Photoresponse and H2 gas sensing properties of highly oriented Al and Al/Sb doped ZnO thin films

Progress in Natural Science: Materials Intemnational 2013;23(6):519- -523Chinese Materials Research SocietyMatlntProgress in Natural Science: Materials Internationalwww .elsevier.com/locate/pnsmiwww.sciencedirect.comORIGINAL RESEARCHPhotoresponse and H2 gas sensing properties of highlyoriented Al and Al/Sb doped ZnO thin filmsHannane Benelmadjath*, Boubekeur Boudine', Aissa Keffous' , Noureddine Gabouze"Laboratoire de Physico-Chimie des Semi Conducteurs, Departement de Physique, Faculte des Sciences Exactes,Universite Mentouri-Route Ain Elbey, 25000 Constantine, Algeria"Laboratoire de Cristallographie, Departement de Physique, Faculte des Sciences Exactes, Universite Mentouri-Route Ain Elbey, 25000Constantine, AlgeriaSilicon Technology Development Unit, 02 Bd, Frantz Fanon, B.P. 140, Algiers, AlgeriaReceived 9 February 2013; accepted 2 July 2013Available online 23 November 2013KEYWORDSAbstract ZnO:Al and ZnO:AI/Sb thin films have been prepared and investigated. The thin films weredeposited on Si substrates by the sol- -gel method. The structural, optical and electrical properties of ZnOZine oxide;films have been investigated by spectrophotometry, ellipsometry, X-ray diffraction and current- voltageCo-doping:characterizations. It is found that the films exhibit wurtzite structure with a highly c-axis orientationThin films;Gas sensing;perpendicular to the surface of the substrate, a high rflectivity in the infrared region and a response toPhotoresponseillumination. Furthermore, it has been found that Si/(ZnO:AI/Sb)/AI photodiode is promising inphotoconduction device while Si/(ZnO:Al)/A1 can be used as gas sensor responding to the low H2concentrations.◎2013 Chinese Materials Research Society. Production an and hosting by EIsevier B.V. All rights reserved.1. Introductionused like silicon and GaN. Among these materials, Al-doped ZnOfilm is an attractive alternative since it presents good electricalIn last decade, sensitive semiconductor oxides have reached anproperties with high transparency.Futhermore,zinc oxide is aimportant place in industrial and technological applications such asII-VI semiconductor with a direct wide band gap of 3.3 eV whichin warning systems and space research. Some materials are widelylargest exciton binding energy of 60 meV compared with ZnS*Corresponding author. Tel./fax: +21331 81 88 72.(20 meV) and GaN (21 meV) [1]. Moreover, ZnO can be preparedE- mail address: benelmadjat _hannane@ yahoo.fr (H. Benelmadjat).at lower temperature than the other two materials. Thanks to thesePeer review under responsibility of Chinese Materials Research Society.properties, ZnO films find applications in solid-state white-lightingindustry, p- n junction UV LED [2], gas sensing [3], blue emiting[4] and photoconductivity [5]， as well as in substituting thecommonly used transparent conductive oxides (TCO) such asELSEVIERProduction and hosting by EIsevierindium tin oxide (TO中国煤化工:en employed1002-0071 ◎2013 Chinese Matrials Research Society. Production and hoting by Elsever B.V. Allri|YHCNMHGhtp://x.doi.org/10. 1016/jpnsc.2013.1 1.001520H. Benelmadjat et al.to prepare pure and doped ZnO films such as spray pyrolysis [7],3. Results and discussionsmolecular beam epitaxy [8], chemical vapor deposition [9], RFmagnetron sputtering [10] and the sol -gel method [11]. The dipXRD studies have been performed in order to determine crystal-coating sol- gel method is easier for elaborating uniform structureslographic phases. Fig. 1 shows the X-ray diffraction diagrams ofwith high surface morphology at lower crystallizing temperatureZnO:Al and ZnO:Al/Sb films. ZnO:AI film diagram (curve a)and lower ccshows two diffraction peakscenteredat 2θ=34.43°and20= 38.23°. The peak centered at 20= 34.438° belongs to (002)structural properties have to be improved, which can be possibleplane of ZnO wurtzite structure, while the other peak centered atby doping of the elements. A variety of dopants have been used2θ= 38.234° is probably due to Al2O3 phase (JCPDS file no.like Cu, Al, In, Ga, Sb and N [12,13]. It is expected that120539). We remark that the peak related to Al2O3 shifts towardintroducing some impurities in ZnO results in high electricalsmall angles when compared with ZnO:Al/Sb and JCPDS filconductivity and high optical transparency in the visible range.Whereas, ZnO:AI/Sb film exhibits (curve b) three diffraction peaksAI+ 3 is the most used impurity for doping ZnO films, and a lot ofcentered at 20=33.29，20= 34.78° and 20= 38.499. The peakstudies discussed the importance of this dopant in optical andcentered at 34.78° related to ZnO wurtzite structure (curve b)electrical properties of ZnO films. In order to improve theseexhibits a shift toward large angle when compared to ZnO:AIXRD pattern. While, we think that the two other peaks situated atdopants like Sc and Ti [14,15]. But very few works have beenaimed on the effect of Sb co-doping in the properties of ZnO:430992) and Al2O3 (JCPDS file no. 120539) phases, respectively.Al films.The crystallites sizes of the two films were estimated usingIn previous work [16], we have studied Sb doped ZnO films.Scherer's formula [17]We have found that Sb doping strongly influences the crstaliteformation, the optical absorption spectra and the optical band gap,D= -0.9h(1)resulting in small sized crystallites and a blue shift of the band gap.B cos() .The results indicated that ZnO:Sb seems suitable to be used inwhere D is the crstallite diameter, λ is the wavelength, θ is theoptoelectronic devices. In this regard, we have introduced fewBragg angle and B is the full-width at half- maximum (FWMH) ofamount of Sb impurities in order to study its influence on thethe peak.properties of ZnO:AI films. In this work, the structural, optical,"he average sizes of crystallites are given in Table 1. Thlight response and gas sensing properties of ZnO:AI and ZnO:AI/Sbcrystallites sizes of ZnO:AI film calculated are found aroundfilms deposited on cleaned Si substrates by the dip coating29 nm while the crystallites sizes of Al2O3 are very smalltechnique have been studied.(8.59 nm), which could explain the shift of the diffraction peaktoward small angles. In the case of ZnO:AI/Sb film, we remark that2. Experimental procedure18-(a): ZnO:AIZinc acetate dehydrate was used as precursor metal salt; the source..-. (b): ZnO:AI/Sbof doping was Al (NO3)3.9H2O (1% mass) and Sb2O3 (0.3%ZnO、b)mass). ZnO:AI and ZnO:AI/Sb solutions were prepared by mixingzinc acetate dehydrate and dopants with 2-methoxyethanol, when(athe solutions turmed milky an equimolar amount of monoethano-lamine (MEA) was added. The solutions were stirred at 60 °C for2 h. The final solutions of 0.35 M were clear and homogeneous.AISThe coatings were made 48 h after the solutions were prepared.60A!O3The post-annealing of the samples was carried out at 35C650 °c for10 min in order to evaporate solvents and organic residuals, thenthe films were annealed at 600 °C for 1 h. The ZnO films weredeposited on n-Si substrate with resistivity of 40 2 cm.°20 25Two structures of Si/(ZnO:AI)/AI and Si/(ZnO:AI/Sb)/AI have20 Idegreebeen prepared, then photodiode structures of (1.2x 1.2 cm2) wererealised with metallic contacts achieved by deposition of 99.99%Fig.1 XRD diagrams of ZnO:Al (curve a) and ZnO:AI/Sb (curve b)pure Al on the two sides of the strucures.films.The crystalline structure of ZnO films was analyzed with X-raydiffraction (XRD) by a Bruker D5000 Advance diffractometer(h= 1.5402 A). Optical properties were carried out using anTable 1 Crytallites sizes of ZnO: Al and ZnO:Al/Sb films.EL01085306 Varian NIR _UV_and a DRE EL X-02Cellipsometer. The ilumination of the20 FWMHCrstalliteCrystallitephotodiodes was done by a Halogen lamp (OSRAM Bellaphot)(deg) (deg)orientationssizes (nm)with a power of 150 W. Gas sensor application was investigated inZnO:AI 34.438 0.286(002) ZnO29.054a stainless steel vacuum chamber (1.43x 10- 3 m3 vol) kept at9 501room temperature (295 K) and connected by a valve to gas bottlesZnO:33 299中国煤化工0.9730.973containing hydrogen gas to be used. The current -voltage char-34.78acteristics 0i the structures were analyzed for -1.5V to +1V38.48YHCNMHG7.292bias voltage change against H2 gas.Photoresponse and H2 gas sensing properties of highly oriented Al and Al/Sb doped ZnO thin films521the crystallites sizes of ZnO are similar to those estimated in thetoward the infrared region, the imaginary part of refractive indexcase of ZnO:AI, indicating that the use of Sb as dopant does notincreases with increasing wavelength, and the variation of theinfluence the grain size of the deposited ZnO film. Whereas, therefractive index leads to a sharp increase in reflection of the film.crystallites sizes of the other phases such as AlSb and Al2O3, areThe effect of Sb co-dopant appears on the reflection spectrumfound of about 50.93 nm and 47.29 nm, respectively. The crystal-(curve a) as a slight shift towards high wavelength when comparedlites sizes values are higher compared with that of ZnO:AI film; itwith ZnO:Al spectrum (curve b). In order to explain this shift,is considered that the presence of Sb may promote the growth ofseveral parameters must be known such as the free electronsthese two phases since Al2O; is known to be formed at aboutconcentration.200°C [18]. The shift of the ZnO peaks could be due to theFrom reflection spectra, free electrons concentration can bedifference between Al and Sb ions diameters, which probablycalculated in the near infrared region from plasma frequency of theinduces a compressive strain in the film.ilms. The minimum value of reflection corresponds to theIn order to explain the shift of the peak related to ZnO:AI/Sbminimum frequency and depends on the free electrons concentra-toward large angles, we have calculated the C parameter of thetion by the following relation:flms using XRD pattern and the following relation:Ne211/(2Omin=l eo(e∞- 1)m*」(4√(4/3)02 +k2 + hk)+ P(a2/c2)where 0min is the minimum value of reflection, N is the freewhere h, k, and I are the miller indices, and a and c are the cellelectrons concentration, Eo is the vacuum permittivity, E∞o is thedielectric constant of ZnO, m* is the effective mass of electron andparameters.The lttice parameter c is found to be 5.152 A for ZnO:AI filme is the charge of electron.The results of N obtained are given inand 5.202 A for ZnO:AI/Sb film, showing that the lttice para-Table 2. We remark that the free electrons concentration N ofmeter of the co-doped film is found to be greater than the doped(ZnO:AI/Sb) is greater than that of ZnO:AI. This indicates that thene. For Sb doped ZnO:AI films the increase in the ltticepresence of Sb ions provides more free electrons. Eq. (4) suggestsparameter c would be expected when Zn++ ions are replaced bymeans that the reflection spectrum will be shifted towards theSb+3 ions due to mismatch in ionic radii [19]. Another possibilityfor the increase in the lattice parameters could be due to thevisible region when the concentration of the free electronsformation of electrically inactive Sbzn- Sb; pairs described byincreases.We can also calculate the resitivity and the Hall mobility of theWardle et al. [20]. In this regard,have calculated the strainsfilms from the following relations:using the following relations [21]:ρ= Rxqd(5(C + C2)Cj]= 2C3(3C13(6euN0.99Cgistalwhere p is the resistivity of the film, Rsq is the square resistance ofand ex = (Co- C)/Co- (a): ZnO:AI---- (b);: ZnO:AI/Sbwhere Ci are the elastic constants of zincite. Co is the strains free50lttce parameter (JCPDS file 36- 1451).We find a strain value of - 0.382 GPa in the case of ZnO:AIfilm and - 4.979 GPa for ZnO:A1/Sb one. According to these\(aestimations, we can note that the strains are compressive. Thisresult explains why the c parameter of the co-doped film is smallerthan the doped one and also the shift of the peak related to ZnO:三20Al/Sb film toward high angle values. We can conclude that, theintroduction of Sb in ZnO:AI lattice causes compressive strains.This is probably due to the great radius of Sb co-dopant (0.245 nmin the caseofSb~ ion).The reflection spectra of ZnO:A1 and ZnO:A1/Sb films mea-50 500 750 1000 1250 1500 1750 2000 2250 250λ [nm|Fig.2. It shows that the two films exhibit high reflection in theinfrared region. An abrupt decrease of the rflction in the nearFig. 2 Reflection spectra of ZnO:AISb (curve a) and ZnO:Al (curve b)infrared region is also observed. It is well known that, when offfilms.Table 2 Free electrons concentrations, resistivity and the Hall mobility of ZnO:AI and ZnO:AI/Sb films.hmin (nm) Wnmin (cm-) x 1012N(cm-3) x 109d (nm) Rn(9.cm) 11 (cm2/V s)ZnO:A1833.524.9980.15中国煤化工ZnO:A/Sb718.064.1867384.00YHCNMH G522H. Benelmadjat et al.the film, d is the thickness of the film estimated by ellipsometrydiode. We remark that, the direct bias current depends strongly onmeasurements (Table 2), e is the charge of electron, μ is the Hallthe presence of ilumination. Curves aL (light) and bL (light) ofmobility of the film and N is the free electrons concentration.Fig. 4 indicate the current obtained under ilumination of the twoThe thickness of ZnO:A1 and ZnO:A1/Sb films measured bystudied structures. It is found that for both structures the photo-llipsometry measurement (shown in Table 2) has been found togenerated current ([ph) increases with increasing bias voltage.be 80.15 nm and 84 nm, values slighly similar.Moreover, we observed also, that the photocurrent (ph=lIigh-From Table 2, we note values of electrical resistivity o0.188 2 cm and 0.315 0 cm for ZnO:AI/Sb and ZnO:Al, respec-dark respectively, is more important in the case of ZnO:AI/Sb filmtively. It clearly appears that the resistivity of the co doped film isthan that of ZnO:AI. This result clearly indicates that thesmaller than the Al-doped one. Moreover, the mobility of (ZnO:introduction of Sb in the ZnO:AI film improves the elctricalAl):Sb film is found to be greater than that of ZnO:A1 film.(photo) response of the structure and increases the photoelectricalWhen Al is introduced in the ZnO matrix, it occupies the sitesperformances of the structure. The results are in good agreementof Zn atoms. The replacement of Zn+2 ions with AI+3 leads to n-with those reported in Table 2.type doping, which results in the increase in carrier concentration.However, when ZnO doped Al is co-doped with a certain4.2. Gas sensor applicationconcentration of Sb we note the increase of arier concentrationand the decrease of resistivity of the films. It is considered that,The Si/(ZnO:AI)/AI and Si/(ZnO:AI/Sb)/AI structures (Fig. 3) havesmall amount of Sb+3 ions provides more free electrons and resultsbeen used to investigate their gas sensing properties. Fig. 5 showsin the decrease in the resistivity of the films.the I- V curves of the ZnO:A1 based sensor under air and againstdifferent H2 gas concentrations. One can observe that, for positive4. Applicationsbias voltage the variation |AI|=|I- Iairl, where lair and I corre-.I. Photodiode application12The photodiode structure realized is shown in Fig. 3. The current-450 ppmvoltage (I-V) characteristics of the structures in the dark and under-- H,gas- 90 pmp8(Hgas-180pmpAlillumination are depicted in Fig. 4. A rectifying behavior can be--Hgas-450ppmobserved in the I _V curves in the dark of Si/(ZnO:A1)/A1 (curve a)and Si(ZnO:AI/Sb)/AI (curve b) diodes, which suggest a Schottkyj180 PlFront contact: Ohmic contact (AI)" 90pm 言ZnO Thin layer--0..51.015Substrate: Silicon (n)Rear contact: Ohmic contact (AI)Bias Voltage [V]ig. 3 Schematic diagram of the photodiode and the gas sensorstructure.Fig.5 I-V characteristics of Si/(ZnO:AI)/AI on air and againstdifferent H2 gas concentrations.60by/45H, gas40v=1.5V当间)203025--0.5~ 00 0.5 1.0 1.5 2.0 2.5 3.0 3| 550 100150200250300350400450 500Fig. 4 Dark and light I-V characteristics of ZnO:Al/Si and ZnO:AI/中国煤化工Sb/Si ZnO:AI/Sb with ilumination (curve aL), without ilumination(curve a), ZnO:A1 with ilumination (curve bL), and without ilumina-ig. 6 Variation ofCNMHGurewithH2gastion (curve b).concentration at bias potential of +1.5 V.Photoresponse and H2 gas sensing properties of highly oriented Al and Al/Sb doped ZnO thin films523oriented ZnO wurtzite structure with the formation of Al2O3 phase-●- H, gas 90 ppmnd AlSb in the case of Sb co-doping. Carrier concentration,-▲H,gas180ppmelectrical resistivity and light response of the films are found to beH, gas 450 Ppmimproved by Sb introduction. So, the Si/(ZnO:Al/Sb)/Al structureis very promising in photoconduction devices. The results of gassensing with several H2 gas concentrations presented in this workshow that, only the developed Si/(ZnO:AI)/AIstructure can beused as gas sensor and responds to H2 gas with high sensitivity tolow H2 concentration. Finally, the results prove that both ZnO:AIand ZnO:AI/Sb films can be used in photovoltaic cells andoptoelectronic devices.0.Bias Voltage [V][1]B.Lin,Z. Fu, Y. Jia, App. Phys. Lett. 79 (2001) 943.[2] D.C. Kim, W.S. Han, B.H. Kong, H.K. Cho, C.H. Hong, Phys. B:Fig. 7 Variation of sensitivity of Si/(ZnO:AI)/AI structure as aCondens. Matter 401- 402 (2007) 386- -390.function of bias voltage for several H2 gas concentration.[3] K. Arshak, I. Gaidan, E.G. Moore, C. Cuniffe, SuprltticesMicrostruct. 42 (2007) 348 -356.with H2 gas, is[4] J. Zhang, H. Feng, W. Hao, T. Wang, J. Sol- _Gel Sci. Technol. 39negative for H2 gas, indicating that, hydrogen behaves as areducing gas on the Si/(ZnO:AI)/AI structure. In addition, it is[5] z. Xu, H. Deng, J. Xie, Y. Li, Y. Li, J. Sol Gel Sci. Technol. 36shown in Fig. 6 that at a bias voltage of +1.5 V the curent(2005) 223- 226.variation |O1| value is first maximum for a gas concentration of[6] T. Minami, Thin Solid Films 516 (2008) 5822- -5828.50 ppm and then decreases with higher concentration of H2[7] J. Benny, K.G. Gopchandran, P.K. Manoj, P. Koshy, V .K. Vaidyan,Bull. Mater. Sci. 22 (1999) 921-926.Whereas, we noted that the Si/(ZnO:Al/Sb)/AI structures are not[8] Y. Feng, Y. Zhou, Y. Liu, G. Zhang, x. Zhang, J. Lumin. 120 (2006)influenced by the presence of H2 gas and no variation in the233- 236.current has been noted. Moreover, the structure was also shown to[9] H. Funakubo, N. Mizutani, M. Yonetsu, A. Saiki, K. Shinozaki,be insensitive to cigarette smoke which contains several gasesJ. Electroeram. 4 (1999) 25 -32.(oxidant and reducing gases). At first glance, it appears that Sb as. [10] w. Lail, C.T. Lee, Mater. Chem. Phys. 110 (2008) 393. -396.co-dopant plays an important role in the phenomenon of H211] H. Benelmadjat, B. Boudine, O. Halimi, M. Sebais, Opt. Laseradsorption. This striking feature is currently under study in orderTechnol. 41 (2009) 630- 633.to determine its (Sb) contribution in the detection of H2 gas.[12] T. Ratana, P. Amomnpitoksuk, T. Ratana, s. Suwanboon, J. AllysFinally, the sensitivity (S= |OI|/I) of the Si/(ZnO:A1)/A1 structureCompd. 470 (2009) 408 412.for three concentrations of hydrogen gas (90, 180 and 450 ppm) as[13] S. Ilican, Y. Caglar, M. Caglar, F. Yakuphanoglu, J. Cui, Physica Ea function of bias voltage has been measured (Fig. 7). It is shown41 (2008) 96-100.that, the sensitivity increases when increasing bias voltage reaches[14] J. Chen, D. Chen, J. He, S. Zhang, Z. Chen, Appl. Surf. Sci. 255(2009) 9413- 9419.a maximum at 0.12 V, corresponding to the maximum sensitivity,[15] M. Jiang, x. Liu, Appl. Surf. Sci. 255 (2008) 3175- -3178.then decreases. In addition it is noted that the sensitivity decreases[16] H. Benelmadjat, N. Touka, B. Harieche, B. Boudine, O. Halimi,with increasing H2 gas concentration. This result clearly indicatesM. Sebais, Opt. Mater. 32 (2010) 764 -767.that the high sensitivity of the structure is observed for low17] A. Davydov, A. Glebkin, Izv. Akad. Nauk. SSSR Neorg. Mater. 8concentration of H2 gas.(1972) 883- 899.[18] s. Bernik, N. Daneu, J. Eur. Ceram. Soc. 27 (2007) 3161-3170.[19] O. Lupan, L. Chow, LK. Ono, B.R. Cuenya, G. Chai, H. Khallaf,5. ConclusionS. Park, A. Shultz, J. Phys. Chem. C (2010) 12401-12408.20] M.G. Wardle, J.P. Goss, P.R. Briddon, Phys. Rev. B 71 (2005)In this paper, ZnO:AI and ZnO:AI/Sb films were prepared with the155205.sol-gel method and deposited by the dip coating technique on Si[21] K.T. Ramakrisha Reddy, TB.S. Reddy, I. Forbes, R.W. Mies, Surf.substrates. Structural characterization by XRD shows a highlyCoat. Technol. 151-152 (2002) 110-113.中国煤化工MHCNMH G