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Transparent semiconducting ZnO:Al thin films prepared by spray pyrolysis
Materials Science in Semiconductor Processing 2 (1999) 45±55
Transparent semiconducting ZnO:Al thin ®lms prepared by spray pyrolysis W.T. Seeber a, *, M.O. Abou-Helal a, S. Barth a, D. Beil a, T. HoÈche a, H.H. A®fy b, S.E. Demian b a
UniversitaÈt Jena, Otto-Schott-Institut, Fraunhoferstr. 6, 07743 Jena, Germany b National Research Center, El-Tahrir St., Dokki, Cairo NRC12311, Egypt
Abstract Aluminum doped zinc oxide (ZnO:Al) ®lms which can be used as transparent electrodes or heating layers have been deposited by the low cost spray pyrolysis technique. Undoped and Al-doped ZnO ®lms deposited using various preparation conditions and on dierent substrates (soda lime glass, quartz glass and crystalline quartz, respectively) have been studied. The eect of substrate type, temperature, deposition time and doping concentration on ZnO:Al thin layers have been investigated by analysing the optical and structural properties of the ®lms. A substrate temperature of 770 K allows the preparation of nanosized ZnO:Al crystals with preferred [002] orientation. Films with optical transmission T>85% and a adjustable resistivity r between 2 and 100 O cm have been obtained. The resistivity value of the ®lms can be adjusted by tuning suitable processing parameters. The feasibility of the spray pyrolysis technique for the preparation of thin semiconducting ZnO:Al ®lms on conventional soda lime glass substrates is demonstrated. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Chemical vapor deposition (CVD); X-ray diraction; Nanocrystalline materials; Optical properties; Resistivity
1. Introduction There exists a worldwide strong interest in realizing inexpensive transparent conducting ®lms to be used, for example, in solar cells, as sensor devices or as coating to heat glass windows. ZnO is one of the few metal oxides which can be used as a transparent conducting oxide. It has some advantages over other possible materials such as In2O3, Cd2SnO4 or SnO2 due to its unique combination of interesting properties: non-toxicity, good electrical, optical and piezoelectric behavior, stability in a hydro-
* Corresponding author. Tel.: +49-3641-948-543; fax: +493641-948-502; e-mail: [email protected]
gen plasma atmosphere and its low price [1±3]. Additionally, ZnO is suitable for many dierent applications, such as opto-electrodes, surface acoustic wave devices (SAW) and sensor materials. Zinc oxide can be doped with a wide variety of ions to meet the demands of several application ®elds. Typical dopants that have been used to produce conducting ®lms of ZnO belongs to the group IIIa elements of the periodic table (B, Al, Ga, In). Thin ®lms of doped ZnO have been prepared with many techniques such as sputtering [4±8], MOCVD [9,10], vapor transport [11], pulsed laser deposition [12], spray pyrolysis or pyrosol process [13,14]. The spray pyrolysis technique (SPT) has some advantages in comparison to the other methods. SPT is quite simple and the required setup is less expensive and ¯exible for process modi®cations. Due to the use
1369-8001/98/$19.00 # 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 9 - 8 0 0 1 ( 9 9 ) 0 0 0 0 7 - 4
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W.T. Seeber et al. / Materials Science in Semiconductor Processing 2 (1998) 45±55
Fig. 1. (a) X-ray diraction patterns of Al-doped ZnO ®lms deposited on soda lime glass at dierent substrate temperatures. The peak positions of a reference sample (undoped ZnO powder, ASTM-®le 36-1451) are also reported ( T ). (b) X-ray diraction patterns of Al doped ZnO ®lms deposited on dierent substrates. Deposition temperature was 870 K and the spray time was 30 min.
of less expensive chemicals at air pressure conditions it is attractive from an economical point of view also. Additionally, by using this technique one can produce large area ®lms without the need of an ultra high vac-
uum and the produced ®lms can be controlled step by step. In a previous paper we have discussed the properties of rare earth ion (RE3+) doped semiconducting ®lms
W.T. Seeber et al. / Materials Science in Semiconductor Processing 2 (1998) 45±55
47
Fig. 2. (a) Scanning electron microscopic picture of ZnO:Al ®lm deposited on soda lime glass. Spray time was 30 min and deposition temperature was 770 K. (b) Scanning electron microscopic picture of ZnO:Al ®lm deposited on soda lime glass. Spray time was 30 min and deposition temperature was 870 K.
by spray pyrolysis [15]. In this paper the doping experiments of ZnO ®lms with Al3+ ions will be presented. To our knowledge to date there exists only limited information about Al-doped ZnO ®lms with adjustable resistivity prepared by SPT using air as carrier gas. It is well known that the SPT processing parameters strongly eect in a complex manner the properties of
the obtained ®lms. Therefore one has to combine various dierent techniques to get detailed results. That is why we applied common ultraviolet/visible/near infrared spectroscopy (UV/VIS/NIR), interference shearing microscopy (IFM), atomic force microscopy (AFM) and X-ray diraction (XRD) to investigate the optical and structural properties of the obtained ZnO:Al ®lms.
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W.T. Seeber et al. / Materials Science in Semiconductor Processing 2 (1998) 45±55
Scanning electron microscopy (SEM) has been employed for ®lm surface characterization and element analysis. An aim of this paper was also to test the usefulness of less expensive soda-lime glass as a substrate material for ZnO ®lms with adjustable resistivity in the 10 O cm region. 2. Experimental procedure Al-doped and undoped ZnO ®lms were prepared by spray pyrolysis on soda-lime slide glass substrates (Menzel-GlaÈser, Germany), on quartz glass or on crystalline SiO2 (nucleation plates from former Kombinat Carl Zeiss Jena, Germany). Zinc acetate (99%, Merck, Germany) was dissolved in a mixture of three quarters water (HPLC grade, conductivity <0.006 mS/cm) and one quarter methanol (p.a., Laborchemie Apolda, Germany). AlCl36H2O (99.9%, Merck, Germany) has been dissolved in HPLC grade water and appropriate parts have been added to a 0.2 M zinc acetate solution to realize an atomic Zn:Al ratio of 96:4. Starting spray solutions with a molarity higher than 0.4 M Zn resulted in almost opaque ®lms as previously observed [15]. The SPT system setup consisted of a heated substrate, a spray nozzle, an air compressor, a solution pump and a gas exhaust. Details on this setup have been published elsewhere [15]. The air pressure to generate a continuous air ¯ow rate was 1.1 bar, the solution rate 0.3 ml minÿ1 and the spraying time ranged between 10 and 70 min. Substrate temperatures were ®xed at 670, 720, 770, 820 and 870 K, respectively. Selected samples were examined with an X-ray diractometer (SIEMENS D5000), an interference shearing microscope ISM (JENAMAP, Carl Zeiss Jena), an atomic force microscope AFM (JENAVAL, Carl Zeiss, equipped with an SIS-ultraobjective) and a scanning electron microscope SEM 940A, Carl Zeiss, equipped with an analytical data management system (LINK). The UV/VIS/NIR-spectra of the samples were recorded by a spectrophotometer (UV-3101PC Shimadzu). A special software package [16] has been used to estimate the ®lm thickness from the transmission spectra. For comparison purposes the ®lm thickness was independently measured with ISM and SEM. No dierences greater than errors in measurements were observed. The resistivity data have been obtained by using conventional comparison of voltage versus applied current (two electrodes) together with the determined thickness value. 3. Results We employed the X-ray diraction technique to get
a ®rst impression of the main crystalline phases and the possible orientation of crystallites in the ®lms prepared at dierent conditions. As an example the X-ray diraction spectra of Al-doped ZnO ®lms prepared at 30 min spraying time and at 670, 770 and 870 K substrate temperature on soda-lime glass are shown in Fig. 1(a). On the other hand Fig. 1(b) gives information on the dependence of the diraction peak intensities on the substrate type (SiO2 glass, quartz crystal, soda lime glass) onto ®lms deposited at 870 K. The diraction patterns have been normalized to the peak with the maximum intensity equal to one. The plots in Fig. 1(a) and (b) give a convenient overview of the changes in relative peak intensities corresponding to the dierent crystal orientations. The peak positions in Fig. 1(a) and (b) correspond very well with the theoretical ZnO patterns (hexagonal wurzite structure, ASTM data®le 36-1451 also reported in Fig. 1, T). Fig. 2 shows two SEM pictures of Al-doped ZnO ®lms prepared at 770 K (Fig. 2a) and 870 K substrate temperature (Fig. 2b), respectively. We can observe that there exists an homogeneous distribution of quite small grains. The sample prepared at a higher deposition temperature shows a smoother surface. This is obviously the result of a decrease of the average crystal size by surface diusion. Interference shearing microscopy was used to get a semiquantitative expression of the homogeneity and topography of the ZnO:Al ®lm prepared at 870 K and 30 min spraying time (see Fig. 3). For the calculation of the average arithmetic roughness, Ra, employing the commercial software package MICROFRINGE of the ISM JENAMAP, a representative area of about 600 600 mm of the ®lm has been used. We found that the average arithmetic roughness, Ra (the arithmetic mean value of the deviations from best ®tted plane), was about 73 nm. The peak to valley value, Rd (dierence between the highest and the lowest measured height), was about 2 mm and indicates some strong inhomogeneities. A quite similar value for the average roughness (70210 nm) was derived from AFM measurements (Fig. 4) using a 5 5 mm ®lm area. Moreover, we found that on average the height of the deposited objects on the surface did not exceed 100 nm. 4. Discussion 4.1. The potential of produced ZnO:Al ®lms for sensor devices The Al-doping procedure caused no additional Xray diraction peaks or at least Al2O3 content was below the detection limit. When compared with the theoretical peaks of randomly distributed crystals, the deposited ®lms exhibit a preferred orientation that
W.T. Seeber et al. / Materials Science in Semiconductor Processing 2 (1998) 45±55
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Fig. 3. Interference shearing microscopic picture of ZnO:Al ®lm deposited on soda lime glass. Spray time was 30 min and deposition temperature was 870 K.
depends on the substrate composition and temperature. For applications like sensor devices the alignment of the crystals on the surface of the semiconducting sensor ®lm has some advantages. For instance, the number, the size and the shape of docking sites of the ZnO-®lm for gaseous traces should be tunable within some limits. It is well known that if the [002]-peak is very strong the grains are strongly oriented in the caxis of the hexagonal ZnO (normal to the substrate surface). This eect should be due to an asymmetric charge distribution around the Zn and O atoms, such that these atoms are linked together along the c-axis as pseudodiatomic molecules ([17] and references cited therein). This means that the bonding between Zn and O atoms along the basal plane is dierent from that in other directions and should eect for instance the volatization of simple ZnO molecules at temperatures well below its melting point [17]. In conclusion we assume that active and sensoric selective sites remain on the surface. Considering the practice-orientated demands to employ sensors at temperatures in the region of 300
K this approach may be useful in future tailoring of the sensor properties. By using quartz glass as a substrate at high temperatures (870 K) the diraction pattern of the ZnO:Al ®lms shows a strong increase of the [002] peak. On the other hand, crystalline SiO2 as substrate material reduces the [100]-alignment. The optical properties of ®lms deposited on the dierent substrates are almost the same. Generally we observed a change in the crystallinity of the ®lms with increasing substrate temperature. With increasing substrate temperature the preferred orientation changes in the order [100], [002] and [101]. This fact may be attributed to the dierent temperature dependence of the growth of the various crystal planes. From the intensity ratio of diraction peaks of ZnO:Al samples to corresponding ASTM values of ZnO, the relative intensity of [100], [002] and [101] peaks dependent on preparation conditions can be estimated (see Table 1). The degree of preferred [002]-orientation could be estimated using the ratio [002]sample/[002]ASTM 36-1451.
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Fig. 4. Atomic force microscopic picture of ZnO:Al ®lm deposited on soda lime glass. Spray time was 30 min and deposition temperature was 870 K.
Table 1 Intensity ratio of diraction peaks as function of substrate temperature. Substrate material: soda-lime glass. Spray time: 30 min Sample ZnO ASTM 36-1451 ZnO undoped ZnO:Al ZnO:Al ZnO:Al
Substrate Intensity Intensity Intensity [100]sample [002]sample [101]sample temperature (K) (relative) [100] (relative) [002] (relative) [101] /[100]ZnO ASTM /[002]ZnO ASTM /[101]ZnO ASTM ±
0.57
0.44
1.0
1.0
1.0
1.0
770 670 770 870
± 1.0 0.56 0.88
1.0 0.5 1.0 0.45
0.9 0.48 0.71 1.0
± 1.75 0.98 1.54
2.27 1.14 2.27 1.02
0.9 0.48 0.71 1.0
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Table 2 Crystallite size as function of substrate temperature. Substrate material: soda-lime glass. Spray time: 30 min Sample
Substrate temperature (K)
Crystallite size from [100] peak (nm)
Crystallite size from [002] peak (nm)
Crystallite size from [101] peak (nm)
Crystallite size from SEM (nm)
ZnO ZnO:Al ZnO:Al ZnO:Al
770 670 770 870
± 15.5 18.5 16.0
24.0 14.0 16.5 14.0
25.5 11.0 16.5 16.5
± 390 120 85
The ZnO:Al ®lm prepared at medium substrate temperature (770 K) and the undoped sample at the same temperature show a similar high degree of [002]-orientation. On the other hand, at higher temperatures the degree of [100]-orientation increases compared to statistical distribution. The same is valid for lower temperatures. It should be mentioned that the degree of [002]-orientation in some Al-doped samples reaches the
values of rare earth ion doped ZnO samples as recently investigated [15]. Assuming a homogeneous strain across crystallites, the size of microcrystallites can be estimated from the full width half maximum (FWHM) values D(2y ) of diffraction peaks. An average crystallite size could be obtained for the ®lms using the well known Scherrer formula for crystallite size broadening of diraction
Fig. 5. Element analysis of ZnO:Al ®lm deposited on soda lime glass. Spray time was 30 min and deposition temperature was 870 K.
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W.T. Seeber et al. / Materials Science in Semiconductor Processing 2 (1998) 45±55
Fig. 6. UV/VIS/NIR transmission curve of undoped and doped ZnO ®lms. In¯uence of substrate temperature (a) and of spray time (b).
peaks [18]: D 0:94l=D 2ycos y
1
Table 2 shows the estimated values. Using the SEM technique we have observed a decrease of size as the temperature increases: a size reduction from 390 to 85 nm. These values are dierent from those estimated from X-ray diraction. It seems
that the primary nanosized crystallites form interconnected aggregates. Moreover, in further investigations we will use cross section transmission microscopy (XTEM) additional to SEM plan view technique to make a meaningful comparison with the values of crystallite size derived by X-ray analysis. We employed element analysis to check the chemical composition of the ®lms. Fig. 5 shows as expected the presence of Zn, O and Al. The other detected elements
W.T. Seeber et al. / Materials Science in Semiconductor Processing 2 (1998) 45±55
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Fig. 7. Film thickness as a function of substrate temperature at constant time (a) and as a function of the spraying time at constant temperature (b). The solid line is a guide to the eye.
like Si, Ca, K and Mg belong to the substrate glass. Interestingly, also Cl could be observed and its concentration seems to be higher in the crystals on the surface. In a ®rst experiment to evaluate the sensor properties of ZnO-®lms by spray pyrolysis we observed a strong decrease of the dc ®lm resistance if the sensor ®lm has been exposed to ethanol in a home made apparatus [19].
4.2. The potential of produced ZnO:Al ®lms as transparent electrodes or to heat windows The energy gap of undoped and Al-doped ZnO is quite similar and equals about 3.39 eV (366 nm). Al3+ ions show no transitions in the visible range, that is why we don't observe any additional absorption bands in the UV/VIS/NIR spectra reported in Fig. 6(a) and
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W.T. Seeber et al. / Materials Science in Semiconductor Processing 2 (1998) 45±55
(b). ZnO:Al ®lms with a transmission up to 87% could be obtained. Fig. 6(a) shows the spectra at constant spraying time but dierent temperature and Fig. 6(b) shows the spectra at constant temperature but dierent time. A substrate temperature above 850 K may provide the basis for an increased transmission if the changes in ®lm thickness are neglected. This correlates with the already mentioned observation of reduced surface roughness at high deposition temperatures too. The relation between the substrate temperature and the ®lm thickness at constant spraying time can be seen in Fig. 7(a). The thickness decreases as the temperature increases. This decrease can be explained by the fact that evaporation of the metal compound occurs before it reaches the substrate and the reaction takes place in the vapor phase with a normally low rate. At relatively low temperature the reaction between the species takes place on the substrate surface with a higher rate of growth and the ®lm thickness is relatively high. The experimental data can be ®tted by the following function: t 3500 ÿ 7:2 T 0:004 T 2
Fig. 8 shows the decrease of electrical resistivity of ZnO:Al ®lms with increasing substrate temperature (Fig. 8a) or spraying time (Fig. 8b). The dierent ®lm thickness has been considered. The decrease in resistivity can be interpreted in terms of crystallite structure enhancement. Subsequent annealing procedures to reduce the resistivity of ZnO thin ®lms, e.g. annealing in 10ÿ4 Torr vacuum at 470 K for 1 h [14], were not applied to keep the ®lm preparation process as simple as possible. The reported ®lms do not meet the resistivity requirements for electrode applications (should be below 5 10ÿ3 O cm). On the other hand, the discussed ZnO:Al ®lms can also be used to heat the surfaces of windows. In ®rst experiments we employed ZnO:2%Al-®lms on a 1 mm thick soda-lime glass. By applying a dc voltage of about 30 V using copper stripes (measured current was about 0.7 mA, electrical input power was about 20 mW) we observed an increase of the surface temperature of about 1 K at
2
with t as the thickness in nm and T as the substrate temperature in K. Fig. 7(b) shows a linear dependency of the thickness on the spraying time up to about 70 min. From this data a deposition rate of about 10 nm/min could be estimated despite the high substrate temperature of 870 K. Obviously, the ®lm roughness for applications like transparent electrodes should be as low as possible to avoid light scattering eects. On the other hand, for employment of ®lms as a front electrode in solar cells a sucient texture to scatter light is necessary. We found that the morphology of our ZnO:Al ®lm surfaces shows a signi®cant local dependence as a result of inhomogeneous vapor phase conditions. There exist areas with an average roughness of about 75 nm (Fig. 3 and Fig. 4a), but also areas with a strong agglomeration of particles up to 2 mm size. It seems that this behavior is introduced by the Al-dopant. Compared to previous results on rare earth doped ZnO ®lms the Aldoping causes an increased inhomogenity of the surface. Maybe there are correlation's to the Cl-content of the vapor phase during the pyrolysis process and the use of chlorides as doping compounds is not advantageous, at least in our spray setup. A detailed study of the chemical nature of the dierent objects on the surface is forthcoming. Nevertheless, the optical quality of ZnO:Al ®lms prepared at temperatures higher than about 750 K is remarkably good (Fig. 6a). This is in agreement with the mentioned low average ®lm roughness of about 70 nm and meets the transmission requirement for electrode applications.
Fig. 8. (a) Electrical resistivity of ZnO:Al ®lms as a function of substrate temperature during pyrolysis. Spray time was 30 min. (b) Electrical resistivity of ZnO:Al ®lms as a function of spray time. Films were deposited at 870 K.
W.T. Seeber et al. / Materials Science in Semiconductor Processing 2 (1998) 45±55
room temperature. Further experiments are in progress. Moreover, a decrease of the resistivity values is possible and will be discussed in a following paper [19]. 5. Conclusion Al-doped ZnO ®lms could be prepared by a low-cost spray pyrolysis technique. Control of the spray process by adjusting the dierent parameters, especially the spraying temperature and the spraying time, is necessary to obtain consistent optical properties in the ®lms. The good quality despite some roughness (about 70 nm) of the deposited layers could be shown by XRD, SEM, AFM and ISM measurements. It seems that the prepared ®lms are built up by nanosized crystals in preferred ([100], [002] or [101]) direction. Further investigations are on the way, because ZnO ®lms doped with Al are good candidates for opto-electronics, photovoltaic and sensor applications. Working under atmospheric conditions accompanied with the use of commercial available chemicals, makes the process easy to use. One can obtain good ®lms of ZnO:Al working at optimized parameters after about 10 min. Resistivity values between 2 and 100 O cm can be tailored even by the use of conventional soda-lime glass substrates by adjusting the substrate temperature and spraying time. Acknowledgements The work has been supported by DAAD (grant for MOA-H) and Alexander von Humboldt-Foundation. We wish to thank T. Kittel, B. Hartmann, R. Keding and coworkers at OSI Jena.
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References [1] Jin ZC, Hamberg J, Granqvist CG. J Appl Phys 1988;64:5117. [2] Yoo JB, Fahrenbruch AL, Bube RH. J Appl Phys 1990;68:4694. [3] Major S, Kumar S, Bhatnagar M, Chopra KL. Appl Phys Lett 1986;49:394. [4] Minami T, Sato H, Nanto H, Takata S. Jpn J Appl Phys 1986;25:9. [5] Schropp REI, Madan A. J Appl Phys 1989;66:5. [6] Kobayashi K, Maeda T, Matsushima S, Okada G. J Mater Sci 1992;27:5953. [7] Konishi R, Noda K, Harada H, Sasakura H. J Cryst Growth 1992;117:939. [8] Martinez MA, Herrero J, Gutierrez MT. Solar Energy Mater Solar Cells 1997;45:75. [9] Roth AP, Webb JB, Williams DF. Phys Rev 1982;25:12. [10] Wenas WW, Yamada A, Konagai M, Takahashi K. Jpn J Appl Phys 1991;30:38. [11] Noritake F, Yamamoto N, Horiguchi Y. J Am Ceram Soc 1991;74:1. [12] Suzuki A, Matsushita T, Wada N, Sakamoto Y, Okuda M. Jpn J Appl Phys 1996;35:L56±L59. [13] Tiburcio-Silver A, Joubert JC, Laveau M. Thin Solid Films 1991;197:195. [14] Song J, Cho W, Lim K, Yoon K, Yu G. First WCPEC, Hawaii, Dec. 5±9, IEEE Proc. CH3365-4/94, 1994. p. 630. [15] Abou-Helal MO, Seeber W. J Non-Cryst Solids 1997;218:139. [16] Software package APAS 1.3. UniversitaÈt Jena, OttoSchott-Institut, Germany, 1995. [17] Siqueiros JM, Diaz JA, Contreras O, Hirata GA, McKittnick J. Proc. of Materials Research Society Symposium, Pittsburgh, 1996. p. 247. [18] Scherrer P. Goettinger Nachr 1918;2:98. [19] Seeber, W, Abou-Helal MO, A®fy H, Barth S. 1998, in preparation.
Transparent semiconducting ZnO:Al thin ®lms prepared by spray pyrolysis W.T. Seeber a, *, M.O. Abou-Helal a, S. Barth a, D. Beil a, T. HoÈche a, H.H. A®fy b, S.E. Demian b a
UniversitaÈt Jena, Otto-Schott-Institut, Fraunhoferstr. 6, 07743 Jena, Germany b National Research Center, El-Tahrir St., Dokki, Cairo NRC12311, Egypt
Abstract Aluminum doped zinc oxide (ZnO:Al) ®lms which can be used as transparent electrodes or heating layers have been deposited by the low cost spray pyrolysis technique. Undoped and Al-doped ZnO ®lms deposited using various preparation conditions and on dierent substrates (soda lime glass, quartz glass and crystalline quartz, respectively) have been studied. The eect of substrate type, temperature, deposition time and doping concentration on ZnO:Al thin layers have been investigated by analysing the optical and structural properties of the ®lms. A substrate temperature of 770 K allows the preparation of nanosized ZnO:Al crystals with preferred [002] orientation. Films with optical transmission T>85% and a adjustable resistivity r between 2 and 100 O cm have been obtained. The resistivity value of the ®lms can be adjusted by tuning suitable processing parameters. The feasibility of the spray pyrolysis technique for the preparation of thin semiconducting ZnO:Al ®lms on conventional soda lime glass substrates is demonstrated. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Chemical vapor deposition (CVD); X-ray diraction; Nanocrystalline materials; Optical properties; Resistivity
1. Introduction There exists a worldwide strong interest in realizing inexpensive transparent conducting ®lms to be used, for example, in solar cells, as sensor devices or as coating to heat glass windows. ZnO is one of the few metal oxides which can be used as a transparent conducting oxide. It has some advantages over other possible materials such as In2O3, Cd2SnO4 or SnO2 due to its unique combination of interesting properties: non-toxicity, good electrical, optical and piezoelectric behavior, stability in a hydro-
* Corresponding author. Tel.: +49-3641-948-543; fax: +493641-948-502; e-mail: [email protected]
gen plasma atmosphere and its low price [1±3]. Additionally, ZnO is suitable for many dierent applications, such as opto-electrodes, surface acoustic wave devices (SAW) and sensor materials. Zinc oxide can be doped with a wide variety of ions to meet the demands of several application ®elds. Typical dopants that have been used to produce conducting ®lms of ZnO belongs to the group IIIa elements of the periodic table (B, Al, Ga, In). Thin ®lms of doped ZnO have been prepared with many techniques such as sputtering [4±8], MOCVD [9,10], vapor transport [11], pulsed laser deposition [12], spray pyrolysis or pyrosol process [13,14]. The spray pyrolysis technique (SPT) has some advantages in comparison to the other methods. SPT is quite simple and the required setup is less expensive and ¯exible for process modi®cations. Due to the use
1369-8001/98/$19.00 # 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 9 - 8 0 0 1 ( 9 9 ) 0 0 0 0 7 - 4
46
W.T. Seeber et al. / Materials Science in Semiconductor Processing 2 (1998) 45±55
Fig. 1. (a) X-ray diraction patterns of Al-doped ZnO ®lms deposited on soda lime glass at dierent substrate temperatures. The peak positions of a reference sample (undoped ZnO powder, ASTM-®le 36-1451) are also reported ( T ). (b) X-ray diraction patterns of Al doped ZnO ®lms deposited on dierent substrates. Deposition temperature was 870 K and the spray time was 30 min.
of less expensive chemicals at air pressure conditions it is attractive from an economical point of view also. Additionally, by using this technique one can produce large area ®lms without the need of an ultra high vac-
uum and the produced ®lms can be controlled step by step. In a previous paper we have discussed the properties of rare earth ion (RE3+) doped semiconducting ®lms
W.T. Seeber et al. / Materials Science in Semiconductor Processing 2 (1998) 45±55
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Fig. 2. (a) Scanning electron microscopic picture of ZnO:Al ®lm deposited on soda lime glass. Spray time was 30 min and deposition temperature was 770 K. (b) Scanning electron microscopic picture of ZnO:Al ®lm deposited on soda lime glass. Spray time was 30 min and deposition temperature was 870 K.
by spray pyrolysis [15]. In this paper the doping experiments of ZnO ®lms with Al3+ ions will be presented. To our knowledge to date there exists only limited information about Al-doped ZnO ®lms with adjustable resistivity prepared by SPT using air as carrier gas. It is well known that the SPT processing parameters strongly eect in a complex manner the properties of
the obtained ®lms. Therefore one has to combine various dierent techniques to get detailed results. That is why we applied common ultraviolet/visible/near infrared spectroscopy (UV/VIS/NIR), interference shearing microscopy (IFM), atomic force microscopy (AFM) and X-ray diraction (XRD) to investigate the optical and structural properties of the obtained ZnO:Al ®lms.
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W.T. Seeber et al. / Materials Science in Semiconductor Processing 2 (1998) 45±55
Scanning electron microscopy (SEM) has been employed for ®lm surface characterization and element analysis. An aim of this paper was also to test the usefulness of less expensive soda-lime glass as a substrate material for ZnO ®lms with adjustable resistivity in the 10 O cm region. 2. Experimental procedure Al-doped and undoped ZnO ®lms were prepared by spray pyrolysis on soda-lime slide glass substrates (Menzel-GlaÈser, Germany), on quartz glass or on crystalline SiO2 (nucleation plates from former Kombinat Carl Zeiss Jena, Germany). Zinc acetate (99%, Merck, Germany) was dissolved in a mixture of three quarters water (HPLC grade, conductivity <0.006 mS/cm) and one quarter methanol (p.a., Laborchemie Apolda, Germany). AlCl36H2O (99.9%, Merck, Germany) has been dissolved in HPLC grade water and appropriate parts have been added to a 0.2 M zinc acetate solution to realize an atomic Zn:Al ratio of 96:4. Starting spray solutions with a molarity higher than 0.4 M Zn resulted in almost opaque ®lms as previously observed [15]. The SPT system setup consisted of a heated substrate, a spray nozzle, an air compressor, a solution pump and a gas exhaust. Details on this setup have been published elsewhere [15]. The air pressure to generate a continuous air ¯ow rate was 1.1 bar, the solution rate 0.3 ml minÿ1 and the spraying time ranged between 10 and 70 min. Substrate temperatures were ®xed at 670, 720, 770, 820 and 870 K, respectively. Selected samples were examined with an X-ray diractometer (SIEMENS D5000), an interference shearing microscope ISM (JENAMAP, Carl Zeiss Jena), an atomic force microscope AFM (JENAVAL, Carl Zeiss, equipped with an SIS-ultraobjective) and a scanning electron microscope SEM 940A, Carl Zeiss, equipped with an analytical data management system (LINK). The UV/VIS/NIR-spectra of the samples were recorded by a spectrophotometer (UV-3101PC Shimadzu). A special software package [16] has been used to estimate the ®lm thickness from the transmission spectra. For comparison purposes the ®lm thickness was independently measured with ISM and SEM. No dierences greater than errors in measurements were observed. The resistivity data have been obtained by using conventional comparison of voltage versus applied current (two electrodes) together with the determined thickness value. 3. Results We employed the X-ray diraction technique to get
a ®rst impression of the main crystalline phases and the possible orientation of crystallites in the ®lms prepared at dierent conditions. As an example the X-ray diraction spectra of Al-doped ZnO ®lms prepared at 30 min spraying time and at 670, 770 and 870 K substrate temperature on soda-lime glass are shown in Fig. 1(a). On the other hand Fig. 1(b) gives information on the dependence of the diraction peak intensities on the substrate type (SiO2 glass, quartz crystal, soda lime glass) onto ®lms deposited at 870 K. The diraction patterns have been normalized to the peak with the maximum intensity equal to one. The plots in Fig. 1(a) and (b) give a convenient overview of the changes in relative peak intensities corresponding to the dierent crystal orientations. The peak positions in Fig. 1(a) and (b) correspond very well with the theoretical ZnO patterns (hexagonal wurzite structure, ASTM data®le 36-1451 also reported in Fig. 1, T). Fig. 2 shows two SEM pictures of Al-doped ZnO ®lms prepared at 770 K (Fig. 2a) and 870 K substrate temperature (Fig. 2b), respectively. We can observe that there exists an homogeneous distribution of quite small grains. The sample prepared at a higher deposition temperature shows a smoother surface. This is obviously the result of a decrease of the average crystal size by surface diusion. Interference shearing microscopy was used to get a semiquantitative expression of the homogeneity and topography of the ZnO:Al ®lm prepared at 870 K and 30 min spraying time (see Fig. 3). For the calculation of the average arithmetic roughness, Ra, employing the commercial software package MICROFRINGE of the ISM JENAMAP, a representative area of about 600 600 mm of the ®lm has been used. We found that the average arithmetic roughness, Ra (the arithmetic mean value of the deviations from best ®tted plane), was about 73 nm. The peak to valley value, Rd (dierence between the highest and the lowest measured height), was about 2 mm and indicates some strong inhomogeneities. A quite similar value for the average roughness (70210 nm) was derived from AFM measurements (Fig. 4) using a 5 5 mm ®lm area. Moreover, we found that on average the height of the deposited objects on the surface did not exceed 100 nm. 4. Discussion 4.1. The potential of produced ZnO:Al ®lms for sensor devices The Al-doping procedure caused no additional Xray diraction peaks or at least Al2O3 content was below the detection limit. When compared with the theoretical peaks of randomly distributed crystals, the deposited ®lms exhibit a preferred orientation that
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Fig. 3. Interference shearing microscopic picture of ZnO:Al ®lm deposited on soda lime glass. Spray time was 30 min and deposition temperature was 870 K.
depends on the substrate composition and temperature. For applications like sensor devices the alignment of the crystals on the surface of the semiconducting sensor ®lm has some advantages. For instance, the number, the size and the shape of docking sites of the ZnO-®lm for gaseous traces should be tunable within some limits. It is well known that if the [002]-peak is very strong the grains are strongly oriented in the caxis of the hexagonal ZnO (normal to the substrate surface). This eect should be due to an asymmetric charge distribution around the Zn and O atoms, such that these atoms are linked together along the c-axis as pseudodiatomic molecules ([17] and references cited therein). This means that the bonding between Zn and O atoms along the basal plane is dierent from that in other directions and should eect for instance the volatization of simple ZnO molecules at temperatures well below its melting point [17]. In conclusion we assume that active and sensoric selective sites remain on the surface. Considering the practice-orientated demands to employ sensors at temperatures in the region of 300
K this approach may be useful in future tailoring of the sensor properties. By using quartz glass as a substrate at high temperatures (870 K) the diraction pattern of the ZnO:Al ®lms shows a strong increase of the [002] peak. On the other hand, crystalline SiO2 as substrate material reduces the [100]-alignment. The optical properties of ®lms deposited on the dierent substrates are almost the same. Generally we observed a change in the crystallinity of the ®lms with increasing substrate temperature. With increasing substrate temperature the preferred orientation changes in the order [100], [002] and [101]. This fact may be attributed to the dierent temperature dependence of the growth of the various crystal planes. From the intensity ratio of diraction peaks of ZnO:Al samples to corresponding ASTM values of ZnO, the relative intensity of [100], [002] and [101] peaks dependent on preparation conditions can be estimated (see Table 1). The degree of preferred [002]-orientation could be estimated using the ratio [002]sample/[002]ASTM 36-1451.
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Fig. 4. Atomic force microscopic picture of ZnO:Al ®lm deposited on soda lime glass. Spray time was 30 min and deposition temperature was 870 K.
Table 1 Intensity ratio of diraction peaks as function of substrate temperature. Substrate material: soda-lime glass. Spray time: 30 min Sample ZnO ASTM 36-1451 ZnO undoped ZnO:Al ZnO:Al ZnO:Al
Substrate Intensity Intensity Intensity [100]sample [002]sample [101]sample temperature (K) (relative) [100] (relative) [002] (relative) [101] /[100]ZnO ASTM /[002]ZnO ASTM /[101]ZnO ASTM ±
0.57
0.44
1.0
1.0
1.0
1.0
770 670 770 870
± 1.0 0.56 0.88
1.0 0.5 1.0 0.45
0.9 0.48 0.71 1.0
± 1.75 0.98 1.54
2.27 1.14 2.27 1.02
0.9 0.48 0.71 1.0
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Table 2 Crystallite size as function of substrate temperature. Substrate material: soda-lime glass. Spray time: 30 min Sample
Substrate temperature (K)
Crystallite size from [100] peak (nm)
Crystallite size from [002] peak (nm)
Crystallite size from [101] peak (nm)
Crystallite size from SEM (nm)
ZnO ZnO:Al ZnO:Al ZnO:Al
770 670 770 870
± 15.5 18.5 16.0
24.0 14.0 16.5 14.0
25.5 11.0 16.5 16.5
± 390 120 85
The ZnO:Al ®lm prepared at medium substrate temperature (770 K) and the undoped sample at the same temperature show a similar high degree of [002]-orientation. On the other hand, at higher temperatures the degree of [100]-orientation increases compared to statistical distribution. The same is valid for lower temperatures. It should be mentioned that the degree of [002]-orientation in some Al-doped samples reaches the
values of rare earth ion doped ZnO samples as recently investigated [15]. Assuming a homogeneous strain across crystallites, the size of microcrystallites can be estimated from the full width half maximum (FWHM) values D(2y ) of diffraction peaks. An average crystallite size could be obtained for the ®lms using the well known Scherrer formula for crystallite size broadening of diraction
Fig. 5. Element analysis of ZnO:Al ®lm deposited on soda lime glass. Spray time was 30 min and deposition temperature was 870 K.
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W.T. Seeber et al. / Materials Science in Semiconductor Processing 2 (1998) 45±55
Fig. 6. UV/VIS/NIR transmission curve of undoped and doped ZnO ®lms. In¯uence of substrate temperature (a) and of spray time (b).
peaks [18]: D 0:94l=D 2ycos y
1
Table 2 shows the estimated values. Using the SEM technique we have observed a decrease of size as the temperature increases: a size reduction from 390 to 85 nm. These values are dierent from those estimated from X-ray diraction. It seems
that the primary nanosized crystallites form interconnected aggregates. Moreover, in further investigations we will use cross section transmission microscopy (XTEM) additional to SEM plan view technique to make a meaningful comparison with the values of crystallite size derived by X-ray analysis. We employed element analysis to check the chemical composition of the ®lms. Fig. 5 shows as expected the presence of Zn, O and Al. The other detected elements
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Fig. 7. Film thickness as a function of substrate temperature at constant time (a) and as a function of the spraying time at constant temperature (b). The solid line is a guide to the eye.
like Si, Ca, K and Mg belong to the substrate glass. Interestingly, also Cl could be observed and its concentration seems to be higher in the crystals on the surface. In a ®rst experiment to evaluate the sensor properties of ZnO-®lms by spray pyrolysis we observed a strong decrease of the dc ®lm resistance if the sensor ®lm has been exposed to ethanol in a home made apparatus [19].
4.2. The potential of produced ZnO:Al ®lms as transparent electrodes or to heat windows The energy gap of undoped and Al-doped ZnO is quite similar and equals about 3.39 eV (366 nm). Al3+ ions show no transitions in the visible range, that is why we don't observe any additional absorption bands in the UV/VIS/NIR spectra reported in Fig. 6(a) and
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(b). ZnO:Al ®lms with a transmission up to 87% could be obtained. Fig. 6(a) shows the spectra at constant spraying time but dierent temperature and Fig. 6(b) shows the spectra at constant temperature but dierent time. A substrate temperature above 850 K may provide the basis for an increased transmission if the changes in ®lm thickness are neglected. This correlates with the already mentioned observation of reduced surface roughness at high deposition temperatures too. The relation between the substrate temperature and the ®lm thickness at constant spraying time can be seen in Fig. 7(a). The thickness decreases as the temperature increases. This decrease can be explained by the fact that evaporation of the metal compound occurs before it reaches the substrate and the reaction takes place in the vapor phase with a normally low rate. At relatively low temperature the reaction between the species takes place on the substrate surface with a higher rate of growth and the ®lm thickness is relatively high. The experimental data can be ®tted by the following function: t 3500 ÿ 7:2 T 0:004 T 2
Fig. 8 shows the decrease of electrical resistivity of ZnO:Al ®lms with increasing substrate temperature (Fig. 8a) or spraying time (Fig. 8b). The dierent ®lm thickness has been considered. The decrease in resistivity can be interpreted in terms of crystallite structure enhancement. Subsequent annealing procedures to reduce the resistivity of ZnO thin ®lms, e.g. annealing in 10ÿ4 Torr vacuum at 470 K for 1 h [14], were not applied to keep the ®lm preparation process as simple as possible. The reported ®lms do not meet the resistivity requirements for electrode applications (should be below 5 10ÿ3 O cm). On the other hand, the discussed ZnO:Al ®lms can also be used to heat the surfaces of windows. In ®rst experiments we employed ZnO:2%Al-®lms on a 1 mm thick soda-lime glass. By applying a dc voltage of about 30 V using copper stripes (measured current was about 0.7 mA, electrical input power was about 20 mW) we observed an increase of the surface temperature of about 1 K at
2
with t as the thickness in nm and T as the substrate temperature in K. Fig. 7(b) shows a linear dependency of the thickness on the spraying time up to about 70 min. From this data a deposition rate of about 10 nm/min could be estimated despite the high substrate temperature of 870 K. Obviously, the ®lm roughness for applications like transparent electrodes should be as low as possible to avoid light scattering eects. On the other hand, for employment of ®lms as a front electrode in solar cells a sucient texture to scatter light is necessary. We found that the morphology of our ZnO:Al ®lm surfaces shows a signi®cant local dependence as a result of inhomogeneous vapor phase conditions. There exist areas with an average roughness of about 75 nm (Fig. 3 and Fig. 4a), but also areas with a strong agglomeration of particles up to 2 mm size. It seems that this behavior is introduced by the Al-dopant. Compared to previous results on rare earth doped ZnO ®lms the Aldoping causes an increased inhomogenity of the surface. Maybe there are correlation's to the Cl-content of the vapor phase during the pyrolysis process and the use of chlorides as doping compounds is not advantageous, at least in our spray setup. A detailed study of the chemical nature of the dierent objects on the surface is forthcoming. Nevertheless, the optical quality of ZnO:Al ®lms prepared at temperatures higher than about 750 K is remarkably good (Fig. 6a). This is in agreement with the mentioned low average ®lm roughness of about 70 nm and meets the transmission requirement for electrode applications.
Fig. 8. (a) Electrical resistivity of ZnO:Al ®lms as a function of substrate temperature during pyrolysis. Spray time was 30 min. (b) Electrical resistivity of ZnO:Al ®lms as a function of spray time. Films were deposited at 870 K.
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room temperature. Further experiments are in progress. Moreover, a decrease of the resistivity values is possible and will be discussed in a following paper [19]. 5. Conclusion Al-doped ZnO ®lms could be prepared by a low-cost spray pyrolysis technique. Control of the spray process by adjusting the dierent parameters, especially the spraying temperature and the spraying time, is necessary to obtain consistent optical properties in the ®lms. The good quality despite some roughness (about 70 nm) of the deposited layers could be shown by XRD, SEM, AFM and ISM measurements. It seems that the prepared ®lms are built up by nanosized crystals in preferred ([100], [002] or [101]) direction. Further investigations are on the way, because ZnO ®lms doped with Al are good candidates for opto-electronics, photovoltaic and sensor applications. Working under atmospheric conditions accompanied with the use of commercial available chemicals, makes the process easy to use. One can obtain good ®lms of ZnO:Al working at optimized parameters after about 10 min. Resistivity values between 2 and 100 O cm can be tailored even by the use of conventional soda-lime glass substrates by adjusting the substrate temperature and spraying time. Acknowledgements The work has been supported by DAAD (grant for MOA-H) and Alexander von Humboldt-Foundation. We wish to thank T. Kittel, B. Hartmann, R. Keding and coworkers at OSI Jena.
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