DSpace at VNU: Pulsed electron beam deposition of transparent conducting Al-doped ZnOfilms

Thin Solid Films 520 (2012) 6455–6458 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf Pulsed electron beam deposition of transparent conducting Al-doped ZnO films Pham Hong Quang a,⁎, Ngo Dinh Sang b, Do Quang Ngoc a a b Hanoi University of Science, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam National University of Civil Engineering, 55 Giai Phong Street, Hai Ba Trung, Hanoi, Viet Nam a r t i c l e i n f o Article history: Received November 2011 Received in revised form July 2012 Accepted July 2012 Available online 16 July 2012 Keywords: Zinc oxide Doped II–VI semiconductors Pulsed electron beam deposition Thin films Microstructure a b s t r a c t Good quality transparent conducting Al-doped ZnO films were deposited on quartz substrates from a high purity target using pulsed electron deposition (PED) Two series of films 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 evaluate the effect of substrate temperature and oxygen pressure on the properties of obtained films, various characterization techniques were employed including X-ray diffraction, stylus profiler, scanning electron microscope, optical spectrophotometer and electrical resistivity For the first 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 series films, 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 ZnO films © 2012 Elsevier B.V All rights reserved 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 −4 Ω.cm With these features, ZnO is a promising material for fabricating a variety of devices such as transparent conducting electrode for flat 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], pulsed filtered cathodic arc deposition [12] and pulsed laser deposition (PLD) [13,14] In particular, the pulsed laser deposition method has been used for growth of transparent conducting ZnO films such as Al, Ga and Er doped ZnO films [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 deposition (PED) has been considered as an alternative deposition technique for the formation of high quality ZnO thin films 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 stoichiometry of target materials is preserved in the film, PED is scalable, simple, ⁎ Corresponding author Tel./fax: +84 438584438 E-mail address: phquang2711@yahoo.com (P.H Quang) 0040-6090/$ – see front matter © 2012 Elsevier B.V All rights reserved doi:10.1016/j.tsf.2012.07.027 and low cost, making this technique become a suitable tool for growing films of complex materials Growth of pure ZnO films 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, the films 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 ZnO film by PED Moreover, several issues concerning the optimal growth conditions in order to get good quality transparent conducting ZnO film have been addressed in this paper Experiment Al-doped ZnO (AZO) thin films 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 × 10 W/cm 2, beam cross section of about × 10 − cm 2, and pulse frequency of 1–10 Hz The distance between target and substrate was 60 mm In order to enhance the uniform erosion and reduce the “conic effect” on the surface 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 discharge voltage was maintained at 14 kV and the pulse frequency was 6456 P.H Quang et al / Thin Solid Films 520 (2012) 6455–6458 maintained at 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 of films, 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 the flow of oxygen gas introduced into the chamber The target used is a high purity commercial wt.% Al2O3 doped ZnO target of diameter in and thickness mm All samples were deposited with 20,000 pulses The crystallinity of the films 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 examined 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 Results and discussion 3.1 Morphology, thickness and crystallinity Fig shows three typical SEM images of the films 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 the films, whatever the condition of deposition However, the density and the size of the particulates are smaller in the films deposited at a higher pressure The origin of these particulates is a matter of discussion They can be directly emitted 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 thin films deposited by PLD that were considered to be micro particles and drops which are produced by ZnO target due to the large energy 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 electron beam parameters include the electron kinetic energy, power density 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 addition, we suggested that the presence of gas ambient can have a significant role in the elimination of this problem The thickness of the films depends strongly on the oxygen pressure The thickness of the films in series AZO-1 are 270, 250, 230 and 220 nm for the films grown at room temperature and at 0.7, 1.3, 2.0 and 2.7 Pa oxygen pressure, respectively The thickness of the films 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 shows the XRD patterns of the AZO-1 films It is clear that the films 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 the film grown at 1.3 Pa indicates that this pressure is the most suitable The degradation of the crystallinity at high oxygen pressure has also been observed by S M Park et al in the growth of AZO films by using PLD [15] and has been attributed to the excess oxygen that might induce defects in the films which, in our opinion, should be the planar type At the substrate temperature of 320 °C, P Zhan et al have obtained crystalline ZnO films 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 favorable value for obtaining the best crystallinity [21] In our case, the effect of oxygen pressure seems to be stronger and more obvious because the films were grown at room temperature, so that the influence of temperature has been eliminated The fact that oxygen pressure has an optimal value originates from the requirement of the average energy per deposited atom which should be about 10–20 eV This energy on one hand is strong enough to complete the disruption of the columnar morphology of the growing film but on the other hand not damage 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 shows the XRD patterns of the films in AZO-2 series First of all, we can see that all films exhibit a wurtzite structure with a high preference for the c-axis orientation perpendicular to the substrate surface The crystallinity evaluated from the intensity and full width at halfmaximum of the (002) peak improves with increasing substrate temperature up to 400 °C Further increasing the substrate temperature leads to a slight degradation in crystallinity The grain size estimated from Scherrer equation is about 30 nm for the best film, i.e the one deposited at 400 °C Hirata et al [15], and P Zhan et al [20] have reported the similar effect of substrate temperature on the crystallinity of the ZnO films 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, the films 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 We can see that the resistivity of AZO-1 films is very high Fig Typical SEM images of the 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 P.H Quang et al / Thin Solid Films 520 (2012) 6455–6458 6457 Table Electrical resistivity of the prepared samples Fig XRD patterns of AZO films deposited at room temperature and various oxygen pressures However, it is still possible to realize that the resistivity is lower for the films deposited at lower oxygen pressure For AZO-2 films, the resistivity decreases dramatically with increasing the substrate temperature up to 400 °C, and then increases slightly again Park et al [14] have observed a similar phenomenon on undoped ZnO films 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 the films grown at high temperature was explained by the contamination of C from quartz substrate Hirata et al [15] reported that resistivity of Ga-doped ZnO films deposited by PLD decreases 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 really activated in matrix oxide to produce extra electrons in the band The resistivity of 3.4 × 10 −2 Ω.cm obtained at the optimal temperature is still quite high for the requirement of transparent conducting oxide films (TCO) Noting that the resistivity of AZO-1 decreases with decreasing the oxygen pressure, we made an additional Oxygen pressure (Pa) Substrate temperature (°C) Electrical resistivity (Ω.cm) 0.7 1.3 2.0 2.7 1.3 1.3 1.3 25 25 25 25 150 400 600 × 10−1 × 100 × 101 × 102 × 10−1 3.4 × 10−2 × 10−2 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 because of mutual charge repulsion while there is little space charge shielding [24] The resistivity of this film is 2.4 × 10 − Ω.cm, that is one order lower than that of the best film of AZO-2 series We believe that the improvement in conducting property of this film 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 pulsed filtered cathodic technique (about × 10 − Ω.cm) and by A O Dikovska [17] using PLD technique (2.4 × 10 − Ω.cm) 3.3 Optical properties The optical transmittance of the films 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 transmittance, e.g the highest value of transmittance (~90%) has been obtained for the film that has the best crystallinity This transmittance is comparable to those obtained by other techniques [12,17] and meets the application requirement of transmittance exceeding 80% The relation 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 in Fig 4a and b, respectively The absorption spectra of AZO-1 films 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 determined 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] Conclusion Al-doped ZnO films have been grown by using Pulsed Electron Deposition technique The properties of the films 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 best film 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 resistivity is still rather high In order to reduce the electrical resistivity, further studies are necessary and will be done in future The obtained results indicate that PED is a suitable technique for growing transparent conducting ZnO films Acknowledgments Fig XRD patterns of AZO films deposited at various substrate temperatures and oxygen pressure of 1.3 Pa The authors would like to acknowledge the financial support by the project NAFOSTED 103.02.59.09 6458 P.H Quang et al / Thin Solid Films 520 (2012) 6455–6458 References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] Fig Absorption spectra of AZO films deposited (a) at room 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