Appl Phys A (2009) 95: 613–620DOI 10.1007/s00339-008-4965-5

Enhanced field emission from pulsed laser depositednanocrystalline ZnO thin films on Re and W

Dattatray J. Late · Pankaj Misra · B.N. Singh ·Lalit M. Kukreja · Dilip S. Joag · Mahendra A. More

Received: 29 April 2008 / Accepted: 20 October 2008 / Published online: 25 November 2008© Springer-Verlag 2008

Abstract Nanocrystalline ZnO thin films have been de-posited on rhenium and tungsten pointed and flat substratesby pulsed laser deposition method. An emission current of1 nA with an onset voltage of 120 V was observed re-peatedly and maximum current density ∼1.3 A/cm2 and9.3 mA/cm2 has been drawn from ZnO/Re and ZnO/Wpointed emitters at an applied voltage of 12.8 and 14 kV, re-spectively. In case of planar emitters (ZnO deposited on flatsubstrates), the onset field required to draw 1 nA emissioncurrent is observed to be 0.87 and 1.2 V/µm for ZnO/Re andZnO/W planar emitters, respectively. The Fowler–Nordheimplots of both the emitters show nonlinear behaviour, typi-cal for a semiconducting field emitter. The field enhance-ment factor β is estimated to be ∼2.15 × 105 cm−1 and2.16 × 105 cm−1 for pointed and 3.2 × 104 and 1.74 × 104

for planar ZnO/Re and ZnO/W emitters, respectively. Thehigh value of β factor suggests that the emission is fromthe nanometric features of the emitter surface. The emissioncurrent–time plots exhibit good stability of emission currentover a period of more than three hours. The post field emis-sion surface morphology studies show no significant dete-rioration of the emitter surface indicating that the ZnO thinfilm has a very strong adherence to both the substrates andexhibits a remarkable structural stability against high-field-induced mechanical stresses and ion bombardment. The re-

D.J. Late · D.S. Joag · M.A. More (�)Center for Advanced Studies in Material Science and CondensedMatter of Physics, Department of Physics, University of Pune,Pune 411007, Indiae-mail: mam@physics.unipune.ernet.inFax: +91-020-25691684

P. Misra · B.N. Singh · L.M. KukrejaRaja Ramnna Centre for Advanced Technology (CAT),Indore 452013, India

sults reveal that PLD offers unprecedented advantages infabricating the ZnO field emitters for practical applicationsin field-emission-based electron sources.

PACS 79.70.+q · 73.61.Jc · 61.46.Hk · 81.15.Fg

1 Introduction

In recent years, amongst various semiconducting materials,ZnO nanostructures have been extensively studied in view oftheir application as cold cathodes [1–5]. This has been due tothe superior physico-chemical properties exhibited by ZnOparticularly, large exciton binding energy (60 meV), lowelectron affinity [6], high thermal stability and oxidation re-sistance in harsh chemical environment [7]. Various effectsdue to the geometrical factors, areal density and morpholog-ical variations have been reported for arrays of ZnO nanos-tructures, primarily with the objective of attaining highervalues of emission current density at relatively lower onsetvoltage, compared to other cathode materials.

In most of these studies, the ZnO nanostructures havebeen synthesized by either a simple thermal evaporationmethod or using the chemical vapour deposition route. In re-cent years, pulsed laser deposition (PLD) method has beenused to prepare high-quality ZnO thin films on various sub-strates [8–11]. The advantage of this method lies in the pos-sibility to make thin films from materials that are difficult toshape under normal conditions [12]. Because of the higherenergy of the ablated particles in the laser-produced plume,relatively high deposition rates could be achieved. Recently,we have efficiently used PLD to synthesize nanocrystallinethin films of LaB6 on tungsten and rhenium substrates andinvestigated their field emission properties [13]. ZnO beinga promising material as cold cathode, we planned to study

614 D.J. Late et al.

the field emission properties of the PLD ZnO thin films. Inthis paper, we report our recent investigations on the PLDnanocrystalline ZnO thin films on rhenium and tungsten tipsas well as flat substrates and their field emission character-istics.

2 Experimental details

2.1 Pulsed laser deposition

The ZnO thin films were deposited on tungsten and rheniumtips and foils by optimised PLD technique. A third harmonicof Q-switched Nd: YAG laser operating at 355 nm, 6 nspulse duration and 10 Hz repetition rate was used to ab-late sintered ZnO pellet target at a fluence of ∼1 J/cm2.The pellet was prepared from ZnO powder (Purity 99.99%)with Poly Vinyl Alcohol as a binder by applying pressureof ∼75 kN/cm2, followed by sintering under nitrogen am-bient at ∼400°C for 8 hours. The W and Re tips were pre-pared by electrochemical polishing Re and W wires (diam-eter 0.125 mm (Re) and 0.25 mm (W), length 5 cm) in 1NKOH solution. The W and Re tips were mounted on a sub-strate holder such that the tip apex is normal to the holderbase and facing towards the target. The distance between thetips and the target was kept ∼5 cm. The chamber was ini-tially evacuated to base pressure of ∼1 × 10−6 mbar usingturbo molecular pump, and depositions were carried out inoxygen ambient pressure of 1 × 10−4 mbar for 60 seconds.During deposition, the substrate temperature was kept con-stant at 600°C. The deposited ZnO film has a thickness of∼1 µm.

The use of flat substrates facilitated structural character-isation of the deposited ZnO films by grazing angle X-rayDiffractometer (Model D-8 Advanced, Brucker AXS). An-alytical Scanning Electron Microscope JEOL 6360A wasused to investigate the surface morphology of the PLD ZnOfilms on tungsten and rhenium tips and foils. These charac-terisations were performed prior to the field emission inves-tigations. The use of tungsten and rhenium as substrates inthe present investigations is due to the fact that these refrac-tory metals are being used as support materials for fabricat-ing electron sources.

2.2 Field emission

The field emission measurements of the ZnO/Re and ZnO/Wpointed emitters were carried out in an all glass conven-tional field emission microscope (FEM) tubes, in separateexperiments. The conventional FEM tube is comprised ofan emitter cathode and a transparent anode with phosphorcoating. The ZnO deposited tips (pointed emitters) werespot welded to the tungsten glass pinch feed through and

fused into the field emission tube. The emission sites couldbe seen directly on the anode screen, which was at a dis-tance of ∼5 cm from the cathode. The field emission stud-ies of ZnO/Re (0.1 cm × 0.5 cm) and ZnO/W (0.5 cm ×0.5 cm) planar emitters were performed in planar diode con-figuration, in separate experiments. The planar emitter waspasted onto a stainless steel stub (spot welded to the glasspinch feedthrough) using vacuum compatible silver paste.A phosphor screen (made on optically transparent conduct-ing glass plate), held at a distance 5 mm from the cathode,was used as anode. The cathode–anode assembly was thenintroduced in the FEM tube.

The FEM tube was then vacuum processed using ul-trahigh vacuum system, comprising of an oil vapour dif-fusion pump with a chevron liquid nitrogen trap, a sput-ter ion pump and a titanium sublimation pump. A pres-sure of 1 × 10−9 mbar was obtained after baking the FEMtube at 250°C for 8 hours. The detailed experimental proce-dure describing mounting of the pointed emitter and vacuumprocessing of the FEM tube is reported elsewhere [13]. Thefield emission I–V and I–t characteristics were measuredat the base pressure of 1 × 10−9 mbar, using a Keithley 485picoammeter and a Spellman high-voltage DC power sup-ply. Special care has been taken to avoid any leakage currentusing shielded cables with proper grounding.

3 Results and discussion

The grazing angle (2°) X-ray diffraction (XRD) patterns ofthe ZnO thin films deposited on W and Re flat substrates areseen in Figs. 1(a) and (b), respectively. In both the cases,a set of well-defined diffractions peaks is seen, revealingcrystalline nature of the ZnO on these substrates. The phaseidentification and plane indexing have been done by com-paring the observed d values with the standard JCPDS datacards for ZnO, W and Re [14]. A careful observation of theXRD pattern reveals that the ZnO film on Re foil has pre-ferred orientation along the [101] direction. Moreover onboth the substrates the growth is observed to be isomorphicas revealed from the set of diffraction planes of ZnO and thesubstrates. The average grain size obtained using Debye–Scherrer formula is found to be ∼50 nm on both the W andRe substrates. Thus the XRD patterns clearly reveal forma-tion of nanocrystalline ZnO thin film under the experimentalconditions of PLD.

The scanning electron microscope (SEM) images of theZnO thin films on the Re tip and flat substrate recordedat different magnifications are shown in Figs. 2(a, b)and 2(c, d). Figures 3(a, b) and 3(c, d) show the SEM imagesof ZnO thin films deposited on W tip and flat substrate, re-spectively. As seen from the micrographs, in both the cases,the entire substrate surface is observed to be covered with

Enhanced field emission from pulsed laser deposited nanocrystalline ZnO thin films on Re and W 615

(a)

(b)

Fig. 1 The XRD of ZnO film deposited on a Re foil and b W foil byPLD

the ZnO crystallites. The nanocrystallites observed on theRe substrate are well isolated and randomly oriented. Theaverage grain size of the ZnO crystallites is found to be ∼50to 100 nm, which is in agreement with that estimated fromthe XRD pattern. However, on W substrate the crystallitesare not well resolved. The unique morphological features ofthe ZnO film on the Re and W substrates are due to the roleof the substrate in the growth mechanism. The (110) planeof W, being a low-index plane, is a smooth plane, and henceit is speculated that the isomorphic growth of ZnO nanocrys-talline film on W substrate will lead to smooth and planarmorphology. This suggests the possible cause for the ZnOnanocrystallites not being well resolved in the SEM image(Fig. 3c). On the pointed substrates the overall surface mor-phology is observed to be rough and characterised by thepresence of nanoscale protrusions, as reflected in the highvalue of field enhancement factor estimated in the followingsection.

The field emission current–voltage (I–V ) characteristicsof the ZnO/Re and ZnO/W pointed emitters are depicted inFig. 4. Interestingly, a current of 1 nA with an onset volt-age of 120 V is observed repeatedly for both the ZnO de-posited Re and W tips. Remarkably, this voltage is unaf-fected even after operating the emitters at much higher volt-ages (15 kV and above) for recording the I–V characteris-tics. With the increase in the applied voltage, the emissioncurrent is found to increase rapidly, and a total emission cur-rent of 13 and 3.28 µA has been drawn from the ZnO/Re andZnO/W pointed emitters at an applied voltage of 12.8 and14 kV, respectively.

The Fowler–Nordheim (F–N) plots [15], i.e., a graph ofln(I/V 2) versus 104/V derived from the I–V curves areshown in the inset of Fig. 4. An interesting nonlinear be-haviour of the F–N plot is observed for both the emitters,which could be clearly understood for two ranges of the ap-plied field, low and high. In the two ranges of the appliedfield, the resolved F–N plots are fairly linear in nature, char-acterised with distinct slopes. Such a nonlinear behaviour ofthe F–N plot, typical for a semiconductor emitter, has beenobserved previously for ZnO and SnO2 emitters [4, 5, 16],which has been attributed to the origin of field-emitted elec-trons. We believe that, in the low-field region, the emissioncurrent is due to the conduction band electrons only, whereasin the high-field region, electrons from both the conductionand the valence bands contribute to the observed emissioncurrent.

The field enhancement factor β is related to the slope (m)

of the F–N plot and is expressed by the equation [15]

β = (−6.8 × 103φ3/2)/m, (1)

where φ is the work function of the emitter material in eV.By taking work function of ZnO as 5.3 eV and using theslope of the low field region of F–N plot, the field enhance-ment factor β was estimated to be ∼2.15 × 105 cm−1 forZnO/Re pointed emitter. Similarly, for the ZnO/W pointedemitter, the β factor was observed to be 2.16 × 105 cm−1.The high value of the field enhancement factor β indicatesthat the emission is from the nanometric protrusions presenton the PLD ZnO thin film. Similar values of the field en-hancement factor β have been observed for ZnO nanos-tructures and PLD LaB6 thin films on W and Re substrates[3, 13].

In the present case, the emission current density J is es-timated to be 1.3 A/cm2 and 9.3 mA/cm2 for ZnO/Re andZnO/W pointed emitters at an applied voltage of 12.8 and14 kV, respectively. For pointed emitters approximating thetip apex to be a hemisphere of radius r , the current densityhas been defined as J = I/2πr2.

The emission current stability is one of the decisive andimportant parameters in the context of practical applicationsof the cold cathode. The emission current stability of the

616 D.J. Late et al.

Fig. 2 SEM images of ZnO film deposited on a, b Re tip and c, d Re foil

ZnO/Re and ZnO/W pointed emitters has been investigatedat the pre-set current value of 2 µA, over a duration of morethan three hours. Figure 5 shows the current–time (I–t) plotsrecorded at the base pressure of 1 × 10−9 mbar. In caseof the ZnO/W pointed emitter, the emission current slowlydecreases with time, exhibiting fluctuations in the form ofspikes, making an excursion to ∼1.2 µA and finally stabil-ising at the set value of 2 µA after one hour. The observedfluctuations are due to the adsorption, desorption and migra-tion of the residual gas molecules on the emitter surface. It isexpected that after continuous operation over a finite dura-tion, the emitter surface could become cleaner due to the ionbombardment-induced desorption of the residual gases fromthe surface. The clean surface thus produced leads to stabil-isation of the emission current at pre-set value of 2 µA afterone hour duration. It is interesting to note that, in case of theZnO/Re pointed emitter, the emission current is observedto be stable over the entire observation period, showing nosigns of degradation. The above results show that emittersurface conditioning is required for the stable operation ofthe PLD ZnO/W pointed emitter.

We have also studied the temporal changes in the fieldemission image during the current stability measurements.Typical field emission images, captured at the beginning ofthe stability measurements, are shown in the inset of Fig. 5.The observed emission pattern comprises of oval-shapedtwin bright spots. It is interesting to note that the numberof emission spots and the image features did not change,however the intensity of the spots was seen to vary abruptly.The intensity becoming temporarily higher is in accordancewith the spikes observed in the field emission current. Thedifference in the behaviour of current stability may be at-tributed to the growth of the ZnO thin film on the substrates.The ZnO growth on Re substrate is characterised by uni-formly distributed and well-resolved nanocrystallites (as re-vealed from the SEM images), whereas on W substrate, theZnO crystallites are not well resolved, and also the surfacemorphology is less uniform (rough). The ‘rough’ morphol-ogy of the emitter may lead to poor stability of the emissioncurrent. It is quite possible that due to ion bombardment, thetrapped gas molecules and some loosely bound ZnO clusterswill get removed and the clean emitter surface thus producedresults in minimisation of the emission current fluctuations.

Enhanced field emission from pulsed laser deposited nanocrystalline ZnO thin films on Re and W 617

Fig. 3 SEM images of ZnO film deposited on a, b W tip and c, d W foil

Fig. 4 Field emission current–voltage (I–V ) characteristics of theZnO/Re and ZnO/W pointed emitters. The inset shows the correspond-ing F–N plots

Fig. 5 Field emission current–time (I–t ) plots of the ZnO/Re andZnO/W pointed emitters. The inset shows the field emission patternof a ZnO/Re and b ZnO/W emitters

618 D.J. Late et al.

Fig. 6 Field emission current–voltage (I–V ) characteristics of theZnO/Re and ZnO/W planar emitters. The inset shows the correspond-ing F–N plot

The post-field emission SEM images (Fig. 8) clearly showthat the ion bombardment effect is more ‘severe’ in case ofZnO/W emitter.

Figure 6 shows the field emission current density-appliedfield (J –F ) characteristics of ZnO/Re and ZnO/W planaremitters. The turn on voltage required to draw a current of1 nA was observed to be 4.36 kV, which corresponds to anapplied field (average field) of 0.87 V/µm for ZnO/Re pla-nar emitter. For ZnO/W planar emitter, the turn on voltagewas found to be 6.0 kV, corresponding to an applied field of1.2 V/µm. As seen from Fig. 6, the emission current is ob-served to increase rapidly with increase in the applied volt-age. For ZnO/Re emitter, a total emission current of 1.2 µAhas been drawn at an applied electric field of 2.7 V/µm,whereas for ZnO/W emitter, a total emission current of 1 µAhas been drawn with an applied field 2.8 V/µm. The valuesof the applied field (required to draw 1 µA) are comparableto those reported in the literature [17]. For the planar emit-ters, the average field is defined as F = V/d , where d is theseparation between cathode and anode (5 mm for both theemitters).

The F–N plots, i.e., graphs of ln(J/F 2) versus (1/F ) de-rived from the J –F characteristics, are shown as the insetsof Fig. 6. The F–N plots exhibit nonlinear characteristic inthe entire range of applied field, which is identical to those ofpointed emitters. The field enhancement factor β , estimatedusing (1) and slope of the F–N plot in the low-field region,has values of 3.2 × 104 and 1.74 × 104 for the ZnO/Re andZnO/W planar emitters, respectively. These values are lowerby an order of magnitude as compared to the pointed emit-ters, suggesting that the nanometric features on pointed sub-strates are sharp and responsible for the observed high cur-rent density. The high value of β indicates that the emission

Fig. 7 Field emission current–time (I–t ) plots of ZnO/Re and ZnO/Wplanar emitters. The inset shows the field emission pattern of a ZnO/Reand b ZnO/W planar emitters

is from nanometric protrusions present on the PLD ZnO thinfilms. The formation of relatively more sharp protrusions onpointed substrates may be attributed to the role of the sub-strate played during the growth mechanism.

The emission current–time (I–t) graphs for the ZnO/Reand ZnO/W planar emitters, recorded at base pressure of1 × 10−9 mbar, are depicted in Fig. 7. As revealed fromthe I–t graphs, the ZnO/Re emitter shows excellent emis-sion current stability, whereas, the ZnO/W emitter exhibitsfairly stable emission current with fluctuations within ±1%of the preset value of the emission current ∼1µA. As com-pared to the pointed emitters, the planar emitters exhibit aremarkable current stability for repeated performance with-out any obvious signs of degradation. The emission currentstability behaviour of the pointed and planar emitters can beattributed to the growth mechanism on these substrates, asdiscussed earlier. The field emission patterns recorded at theoutset of the current stability measurement are shown as theinset of Figs. 7(a) and (b).

The post-field emission surface morphology of the ZnOthin films on Re (Fig. 8(a–b)) and W (Fig. 8(c–d)) tips showsno significant deterioration of the surface after the long-termoperation of the emitter. This signifies that the ZnO thinfilms obtained by PLD on Re and W tips have very good ad-hesion to the substrate and are resistant to ion bombardmentduring the electron emission. This is especially important forfabricating field emission electron sources delivering veryhigh current density with good stability for a continuous op-eration over duration of more than three hours. The sharpprotrusions with a nanometer-scale radius of curvature andhigh mechanical stiffness offer unprecedented advantages,thus providing a robust material for tiny electron sources.

Enhanced field emission from pulsed laser deposited nanocrystalline ZnO thin films on Re and W 619

Fig. 8 Post field emission SEM images of the ZnO/Re a, b and ZnO/W c, d pointed emitters

Further, we believe that the ease with which these nanos-tructures can be deposited and the smaller size of emittingarea, higher beam current and, importantly, longer lifetimeare promising features for making field emission-based de-vices.

4 Conclusions

In conclusion, well-adherent nanocrystalline ZnO thin filmshave been obtained using optimized PLD technique on rhe-nium and tungsten pointed and flat substrates. The nanocrys-talline ZnO thin film growth is observed to be isomorphicon both the Re and W flat substrates. The onset voltage, re-quired to draw 1 nA current from both pointed emitters isrepeatedly observed to be ∼120 V. For the planar emitters,the onset field required is observed to be 0.86 and 1.2 V/µmfor ZnO/Re and ZnO/W planar emitters. Very high currentdensity ∼1.3 A/cm2 and 9.3 mA/cm2 has been drawn fromZnO/Re and ZnO/W pointed emitters at an applied voltageof 12.8 and 14 kV, respectively. The F–N plots correspond-ing to both the pointed and planar emitters, show nonlin-ear behaviour, typical for a semiconducting emitter, whichhave been interpreted on the basis of the theory of electron

emission from semiconductor. The high value of the fieldenhancement factor suggests that the emission is from ZnOnanocrystallites. The observed superior field emission char-acteristics of the PLD nanocrystalline ZnO thin film are dueto the nanometric crystallites offering high value of field en-hancement factor coupled with the mechanical robustnessagainst ion bombardment and high field-induced stresses.

Acknowledgements The work performed by D.J. Late is supportedby Bhabha Atomic Research Center (BARC) by granting Senior Re-search fellowship. The authors are thankful to Centre for Nanomateri-als and Quantum Systems (CNQS), University of Pune, for providingthe XRD facility.

References

1. C.J. Lee, T.J. Lee, S.C. Lyu, Y. Zhang, Appl. Phys. Lett. 81, 3648(2002)

2. W. Wang, B. Zeng, J. Yang, B. Poudel, J. Huang, M.J. Naughton,Z. Ren, Adv. Mater. 18, 3275 (2006)

3. N.S. Ramgir, D.J. Late, A.B. Bhise, M.A. More, I.S. Mulla, K. Vi-jaymohanan, D.S. Joag, Nanotechnol. 17, 2730 (2006)

4. N.S. Ramgir, I.S. Mulla, K. Vijaymohanan, D.J. Late, A.B. Bhise,M.A. More, D.S. Joag, Appl. Phys. Lett. 88, 42107 (2006)

5. A.A. Al-Tabbakh, M.A. More, D.S. Joag, N.S. Ramgir, I.S. Mulla,K. Vijayamohanan, Appl. Phys. Lett. 90, 162102 (2007)

620 D.J. Late et al.

6. P. Misra, L.M. Kukreja, Thin Solid Films 485, 42 (2005)7. Q. Zhao, H.Z. Zhang, Y.W. Zhu, S.Q. Feng, X.C. Sun, J. Xu,

D.P. Yu, Appl. Phys. Lett. 86, 203115 (2005)8. J.L. Zhao, X.M. Li, J.M. Bian, W.D. Yu, X.D. Gao, J. Cryst.

Growth 276, 507 (2005)9. F.K. Shan, G.X. Liu, W.J. Lee, G.H. Lee, I.S. Kim, B.C. Shin,

Y.C. Kim, J. Cryst. Growth 277, 284 (2005)10. S. Christoulakis, M. Suchea, E. Koudoumas, M. Katharakis,

N. Katsarakis, G. Kiriakidis, Appl. Surf. Sci. 252, 5351 (2006)11. K. Ramamoorthy, C. Sanjeeviraja, M. Jayachandran, K. Sankara-

narayanan, P. Misra, L.M. Kukreja, Curr. Appl. Phys. 6, 103(2006)

12. D.B. Chrisey, G.K. Hubler (eds.), Pulsed Laser Deposition of ThinFilms (Wiley, New York, 1994), p. 417

13. D.J. Late, M.A. More, D.S. Joag, P. Misra, B.N. Singh,L.M. Kukreja, Ultramicroscopy 107, 825 (2007)

14. Powder Diffraction File, International Centre for Diffraction Data,Swarthmore, PA, JCPDS File No. 5-664, 4-806 and 5-702

15. R.H. Fowler, L.W. Nordheim, Proc. R. Soc. Lond. Ser. A 119, 173(1928)

16. A.B. Bhise, D.J. Late, M.A. More, D.S. Joag, N.S. Ramgir,I.S. Mulla, K. Vijayamohanan, Appl. Surf. Sci. 253, 9159 (2007)

17. S.H. Jo, D. Banerjee, Z.F. Ren, Appl. Phys. Lett. 85, 1407 (2004)