The present paper is based on study of polycrystalline ZnO:Co thin films deposited on glass substrates by pulsed laser deposition (PLD) technique using pulsed Nd-YAG laser with wavelength ( = 532 nm) and duration (7 ns) and energy fluence (1.4 J/cm 2) with different doping (1 wt. %, 3 wt. % and 5 wt. %). The X-Ray diffraction patterns of the films showed that the ZnO films and ZnO:Co films exhibit wurtzite crystal structure and high crystalline quality. The root mean square (RMS) surface roughness of Co doped ZnO thin films was estimated using atomic force microscopy (AFM) found to be 50.95 nm, 55.78 nm, 56.94 nm and 67.88 nm for pure,1 wt. %, 3 wt. % and 5 wt. % Co doping concentrations respectively. Through the electrical properties, electrical D.C conductivity at temperature range (27-300) ºC for ZnO:Co films as studied which are realized that these films have two activation energies. Hall effect is studied to estimate the type of carriers, from the result we deduced that the ZnO:Co thin films are n-type. The films exhibited good sensitivity to the ethanol vapors with quick response-recovery characteristics and it was found that the sensitivity for detecting (80) ppm, for ethanol vapor was of (27.5), (31.75), (79.0) and (53.1) at an operating temperature of (50) C for ZnO:Co thin films.
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Habubi, N.F.

Haidar, A.A.

Yousif, A.A.

Electronic Sumy State University Institutional Repository

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ Vol. 4 No 2, 02007(6pp) (2012) Том 4 № 2, 02007(6cc) (2012)   2077-6772/2012/4(2)02007(6) 02007-1  2012 Sumy State University Nanostructure Zinc Oxide with Cobalt Dopant by PLD for Gas Sensor Applications  A.A. Yousif1, N.F. Habubi1,*, A.A. Haidar2  1 Department of Physics, College of Education, University of Al-Mustansiriyah, Baghdad, Iraq 2 School of Applied Sciences, University of Technology, Baghdad, Iraq  (Received 17 February 2012; published online 07 May 2012)  The present paper is based on study of polycrystalline ZnO:Co thin films deposited on glass substrates by pulsed laser deposition (PLD) technique using pulsed Nd-YAG laser with wavelength (  = 532 nm) and duration (7 ns) and energy fluence (1.4 J/cm 2) with different doping (1 wt. %, 3 wt. % and 5 wt. %). The X-Ray diffraction patterns of the films showed that the ZnO films and ZnO:Co films exhibit wurtzite crystal structure and high crystalline quality. The root mean square (RMS) surface roughness of Co doped ZnO thin films was estimated using atomic force microscopy (AFM) found to be 50.95 nm, 55.78 nm, 56.94 nm and 67.88 nm for pure,1 wt. %, 3 wt. % and 5 wt. % Co doping concentrations  respectively.  Through the electrical properties, electrical D.C conductivity at temperature range (27-300) ºC for ZnO:Co films as stud-ied which are realized that these films have two activation energies. Hall effect is studied to estimate the type of carriers, from the result we deduced that the ZnO:Co thin films are n-type. The films exhibited good sensitivity to the ethanol vapors with quick response-recovery characteristics and it was found that the sensitivity for detecting (80) ppm, for ethanol vapor was of (27.5), (31.75), (79.0) and (53.1) at an operating temperature of (50) C for ZnO:Co thin films.   Keywords: Pulsed-Laser, Zinc Oxide Thin Films, Structural, Electrical, Morphology, Gas Sensor.    PACS numbers: 78.66.Bz, 81.40.Vw.   ______________ * nadirfadhil@yahoo.com 1. INTRODUCTION  Zinc oxide (ZnO) is an n-type semiconductor of wurtzite structure, with a direct band gap of about 3.37 eV at room temperature [1, 2]. It is a quite im-portant oxide which exhibits near ultraviolet emission and transparent conductivity. ZnO is piezoelectric due to its non-central symmetry, a property that makes it a major candidate for building electromechanical coupled sensors and transducers [3]. ZnO is also one of the most widely applied oxide gas sensing materials [4]. Various deposition techniques have been developed for deposit-ing ZnO thin films, including chemical vapor deposi-tion, radio frequency sputtering, magnetron sputtering, sol-gel, ion-beam-assisted or molecular-beam epitaxy, and pulsed laser deposition (PLD).[5] Amongst them, the pulsed laser deposition technique is interesting because it offers an easy way to add other elements for alloying or for doping purposes. An important point is that PLD gives the advantage of carrying out the growth in a high-O partial pressure for better control of possible oxygen deficiency [6]. The possibility to tune the deposition parameters in order to control the mor-phology of the deposited films, for example passing from smooth to rough films or to nanostructures, is a fundamental issue towards their functional use in the different technological fields. Here we report on the growth of zinc oxide films with different morphologies by pulsed laser deposition, using different substrate temperatures. The possible growth mechanisms are discussed in order to explain the different observed morphologies. [7-9] In this study we prepared Co-doped ZnO thin films deposited by PLD on glass substrates at 400 C temperature. We also investigates the influence of laser (1.4) J/cm2 applied during the deposition pro-cess on the structural, optical, electrical and sensing properties of the films. 2. EXPERIMENT DETAILS   ZnO:Co thin films were synthesized by pulsed laser deposition system using a second harmonic Nd:YAG laser. Thin films were grown in a vacuum chamber with background pressure of ~ 1 × 10 – 3 mbar. The Nd:YAG laser was operated at the wavelength of  (  = 533 nm) with the repetition rate of (10 Hz) and pulse duration of (7 ns). The target to substrate dis-tance was (3 cm). X-ray diffraction measurement has been done and compared with the ASTM (American Society of Testing Materials) cards, using Philips PW 1840 X-ray diffract meter of  = 1.54 Å from Cu-K . The morphological features of the various films were investigated with a JEOL JSM-6360 equipped with a EDAX detector. The microstructures of the films were analyzed using Atomic Force Microscopy (AFM-Digital Instruments Nan Scope) working in tapping mode. The resistivity is conventionally calculated from measured electrical resistance. The activation energy is calculat-ed from measuring the conductivity as a function of temperature using a cryostat. The temperature read out is by (MANFREDI L7C). The bias voltage was sup-plied by (FARNELL E 350) power supply. The current read out is by (Kithley-616 Digital Electrometer) mul-timeter. Hall coefficient (RH), determine the type of charge carriers and estimate their concentration and mobility measurement. The measurement begins after the vertical application of magnetic field density of 0.25 Tesla (Wb/m2) on the film. At this point, the measure-ment is made by changing the current flowing in the film which, in turn, is achieved by changing the applied po-tential difference through the power supply. Power sup-ply type Tandem. They are connected in series with an Ammeter type Kithley-616 Digital Electrometer; the other two electrodes are connected with the same type of copper wires, to voltmeter type Kithley. 177 Micro Voh Dmm. The sensing measurements were carried out by  A.A. YOUSIF, N.F. HABUBI, A.A. HAIDAR J. NANO- ELECTRON. PHYS. 4, 02007 (2012)   02007-2 measuring the variation in resistivity through measur-ing the output current resulting from exposing the thin film surface to the gas or chemical vapor (methanol and ethanol) Ethanol (laboratory reagents 99.9 %) and meth-anol (laboratory reagents 99.95 %) were evaporated by heating them to 50 C, the temperature was recorded by a k-type thermocouple (XB 9208B). The bias voltage was supplied by (FARNELL E350) power supply. The output current was recorded by (Kithley-619 Electrometer) mul-timeter. The thin film surface exposed to 80 ppm vapor concentrations in vacuum of ethanol and methanol. The measuring carried out with an applied voltage constant to (5) volts at temperature constant of 50 C.  3. RESULTS AND DISCUSSION  3.1 Structural Properties  Fig. 1 shows the XRD patterns of ZnO:Co films. It can be seen that the structure of all the films are hexag-onal with a strong (100) preferred orientation. No diffraction peaks of Co or other impurity phases are found in these samples. The angle of the dominant peak corresponding to (100) plane is at 2θ = 31.7  for the undoped ZnO sample and it increased as the concentra-tion of Co impurity increased [10].   0100200300400500600700800900100030 31 32 33 34 35 36 37 38Intensity (arb. units)2θ (Degree)(100)(101)(002)ZnO pure     0100200300400500600700800900100030 31 32 33 34 35 36 37 38Intensity (arb. units)2θ (Degree)(100)(101)(002)ZnO:Co (1%) 0100200300400500600700800900100030 31 32 33 34 35 36 37 38Intensity (arb. units)2θ (Degree)(100)(002)(101)ZnO:Co (3%)   0100200300400500600700800900100030 31 32 33 34 35 36 37 38Intensity (arb. units)2θ (Degree)(100(101)(002ZnO:Co (5%)  Fig. 1 – XRD spectrum of ZnO pure and cobalt-doped ZnO thin films deposited on glass substrate   3.2 Morphology Properties  Fig. 2 show the AFM images of samples which indi-cate the formation of grains, hence revealing the crys-talline nature of the thin films. The surface morphology of the undoped and ZnO:Co films as observed from the AFM micrographs. Fig. 2, left pictures proves that the grains are uniformly distributed within the scanning area (10 × 10)µm, with individual columnar grains ex-tending upwards. The root mean square (RMS) surface roughness of Co doped ZnO films are found to be 50.95 nm, 55.78 nm, 56.94 nm and 67.88 nm for pure, 1wt. %, 3wt. % and 5wt. % Co doping concentrations  respectively (can be see Table 1), i.e. the root mean square (RMS) surface roughness exponentially increas-es with the dopant concentration. The 3D-AFM right pictures of pure and dopant are shown in Fig. 2. It is seen that no segregation of dopants is observed in the grain boundaries of the doped films. From AFM images we found that the presence of the ZnO:Co films increases the (RMS) roughness in the films Figures. The higher the root mean square (RMS) rough-ness was found for ZnO:Co films which have compressive stress and lower root mean square (RMS) roughness for ZnO film which have tensile stress [11-13].  Table 1 – AFM characteristics of the ZnO (undoped and Co doped) films deposited at 400 C substrate temperature, 1.4 J/cm 2 leaser energy and 10 – 1  mbar Oxygen pressure  Co dopant content in the target, wt. % RMS [nm] taken from 10 × 10 m ZnO-pure 50.95 ZnO:Co (1 %) 55.78 ZnO:Co (3 %) 56.94 ZnO:Co (5 %) 67.88  3.3 Electrical Properties  3.3.1 D.C. Electrical Conductivity  The conductivity of ZnO:Co films decreased as the value of 1/T increases at higher temperature (T > 27 C), suggesting a thermally activated conduction in this temperature range. The conductivity for undoped about 1.587 (Ω.cm) – 1 of Co increases in doping have concen-trations (1wt. % and 3wt. %) decreases the conductivity  NANOSTRUCTURE ZINC OXIDE WITH COBALT DOPANT… J. NANO- ELECTRON. PHYS. 4, 02007 (2012)    02007-3            Pure ZnO            ZnO:Co (1 %)              ZnO:Co (3 %)         ZnO:Co (5 %)  Fig. 2 – The AFM images of samples  NANOSTRUCTURE ZINC OXIDE WITH COBALT DOPANT… J. NANO- ELECTRON. PHYS. 4, 02007 (2012)    02007-4  to 1.052 (Ω.cm) – 1 of concentrations (5wt. %) for sam-ples is presented in Table 2 [14, 15].  Table 2 – Electrical properties of ZnO thin films prepared at different Co dopant concentrations  Sample , (Ω.cm) , (Ώ.cm) – 1 at RT │RH│ N, (cm) – 3 , (cm2/ v.s) ZnO-pure 0.630 1.587 48.8 1.28  × 1017 77.13 ZnO:Co (1%) 0.351 2.845 17 3.6  × 1017 48.36 ZnO:Co (3%) 0.366 2.731 0.13 4.56  × 1019 0.374 ZnO:Co (5%) 0.949 1.052 36 1.7  × 1017 37.87  3.2.2 Activation Energy   Fig. 3 shows the electrical activation energy of ZnO:Co films are calculated from lnσ versus (1000/T) plots for different Co-doping concentrations. As shown in Table 3. The activation energy depends on the dop-ing and it will be increased with increasing doping un-til 3wt. %, then, began to decrease. The value of activa-tion energy for the as deposited ZnO film and for the   Table 3 – Activation energy of ZnO thin films prepared at different Co dopant concentrations  Sample Ea1 (eV) Ea2 (eV) ZnO-pure 0.28 0.034 ZnO:Co (1 %) 0.38 0.037 ZnO:Co (3 %) 0.43 0.043 ZnO:Co (5 %) 0.22 0.03  Co-doped ZnO films its values. The activation energy depends on the donor carrier concentration and the im-purity energy levels. An increase in donor carrier con-centration brings the Fermi level up in the energy gap and results in the decrease of activation energy [16].  00.511.522.531 1.5 2 2.5 3 3.51000/T (K-1)Lnδ ( Ω.cm)-1ZnO - pure Ea1=0.28 eVEa2=0.034 eV    00.511.522.531 1.5 2 2.5 3 3.51000/T (K-1)Lnδ ( Ω.cm)-1ZnO:Co (1%)Ea1=0.38 eVEa2=0.037 eV  00.511.522.531 1.5 2 2.5 3 3.51000/T (K-1)Lnδ (Ω.cm)-1ZnO:Co (3%)Ea1=0.43 eVEa2=0.043 eV    00.511.522.531 1.5 2 2.5 3 3.51000/T (K-1)Lnδ (Ω.cm)- 1ZnO:Co (5%)Ea1=0.22 eVEa2=0.03 eV Fig. 3 – Ln  versus 1000/T for pure ZnO and ZnO:Co films at different doping concentrations and temperatures  3.3.3 Hall Effect  The results obtained from Hall effect show that the pure ZnO and ZnO:Co films were n-type, this goes in agreement with the previous work [17-19]. Hall voltage values decrease when the current increases, as shown in Fig. 4. It is noticed that the field applied to the film ends has a proportional relationship with Hall voltage. The values of RH at different concentrations of Co are given in Table 2, the variations of N and H with Co concentration in ZnO:Co films are shown in Table 2. It is seen from this table that N increase with increasing Co concentration. Maximum values of N (4.56 × 1019 cm – 3) are obtained for 3wt. % of Co, but the N decreas-es when increasing Co doping to Co 5wt. %. The mobility H of the ZnO:Co films decreases with in-creasing dopant concentration as in Table 2, and reach-es a minimum around Co 3wt. % with a value of 0.374 cm2 V – 1 S – 1.  3.4 Gas Sensor  Fig. 5 and Fig. 6, show the sensitivity as functions of operation time at a temperature of 50 C. Zinc oxide pure and ZnO:Co films were placed under 80 ppm eth-anol and methanol vapor concentration. Fig. 5 show, an increase in sensitivity value as the doping percentage increase except 1wt. % Co doping which was less   NANOSTRUCTURE ZINC OXIDE WITH COBALT DOPANT… J. NANO- ELECTRON. PHYS. 4, 02007 (2012)    02007-5  -1500-1200-900-600-30000 500 1000 1500 2000 2500Current (nA)VH (mV)ZnO-pure    -350-300-250-200-150-100-5000 200 400 600 800 1000 1200 1400 1600 1800Current (nA)VH (mV)ZnO:Co (1%) -67.5-67-66.5-66-65.5-65-64.50 200 400 600 800 1000 1200 1400 1600 1800Current (nA)VH (mV)ZnO:Co (3%)   -1000-900-800-700-600-500-400-300-200-10000 500 1000 1500 2000 2500Current (nA)VH (mV)ZnO:Co (5%) Fig. 4 – Relation between Hall voltage (VH) and the current (I) of ZnO thin films prepared at different Co dopant concentrations  01020304050607080900 100 200 300 400 500 600 700Time (sec)SensitivityZnO pureZnO:Co (1%)ZnO:Co (3%)ZnO:Co (5%)  Fig. 5 – Sensitivity for as-deposited ZnO:Co as function of operation time for Ethanol  than the pure while the response 5wt. % and 3 % were above the pure value. Furthermore, ethanol gas sensor showed high sensor signal and fast response at concen-tration 80 ppm with operating temperature of 50 C in ZnO:Co thin films. The increase in sensitivity values are higher than that of undoped ZnO film [20, 21]. Fig. 6, show the sensitivity as a functions of opera-tion time at a temperature of 50 C for the various im-purity for ZnO:Co films under 80 ppm methanol vapor concentration, were increase in the sensitivity as the dopant concentration of Co increases.  4. CONCLUSION  The structures, morphology, electrical and sensing properties of ZnO: Co films for an ethanol and metha-nol vapor gas sensor obtained by PLD system were in-vestigated: we conclude the following: 01234567891011120 100 200 300 400 500 600 700Time (sec)SensitivityZnO pureZnO:Co (1%)ZnO:Co (3%)ZnO:Co (5%)  Fig. 6 – Sensitivity for as-deposited ZnO:Co as a function operation time for  methanol  1. The crystal structure of the films is hexagonal wurtzite. All films are polycrystalline and (100) oriented. 2. It was found that doped ZnO films with good structural and morphological properties can be deposit-ed at x  0.05. The film deposited at an optimum alu-mina concentration 0.05 exhibited more intensity main-taining average RMS roughness of (50.95-67.88) nm respectively. The increase of the dopant concentration into the target increases the toughness of the as depos-ited films.  3. By studying the electrical properties, electrical D.C conductivity at temperature range (27-300) ºC for pure and different doping (1 wt. %, 3 wt. % and 5 wt. %), we realized that these films have two activa-tion energies. Hall effect is studied to estimate the type of carriers, from the result we deduced that the pure ZnO and the doped ZnO:Co films are n-type.  4. The pure ZnO and ZnO: Co thin films sensors  A.A. YOUSIF, N.F. HABUBI, A.A. HAIDAR J. NANO- ELECTRON. PHYS. 4, 02007 (2012)   02007-6 show good measurable response to methanol in a con-centration of (80) ppm, with a maximum response at operation temperature of (50) C were (4.0), (4.84), (5.5) and (8.45), respectively. 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Nanostructure Zinc Oxide with Cobalt Dopant by PLD for Gas Sensor Applications

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