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A Study on the Electrical Characteristic of n-ZnO/p-Si Heterojunction Diode Prepared by Vacuum Coating Technique

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A Study on the Electrical Characteristic of n-ZnO/p-Si Heterojunction Diode Prepared by Vacuum Coating Technique
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  A study on the electrical characteristicof n-ZnO/p-Si heterojunction diode preparedby vacuum coating technique Shashikant Sharma a , C. Periasamy a,b, ⇑ a Department of Electronics and Communication Engineering, Malaviya National Institute of Technology, Jaipur, Rajasthan 302017, India b Materials Research Centre, Malaviya National Institute of Technology, Jaipur, Rajasthan 302017, India a r t i c l e i n f o  Article history: Received 21 March 2014Accepted 3 May 2014Available online 14 May 2014 Keywords: n-ZnO/p-Si heterojunctionElectrical propertiesBarrier heightRichardson constantVacuum coating technique a b s t r a c t This article reports fabrication and characterization of n-ZnO/p-Siheterojunction diode using vacuum coating technique. Structuralproperties, surface morphology and quality of thin film have beenstudied using XRD, AFM and EDX measurements. The temperaturedependent electrical junction properties were investigated byCurrent–Voltage–Temperature (I–V–T) measurement. Barrierheight and ideality factor obtained from I–V measurement hasshown the variations of 0.66–0.79eV and 3.50–3.14 respectivelyfor the temperature range of 25–120  C. The temperature depen-dence of series resistance for n-ZnO/p-Si heterojunction diode hasalso been studied. Temperature dependent I–V measurement givesmean barrier height of 194meV and Richardson constant of 6.61   10  7 Acm  2 K  2 , which has shown significant deviationfrom standard theoretical values for these parameters. Consider-ation of Gaussian distribution with a standard deviation of  r 0  =0.176 gives modified barrier height and Richardson constantof 1.25eV and 39.18Acm  2 K  2 respectively. Value obtained forRichardson constant from modified Richardson plot has shownclose relevance with its theoretical value (i.e., 32Acm  2 K  2 ) forZnO. Results confirm that the temperature dependent I–V http://dx.doi.org/10.1016/j.spmi.2014.05.0110749-6036/   2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Department of Electronics and Communication Engineering, Malaviya National Institute of Technology, Jaipur, Rajasthan 302 017, India. Tel.: +91 1412713232; fax: +91 01412529029. E-mail address:  cpsamy.ece@mnit.ac.in (C. Periasamy).Superlattices and Microstructures 73 (2014) 12–21 Contents lists available at ScienceDirect Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices  characteristics of n-ZnO/p-Si heterojunction obey the theory of thermionic emission with Gaussian distribution.   2014 Elsevier Ltd. All rights reserved. 1. Introduction Inrecentyears, ZnOhasdrawnaglobalresearchinterestofresearchersduetoitsuniqueelectronicandoptoelectronicproperties[1].Thewidebandgap(3.37eV),largeexcitonbindingenergy(60meV),high electron mobility, good transparency, high thermal and mechanical stability, large saturationvelocity and low growth cost ensures its suitability for different optoelectronic and nanoelectronicdevice applications [2–4]. Nanostructured ZnO is an important material for many high technologicalapplications including diodes, solar cells, sensors, actuators, transparent conducting films, photovol-taic devices and so on. Efforts have been devoted by some researchers to produce p-type ZnO thinfilms inorder to fully utilizethe potential of ZnO. But the difficulty inproducing stable andhigh qual-ity p-type doping of ZnO has led researchers to grow n-ZnO on different p-type substrates to ensurethe usability of ZnO thin films in different photonic and optoelectronic devices. Different heterojunc-tion devices such as n-ZnO/p-Si, n-ZnO/p-SiC, p-SrCu 2 O 2 /n-ZnO, p-ZnRh 2 O 4 /n-ZnO and p-NiO/n-ZnOhave been reported in past [5–8]. Among these, n-ZnO/p-Si heterojunction is a suitable choice dueto its cost effectiveness and suitability with mature silicon ICs.Some work on electrical and optoelectronic properties of n-ZnO/p-Si heterostructures using differ-ent fabrication methods have been reported in past [9–11]. However, no significant work for the esti-mation of barrier height and Richardson constant considering the barrier inhomogeneities at theinterface of n-ZnO/p-Si heterojunction diode has been reported in best of our knowledge. Majumdarand Banerji reported p-ZnO/n-Si heterojunction diode using pulsed laser deposition technique [12].They observed that barrier inhomogeneity at ZnO/Si heterojunction diode is the reason for the devi-ation of temperature dependent characteristics from its pure thermionic emission (TE) diffusion the-ory. In addition, Chirakkara and Krupanidhi [11] suggested that TE theory with Gaussian distributionof barrier heights is favorable method to explain most of the electrical abnormalities of heterostruc-tures at low and high temperatures. Al-Heniti et al. [13] demonstrated temperature dependence of n-ZnO/p-Si p–n heterojunction diode with barrier height of 0.864eV but estimation of Richardsonconstant has not been reported. Asil et al. investigated Richardson constant of 6.2Acm  2 K  2 whichis very less than the theoretical value (32Acm  2 K  2 ) for ZnO [14].ThisworkreportstemperaturedependentI–V–Tcharacteristicsofn-ZnO/p-Siheterojunctiondiodeusing vacuum coating technique. Estimation of series resistance, reverse saturation current, idealityfactor, barrier height and Richardson constant have been done for a given temperature range of 25–120  C. Gaussian distribution function with standard deviation of   r 0  around the mean barrier heighthas been used to solve the problem of barrier inhomogeneity at n-ZnO/p-Si interface. 2. Experimental details  2.1. ZnO thin film preparation Nanostructured Zinc Oxide (ZnO) thin film was deposited on p-Si (100) substrate using vacuumcoating method in order to fabricate n-ZnO/p-Si heterojunction diode. p-type Si (100) substrate withboron doping concentration of    7   10 15 cm  3 and resistivity of 8–10 X cmwas used as substrate forthe fabrication of n-ZnO/p-Si heterojunction diode. ZnO powder with 99.99% purity fromMERK-Chemical limited, Mumbai, India was used as source material. Acetone, isopropyl alcohol andde-ionized water (from Milli-Q water plant of Millipore, USA, resistivity   18M X cm) were used insequence for wafer cleaning. For controlled growth of ZnO nanowires, a 20nm thick seed layer of Al doped ZnO (AZO with   1% Al) was deposited on silicon wafer using vacuum coating technique. S. Sharma, C. Periasamy/Superlattices and Microstructures 73 (2014) 12–21  13  Thevacuumcoatingunit(modelno12A4DofHINDVAC,India)wasusedforAZOandZnOlayergrowthin sequence. The base pressure, DC power and evaporation time was 10  3 mPa, 45W and 40minrespectively during the deposition. The distance between source and substrate was 18cm and depo-sition was done at roomtemperature (27  C). ZnO and Al powders were mixed with polyvinyl alcohol(PVA) in appropriate amount (by weight) for about 1h using agate mortar and pestle sets. ZnO and AldopedZnOpelletsweremadeusingahotpressuresetup.Conventionalmolybdenumboatwasusedasheating filament for evaporation of AZO/ZnO pellets during the deposition of the film. The measuredthickness of ZnO thin film was   300nm. Then, the sample was subjected to rapid thermal annealing(RTA) at 600  C in O 2  and Ar ambient so that the quality and conductivity of ZnO thin film nanowirescan be improved. After annealing, when temperature got reached at room temperature (27  C), struc-tural and morphological properties of ZnO thin film were studied.  2.2. Thin film characterization The crystal structure and morphology of the sample was characterized with the help of X-ray dif-fraction (XRD) (18kW Cu-rotating anode based X-ray diffractometer, model: XDMAX, PC-20, Rigaku,Tokyo, Japan) and Atomic Force Microscopy (AFM) (model Solver PRO-47 from NT-MDT Co., Russia).EDX facility available with the ZEISS SUPRA-40 model was used for Energy-Dispersive X-ray analysis.Subsequently, the potential of n-ZnO/p-Si heterojunction for optoelectronic device applications wastested.Fig. 2(a) inset shows the schematic diagram of I–V measurement setup for n-ZnO/p-Si heterojunc-tiondiode.OhmiccontactsforI–VmeasurementwereobtainedbydepositingAldotsonZnOthinfilm.A  200nmthickAllayerwasdepositedonbacksideofp-Sisubstratetoobtaintheohmiccontactfromthe back. After ensuring ohmic characteristics for both the Al/Si and Al/ZnO semiconductor interface(refer Fig. 2(a)), the I–V characteristics of the n-ZnO/p-Si heterojunction were studied further. Thetemperature dependent I–V characteristics of n-ZnO/p-Si heterojunction diode were studied usingsemiconductor parameter analyzers from Agilent Technologies (model: B1500A). Temperature wasvaried from 25  C to 120  C for a voltage range of    5 to 5V. 3. Results and discussion  3.1. Structural and surface morphology study Fig. 1(a) depicts the X-ray diffraction (XRD) analysis of nanostructures n-ZnO thin film. The XRDpattern of ZnO thin film (Fig. 1(a)) showed a unique a unique peak at 2 h  =34.41   corresponding tothe (002) reflexion of ZnO and confirmed a good single crystalline nature of ZnO thin film. XRD pat-tern indicates a preferential growth, oriented along the  c  -axis in accordance with previous reportsavailable in the literature [5,10,12]. The Atomic Force Microscope (AFM) image clearly (insetFig. 1(a)) demonstrates that vertical ZnO nanowires with excellent surface morphology can be grownon p-Si substrate using vacuumcoating technique. The average diameter and length of the ZnO nano-wireswasfoundtobeintherangeof40–50nmand300–400nmrespectively.ThisfeatureofZnOthinfilmensuresitssuitabilityfornanoelectronicdeviceapplications.Energy-DispersiveX-ray(EDX)spec-tra shown in Fig. 1(b) confirmthe presence of Zn, Al and O on the sample. The inset in Fig. 1(b) shows the elements (by corresponding wt.%) present in the prepared thin film.  3.2. Temperature dependent characteristics for n-ZnO/p-Si heterojunction Fig. 2(b) shows the experimentally measured I–V characteristics for n-ZnO/p-Si heterojunctiondiode and confirms the rectifying nature of the contact. The turn-on voltage of the n-ZnO/p-Si hetero- junction is estimated to be 0.8V at room temperature. Conventional thermionic emission model hasbeen used for the analysis of temperature dependent I–V characteristics of n-ZnO/p-Si heterojunctionto extract the diode parameters. Using standard thermionic emission equation for ZnO/Si heterojunc-tion, the relation between voltage and current can be expressed as: 14  S. Sharma, C. Periasamy/Superlattices and Microstructures 73 (2014) 12–21  I   ¼  I  0  exp  qV  g kT    1   ð 1 Þ where  q  represents charge of electron,  V   is biasing voltage,  k  is Boltzmann constant,  I  0  reverse satura-tion current and  g  is the ideality factor described as: g  ¼  qkT  @  V  @  ln ð I  Þ   ð 2 Þ where ( o  ln( I  ))/ o V  ) can be obtained from the linear region slop of ln( I  )  V   plot,  A  is contact area whichis   0.785   10  2 cm 2 in our case,  A  is Richardson constant (  A  =32Acm  2 k  2 ) and  / B , eff   is the effec-tive barrier height at zero bias described as: / B ; eff   ¼  kT q  ln  AA  T  2 I  0 !  ð 3 Þ Fig. 1.  (a) XRD spectra of ZnO thin film (inset shows two-dimensional view AFM image of ZnO on Si) and (b) EDX spectra forZnO thin film (inset shows elements by corresponding atomic wt.% for ZnO/Si heterojunction). S. Sharma, C. Periasamy/Superlattices and Microstructures 73 (2014) 12–21  15  The proportionality of reverse current with temperature (refer Eq. (3)) clearly shows that reversecurrent is highly temperature dependent. Temperature dependent I–V curve shown in Fig. 2(b) alsoconfirms this phenomenon. It is also observed in Fig. 2(b) that the reverse current is slightly non-sat-uratingwiththereversebiasvoltage.Thisnon-saturationofcurrentwithappliedreversebiasisduetospatial inhomogeneity of barrier height explained by Yıldız et al. [15]. However, the saturated down-ward curvature in the temperature dependent I–V–T characteristics at forward bias region can beattributed to the large surface state density near the bottom of the conduction band and the seriesresistance of the device. Aydogan et al. [16] reported that series resistance is one of the reasons fornon linear region I–V characteristics. The method introduced by Cheung and Cheung [17], verifiedby Altindal et al. [18] and Werner [19] have been usedto analyze the temperature dependenceof ser- ies resistance for I–V characteristics. Series resistance for given n-ZnO/p-Si heterojunction diode canbe described as [16]: Fig. 2.  (a)OhmiccontactplotforAl/ZnOandAl/Siinterfaces(insetshowsschematicdiagramofheterojunctionbasedonn-ZnO/p-Sidevicestructure)and(b)experimental lnI–Vcharacteristicsofn-ZnO/p-Siheterojunctionfordifferenttemperatureranges.16  S. Sharma, C. Periasamy/Superlattices and Microstructures 73 (2014) 12–21
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