Two series offilms were made, one deposited at room temperature but at four pressures, viz., 0.7, 1.3, 2.0 and 2.7 Pa of oxygen and one deposited at 1.3 Pa oxygen pressure but at the subs
Trang 1Pulsed electron beam deposition of transparent conducting Al-doped ZnO films
a
Hanoi University of Science, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam
b
National University of Civil Engineering, 55 Giai Phong Street, Hai Ba Trung, Hanoi, Viet Nam
a b s t r a c t
a r t i c l e i n f o
Article history:
Received 9 November 2011
Received in revised form 9 July 2012
Accepted 9 July 2012
Available online 16 July 2012
Keywords:
Zinc oxide
Doped II–VI semiconductors
Pulsed electron beam deposition
Thin films
Microstructure
Good quality transparent conducting Al-doped ZnOfilms were deposited on quartz substrates from a high purity target using pulsed electron deposition (PED) Two series offilms were made, one deposited at room temperature but at four pressures, viz., 0.7, 1.3, 2.0 and 2.7 Pa of oxygen and one deposited at 1.3 Pa oxygen pressure but at the substrate temperature ranged from room temperature to 600 °C In order to eval-uate the effect of substrate temperature and oxygen pressure on the properties of obtainedfilms, various characterization techniques were employed including X-ray diffraction, stylus profiler, scanning electron mi-croscope, optical spectrophotometer and electrical resistivity For thefirst series films, the optimal oxygen pressure of 1.3 Pa was found to bring about the appropriate energetic deposition atoms which results in the best crystallinity For the second seriesfilms, the lowest resistivity was obtained in the film grown at
400 °C An attempt was made to reduce the resistivity by lowering the oxygen pressure to 0.5 Pa which was the lower limit of working pressure of the PED system The obtained results indicate that PED is a suitable technique for growing transparent conducting ZnOfilms
© 2012 Elsevier B.V All rights reserved
1 Introduction
Recently, much attention has been paid on zinc oxide which is a
highly transparent semiconductor in the visible region with a wide
band gap of ~3.37 eV at room temperature and a high excitonic binding
energy of ~60 meV[1] When doped with group-III elements, such as Al,
Ga and In, its resistivity could be reduced to 2–4×10−4Ω.cm With
these features, ZnO is a promising material for fabricating a variety of
devices such as transparent conducting electrode forflat panel displays
and solar cell, optoelectronic devices[2–7] There are a wide variety of
methods for producing ZnO for fundamental studies or fabrication
of devices including RF sputtering[8], molecular beam epitaxy[9],
metal-organic chemical vapor deposition[10], vapor transport[11],
pulsedfiltered cathodic arc deposition[12]and pulsed laser
deposi-tion (PLD)[13,14] In particular, the pulsed laser deposition method
has been used for growth of transparent conducting ZnOfilms such
as Al, Ga and Er doped ZnOfilms[15–18] However, limitations of this
technique include high cost of laser source, eye-safety requirements, as
well as issues related to scalability More recently, pulsed electron
depo-sition (PED) has been considered as an alternative depodepo-sition technique
for the formation of high quality ZnO thinfilms In PED technique instead
of photons, energetic electrons are used to ablate the ceramic target
Therefore, this method will work with materials that are transparent to
ultraviolet photons Besides the advantage as in PLD that the
stoichiom-etry of target materials is preserved in thefilm, PED is scalable, simple,
and low cost, making this technique become a suitable tool for growing films of complex materials
Growth of pure ZnOfilms by PED has been reported by H L Porter
et al.[19], M Nistor et al.[20]and P Zhan et al.[21] In these works, thefilms with high crystalline and optical quality have been obtained and the effect of substrate temperature and oxygen pressure on the crystallinity and surface morphology has been studied In this paper,
we report the growth of transparent conducting Al doped ZnOfilm
by PED Moreover, several issues concerning the optimal growth con-ditions in order to get good quality transparent conducting ZnOfilm have been addressed in this paper
2 Experiment Al-doped ZnO (AZO) thinfilms were deposited on glass and quartz substrates using PED technique The pulsed electron gun used in our experiments is a commercial source, PEBS-20 manufactured by Neocera, Inc It has the following technical specifications: discharge voltage of 5–20 kV, beam energy of 0.2–0.8 J, pulse duration of
100 ns, maximum power density of 1.3 × 108W/cm2, beam cross section of about 6 × 10−2cm2, and pulse frequency of 1–10 Hz The distance between target and substrate was 60 mm In order to en-hance the uniform erosion and reduce the“conic effect” on the sur-face of target, a target rastering feature was made by the movement
of the target stage at the X–Y plane about the central axis of the stage Start angle, end angle, and desired speed of rastering can be selected by the user using computer In all our experiments, the dis-charge voltage was maintained at 14 kV and the pulse frequency was
⁎ Corresponding author Tel./fax: +84 438584438.
E-mail address: phquang2711@yahoo.com (P.H Quang).
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Trang 2maintained at 5 Hz We chose the discharge voltage as 14 kV because,
according to M Strikovski et al.[22], this is the optimal potential for
obtaining the maximum deposition rate We made two series offilms,
one deposited at room temperature but at four pressures, viz., 0.7, 1.3,
2.0 and 2.7 Pa of oxygen (named AZO-1), and one deposited at 1.3 Pa
oxygen pressure but at the substrate temperature ranged from room
temperature to 600 °C (named AZO-2) During deposition process, the
pressure was maintained by controlling the balance between the rate
of high vacuum pump and theflow of oxygen gas introduced into the
chamber The target used is a high purity commercial 2 wt.% Al2O3
doped ZnO target of diameter 2 in and thickness 3 mm All samples
were deposited with 20,000 pulses
The crystallinity of thefilms was characterized by X-ray diffraction
using Cu-α radiation (XRD, Bruker D5005) in a special configuration
where the incident angle was kept at 1.5° Thickness was examined
by stylus profiler (VEECO-Dektak D150) Surface morphology was
ex-amined by a scanning electron microscopy (SEM, Jeol— JSM5410LV)
at a voltage of 15 keV The optical properties were measured using
UV–vis spectrophotometer (UV — Shimadzu 2450) in the wavelength
range from 200 to 800 nm The electrical resistivity was determined
by four probe technique at room temperature
3 Results and discussion
3.1 Morphology, thickness and crystallinity
Fig 1shows three typical SEM images of thefilms grown at different
conditions As seen in these pictures, particulates in the range from 50
to about 100 nm are present on the surface of thefilms, whatever the
condition of deposition However, the density and the size of the
partic-ulates are smaller in thefilms deposited at a higher pressure The origin
of these particulates is a matter of discussion They can be directly
emit-ted by the target during electron beam ablation, or they can be formed
in the gas phase, during the transport of species from the target to the
substrate A Yousif et al observed some bright spots on the surface of
ZnO thinfilms deposited by PLD that were considered to be micro
par-ticles and drops which are produced by ZnO target due to the large
en-ergy density of laser[23] M Nistor et al also observed this problem[20]
and suggested the possibility to reduce it by careful optimization of the
electron beam parameters in relation to the target materials The
elec-tron beam parameters include the elecelec-tron kinetic energy, power
den-sity and beam cross section We agree with this suggestion because
we believe that the particulates are caused by high evaporation rate of
target materials due to the pulse feature of the electron source In
addi-tion, we suggested that the presence of gas ambient can have a signi
fi-cant role in the elimination of this problem
The thickness of thefilms depends strongly on the oxygen
pres-sure The thickness of thefilms in series AZO-1 are 270, 250, 230
and 220 nm for thefilms grown at room temperature and at 0.7,
1.3, 2.0 and 2.7 Pa oxygen pressure, respectively The thickness of
thefilms in AZO-2 series is almost identical with the value of about
250 nm The decrease of the thickness with oxygen pressure can be
easily explained by the interaction between incoming ions in plasma flux and gas atoms, resulting in a decrease of current of ions arriving
at the substrate
Fig 2shows the XRD patterns of the AZO-1films It is clear that thefilms grown at lower oxygen pressure (i.e 0.7 and 1.3 Pa) exhibit
a wurtzite structure with a high preference for the (002) orientation whereas the films grown at higher oxygen pressure (i.e 2.0 and 2.7 Pa) exhibit an amorphous structure The highest intensity of (002) peaks for thefilm grown at 1.3 Pa indicates that this pressure
is the most suitable The degradation of the crystallinity at high oxy-gen pressure has also been observed by S M Park et al in the growth
of AZOfilms by using PLD[15]and has been attributed to the excess oxygen that might induce defects in thefilms which, in our opinion, should be the planar type At the substrate temperature of 320 °C, P Zhan et al have obtained crystalline ZnOfilms grown by PED at any oxygen pressure in the range from 1.1 Pa to 2.4 Pa but the authors have also reported that the oxygen pressure of 1.6 Pa is the most fa-vorable value for obtaining the best crystallinity[21] In our case, the effect of oxygen pressure seems to be stronger and more obvious because thefilms were grown at room temperature, so that the influ-ence of temperature has been eliminated The fact that oxygen pressure has an optimal value originates from the requirement of the average en-ergy per deposited atom which should be about 10–20 eV This energy
on one hand is strong enough to complete the disruption of the colum-nar morphology of the growingfilm but on the other hand do not dam-age the surface It is well-known that in PED and PLD techniques, with the presence of background gasses during ablation, the species arriving
at the substrate loses their average velocity as they undergo through scattering, thermalization and deceleration[24]
Fig 3shows the XRD patterns of thefilms in AZO-2 series First of all,
we can see that allfilms exhibit a wurtzite structure with a high prefer-ence for the c-axis orientation perpendicular to the substrate surface The crystallinity evaluated from the intensity and full width at half-maximum of the (002) peak improves with increasing substrate temper-ature up to 400 °C Further increasing the substrate tempertemper-ature leads to
a slight degradation in crystallinity The grain size estimated from Scherrer equation is about 30 nm for the bestfilm, i.e the one deposited
at 400 °C Hirata et al.[15], and P Zhan et al.[20]have reported the sim-ilar effect of substrate temperature on the crystallinity of the ZnOfilms grown by PLD and PED P Zhan et al attributed the promotion of the (002) peak to the increase of adatom mobility as increasing substrate temperature P Zhan et al also explained the degradation of (002) peak at high temperature by structure zone model proposed by J A Thormton[25]and recently extended by A Anders[26] According to this model, thefilms deposited at high temperature have zone three structures which contain much more randomly oriented grains formed
by secondary nucleation and recrystallization
3.2 Electrical properties The electrical resistivity of both AZO-1 and AZO-2 is presented in Table 1 We can see that the resistivity of AZO-1films is very high
films grown at different conditions: (a) at 2.7 Pa and room temperature, (b) at 1.3 Pa and room temperature, and (c) at 1.3 Pa and 600 °C.
Trang 3However, it is still possible to realize that the resistivity is lower for
thefilms deposited at lower oxygen pressure For AZO-2 films, the
re-sistivity decreases dramatically with increasing the substrate
temper-ature up to 400 °C, and then increases slightly again Park et al.[14]
have observed a similar phenomenon on undoped ZnOfilms grown
by PLD and attributed the decrease of resistivity to the increase of
both carrier concentration and carrier mobility The slight increase
in resistivity of thefilms grown at high temperature was explained
by the contamination of C from quartz substrate Hirata et al.[15]
reported that resistivity of Ga-doped ZnOfilms deposited by PLD
de-creases with increasing deposited temperature up to 300 °C which is
their highest investigated temperature Since the facts that there is a
close relation between the crystallinity and conductivity and our
films are ZnO doped with Al, we suggest that the enhancement in
conductivity is mainly due to the increase of number of Al atoms
re-ally activated in matrix oxide to produce extra electrons in the
band The resistivity of 3.4 × 10−2Ω.cm obtained at the optimal
tem-perature is still quite high for the requirement of transparent
conducting oxidefilms (TCO) Noting that the resistivity of AZO-1
de-creases with decreasing the oxygen pressure, we made an additional
film at 400 °C and at an oxygen pressure of 0.5 Pa which was the lower critical working pressure of our system In PED technique, a too-low gas pressure may cause a divergence of electron beams be-cause of mutual charge repulsion while there is little space charge shielding[24] The resistivity of thisfilm is 2.4×10−3Ω.cm, that is one order lower than that of the bestfilm of AZO-2 series We believe that the improvement in conducting property of thisfilm has both contributions from the presence of oxygen vacancies and Al‐doped atoms However, this resistivity is still one order higher than that obtained by A Anders et al.[12]using pulsedfiltered cathodic tech-nique (about 5 × 10−4Ω.cm) and by A O Dikovska[17]using PLD technique (2.4 × 10−4Ω.cm)
3.3 Optical properties The optical transmittance of thefilms in both series AZO-1 and AZO-2
is very high (more than 80%) in the range of 350–900 nm It is worth to note that there is a relation between the crystallinity and the transmit-tance, e.g the highest value of transmittance (~90%) has been obtained for thefilm that has the best crystallinity This transmittance is compara-ble to those obtained by other techniques[12,17]and meets the applica-tion requirement of transmittance exceeding 80% The relaapplica-tion between the crystallinity and the optical property can be seen more clearly from the absorption spectra The absorption spectra of AZO-1 and AZO-2 films are shown inFig 4a and b, respectively The absorption spectra
of AZO-1films have a tail in the transparent zone All AZO-2 films have
a typical absorption spectrum of a direct band semiconductor with a sharp absorption edge The value of band gap about 3.2 eV can be deter-mined by extrapolating the square of the absorption coefficient versus the photon energy curve This value is somewhat lower than the value
of 3.37 eV measured on pure ZnO materials[1]
4 Conclusion Al-doped ZnOfilms have been grown by using Pulsed Electron De-position technique The properties of thefilms such as morphology, thickness, crystallinity, electrical resistivity and transmittance were found to depend strongly on the deposition conditions In terms of electrical conducting requirement, the bestfilm was obtained at the lowest oxygen pressure and at 400 °C The transmittance of the best films has met the application requirement of TCO films while the re-sistivity is still rather high In order to reduce the electrical rere-sistivity, further studies are necessary and will be done in future The obtained results indicate that PED is a suitable technique for growing transpar-ent conducting ZnOfilms
Acknowledgments The authors would like to acknowledge thefinancial support by the project NAFOSTED 103.02.59.09
Fig 2 XRD patterns of AZO films deposited at room temperature and various oxygen
pressures.
Fig 3 XRD patterns of AZO films deposited at various substrate temperatures and
ox-Table 1 Electrical resistivity of the prepared samples.
Oxygen pressure (Pa) Substrate temperature (°C) Electrical resistivity (Ω.cm)
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Fig 4 Absorption spectra of AZO films deposited (a) at room temperature and various
oxygen pressures and (b) at various substrate temperatures and oxygen pressure of
1.3 Pa.