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Characterization of r.f.-sputtered ZnO thin films by X-ray diffraction and scanning electron microscopy
Thin Solid Films, 94 (1982) 7-- 14 PREPARATION AND CHARACTERIZATION
CHARACTERIZATION OF R.F.-SPUTTERED ZnO THIN FILMS BY X-RAY D I F F R A C T I O N AND SCANNING ELECTRON MICROSCOPY S. SEN*, D. J. LEARYt AND C. L. BAUER
Center for the Joining of Materials, Carnegie-Mellon University, Pittsburgh, PA 15213 (U.S.A.) (Received October 2, 1981 ; accepted January 7, 1982)
Thin films of ZnO, ranging in thickness from 0.08 to 6 ~tm, have been prepared by r.f. sputtering on substrates of either quartz or glass under various deposition conditions and subsequently characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results from XRD indicate that the grain size increases from 0.01 to 0.5 t~m as the film thickness is increased from 0.08 to 6 lam for depositions at 65 °C and increases from 0.1 to 0.3 pm as the deposition temperature is increased from 65 to 480 °C for a constant film thickness of 2 ~tm, whereas the lattice strain and dislocation density decrease slightly under similar conditions. Results from SEM indicate that the particle size parallel to the plane of the film is approximately equal to the mean grain size perpendicular to the plane of the film, suggesting that growth proceeds by the nucleation of new grains rather than by the elongation of columnar grains in the growth direction. General observations indicate that microstructural parameters, such as grain size, grain shape and lattice strain, depend sensitively on the exact nature of the deposition conditions.
I. INTRODUCTION
Because thin films of ZnO are commonly used in modern solid state devices such as acoustic wave devices 1, photoconducting junction devices 2 and, recently, integrated gas sensing devices 3, it is important to optimize the desired material properties by careful control of microstructural parameters such as grain size, grain shape, lattice strain and dislocation density. These parameters, in turn, depend on the method of production and the exact nature of the deposition conditions. Thin films of ZnO are usually produced by either chemical vapor deposition 4' 5 or r.f. sputtering 6"7. Although films of acceptable quality may be produced routinely by either of these techniques, sputtering affords a greater degree of flexibility, since the chemical composition can be controlled accurately and the choice of substrate and temperature is not limited by the range of a specific chemical reaction. The resultant films are generally characterized by a fine grain size and a high degree of * Present address: Indian Association for the Cultivation of Science, Calcutta 700032, India. f Present address: Hewlett Packard Corporation, Fort Collins, CO 80525, U.S.A. 0040-6090/82/0000-0000/$02.75
© Elsevier Sequoia/Printed in The Netherlands
S. SEN, 1). J. LEARY, ('. I,. B A U E R
preferred orientation s'';, which are known to affect the electrical properties of r.f.sputtered Z n O thin films greatly 1°. The purpose of this particular investigation is to produce thin films of Z n O by r.f. sputtering as a function of various deposition variables, such as film thickness and substrate temperature, and to measure the resultant grain size, strain, dislocation density and related parameters by a combination of X-ray diffraction (XRD) and scanning electron microscopy (SEM). An investigation of this nature should provide a more quantitative basis for systematic variation of these parameters and thereby allow further optimization ot concomitant electrical properties of ZnO, especially for device applications. 2.
EXPERIMENTAL PRO('E1)URE
Thin films were produced from targets 6 in in diameter of 99.99 wt.0,, ZnO, obtained from the Materials Research Corporation in the form of pressed and sirltered (1200'C for 12 h) discs, using a Perkin-Elmer model 2400-6J sputtering system. This system contains a helium closed-cycle cryopump, capable of attaining base pressures of 5 nPa, and an r.f. power supply which is coupled, without external d.c. bias, to the anode (substrate) and cathode (target) through a tunable impedancematching network in order to minimize reflected r.f. power, i.e.d.c, charging of the target. In addition, the walls of the chamber and the substrate were maintained at equal (ground) potential in order to minimize backsputtering onto the substrate. The composition of the plasma was fixed at 11 at.!'4; O and 80 at.'~0 Ar by adjustment of the partial pressures before the commencement of sputtering and a (total) pressure of 3 Pa was maintained throughout the sputtering operation. Under these conditions, thin films were produced at a deposition rate of about 22 nm rain 1 at 80 W on substrates of either quartz or glass, positioned 8.5 cm from the target and maintained at constant temperatures ranging from 65 to 480'C. The substrate temperatures were measured in .situ by a thermocouple probe and the resulting film thicknesses were measured either by optical interferometry or standard profilometry. Further details concerning the sputtering operation and subsequent characterization of the concomitant electrical properties are reported elsewhere 11. Specimens were examined at room temperature with a horizontal Seeman .... Bohlin X-ray diffractometer operated at 30 kV and 25 mA with Cu K~ radiation filtered by a thin foil of nickel and collimated by 1.2 m m divergent and 0.2 mm receiving slits. Resolution of the diffraction peaks is ffchieved to _+0.0125' of 20 and the concomitant integral breadth is obtained by numerical integration of the area under a given peak, after correction for the background intensity, and subsequent division by the maximum peak intensity. The true integral breadth may then be obtained from the empirical expression L2' 13 Bt = Bm-Bi2/B~
(1)
where Bm and Bi denote the measured and the instrumental integral breadth respectively. Assuming that line broadening is due to both the small grain size and the non-uniform lattice strain within these grains, the mean grain size D and lattice strain ~:may then be extracted from the expression 13, 14 BI2D cos 0 m = )~Bt + 16e2D sin 0 m tan 0n,
(2)
X R D A N D SEM OF R . F . - S P U T T E R E D
ZnO
9
(where 2 and 0 m respectively denote the wavelength of the incident radiation and the value of the Bragg angle at maximum peak height) by substituting B t and 0 m for diffraction from the (0002) and (0004) planes and simultaneously solving for the corresponding values of D and ~. The corresponding values of the dislocation density p may then be obtained, assuming the absence of extensive polygonization or dislocation pile-ups, from the expression 15,16 p = K~/bD
(3)
where K is a dimensionless constant which depends on the strain distribution and b denotes the magnitude of the Burgers' vector. (Throughout this investigation, it was assumed that the strain distribution function is parabolic in nature, which yields a value for K of 7.76 14.) For very weak intensities, corresponding to very thin films (i.e. 0.08 gin), only diffraction from the (0002) planes could be measured accurately, even with larger divergent and receiving slits of 2.4 mm and 1.0 mm respectively. In this case, the true integral breadth is obtained under conditions identical with those described previously and D and e are computed from the expressions 13 O-
2
(4)
B t COS 0 m
and ---- B t cot 0 m
(5)
in which it is assumed that contributions to the line broadening may be separated into independent contributions stemming from the grain size and the lattice strain. However, it was only necessary to rely on eqns. (4) and (5) when multiple-order diffraction could not be observed, i.e. for very thin films. According to the aforementioned expressions, various microstructural parameters may be extracted from the diffracted intensity from known crystallographic planes as a function of 20. Specimens were also characterized by the deposition of a thin layer of gold on the film surface and subsequent examination in a J E O L JSM-35 scanning electron microscope with the stage tilted at about 45 ° from the incident electron beam in order to optimize depth resolution. In this manner, the spatial variation in surface topology may be determined and compared with the more indirect results from XRD. Results obtained from examination of ZnO by both XRD and SEM are presented in the following section. 3.
E X P E R I M E N T A L RESULTS
Since r.f.-sputtered films of ZnO exhibit strong preferred orientation along the [0001] crystallographic axis, detailed examination was limited to diffraction from (0002) and (0004) planes. This preferred orientation was observed for all films examined in the present investigation, regardless of film thickness and/or substrate temperature. Typical results for a standard sample of ZnO powder are presented in Figs. l(a) and l(b) for diffraction from (0002) and (0004) planes respectively. In this particular case, higher order reflections were used to compute values of the lattice parameters of a = 0.3252 nm and c = 0.5203 nm, in good agreement with the reported values of 0.3249 nm and 0.5205 nm respectively 17. Typical results for thin
10
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Fig. 1. V a r i a t i o n in the relative diffracted intensity I with twice the Bragg angle 20 for Z n O : (a) diffraction f r o m (0002) p h m e s of a reference powder a n d a 6 tam film deposited at 65 C: (b) d i f f r a c t i o n f r o m (IX)04) planes of a reference powder and a 6 lain film deposited at 65 ('. T h e m a x i m u m value of I [peak position) corresponds to values of 20 m of 34.56 a n d 7 2 . 6 9 for diffraction from the (00021 a n d (0004) planes
respectiveb,,
films of ZnO, deposited under the conditions described in Section 2 to a thickness of 6 lam, are also presented in Figs. l(a) and l(b) for diffraction from (0002) and (0004) planes respectively. Also, several specimens were doped with up to 1 at.?~, Pd, but neither the resulting grain size nor the lattice strain was significantly affected. The instrumental linebreadth B~ was determined from the standard samples of ZnO powder to be 0.0025 rad and 0.0032 rad for diffraction from the (0002) and (0004) planes respectively. These values are combined with the corresponding measured values according to eqn. (1) in order to extract the true linebreadth B, of
11
XRD AND SEM OF R.F.-SPUTTERED Z r l O
ZnO thin films prepared under various deposition conditions. Typical values for Bt range from 0.0113 rad and 0.0188 rad for the (0002) and (0004) planes respectively in thin films (0.20 ~m) to 0.0070 rad and 0.0163 rad respectively in thicker films (6 ~m). Quantitative values of the true linebreadth Bt, as extracted from eqn. (l), were obtained from diffraction spectra similar to those presented in Fig. 1 as a function of film thickness and substrate temperature. These data were inserted in eqn. (2) in order to separate the grain size and the lattice strain, and the results are presented in Figs. 2 and 3 as functions of film thickness and substrate temperature respectively. The dislocation density, as extracted from eqn. (3), was determined to range between 1 × 1011 c m c m - 3 for thin films to 3 x 101° c m c m - 3 for thicker films. Various films of ZnO were also examined by SEM, which provides a global view of the surface topology. Typical results are presented in Fig. 4 for films of thickness 0.20 pm, 1.3 pm and 6.0 ~tm, which are characterized by particle sizes of 0.01 tam, 0.07 pm and 0.6 pm respectively. These results, and others, are analyzed in Section 4. 06
]
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2
3
4
5
6
7
8 0
d (/~m)
Fig. 2. Variation in the mean grain size D (O) and lattice strain e (A) with thickness d for films of ZnO deposited onto either glass or quartz substrates maintained at 65 "C. 0"5 /
I
I
I
L
I
[
I0
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00
5
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500 400 T (°C)
i
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500
600
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Fig. 3. Variation in the mean grain size D (C)) and lattice strain e (A) with substrate temperature Tduring deposition for films of Z n O measuring 2 ~m in thickness.
12
[al
S. SEN, I). J. I,EARY, ('. L. BAUER
tb)
(cl Fig. 4. C h a r a c t e r i z a t i o n of surface m o r p h o l o g y by SEM for tilms of Z n O deposited o n t o substrates m a i n t a i n e d at 65 C : (a) particle size of 0.01 g m in a film of thickness 0.201am on a glass substrate: (b'~ particle size of 0.07 ~m in a film of thickness 1.3 gm on a glass substrate: (el particle size of 0.6 ~am in a film of thickness 6 g m on a q u a r t z substrate. [Magnifications: (a) 52 800 x : (b) 2 0 0 0 0 × : (c) 5280 × .)
4. DISCUSSION OF RESULTS
The strain associated with deposited thin films of Z n O seems to vary with substrate species, even under identical deposition conditions (~f Fig. 2). Since this strain is caused mainly by dislocations and other lattice defects, it may be concluded that Z n O films deposited onto quartz substrates contain a larger density of lattice defects than those deposited under similar conditions onto glass substrates. These defects may have been produced by dissimilar thermal expansion and contraction ot the Z n O and substrate during heating and cooling. Moreover, the larger strain associated with Z n O on quartz substrates may be explained by the fact that the thermal expansion coefficient ofpolycrystalline Z n O (3.2 x 10 " 'C ~) corresponds more closely to that of glass (8.9 x 10 6 ' C i) than to that of fused quartz (0.55×10 ~ C 1). According to Figs. 2 and 3, the grain size increases and the lattice strain decreases (slightly) with increasing film thickness and/or substrate temperature. These variations could be due to either evolution of the growth morphology or grain boundary movement during the deposition process. Since grain growth in thin films
XRD AND SEM OF R.F.-SPUTTERED Z n O
13
is suppressed by surface pinning TM and additions of up to 1 at.~o Pd did not noticeably affect the resultant grain size, it is more likely that the observed increases in grain size (and concomitant decreases in lattice strain) are a result of increasing nucleus size as the temperature of the substrate is increased and of the gradual coalescence of grains as the film thickness is increased. The fact that the films are highly strained compared with vapor-deposited films 19-21 may also provide a driving force for such grain refinement. The dislocation density in these films, ranging from 3 x 101° to 10 x 1010 cm cm - 3 is also rather high compared with that in vapor-deposited films 19-21 which, of course, is consistent with the large measured value of the lattice strain. Although the actual dislocation arrangement cannot be determined by either XRD or SEM, the mean distance between dislocations of less than 100 nm produces a highly strained matrix. It is probable that such a highly strained and defected matrix does not give rise to optimum electrical properties for device applications. Since the grain size in the present investigation was determined by diffraction from planes parallel to the film surface, D provides a measure of the mean (longitudinal) grain size perpendicular to the film surface whereas the particle size provides an upper limit of the (transverse) grain size parallel to the film surface. Therefore it may be concluded that grains are either equiaxed or only slightly elongated perpendicular to the film surface, i.e. in the growth direction. Although occasional columnar grains were indeed observed in films examined in a plane parallel to the growth direction, the fact that the transverse grain size is approximately equal to the particle size and varies in proportion to the film thickness with a ratio of about ! : 13 suggests that grains (on the average) are not greatly elongated in the growth direction. Similar r.f.-sputtered films of ZnO have been produced in both reactive and non-reactive conditions and subsequently characterized by reflection electron diffraction, XRD and SEM 8'2~'22. These results also indicate that the [0001] crystallographic directions are preferentially oriented perpendicular to the substrate surface, but in one case z2 the measured grain size was significantly smaller than the particle size. Disagreement on the ratio of grain size to particle size may be due to the fact that the contributions from grain size and strain were systematically separated in the present investigation as well as to differences in the substrate and deposition conditions, such as power level and deposition rate. Such differences could give rise to the reported variations in the microstructural parameters of ZnO films. 5. SUMMARIZING REMARKS
Thin films of ZnO, ranging in thickness from 0.08 to 6 gm, have been prepared by r.f. sputtering on substrates of either quartz or glass at temperatures ranging from 65 to 480 °C and subsequently characterized by XRD and SEM. Results from XRD indicate that the grain size increases from 0.01 to 0.5 gm as the film thickness is increased from 0.08 to 6 gm for depositions at 65 °C, and increases from 0.1 to 0.3 p,m as the deposition temperature is increased from 65 to 480 °C for a constant film thickness of 2 gm, whereas the lattice strain and dislocation density decrease slightly under similar conditions. Results from SEM indicate that the particle size parallel to the plane of the film is approximately equal to the mean grain size perpendicular to
14
S. SEN, 1), J. LEARY, ('. L. BAUER
the plane of the film, suggesting that growth proceeds by the nucleation of new grains rather than by the elongation of columnar grains in the growth direction. General observations indicate that microstructural parameters, such as the grain size, grain shape and lattice strain, depend sensitively on the exact nature of the deposition conditions. ACKNOWLEDGMENT
Support of this research by the Materials Research Laboratory Section, Division of Materials Research, National Science Foundation, under Grant DMR 78-24699 is gratefully acknowledged. REFERENCES
1 2 3 4 5 6 7 8
9 10 1I 12 13 14 15 16 17 18 19 20 21 22
B.T. Khnri-Yakub and G. S. Kino, Appl. Phys. Le;;., 25 (1974) 255. K. Wasa and S. Hayakawa, Jim. J. Appl. Phys., lO( 19711 1732. J, C. Yen, J. Vac. Sci. Technol., 12 (1975) 47. M. Kasuga and S. Ishihara, Jpn. J. Appl. Phys., 15 ( 1976} 1835. J. Aranovich, A. A r m a n d o and R. Bube, J. I'ac. Sci. Technol., 16 (1979) 994. G . A . Rozgonyi and W. J. Polito, Appl. Phys. Lett., 8 (1966) 220. M . L . Liberman and R. C, Medrud, J. Eh,ctrochenl. Sot., 116 (1969) 242. B . T . Khuri-Yakub, G. S. Kino and P. Galle, J. Appl. Phys., 46 (1975) 3266. H . W . Lehmann and R. Widmer, Jpn. J. Appl. Phys., Suppl. 2 (1974) 741. D.J. Leary, Ph,D. Thesis, Carnegie-Mellon University, 1979. D.J. Leafy and J. O. Barnes, J. Electrochem. Sot., 127(198(}) 1636. C . N . J . WagnerandE. N. Aqua, Adr.~'-RavAnal.,7(1963)46. S. Sen, S. K. ttalder and S. P. Sen Gupta, J. Phys. Soc. Jpn., 38 ( 1975i 1641. N . C . Halder and C. N. J. Wagner, Acta ('rystallogr., 20 (1966) 312. G . K . Williamson and R. E. Smallman, Philos. Ma~,,., 1 (1956) 34. A. Gangulee, J. Appl. Phys.,43(1972) 867. H.E. Swanson and E. Tatge, Natl. Bur. Stand. (U.S.), ('ire., 2 (1953) 25. C . L . Bauer, Can. Metall. Q., 13 (1974) 303. S. Sen, R. K. Nandi and S. P. Sen Gupta, Thin Solid Films, 48 (1978) 1. S. Sen, R. K. Nandi and S. P. Sen Gupta, Thin Solid Fih~ts, 51 (1978) 141. S. Sen and S. P. Sen Gupta, J. b2w. Sci. Technol., 16 (1979) 42. S. Maniv and Z. Zangvil, J. App/. Phvs'., 49 (1978) 2787.
CHARACTERIZATION OF R.F.-SPUTTERED ZnO THIN FILMS BY X-RAY D I F F R A C T I O N AND SCANNING ELECTRON MICROSCOPY S. SEN*, D. J. LEARYt AND C. L. BAUER
Center for the Joining of Materials, Carnegie-Mellon University, Pittsburgh, PA 15213 (U.S.A.) (Received October 2, 1981 ; accepted January 7, 1982)
Thin films of ZnO, ranging in thickness from 0.08 to 6 ~tm, have been prepared by r.f. sputtering on substrates of either quartz or glass under various deposition conditions and subsequently characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results from XRD indicate that the grain size increases from 0.01 to 0.5 t~m as the film thickness is increased from 0.08 to 6 lam for depositions at 65 °C and increases from 0.1 to 0.3 pm as the deposition temperature is increased from 65 to 480 °C for a constant film thickness of 2 ~tm, whereas the lattice strain and dislocation density decrease slightly under similar conditions. Results from SEM indicate that the particle size parallel to the plane of the film is approximately equal to the mean grain size perpendicular to the plane of the film, suggesting that growth proceeds by the nucleation of new grains rather than by the elongation of columnar grains in the growth direction. General observations indicate that microstructural parameters, such as grain size, grain shape and lattice strain, depend sensitively on the exact nature of the deposition conditions.
I. INTRODUCTION
Because thin films of ZnO are commonly used in modern solid state devices such as acoustic wave devices 1, photoconducting junction devices 2 and, recently, integrated gas sensing devices 3, it is important to optimize the desired material properties by careful control of microstructural parameters such as grain size, grain shape, lattice strain and dislocation density. These parameters, in turn, depend on the method of production and the exact nature of the deposition conditions. Thin films of ZnO are usually produced by either chemical vapor deposition 4' 5 or r.f. sputtering 6"7. Although films of acceptable quality may be produced routinely by either of these techniques, sputtering affords a greater degree of flexibility, since the chemical composition can be controlled accurately and the choice of substrate and temperature is not limited by the range of a specific chemical reaction. The resultant films are generally characterized by a fine grain size and a high degree of * Present address: Indian Association for the Cultivation of Science, Calcutta 700032, India. f Present address: Hewlett Packard Corporation, Fort Collins, CO 80525, U.S.A. 0040-6090/82/0000-0000/$02.75
© Elsevier Sequoia/Printed in The Netherlands
S. SEN, 1). J. LEARY, ('. I,. B A U E R
preferred orientation s'';, which are known to affect the electrical properties of r.f.sputtered Z n O thin films greatly 1°. The purpose of this particular investigation is to produce thin films of Z n O by r.f. sputtering as a function of various deposition variables, such as film thickness and substrate temperature, and to measure the resultant grain size, strain, dislocation density and related parameters by a combination of X-ray diffraction (XRD) and scanning electron microscopy (SEM). An investigation of this nature should provide a more quantitative basis for systematic variation of these parameters and thereby allow further optimization ot concomitant electrical properties of ZnO, especially for device applications. 2.
EXPERIMENTAL PRO('E1)URE
Thin films were produced from targets 6 in in diameter of 99.99 wt.0,, ZnO, obtained from the Materials Research Corporation in the form of pressed and sirltered (1200'C for 12 h) discs, using a Perkin-Elmer model 2400-6J sputtering system. This system contains a helium closed-cycle cryopump, capable of attaining base pressures of 5 nPa, and an r.f. power supply which is coupled, without external d.c. bias, to the anode (substrate) and cathode (target) through a tunable impedancematching network in order to minimize reflected r.f. power, i.e.d.c, charging of the target. In addition, the walls of the chamber and the substrate were maintained at equal (ground) potential in order to minimize backsputtering onto the substrate. The composition of the plasma was fixed at 11 at.!'4; O and 80 at.'~0 Ar by adjustment of the partial pressures before the commencement of sputtering and a (total) pressure of 3 Pa was maintained throughout the sputtering operation. Under these conditions, thin films were produced at a deposition rate of about 22 nm rain 1 at 80 W on substrates of either quartz or glass, positioned 8.5 cm from the target and maintained at constant temperatures ranging from 65 to 480'C. The substrate temperatures were measured in .situ by a thermocouple probe and the resulting film thicknesses were measured either by optical interferometry or standard profilometry. Further details concerning the sputtering operation and subsequent characterization of the concomitant electrical properties are reported elsewhere 11. Specimens were examined at room temperature with a horizontal Seeman .... Bohlin X-ray diffractometer operated at 30 kV and 25 mA with Cu K~ radiation filtered by a thin foil of nickel and collimated by 1.2 m m divergent and 0.2 mm receiving slits. Resolution of the diffraction peaks is ffchieved to _+0.0125' of 20 and the concomitant integral breadth is obtained by numerical integration of the area under a given peak, after correction for the background intensity, and subsequent division by the maximum peak intensity. The true integral breadth may then be obtained from the empirical expression L2' 13 Bt = Bm-Bi2/B~
(1)
where Bm and Bi denote the measured and the instrumental integral breadth respectively. Assuming that line broadening is due to both the small grain size and the non-uniform lattice strain within these grains, the mean grain size D and lattice strain ~:may then be extracted from the expression 13, 14 BI2D cos 0 m = )~Bt + 16e2D sin 0 m tan 0n,
(2)
X R D A N D SEM OF R . F . - S P U T T E R E D
ZnO
9
(where 2 and 0 m respectively denote the wavelength of the incident radiation and the value of the Bragg angle at maximum peak height) by substituting B t and 0 m for diffraction from the (0002) and (0004) planes and simultaneously solving for the corresponding values of D and ~. The corresponding values of the dislocation density p may then be obtained, assuming the absence of extensive polygonization or dislocation pile-ups, from the expression 15,16 p = K~/bD
(3)
where K is a dimensionless constant which depends on the strain distribution and b denotes the magnitude of the Burgers' vector. (Throughout this investigation, it was assumed that the strain distribution function is parabolic in nature, which yields a value for K of 7.76 14.) For very weak intensities, corresponding to very thin films (i.e. 0.08 gin), only diffraction from the (0002) planes could be measured accurately, even with larger divergent and receiving slits of 2.4 mm and 1.0 mm respectively. In this case, the true integral breadth is obtained under conditions identical with those described previously and D and e are computed from the expressions 13 O-
2
(4)
B t COS 0 m
and ---- B t cot 0 m
(5)
in which it is assumed that contributions to the line broadening may be separated into independent contributions stemming from the grain size and the lattice strain. However, it was only necessary to rely on eqns. (4) and (5) when multiple-order diffraction could not be observed, i.e. for very thin films. According to the aforementioned expressions, various microstructural parameters may be extracted from the diffracted intensity from known crystallographic planes as a function of 20. Specimens were also characterized by the deposition of a thin layer of gold on the film surface and subsequent examination in a J E O L JSM-35 scanning electron microscope with the stage tilted at about 45 ° from the incident electron beam in order to optimize depth resolution. In this manner, the spatial variation in surface topology may be determined and compared with the more indirect results from XRD. Results obtained from examination of ZnO by both XRD and SEM are presented in the following section. 3.
E X P E R I M E N T A L RESULTS
Since r.f.-sputtered films of ZnO exhibit strong preferred orientation along the [0001] crystallographic axis, detailed examination was limited to diffraction from (0002) and (0004) planes. This preferred orientation was observed for all films examined in the present investigation, regardless of film thickness and/or substrate temperature. Typical results for a standard sample of ZnO powder are presented in Figs. l(a) and l(b) for diffraction from (0002) and (0004) planes respectively. In this particular case, higher order reflections were used to compute values of the lattice parameters of a = 0.3252 nm and c = 0.5203 nm, in good agreement with the reported values of 0.3249 nm and 0.5205 nm respectively 17. Typical results for thin
10
s. SEN, 1). J. LEARY, (". 1,. BAUER
I 0Cl~
-T
-
-
/, 1
I
/
/ / .~ 6c
~FILM
5c / e 4c //
/
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2c
/; /~o~DER
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'
',
\
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.... 35
i 34
la)
20 (°)
5
4
a >, B ,2
L 713 (b)
72 2 0 (o)
Fig. 1. V a r i a t i o n in the relative diffracted intensity I with twice the Bragg angle 20 for Z n O : (a) diffraction f r o m (0002) p h m e s of a reference powder a n d a 6 tam film deposited at 65 C: (b) d i f f r a c t i o n f r o m (IX)04) planes of a reference powder and a 6 lain film deposited at 65 ('. T h e m a x i m u m value of I [peak position) corresponds to values of 20 m of 34.56 a n d 7 2 . 6 9 for diffraction from the (00021 a n d (0004) planes
respectiveb,,
films of ZnO, deposited under the conditions described in Section 2 to a thickness of 6 lam, are also presented in Figs. l(a) and l(b) for diffraction from (0002) and (0004) planes respectively. Also, several specimens were doped with up to 1 at.?~, Pd, but neither the resulting grain size nor the lattice strain was significantly affected. The instrumental linebreadth B~ was determined from the standard samples of ZnO powder to be 0.0025 rad and 0.0032 rad for diffraction from the (0002) and (0004) planes respectively. These values are combined with the corresponding measured values according to eqn. (1) in order to extract the true linebreadth B, of
11
XRD AND SEM OF R.F.-SPUTTERED Z r l O
ZnO thin films prepared under various deposition conditions. Typical values for Bt range from 0.0113 rad and 0.0188 rad for the (0002) and (0004) planes respectively in thin films (0.20 ~m) to 0.0070 rad and 0.0163 rad respectively in thicker films (6 ~m). Quantitative values of the true linebreadth Bt, as extracted from eqn. (l), were obtained from diffraction spectra similar to those presented in Fig. 1 as a function of film thickness and substrate temperature. These data were inserted in eqn. (2) in order to separate the grain size and the lattice strain, and the results are presented in Figs. 2 and 3 as functions of film thickness and substrate temperature respectively. The dislocation density, as extracted from eqn. (3), was determined to range between 1 × 1011 c m c m - 3 for thin films to 3 x 101° c m c m - 3 for thicker films. Various films of ZnO were also examined by SEM, which provides a global view of the surface topology. Typical results are presented in Fig. 4 for films of thickness 0.20 pm, 1.3 pm and 6.0 ~tm, which are characterized by particle sizes of 0.01 tam, 0.07 pm and 0.6 pm respectively. These results, and others, are analyzed in Section 4. 06
]
I
I
I
/
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12
05
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04
T
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E (on gloss)
O.t
4
2
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I
2
3
4
5
6
7
8 0
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Fig. 2. Variation in the mean grain size D (O) and lattice strain e (A) with thickness d for films of ZnO deposited onto either glass or quartz substrates maintained at 65 "C. 0"5 /
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600
7O8
Fig. 3. Variation in the mean grain size D (C)) and lattice strain e (A) with substrate temperature Tduring deposition for films of Z n O measuring 2 ~m in thickness.
12
[al
S. SEN, I). J. I,EARY, ('. L. BAUER
tb)
(cl Fig. 4. C h a r a c t e r i z a t i o n of surface m o r p h o l o g y by SEM for tilms of Z n O deposited o n t o substrates m a i n t a i n e d at 65 C : (a) particle size of 0.01 g m in a film of thickness 0.201am on a glass substrate: (b'~ particle size of 0.07 ~m in a film of thickness 1.3 gm on a glass substrate: (el particle size of 0.6 ~am in a film of thickness 6 g m on a q u a r t z substrate. [Magnifications: (a) 52 800 x : (b) 2 0 0 0 0 × : (c) 5280 × .)
4. DISCUSSION OF RESULTS
The strain associated with deposited thin films of Z n O seems to vary with substrate species, even under identical deposition conditions (~f Fig. 2). Since this strain is caused mainly by dislocations and other lattice defects, it may be concluded that Z n O films deposited onto quartz substrates contain a larger density of lattice defects than those deposited under similar conditions onto glass substrates. These defects may have been produced by dissimilar thermal expansion and contraction ot the Z n O and substrate during heating and cooling. Moreover, the larger strain associated with Z n O on quartz substrates may be explained by the fact that the thermal expansion coefficient ofpolycrystalline Z n O (3.2 x 10 " 'C ~) corresponds more closely to that of glass (8.9 x 10 6 ' C i) than to that of fused quartz (0.55×10 ~ C 1). According to Figs. 2 and 3, the grain size increases and the lattice strain decreases (slightly) with increasing film thickness and/or substrate temperature. These variations could be due to either evolution of the growth morphology or grain boundary movement during the deposition process. Since grain growth in thin films
XRD AND SEM OF R.F.-SPUTTERED Z n O
13
is suppressed by surface pinning TM and additions of up to 1 at.~o Pd did not noticeably affect the resultant grain size, it is more likely that the observed increases in grain size (and concomitant decreases in lattice strain) are a result of increasing nucleus size as the temperature of the substrate is increased and of the gradual coalescence of grains as the film thickness is increased. The fact that the films are highly strained compared with vapor-deposited films 19-21 may also provide a driving force for such grain refinement. The dislocation density in these films, ranging from 3 x 101° to 10 x 1010 cm cm - 3 is also rather high compared with that in vapor-deposited films 19-21 which, of course, is consistent with the large measured value of the lattice strain. Although the actual dislocation arrangement cannot be determined by either XRD or SEM, the mean distance between dislocations of less than 100 nm produces a highly strained matrix. It is probable that such a highly strained and defected matrix does not give rise to optimum electrical properties for device applications. Since the grain size in the present investigation was determined by diffraction from planes parallel to the film surface, D provides a measure of the mean (longitudinal) grain size perpendicular to the film surface whereas the particle size provides an upper limit of the (transverse) grain size parallel to the film surface. Therefore it may be concluded that grains are either equiaxed or only slightly elongated perpendicular to the film surface, i.e. in the growth direction. Although occasional columnar grains were indeed observed in films examined in a plane parallel to the growth direction, the fact that the transverse grain size is approximately equal to the particle size and varies in proportion to the film thickness with a ratio of about ! : 13 suggests that grains (on the average) are not greatly elongated in the growth direction. Similar r.f.-sputtered films of ZnO have been produced in both reactive and non-reactive conditions and subsequently characterized by reflection electron diffraction, XRD and SEM 8'2~'22. These results also indicate that the [0001] crystallographic directions are preferentially oriented perpendicular to the substrate surface, but in one case z2 the measured grain size was significantly smaller than the particle size. Disagreement on the ratio of grain size to particle size may be due to the fact that the contributions from grain size and strain were systematically separated in the present investigation as well as to differences in the substrate and deposition conditions, such as power level and deposition rate. Such differences could give rise to the reported variations in the microstructural parameters of ZnO films. 5. SUMMARIZING REMARKS
Thin films of ZnO, ranging in thickness from 0.08 to 6 gm, have been prepared by r.f. sputtering on substrates of either quartz or glass at temperatures ranging from 65 to 480 °C and subsequently characterized by XRD and SEM. Results from XRD indicate that the grain size increases from 0.01 to 0.5 gm as the film thickness is increased from 0.08 to 6 gm for depositions at 65 °C, and increases from 0.1 to 0.3 p,m as the deposition temperature is increased from 65 to 480 °C for a constant film thickness of 2 gm, whereas the lattice strain and dislocation density decrease slightly under similar conditions. Results from SEM indicate that the particle size parallel to the plane of the film is approximately equal to the mean grain size perpendicular to
14
S. SEN, 1), J. LEARY, ('. L. BAUER
the plane of the film, suggesting that growth proceeds by the nucleation of new grains rather than by the elongation of columnar grains in the growth direction. General observations indicate that microstructural parameters, such as the grain size, grain shape and lattice strain, depend sensitively on the exact nature of the deposition conditions. ACKNOWLEDGMENT
Support of this research by the Materials Research Laboratory Section, Division of Materials Research, National Science Foundation, under Grant DMR 78-24699 is gratefully acknowledged. REFERENCES
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9 10 1I 12 13 14 15 16 17 18 19 20 21 22
B.T. Khnri-Yakub and G. S. Kino, Appl. Phys. Le;;., 25 (1974) 255. K. Wasa and S. Hayakawa, Jim. J. Appl. Phys., lO( 19711 1732. J, C. Yen, J. Vac. Sci. Technol., 12 (1975) 47. M. Kasuga and S. Ishihara, Jpn. J. Appl. Phys., 15 ( 1976} 1835. J. Aranovich, A. A r m a n d o and R. Bube, J. I'ac. Sci. Technol., 16 (1979) 994. G . A . Rozgonyi and W. J. Polito, Appl. Phys. Lett., 8 (1966) 220. M . L . Liberman and R. C, Medrud, J. Eh,ctrochenl. Sot., 116 (1969) 242. B . T . Khuri-Yakub, G. S. Kino and P. Galle, J. Appl. Phys., 46 (1975) 3266. H . W . Lehmann and R. Widmer, Jpn. J. Appl. Phys., Suppl. 2 (1974) 741. D.J. Leary, Ph,D. Thesis, Carnegie-Mellon University, 1979. D.J. Leafy and J. O. Barnes, J. Electrochem. Sot., 127(198(}) 1636. C . N . J . WagnerandE. N. Aqua, Adr.~'-RavAnal.,7(1963)46. S. Sen, S. K. ttalder and S. P. Sen Gupta, J. Phys. Soc. Jpn., 38 ( 1975i 1641. N . C . Halder and C. N. J. Wagner, Acta ('rystallogr., 20 (1966) 312. G . K . Williamson and R. E. Smallman, Philos. Ma~,,., 1 (1956) 34. A. Gangulee, J. Appl. Phys.,43(1972) 867. H.E. Swanson and E. Tatge, Natl. Bur. Stand. (U.S.), ('ire., 2 (1953) 25. C . L . Bauer, Can. Metall. Q., 13 (1974) 303. S. Sen, R. K. Nandi and S. P. Sen Gupta, Thin Solid Films, 48 (1978) 1. S. Sen, R. K. Nandi and S. P. Sen Gupta, Thin Solid Fih~ts, 51 (1978) 141. S. Sen and S. P. Sen Gupta, J. b2w. Sci. Technol., 16 (1979) 42. S. Maniv and Z. Zangvil, J. App/. Phvs'., 49 (1978) 2787.