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The growth of transparent conducting ZnO films by pulsed laser ablation
Surface and Coatings Technology 177 – 178 (2004) 271–276
The growth of transparent conducting ZnO films by pulsed laser ablation S.J. Henleya,*, M.N.R. Ashfolda, D. Chernsb b
a School of Chemistry, University of Bristol, Tyndall Avenue, Bristol BS8 1TS, UK H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK
Abstract The structure of undoped, Al-doped ZnO (AZO) and Ga-doped ZnO (GZO) thin films grown on sapphire and NaCl substrates by 193 nm pulsed laser ablation of a ZnO target in a low background pressure of oxygen was investigated using transmission electron microscopy (TEM) and X-ray diffraction (XRD). The films on sapphire grew with the polar (0002) orientation. The samples deposited on NaCl, at substrate temperatures above 570 K, presented a mixture of polar and non-polar orientations. All samples demonstrated improved crystalline quality, as measured by the FWHM of the ZnO (0002) rocking curve, with increasing substrate temperature. The best crystalline quality was observed for the undoped films. The inclusion of Al or Ga into the lattice degraded the crystallinity of the films, but allowed production of highly conductive films. AZO and GZO film resistivities were measured using a four-point probe method and were found to decrease with increasing deposition temperature. Film thickness was determined using variable angle spectroscopic ellipsometry. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Al-doped ZnO (AZO) films; Ga-doped ZnO (GZO) films; Pulsed laser ablation (PLA)
1. Introduction Transparent conducting oxide (TCO) films are currently of great commercial and scientific interest. Traditionally indium tin oxide (ITO) has been the TCO of choice but it is expensive due to the low natural abundance of indium. With appropriate doping, transparent ZnO conducting films can be produced. From a purely financial view point doped ZnO TCO films are preferable to ITO as the natural abundance of Zn is 1000= higher than that of indium. As-grown, nominally undoped, ZnO usually demonstrates n-type conductivity due to the presence of either Zn interstitials or O vacancies w1x. The literature is still unclear, however, as to which of these intrinsic defects are the cause of the conductivity. Unfortunately the as-grown films are not suitable for device applications as the resistivity is too high and the reoxidation of the Zn rich films at ambient temperatures removes the source of the conductivity. To introduce stable n-type conductivity the two dopants that are used most typically are Ga and Al w2–9x, although doping with other elements such as B w10x, In *Corresponding author. Present address: Advanced Technology Institute, School of Electronics & Physical Sciences, University of Surrey, Guildford GU2 7XH, UK; Tel.: q44-1483-686088. E-mail address: [email protected] (S.J. Henley).
w11x and Zr w3x has also been investigated. Al doped ZnO (AZO) and Ga doped ZnO (GZO) films can be used as electrodes for flat panel displays, low emissivity glass, as thin film solar cells and as anode material for organic light emitting diodes w3x. ZnO primarily crystallizes in the wurtzite structure with lattice parameters as0.325 nm and cs0.521 nm. Films grown by a variety of techniques generally grow with the c-axis perpendicular to the substrate surface. For example, caxis aligned material has been observed in growth by spray pyrolysis w12,13x, ion beam assisted sputter deposition w6,14x, by RF and DC magnetron sputtering w4,5,15x and pulsed laser ablation (PLA) w7,16–18x. PLA has been shown to yield high quality c-axis oriented ZnO films at lower substrate temperatures than many other techniques w16,19x. The films deposited by PLA in vacuum are typically Zn rich due to recondensation of Zn onto the surface of the target w20x. For this reason ZnO films are typically grown in a background pressure of oxygen w2x. In this paper we report results obtained from nominally undoped ZnO, AZO and GZO films grown by 193 nm PLA at different substrate temperatures and background pressures of O2. The crystallinity of the films, with different growth parameters, and its effect on the resistivity will be discussed.
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.09.005
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Fig. 1. XRD 2u scans of ZnO films grown on sapphire substrates in the vicinity of the ZnO (0002) reflection. (a) 2u scans of undoped ZnO as a function of Ts. (b) 2u scans of GZO films as a function of pO2.
2. Experimental details The films were grown by PLA of commercially available ZnO hot pressed disks (Cerac). The nominally undoped ZnO films were deposited by ablating a 99.999% pure ZnO target. The AZO and GZO films were produced by ablating ZnO targets containing 2% Al2O3 and 5% Ga by weight, respectively. Targets with these compositions were chosen as other groups have shown that these levels of dopant are required to obtain good conductivities w5,7x. A Lambda-Physik COMPex 201 excimer laser operating at 193 nm was employed for the ablation. Thin films were grown using a laser pulse energy of 100 mJ focused onto the target producing a fluence of approximately 10 J cmy2. The target was rotated to avoid repeated ablation from the same spot. The growth chamber was evacuated using a turbo pump and then back filled with a steady flow of oxygen using a mass flow controller so as to maintain a constant background pressure. The substrate was positioned 15 cm away from the target and could be heated up to 500 8C. The typical growth run consisted of 9000 laser shots with a repetition rate of 10 Hz. The majority of the films were deposited on (0001) sapphire substrates that were only polished on one side. Films for TEM analysis were grown on NaCl substrates. The TEM analysis was carried out on a Philips EM430 microscope operating at 250 kV. Electron transparent samples could be readily made from the films by dissolving off the NaCl substrate in distilled water. The free-floating film was then caught on a standard TEM mesh grid. The sheet resistances of the films grown on sapphire were measured using a four-point probe method. In order to obtain the resisitivity it is necessary to know the film thickness. Variable angle spectroscopic ellipsometry (VASE) measurements were performed using a Woolam M–2000U spectroscopic ellipsometer to determine the film thickness in a
manner similar to that reported by Sun et al. w21x but using four fitting angles in the range 60–758 and Cauchy equations for the dispersion. The variation in crystallinity of the films was explored using X-ray diffraction (XRD) in a Bruker AXS D8 advance powder diffractometer. The extent of orientation of the films was assessed from the full width at half maximum (FWHM) of the v rocking curves of the ZnO (0002) reflection. 3. Results and discussion The first stage of optimisation of the growth of ZnO films was to study the dependence of their crystallinity as a function of substrate temperature (Ts). For this purpose a set of undoped ZnO samples were deposited at different temperatures in the range 343 K-Ts-713 K on sapphire substrates in an oxygen background pressure (pO2) of 1.3 Pa. Their crystallinity was investigated using XRD. The only ZnO related reflections observed were the (0002) and (0004) basal plane reflections of wurtzite ZnO. This indicates that the samples, although polycrystalline, grew with the (0001) direction perpendicular to the substrate; similar texturing of the films has also been observed by other groups w7,16–18x. Fig. 1a shows segments of the 2u scans in the vicinity of the (0002) reflection. The peak is observed to shift towards higher angle and to become narrower with increasing Ts. Such a shift is equivalent to a decrease in c-lattice parameter from 0.526 to 0.518 nm as Ts increases from 343 to 713 K. The standard value for unstrained wurtzite ZnO is 0.521 nm, so the shift observed involves a change of strain (according to the biaxial strain model) from q1 to y0.5%. Similar sets of samples were deposited using the Al and Ga containing targets and XRD 2u scans carried out on these films, also. These results are not presented here but showed the same trend with Ts as the undoped films.
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w19x, but to summarise briefly the pattern demonstrates the presence of crystal grains with a mixture of polar and non-polar orientations, unlike the growth on sapphire which typically demonstrated improved (0001) texturing with increasing Ts. The FWHM of the XRD (0002) reflection is often used to determine the grain size (dg) of the films w5x using the Scherrer formula: dgs
Fig. 2. TEM images of two samples of undoped ZnO grown on NaCl at Tss293 and 473 K. At both temperatures pO2 was 1.3 Pa. (a) SAED pattern taken from an area on the sample grown at Tss293 K. (b) SAED pattern from the sample grown at Tss473 K. This pattern, although taken from many hundreds of crystal grains, is a spot pattern with four-fold symmetry. (c) and (d) BF TEM images of regions of the low and high temperature samples, respectively.
For example, the peak shift observed in the AZO films would imply a shift of the c-lattice parameter from 0.525 to 0.502 nm over the same range of increasing Ts. At Tss713 K the strain along the c axis is calculated as y3.6%. The deduced increase in compressive strain in the AZO films is likely to be a consequence of the inclusion of Al into the ZnO lattice. The ionic radius of Al3q is 53 pm, smaller than that of Zn2q at 72 pm. For comparison the ionic radius of Gal3q is 62 pm. As a substitutional point defect on the Zn lattice site, Al would be expected to contract the ZnO lattice, as is observed. In addition to the XRD analysis of the crystallinity of the undoped ZnO, samples were grown at different Ts on (001) NaCl substrates so that TEM analysis could be carried out on the films. Films were deposited at both Tss293 and 473 K, with pO2s1.3 Pa. Fig. 2a shows selected area electron diffraction (SAED) patterns taken from a region of the low temperature sample containing many crystal grains. The ring pattern is that of polycrystalline wurtzite ZnO. Even at room temperature, however, there is evidence that oriented growth is occurring as the (0002) ring, which should lie between the first two visible rings, is absent. Fig. 2b shows the SAED pattern from the sample grown at the higher Ts. Though sampling many hundreds of crystal grains, this is a spot pattern with four-fold symmetry and thus indicative of epitaxial growth. The nature of epitaxy and the indexing of the spot pattern is discussed elsewhere
kl
Žcos u.yb2yb20
(1)
where ls0.154056 nm, k is the correction factor (taken as 0.9), u is obtained from the peak position and b and b0 are the peak width and the width introduced by instrumental broadening, respectively. XRD 2u scans were performed on the films grown on NaCl prior to dissolution of the substrate. Substituting the measured width of the (0002) peak into Eq. (1) along with the experimental broadening term, determined from the measured width of the (0006) peak from a single crystal piece of sapphire, yielded dgs29 nm. Bright field (BF) TEM images of the films grown at Tss293 and 473 K are shown in Fig. 2c,d. Examination of these images suggests that the dg value derived using Eq. (1) is somewhat high; indeed the grain size of the sample grown at room temperature, if anything, appears slightly larger than in the film grown at higher Ts. Such behaviour was not expected and may indicate that, after the initial onset of epitaxial growth the grain size is smaller than that observed in growth unfettered by the substrate. Unfortunately the (0002) reflection was not evident in XRD 2u scans of the room temperature film, so no comparison with Eq. (1) could be made. Applying the Scherrer formula to the XRD 2u scans presented in Fig. 1 allows estimation of the grain size, as a function of Ts. The calculated value for the grain size of the undoped films increases from 12 nm at Tss343 K up to 38 nm at Tss713 K. Similar analysis of the AZO films yielded dg values increasing from 7 to 23 nm. This smaller calculated grain size (compared to the undoped material) could be an indication that Al atoms are having a negative effect on the crystalline quality of the ZnO lattice. The high temperature GZO films, in contrast, had a calculated grain size of 36 nm, very similar to that of undoped ZnO, indicating that the Ga3q ions fit into the ZnO lattice more easily than the Al at the higher substrate temperatures. The effect of pO2 on the crystallinity of the GZO film was investigated by depositing films at a range of pO2 from f0 (i.e. no addition of oxygen) to 5.2 Pa at Tss723 K. XRD 2u scans, in the vicinity of the (0002) reflection, were carried out on these films. The results are shown in Fig. 1b. The sample prepared in 0 Pa of O2 showed little evidence of crystallinity from the XRD measurements. This is to be expected, as the film is
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Fig. 3. FWHM of ZnO (0002) v rocking curves of (a) undoped and (b) GZO and AZO samples as a function of Ts. All samples were grown with pO2s1.3 Pa.
likely to deviate significantly from its correct stoichiometry w20x. The optimal pO2 (with regards to total reflected intensity of the (0002) peak) for our experimental setup was found to be 2.6 Pa, but the FWHM of the (0002) reflection was not observed to differ between samples grown with pO2)0 Pa. This may indicate little or no change in the grain size of the film. The reduction in intensity of the (0002) reflection between samples could be a consequence of scattering from point defects in the films. In order to determine the extent of c-axis orientation of the films, the v rocking curves of the (0002) peaks were measured. The FWHM of the rocking curves for the undoped ZnO films are plotted in Fig. 3a and those for the AZO and GZO films are shown in Fig. 3b. The FWHM of the (0002) rocking curve in the case of the undoped ZnO films is seen to reduce significantly with increasing Ts, with the narrowest FWHM (0.578) obtained at the highest Ts used. The AZO and GZO films showed inferior orientation compared to the undoped samples grown at the same value of Ts. As Fig. 3b shows the rocking curves of the GZO films exhibit a larger FWHM than those of the corresponding AZO film at the low Ts values but with increasing Ts, the GZO films show the better alignment. The lowest FWHM obtained for the GZO and AZO films are 0.98 and 1.98, respectively. The disruption to the growth of the ZnO introduced by the dopants is more significant at lower Ts for the films grown from the target containing 5% Ga by weight than those from the 2% Al2O3 by weight target. This is understandable, since the impurity density will be higher in the film grown from the Ga containing target. The ionic radius of Ga3q is smaller than that of Zn2q but larger than that of Al3q. As a substitutional donor Ga should thus disrupt the ZnO lattice less than an Al ion. The observation that at higher
Ts values, the FWHM of the rocking curves of the GZO films are less than those of the corresponding AZO films is consistent with the view that although the GZO films are expected to have a higher impurity density, the Ga impurities are accommodated into the ZnO lattice more easily. After demonstrating growth of crystalline doped and undoped ZnO thin films, the resistivities (r) of the films were measured using a four-point probe method. This technique allows measurement, after suitable corrections for the finite size of the samples, of the sheet resistance (RS) of the films. In order to obtain the resistivity, the film thickness (t) must be known (rsRSt). The thickness of the films was estimated using spectroscopic ellipsometry. As neither the optical parameters of the material nor the thickness were known before fitting, the ellipsometric data for each sample was measured at four different incident angles to increase the reliability of the fitting. For all sets of films the measured thickness was from f100 nm at the lower temperatures to f80 nm at the highest temperatures. This is likely to be a consequence of the improved packing of material due to better crystallinity. There may also be more desorption of incident material from the substrate at higher Ts. Fig. 4 displays the RS data for the AZO and GZO films as a function of Ts. No data is presented for the undoped ZnO material as the resisitivity of all the films was too high to measure with the equipment available. It should be noted that the GZO data (Fig. 4b) is plotted on a logarithmic scale as RS changes by two orders of magnitude between high and low Ts. GZO and AZO films grown at the highest Ts had similar RS values giving values of rf1.6=10y3 V cm and rf9.7=10y4 V cm, respectively. These are slightly larger than the best values reported for AZO and GZO films, which are typically in the range of 1.4–
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Fig. 4. GFour-point probe measurements of the sheet resistance of (a) AZO and (b) GZO samples as a function of Ts. All samples were grown with pO2s1.3 Pa. Note the logarithmic scale in (b).
2.0=10y4 V cm irrespective of deposition method w22x. It should be noted, however, that our films are thinner than most of those from similar sets of data presented in the literature. The resistivity of AZO films, produced by ion beam sputtering, has been shown to increase by an order of magnitude as the film thickness decreases from 200 to 100 nm w6x. This effect is attributed to a reduction of the crystallinity at the substrateyfilm interface with increased scattering from crystallographic defects w6x. Considering this effect, our resistivity results are comparable with the best in the literature w22x. It is likely that these best values are the lowest resistivities that can be obtained with conventional doping as, in this range, ionised impurity scattering limits the conductivity. 4. Conclusions The crystalline quality, extent of oriented growth and conductivity of undoped ZnO films and of Al and Ga doped ZnO films, grown by 193 nm PLA, have been studied as a function of substrate temperature and background O2 pressure. The GZO films deposited at high Ts were shown to be of better crystalline quality than the AZO films, even though they were grown from a target containing 5% Ga compared to only 2% Al2O3 for the AZO films. This we attribute to differences in the ionic radii of the dopants. The ionic radius of Ga3q atoms is closer to that of Zn2q, so Ga atoms cause less distortion of the ZnO lattice when they sit as substitutional impurities on metal lattice sites. The undoped ZnO films demonstrated better crystalline quality than either doped material, as was to be expected, but were shown to have very low conductivities. The undoped ZnO grown on NaCl demonstrated significant epitaxial growth at Ts)300 8C, suggesting that this is the suitable minimum temperature for growth of high quality
ZnO films by PLA. The crystal grains produced at this temperature presented a mixture of polar and non-polar orientations w19x, unlike the growth on sapphire, which only demonstrated improved (0001) texturing. The addition of Ga or Al enabled the production of conducting films. The lowest resistivity value for the AZO film (grown at the highest Ts) was 9.7=10y4 V cm; For GZO, the best resistivity was slightly larger at 1.6=10y3 V cm. The resistivity of the GZO films was found to increase by two order of magnitudes as Ts was decreased to near room temperature, whereas that of the AZO films only increased by a factor of 4 for the same decrease in Ts. We suggest that, although Ga is a better dopant for producing high quality n-type ZnO, Al maybe more economically viable as lower process temperatures can be used and the conductivity of the films can be expected to be more reproducible. Acknowledgments This work was supported by a DTI LINK OSDA Programme: AEROFED. The authors thank Dr N.A. Fox and K.N. Rosser for their encouragement of this work. S.J.H. would like to thank Dr R. Vincent for many useful discussions regarding the analysis of the ED patterns. References w1x A.F. Kohan, G. Ceder, D. Morgan, C.G. Van de Walle, Phys. Rev. B 16 (2000) 15 019. w2x A.V. Singh, R.M. Mehra, N. Buthrath, A. Wakahara, A. Yoshida, J. Appl. Phys. 90 (2001) 5661. w3x H. Kim, J.S. Horwitz, W.H. Kim, A.J. Makinen, ¨ Z.H. Kafafi, D.B. Chrisey, Thin Solid Films 420–421 (2002) 539. w4x T. Minami, K. Oohashi, S. Takata, Thin Solid Films 193–194 (1990) 721. w5x M. Chen, Z.L. Pei, X. Wang, C. Sun, L.S. Wen, J. Mater. Res. 16 (2001) 2118.
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w6x Y. Qu, T.A. Gessert, K. Ramanathan, R.G. Dhere, R. Noufi, T.J. Coutts, J. Vac. Sci. Technol. A 11 (1993) 996. w7x M. Hiramatsu, K. Imaeda, N. Horio, M. Nawata, J. Vac. Sci. Technol. A 16 (1998) 669. w8x G.A. Hirata, J. McKittrick, T. Cheeks, J.M. Siqueiros, J.A. Diaz, O. Contreras, et al., Thin Solid Films 288 (1996) 29. w9x K.T.R. Reddy, T.B.S. Reddy, I. Forbes, R.W. Miles, Surf. Coat. Technol. 151 (2002) 110. w10x A. Yamada, B. Sang, M. Konagai, Appl. Surf. Sci. 112 (1997) 216. w11x K. Tominaga, N. Umezu, I. Mori, T. Ushiro, I. Nakabayashi, J. Vac. Sci. Technol. A 16 (1997) 1213. w12x B.J. Lokhande, M.D. Uplane, Appl. Surf. Sci. 167 (2000) 243. w13x M. Nurul Islam, T.B. Ghosh, K.L. Chopra, H.N. Acharya, Thin Solid Films 280 (1995) 20.
w14x F. Quaranta, A. Valentini, F.R. Rizzi, G. Casamassima, J. Appl. Phys. 74 (1993) 244. w15x A. Hachigo, H. Nakahata, K. Higaki, S. Fujii, S. Shikata, Appl. Phys. Lett. 65 (1994) 2556. w16x V. Craciun, J. Elders, J.G.E. Gardeniers, J. Gretovsky, I.W. Boyd, Thin Solid Films 259 (1995) 1. w17x J.H. Choi, H. Tabata, T. Kawai, J. Cryst. Growth 226 (2001) 493. w18x R.D. Vispute, V. Talyansky, Z. Trajanovic, S. Choopun, M. Downes, R.P. Sharma, et al., Appl. Phys. Lett. 70 (1997) 2735. w19x S.J. Henley, M.N.R. Ashfold, D. Cherns, Thin Solid Films 422 (2002) 69. w20x F. Claeyssens, A. Cheesman, S.J. Henley, M.N.R. Ashfold, J. Appl. Phys. 92 (2002) 6886. w21x X.W. Sun, H.S. Kwok, J. Appl. Phys. 86 (1999) 408. w22x K. Ellmer, J. Phys. D: Appl. Phys. 34 (2001) 3097.
The growth of transparent conducting ZnO films by pulsed laser ablation S.J. Henleya,*, M.N.R. Ashfolda, D. Chernsb b
a School of Chemistry, University of Bristol, Tyndall Avenue, Bristol BS8 1TS, UK H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK
Abstract The structure of undoped, Al-doped ZnO (AZO) and Ga-doped ZnO (GZO) thin films grown on sapphire and NaCl substrates by 193 nm pulsed laser ablation of a ZnO target in a low background pressure of oxygen was investigated using transmission electron microscopy (TEM) and X-ray diffraction (XRD). The films on sapphire grew with the polar (0002) orientation. The samples deposited on NaCl, at substrate temperatures above 570 K, presented a mixture of polar and non-polar orientations. All samples demonstrated improved crystalline quality, as measured by the FWHM of the ZnO (0002) rocking curve, with increasing substrate temperature. The best crystalline quality was observed for the undoped films. The inclusion of Al or Ga into the lattice degraded the crystallinity of the films, but allowed production of highly conductive films. AZO and GZO film resistivities were measured using a four-point probe method and were found to decrease with increasing deposition temperature. Film thickness was determined using variable angle spectroscopic ellipsometry. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Al-doped ZnO (AZO) films; Ga-doped ZnO (GZO) films; Pulsed laser ablation (PLA)
1. Introduction Transparent conducting oxide (TCO) films are currently of great commercial and scientific interest. Traditionally indium tin oxide (ITO) has been the TCO of choice but it is expensive due to the low natural abundance of indium. With appropriate doping, transparent ZnO conducting films can be produced. From a purely financial view point doped ZnO TCO films are preferable to ITO as the natural abundance of Zn is 1000= higher than that of indium. As-grown, nominally undoped, ZnO usually demonstrates n-type conductivity due to the presence of either Zn interstitials or O vacancies w1x. The literature is still unclear, however, as to which of these intrinsic defects are the cause of the conductivity. Unfortunately the as-grown films are not suitable for device applications as the resistivity is too high and the reoxidation of the Zn rich films at ambient temperatures removes the source of the conductivity. To introduce stable n-type conductivity the two dopants that are used most typically are Ga and Al w2–9x, although doping with other elements such as B w10x, In *Corresponding author. Present address: Advanced Technology Institute, School of Electronics & Physical Sciences, University of Surrey, Guildford GU2 7XH, UK; Tel.: q44-1483-686088. E-mail address: [email protected] (S.J. Henley).
w11x and Zr w3x has also been investigated. Al doped ZnO (AZO) and Ga doped ZnO (GZO) films can be used as electrodes for flat panel displays, low emissivity glass, as thin film solar cells and as anode material for organic light emitting diodes w3x. ZnO primarily crystallizes in the wurtzite structure with lattice parameters as0.325 nm and cs0.521 nm. Films grown by a variety of techniques generally grow with the c-axis perpendicular to the substrate surface. For example, caxis aligned material has been observed in growth by spray pyrolysis w12,13x, ion beam assisted sputter deposition w6,14x, by RF and DC magnetron sputtering w4,5,15x and pulsed laser ablation (PLA) w7,16–18x. PLA has been shown to yield high quality c-axis oriented ZnO films at lower substrate temperatures than many other techniques w16,19x. The films deposited by PLA in vacuum are typically Zn rich due to recondensation of Zn onto the surface of the target w20x. For this reason ZnO films are typically grown in a background pressure of oxygen w2x. In this paper we report results obtained from nominally undoped ZnO, AZO and GZO films grown by 193 nm PLA at different substrate temperatures and background pressures of O2. The crystallinity of the films, with different growth parameters, and its effect on the resistivity will be discussed.
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.09.005
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Fig. 1. XRD 2u scans of ZnO films grown on sapphire substrates in the vicinity of the ZnO (0002) reflection. (a) 2u scans of undoped ZnO as a function of Ts. (b) 2u scans of GZO films as a function of pO2.
2. Experimental details The films were grown by PLA of commercially available ZnO hot pressed disks (Cerac). The nominally undoped ZnO films were deposited by ablating a 99.999% pure ZnO target. The AZO and GZO films were produced by ablating ZnO targets containing 2% Al2O3 and 5% Ga by weight, respectively. Targets with these compositions were chosen as other groups have shown that these levels of dopant are required to obtain good conductivities w5,7x. A Lambda-Physik COMPex 201 excimer laser operating at 193 nm was employed for the ablation. Thin films were grown using a laser pulse energy of 100 mJ focused onto the target producing a fluence of approximately 10 J cmy2. The target was rotated to avoid repeated ablation from the same spot. The growth chamber was evacuated using a turbo pump and then back filled with a steady flow of oxygen using a mass flow controller so as to maintain a constant background pressure. The substrate was positioned 15 cm away from the target and could be heated up to 500 8C. The typical growth run consisted of 9000 laser shots with a repetition rate of 10 Hz. The majority of the films were deposited on (0001) sapphire substrates that were only polished on one side. Films for TEM analysis were grown on NaCl substrates. The TEM analysis was carried out on a Philips EM430 microscope operating at 250 kV. Electron transparent samples could be readily made from the films by dissolving off the NaCl substrate in distilled water. The free-floating film was then caught on a standard TEM mesh grid. The sheet resistances of the films grown on sapphire were measured using a four-point probe method. In order to obtain the resisitivity it is necessary to know the film thickness. Variable angle spectroscopic ellipsometry (VASE) measurements were performed using a Woolam M–2000U spectroscopic ellipsometer to determine the film thickness in a
manner similar to that reported by Sun et al. w21x but using four fitting angles in the range 60–758 and Cauchy equations for the dispersion. The variation in crystallinity of the films was explored using X-ray diffraction (XRD) in a Bruker AXS D8 advance powder diffractometer. The extent of orientation of the films was assessed from the full width at half maximum (FWHM) of the v rocking curves of the ZnO (0002) reflection. 3. Results and discussion The first stage of optimisation of the growth of ZnO films was to study the dependence of their crystallinity as a function of substrate temperature (Ts). For this purpose a set of undoped ZnO samples were deposited at different temperatures in the range 343 K-Ts-713 K on sapphire substrates in an oxygen background pressure (pO2) of 1.3 Pa. Their crystallinity was investigated using XRD. The only ZnO related reflections observed were the (0002) and (0004) basal plane reflections of wurtzite ZnO. This indicates that the samples, although polycrystalline, grew with the (0001) direction perpendicular to the substrate; similar texturing of the films has also been observed by other groups w7,16–18x. Fig. 1a shows segments of the 2u scans in the vicinity of the (0002) reflection. The peak is observed to shift towards higher angle and to become narrower with increasing Ts. Such a shift is equivalent to a decrease in c-lattice parameter from 0.526 to 0.518 nm as Ts increases from 343 to 713 K. The standard value for unstrained wurtzite ZnO is 0.521 nm, so the shift observed involves a change of strain (according to the biaxial strain model) from q1 to y0.5%. Similar sets of samples were deposited using the Al and Ga containing targets and XRD 2u scans carried out on these films, also. These results are not presented here but showed the same trend with Ts as the undoped films.
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w19x, but to summarise briefly the pattern demonstrates the presence of crystal grains with a mixture of polar and non-polar orientations, unlike the growth on sapphire which typically demonstrated improved (0001) texturing with increasing Ts. The FWHM of the XRD (0002) reflection is often used to determine the grain size (dg) of the films w5x using the Scherrer formula: dgs
Fig. 2. TEM images of two samples of undoped ZnO grown on NaCl at Tss293 and 473 K. At both temperatures pO2 was 1.3 Pa. (a) SAED pattern taken from an area on the sample grown at Tss293 K. (b) SAED pattern from the sample grown at Tss473 K. This pattern, although taken from many hundreds of crystal grains, is a spot pattern with four-fold symmetry. (c) and (d) BF TEM images of regions of the low and high temperature samples, respectively.
For example, the peak shift observed in the AZO films would imply a shift of the c-lattice parameter from 0.525 to 0.502 nm over the same range of increasing Ts. At Tss713 K the strain along the c axis is calculated as y3.6%. The deduced increase in compressive strain in the AZO films is likely to be a consequence of the inclusion of Al into the ZnO lattice. The ionic radius of Al3q is 53 pm, smaller than that of Zn2q at 72 pm. For comparison the ionic radius of Gal3q is 62 pm. As a substitutional point defect on the Zn lattice site, Al would be expected to contract the ZnO lattice, as is observed. In addition to the XRD analysis of the crystallinity of the undoped ZnO, samples were grown at different Ts on (001) NaCl substrates so that TEM analysis could be carried out on the films. Films were deposited at both Tss293 and 473 K, with pO2s1.3 Pa. Fig. 2a shows selected area electron diffraction (SAED) patterns taken from a region of the low temperature sample containing many crystal grains. The ring pattern is that of polycrystalline wurtzite ZnO. Even at room temperature, however, there is evidence that oriented growth is occurring as the (0002) ring, which should lie between the first two visible rings, is absent. Fig. 2b shows the SAED pattern from the sample grown at the higher Ts. Though sampling many hundreds of crystal grains, this is a spot pattern with four-fold symmetry and thus indicative of epitaxial growth. The nature of epitaxy and the indexing of the spot pattern is discussed elsewhere
kl
Žcos u.yb2yb20
(1)
where ls0.154056 nm, k is the correction factor (taken as 0.9), u is obtained from the peak position and b and b0 are the peak width and the width introduced by instrumental broadening, respectively. XRD 2u scans were performed on the films grown on NaCl prior to dissolution of the substrate. Substituting the measured width of the (0002) peak into Eq. (1) along with the experimental broadening term, determined from the measured width of the (0006) peak from a single crystal piece of sapphire, yielded dgs29 nm. Bright field (BF) TEM images of the films grown at Tss293 and 473 K are shown in Fig. 2c,d. Examination of these images suggests that the dg value derived using Eq. (1) is somewhat high; indeed the grain size of the sample grown at room temperature, if anything, appears slightly larger than in the film grown at higher Ts. Such behaviour was not expected and may indicate that, after the initial onset of epitaxial growth the grain size is smaller than that observed in growth unfettered by the substrate. Unfortunately the (0002) reflection was not evident in XRD 2u scans of the room temperature film, so no comparison with Eq. (1) could be made. Applying the Scherrer formula to the XRD 2u scans presented in Fig. 1 allows estimation of the grain size, as a function of Ts. The calculated value for the grain size of the undoped films increases from 12 nm at Tss343 K up to 38 nm at Tss713 K. Similar analysis of the AZO films yielded dg values increasing from 7 to 23 nm. This smaller calculated grain size (compared to the undoped material) could be an indication that Al atoms are having a negative effect on the crystalline quality of the ZnO lattice. The high temperature GZO films, in contrast, had a calculated grain size of 36 nm, very similar to that of undoped ZnO, indicating that the Ga3q ions fit into the ZnO lattice more easily than the Al at the higher substrate temperatures. The effect of pO2 on the crystallinity of the GZO film was investigated by depositing films at a range of pO2 from f0 (i.e. no addition of oxygen) to 5.2 Pa at Tss723 K. XRD 2u scans, in the vicinity of the (0002) reflection, were carried out on these films. The results are shown in Fig. 1b. The sample prepared in 0 Pa of O2 showed little evidence of crystallinity from the XRD measurements. This is to be expected, as the film is
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Fig. 3. FWHM of ZnO (0002) v rocking curves of (a) undoped and (b) GZO and AZO samples as a function of Ts. All samples were grown with pO2s1.3 Pa.
likely to deviate significantly from its correct stoichiometry w20x. The optimal pO2 (with regards to total reflected intensity of the (0002) peak) for our experimental setup was found to be 2.6 Pa, but the FWHM of the (0002) reflection was not observed to differ between samples grown with pO2)0 Pa. This may indicate little or no change in the grain size of the film. The reduction in intensity of the (0002) reflection between samples could be a consequence of scattering from point defects in the films. In order to determine the extent of c-axis orientation of the films, the v rocking curves of the (0002) peaks were measured. The FWHM of the rocking curves for the undoped ZnO films are plotted in Fig. 3a and those for the AZO and GZO films are shown in Fig. 3b. The FWHM of the (0002) rocking curve in the case of the undoped ZnO films is seen to reduce significantly with increasing Ts, with the narrowest FWHM (0.578) obtained at the highest Ts used. The AZO and GZO films showed inferior orientation compared to the undoped samples grown at the same value of Ts. As Fig. 3b shows the rocking curves of the GZO films exhibit a larger FWHM than those of the corresponding AZO film at the low Ts values but with increasing Ts, the GZO films show the better alignment. The lowest FWHM obtained for the GZO and AZO films are 0.98 and 1.98, respectively. The disruption to the growth of the ZnO introduced by the dopants is more significant at lower Ts for the films grown from the target containing 5% Ga by weight than those from the 2% Al2O3 by weight target. This is understandable, since the impurity density will be higher in the film grown from the Ga containing target. The ionic radius of Ga3q is smaller than that of Zn2q but larger than that of Al3q. As a substitutional donor Ga should thus disrupt the ZnO lattice less than an Al ion. The observation that at higher
Ts values, the FWHM of the rocking curves of the GZO films are less than those of the corresponding AZO films is consistent with the view that although the GZO films are expected to have a higher impurity density, the Ga impurities are accommodated into the ZnO lattice more easily. After demonstrating growth of crystalline doped and undoped ZnO thin films, the resistivities (r) of the films were measured using a four-point probe method. This technique allows measurement, after suitable corrections for the finite size of the samples, of the sheet resistance (RS) of the films. In order to obtain the resistivity, the film thickness (t) must be known (rsRSt). The thickness of the films was estimated using spectroscopic ellipsometry. As neither the optical parameters of the material nor the thickness were known before fitting, the ellipsometric data for each sample was measured at four different incident angles to increase the reliability of the fitting. For all sets of films the measured thickness was from f100 nm at the lower temperatures to f80 nm at the highest temperatures. This is likely to be a consequence of the improved packing of material due to better crystallinity. There may also be more desorption of incident material from the substrate at higher Ts. Fig. 4 displays the RS data for the AZO and GZO films as a function of Ts. No data is presented for the undoped ZnO material as the resisitivity of all the films was too high to measure with the equipment available. It should be noted that the GZO data (Fig. 4b) is plotted on a logarithmic scale as RS changes by two orders of magnitude between high and low Ts. GZO and AZO films grown at the highest Ts had similar RS values giving values of rf1.6=10y3 V cm and rf9.7=10y4 V cm, respectively. These are slightly larger than the best values reported for AZO and GZO films, which are typically in the range of 1.4–
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Fig. 4. GFour-point probe measurements of the sheet resistance of (a) AZO and (b) GZO samples as a function of Ts. All samples were grown with pO2s1.3 Pa. Note the logarithmic scale in (b).
2.0=10y4 V cm irrespective of deposition method w22x. It should be noted, however, that our films are thinner than most of those from similar sets of data presented in the literature. The resistivity of AZO films, produced by ion beam sputtering, has been shown to increase by an order of magnitude as the film thickness decreases from 200 to 100 nm w6x. This effect is attributed to a reduction of the crystallinity at the substrateyfilm interface with increased scattering from crystallographic defects w6x. Considering this effect, our resistivity results are comparable with the best in the literature w22x. It is likely that these best values are the lowest resistivities that can be obtained with conventional doping as, in this range, ionised impurity scattering limits the conductivity. 4. Conclusions The crystalline quality, extent of oriented growth and conductivity of undoped ZnO films and of Al and Ga doped ZnO films, grown by 193 nm PLA, have been studied as a function of substrate temperature and background O2 pressure. The GZO films deposited at high Ts were shown to be of better crystalline quality than the AZO films, even though they were grown from a target containing 5% Ga compared to only 2% Al2O3 for the AZO films. This we attribute to differences in the ionic radii of the dopants. The ionic radius of Ga3q atoms is closer to that of Zn2q, so Ga atoms cause less distortion of the ZnO lattice when they sit as substitutional impurities on metal lattice sites. The undoped ZnO films demonstrated better crystalline quality than either doped material, as was to be expected, but were shown to have very low conductivities. The undoped ZnO grown on NaCl demonstrated significant epitaxial growth at Ts)300 8C, suggesting that this is the suitable minimum temperature for growth of high quality
ZnO films by PLA. The crystal grains produced at this temperature presented a mixture of polar and non-polar orientations w19x, unlike the growth on sapphire, which only demonstrated improved (0001) texturing. The addition of Ga or Al enabled the production of conducting films. The lowest resistivity value for the AZO film (grown at the highest Ts) was 9.7=10y4 V cm; For GZO, the best resistivity was slightly larger at 1.6=10y3 V cm. The resistivity of the GZO films was found to increase by two order of magnitudes as Ts was decreased to near room temperature, whereas that of the AZO films only increased by a factor of 4 for the same decrease in Ts. We suggest that, although Ga is a better dopant for producing high quality n-type ZnO, Al maybe more economically viable as lower process temperatures can be used and the conductivity of the films can be expected to be more reproducible. Acknowledgments This work was supported by a DTI LINK OSDA Programme: AEROFED. The authors thank Dr N.A. Fox and K.N. Rosser for their encouragement of this work. S.J.H. would like to thank Dr R. Vincent for many useful discussions regarding the analysis of the ED patterns. References w1x A.F. Kohan, G. Ceder, D. Morgan, C.G. Van de Walle, Phys. Rev. B 16 (2000) 15 019. w2x A.V. Singh, R.M. Mehra, N. Buthrath, A. Wakahara, A. Yoshida, J. Appl. Phys. 90 (2001) 5661. w3x H. Kim, J.S. Horwitz, W.H. Kim, A.J. Makinen, ¨ Z.H. Kafafi, D.B. Chrisey, Thin Solid Films 420–421 (2002) 539. w4x T. Minami, K. Oohashi, S. Takata, Thin Solid Films 193–194 (1990) 721. w5x M. Chen, Z.L. Pei, X. Wang, C. Sun, L.S. Wen, J. Mater. Res. 16 (2001) 2118.
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