurement and stated that the conductivity change of the sensors depends on the change of carrier concentration which results from the adsorption or desorption of oxidizing or reducing gases on the surface. From the reported gas sensing mechanism it can be presumed that conversion of these carriers either from n to p type (or vice-versa), plays a significant role in selective detection of a gas, which still remains as one of the major problems in most of the semiconducting metal oxide gas sensors. In the present work we also noticed an unusual behavior of gas response in presence of hydrogen gas (reducing gas) at lower operating temperatures. Fig. 5(aed) shows resistance transients during hydrogen sensing (1660 ppm) at various operating temperatures, ranging 200 to 300 C. As shown in
5
Fig. 5 the resistance decreases at 300 C (Fig. 5a), indicating the n-type response of the sensor. It may be noted that at 300 C the sensor can respond up to 1 ppm or even less amount of hydrogen gas and at each concentration level it exhibits this n-type response, as shown in Fig. 6(a). Response % estimated using Eq. 2 is marked in the figure. However, decreasing the temperature to 250 C (Fig. 5b) the resistance plot shows an unusual behavior. The resistance first increases on exposure of hydrogen gas, then it decrease subsequently, indicating initially a n-p transition. Further decrease in temperature to 225 C (Fig. 5c), results in the enhancement of this p-type response and finally at 200 C (Fig. 5d) the sensor shows fully p-type behavior. As shown in Fig. 6(b) the response at 200 C is also measured at various test gas concentrations. It is found that for each test gas concentrations, the response is p-type. Response % is estimated using Eq. 3 and is marked in the figure. For both n- and p- type response (as shown in Fig. 6(a) and (b)), response %, response and recovery time (tres or trec) is estimated from the resistance transients at each ppm level and is tabulated in Table 2. Note that the response % almost linearly decreases with decrease in gas concentration and the response and recovery time is systematically increased with the decrease in test gas concentration. To confirm the carrier reversal further, the gas sensing characteristics is also studied in an oxidizing gas (NO2) environment. Fig. 7 shows typical resistance transients in presence of 5 ppm of NO2 in the identical range of operating temperature (300e200) C. However, in this case no such carrier reversal is noticed. As a typical n-type semiconducting gas sensor as expected, the resistance increases at each temperature upon exposure of the oxidizing gas and get saturated probably after complete reaction with the adsorbed oxygen species. The same trend was also observed by Y. S. Kim et al. [30]. This anomalous behavior has been attributed to the
Fig. 5 e Resistance transients of indium doped ZnO thin film towards the detection of 1660 ppm of H2 (reducing gas) at operating temperatures (a) 300 C, (b) 250 C, (c) 225 C, and (d) 200 C. The n to p transition is clearly identified with the reduction of sensor operating temperatures.
Please cite this article in press as: Pati S, et al., n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.075
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Fig. 6 e Resistance transients of indium doped ZnO thin film gas sensor recorded in presence of various H2 gas concentration: (a) at 300 C showing n-type behavior, and (b) at 200 C showing p-type behavior.
higher surface-to-volume ratio and active adsorption sites of the WO3 nanorod based films. At these adsorption sites the adsorbates might act as active scattering centers and suppress the electrical conduction of the free carriers, which results in the increase in resistance for both oxidizing and reducing gases.
Discussion For n-type semiconducting metal oxide thin film gas sensors [8], it is known that depending on the sensor operating tem 2 perature, various forms of oxygen (O 2 , O , O ) take part in gas sensing. At lower temperature (~200 C) molecular oxygen (O 2) is known to have the dominant role and its reaction with the H2 gas molecule can be expressed as follows [27]: H2 þ O 2 þ e ¼ 2OH
(4)
H2 þ O ¼ H2 O þ e
(5)
H2 þ 2O2 ¼ 2OH þ 2e
(6)
However, in the temperature range (200e250) C, the complex behavior (shown in Fig. 5(b,c)) is presumed to be the result of n-p transition, as in this temperature range there is a probability of the presence of both the molecular and atomic oxygen. It is noteworthy that, the surface conductivity (or resistance) mostly depends on the majority carrier concentration (electron or hole). Indium doped ZnO being an n-type semiconductor, if the concentration of electron is more, after reaction with hydrogen gas, it will show a decrease in resistance and higher hole concentration will lead to an increase in resistance. Thus, the nature of the measured resistance
As the electrons are trapped for this reaction to proceed, the conductivity of the material decreases leading to the increase in resistance (see in Fig. 5(d)). However, the reaction of H2 with atomic oxygen (O and O2) (see Eqs. 5 and 6) releases electrons trapped by the oxygen molecules to the conduction band of ZnO. At higher operating temperatures (T 250 oC) these are dominant reactions which result increase in electron concentration in conduction band. Therefore, as shown in Fig. 5(a), resistance decreases with time.
Table 2 e Values of response %, response time (tres) and recovery time (trec) estimated from the recorded resistance transients of indium doped ZnO thin film at 300 C and 200 C. The test gas concentration was kept in the range 50e1660 ppm. Gas conc. (ppm)
1660 1000 500 200 100 50
300 C (n-type response)
200 C (p-type response)
S (%)
tres (s)
trec (s)
S (%)
tres (s)
trec (s)
89 77 65 54 43 41
23 28 35 40 46 53
51 59 64 69 74 79
13 11 8 5 3 2.5
63 71 89 98 111 119
65 74 95 103 115 123
Fig. 7 e Resistance transients of indium doped ZnO thin film gas sensor towards the detection of 5 ppm of NO2 (oxidizing gas) at different operating temperatures (300 C, 250 C, 225 C, and 200 C), showing n-type behavior of the sensor.
Please cite this article in press as: Pati S, et al., n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.075
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e8
transients can be used as an effective tool for selective detection of hydrogen gas at lower operating temperature. However, further investigation is needed to better understand the origin of such anamoly.
Conclusion In summary, indium doped ZnO thin films were synthesized using solegel spin coating technique. Substitution of indium in ZnO lattice was confirmed by x-ray diffraction and XPS analysis in conjunction with microstructure characterization. Elemental analysis of the grown films was also performed using XPS measurement. Gas sensing performance of these thin films was investigated in both reducing (H2) and oxidizing (NO2) gas environment by varying the operating temperatures and test gas concentration. An unusual behavior (p-type behavior) of gas response was observed in presence of H2 gas at lower operating temperature (~200 C). However, the response switched to n-type at ~300 C, with a p-n transition in the temperature range (200e300) C. For a given operating temperature the type of response remain unaltered at each concentration of gas. However, no such carrier reversal is noticed in presence of oxidising gas, NO2. This transition behavior in presence of H2 gas is attributed to the participation of a variety of surface adsorbed oxygen species in gas sensing at different operating temperatures. We have postulated that the nature of adsorbed oxygen species controls the carrier reversal phenomenon during H2 gas sensing.
Acknowledgments The above research work was partially supported by the research grants from IBSA-DST (Grant No. INT/ IBSA/04/2011/ 27.04.2011). One of the authors (S Pati) wishes to acknowledge AICTE and BIET, Bhadrak, Orissa, India for research fellowship.
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Please cite this article in press as: Pati S, et al., n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.075
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n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors Sumati Pati*, P. Banerji, S.B. Majumder Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India
article info
abstract
Article history:
Selectivity has been the central issue in the research area of semiconducting metal oxide
Received 18 April 2014
gas sensors and still remains a challenge for the researchers. In the present work, we
Received in revised form
report a promising hydrogen (reducing gas) sensing characteristics of nanocrystalline in-
9 July 2014
dium doped zinc oxide thin film sensing elements. At sensor operating temperature in the
Accepted 14 July 2014
range (200e300) C, ‘n’ to ‘p’ type carrier reversal is detected in the measured resistance
Available online xxx
transients. Furthermore, for a given operating temperature the type of carriers remain unaltered for each concentrations of gas. Thus, temperature plays a crucial role for this
Keywords:
transition irrespective of the gas concentration. However, such carrier reversal is not
Thin film
observed in NO2 (oxidizing gas) environment even with the variation of operating tem-
Indium doping
perature and gas concentration. This is attributed to the gas sensing mechanism related to
Carrier reversal
the surface conductivity and the underlying variations of the carrier type. It is argued that,
Gas sensor
it can pave the way for selective detection of hydrogen at lower operating temperatures.
Selectivity
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction For a variety of applications, selective and low temperature detection of flammable hydrogen gas (lower explosion limit is 4% in air) is highly desirable. Large number of studies are devoted to detect hydrogen in various industrial applications, as well as automobile, space and aeronautics sectors [1,2]. Factors such as lower cost, ease of production and operation, simplicity in use, make semiconducting metal oxides (especially ZnO) as attractive hydrogen gas sensors. Undoped ZnO exhibits n-type conductivity due to the presence of intrinsic 00 defects such as oxygen vacancies (VO ,) and zinc interstitials 00 (Zni ). Doping with higher valence impurities (viz. aluminum (Al), tin (Sn), iron (Fe), indium (In) etc.) further enhances its
conductivity [3e5]. Recently thin film type sensing elements are opted to make miniaturized integrated gas sensors. Since the gas sensing mechanism in these thin films involve surface oxidation, electron exchange and desorption of the reaction product; the measured resistance transients (during response and recovery) depends on a variety of factors interrelated to each other. For example, the response is related to the type (n or p) and concentration of intrinsic charge carriers. During gas detection, depending on the sensing material, the characteristics of the sensing layer is changed in various ways and the transducer subsequently transforms it to an electrical signal. When the chemi-resistive gas sensors are heated in ambient air oxygen is chemi-adsorbed on the sensor surface. When exposed to reducing gases at constant temperature the sensor resistance decreases for ‘n’- type semiconductors, whereas it
* Corresponding author. Tel. þ91 09438855121. E-mail addresses: [email protected], [email protected] (S. Pati). http://dx.doi.org/10.1016/j.ijhydene.2014.07.075 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Pati S, et al., n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.075
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increases when the dominant carriers are of ‘p’- type [6]. In contrast, some researchers have reported the increase in resistance of n-type semiconducting metal oxides in presence of reducing gases under different operating conditions. Various mechanisms are proposed to explain the observed behaviour [7e10]. In one of our previous works [10] we have reported ‘n’ to ‘p’ type transition behavior of undoped ZnO thin films during butane detection at higher operating temperature. Reviewing all these research works we felt that the origin and the nature of this n to p transition behavior is poorly understood and needs further investigation. To the best of our knowledge, for hydrogen sensing there is no report on the study of n to p transition behavior in indium doped ZnO thin films grown by economic solegel deposition technique. In this work we have studied both the reducing (H2) and oxidizing (NO2) gas sensing characteristics of indium doped ZnO thin films.n to p carrier reversal was observed during H2 sensing (for various test gas concentration) at relatively lower operating temperature regimes. The plausible mechanism related to this n-p transition is discussed.
Experimental details Zinc acetate dihydrate (purity 98%) and indium nitrate dihydrate (purity 99.99%) powders were first mixed in 97: 3 molar ratios to prepare 3 wt. % indium doped ZnO precursor. The mixed powder was dissolved in a mixture of 2-methoxyethanol and MEA (monoethanolamine) solution at room temperature. The solution was heated to 60 c and continuously stirred for 2 h to yield clear and homogeneous solution. The solution (concentration ~0.4 M) was spin cast onto quartz substrates using a spin coater unit (SCU 2007, apex instruments co.). The spin speed and spin time was kept at 3000 rpm for 30 s. Prior to the film deposition, the quartz substrates were ultrasonically cleaned for five minutes each in trichloro ethylene, followed by acetone and methanol. The substrates were then rinsed in deionized water (for three minutes), subsequently dried in flowing N2 and used for film deposition [11]. After deposition, the films were heat treated at 300 C for 5 min to evaporate the solvent and to remove the organic residuals. The coating and firing cycle was repeated to yield films with desired thickness. The films were finally annealed at 600 C for 1 h in air. The phase formation behavior of the deposited thin films were studied by X-ray diffraction (Ultima III, Rigaku, Japan) analyses using Cu Ka radiation in 2q range 20e80 at a scanning rate 3 min1. For X-ray diffraction measurements, the accelerating voltage and current were maintained at 40 kV and 30 mA respectively. The micro structural characteristics of the films were investigated using field emission scanning electron microscope (FESEM) (SUPRA-40, Carl Zeiss, Germany). To obtain clear image, electron potential and working distance were maintained to be 5.00 kV and 2.0e5.0 mm respectively. The elemental analysis was carried out by the X-ray photoelectron spectroscopy (XPS) (PHI 5000 Versa probe II) at a pressure of <106 Pa; pass energy of 58.700 eV, electron take off angle 45 and overall resolution 0.51 eV. To measure the gas sensing characteristics of the synthesized films the surface of the films were sputter coated with gold
electrodes. The gas sensing performance was characterized using an automated dynamic flow gas sensing measurement set-up developed in our laboratory. The details of the measurement set up is reported, elsewhere [12].
Results and discussion Structural characterization Fig. 1 shows the X-ray diffraction pattern of solution synthesized indium doped ZnO thin films deposited on quartz substrate. The presence of a strong peak (002) at 2Ø ~34.8 indicates the textured growth of the film along c-axis (perpendicular to the substrate). As reported in the literature this predominant growth could be due to the highest packing density of Zn atoms along (002) planes [13,14]. No other impurity phase has been detected in these textured films. This confirms the substitutional doping of indium atoms into ZnO lattice. From the XRD pattern, the average crystallite size (D) was estimated using the DebyeeScherer relation [15]: D ¼ 0:9l =bcosq
(1)
where l (¼0.154 nm) is the wavelength of the X-ray radiation used, q is the Bragg diffraction angle of the XRD peak and b (measured in radian) is the broadening of the diffraction line at half maxima. Using this relation the crystallite size is estimated to be ~14.3 nm. The other relevant parameters estimated from the XRD pattern are given in the inset of Fig. 1.
Micro structural characterization Fig. 2 shows a typical surface morphology of the indium doped ZnO thin films deposited on quartz substrate. As observed the film exhibits uniform and granular surface morphology. In addition, it also reflects the presence of micro pores or small voids uniformly throughout the film. From the cross-sectional micrograph (Fig. 3), the film thickness is estimated ~310 nm. The thickness uniformity is also apparent in the cross
Fig. 1 e XRD pattern of solegel derived indium doped ZnO thin film deposited on fused quartz substrate. The relevant parameters estimated from XRD pattern is shown in the inset.
Please cite this article in press as: Pati S, et al., n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.075
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e8
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Fig. 2 e FESEM micrograph showing the surface morphology of indium doped ZnO thin film.
sectional micrograph. This uniformity in surface and cross sectional morphology is attributed to the orderly arrangement of indium atoms inside ZnO lattice.
X-ray photoelectron spectroscopic studies Further, X-ray photoelectron spectroscopy (XPS) measurement was performed to investigate the quality and composition of the film. Fig. 4 shows the XPS spectra of (a) zinc, (b) indium, and (c) oxygen. The elemental binding energies of these elements were corrected based on Carbon (C) 1S binding energy at 284.5 eV. Fig. 4(a) shows two peaks at 1021.37 eV and 1044.48 eV which are typical corresponding to Zn 2p3/2 and Zn 2p5/2 respectively [16]. Incorporation of indium in ZnO lattice is confirmed from the presence of two characteristic peaks of In 3d3/2 and In 3d5/2 at 444.39 eV and 451.89 eV respectively, (Fig. 4(b)). Fig. 4(c) shows O1s peak at binding energy ~530.24 eV which was deconvoluted into two peaks at binding
Fig. 4 e XPS spectrum of indium doped ZnO thin film for (a) zinc, (b) indium, and (c) oxygen.
energies of 530.24 eV and 530.52 eV. The atomic ratio of Zn:In:O matches well with the nominal film composition.
Gas sensing characteristics
Fig. 3 e FESEM micrograph showing the cross-sectional morphology and the thickness uniformity of the indium doped ZnO thin film.
The grown indium doped ZnO thin films are characterized in terms of their reducing (H2) and oxidizing (NO2) gas sensing characteristics. It is well known that ZnO is an n-type semiconductor and this n-type conductivity increases further by doping it with indium. The gas sensing mechanism for a typical n-type semiconducting metal oxide gas sensor is well explained in the literature [17e19]. In brief, when the sensor is kept at elevated temperature oxygen is adsorbed on it by
Please cite this article in press as: Pati S, et al., n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.075
4
Sensing material
Gas type
Observed anomalous behavior
Underlying mechanism for this anomaly
Ref
a-Fe2O3
H2S
nep transition sensing behavior induced by the variation of working temperature and pen transition sensing behavior related to the increase of H2S concentration
[7]
In2O3
CO, H2
SnO2
NH3 , NO2
ZnO
C4H10
a-Fe2O3
O2
At Toper <250e300 C the decrease of In2O3 film conductivity took place Gas response of the SnO2 films to NH3 and NO2 gases shows an increase and decrease in resistance, respectively. At sensor operating temperature in the range 350e380 C, ‘n’ to ‘p’ type carrier reversal in ZnO thin films were observed during butane sensing. Switching from n- to p- type conductivity
ZnO
NO2
n-p transition for ZnO nanotubes on exposure of NO2 gas
TiO2/ Poly (vinyl acetate) composite ZnO
NO2
Unusual response (n- to p- inversion) patterns were observed at high NO2 concentrations (> 12.5 ppm)
Large density of unstable surface states resulting from high surface-to-volume ratio would be beneficial for the formation of a surface inversion layer and account for the nep transition The mechanism of CO and H2 interactions with chemisorbed molecular oxygen has been proposed to explain the observed effect. The observed p-type conductivity is attributed to the holes generated by these interstitial oxygen ions. The cracking of alkanes and reaction of the lattice oxygen of ZnO with the highly reactive species (formed after cracking), explained the observed carrier reversal The oxygen adsorption and formation of a surface inversion layer and, therefore, to the inversion of the surface conduction type The changes of majority carrier density can lead to the inversion of the type of mobile carriers at the surface This is attributed to the surface-trap limited conduction facilitated by the high surface-to-volume ratio of this material
H2
The conductivity type of both the unannealed and annealed ZnO films converted from p-type to n-type while increasing the operating temperature
TeO2
ethanol
n- to p- type conductivity is induced by the variation of the ethanol concentration
SnO2
H2
p- and ndoped ZnO
O2 or H2
Fe-doped SnO2
O2
The change of work function in the same concentration of hydrogen shows different trends according to the variation of the film temperature Exposure of p-type ZnO films to relatively high H2 concentrations (>1000 ppm) inverts the majority carriers from holes to electrons in a reversible way It responded as a p-type semiconductor to oxygen concentrations of up to 10% at 300 C. As the temperature increased to 400 C, the material responded as an n-type semiconductor.
The origin of the p-type conductivity in the unannealed and annealed ZnO films should be attributed to oxygen related defects and zinc vacancies related defects, respectively. For the n-type semiconductor, the conductance increases due to the increase of electron concentration in the semiconductor and for the p- type semiconductor, the conductance decreases because of the combination of holes with electrons released from the surface reaction. The conductivity change of the sensors depends on the change of carrier concentration which results from the adsorption or desorption of oxidizing or reducing gases on the surface The sensor response to O2 or H2 was found to be profoundly dependent on the carrier type and concentration As the operating temperature increases, a greater number of electrons jump into the conduction band from the valence band, thus switches from p- (hole dominated) to n-type (electron dominated).
[8] [9] [10]
[23] [24] [25]
[26]
[6]
[27]
[28]
[29]
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e8
Please cite this article in press as: Pati S, et al., n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.075
Table 1 e Summary of the anomalous behavior in gas sensing reported in recent literature.
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trapping electrons from the conduction band of metal oxide, thus leading to an increase in resistance. Now the resistance (Ra) attains saturation with the saturation of oxygen adsorption. Reaction of reducing gas with this adsorbed oxygen brings back the trapped electrons to the conduction band which leads to a decrease in resistance. When all the trapped electrons are returned back, the resistance (Rg) get saturated. The sensor response (S) is estimated from the following relation [19]: S¼
Ra Rg 100 for n type sensorÞ Ra
(2)
S¼
Rg Ra 100 for p type sensorÞ Rg
(3)
During test gas detection, the response and recovery time of these n-type sensors were estimated as the time required for 63% change of their base resistance, resistance measured in air (response time (tres)) or the resistance measured in test gas (recovery time (trec)). For p-type response the above is reversed [20]. From this gas sensing mechanism of n-type semiconducting metal oxides, it is known that the resistance of the sensor decreases on exposure of reducing gases (e.g. H2, CO) whereas it increases in presence of oxidizing gases (e.g. O3, NO2) [21,22]. However in numerous literature reports anomalous behavior have been reported in various sensor materials [6e10,23e29]. Table 1 summarizes some of these reported results. As outlined in Table 1 the origin of such anomaly is widely varied. Thus, Korotcenkov et al. [8] observed the acceptor-like effect in In2O3 thin films and suggested the participation of surface molecular oxygen and chemisorbed water (OHgroups) in conversion of reducing gases, for the unusual behavior. They considered the conversion of chemisorbed oxygen from molecular form to atomic one during the low temperature interaction with reducing gases. Such process results in the increase of total amount of surface charge and, therefore, leads to the decrease of the In2O3 film conductivity. Wang et al. [24] reported n-p transition for ZnO nanotubes on exposure to NO2 gas. They explained that since the surface conduction of semiconducting oxides arises from the contributions of both electrons and holes, the changes of majority carrier density can lead to the inversion of the type of mobile carriers at the surface. The n to p transition occurs when the electron concentration is smaller than the hole concentration. B Yea et al. [27] explained this change in conductivity using work function measurement and stated that the conductivity change of the sensors depends on the change of carrier concentration which results from the adsorption or desorption of oxidizing or reducing gases on the surface. From the reported gas sensing mechanism it can be presumed that conversion of these carriers either from n to p type (or vice-versa), plays a significant role in selective detection of a gas, which still remains as one of the major problems in most of the semiconducting metal oxide gas sensors. In the present work we also noticed an unusual behavior of gas response in presence of hydrogen gas (reducing gas) at lower operating temperatures. Fig. 5(aed) shows resistance transients during hydrogen sensing (1660 ppm) at various operating temperatures, ranging 200 to 300 C. As shown in
5
Fig. 5 the resistance decreases at 300 C (Fig. 5a), indicating the n-type response of the sensor. It may be noted that at 300 C the sensor can respond up to 1 ppm or even less amount of hydrogen gas and at each concentration level it exhibits this n-type response, as shown in Fig. 6(a). Response % estimated using Eq. 2 is marked in the figure. However, decreasing the temperature to 250 C (Fig. 5b) the resistance plot shows an unusual behavior. The resistance first increases on exposure of hydrogen gas, then it decrease subsequently, indicating initially a n-p transition. Further decrease in temperature to 225 C (Fig. 5c), results in the enhancement of this p-type response and finally at 200 C (Fig. 5d) the sensor shows fully p-type behavior. As shown in Fig. 6(b) the response at 200 C is also measured at various test gas concentrations. It is found that for each test gas concentrations, the response is p-type. Response % is estimated using Eq. 3 and is marked in the figure. For both n- and p- type response (as shown in Fig. 6(a) and (b)), response %, response and recovery time (tres or trec) is estimated from the resistance transients at each ppm level and is tabulated in Table 2. Note that the response % almost linearly decreases with decrease in gas concentration and the response and recovery time is systematically increased with the decrease in test gas concentration. To confirm the carrier reversal further, the gas sensing characteristics is also studied in an oxidizing gas (NO2) environment. Fig. 7 shows typical resistance transients in presence of 5 ppm of NO2 in the identical range of operating temperature (300e200) C. However, in this case no such carrier reversal is noticed. As a typical n-type semiconducting gas sensor as expected, the resistance increases at each temperature upon exposure of the oxidizing gas and get saturated probably after complete reaction with the adsorbed oxygen species. The same trend was also observed by Y. S. Kim et al. [30]. This anomalous behavior has been attributed to the
Fig. 5 e Resistance transients of indium doped ZnO thin film towards the detection of 1660 ppm of H2 (reducing gas) at operating temperatures (a) 300 C, (b) 250 C, (c) 225 C, and (d) 200 C. The n to p transition is clearly identified with the reduction of sensor operating temperatures.
Please cite this article in press as: Pati S, et al., n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.075
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Fig. 6 e Resistance transients of indium doped ZnO thin film gas sensor recorded in presence of various H2 gas concentration: (a) at 300 C showing n-type behavior, and (b) at 200 C showing p-type behavior.
higher surface-to-volume ratio and active adsorption sites of the WO3 nanorod based films. At these adsorption sites the adsorbates might act as active scattering centers and suppress the electrical conduction of the free carriers, which results in the increase in resistance for both oxidizing and reducing gases.
Discussion For n-type semiconducting metal oxide thin film gas sensors [8], it is known that depending on the sensor operating tem 2 perature, various forms of oxygen (O 2 , O , O ) take part in gas sensing. At lower temperature (~200 C) molecular oxygen (O 2) is known to have the dominant role and its reaction with the H2 gas molecule can be expressed as follows [27]: H2 þ O 2 þ e ¼ 2OH
(4)
H2 þ O ¼ H2 O þ e
(5)
H2 þ 2O2 ¼ 2OH þ 2e
(6)
However, in the temperature range (200e250) C, the complex behavior (shown in Fig. 5(b,c)) is presumed to be the result of n-p transition, as in this temperature range there is a probability of the presence of both the molecular and atomic oxygen. It is noteworthy that, the surface conductivity (or resistance) mostly depends on the majority carrier concentration (electron or hole). Indium doped ZnO being an n-type semiconductor, if the concentration of electron is more, after reaction with hydrogen gas, it will show a decrease in resistance and higher hole concentration will lead to an increase in resistance. Thus, the nature of the measured resistance
As the electrons are trapped for this reaction to proceed, the conductivity of the material decreases leading to the increase in resistance (see in Fig. 5(d)). However, the reaction of H2 with atomic oxygen (O and O2) (see Eqs. 5 and 6) releases electrons trapped by the oxygen molecules to the conduction band of ZnO. At higher operating temperatures (T 250 oC) these are dominant reactions which result increase in electron concentration in conduction band. Therefore, as shown in Fig. 5(a), resistance decreases with time.
Table 2 e Values of response %, response time (tres) and recovery time (trec) estimated from the recorded resistance transients of indium doped ZnO thin film at 300 C and 200 C. The test gas concentration was kept in the range 50e1660 ppm. Gas conc. (ppm)
1660 1000 500 200 100 50
300 C (n-type response)
200 C (p-type response)
S (%)
tres (s)
trec (s)
S (%)
tres (s)
trec (s)
89 77 65 54 43 41
23 28 35 40 46 53
51 59 64 69 74 79
13 11 8 5 3 2.5
63 71 89 98 111 119
65 74 95 103 115 123
Fig. 7 e Resistance transients of indium doped ZnO thin film gas sensor towards the detection of 5 ppm of NO2 (oxidizing gas) at different operating temperatures (300 C, 250 C, 225 C, and 200 C), showing n-type behavior of the sensor.
Please cite this article in press as: Pati S, et al., n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.075
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e8
transients can be used as an effective tool for selective detection of hydrogen gas at lower operating temperature. However, further investigation is needed to better understand the origin of such anamoly.
Conclusion In summary, indium doped ZnO thin films were synthesized using solegel spin coating technique. Substitution of indium in ZnO lattice was confirmed by x-ray diffraction and XPS analysis in conjunction with microstructure characterization. Elemental analysis of the grown films was also performed using XPS measurement. Gas sensing performance of these thin films was investigated in both reducing (H2) and oxidizing (NO2) gas environment by varying the operating temperatures and test gas concentration. An unusual behavior (p-type behavior) of gas response was observed in presence of H2 gas at lower operating temperature (~200 C). However, the response switched to n-type at ~300 C, with a p-n transition in the temperature range (200e300) C. For a given operating temperature the type of response remain unaltered at each concentration of gas. However, no such carrier reversal is noticed in presence of oxidising gas, NO2. This transition behavior in presence of H2 gas is attributed to the participation of a variety of surface adsorbed oxygen species in gas sensing at different operating temperatures. We have postulated that the nature of adsorbed oxygen species controls the carrier reversal phenomenon during H2 gas sensing.
Acknowledgments The above research work was partially supported by the research grants from IBSA-DST (Grant No. INT/ IBSA/04/2011/ 27.04.2011). One of the authors (S Pati) wishes to acknowledge AICTE and BIET, Bhadrak, Orissa, India for research fellowship.
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Please cite this article in press as: Pati S, et al., n- to p- type carrier reversal in nanocrystalline indium doped ZnO thin film gas sensors, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.07.075