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A novel approach to prepare zinc oxide films: excimer laser irradiation of sol–gel derived precursor films
Thin Solid Films 357 (1999) 151±158 www.elsevier.com/locate/tsf
A novel approach to prepare zinc oxide ®lms: excimer laser irradiation of sol±gel derived precursor ®lms T. Nagase*, T. Ooie, J. Sakakibara Shikoku National Industrial Research Institute, 2217-14 Hayashi, Takamatsu, Kagawa 761-0395, Japan Received 30 March 1999; received in revised form 13 July 1999; accepted 13 July 1999
Abstract Crystalline zinc oxide ®lms with a c-axis orientation were prepared by a new approach using KrF excimer laser irradiation of sol±gel derived precursor ®lms on glass substrates. The structural characteristics, optical and electrical properties of the laser-irradiated ®lms were investigated and compared with those of heat-treated ®lms. Laser irradiation gives two kinds of crystalline zinc oxide ®lms; irradiation at low energy ¯uence produces low crystallinity with weak orientation, while irradiation at high energy ¯uence produces high crystallinity with strong orientation. The differences in crystallinity and orientation are explained in terms of the thermal effect caused by laser irradiation. Laser processing characteristically created oxygen vacancy in contrast to conventional heat-treatment, resulting in a decrease in electrical resistivity of the ®lms. In addition, ®lm irradiated at high energy ¯uence shows a band structure with indirect band gap, although the other prepared ®lms and a typical ZnO crystal show a band structure with direct band gap. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Crystallization; Laser irradiation; Vacancies; Zinc oxide
1. Introduction Zinc oxide (ZnO) is an n-type semiconductor with a wide band gap of 3.3 eV, showing attractive electrical and optical properties. ZnO thin ®lms with a c-axis orientation are used as surface acoustic wave devices. ZnO ®lms doped with aluminum or indium have been vigorously studied with the aim of application to transparent conducting electrodes [1±5]. Recently, luminescence from ZnO ®lms has attracted wide attention owing to its potential applications to UV diode lasers [6,7] and as a phosphor for low-voltage luminescence in ¯at panel displays [8,9]. The electrical and optical properties of ZnO ®lms depend strongly on their chemical composition and crystallographic characteristics. Non-stoichiometric undoped ZnO ®lms usually show low electrical resistivity due to oxygen vacancy and zinc interstitials. High crystallinity and orientation of the undoped ZnO ®lms also contribute to low electrical resistivity [10]. In addition, the undoped ZnO ®lms show green and orange photoluminescence, which result from oxygen-poor and oxygen-rich states, respectively [11]. Therefore, technology to develop methods for control-
* Corresponding author. Tel.: 181-87-869-3526; fax: 181-87-8693551. E-mail address: [email protected] (T. Nagase)
ling the chemical composition and the crystallinity of zinc oxide ®lms is highly desirable. Various methods for the preparation of functional ZnO ®lms have been extensively studied including sputtering [12,13], chemical vapor deposition (CVD) [14±16], laser molecular beam epitaxy (MBE) [6,7], thermal deposition [17], galvanostatic deposition [18], spray pyrolysis [11,19] and sol±gel [4,5,10,20,21] methods. In most of these methods, the chemical composition of the ZnO ®lms is controlled by adjusting the surrounding atmosphere during their formation. Crystallinity and orientation are controlled by adjusting heating-conditions and selection of substrates. Crystalline ZnO ®lms with a c-axis orientation on glass substrates have been prepared by the sol±gel method [5,10,20] as well as by sputtering [12] etc. High-quality ZnO ®lms with epitaxial growth on sapphire substrates have been prepared by sputtering [13] and laser MBE methods [6,7]. The present paper proposes the preparation of ZnO ®lms by a novel process involving KrF excimer laser irradiation of sol±gel derived precursor ®lms. This method has several merits. From a practical standpoint, the sol±gel method has the advantage of the elimination of the need for vacuum apparatus and has the potential for preparation of ®lms of large area and complicated forms on various substrates. In addition, it is easy to control the coordination structure to
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00645-8
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resistivity of the laser-irradiated ®lms are discussed in terms of their structural characteristics and resultant oxygen vacancy. 2. Experimental
Fig. 1. UV-visible absorption spectra of dried ®lms and quartz glass substrate.
produce amorphous precursor ®lms suitable for laser irradiation. The sol±gel method, however, has the disadvantage of requiring relatively high heating-temperatures for crystallization; for example extensively crystallized ZnO ®lms require temperatures above 673 K [10,21]. On the other hand, excimer laser irradiation is expected to produce instantaneously a thermal in¯uence on precursor ®lms with little in¯uence on substrates, owing to selective excitation of the ®lms. In addition, laser irradiation is expected to have a direct photo-induced in¯uence also, since electronic excitation by laser irradiation produces defects in alkali halide crystals [22]. This will bring about rapid crystallization of the precursor ®lms and atom dissipation from the ®lms. Excimer laser irradiation on oxide ®lms has been studied by several authors; Imai et al., have reported its application on sol±gel derived indium oxide ®lms [23] and SzoÈreÂnyi et al., have reported it on indium-tin-oxide (ITO) ®lms obtained by dc sputtering [24,25]. Laser irradiation also has the additional merit of being able to change structural characteristics locally by control of beam size and positions or by using masks. In the present paper, we report the effects of the energy ¯uence of laser and ®lm thickness on structural characteristics and physical properties of sol±gel derived zinc oxide ®lms. The effects of laser irradiation were investigated and compared with those of conventional heat-treatment. We found that laser irradiation produces crystallized zinc oxide with oxygen vacancy. The crystallization process by laser irradiation is discussed in terms of its thermal and direct photo-induced effects. Band structure and electrical
A coating solution, containing 0.6 mol/l zinc acetate and 0.6 mol/l monoethanolamine in 2-methoxymethanol, was prepared as reported by Ohyama et al. [21]. The solution was spun on a quartz glass substrate (SiO2 purity: 99.99%) set on spin-coating equipment rotating at a speed of 2000 rev./min, to form a wet ®lm. Wet ®lm samples were dried at either 473 or 573 K for 10 min on a hot plate. The coatingdrying procedure was repeated 1, 3 or 6 times and 3, 6 or 9 times for the drying temperatures of 473 and 573 K, respectively, in order to obtain dried ®lms with different thicknesses. The dried ®lms were designated according to their drying temperatures and effective ®lm thickness, e.g. D(473 K±95 nm). The effective ®lm thickness was evaluated optically as described below. Resultant dried-®lm samples were set on a hot plate at 473 K in air and then subjected to irradiation by KrF excimer laser (l 248 nm, 22 ns FWHM) using a Lambda Physik type LPX-305icc laser source. A rectangular area of approximately 1:2 £ 0:8 cm 2 was illuminated with constant energy distribution over the whole area using an Optec type beam homogenizer. The number of impinging shots was ®xed at ®ve shots and the frequency of the laser at 1 Hz, while energy ¯uence (Ef) of the laser was varied between 50 and 170 mJ/cm 2. Conventional heat-treatment of similarly prepared dried®lm samples was carried out in an electric furnace at 673 or 773 K for 1 h in air, for comparison with those subjected to laser processing. The prepared ®lms were subjected to X-ray diffraction analysis with Cu Ka radiation using a Rigaku type RINT1200 diffractometer. Minute pro®les around the (002) peak were also measured in order to determine the c-axis lattice constants of crystallized ZnO ®lms. In addition, 2u values of the (002) peak were determined using the FWHM middle point method with a correction by external standard of silicon powder. Residual organic constituents in the ®lms were examined by IR spectroscopy using a Perkin±Elmer type iSeries FTIR microscopic spectrometer with Ge-ATR objective. Re¯ection and transmission spectra in the UV-visible region were obtained using a Shimadzu type UV-3100PC spectrometer. Optical absorbance was evaluated from these spectra. Sheet resistance of the ®lms was measured by the four-probe method using a Kyowa Riken type K-705RS resistance meter. Film thickness was evaluated from the re¯ection spectra using the refractive index of ZnO crystal (lit., 2.0) or from the absorbance at 250 nm. The thicknesses of crystalline ZnO ®lms heat-treated at 773 K were used as values of effective ®lm thickness. The values for samples derived
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regardless of the drying temperatures. FTIR measurement showed that the amount of residual acetate group varied depending on the drying temperatures. For ®lms dried at 473 K two obvious absorption bands (1580 and 1450± 1420 cm 21) corresponding to the acetate group were observed, while for ®lms dried at 573 K they were barely observed. This shows that most of the organic constituents are removed by drying at 573 K. Fig. 1 shows UV-visible absorption spectra of dried ®lms and the quartz glass substrate. The spectra show that the photon energy of the KrF excimer laser (l 248 nm) can be selectively absorbed by the zinc oxide ®lms, but not by the substrate. In addition, the absorption spectra of the dried ®lms are similar to that of a typical ZnO crystal, suggesting that the amorphous zinc oxide in the dried ®lms has a band structure similar to that of ZnO crystal. 3.2. Effect of energy ¯uence of laser on crystallization to ZnO ®lms
Fig. 2. XRD patterns of dried ®lms and heat-treated ®lms. The ®lms shown in (a) and (b) were derived from D(473 K±95 nm) and D(573 K±45 nm), respectively.
from ®lms dried at 473 K were determined as 35, 95, and 190 nm and those derived from ®lms dried at 573 K as 45, 110, and 160 nm, depending on the coating times.
3. Results and discussion 3.1. Identi®cation of dried ®lms All the dried-®lm precursors were in an amorphous phase
Crystallization to ZnO in heat-treated ®lms is described before that in laser-irradiated ®lms. Although the thickness of dried ®lms produced no signi®cant difference in crystallinity or orientation of ZnO, the drying temperatures markedly in¯uenced crystallization to ZnO. XRD patterns of heat-treated samples derived from ®lms dried at different temperatures are shown in Fig. 2a,b. Heat-treatment of D(473 K±95 nm) at temperatures at and above 673 K produce remarkable crystallization to the ZnO phase with a strong orientation along the c-axis (Fig. 2a), while that of D(573 K±45 nm) under the same conditions gives little crystallization (Fig. 2b). These results suggest that the residual organic constituents exert a marked in¯uence on the crystallization of amorphous zinc oxide in the thermal processing. The residual organic constituents probably cause a large structural relaxation of the zinc oxide matrix during their evaporation, leading to crystallization of ZnO with strongly preferred orientation [21]. Fig. 3a,b show XRD patterns of the laser-irradiated ®lms derived from the precursors D(473 K±95 nm) and D(573 K± 110 nm), respectively. Only ®lms irradiated at an Ef of 50 mJ/cm 2 remain in the amorphous phase for either precursor. Films irradiated at an Ef of 100 or 130 mJ/cm 2 give XRD patterns with small diffraction peaks around 31.7, 34.4, and 36.28, which correspond to the (100), (002), and (101) diffraction peaks of hexagonal wurtzite-type ZnO crystal, respectively. These patterns show the formation of ZnO crystals with relatively low crystallinity and weak crystallographic orientation along the c-axis. Irradiation at high Ef $ 150 mJ/cm 2 results in dramatic crystallization to ZnO strongly oriented along the c-axis. These results indicate that laser irradiation gives two kinds of crystalline ZnO ®lms: ®lm with low crystallinity and weak orientation (at 100 # Ef # 130 mJ/cm 2), and ®lm with high crystallinity and strong orientation (at Ef $ 150 mJ/cm 2). Organic constituents were not detected in the ®lms irradiated at Ef $ 100
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Fig. 4. c-Axis lattice constants of ZnO in the heat-treated ®lms and the laser-irradiated ®lms. (O) and (´´´) denote reference ZnO powder and the JCPDS data, respectively. (A) and (k) denote the heat-treated ®lms obtained at a heating temperature of 773 K from D(473 K±95 nm) and D(573 K±45 nm), respectively. (X) and (W) denote the laser-irradiated ®lms obtained at various energy ¯uences from D(473 K±95 nm) and D(573 K±110 nm), respectively.
laser processing except for the ®lms obtained at an Ef of 130 mJ/cm 2. This suggests that the residual organic constituents in the dried ®lms exert only a small effect on the crystallization of zinc oxide during laser processing, especially at high energy ¯uence. The laser irradiation successfully crystallized amorphous zinc oxide to ZnO at a low substrate temperature of 473 K, equivalent to the lower drying temperature. Moreover, the substrate temperature is 200 K lower than that required for thermal processing. Laser processing may have the potential to proceed with a substrate temperature for crystallization as low as room temperature, as Imai et al., demonstrated for indium oxide ®lms [23]. 3.3. c-Axis lattice constants of ZnO ®lms
Fig. 3. XRD patterns of dried ®lms and laser-irradiated ®lms obtained at various energy ¯uences. The ®lms shown in (a) and (b) were derived from D(473 K±95 nm) and D(573 K±110 nm), respectively.
mJ/cm 2. Unlike the above cases involving thermal processing, the in¯uence of drying temperatures is small in the
The c-axis lattice constants (C0) are plotted in Fig. 4. Reagent ZnO powder (Wako Ltd., special grade), which was measured as a reference, gives a C0 value close to that in the JCPDS data ®le [26]. The heat-treated ®lms give higher C0 values than the ZnO powder. Bao et al., have also reported slightly higher C0 values for ZnO ®lms prepared on quartz glass substrates by a sol±gel method followed by heat-treatment [10]. The laser-irradiated ®lms give lower C0 values than the heat-treated ®lms. The lattice constants show a tendency to decrease with increasing energy ¯uence of the laser. The lower C0 values can be attributed to the resultant oxygen
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correlated with the energy ¯uence (Fig. 3), we can conclude that laser irradiation at high energy ¯uence brings about crystallization to a strongly oriented ZnO phase accompanied by large resultant oxygen vacancy. The heat-treated ®lms probably possess little oxygen vacancy based on sheet resistance results (in Section 3.6). The above results show that laser processing can characteristically produce larger oxygen vacancy more ef®ciently than does thermal processing. 3.4. Effect of ®lm thickness on crystallization to ZnO ®lms The XRD results of laser-irradiated ®lms derived from the precursors of different ®lm thicknesses dried at 473 K are given in Fig. 5. The XRD patterns vary depending on the ®lm thickness as well as the Ef of the laser. In the irradiation at an Ef of 100 mJ/cm 2 (Fig. 5a), the ®lms derived from D(473 K±95 nm) and D(473 K±190 nm) each demonstrated weak diffraction peaks corresponding to (002) and (101) planes: this indicates that the ®lms have low crystallinity with weak orientation. In addition, the thicker ®lm has a stronger (002) peak than the thinner ®lm, but the c-axis lattice constants of each are nearly the same. The irradiation at an Ef of 150 mJ/cm 2 results in dramatic crystallization, especially for the thinner ®lm (Fig. 5b). Contrary to the case at an Ef of 100 mJ/cm 2, the thinner ®lm gives a stronger (002) diffraction peak than the thicker ®lm. The thicker ®lm has a C0 value similar to the ®lms irradiated at an Ef of 100 mJ/cm 2 and also has a (101) diffraction peak in addition to the (002) peak. On the other hand, the thinner ®lm has only a very strong (002) peak with a lower C0 value than the above three ®lms. An even lower C0 value was seen in the thinnest ®lm obtained from D(473 K±35 nm) at an Ef of 150 mJ/cm 2. Similar results were observed for laser-irradiated ®lms derived from precursors dried at 573 K. 3.5. Crystallization process by laser irradiation Fig. 5. XRD patterns of the laser-irradiated ®lms with different ®lm thicknesses. The laser-irradiated ®lms shown in (a) and (b) were obtained at energy ¯uences of 100 and 150 mJ/cm 2, respectively. Drying temperature of the precursor ®lms was 473 K.
vacancy, since atom vacancy usually causes a decrease in lattice constants and, furthermore, oxygen atoms dissipate from the zinc oxide matrix more easily than zinc atoms. The creation of oxygen vacancy was also supported by energy dispersive X-ray (EDX) analysis in transmission electron microscopic observation of sections of representative ®lms. The EDX results indicated that crystallized laser-irradiated ®lms with lower C0 values showed smaller intensity ratios of O Ka to Zn Ka X-rays than did heat-treated ®lm. The detailed EDX results will be reported in the near future. Therefore, the decrease in C0 values means that an amount of the resultant oxygen vacancy increases with increasing energy ¯uence. Since the intensity of the (002) peak is also
The effect of the laser irradiation can be ascribed mainly to the thermal effect caused by the absorption of excimer laser energy. SzoÈreÂnyi et al., have simulated the temperature pro®les of ITO ®lm on a glass substrate with a single pulse laser irradiation, using a ®nite difference technique. They have estimated a steep (about 30 ns) increase in surface temperatures and a relatively slow (above 100 ns) increase in interfacial temperatures [24]. They have also estimated that thinner ®lms reach higher maximum interfacial temperatures than thicker ®lms [25]. Similar temperature changes may be expected to take place with each irradiation on the present zinc oxide ®lms. The laser irradiation gives two kinds of crystalline ZnO ®lms: a strongly oriented ®lm with large resultant oxygen vacancy and a low crystallized ®lm with weak orientation. The former tends to be produced in relatively thin precursors at high energy ¯uence of the laser. The formation of the strongly oriented ®lm needs nucleation at the ZnO/glass interface, since oriented crystallization in a supported amor-
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Fig. 6. UV-visible absorption spectra of the laser-irradiated ®lms with various ®lm thicknesses at energy ¯uence of 100 or 150 mJ/cm 2. Narrow and bold lines denote the ®lms irradiated at 100 and 150 mJ/cm 2, respectively. Drying temperature of the precursor ®lms was 473 K.
phous ®lm generally starts by nucleation at the ®lmsubstrate interface followed by crystal growth perpendicular to the substrate. For a strongly oriented ®lm with large oxygen vacancy, the temperature at the ZnO/glass interface must increase above the critical point of nucleation for the oriented crystallization. Once the nuclei of ZnO crystals form at the interface, the crystallization will progress easily because the bulk ®lm temperature is considered to be higher than the interface temperature for about 100 ns [24]. In addition, the crystallization at markedly high temperature is probably accompanied by the dissipation of oxygen atoms from the ®lm because of strong lattice vibration, resulting in zinc oxide crystals with oxygen vacancy. These considerations suggest that there is a threshold of energy ¯uence to form a strongly oriented ®lm (in other words, to form crystal nuclei at the interface). The threshold energy ¯uence probably depends on the thickness (i.e., deposited amount) and speci®c heat of ZnO ®lm; around 130 mJ/cm 2 for the precursor D(473 K±95 nm), but higher than 150 mJ/cm 2 for the thicker precursor D(473 K±190 nm). The low crystalline ®lm with weak orientation is formed with relatively low energy ¯uence. The energy ¯uence is probably insuf®cient to raise the ®lm temperature up to that for the overall crystallization described above. It is note-
Fig. 7. Electrical resistivity of the crystalline zinc oxide ®lms as a function of c-axis lattice constants. (A) and (k) denote the heat-treated ®lms obtained at a heating temperature of 773 K from D(473 K±95 nm) and D(573 K±45 nm), respectively. (X) and (W) denote the laser-irradiated ®lms obtained at various energy ¯uences from D(473 K±95 nm) and D(573 K±110 nm), respectively. (O) and (P) denote the laser-irradiated ®lms obtained at 150 mJ/cm 2 from D(473 K±35 nm) and D(473 K±95 nm), respectively.
worthy that for laser irradiation at 100 mJ/cm 2 (Fig. 5a) the ®lm derived from D(473 K±190 nm) gives a higher intensity XRD diffraction peak than that derived from D(473 K±95 nm). This implies that the crystallization proceeds by a direct photo-induced effect as well as by the thermal effect, since the temperature achieved by laser irradiation is lower for the thicker ®lm at the same energy ¯uence. Laser-induced electronic excitation most likely makes the Zn-O bond active and then promotes the nucleation and crystallization. Such nucleation and crystallization may progress throughout the precursors. The activation of the Zn-O bond probably also contributes to the creation of oxygen vacancy, even in the above case of irradiation at high energy ¯uence. 3.6. Optical and electrical properties of laser-irradiated ®lms The ®lms laser-irradiated at an Ef of 50 mJ/cm 2 exhibited UV-visible absorption spectra similar to those of the dried ®lms, and hence have a band structure with band gap similar to that of a typical ZnO crystal. Fig. 6 shows the absorption spectra of the laser-irradiated ®lms derived from D(473 K± 35 nm), D(473 K±95 nm), and D(473 K±190 nm) at an Ef of 100 or 150 mJ/cm 2. The ®lms laser-irradiated at an Ef of 100 mJ/cm 2 also exhibit absorption spectra similar to those of the dried ®lms in Fig. 1. In the ®lms laser-irradiated at an Ef
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of 150 mJ/cm 2, only that derived from D(473 K±190 nm) exhibits an adsorption spectrum similar to those of the dried ®lms and the ®lms laser-irradiated at an Ef # 100 mJ/cm 2, except for some light scattering caused by particle growth. On the other hand, the ®lms derived from D(473 K±35 nm) and D(473 K±95 nm) exhibit remarkably deformed spectra with decreased intensity of absorbance (especially between 300 and 375 nm in wavelength), in comparison with those of the other ®lms. This suggests that these two ®lms have a band structure with indirect band gap, which may result mainly from their large oxygen vacancy. The dried-®lm precursors and the heat-treated ®lms show high sheet resistance above the equipment ef®ciency limit (5 MV/A), mainly due to lack of any oxygen vacancy. On the other hand, some crystalline laser-irradiated ®lms showed a decrease in the sheet resistance, which varied widely (2±3 orders) mainly dependent on the energy ¯uence and ®lm thickness. The decrease in the resistance can be explained by the fact that the crystalline laser-irradiated ®lms have oxygen vacancy (as mentioned in Section 3.3), producing conduction electrons. Fig. 7 shows the electrical resistivity of typical crystalline zinc oxide ®lms as a function of their C0 values. For ®lms with high sheet resistance above the limit (5 MV/A), their resistivity is tentatively plotted at the position of 300 Vcm for convenience. The C0 values in the horizontal axis correlate with the amount of oxygen vacancy created and hence carrier concentration. Therefore, this ®gure suggests that not all laser-irradiated ®lms have the same electronic mobility. Among the ®lms with C0 between 0.5209 and 0.5217 nm, the ®lms with higher crystallinity and orientation show higher electronic mobility. Furthermore, it is also noteworthy that three ®lms with C0 between 0.5191 and 0.5194 nm show generally higher resistivity than the laser-irradiated ®lms with C0 between 0.5209 and 0.5217 nm, although the three ®lms have larger amounts of oxygen vacancy. The three ®lms had a band structure with indirect band gap or an intermediate band structure, while the ®lms with higher C0 had a band structure with direct band gap. These results indicate that the band structure with indirect band gap is accompanied by lower electrical mobility. We will further investigate properties involving the Hall effect and photoluminescence, in order to control precisely the physical properties of zinc oxide ®lms by this new process using laser irradiation. 4. Conclusion KrF excimer laser irradiation on sol±gel derived ®lms gives two kinds of crystalline zinc oxide ®lms depending on the energy ¯uence of the laser and ®lm thickness. Irradiation at low energy ¯uence produces low crystallinity with weak orientation, while irradiation at high energy ¯uence produces high crystallinity with strong orientation. The thresholds of the energy ¯uence increase with ®lm thick-
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ness. The laser-induced crystallization is accompanied by the creation of oxygen vacancy, in contrast to conventional heat-treatment. The optical and electrical properties of the laser-irradiated ®lms vary remarkably depending on their structural characteristics and amount of oxygen vacancy. Film obtained at low energy ¯uence retains a band structure with direct band gap, while that obtained at high energy ¯uence is transformed to a band structure with indirect band gap. The laser-irradiated ®lms show low electrical resistivity depending on the band structure as well as the amount of oxygen vacancy and crystallinity. In conclusion, excimer laser irradiation of sol±gel derived ®lms is an attractive and promising method for the easy preparation of zinc oxide ®lms with unique optical and electrical properties, without the need for special vacuum equipment. Acknowledgements We would like to thank Dr. T. Shikama of Takamatsu National College of Technology for measurement of sheet resistance and Professor N. Mizutani in Tokyo Institute of Technology, Professor M. Nakatsuka in Osaka Univ. and Dr. K. Ooi in Shikoku National Industrial Research Institute for helpful comments concerning this work. References [1] S.Y. Myong, S.J. Baik, C.H. Lee, W.Y. Cho, K.S. Lim, Jpn. J. Appl. Phys. 36 (1997) L1078. [2] M.N. Islam, T.B. Ghosh, K.L. Chopra, H.N. Acharya, Thin Solid Films 280 (1996) 20. [3] C.H. Lee, J.S. Song, K.S. Lim, Sol. Energy Mater. Solar Cell 43 (1996) 37. [4] Y. Ohya, H. Saiki, T. Tanaka, Y. Takahashi, J. Am. Ceram. Soc. 79 (1996) 825. [5] Y. Ohya, H. Saiki,, Y. Takahashi, J. Mater. Sci. 29 (1994) 4099. [6] Y. Segawa, A. Ohtomo, M. Kawasaki, H. Koinuma, Z.K. Tang, P. Yu, G.K.L. Wong, Phys. Stat. Sol. B 202 (1997) 669. [7] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 (1997) 459. [8] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983. [9] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J. Caruso, M.J. Hampden-Smith, T.T. Kodas, J. Lumin. 75 (1997) 11. [10] D. Bao, H. Gu, A. Kuang, Thin Solid Films 312 (1998) 37. [11] S.A. Studenikin, N. Golego, M. Cocivera, J. Appl. Phys. 84 (1998) 2287. [12] T. Yamamoto, T. Shiosaki, A. Kawabata, J. Appl. Phys. 63 (1980) 3113. [13] O. Yamazaki, T. Mitsuyu, K. Wasa, IEEE Trans. Sonics and Ultrasonics SU-27 (1980) 369. [14] T. Maruyama, J. Shionoya, J. Mater. Sci. Lett. 11 (1992) 170. [15] M.F. Ogawa, Y. Natsume, T. Hirayama, H. Sakata, J. Mater. Sci. Lett. 9 (1990) 1351. [16] M.F. Ogawa, Y. Natsume, T. Hirayama, H. Sakata, J. Mater. Sci. Lett. 9 (1990) 1354. [17] J.H. Jean, J. Mater. Sci. Lett. 9 (1990) 127. [18] M. Izaki, T. Omi, J. Electrochem. Soc. 144 (1997) 1949.
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A novel approach to prepare zinc oxide ®lms: excimer laser irradiation of sol±gel derived precursor ®lms T. Nagase*, T. Ooie, J. Sakakibara Shikoku National Industrial Research Institute, 2217-14 Hayashi, Takamatsu, Kagawa 761-0395, Japan Received 30 March 1999; received in revised form 13 July 1999; accepted 13 July 1999
Abstract Crystalline zinc oxide ®lms with a c-axis orientation were prepared by a new approach using KrF excimer laser irradiation of sol±gel derived precursor ®lms on glass substrates. The structural characteristics, optical and electrical properties of the laser-irradiated ®lms were investigated and compared with those of heat-treated ®lms. Laser irradiation gives two kinds of crystalline zinc oxide ®lms; irradiation at low energy ¯uence produces low crystallinity with weak orientation, while irradiation at high energy ¯uence produces high crystallinity with strong orientation. The differences in crystallinity and orientation are explained in terms of the thermal effect caused by laser irradiation. Laser processing characteristically created oxygen vacancy in contrast to conventional heat-treatment, resulting in a decrease in electrical resistivity of the ®lms. In addition, ®lm irradiated at high energy ¯uence shows a band structure with indirect band gap, although the other prepared ®lms and a typical ZnO crystal show a band structure with direct band gap. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Crystallization; Laser irradiation; Vacancies; Zinc oxide
1. Introduction Zinc oxide (ZnO) is an n-type semiconductor with a wide band gap of 3.3 eV, showing attractive electrical and optical properties. ZnO thin ®lms with a c-axis orientation are used as surface acoustic wave devices. ZnO ®lms doped with aluminum or indium have been vigorously studied with the aim of application to transparent conducting electrodes [1±5]. Recently, luminescence from ZnO ®lms has attracted wide attention owing to its potential applications to UV diode lasers [6,7] and as a phosphor for low-voltage luminescence in ¯at panel displays [8,9]. The electrical and optical properties of ZnO ®lms depend strongly on their chemical composition and crystallographic characteristics. Non-stoichiometric undoped ZnO ®lms usually show low electrical resistivity due to oxygen vacancy and zinc interstitials. High crystallinity and orientation of the undoped ZnO ®lms also contribute to low electrical resistivity [10]. In addition, the undoped ZnO ®lms show green and orange photoluminescence, which result from oxygen-poor and oxygen-rich states, respectively [11]. Therefore, technology to develop methods for control-
* Corresponding author. Tel.: 181-87-869-3526; fax: 181-87-8693551. E-mail address: [email protected] (T. Nagase)
ling the chemical composition and the crystallinity of zinc oxide ®lms is highly desirable. Various methods for the preparation of functional ZnO ®lms have been extensively studied including sputtering [12,13], chemical vapor deposition (CVD) [14±16], laser molecular beam epitaxy (MBE) [6,7], thermal deposition [17], galvanostatic deposition [18], spray pyrolysis [11,19] and sol±gel [4,5,10,20,21] methods. In most of these methods, the chemical composition of the ZnO ®lms is controlled by adjusting the surrounding atmosphere during their formation. Crystallinity and orientation are controlled by adjusting heating-conditions and selection of substrates. Crystalline ZnO ®lms with a c-axis orientation on glass substrates have been prepared by the sol±gel method [5,10,20] as well as by sputtering [12] etc. High-quality ZnO ®lms with epitaxial growth on sapphire substrates have been prepared by sputtering [13] and laser MBE methods [6,7]. The present paper proposes the preparation of ZnO ®lms by a novel process involving KrF excimer laser irradiation of sol±gel derived precursor ®lms. This method has several merits. From a practical standpoint, the sol±gel method has the advantage of the elimination of the need for vacuum apparatus and has the potential for preparation of ®lms of large area and complicated forms on various substrates. In addition, it is easy to control the coordination structure to
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00645-8
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resistivity of the laser-irradiated ®lms are discussed in terms of their structural characteristics and resultant oxygen vacancy. 2. Experimental
Fig. 1. UV-visible absorption spectra of dried ®lms and quartz glass substrate.
produce amorphous precursor ®lms suitable for laser irradiation. The sol±gel method, however, has the disadvantage of requiring relatively high heating-temperatures for crystallization; for example extensively crystallized ZnO ®lms require temperatures above 673 K [10,21]. On the other hand, excimer laser irradiation is expected to produce instantaneously a thermal in¯uence on precursor ®lms with little in¯uence on substrates, owing to selective excitation of the ®lms. In addition, laser irradiation is expected to have a direct photo-induced in¯uence also, since electronic excitation by laser irradiation produces defects in alkali halide crystals [22]. This will bring about rapid crystallization of the precursor ®lms and atom dissipation from the ®lms. Excimer laser irradiation on oxide ®lms has been studied by several authors; Imai et al., have reported its application on sol±gel derived indium oxide ®lms [23] and SzoÈreÂnyi et al., have reported it on indium-tin-oxide (ITO) ®lms obtained by dc sputtering [24,25]. Laser irradiation also has the additional merit of being able to change structural characteristics locally by control of beam size and positions or by using masks. In the present paper, we report the effects of the energy ¯uence of laser and ®lm thickness on structural characteristics and physical properties of sol±gel derived zinc oxide ®lms. The effects of laser irradiation were investigated and compared with those of conventional heat-treatment. We found that laser irradiation produces crystallized zinc oxide with oxygen vacancy. The crystallization process by laser irradiation is discussed in terms of its thermal and direct photo-induced effects. Band structure and electrical
A coating solution, containing 0.6 mol/l zinc acetate and 0.6 mol/l monoethanolamine in 2-methoxymethanol, was prepared as reported by Ohyama et al. [21]. The solution was spun on a quartz glass substrate (SiO2 purity: 99.99%) set on spin-coating equipment rotating at a speed of 2000 rev./min, to form a wet ®lm. Wet ®lm samples were dried at either 473 or 573 K for 10 min on a hot plate. The coatingdrying procedure was repeated 1, 3 or 6 times and 3, 6 or 9 times for the drying temperatures of 473 and 573 K, respectively, in order to obtain dried ®lms with different thicknesses. The dried ®lms were designated according to their drying temperatures and effective ®lm thickness, e.g. D(473 K±95 nm). The effective ®lm thickness was evaluated optically as described below. Resultant dried-®lm samples were set on a hot plate at 473 K in air and then subjected to irradiation by KrF excimer laser (l 248 nm, 22 ns FWHM) using a Lambda Physik type LPX-305icc laser source. A rectangular area of approximately 1:2 £ 0:8 cm 2 was illuminated with constant energy distribution over the whole area using an Optec type beam homogenizer. The number of impinging shots was ®xed at ®ve shots and the frequency of the laser at 1 Hz, while energy ¯uence (Ef) of the laser was varied between 50 and 170 mJ/cm 2. Conventional heat-treatment of similarly prepared dried®lm samples was carried out in an electric furnace at 673 or 773 K for 1 h in air, for comparison with those subjected to laser processing. The prepared ®lms were subjected to X-ray diffraction analysis with Cu Ka radiation using a Rigaku type RINT1200 diffractometer. Minute pro®les around the (002) peak were also measured in order to determine the c-axis lattice constants of crystallized ZnO ®lms. In addition, 2u values of the (002) peak were determined using the FWHM middle point method with a correction by external standard of silicon powder. Residual organic constituents in the ®lms were examined by IR spectroscopy using a Perkin±Elmer type iSeries FTIR microscopic spectrometer with Ge-ATR objective. Re¯ection and transmission spectra in the UV-visible region were obtained using a Shimadzu type UV-3100PC spectrometer. Optical absorbance was evaluated from these spectra. Sheet resistance of the ®lms was measured by the four-probe method using a Kyowa Riken type K-705RS resistance meter. Film thickness was evaluated from the re¯ection spectra using the refractive index of ZnO crystal (lit., 2.0) or from the absorbance at 250 nm. The thicknesses of crystalline ZnO ®lms heat-treated at 773 K were used as values of effective ®lm thickness. The values for samples derived
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regardless of the drying temperatures. FTIR measurement showed that the amount of residual acetate group varied depending on the drying temperatures. For ®lms dried at 473 K two obvious absorption bands (1580 and 1450± 1420 cm 21) corresponding to the acetate group were observed, while for ®lms dried at 573 K they were barely observed. This shows that most of the organic constituents are removed by drying at 573 K. Fig. 1 shows UV-visible absorption spectra of dried ®lms and the quartz glass substrate. The spectra show that the photon energy of the KrF excimer laser (l 248 nm) can be selectively absorbed by the zinc oxide ®lms, but not by the substrate. In addition, the absorption spectra of the dried ®lms are similar to that of a typical ZnO crystal, suggesting that the amorphous zinc oxide in the dried ®lms has a band structure similar to that of ZnO crystal. 3.2. Effect of energy ¯uence of laser on crystallization to ZnO ®lms
Fig. 2. XRD patterns of dried ®lms and heat-treated ®lms. The ®lms shown in (a) and (b) were derived from D(473 K±95 nm) and D(573 K±45 nm), respectively.
from ®lms dried at 473 K were determined as 35, 95, and 190 nm and those derived from ®lms dried at 573 K as 45, 110, and 160 nm, depending on the coating times.
3. Results and discussion 3.1. Identi®cation of dried ®lms All the dried-®lm precursors were in an amorphous phase
Crystallization to ZnO in heat-treated ®lms is described before that in laser-irradiated ®lms. Although the thickness of dried ®lms produced no signi®cant difference in crystallinity or orientation of ZnO, the drying temperatures markedly in¯uenced crystallization to ZnO. XRD patterns of heat-treated samples derived from ®lms dried at different temperatures are shown in Fig. 2a,b. Heat-treatment of D(473 K±95 nm) at temperatures at and above 673 K produce remarkable crystallization to the ZnO phase with a strong orientation along the c-axis (Fig. 2a), while that of D(573 K±45 nm) under the same conditions gives little crystallization (Fig. 2b). These results suggest that the residual organic constituents exert a marked in¯uence on the crystallization of amorphous zinc oxide in the thermal processing. The residual organic constituents probably cause a large structural relaxation of the zinc oxide matrix during their evaporation, leading to crystallization of ZnO with strongly preferred orientation [21]. Fig. 3a,b show XRD patterns of the laser-irradiated ®lms derived from the precursors D(473 K±95 nm) and D(573 K± 110 nm), respectively. Only ®lms irradiated at an Ef of 50 mJ/cm 2 remain in the amorphous phase for either precursor. Films irradiated at an Ef of 100 or 130 mJ/cm 2 give XRD patterns with small diffraction peaks around 31.7, 34.4, and 36.28, which correspond to the (100), (002), and (101) diffraction peaks of hexagonal wurtzite-type ZnO crystal, respectively. These patterns show the formation of ZnO crystals with relatively low crystallinity and weak crystallographic orientation along the c-axis. Irradiation at high Ef $ 150 mJ/cm 2 results in dramatic crystallization to ZnO strongly oriented along the c-axis. These results indicate that laser irradiation gives two kinds of crystalline ZnO ®lms: ®lm with low crystallinity and weak orientation (at 100 # Ef # 130 mJ/cm 2), and ®lm with high crystallinity and strong orientation (at Ef $ 150 mJ/cm 2). Organic constituents were not detected in the ®lms irradiated at Ef $ 100
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Fig. 4. c-Axis lattice constants of ZnO in the heat-treated ®lms and the laser-irradiated ®lms. (O) and (´´´) denote reference ZnO powder and the JCPDS data, respectively. (A) and (k) denote the heat-treated ®lms obtained at a heating temperature of 773 K from D(473 K±95 nm) and D(573 K±45 nm), respectively. (X) and (W) denote the laser-irradiated ®lms obtained at various energy ¯uences from D(473 K±95 nm) and D(573 K±110 nm), respectively.
laser processing except for the ®lms obtained at an Ef of 130 mJ/cm 2. This suggests that the residual organic constituents in the dried ®lms exert only a small effect on the crystallization of zinc oxide during laser processing, especially at high energy ¯uence. The laser irradiation successfully crystallized amorphous zinc oxide to ZnO at a low substrate temperature of 473 K, equivalent to the lower drying temperature. Moreover, the substrate temperature is 200 K lower than that required for thermal processing. Laser processing may have the potential to proceed with a substrate temperature for crystallization as low as room temperature, as Imai et al., demonstrated for indium oxide ®lms [23]. 3.3. c-Axis lattice constants of ZnO ®lms
Fig. 3. XRD patterns of dried ®lms and laser-irradiated ®lms obtained at various energy ¯uences. The ®lms shown in (a) and (b) were derived from D(473 K±95 nm) and D(573 K±110 nm), respectively.
mJ/cm 2. Unlike the above cases involving thermal processing, the in¯uence of drying temperatures is small in the
The c-axis lattice constants (C0) are plotted in Fig. 4. Reagent ZnO powder (Wako Ltd., special grade), which was measured as a reference, gives a C0 value close to that in the JCPDS data ®le [26]. The heat-treated ®lms give higher C0 values than the ZnO powder. Bao et al., have also reported slightly higher C0 values for ZnO ®lms prepared on quartz glass substrates by a sol±gel method followed by heat-treatment [10]. The laser-irradiated ®lms give lower C0 values than the heat-treated ®lms. The lattice constants show a tendency to decrease with increasing energy ¯uence of the laser. The lower C0 values can be attributed to the resultant oxygen
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correlated with the energy ¯uence (Fig. 3), we can conclude that laser irradiation at high energy ¯uence brings about crystallization to a strongly oriented ZnO phase accompanied by large resultant oxygen vacancy. The heat-treated ®lms probably possess little oxygen vacancy based on sheet resistance results (in Section 3.6). The above results show that laser processing can characteristically produce larger oxygen vacancy more ef®ciently than does thermal processing. 3.4. Effect of ®lm thickness on crystallization to ZnO ®lms The XRD results of laser-irradiated ®lms derived from the precursors of different ®lm thicknesses dried at 473 K are given in Fig. 5. The XRD patterns vary depending on the ®lm thickness as well as the Ef of the laser. In the irradiation at an Ef of 100 mJ/cm 2 (Fig. 5a), the ®lms derived from D(473 K±95 nm) and D(473 K±190 nm) each demonstrated weak diffraction peaks corresponding to (002) and (101) planes: this indicates that the ®lms have low crystallinity with weak orientation. In addition, the thicker ®lm has a stronger (002) peak than the thinner ®lm, but the c-axis lattice constants of each are nearly the same. The irradiation at an Ef of 150 mJ/cm 2 results in dramatic crystallization, especially for the thinner ®lm (Fig. 5b). Contrary to the case at an Ef of 100 mJ/cm 2, the thinner ®lm gives a stronger (002) diffraction peak than the thicker ®lm. The thicker ®lm has a C0 value similar to the ®lms irradiated at an Ef of 100 mJ/cm 2 and also has a (101) diffraction peak in addition to the (002) peak. On the other hand, the thinner ®lm has only a very strong (002) peak with a lower C0 value than the above three ®lms. An even lower C0 value was seen in the thinnest ®lm obtained from D(473 K±35 nm) at an Ef of 150 mJ/cm 2. Similar results were observed for laser-irradiated ®lms derived from precursors dried at 573 K. 3.5. Crystallization process by laser irradiation Fig. 5. XRD patterns of the laser-irradiated ®lms with different ®lm thicknesses. The laser-irradiated ®lms shown in (a) and (b) were obtained at energy ¯uences of 100 and 150 mJ/cm 2, respectively. Drying temperature of the precursor ®lms was 473 K.
vacancy, since atom vacancy usually causes a decrease in lattice constants and, furthermore, oxygen atoms dissipate from the zinc oxide matrix more easily than zinc atoms. The creation of oxygen vacancy was also supported by energy dispersive X-ray (EDX) analysis in transmission electron microscopic observation of sections of representative ®lms. The EDX results indicated that crystallized laser-irradiated ®lms with lower C0 values showed smaller intensity ratios of O Ka to Zn Ka X-rays than did heat-treated ®lm. The detailed EDX results will be reported in the near future. Therefore, the decrease in C0 values means that an amount of the resultant oxygen vacancy increases with increasing energy ¯uence. Since the intensity of the (002) peak is also
The effect of the laser irradiation can be ascribed mainly to the thermal effect caused by the absorption of excimer laser energy. SzoÈreÂnyi et al., have simulated the temperature pro®les of ITO ®lm on a glass substrate with a single pulse laser irradiation, using a ®nite difference technique. They have estimated a steep (about 30 ns) increase in surface temperatures and a relatively slow (above 100 ns) increase in interfacial temperatures [24]. They have also estimated that thinner ®lms reach higher maximum interfacial temperatures than thicker ®lms [25]. Similar temperature changes may be expected to take place with each irradiation on the present zinc oxide ®lms. The laser irradiation gives two kinds of crystalline ZnO ®lms: a strongly oriented ®lm with large resultant oxygen vacancy and a low crystallized ®lm with weak orientation. The former tends to be produced in relatively thin precursors at high energy ¯uence of the laser. The formation of the strongly oriented ®lm needs nucleation at the ZnO/glass interface, since oriented crystallization in a supported amor-
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Fig. 6. UV-visible absorption spectra of the laser-irradiated ®lms with various ®lm thicknesses at energy ¯uence of 100 or 150 mJ/cm 2. Narrow and bold lines denote the ®lms irradiated at 100 and 150 mJ/cm 2, respectively. Drying temperature of the precursor ®lms was 473 K.
phous ®lm generally starts by nucleation at the ®lmsubstrate interface followed by crystal growth perpendicular to the substrate. For a strongly oriented ®lm with large oxygen vacancy, the temperature at the ZnO/glass interface must increase above the critical point of nucleation for the oriented crystallization. Once the nuclei of ZnO crystals form at the interface, the crystallization will progress easily because the bulk ®lm temperature is considered to be higher than the interface temperature for about 100 ns [24]. In addition, the crystallization at markedly high temperature is probably accompanied by the dissipation of oxygen atoms from the ®lm because of strong lattice vibration, resulting in zinc oxide crystals with oxygen vacancy. These considerations suggest that there is a threshold of energy ¯uence to form a strongly oriented ®lm (in other words, to form crystal nuclei at the interface). The threshold energy ¯uence probably depends on the thickness (i.e., deposited amount) and speci®c heat of ZnO ®lm; around 130 mJ/cm 2 for the precursor D(473 K±95 nm), but higher than 150 mJ/cm 2 for the thicker precursor D(473 K±190 nm). The low crystalline ®lm with weak orientation is formed with relatively low energy ¯uence. The energy ¯uence is probably insuf®cient to raise the ®lm temperature up to that for the overall crystallization described above. It is note-
Fig. 7. Electrical resistivity of the crystalline zinc oxide ®lms as a function of c-axis lattice constants. (A) and (k) denote the heat-treated ®lms obtained at a heating temperature of 773 K from D(473 K±95 nm) and D(573 K±45 nm), respectively. (X) and (W) denote the laser-irradiated ®lms obtained at various energy ¯uences from D(473 K±95 nm) and D(573 K±110 nm), respectively. (O) and (P) denote the laser-irradiated ®lms obtained at 150 mJ/cm 2 from D(473 K±35 nm) and D(473 K±95 nm), respectively.
worthy that for laser irradiation at 100 mJ/cm 2 (Fig. 5a) the ®lm derived from D(473 K±190 nm) gives a higher intensity XRD diffraction peak than that derived from D(473 K±95 nm). This implies that the crystallization proceeds by a direct photo-induced effect as well as by the thermal effect, since the temperature achieved by laser irradiation is lower for the thicker ®lm at the same energy ¯uence. Laser-induced electronic excitation most likely makes the Zn-O bond active and then promotes the nucleation and crystallization. Such nucleation and crystallization may progress throughout the precursors. The activation of the Zn-O bond probably also contributes to the creation of oxygen vacancy, even in the above case of irradiation at high energy ¯uence. 3.6. Optical and electrical properties of laser-irradiated ®lms The ®lms laser-irradiated at an Ef of 50 mJ/cm 2 exhibited UV-visible absorption spectra similar to those of the dried ®lms, and hence have a band structure with band gap similar to that of a typical ZnO crystal. Fig. 6 shows the absorption spectra of the laser-irradiated ®lms derived from D(473 K± 35 nm), D(473 K±95 nm), and D(473 K±190 nm) at an Ef of 100 or 150 mJ/cm 2. The ®lms laser-irradiated at an Ef of 100 mJ/cm 2 also exhibit absorption spectra similar to those of the dried ®lms in Fig. 1. In the ®lms laser-irradiated at an Ef
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of 150 mJ/cm 2, only that derived from D(473 K±190 nm) exhibits an adsorption spectrum similar to those of the dried ®lms and the ®lms laser-irradiated at an Ef # 100 mJ/cm 2, except for some light scattering caused by particle growth. On the other hand, the ®lms derived from D(473 K±35 nm) and D(473 K±95 nm) exhibit remarkably deformed spectra with decreased intensity of absorbance (especially between 300 and 375 nm in wavelength), in comparison with those of the other ®lms. This suggests that these two ®lms have a band structure with indirect band gap, which may result mainly from their large oxygen vacancy. The dried-®lm precursors and the heat-treated ®lms show high sheet resistance above the equipment ef®ciency limit (5 MV/A), mainly due to lack of any oxygen vacancy. On the other hand, some crystalline laser-irradiated ®lms showed a decrease in the sheet resistance, which varied widely (2±3 orders) mainly dependent on the energy ¯uence and ®lm thickness. The decrease in the resistance can be explained by the fact that the crystalline laser-irradiated ®lms have oxygen vacancy (as mentioned in Section 3.3), producing conduction electrons. Fig. 7 shows the electrical resistivity of typical crystalline zinc oxide ®lms as a function of their C0 values. For ®lms with high sheet resistance above the limit (5 MV/A), their resistivity is tentatively plotted at the position of 300 Vcm for convenience. The C0 values in the horizontal axis correlate with the amount of oxygen vacancy created and hence carrier concentration. Therefore, this ®gure suggests that not all laser-irradiated ®lms have the same electronic mobility. Among the ®lms with C0 between 0.5209 and 0.5217 nm, the ®lms with higher crystallinity and orientation show higher electronic mobility. Furthermore, it is also noteworthy that three ®lms with C0 between 0.5191 and 0.5194 nm show generally higher resistivity than the laser-irradiated ®lms with C0 between 0.5209 and 0.5217 nm, although the three ®lms have larger amounts of oxygen vacancy. The three ®lms had a band structure with indirect band gap or an intermediate band structure, while the ®lms with higher C0 had a band structure with direct band gap. These results indicate that the band structure with indirect band gap is accompanied by lower electrical mobility. We will further investigate properties involving the Hall effect and photoluminescence, in order to control precisely the physical properties of zinc oxide ®lms by this new process using laser irradiation. 4. Conclusion KrF excimer laser irradiation on sol±gel derived ®lms gives two kinds of crystalline zinc oxide ®lms depending on the energy ¯uence of the laser and ®lm thickness. Irradiation at low energy ¯uence produces low crystallinity with weak orientation, while irradiation at high energy ¯uence produces high crystallinity with strong orientation. The thresholds of the energy ¯uence increase with ®lm thick-
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ness. The laser-induced crystallization is accompanied by the creation of oxygen vacancy, in contrast to conventional heat-treatment. The optical and electrical properties of the laser-irradiated ®lms vary remarkably depending on their structural characteristics and amount of oxygen vacancy. Film obtained at low energy ¯uence retains a band structure with direct band gap, while that obtained at high energy ¯uence is transformed to a band structure with indirect band gap. The laser-irradiated ®lms show low electrical resistivity depending on the band structure as well as the amount of oxygen vacancy and crystallinity. In conclusion, excimer laser irradiation of sol±gel derived ®lms is an attractive and promising method for the easy preparation of zinc oxide ®lms with unique optical and electrical properties, without the need for special vacuum equipment. Acknowledgements We would like to thank Dr. T. Shikama of Takamatsu National College of Technology for measurement of sheet resistance and Professor N. Mizutani in Tokyo Institute of Technology, Professor M. Nakatsuka in Osaka Univ. and Dr. K. Ooi in Shikoku National Industrial Research Institute for helpful comments concerning this work. References [1] S.Y. Myong, S.J. Baik, C.H. Lee, W.Y. Cho, K.S. Lim, Jpn. J. Appl. Phys. 36 (1997) L1078. [2] M.N. Islam, T.B. Ghosh, K.L. Chopra, H.N. Acharya, Thin Solid Films 280 (1996) 20. [3] C.H. Lee, J.S. Song, K.S. Lim, Sol. Energy Mater. Solar Cell 43 (1996) 37. [4] Y. Ohya, H. Saiki, T. Tanaka, Y. Takahashi, J. Am. Ceram. Soc. 79 (1996) 825. [5] Y. Ohya, H. Saiki,, Y. Takahashi, J. Mater. Sci. 29 (1994) 4099. [6] Y. Segawa, A. Ohtomo, M. Kawasaki, H. Koinuma, Z.K. Tang, P. Yu, G.K.L. Wong, Phys. Stat. Sol. B 202 (1997) 669. [7] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 (1997) 459. [8] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983. [9] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J. Caruso, M.J. Hampden-Smith, T.T. Kodas, J. Lumin. 75 (1997) 11. [10] D. Bao, H. Gu, A. Kuang, Thin Solid Films 312 (1998) 37. [11] S.A. Studenikin, N. Golego, M. Cocivera, J. Appl. Phys. 84 (1998) 2287. [12] T. Yamamoto, T. Shiosaki, A. Kawabata, J. Appl. Phys. 63 (1980) 3113. [13] O. Yamazaki, T. Mitsuyu, K. Wasa, IEEE Trans. Sonics and Ultrasonics SU-27 (1980) 369. [14] T. Maruyama, J. Shionoya, J. Mater. Sci. Lett. 11 (1992) 170. [15] M.F. Ogawa, Y. Natsume, T. Hirayama, H. Sakata, J. Mater. Sci. Lett. 9 (1990) 1351. [16] M.F. Ogawa, Y. Natsume, T. Hirayama, H. Sakata, J. Mater. Sci. Lett. 9 (1990) 1354. [17] J.H. Jean, J. Mater. Sci. Lett. 9 (1990) 127. [18] M. Izaki, T. Omi, J. Electrochem. Soc. 144 (1997) 1949.
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