Structure and Formation Mechanism of Sn-Doped ZnO Nanoneedles

WANG Jie ZHUANG Hui-Zhao XUE Cheng-Shan LI Jun-Lin XU Peng

(Institute of Semiconductors,Shandong Normal University,Jinan 250014,P.R.China)

Structure and Formation Mechanism of Sn-Doped ZnO Nanoneedles

WANG Jie ZHUANG Hui-Zhao*XUE Cheng-Shan LI Jun-Lin XU Peng

(Institute of Semiconductors,Shandong Normal University,Jinan 250014,P.R.China)

We synthesized Sn-doped ZnO nanoneedles on Si(111)substrates in two steps:sputtering and thermal oxidation.First,a thin layer of the Sn∶Zn films was deposited onto the Si(111)substrates in a JCK-500A radio-frequency magnetron sputtering system.Sn-doped ZnO nanoneedles were then grown by simple thermal oxidation of the asdeposited films at 650℃in Ar atmosphere.The structural,componential,and optical properties of the samples were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),high-resolution transmission electron microscopy(HRTEM),energy dispersive X-ray(EDX)spectroscopy,and photoluminescence(PL)spectroscopy.The results reveal that the ZnO nanoneedles doped with 2.5%(x,atomic ratio)Sn are single crystalline with a wurtzite hexagonal structure.The lengths of the grown nanoneedles vary between 1 and 3 μm.The root diameters of the needles range between 200 and 500 nm while the tips have an average diameter of about 40 nm.Moreover,most of the Sn-doped ZnO nanoneedles are of high crystal quality.Room temperature PL spectroscopy shows a blue-shift from the bulk bandgap emission,which can be attributed to a Sn composition in the nanoneedles as detected by EDX.Based on the reaction conditions,the growth mechanism of the Sn-doped ZnO nanoneedles was also discussed.

Nanostructure;ZnO;Sn-doping;Sputtering;Optical property;Formation mechanism

Recently,much attention has been focused on the research of quasi-one-dimensional nanostructured semiconductor materials due to theirnovel physical,chemical properties and potential applications in nanodevices[1-3].As a II-IV compound,ZnO is an important semiconductorwith a wide direct band gap(3.37 eV)and a large exciton binding energy of 60 meV,which is muchhigherthan those of othermaterials,such as ZnS(40 meV)and GaN(25 meV)[4].Overthe past decade,tremendous efforts have been made to synthesize nanoscaled or microscaled ZnO crystals.Up to now,ZnO nanostructures with various sizes and morphologies have beensuccessfully synthesizedand reported in the literature,such as nanowires[5],nanorods[6],nanobelts[7]，nanoprism[8],nanoneedles[9],and nanopropellers[10].As is well known, impurity-doping in semiconductors with selective elements greatly affects the basic physical properties,such as the electrical,optical,and magnetic properties,which are crucial for their practical application[11],and the doping effect has attracted extraordinary attention.Recently,various doped ZnO nanostructures with different elements(e.g.,Al,As,In,Sn,Mg,and Sb) have been achieved[12-17].It was reported that Sn substituting Zn is agoodcandidate as ann-type dopantin ZnOwithoutaffecting lattice constant,because ionic radii of Sn4+(0.071 nm)and Zn2+(0.074 nm)are almost similar[18],the Zn positions can be easily substitutedby Snundercertainconditions.Inaddition,Sn-doped ZnO nanostructures can improve field emission characteristics significantly in comparison with undoped ones[19].By far,Sndoped ZnO nanostructures have been successfully fabricated by a lot of methods,such as nanowires by vapor-liquid-solid(VLS) method[20],single-crystal ZnO nanobelts by chemical vapor deposition(CVD)method[18,21],and oriented nanoplate arrays by thermal treatment of ZnO-SnO2powders[22].Nevertheless,most of them have some drawbacks,involving long reaction time, high growth temperatures orimpure metal particles and the outcomes are poorin performances,hence influencing some applications of the ZnO nanostructures.In order to avoid these defects and improve the quality of Sn-doped ZnO nanostructures, we attempted a novel route to synthesize high-quality Sn-doped ZnO nanoneedles on Si(111)substrates by thermal oxidation of Sn:Zn films deposited by sputtering.The as-synthesized Sndoped ZnO nanoneedles possess single crystal structures and show good characteristics.This growth method allows a continuous synthesis and the synthesis process is simple and inexpensive,suitable for commercial scale production.Details of the growthof Sn-dopedZnOnanoneedles will also be discussed.

1 Experimental

1.1 Preparation of ZnO nanoneedles

Sn-doped ZnO nanoneedles were synthesized by annealing Sn:Zn thin films under flowing Ar at 650℃ in a traditional quartz tube.The whole experiment course chiefly consists of two differentstages.

First,Zn thin films doped with Sn were deposited on Si(111) substrates by sputtering a Zn target with purity of 99.99%and a Sn target with a purity of 99.999%by using a JCK-500A radiofrequency(RF)magnetron sputtering system.The Sn target and Zn target were employed to direct-current(DC)and radio-frequency magnetron sputtering,respectively.Next 30 cycles of this process were performed fora total deposition time of about 6 min,after which the total thickness of the Sn-doped Zn filmswas about 900 nm.In a single sputtering cycle,Sn films were deposited with a thickness of about 5 nm,and then undoped Zn layer was deposited with the thickness of about 20 nm.Details of the sputtering parameters are giveninTable 1.

Table 1 Sputtering parameters of the Sn:Zn thin films

Second,the as-deposited Sn:Zn thin films were oxidated in a conventional tube furnace.When the furnace reached the equilibrium temperature of 650℃,flowing Ar gas was introduced into the tube at a flow rate of 500 cm3·min-1(standard state)to flush out the residual airfor5 min.Then a quartz boat with the samples was placed into the constant-temperature region.Meanwhile,Ar(99.999%)flow rate was set to be 50-100 cm3·min-1under the ambient pressure for 15 min.After reaction,a white layerwas foundonthe substrate surface.

1.2 Methods of characterization

The structure,morphology,composition,and optical property of the synthesized samples were characterized by X-ray diffraction(XRD,A Rigaku D/max-rB X-ray diffraction meterwith Cu Kα-line,0.154178 nm,40 kV,100 mA,Japan),scanning electron microscopy(SEM,Hitachi S-570,Japan),transmissionelectron microscopy(TEM,Hitachi H-800,Japan),high resolution transmission electron microscopy(HRTEM,Tecnai F30),energy dispersive X-ray spectroscopy(EDX)attached to the HRTEM instrument,and fluorescence spectrophotometer(LS50-B)with a Xe lamp as the excitation light source at room temperature (λ=325 nm).

2 Results and discussion

2.1 SEM characterization

Fig.1 presents the representative SEM images of the as-synthesized products at different magnifications.Fig.1(a)demonstrates that sea urchin-like assembly structures are grown on the substrate surface with a high density and coverthe substrate fully with whiskers arising from them.The magnified SEM image (Fig.1(b))shows thatmostof the whiskers exhibitsharpened tips with wider bases and are partially aligned to the substrate surface,forming needle-like structures(nanoneedles).The lengths of the grown nanoneedles are varying from each otherfrom 1 to 3 μm.The rootdiameters of the needles are in the range of 200-500 nm,while the tips have anaverage diameterof about40 nm.

2.2 XRD characterization

Fig.2 shows the typical XRD pattern of the Sn-doped ZnO nanoneedles.All the diffraction peaks in the panel can be assigned to hexagonal wurtzite ZnO,which confirms that the nanoneedles have hexagonal wurtzite structure(JCPDS No.36-1451).The(002)diffractionpeakat2θ=34.5°has aslightshift of about 0.1°toward the larger diffraction angle from 34.4°(the standard for bulk ZnO),indicating the substitution of smaller Sn4+for Zn2+.No diffraction peaks from metallic Zn and Sn or other phase have been found,revealing that Sn doping has not changed the wurtzite structure of ZnO because of its small content,which is also compatible with results from the substitution of Sn for Zn site in previous reports[19,21].The strong intensity and narrow width of the diffraction peaks indicate that the asgrown nanoneedles are of high crystallinity.

2.3 TEM and HRTEM characterization

To further illuminate the detailed microstructures of the Sndoped ZnO nanoneedles,TEM,HRTEM,and corresponding selected area electron diffraction(SAED)measurements were performed.As showninFig.3(a),a TEMimage with a general morphology of nanoneedle reveals that the Sn-doped ZnO nanoneedle with a length of over1 μm and its surface is slightly rough, which may be attributed to the defects from Sn doping during growth.The inset in the upper-right corner(Fig.3(a))shows the SAED pattern,which can be indexed as a hexagonal structure with no second phase detected.Therefore,we can conclude that the Sn doping does not perturb significantly the lattice structure of the ZnO matrix,and the as-synthesized Sn-doped ZnO nanoneedles are single crystalline.Fig.3(b)shows the HRTEM lattice image of the ZnO nanoneedles.The interplanar spacing is about 0.518 nm,which is slightly smaller than the value of the (0001)plane(0.52 nm)spacing of wurtzite ZnO,indicating that the growth direction of the ZnO nanoneedles is parallel to the (0001)plane.This can confirm that Sn ions are averagely adulterated into the crystal lattice of ZnO,which is compatible with results of XRD.This confirms that introduction of Sn4+in the ZnO lattice does not affect the growth direction.The visible lattice fringes also suggest that the as-fabricated nanoneedles are single-crystal,whichis consistedwiththe XRD results.

2.4 Component characterization

Corresponding chemical composition of the synthesized ZnO nanoneedles was determined by EDX spectrum.Fig.3(c)shows a typical EDX spectrum of the edge of the nanoneedles.It is clearly that only the Zn,Sn,Cu,C,and O peaks are observed (the Cu peak comes from the TEM sample grid and the C peak comes mainly from atmospheric contamination due to the exposure of the sample to air),confirming that the compositions of the products are Sn-doped ZnO without impurity.Much the same results were obtained from different areas of the nanonee-dles for several times.According to the results of quantitative calculation,the contentof Sninthe nanoneedles in mostof them is identified to be about 2.5%from the intensity ratios of the Zn, Sn,and O peaks.The results reveal that the Sn atoms might be dopedinto the nanoneedles inthe formof substitution.

2.5 Optical properties

Fig.4 shows the measurementof PLspectrumatroomtemperature.Both the Sn-doped and undoped samples show two emission bands,a narrow ultraviolet(UV)emission and a broad greenemission.The UV emission is originated fromthe exitonic recombination corresponding to the nearband-edge emission of band gap ZnO[13].Compared with the undoped one,the UV peak of the doped sample shifts to 380 nm from 387 nm.According to Eq.(1),

where Eνis the banding energy(eV),λ is the wavelength,the peaks at 380 and 387 nm corresponding to Eν=3.26 eV and Eν= 3.20 eV,indicating that the band-gap has been widened by the incorporation of the Sn substitution because of Burstein-Moss effect with an increasing in band-gap value[23],in agreement with the XRD and EDX results.The green emission is usually attributed to the recombination of the holes with the electron occupying the single ionized oxygen vacancy(VO)[14].At highertemperature the numbers of molten Zn/Sn atoms increase,combined with oxygen to form Sn-doped ZnO nanostrucures.In our experiment,because ZnO nanoneedles were fabricated at 650℃in Aratomosphere and no oxygen was introduced,a quantity of oxygen vacancies could also easily be produced.Thus,the green emission centered at 508 nm would be a result of the existence of the oxygen vacancies in the ZnO nanoneedles.In ourcase,an interesting phenomenon is that the green band in the Sn-doped ZnO is much strongerthan the UV band.This may be attributed to the excess carriers supplied by the impurities to the conduction band contribute to decrease the electrical conductivity of ZnO.So when the PL source excites the nanoneedles,the amountof excitons decreases significantly.In addition,the probability of forming a new band structure deformation increases with the increase in carrier concentration.This will give rise to some new defects,suchas Zn and Ovacancies,which should result in the increase of green emission.Therefore,we expect that the ZnO nanoneedles with strengthened green light emission would be a promising material forapplications in optoelectronic nanodevices.But still,much effort is needed to investigate the PLmechanismof the ZnOnanoneedles.

2.6 Formation mechanism

In ourcase,the growth of ZnOnanoneedles was done without catalyst.Therefore,the formation mechanismof the nanoneedles was thought to be controlled by the traditional vapor-solid(VS) mechanism.This mechanism consists of two stages:one nucleation and another growth.At the temperature of 650℃(higher thanthe melting points of Snat231.9℃andZn at419.5℃),the Sn:Zn thin films are melted and aggregated to form the microsized Zn-Sn alloy droplets on the film surface.Meantime,trace amount of O2from Ar may solve into the outer surface of the previously formed Zn-Sn droplets to form Sn-Zn-O eutectic droplets.Zn is more active than Sn in chemical property[24],as the supersaturated state of aggregated droplets appeared;ZnO began to diffuse through the liquid phase,where ZnO crystalline nuclei were formed.With the oxygen gases continuously solving into the liquid Zn-Sn droplets,these ZnO nuclei individually underwent a very rapid root growth process[25],consuming the Zn from the high mobility Zn-Sn liquid droplets to form nanobelts in randomly directions.At the same time,Sn was doped into the ZnO particles through the process of Sn substitute Zn atom in ZnO.Due to the reduction of Zn from the micropaticles,the diameter of them will gradually decrease from the root to the top till the Zn source is almost used up,which results in the creation of ZnOnanoneedles.The size andshape of Sn-Zn-Odroplets are presumably dominantfactors forthe morphology and crystallinity of Sn-dopedZnOnanoneedles.

3 Conclusions

In summary,high density Sn-doped ZnO nanoneedles have been synthesized on Si(111)substrates by thermal oxidation of Sn:Zn thin films in a tube furnace at 650℃in Ar atmosphere. The nanoneedles are hexagonal wurtzite single-crystal Sn-doped ZnO with lengths up to 1-3 μm,and the root and tip diameters are 200-500 nm and 40 nm,respectively.PL shows that the Sndoped ZnO nanoneedles exhibit a clearblue shift from the bulk band gap emission,which is result from Burstein-Moss effect with an increasing in band-gap value.It is reasonable to expect thatthe ZnOnanoneedles withstrengthenedgreenlightemission would be a promising material forapplications in optoelectronic nanodevices.

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Sn摻雜ZnO納米針的結構及其生長機制

王 杰 莊惠照*薛成山 李俊林 徐 鵬

(山東師范大學半導體研究所,濟南 250014)

利用包括磁控濺射和熱氧化的兩步法在Si(111)襯底上制備了Sn摻雜ZnO納米針.首先用磁控濺射法在Si(111)襯底上制備Sn:Zn薄膜,然后在650℃的Ar氣氛中對薄膜進行熱氧化,制備出Sn摻雜ZnO納米針.樣品的結構、成分和光學性質采用X射線衍射(XRD)、掃描電子顯微鏡(SEM)、透射電子顯微鏡(TEM)、高分辨透射電子顯微鏡(HRTEM)、能量散射X射線(EDX)譜和光致發光(PL)光譜等技術手段進行分析.結果表明,制備的樣品為具有六方纖鋅礦結構的單晶Sn摻雜ZnO納米針,Sn摻雜量為2.5%(x,原子比),底部和頭部直徑分別為200-500 nm和40 nm,長度為1-3 μm,結晶質量較高.室溫光致發光光譜顯示紫外發光峰比純ZnO的發光峰稍有藍移,這可歸因于能譜分析中探測到的Sn的影響.基于本實驗的實際條件,簡單探討了Sn摻雜ZnO納米針的生長機制.

納米結構;ZnO;Sn摻雜; 濺射;光學特性;生長機制

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Received:June 27,2010;Revised:July 23,2010;Published on Web:September 10,2010.

*Corresponding author.Email:zhuanghuizhao@sdnu.edu.cn;Tel:+86-531-86182624;Fax:+86-531-86180017.The project was supported by the National Natural Science Foundation of China(90201025,90301002).

國家自然科學基金重大研究項目(90201025,90301002)資助

?Editorial office of Acta Physico-Chimica Sinica