Characterization and sensing properties of ZnO film prepared by single source chemical vapor deposition

Advanced Powder Technology xxx (2016) xxx–xxx

Contents lists available at ScienceDirect

Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt

Original Research Paper

Characterization and sensing properties of ZnO film prepared by single source chemical vapor deposition Germán Escalante a,1, Héctor Juárez b, Paloma Fernández a,⇑ a b

Depto. de Física de Materiales, Facultad de Físicas, Univ. Complutense, 28040 Madrid, Spain Centro de Investigación en Dispositivos Semiconductores, Univ. Autónoma de Puebla, 72570 Puebla, Mexico

a r t i c l e

i n f o

Article history: Received 11 April 2016 Received in revised form 1 July 2016 Accepted 7 July 2016 Available online xxxx Keywords: ZnO film SSCVD X-ray diffraction Cathodoluminescence Sensing

a b s t r a c t A novel deposition technique has been used to grow ZnO films. Good quality films were obtained on glass substrates by single source chemical vapor deposition (SSCVD), for gas sensing applications. The properties of ZnO films were investigated at different deposition temperatures 300, 350 and 400 °C. X-ray diffraction results show that all deposited films were polycrystalline. The morphological, structural, optical and electrical properties of the films have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), cathodoluminescence (CL) and Hall effect techniques. The morphology of the deposited films evolves from columnar grains, to parallel plates as the substrate temperature increases. A significant increase in the relative intensities of the green and red emission with increasing deposition temperature has been observed. Electrical properties, relevant for gas sensing behavior have been investigated as well. In the particular case of CO an operating temperature of 300 °C seems to yield the best sensitivity. Ó 2016 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.

1. Introduction Gas sensors based on semiconducting metal oxides have gained special importance in the last years. The diversity of applications such as detection of explosive and inflammable gases, environmental monitoring, health care, and air-quality detection has driven relevant studies [1]. Nevertheless, the mechanism of the gas sensing is something complex and still under controversy. There is a general agreement on the preponderant role of the surface properties and surface reactions on these materials, and consequently, on the importance for the sensing performances of the morphology and the microstructure of materials, namely grain size, crystal structure, surface area, porosity, etc. [2]. The gas sensing characteristics of numerous materials such as ZnO, SnO2, TiO2, and WO3 have been reported in the literature [3–6]. Particularly, ZnO offers the advantages of being nontoxic and easily obtained. ZnO is characterized as a wide band gap semiconductor (3.3 eV) with a large exciton binding energy (60 meV) and n-type conductivity [7]. Many techniques have been used to produce ZnO films, such as pulsed laser deposition (PLD) [8], sputtering [9], spray ⇑ Corresponding author. Fax: +34 913944547. E-mail address: [email protected] (P. Fernández). Formerly at Centro de Investigación en Dispositivos Semiconductores, Univ. Autónoma de Puebla, 72570 Puebla, Mexico. 1

pyrolysis [10] and atmospheric pressure chemical vapor deposition (APCVD) [11]. In the present work, single source chemical vapor deposition (SSCVD) reveals as a useful technique for preparing films, offering the simplicity of having all film components contained within one molecule [12], and some additional advantages as the use of non-severe deposition conditions. This growth method offers then the possibility of producing high-quality films using simple deposition equipment. The present work is focused on the study of the structural, optical and electrical properties of ZnO films deposited by SSCVD at atmospheric pressure. Also, the study of gas sensing properties of the ZnO film sensor using aluminum electrodes with interdigital structure is reported. 2. Experimental 2.1. Films deposition ZnO films were deposited on glass substrates using zinc acetate dehydrate (Zn(CH3COO)2
2H2O) as a single-source precursor. The temperature of the precursor was stabilized at 210 °C during the deposition. The substrates were previously cleaned by 10 min. in a piranha solution (50% sulfuric acid and 50% hydrogen peroxide) [13], rinsed in deionized water (18.2 MO cm), and dried. The distance between the source and substrate was 50 mm. For the

http://dx.doi.org/10.1016/j.apt.2016.07.005 0921-8831/Ó 2016 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.

Please cite this article in press as: G. Escalante et al., Characterization and sensing properties of ZnO film prepared by single source chemical vapor deposition, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.07.005

2

G. Escalante et al. / Advanced Powder Technology xxx (2016) xxx–xxx

experiments reported in this work, three different substrate temperatures were selected, 300, 350 and 400 °C, while the deposition time was kept in 5 min. An oxygen gas flow was used as oxidant agent and the flow rate was 50 sccm (standard cc/min at 1 atm for 25 °C). Fig. 1a shows a scheme of the deposition system, which has been more extensively described in our previous work [14]. As mentioned, the main advantage of this method, respect to other deposition techniques, is the possibility to work at lower deposition temperatures and the simplicity of the experimental setup, allowing to obtain good quality films faster, cheaper and easier than in more complex reactors. 2.2. Films characterization X Ray Diffraction (XRD) measurements were carried out by using a Bruker D8 Discover diffractometer with a X-ray source of Cu Ka radiation (k = 0.15406 nm) at 40 kV and 40 mA. The thicknesses of the films deposited were measured by means of a stylus profilometer. Surface morphologies of the films were observed by using a FEI Inspect Scanning Electron Microscope (SEM) and a Nanotec-AFM operated at room temperature. Cathodoluminescence (CL) measurements were carried out at room temperature on a Hitachi 2500 SEM at an operation voltage of 20 kV. CL spectra were obtained with a Hamamatsu PMA-11 CCD camera. The electrical properties of the films were obtained from the Hall effect measurement at room temperature in the van der Pauw configuration using the Ecopia Bridge Technology HMS-5300 system equipped with a permanent magnet yielding a field of 0.55 T. 2.3. Details of gas sensing system and sensor fabrication Sensing measurements were performed using the gas sensing system described elsewhere [15]. The system consists of a heater fixed on the base plate inside a chamber; a thermocouple and a temperature controller were used to control the heater. The gas is inserted into the chamber from a mass flow controller to control the gas flow. A digital multimeter (Keithley 2001) connected through external leads is used to measure the resistance. Sensors were fabricated by depositing a ZnO film on glass substrates with an area of 100 mm2, supplied with interdigital aluminum electrodes on top of the film as shown in Fig. 1b. The gas sensing properties were evaluated for three different operating temperatures 100, 200 and 300 °C, by measuring the changes of sensor resistance in presence of air and CO gas, respectively. The sensor was exposed at different CO concentrations ranging from 0 to 200 ppm. The sensitivity in the experiment was defined as [16]:

S¼

Ra  Rg  100% Ra

ð1Þ

where Rɑ is the resistance in air and Rg is the resistance in the test gas. 3. Results and discussion 3.1. Morphological characterization The morphological characteristics of the ZnO films were investigated by SEM and AFM. The morphology of the deposited films evolves from columnar grains, to parallel plates as the substrate temperature increases. Fig. 2 shows SEM images of the ZnO films deposited on glass substrates; at the three different temperatures used. The film deposited at 300 °C (Fig. 2a) shows columnar grains growing perpendicular to substrate. The columnar structures are common for low thermal mobility species, for which, the initial

Fig. 1. (a) Schematic drawing of the SSCVD system; (b) Schematic illustration of the sensor fabrication.

stages of film formation result in a random distribution of small crystallites, acting as a nucleus for further growth [17]. By increasing the deposition temperature to 350 °C, a higher densification in the film is observed (Fig. 2b), this can be attributed to enhanced surface diffusion, resulting in more homogeneous grains. Regarding the film thickness, it increases from 540 nm, 650 nm to 790 nm for deposition temperatures of 300, 350 and 400 °C, respectively. In the particular case of a deposition temperature of 400 °C (Fig. 2c), plate-like ZnO structures are uniformly distributed over the surface of the film with lateral dimensions of 200–600 nm and thicknesses of 50 nm. At this temperature, the higher ZnO volatility, and the subsequent high vapor pressure, would prevent the nucleation and growth of crystal orientations different from those with the lower energy [18]. Consequently, the plates are preferentially oriented along (0 0 0 1) directions. Kaneti et al. demonstrated that (0 0 0 1) planes of ZnO plates exposed to the gas species, show higher sensitivity in the detection due to higher surface area over other planes [19]. Surface roughness determines the effective surface area of the films and hence it is an important parameter for sensor applications, playing a major role on sensitivity. In the present work, Atomic Force Microscopy (AFM) was used to investigate the surface roughness of the films over an area of 100 lm2. Fig. 3 shows the AFM surface images of the ZnO films deposited at different temperatures. The root-mean square (RMS) surface roughness of the ZnO films deposited at different deposition temperatures is determined from the AFM measurements shown in Table 1. This increase in surface roughness is consistent with an adherence increase from particles in the gas phase, which would render a higher deposition ratio.

Please cite this article in press as: G. Escalante et al., Characterization and sensing properties of ZnO film prepared by single source chemical vapor deposition, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.07.005

3

G. Escalante et al. / Advanced Powder Technology xxx (2016) xxx–xxx

(a)

500 nm

5 µm

(b)

500 nm

5 µm 50 nm

(c)

(0001)

500 nm

planes, respectively, of a hexagonal wurtzite structure of ZnO with lattice parameters of ɑ = 0.325 nm and c = 0.520 nm [20]. In the films deposited at 300 °C and 350 °C, no clear no preferential orientation is observed, though in the case of the film deposited at 350 °C the (1 0 1) peak is slightly higher. When the substrate temperature is increased to 400 °C a self-texture effect is observed. According to the work of Fujimura et al. [21] this is due to the fact that at the higher deposition temperature the equilibrium configuration is achieved, and hence the film shows a (0 0 2) texture, corresponding to the lowest surface energy plane the (0 0 2) plane (9.9 eV/nm2) [22]. When the temperature of the substrate is raised to 400 °C, a higher energy is available for the atoms to migrate towards equilibrium positions, hence, the most frequently observed (0 0 2) orientation is obtained. These results are in agreement with those of Lu et al. who pointed out that a high deposition temperature is favorable for the diffusion of atoms adsorbed on the substrate and accelerates the migration of atoms to energetically favorable positions, resulting in the enhancement of crystallinity and c-axis orientation of the film [23]. A more detailed analysis of the (0 0 2) diffraction peak have been performed to obtain information on crystallite sizes and strains in the films. Fig. 5 shows an enlarged view of the (0 0 2) diffraction peaks of the films deposited. The vertical dashed line indicates the theoretical position of (0 0 2) for ZnO wurtzite structure [20]. At the lower deposition temperatures (300 °C and 350 °C), a slight shift of the (0 0 2) peak to lower diffraction angles is observed, however, as the deposition temperature increases the (0 0 2) peak position, approaches the standard 2h value of the bulk ZnO (2h = 34.42°). A peak shift towards lower angles indicates an increase in the corresponding interplanar distance, which in particular for the (0 0 2) peak is associated to an increase in the c lattice parameter. As has been discussed in connection with the description of the morphologies observed, the films grown at lower substrates temperatures present higher residual strains since the atoms do not have energy enough to migrate towards the equilibrium positions. The decrease of the lattice parameter along the caxis for the films grown at 400 °C, would then be associated to the release of the strains when the atoms reach the equilibrium positions. From the (0 0 2) peaks, we can also estimate mean crystallite size and residual strains either due to substrate misfit or intrinsic defects. Lattice parameters of ZnO hexagonal structure, a and c, were calculated using the Bragg law (k = 2d
sin h) [24] and the plane-spacing equation [25]:

1 2 dhkl

2

¼

2

4 h þ hk þ k 3 a2

!

2

þ

l c2

ð2Þ

where a and c are the lattice constants; h, k, l are Miller indices. The strain ez in ZnO films along c-axis is given by the following equation [26]:

5 µm

ez ¼ Fig. 2. SEM images of ZnO films deposited on glass substrates at a deposition temperature of (a) 300 °C, (b) 350 °C and (c) 400 °C; the insets show a highmagnification of SEM images.

3.2. Structural characterization To investigate the structural properties of the ZnO films XRD measurements were carried out. XRD patterns of the ZnO films deposited by SSCVD at different temperatures as well as the standard pattern of ZnO (JCPDS card no. 00-036-1451), are shown in Fig. 4. The films deposited show diffraction peaks at 31.7°, 34.4° and 36.2°, which can be attributed to the (1 0 0), (0 0 2) and (1 0 1)

c  c0  100% c0

ð3Þ

where c is the lattice parameter of the strained ZnO films calculated from XRD data and c0 is the unstrained lattice parameter for bulk ZnO. The results obtained for lattice parameters and strains are summarized in Table 2. According to the above equation the strain can be negative (compressive) or positive (tensile). The strain values show the presence of tensile strain in the ZnO films deposited, in agreement with the larger c lattice parameter observed in our samples. The reduction of strain with increasing deposition temperature can be attributed to the restructuring in the crystal lattice. Thus, the values of the lattice parameters of the deposited film at 400 °C approximate the ideal values.

Please cite this article in press as: G. Escalante et al., Characterization and sensing properties of ZnO film prepared by single source chemical vapor deposition, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.07.005

4

G. Escalante et al. / Advanced Powder Technology xxx (2016) xxx–xxx

(a)

(b)

410 nm

2 µm

450 nm

2 µm

0 nm

(c)

745 nm

2 µm

0 nm

0 nm

Fig. 3. AFM images showing the surface morphology of the ZnO films deposited at a deposition temperature of (a) 300 °C, (b) 350 °C and (c) 400 °C. Scanned area: 10 lm  10 lm.

Table 1 AFM measurements for the ZnO films deposited at different deposition temperatures. Deposition temperature (°C)

Roughness RMS (nm)

Max. height (nm)

300 350 400

43.2 51.9 58.3

291.3 394.1 437.5

300°C

350°C

Intensity (arb.units.)

400°C

Intensity (arb.units.)

400ºC

350ºC

34.42º

34.0

300ºC (101) (100)

32

(002)

34

36

38

Table 2 Structural parameters of ZnO films deposited at different temperatures; XRD results for the (0 0 2) diffraction peak; lattice parameters (ɑ, c), strain along c-axis ez, FWHM and crystallite size D.

40

2 Theta (degree) Fig. 4. XRD diffractograms of ZnO films deposited at different temperatures.

The average crystallite size of the ZnO films was calculated from of the XRD peak (see Fig. 5) using Scherrer’s equation [24]:

0:9k D¼ b cos h

34.8

Fig. 5. The magnified (0 0 2) diffraction peak of the ZnO films deposited, showing shifting of the center of diffraction (vertical line) towards the left.

JCPDS 036-1451

30

34.4

2 Theta (degree)

ð4Þ

Deposition temperature (°C)

a (nm)

c (nm)

ez (%)

FWHM (0 0 2)

D (nm)

300 350 400

0.3263 0.3261 0.3251

0.5224 0.5222 0.5209

0.326 0.288 0.038

0.224 0.421 0.467

44 36 34

where b is the full width at half-maximum (FWHM), k is the wavelength of the X-ray, h is the Bragg diffraction angle. The calculated values for the crystallite size of the ZnO films deposited are summa-

Please cite this article in press as: G. Escalante et al., Characterization and sensing properties of ZnO film prepared by single source chemical vapor deposition, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.07.005

5

400ºC

1.5

2.0

2.5

3.0

Fig. 6. Normalized cathodoluminescence (CL) spectra of ZnO films deposited at different temperatures.

2

20

10

10 7

ρ μ

1

6

n

10

0

10

5 19

4

10

3

-1

10

2 1

-2

3.4. Electrical characterization

3.5

Photon Energy (eV)

10

300

350

400

Carrier concentration (cm-3)

Regarding the cathodoluminescence properties, spatially and spectrally resolved measurements have been performed. The normalized CL spectra of the ZnO films deposited at different temperatures are shown in Fig. 6. The CL measurements were obtained at room temperature and an electron beam energy of 20 keV. All ZnO films deposited show three main peaks at 3.23, 2.37 and 1.91 eV. The peak located at 3.23 eV corresponds to the near-band edge emission, originating from free and bound exciton and shallow DAP (donor–acceptor pair) recombination [28]. As shown in Fig. 6, the peak position of this band is shifted to a higher energy with increasing deposition temperature. The change in the peak position is directly correlated to the decreasing stress along the c-axis due to the lattice distortion [29] which has been already described (see results in Table 1). In fact a relative decrease of the lower energy components associated to defects and stresses is clearly observed in the spectra when the deposition temperature is increased. The peaks located at 2.37 and 1.91 eV corresponding to green and red emission, show an increase in their relative intensity with increasing deposition temperature. Recent studies have demonstrated that the visible-light emission of the ZnO is associated to native defects [30] such as oxygen vacancy (VO), antisite oxygen (OZn), interstitial zinc (Zni), zinc vacancy (VZn) and interstitial oxygen (Oi) [31–33]. The increase of the relative intensity of these emissions would then be related to the competition between the different recombination mechanisms.

300ºC 350ºC

Moblility (cm2/V·s)

3.3. Cathodoluminescence measurements

Resistivity (Ω·cm)

rized in Table 2. The full width at half-maximum (FWHM) of the (0 0 2) diffraction peak at a deposition temperature of 300, 350, and 400 °C, respectively, are 0.224, 0.421, and 0.467. Since the FWHM of the (0 0 2) diffraction peak varies inversely to the crystallite size of the film, we can conclude that the crystallite size of the ZnO films decreases by increasing the deposition temperature. This decrease in crystallite size is particularly important from the point of view of the increase in sensor sensitivity as has been reported by some authors [27].

Normalized CL Intensity (arb. units.)

G. Escalante et al. / Advanced Powder Technology xxx (2016) xxx–xxx

18

10

Deposition temperature (°C) The room temperature electrical properties of the ZnO films deposited at different temperatures are shown in Fig. 7. Increasing deposition temperature induces a steep rise in both carrier concentration and carrier mobility, which occurs for ZnO films deposited. The increase in electron concentration for films deposited can be attributed to the higher concentration of intrinsic donor defects i.e. oxygen vacancies or zinc interstitial atoms generated at the higher deposition temperatures. The reduced mobility observed in the samples grown at the lowest temperature could be a consequence of a higher density of dispersion centers due to the smaller grain size as shown in Fig. 2a. Resistivity measurements show a decrease in resistivity with increasing deposition temperature, the minimum value (2.75  102 X cm) is obtained for the film deposited at 400 °C. The values for films deposited at 300 and 350 °C are 58.2 and 1.78  101 X cm respectively. At a deposition temperature of 300 °C the concentration of donor defects associated to the emission in the visible region is smaller, therefore the carrier concentration is lower and the resistivity higher, which correlates well to the CL spectra shown. Following up this argument, for deposition temperatures of 350 and 400 °C, the concentration of native defects should be similar according to CL spectra which should render similar values for the resistivity. However, the resistivity is inversely proportional to the carrier mobility, hence an increase of the mobility, and consequently a decrease in the resistivity, associated to an increase of the film quality should be expected for the films grown at 400 °C.

Fig. 7. Room-temperature variations of electrical resistivity (q), mobility (l), and electron concentration (n) of ZnO films deposited at different temperatures (curves are just eye-guides).

3.5. Sensor characterization It is well known that the sensing mechanism of ZnO is mainly controlled by the surface [34]. Its gas sensitivity is closely related to grain size, surface states structure, adsorbed oxygen concentration, activation energy of oxygen adsorption and lattice defects [35]. Accordingly, the films which presumably have the better sensing properties are those grown at 400 °C. In particular the higher conductivity and effective surface area observed in these samples would be related to the better sensitivity. Room temperature measurements have been performed to monitor the sensor resistance within the whole CO concentration range. The value obtained is 9.58 kX, all over the range. Fig. 8 shows the sensor resistance at three different operating temperatures (100, 200 and 300 °C) for a CO concentration ranging from 0 to 200 ppm. From this figure we can see that for all operating temperatures the resistance decreases with increasing concentration of CO indicating a good sensing behavior, characteristic for n-type semiconductor oxide gas sensors [36]. It is accepted that the main mechanism behind semiconductor sensors is related to the chemisorption of oxygen on the negatively charged faces, c-

Please cite this article in press as: G. Escalante et al., Characterization and sensing properties of ZnO film prepared by single source chemical vapor deposition, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.07.005

6

G. Escalante et al. / Advanced Powder Technology xxx (2016) xxx–xxx

8.5

Resistance (kΩ)

ity did not exhibit significant differences for all the operating temperatures under 5 ppm CO concentration. However, an increase in the sensitivity was observed when the CO concentration is increased to 50 ppm, and reached a maximum for the concentration of 200 ppm at an operating temperature of 300 °C. According to Hsiao et al. [34] the sensitivity increase due to the enhancement of the CO oxidation process. The resistance decrease would be associated to the electrons donated to ZnO by the CO during oxidation, according to Takata et al. [38], one or two electrons depending on the temperature regime. In the work of Gong et al. [37] a similar behavior has also been described. At low CO concentration no significant difference on sensitivity at the different operation temperatures is found, however, at the higher operation temperature the sensitivity is considerably improved, as shown in Fig. 9.

Operating temperature: 100°C 200°C 300°C

9.0

8.0

7.5

7.0

6.5

05

50

100

150

200

CO concentration (ppm) Fig. 8. Resistance variation versus CO concentration for different operating temperatures for the ZnO film deposited at 400 °C.

18

Operating temperature: 100 ºC 200 ºC 300 ºC

Sensitivity (%)

15

12

9

6

4. Conclusions ZnO films were obtained by SSCVD at deposition temperatures 300, 350 and 400 °C. The results show significant changes on the morphology and film properties for the different deposition temperatures. According to X-ray diffraction with increasing deposition temperature, the crystallinity of the films increases because the species have enough energy to migrate to the active sites of equilibrium. SEM images were obtained corresponding to well defined polycrystalline formations (plates) compared to those obtained at lower temperatures, which columnar grains are observed. There is a correlation between the measurements of CL and the resistivity of the films, the greater the amount of native defects, the higher the decrease on resistivity. The resistivity also considerably decreased due to scattering of the carriers by a greater number of boundary of grains. The film deposited at 400 °C presumably has the better sensing properties due to the higher conductivity and effective surface area; however the maximum sensitivity was obtained when the sensor was exposed to CO concentration of 200 ppm and an operating temperature of 300 °C.

3

Acknowledgments 0 05

50

100

150

200

CO concentration (ppm) Fig. 9. Sensitivity variation as function of CO concentration of ZnO film at different operating temperatures.

type in the case of wurtzite ZnO, the reaction between the adsorbed oxygen and the gas to be detected being responsible for the change in resistivity [37]. In ZnO Takata et al. [38] described three different reactions to account for the resistivity change at different operation temperature. In the lowest temperature regime, below 100 °C, the most stable form for the oxygen ion would be  O-2, and the corresponding reaction 2CO + O 2 ? 2CO2 + e . At tem2peratures above 300 °C the most stable ion is O and the reaction occurring at the surface 2CO + O2 ? CO2 + 2e, where two instead of one electrons are liberated, hence causing a higher resistance decrease. In the intermediate regime, between 100 and 300 °C, the ion O is the most stable, giving raise to changes in resistance similar to those in the low temperature regime, through the reaction 2CO + O ? CO2 + e. This is the behavior observed from the curves in Fig. 8, where the changes in resistivity at 100 and 200 °C operation temperature are similar, while at the highest temperature, 300 °C, the decrease in the resistance is more pronounced [16,39]. Fig. 9 shows the sensitivity of the sensor as a function of CO concentration for the various operating temperature. The sensitiv-

This work was partially supported by the Spanish Ministry Economy and Competitiveness (TEC2011-22422, TEC2014-52642C2-1-R, MAT 2012-31959 and CSD2009-00013), UCM-Banco Santander, program GR3/14. G. Escalante acknowledges the priceless assistance from Dra. María de la Luz Olvera (CINVESTAV-SEES of Mexico) for her assistance in the realized sensing measurements. References [1] R. Zhou, G. Hu, R. Yu, C. Pan, Z.L. Wang, Piezotronic effect enhanced detection of flammable/toxic gases by ZnO micro/nanowire sensors, Nano Energy 12 (2015) 588–596. [2] M.E. Franke, T.J. Koplin, U. Simon, Metal and metal oxide nanoparticles in chemiresistors: does the nanoscale matter?, Small 2 (2006) 36–50 [3] P.K. Basu, P. Bhattacharyya, N. Saha, H. Saha, S. Basu, The superior performance of the electrochemically grown ZnO thin films as methane sensor, Sens. Actuators, B 133 (2008) 357–363. [4] J. Zhang, S. Wang, Y. Wang, Y. Wang, B. Zhu, H. Xia, X. Guo, S. Zhang, W. Huang, S. Wu, NO2 sensing performance of SnO2 hollow-sphere sensor, Sens. Actuators, B 135 (2009) 610–617. [5] I. Hayakawa, Y. Iwamoto, K. Kikuta, S. Hirano, Gas sensing properties of platinum dispersed-TiO2 thin film derived from precursor, Sens. Actuators, B 62 (2000) 55–60. [6] C.J. Jin, T. Yamazaki, Y. Shirai, T. Yoshizawa, T. Kikuta, N. Nakatani, H. Takeda, Dependence of NO2 gas sensitivity of WO3 sputtered films on film density, Thin Solid Films 474 (2005) 255–260. [7] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog˘an, V. Avrutin, S.-J. Cho, H. Morkoç, A comprehensive review of ZnO materials and devices, J. Appl. Phys. 98 (2005) 041301.1–041301.103. [8] Y. Sun, G.M. Fuge, M.N.R. Ashfold, Growth mechanisms for ZnO nanorods formed by pulsed laser deposition, Superlattices Microstruct. 39 (2006) 33–40.

Please cite this article in press as: G. Escalante et al., Characterization and sensing properties of ZnO film prepared by single source chemical vapor deposition, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.07.005

G. Escalante et al. / Advanced Powder Technology xxx (2016) xxx–xxx [9] C. Guillén, J. Herrero, Optical, electrical and structural characteristics of Al:ZnO thin films with various thicknesses deposited by DC sputtering at room temperature and annealed in air or vacuum, Vacuum 84 (2010) 924–929. [10] S. Golshahi, S.M. Rozati, R. Martins, E. Fortunato, P-type ZnO thin film deposited by spray pyrolysis technique: the effect of solution concentration, Thin Solid Films 518 (2009) 1149–1152. [11] M. Pacio, H. Juárez, G. Escalante, G. García, T. Díaz, E. Rosendo, Study of (1 0 0) orientated ZnO films by APCVD system, Mater. Sci. Eng., B 174 (2010) 38–41. [12] E.Y.M. Lee, N.H. Tran, J.J. Russell, R.N. Lamb, Cathodoluminescence of zinc sulfide films grown by single source chemical vapor deposition, J. Phys. Chem. B 108 (2004) 8355–8358. [13] M.M. Heyns, T. Bearda, I. Cornelissen, S. De Gendt, R. Degraeve, G. Groeseneken, C. Kenens, D.M. Knotter, L.M. Loewenstein, P.W. Mertens, S. Mertens, M. Meuris, T. Nigam, M. Schaekers, I. Teerlinck, W. Vandervorst, R. Vos, K. Wolke, Cost-effective cleaning and high-quality thin gate oxides, IBM J. Res. Dev. 43 (1999) 339–350. [14] G. Escalante1, H. Juárez, M. Pacio, G. García, T. Díaz, E. Rosendo, A. Coyopol, Low temperature ZnO films deposited by SSCVD, IOP Conf. Ser. Mater. Sci. Eng. 45 (2013) 012012. [15] H. Gómez, J.L. González, G. Alberto, J. Rodríguez, A. Maldonado, M. de la Luz Olvera, D. Roberto, M. Avendaño, L. Castañeda, Chromium and rutheniumdoped zinc oxide thin films for propane sensing applications, Sensors 13 (2013) 3432–3444. [16] J.F. Chang, H.H. Kuo, I.C. Leu, M.H. Hon, The effects of thickness and operation temperature on ZnO:Al thin film CO gas sensor, Sens. Actuators, B 84 (2002) 258–264. [17] X.Y. Zhou, H. Deng, B. Jiang, Y. Li, E.X. Wang, Self-organized ZnO thin film design and morphology control, Chin. J. Lumin. 25 (2004) 9–13. [18] T.A. Polley, W.B. Carter, D.B. Poker, Deposition of zinc oxide thin films by combustion CVD, Thin Solid Films 357 (1999) 132–136. [19] Y.V. Kaneti, Z. Zhang, J. Yue, Q.M.D. Zakaria, C. Chen, X. Jiang, A. Yua, Crystal plane-dependent gas-sensing properties of zinc oxide nanostructures: experimental and theoretical studies, Phys. Chem. Chem. Phys. 16 (2014) 11471–11480. [20] JCPDS (Joint Committee on Powder Diffraction Standards) card no. 00-0361451. [21] N. Fujimura, T. Nishihara, S. Goto, J. Xu, T. Ito, Control of preferred orientation for ZnOx films: control of self-texture, J. Cryst. Growth 130 (1993) 269–279. [22] N.H. Tran, A.J. Hartmann, R.N. Lamb, Structural order of nanocrystalline ZnO films, J. Phys. Chem. B 103 (1999) 4264–4268. [23] J.G. Lu, T. Kawaharamura, H. Nishinaka, Y. Kamada, T. Ohshima, S. Fujita, ZnObased thin films synthesized by atmospheric pressure mist chemical vapor deposition, J. Cryst. Growth 299 (2007) 1–10.

7

[24] A good review of diffraction basic principles can be found in B.D. Cullity, Elements of X-ray Diffraction, second ed., Addison-Wesley, Philippines, 1978. [25] W.L. Bond, Crystal Technology, first ed., John Wiley, New York, 1976. [26] H.C. Ong, A.X.E. Zhu, G.T. Du, Dependence of the excitonic transition energies and mosaicity on residual strain in ZnO thin films, Appl. Phys. Lett. 80 (2002) 941–943. [27] M. Bender, E. Fortunato, P. Nunes, I. Ferreira, A. Marques, R. Martins, N. Katsarakis, V. Cimalla, G. Kiriakidis, Highly sensitive ZnO ozone detectors at room temperature, Jpn. J. Appl. Phys. 42 (2003) L435–L437. [28] V. Kumar, H.C. Swart, S. Som, V. Kumar, A. Yousif, A. Pandey, S.K. K Shaat, O.M. Ntwaeaborwa, The role of growth ambient on the structural and optical quality of defect free ZnO films for strong ultraviolet emission, Laser Phys. 24 (2014) 105704.1–105704.10. [29] P. Sagar, P.K. Shishodia, R.M. Mehra, H. Okada, A. Wakahara, A. Yoshida, Photoluminescence and absorption in sol–gel-derived ZnO films, J. Lumin. 126 (2007) 800–806. [30] W.S. Shi, O. Agyeman, C.N. Xu, Enhancement of the light emissions from zinc oxide films by controlling the post-treatment ambient, J. Appl. Phys. 91 (2002) 5640–5644. [31] H.J. Egelaaf, D. Oelkrug, Luminescence and nonradiative deactivation of excited states involving oxygen defect centers in polycrystalline ZnO, J. Cryst. Growth 161 (1996) 190–194. [32] B. Lin, Z. Fu, Y. Jia, Green luminescent center in undoped zinc oxide films deposited on silicon substrates, Appl. Phys. Lett. 79 (2001) 943–945. [33] K.H. Tam, C.K. Cheung, Y.H. Leung, A.B. Djurii, C.C. Ling, C.D. Beling, S. Fung, W. M. Kwok, W.K. Chan, D.L. Phillips, L. Ding, W.K. Ge, Defects in ZnO nanorods prepared by a hydrothermal method, J. Phys. Chem. B 110 (2006) 20865– 20871. [34] C.C. Hsiao, L.S. Luo, A rapid process for fabricating gas sensors, Sensors 14 (2014) 12219–12232. [35] G. Korotcenkov, Metal oxides for solid-state gas sensors: what determines our choice?, Mater Sci. Eng., B 139 (2007) 1–23. [36] S.M. Chou, L.G. Teoh, W.H. Lai, Y.H. Su, M.H. Hon, ZnO:Al thin film gas sensor for detection of ethanol vapor, Sensors 6 (2006) 1420–1427. [37] H. Gong, J.Q. Hu, J.H. Wang, C.H. Ong, F.R. Zhu, Nanocrystalline Cu doped ZnO thin film gas sensor for CO, Sens. Actuators, B 115 (2006) 247–251. [38] M. Takata, D. Tsubone, H. Yanagida, Dependence of electrical conductivity of ZnO on degree of sensing, J. Am. Ceram. Soc. 59 (1976) 4–8. [39] N.H. Al-Hardan, M.J. Abdullah, A. Abdul Aziz, H. Ahmad, L.Y. Low, ZnO thin films for VOC sensing applications, Vacuum 85 (2010) 101–106.

Please cite this article in press as: G. Escalante et al., Characterization and sensing properties of ZnO film prepared by single source chemical vapor deposition, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.07.005