Fabrication of nanowires of Al-doped ZnO using nanoparticle assisted pulsed laser deposition (NAPLD)for device
applications
S.Thanka Rajan a ,B.Subramanian a ,⇑,A.K.Nanda Kumar a ,M.Jayachandran a ,M.S.Ramachandra Rao b
a ECMS Division,CSIR –Central Electrochemical Research Institute,Karaikudi 630006,India b
Department of Physics,Indian Institute of Technology Madras,Chennai 600036,India
a r t i c l e i n f o Article history:
Received 24June 2013
Received in revised form 30August 2013Accepted 7September 2013
Available online 26September 2013Keywords:NAPLD Thin films Al doped ZnO Nanowires
a b s t r a c t
Aluminium doped zinc oxide (AZO)nanostructures have been successfully synthesized on sapphire sub-strates by using nanoparticle assisted pulsed laser deposition (NAPLD)in Ar atmosphere without using any catalyst.The growth of the AZO nanowires has been investigated by varying the argon flow rates.The coatings have been characterized by X-ray diffraction (XRD),Field emission scanning electron microscopy (FESEM),Atomic force microscopy (AFM),Diffuse Reflectance Spectroscopy (DRS),Laser Raman spectroscopy and Photoluminescence spectroscopy.The results of XRD indicate t
hat the deposited films are crystalline ZnO with hexagonal wurtzite structure with (002)preferred orientation.FESEM images also clearly reveal the hexagonal structure and the formation of nanowires with aspect ratios between 15and 20.The surface roughness value of 9.19nm was observed from AFM analysis.The optical properties of the sample showed that under excitation with k =325nm,an emission band was observed in UV and visible region.The characteristic Raman peaks were detected at 328,380,420,430cm À1.
Ó2013Elsevier B.V.All rights reserved.
1.Introduction
Zinc oxide (ZnO)is one of the most important metal oxide semi-conductors.This material has good electrical and optical properties and it is chemically stable.It has a wide direct band gap of 3.37eV with a large binding energy of 60meV [1].Undoped ZnO thin films generally exhibit n-type conductivity due to intrinsic donors,such as oxygen vacancies and Zn interstitials [2].It has an open struc-ture,with a hexagonal close-packed lattice where Zn atoms occupy half of the tetrahedral sites,while all the octahedral sites are empty [3].This crystal structure offers plenty of sites to accommodate intrinsic defect and extrinsic dopants.The wurtzite structure of ZnO can be described as a series of alternating
planes of tetrahe-drally coordinated O 2Àand Zn 2+ions stacked along the c -axis;this characteristic polarity of the surfaces gives rise to different nano-structures (nanorods,nanowires,nanobelts,nanotubes)under proper growth conditions [4].Such nanostructures show different and superior properties over their bulk counterparts due to their small size and large surface to volume ratio [5].Impurity-doped ZnO films also show stable electrical and optical properties.
Aluminium doping in zinc oxide (AZO)is a promising material due to its high conductivity and good optical properties [6].The choice of Al as donor dopant for ZnO over higher valence ions is lar-gely owing to its ease of incorporation in the ZnO structure;more-over it decreases the resistivity of the host ZnO without impairing the optical transmission in thin film form [7].Both ZnO:Al and Al 2O 3are transparent to visible light,making them interesting can-didates for optical applications.Nowadays,pure ZnO or AZO are being actively investigated as alternative materials to indium tin oxide (ITO)because it is nontoxic,inexpensive and have long term environmental stability [8–10].
Several deposition techniques have been routinely used to grow AZO nanostructures.The most common methods are magnetron sputtering [11,12],pulsed laser deposition (PLD)[13],chemical va-por deposition [14],and chemical spray [15].PLD is an attractive method,compared to magnetron sputtering and reactive RF sput-tering techniques,for deposition of ZnO thin films with high struc-tural
homogeneity and crystalline quality [16].Compared with other techniques PLD has many advantages such as (i)the compo-sition of the films are close to that of the target,(ii)surface of the film is smooth and (iii)good quality film can be deposited [17].Nanoparticle assisted pulsed laser deposition (NAPLD)is a rela-tively new modification of the PLD technique in which nanowires can be grown at high temperatures and high pressures without using any catalyst.This technique also gives films with almost the same composition as that of the target.This advantage scores highly when compared to the other modified deposition tech-niques [18].In NAPLD the nanoparticles that are formed in the background gas by laser ablation are used for the subsequent growth of the nanowires [18].The initial nanoparticles formed in the laser ablation plume by the condensation are transported onto the substrate and act as starting materials for nano-crystal growth [19].In this work,we report the successful synthesis of AZO nano
0925-8388/$-see front matter Ó2013Elsevier B.V.All rights reserved./10.1016/j.jallcom.2013.09.046
Corresponding author.Tel.:+914565241538;fax:+914565227713.
E-mail address:subramanianb3@gmail (B.Subramanian).
wires on sapphire substrates by NAPLD at different argonflow rates without any catalyst.The structural and optical properties of the nanowires are also discussed in this paper.
2.Experimental
The depositions of Aluminium doped ZnOfilms were carried out in the NAPLD system.The target preparation procedure is shown in Fig.1.The AZO target was prepared by mixing98mol%ZnO and2mol%Al2O3(99.99%pure,Sigma Aldrich). The sapphire substrate was cleaned byfirst boiling it in trichloroethylene and then ultrasonically cleaned in acetone for3min followed by milli pore water.AZO thin films were deposited on sapphire substrates using frequency triplet,Q switched Nd:YAG laser.Fig.2shows a schematic diagram of the NAPLD system.The target was loaded into the target holder and the substrate was stuck on the substrate holder using silver paste.The target and substrate holder are placed inside a fur-nace,so the whole atmosphere with target and substrate are heated.When the tem-perature reached1000°C,the laser was switched on for ablating the target and Ar gas was let in.The AZOfilms were deposited on sapphire substrate for different ar-gonflow rates(200,300,400and500sccm)for30min.The optimized deposition parameters and conditions are described in the Table1.
Thefilms were characterized by Bruker D8Advance to study the phases and for grain size measurement.The surface morphology of the preparedfilms was studied using Quanta3D FESEM.Atomic force microscopy for topographic studies was done by Agliant technologies(model5500).PL spectral analyses were performed using Cary Eclipse(Varian)and Raman spectroscopy for micro-structural analysis was done by Renishaw in via laser Raman microscope.Hall Effect measurements(Eco-pia,HMS3000)were made on the AZOfilms at a constant magneticfield of 1.02T.Diffuse reflectance spectra of thefilms were recorded using Ocean Optics (USB2000)spectrophotometer.BaSO4powder compact was used as a standard ref-erence.The diffuse reflectance(R)was measured as a function of wavelength rang-ing from300to700nm.
3.Results and discussion
3.1.Structural and compositional analysis
The XRD patterns of the AZO thinfilm for different Argonflow rates(200–500sccm)on sapphire substrates are shown in Fig.3. It was observed that the wurtzite structure of the ZnO is unaffected by the doping of2mol%Aluminium.The diffraction patterns were equivalent,since the Al doping did not show any significant shifts shift is because of the decrease of the lattice constants a and c with increasreactive materials studies
ing argonflow rate.At lowflow rates of the inert gas,the growth mechanism is lateral growth over the substrate surface with uniform coverage.Therefore,a strain can be induced in the lattice of the AZO owing to the strained interface.At higherflow rates,the mechanism varies to condensation in the vapor phase leading to perfect needle type nano wire,which are under signifi-cantly lower stress,with lattice parameters approaching that of an ideal single crystal,although not exactly,owing to the incorpora-tion of Al into the Zn sublattice.Therefore,due to the preferential growth along the[0001]direction,a strain can be induced in the lattice of the AZO by the strained substrate/film interface.This might have led to a change in lattice constants.High texture in (002)will determine the quality of the nano wire.At400sccm flow rate,the(004)line at72°is detected and intensity of the (002)also increased,indicating that the quality of thefilm was im-proved when the argonflow rate was increased.This shows that the crystallinity of the AZO thinfilms increased withflow rate. AZOfilms become polycrystalline,which means thefilm is com-posed of many grains with crystallographic orientations(100), (002),(101),(110),and(004)as indicated in Fig.3.
The grain size(D)for various argonflow rates was calculated from the standard Scherer’s formula,expressed as
D¼
0:94k
b cos h
ð1Þ
where k is the wavelength of Cu K a X-radiation(1.5406Å),b is the Full width at half maximum(FWHM)value for a particular orienta-tion calculated from the XRD pattern and h is the Braggs’angle.The calculated grain size for different argonflow rates(200–500sccm) for(002)peak were77.8,60,86.8and181.5nm respectively. The general trend seems to be that the crystallinity of thefilm also increases with the argonflow rate.
Fig.4a–e compares the surface morphology of the ZnOfilms grown under different Arflow rates observed by FE-SEM.Films deposited at200sccm the lowflow rate regime–show a rather dense formation of ZnO grains growing laterally on the substrate surface with uniform coverage(Fig.4a).There is no evidence of the formation of thin ZnO wires.The wire-type morphology ap-pears only with increasingflow rates.Fig.4b shows the morphol-ogy offilm coated at300sccm.Some agglomeration is observed along with clusters randomly dispersed on the substrate.Interest-ingly,appearance of a thin needle-like structure is also seen.Fig.4c shows a SEM image of thefilm deposited at400sccm of Ar.Whi
le a number of vertically growing nano rods can be discerned in the microstructure,observation of the crystals along a direction nor-mal to{0001}shows afiner structure of the nano rods that consist of a layered arrangement of ZnO crystals,but neatly stacked along the c axis,suggestive of growth by oriented attachment mecha-nism[20].Such a layered growth produces corrugated10 10side walled nano rods as the layered structure is partially fused be-tween the stacks,probably by diffusional sintering.Clearly,this seems to be a preliminary step towards the perfect needle type nano structures observed at500sccm,shown in Fig.4d and e. Thefilm at500sccm,is homogeneous and comprises of nanowires and nanorods of diameters ranging from150to250nm and the lengths from3to10l m grown along the c-axis,clearly showing the hexagonal lattice of ZnO with wurtzite structure.
They appear well aligned in a vertical plane of the substrate and have perfect hexagonal shape.Based on these observations,it is evident that the inert gas(Ar)flow rate plays a crucial role in con-trolling the morphology of the ZnOfilms.At lowflow rates,the partial pressure of Ar in the vapor is too low to produce any con-densation within the vapor and hence,uniform coverage of the substrate is seen.With increasingflow rate of the Ar gas,the ener-getic Zn and O species from the ablated target rapidly lose their
Fig.1.Flow chart of target preparation.
612S.Thanka Rajan et al./Journal of Alloys and Compounds584(2014)611–616
energy by colliding with the colder Ar atoms and nucleate into nano particles within the vapor phase which subsequently act as seeds for the nanocrystalline growth.This mechanism is akin to the inert gas condensation technique and explains the formation of perfectly grown ZnO wire under optimized Ar flow rate
conditions.The gradual change from uniformly grown films with lateral spread to vertical nano rods by varying the inert gas flow rate also introduces a slight straining of the lattice which is also reflected in changes in lattice parameter observed by XRD.The nano rods deposited at 500sccm have a low defect density and hence an unstrained lattice,whereas those deposited at 300sccm have a considerable strain due to bonding with the substrate.
AFM studies were carried out to investigate the effect of Al dop-ing concentration on surface roughness of the films.Fig.5shows that the surface of the film is covered with grains of diameter about 50nm.The root mean square (rms)surface roughness of the film was measured to be 9.19nm.
The elemental composition analyses were carried out on Al doped ZnO nanowires using Energy dispersive X-ray (EDX)analysis and is shown in Fig.6.The presence of Zn,Al and O was observed in th
e AZO samples.No impurity phases were detected on the surface of the film.The different Hall parameters such as Hall mobility (cm 2V À1s À1),resistivity (X cm)and carrier concentration (cm À3)have been measured for the films deposited at 500sccm.From the previous observation it is optimized that 500sccm shows high X-ray diffraction intensity and the SEM images also reveal that 500sccm is an apt parameter for our studies.Analysis of the equi-librium defect concentration in Al doped ZnO reveals that for each Al 3+that substitutes for one Zn 2+,a free electron is released into the crystal to enhance its conductivity,according to the relation (in Kröger–Vink notation):
Al 2O 3ðZnO Þ 2Al  Zn þ2O o þ2e 0þ1
2
O 2
ð2Þ
In this case,the Al dopant concentration has been fixed at 2mol%,and we assume that the O 2partial pressure does not vary significantly with the Ar flow rate.The electronic charge conduc-tion should control the net resistivity of the film at low frequen-cies.The carrier concentration of the AZO film deter
mined by Hall effect measurement is 8.07Â1019.Hall mobility and resistiv-ity were measured as 25.28cm 2V À1s À1and 1.028Â10À2X cm,which agrees clearly with the values reported by Pin-Chuan Yao et al.[21].
3.2.Optical properties
Raman scattering is an effective technique to investigate the crystallization,structure and defects chemistry of materials.Laser Raman spectra of the nanostructured AZO films are shown in Fig.7.Wurtzite structure belongs to the space group C 46v with two formulae units per primitive cell,where all atoms occupy C 3v sites.Twelve vibrational modes exist in the ZnO unit cell;one longitudinal acoustic (LA),two transverse acoustic (TA),
three
Fig.2.Schematic set up of nanoparticle assisted pulsed laser deposition.
Table 1
Deposition parameters and conditions.Specification
Parameters Laser
Nd:YAG (355nm)Repetition rate 10Hz 4ns
140mJ/pulse 2–4J cm À2Sintered AZO Sapphire 1000°C Argon $2mbar
200–500sccm
XRD pattern of AZO films grown on sapphire substrate at different flow
longitudinal-optical(LO),and six transverse optical(TO).A1and E1 symmetries are polar and split into LO and TO components with different frequencies[22].
The Raman peaks near328,380,420,430and465cmÀ1can be attributed to the wurtzite crystal structure of AZO.The dominant peak at430cmÀ1indicates the Wurtzite structure of ZnO and is attributed to the high E2mode of non polar optical phonons.E2 high mode is for ZnO hexagonal structure with vibrations of O sub-lattice.The FWHM of the E2line is about8cmÀ1which is another indication of the high quality of the NAPLD synthesized nanocrys-talline ZnOfilms.The peak at380,420and465cmÀ1corresponds to A1(TO),E1(TO)and A1(LO)respectively.The peak A1measured at328cmÀ1is related to multiple phonon scattering process.A slight shift was also observed which was due to the doping of Al
Fig.4.FESEM images for AZOfilm at(a)200sccm,(b)300sccm,(c)400sccm,(d and e)500sccm.
Fig.5.A representative AFM image of AZOfilm.
Fig.6.Energy dispersive X-ray analysis of AZO nanowires on sapphire substrate.
without introduction of any additional stress within the samples shown in XRD.
Diffuse reflectance spectral studies in the UV–visible region were carried out to estimate the optical band gap of the nanostruc-turedfilm.The optical diffuse reflection spectra of AZO samples differentflow
rates(200–500sccm)on sapphire substrate are dis-played in Fig.8.The energy band gap E g can be determined from onset of the linear increase in the diffuse reflectance.From
2wefind that the band gap of the AZOfilm lies in the range 3.40–3.44eV using Einstein’s energy relation:
1:24
kðl mÞ
where E is the band gap and k is the wavelength.
The band gap increases with increasing Arflow rate.The in-crease in E g may be due to the increase of electron concentration. Generally,the values for band gap of AZOfilms are slightly higher. The conduction electrons in the ZnOfilms are supplied from donor sites associated with oxygen vacancies or excess metal ions[23] The band gap of ZnO is in the near UV region which is3.37 The AZO reflection starts at about380nm and the samples exhib-ited absorption peaks in the visible region at about660nm in both spectra.
The photo luminescent emission spectrum of AZOfilms for the various argonflow rates are shown in Fi
g.9.The PL peaks for all the samples are almost same in position but different in intensity.They show near-band edge emission around373nm which is the UV re-gion;this emission is due to ZnO.The visible emission was ob-served at around405nm,468nm and533nm due to defect emission.The blue peak at468nm comes from the electron transi-tion from Zn interstitial level to the top of the valance band.The green emission observed at533nm resulted from intrinsic defects. Deep level emissions are associated with intrinsic defects such as oxygen vacancies or zinc interstitials[24].
From Table3wefind that the calculated band gaps are approx-imately equal to the band gap of ZnO.So the ZnO phase is con-firmed with the band gap in the UV absorption region.However, the UV emission intensity increases and the deep level emission intensity decreases as argonflow rate increases from200to 500sccm suggesting that the crystals grown at higherflow rates have lower defect densities in their lattice.This observation is in agreement with the XRD results suggesting that defect densities decrease with increasing Arflow rates,along with the correspond-ing change in the lattice parameter.
Fig.7.Laser Raman spectra of AZOfilms on sapphire substrates.
Fig.8.DRS patterns of AZOfilms on sapphire substrates.
Fig.9.PL emission spectra of AZOfilms on sapphire substrates.
Table3
Band gap values of AZOfilm on sapphire substrate determined from the PL spectrum.
Rgonflow(sccm)Wavelength(nm)Band gap(eV)
200364.2  3.32
300365.9  3.32
400364.9  3.31
500355.7  3.33

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