- Email: [email protected]
Fabrication of PbTiO3 thin films by laser metalorganic chemical vapor deposition
22% __ __ @
Nuclear Instruments
and Methods in Physics Research B 121 (1997) 408-411
NOMB
Beam Interactions with Meter&Is S Atoms
EISEVIER
Fabrication of PbTiO, thin films by laser metalorganic chemical vapor deposition Koji Tokita Materials
*,Fumio Okada
and Components Lahorutory, Jupun Energy Corporation.
3-17-35
NibMinumi.
Todushi Saituma 335, Jupun
Abstract PbTiO, thin films were fabricated from their parent metalorganic compounds, (C,H,),PbOCH,C(CH& and Ti(OiC3H7)4, by using a laser metalorganic chemical vapor deposition method with and without heating substrates. When the films were fabricated on Pt(lOO)/MgO(100) substrates heated to 773 K, irradiation from an ArF laser enhanced c-axis orientation of the films. This may result from the temperature rise of substrate surface by laser irradiation. In room temperature deposition of the films on MgO(l00) substrates, the metalorganic compounds were alternately introduced into the reactor aiming at layer by layer deposition of PbO and TiO, films. Although oxide films with stoichiometric composition of Pb and Ti were obtained even at room temperature, no crystalline structure was observed without thermally annealing the films after deposition. The annealed films had lattice constants of 0.393 nm for a-axis and 0.399 nm for c-axis. These values are close to those of the cubic structure.
1. Introduction Studies on fabrication of metal oxide thin films by the metalorganic chemical vapor deposition combined with laser irradiation (laser MOCVD) were started by Collins et al. in early 80’s. Fabrication of SiO, [I], ZnO 121, AlzO, [3] and other oxide films by using this method has already been reported. However, oxide films with good qualities have not been obtained without heating substrates, and the role and effect of laser irradiation have not been fully understood yet. The difficulties in this method can be summarized as follows: although accurate adjustment of deposition conditions is required to obtain stoichiometric films, laser irradiation sometimes changes the metal composition in complex films such as PbTiO, and PbZr,Ti , _ n03; effects of laser irradiation tend to fade out at high substrate temperature region where thermal reaction becomes dominant. As such, the effect of laser irradiation at high temperature remains obscure. In our earlier studies on room temperature deposition of oxide thin films by a laser MOCVD method, crystalline PbO and amorphous TiO, films were successfully obtained in the laser irradiated area of the substrates [4,5]. However, no crystalline PbTiO, films have been obtained unless the films are thermally annealed after room temperature deposition. In addition, laser effects in high temperature deposition were not investigated. In this research,
Corresponding author. [email protected] l
0168-583X/97/$17.00
Fax:
+ 8 I-48-442-1844;
email:
therefore, two kinds of experiments were conducted to complement the earlier results: one was fabrication of PbTiO, thin films by laser MOCVD method at the substrate temperature of 773 K to investigate the laser effect at high temperature region; and the other was room temperature deposition of the films by an improved laser MOCVD method in which the metalorganic compounds are alternately introduced into the reactor so that PbO and TiO, layers were deposited in turn. The results of these experiments will be reported in this paper.
2. Experimental The laser MOCVD apparatus and the representative experimental conditions are shown in Fig. I and Table I, respectively. A horizontal reactor in which gases flow parallel to the substrate surface was used in the experiments. Irradiation from an ArF laser (Lambda Physik, LPX2lOicc) was utilized perpendicularly to the substrates during fabrication of the films. Laser power was adjusted to 15 mJ/cm2, and the repetition rate of the laser was 33 Hz. The irradiated area of the substrates was 5 X 15 mm*. Metalorganic compounds, Ti(O-iC3H7)4 (tetraisopropoxy-titanium: ITIP) and (C2H5),PbOCH2C(CH,), (triethylpentoxy-lead: TEPOL), were sealed in stainless bottles and kept in thermostatic ovens to get appropriate vapor pressures. High purity Ar (purity of 99.9999%) was used as carrier and window-purge gas, and pure oxygen (purity of 99.9%) was used as oxidation gas. Flow rates of the
Copyright 0 1997 Elsevier Science B.V. All rights reserved
PfI’SO168-583X(96)00599-X
K. Tokitu. F. Okudu/Nucl.
In,%-. und Mcth. in Phys. Res. B 121 (1997) 408-411
Fio I. Schematic diagram of the apparatus for laser MOCVD exieriments (MFC: mass flow controller, TTIP: Ti(O-iC,H,),. TEPOL: (CzHs),PbOCH,C(CH,),).
gases were controlled by using mass flow controllers. In order to avoid condensation of the metalorganic compounds between the stainless bottles and reactor, transfer tubing was heated by using ribbon heaters. Pt(lO0) coated MgO(100) substrates of 25 X 25 mm2 in area and 0.5 mm in thickness were used in the experiments conducted at 773 K so that the electrical properties of the films could be measured in the future, while bare MgO(lO0) substrates were used in room temperature experiments. The substrates were placed on a SiO, substrate holder. Fabrication experiments were conducted in two discrete temperature regions: one was high temperature region, specifically at 773 K, where thermal decomposition of the metalorganic compounds occurs irrespective of laser irradiation; and the other was room temperature region where thermal decomposition of them could not be expected. The metalorganics and oxidation gases were mixed and continuously flown in high temperature experiments. The film quality of the laser irradiated and non-irradiated area were compared in these experiments. In the case of room temperature experiments, an improved MOCVD method was applied: TEPOL and TRIP were alternately introduced into the reactor so that PbO and TiO, layers were deposited in turn. Hirai et al. have already reported that epitaxial growth of PbTiO, films was
409
FioD. 2. Typical flow sequences of metalorganics. TEPOL and TRIP are alternately introduced, while O2 was continuously introduced into the reactor. An ArF excimer laser was irradiated all through the sequence.
achieved at relatively low temperature, 783 K, by using this methods [6]. Prior to deposition of PbO-TiO, multilayered films, growth rates of PbO and TiO, films were measured to determine an appropriate flow cycle of the source gases. In these measurements, TEPOL or ‘ITIP was continuously introduced to the reactor with 0, gas, and the laser was continuously irradiated at 33 Hz. Growth rates of PbO and TiO, were measured from the thickness of the films and from deposition period. Then an appropriate supply time of TFPOL and TTIP was estimated for monolayer deposition of PbO and TiO,. Typical sequences determined by this procedure are shown in Fig. 2. Most of film deposition were carried out over 120 introduction cycles. Some films were annealed in O2 atmosphere at 973 K after deposition. The thickness, composition and quality of the films were measured by a field emission scanning electron microscope (FESEM), an energy dispersive X-ray spectrometer (EDXI and an X-ray diffractometer (XRD), respectively.
3. Results and discussion Table I Representative
3.1. Fabrication experimental
Bath temperature TEPOL TTIP Carrier gas flow rate TEPOL I-TIP O? gas flow rate Reactor pressure Substrate materiel Substrate temperature Laser wavelength fluence repetition rate
conditions
of PbTiO,
films at 773 K
for the Laser MOCVD
324 K 322 K 150 cm3/min. 150 cm’/min. 500 cm’/min. 78- 150 Pa Pt(lOO)/Mg0(100), 773 K, RT 193 nm IO mJ/cm’ 33 Hz
Mg0(lCO)
In the case of fabrication at the substrate temperature of 773 K, perovskite PbTiO, films were obtained in both laser irradiated and non-irradiated area. Surface and cross sectional FESEM images of laser irradiated area are shown in Fig. 3a and b, respectively. As is seen in Fig. 3a, the rectangular grains well aligned on the surface are the characteristics of the films obtained at high substrate temperature. The size of each grain ranges from 0.1 to 0.4 mm. The cross sectional image, Fig. 3b, indicates that the surface of the film is smooth and that there is no void in the film. The growth rate is calculated to be 75 nm/hr from the thickness of the film and from fabrication period. Surface and cross sectional FESEM images of non-irradia-
V. LASER PROCESS/APPLICATIONS
K. Tokitu, F. Okada / Nucl. Instr. cd
410
Meth. in Phys. Rex 6 12 I 11997) 408-411 1
.5 ‘I
1
i=
-laser Irradiated area ......... mermal MOCVD area
E
0.6
20
21
22
23
24
25
2 theta (deg.) Fig. 4. X-ray diffraction patterns of laser irradiated and non-irradiated area(tbe area of thermal fabrication) of PbTiO, thin films. Only diffraction peaks from 6X31)and (hO0) were observed. Tbe solid and dashed lines represent the laser irradiated and non-irradiated area, respectively. Vertical axis is normalized by the peak intensity of (001). CuKa X-ray was used in the measurements.
0.3 ,u m
(b) cross section
Fig. 3. FESEM photographs of (a) surface and (b) cross-section of PbTiO, film fabricated on P~lOO)/Mg~lGO) substrate with an ArF excimer laser irradiation. Substrate temperature was kept at 773 K. The laser fluence and repetition rate are 1.5mJ/cm* and 33 Hz, respectively.
ted area were identical with the irradiated area, and the growth rate was the same. The atomic ratio measured by EDX also indicates that both area had the same composition, Pb/(Pb + Ti) = 0.57. All of these results suggest that photochemical reaction was minor in the film formation process. Laser irradiation in low temperature fabrication process usually causes change in morphology, composition and growth rate of the films mainly due to photochemical reaction of metalorganic compounds. However, there was no obvious traces of laser irradiation in surface morphology, growth rate, and composition of PbTiO, films fabricated at 773 K.
The influence of laser irradiation was barely seen in XRD patterns of the films. XRD patterns of the two area are shown in Fig. 4. Note that normalized peak intensity is used as vertical axis in the figure. The relative peak intensity of (100) is decreased by laser irradiation as is shown by the solid line. Although the narrow 28 region is shown in the figure to emphasize (001) and (100) peaks, all of the other peaks which appear in larger 28 region were also assigned to (001) and (hO0). Degrees of the c-axis orientation, which are defined as 1(001)/{1(100) + 1(001)), of laser irradiated and non-irradiated area are calculated to be 0.92 and 0.88, respectively. Laser irradiation actually increased the degree of c-axis orientation by 4%. Funakubo et al. have reported that c-axis orientation of PbTiO, films increases with increasing the substrate temperature, and that 5% increase in c-axis orientation was observed between the substrate temperature of 793 K and 893 K [7]. They concluded that degree of c-axis orientation was the function of thermal stress applied to the films by means of differences in thermal expansion between the films and substrates. Considering their results, it might be deduced that 4% increase observed in our experiments corresponds to the temperature rise of the substrate by 80 K. From these facts, we believe that laser irradiation results in temperature rise of the substrate surface, and that
Table 2 The growth rates of TiO, and PbO films and layers obtained by continuous or alternate introduction procedure of tbe precursors. M&100) substrates were irradiated by an ArF excimer laser at room temperature. The laser fluence and repetition rate were I5 mJ/cm* and 33 Hz, respectively. Deposited film
Precursor
Carrier gas flow rate (cm3/min)
Introduction sequence
Measured growth rate (nm/sJ
TiO, PbO TiO, layer in PbO-TiO, PbO layer in PbO-TiO,
TIIP TEPOL ITIP TEPOL
150 150 150 150
continuous continuous alternate ( IO s) alternate (20 s)
0.028 0.013 0.026 0.030
K. Tokitu. F. Ok&a /Nucl.
Instr. und Meth. in Phys. Res. B 121 (1997) 408-411
photochemical reaction is negligible in high temperature fabrication process. 3.2. Room temperature
deposition
When TEPOL and 0, were continuously introduced into the reactor with continuous irradiation of the laser at 33 Hz, PbO films were grown at the rate of 0.013 rim/s in room temperature experiments. Assuming that PbO monolayer is 0.25 nm in thickness, equivalent supply time of TEPOL for monolayer deposition of PbO was estimated to be 20 s. By using the same procedure, the supply time of TTIP for monolayer deposition of TiO, was determined to be 10 s. Then, a PbO-TiO, multilayered thin film was fabricated on MgO(lO0) substrate with the flow sequences shown in Fig. 2. However, the film composition, Pb/(Pb + Ti) = 0.7 I, was far from stoichiometry. The overall growth rate of this film was measured to be 0.83 nm/cycle. By using the film composition and the overall growth rate, individual growth rates of PbO and TiO, layers in PbOTiO, multilayered deposition process was calculated to be 0.030 rim/s and 0.026 rim/s,, respectively. The former rate is more than two times larger than that in PbO deposition by continuous introduction of TEPOL. In contrast, the latter rate was in good agreement with that of TiO, deposition in continuous introduction procedure. These results are summarized in Table 2. The origin of the difference in growth rates between alternate and continuous introduction procedures is not clear, but the followings might be the possible reasons: sticking probability of TEPOL on TiO, is larger than that on PbO; photo-catalytic dissociation of TEPOL is promoted on TiO, surface. PbO-TiO, multilayered films with stoichiometric composition were obtained by reducing the supply time of TEPOL. XRD pattern of the films indicated that as-deposited films were amorphous. However, crystalline films were obtained after annealing the amorphous ones at 973 K. An example of the grazing angle XRD pattern of the
41 I
films is shown in Fig. 5. The lattice constants of a- and c-axis of the film were measured to be 0.396 nm and 0.399 nm, respectively. The films have longer a-axis and shorter c-axis than the normal tetragonal structure which has aand c-axis of 0.390 and 0.415 nm, respectively. Therefore, the film is rather close to cubic structure of PbTiO,. Interdiffusion of Mg atoms into the PbTiO, film might be the possible origin of this deformation, but further investigation should be needed to clarify the origin.
4. Conclusion PbTiO, thin films were fabricated by laser MOCVD method with and without heating substrates. When the films were fabricated at 773 K, laser irradiation enhanced c-axis orientation. This may result from temperature rise of the substrate surface due to laser irradiation. In the case of deposition at room temperature, alternate introduction of TEPOL and TTIP were employed to achieve layer by layer deposition of PbO and TiO,. However, no crystalline films was obtained without thermally annealing the films. The annealed films had lattice constants of 0.396 nm for a-axis and 0.399 nm for c-axis indicating that the films were deformed and was close to the cubic structure.
Acknowledgements
This work was conducted under the program “Advanced Chemical Processing Technology”, consigned to the Advanced Chemical Processing Technology Research Association from the New Energy and Industrial Technology Development Organization, which is carried out under the Industrial Science and Technology Frontier Program enforced by the Agency of Industrial Science and Technology, the Ministry of International Trade and Industry.
References
2 theta (deg.) Fig. 5. X-ray diffraction pattern of the annealed film fabricated by alternate introduction procedure. The film was annealed at 973 K in 0, atmosphere for 3 h. Deformed PbTiO, thin film was obtained by this method. CoKa X-ray was used in the measurement.
[l] P. K. Boyer, G. A. Roche, W. H. Ritchie and G. J. Collins, Appl. Phys. Lett. 40 (I 982) 716. 121 R. Solanki and G. J. Collins, Appl. Phys. Lett. 42 (1983) 662. [3] R. Solanki, W. H. Ritchie and G. J. Collins, Appl. Phys. Lett. 43 (1983) 454. [4] K. Tokita, F. Okada and H. Segawa, Rev. Laser Eng. 23 ( 1995) 355 [in Japanese]. [S] K. Tokita and F. Okada, J. Appl. Phys., to be published. [61 T. Hirai, T. Goto, H. Matsuhashi, S. Tanimoto and Y. Tarui, Japan. J. Appl. Phys. 32 (1993) 4078. [7] M. Otsu, H. Funakubo, K. Shinozaki and N. Mizutani, in: Advanced Materials ‘93, l/B: Trans. Mat. Res. Sot. Japan., Vol. 14 B (Elsevier, Amsterdam, 1994) p. 42.
V. LASER PROCESS/APPLICATIONS
Nuclear Instruments
and Methods in Physics Research B 121 (1997) 408-411
NOMB
Beam Interactions with Meter&Is S Atoms
EISEVIER
Fabrication of PbTiO, thin films by laser metalorganic chemical vapor deposition Koji Tokita Materials
*,Fumio Okada
and Components Lahorutory, Jupun Energy Corporation.
3-17-35
NibMinumi.
Todushi Saituma 335, Jupun
Abstract PbTiO, thin films were fabricated from their parent metalorganic compounds, (C,H,),PbOCH,C(CH& and Ti(OiC3H7)4, by using a laser metalorganic chemical vapor deposition method with and without heating substrates. When the films were fabricated on Pt(lOO)/MgO(100) substrates heated to 773 K, irradiation from an ArF laser enhanced c-axis orientation of the films. This may result from the temperature rise of substrate surface by laser irradiation. In room temperature deposition of the films on MgO(l00) substrates, the metalorganic compounds were alternately introduced into the reactor aiming at layer by layer deposition of PbO and TiO, films. Although oxide films with stoichiometric composition of Pb and Ti were obtained even at room temperature, no crystalline structure was observed without thermally annealing the films after deposition. The annealed films had lattice constants of 0.393 nm for a-axis and 0.399 nm for c-axis. These values are close to those of the cubic structure.
1. Introduction Studies on fabrication of metal oxide thin films by the metalorganic chemical vapor deposition combined with laser irradiation (laser MOCVD) were started by Collins et al. in early 80’s. Fabrication of SiO, [I], ZnO 121, AlzO, [3] and other oxide films by using this method has already been reported. However, oxide films with good qualities have not been obtained without heating substrates, and the role and effect of laser irradiation have not been fully understood yet. The difficulties in this method can be summarized as follows: although accurate adjustment of deposition conditions is required to obtain stoichiometric films, laser irradiation sometimes changes the metal composition in complex films such as PbTiO, and PbZr,Ti , _ n03; effects of laser irradiation tend to fade out at high substrate temperature region where thermal reaction becomes dominant. As such, the effect of laser irradiation at high temperature remains obscure. In our earlier studies on room temperature deposition of oxide thin films by a laser MOCVD method, crystalline PbO and amorphous TiO, films were successfully obtained in the laser irradiated area of the substrates [4,5]. However, no crystalline PbTiO, films have been obtained unless the films are thermally annealed after room temperature deposition. In addition, laser effects in high temperature deposition were not investigated. In this research,
Corresponding author. [email protected] l
0168-583X/97/$17.00
Fax:
+ 8 I-48-442-1844;
email:
therefore, two kinds of experiments were conducted to complement the earlier results: one was fabrication of PbTiO, thin films by laser MOCVD method at the substrate temperature of 773 K to investigate the laser effect at high temperature region; and the other was room temperature deposition of the films by an improved laser MOCVD method in which the metalorganic compounds are alternately introduced into the reactor so that PbO and TiO, layers were deposited in turn. The results of these experiments will be reported in this paper.
2. Experimental The laser MOCVD apparatus and the representative experimental conditions are shown in Fig. I and Table I, respectively. A horizontal reactor in which gases flow parallel to the substrate surface was used in the experiments. Irradiation from an ArF laser (Lambda Physik, LPX2lOicc) was utilized perpendicularly to the substrates during fabrication of the films. Laser power was adjusted to 15 mJ/cm2, and the repetition rate of the laser was 33 Hz. The irradiated area of the substrates was 5 X 15 mm*. Metalorganic compounds, Ti(O-iC3H7)4 (tetraisopropoxy-titanium: ITIP) and (C2H5),PbOCH2C(CH,), (triethylpentoxy-lead: TEPOL), were sealed in stainless bottles and kept in thermostatic ovens to get appropriate vapor pressures. High purity Ar (purity of 99.9999%) was used as carrier and window-purge gas, and pure oxygen (purity of 99.9%) was used as oxidation gas. Flow rates of the
Copyright 0 1997 Elsevier Science B.V. All rights reserved
PfI’SO168-583X(96)00599-X
K. Tokitu. F. Okudu/Nucl.
In,%-. und Mcth. in Phys. Res. B 121 (1997) 408-411
Fio I. Schematic diagram of the apparatus for laser MOCVD exieriments (MFC: mass flow controller, TTIP: Ti(O-iC,H,),. TEPOL: (CzHs),PbOCH,C(CH,),).
gases were controlled by using mass flow controllers. In order to avoid condensation of the metalorganic compounds between the stainless bottles and reactor, transfer tubing was heated by using ribbon heaters. Pt(lO0) coated MgO(100) substrates of 25 X 25 mm2 in area and 0.5 mm in thickness were used in the experiments conducted at 773 K so that the electrical properties of the films could be measured in the future, while bare MgO(lO0) substrates were used in room temperature experiments. The substrates were placed on a SiO, substrate holder. Fabrication experiments were conducted in two discrete temperature regions: one was high temperature region, specifically at 773 K, where thermal decomposition of the metalorganic compounds occurs irrespective of laser irradiation; and the other was room temperature region where thermal decomposition of them could not be expected. The metalorganics and oxidation gases were mixed and continuously flown in high temperature experiments. The film quality of the laser irradiated and non-irradiated area were compared in these experiments. In the case of room temperature experiments, an improved MOCVD method was applied: TEPOL and TRIP were alternately introduced into the reactor so that PbO and TiO, layers were deposited in turn. Hirai et al. have already reported that epitaxial growth of PbTiO, films was
409
FioD. 2. Typical flow sequences of metalorganics. TEPOL and TRIP are alternately introduced, while O2 was continuously introduced into the reactor. An ArF excimer laser was irradiated all through the sequence.
achieved at relatively low temperature, 783 K, by using this methods [6]. Prior to deposition of PbO-TiO, multilayered films, growth rates of PbO and TiO, films were measured to determine an appropriate flow cycle of the source gases. In these measurements, TEPOL or ‘ITIP was continuously introduced to the reactor with 0, gas, and the laser was continuously irradiated at 33 Hz. Growth rates of PbO and TiO, were measured from the thickness of the films and from deposition period. Then an appropriate supply time of TFPOL and TTIP was estimated for monolayer deposition of PbO and TiO,. Typical sequences determined by this procedure are shown in Fig. 2. Most of film deposition were carried out over 120 introduction cycles. Some films were annealed in O2 atmosphere at 973 K after deposition. The thickness, composition and quality of the films were measured by a field emission scanning electron microscope (FESEM), an energy dispersive X-ray spectrometer (EDXI and an X-ray diffractometer (XRD), respectively.
3. Results and discussion Table I Representative
3.1. Fabrication experimental
Bath temperature TEPOL TTIP Carrier gas flow rate TEPOL I-TIP O? gas flow rate Reactor pressure Substrate materiel Substrate temperature Laser wavelength fluence repetition rate
conditions
of PbTiO,
films at 773 K
for the Laser MOCVD
324 K 322 K 150 cm3/min. 150 cm’/min. 500 cm’/min. 78- 150 Pa Pt(lOO)/Mg0(100), 773 K, RT 193 nm IO mJ/cm’ 33 Hz
Mg0(lCO)
In the case of fabrication at the substrate temperature of 773 K, perovskite PbTiO, films were obtained in both laser irradiated and non-irradiated area. Surface and cross sectional FESEM images of laser irradiated area are shown in Fig. 3a and b, respectively. As is seen in Fig. 3a, the rectangular grains well aligned on the surface are the characteristics of the films obtained at high substrate temperature. The size of each grain ranges from 0.1 to 0.4 mm. The cross sectional image, Fig. 3b, indicates that the surface of the film is smooth and that there is no void in the film. The growth rate is calculated to be 75 nm/hr from the thickness of the film and from fabrication period. Surface and cross sectional FESEM images of non-irradia-
V. LASER PROCESS/APPLICATIONS
K. Tokitu, F. Okada / Nucl. Instr. cd
410
Meth. in Phys. Rex 6 12 I 11997) 408-411 1
.5 ‘I
1
i=
-laser Irradiated area ......... mermal MOCVD area
E
0.6
20
21
22
23
24
25
2 theta (deg.) Fig. 4. X-ray diffraction patterns of laser irradiated and non-irradiated area(tbe area of thermal fabrication) of PbTiO, thin films. Only diffraction peaks from 6X31)and (hO0) were observed. Tbe solid and dashed lines represent the laser irradiated and non-irradiated area, respectively. Vertical axis is normalized by the peak intensity of (001). CuKa X-ray was used in the measurements.
0.3 ,u m
(b) cross section
Fig. 3. FESEM photographs of (a) surface and (b) cross-section of PbTiO, film fabricated on P~lOO)/Mg~lGO) substrate with an ArF excimer laser irradiation. Substrate temperature was kept at 773 K. The laser fluence and repetition rate are 1.5mJ/cm* and 33 Hz, respectively.
ted area were identical with the irradiated area, and the growth rate was the same. The atomic ratio measured by EDX also indicates that both area had the same composition, Pb/(Pb + Ti) = 0.57. All of these results suggest that photochemical reaction was minor in the film formation process. Laser irradiation in low temperature fabrication process usually causes change in morphology, composition and growth rate of the films mainly due to photochemical reaction of metalorganic compounds. However, there was no obvious traces of laser irradiation in surface morphology, growth rate, and composition of PbTiO, films fabricated at 773 K.
The influence of laser irradiation was barely seen in XRD patterns of the films. XRD patterns of the two area are shown in Fig. 4. Note that normalized peak intensity is used as vertical axis in the figure. The relative peak intensity of (100) is decreased by laser irradiation as is shown by the solid line. Although the narrow 28 region is shown in the figure to emphasize (001) and (100) peaks, all of the other peaks which appear in larger 28 region were also assigned to (001) and (hO0). Degrees of the c-axis orientation, which are defined as 1(001)/{1(100) + 1(001)), of laser irradiated and non-irradiated area are calculated to be 0.92 and 0.88, respectively. Laser irradiation actually increased the degree of c-axis orientation by 4%. Funakubo et al. have reported that c-axis orientation of PbTiO, films increases with increasing the substrate temperature, and that 5% increase in c-axis orientation was observed between the substrate temperature of 793 K and 893 K [7]. They concluded that degree of c-axis orientation was the function of thermal stress applied to the films by means of differences in thermal expansion between the films and substrates. Considering their results, it might be deduced that 4% increase observed in our experiments corresponds to the temperature rise of the substrate by 80 K. From these facts, we believe that laser irradiation results in temperature rise of the substrate surface, and that
Table 2 The growth rates of TiO, and PbO films and layers obtained by continuous or alternate introduction procedure of tbe precursors. M&100) substrates were irradiated by an ArF excimer laser at room temperature. The laser fluence and repetition rate were I5 mJ/cm* and 33 Hz, respectively. Deposited film
Precursor
Carrier gas flow rate (cm3/min)
Introduction sequence
Measured growth rate (nm/sJ
TiO, PbO TiO, layer in PbO-TiO, PbO layer in PbO-TiO,
TIIP TEPOL ITIP TEPOL
150 150 150 150
continuous continuous alternate ( IO s) alternate (20 s)
0.028 0.013 0.026 0.030
K. Tokitu. F. Ok&a /Nucl.
Instr. und Meth. in Phys. Res. B 121 (1997) 408-411
photochemical reaction is negligible in high temperature fabrication process. 3.2. Room temperature
deposition
When TEPOL and 0, were continuously introduced into the reactor with continuous irradiation of the laser at 33 Hz, PbO films were grown at the rate of 0.013 rim/s in room temperature experiments. Assuming that PbO monolayer is 0.25 nm in thickness, equivalent supply time of TEPOL for monolayer deposition of PbO was estimated to be 20 s. By using the same procedure, the supply time of TTIP for monolayer deposition of TiO, was determined to be 10 s. Then, a PbO-TiO, multilayered thin film was fabricated on MgO(lO0) substrate with the flow sequences shown in Fig. 2. However, the film composition, Pb/(Pb + Ti) = 0.7 I, was far from stoichiometry. The overall growth rate of this film was measured to be 0.83 nm/cycle. By using the film composition and the overall growth rate, individual growth rates of PbO and TiO, layers in PbOTiO, multilayered deposition process was calculated to be 0.030 rim/s and 0.026 rim/s,, respectively. The former rate is more than two times larger than that in PbO deposition by continuous introduction of TEPOL. In contrast, the latter rate was in good agreement with that of TiO, deposition in continuous introduction procedure. These results are summarized in Table 2. The origin of the difference in growth rates between alternate and continuous introduction procedures is not clear, but the followings might be the possible reasons: sticking probability of TEPOL on TiO, is larger than that on PbO; photo-catalytic dissociation of TEPOL is promoted on TiO, surface. PbO-TiO, multilayered films with stoichiometric composition were obtained by reducing the supply time of TEPOL. XRD pattern of the films indicated that as-deposited films were amorphous. However, crystalline films were obtained after annealing the amorphous ones at 973 K. An example of the grazing angle XRD pattern of the
41 I
films is shown in Fig. 5. The lattice constants of a- and c-axis of the film were measured to be 0.396 nm and 0.399 nm, respectively. The films have longer a-axis and shorter c-axis than the normal tetragonal structure which has aand c-axis of 0.390 and 0.415 nm, respectively. Therefore, the film is rather close to cubic structure of PbTiO,. Interdiffusion of Mg atoms into the PbTiO, film might be the possible origin of this deformation, but further investigation should be needed to clarify the origin.
4. Conclusion PbTiO, thin films were fabricated by laser MOCVD method with and without heating substrates. When the films were fabricated at 773 K, laser irradiation enhanced c-axis orientation. This may result from temperature rise of the substrate surface due to laser irradiation. In the case of deposition at room temperature, alternate introduction of TEPOL and TTIP were employed to achieve layer by layer deposition of PbO and TiO,. However, no crystalline films was obtained without thermally annealing the films. The annealed films had lattice constants of 0.396 nm for a-axis and 0.399 nm for c-axis indicating that the films were deformed and was close to the cubic structure.
Acknowledgements
This work was conducted under the program “Advanced Chemical Processing Technology”, consigned to the Advanced Chemical Processing Technology Research Association from the New Energy and Industrial Technology Development Organization, which is carried out under the Industrial Science and Technology Frontier Program enforced by the Agency of Industrial Science and Technology, the Ministry of International Trade and Industry.
References
2 theta (deg.) Fig. 5. X-ray diffraction pattern of the annealed film fabricated by alternate introduction procedure. The film was annealed at 973 K in 0, atmosphere for 3 h. Deformed PbTiO, thin film was obtained by this method. CoKa X-ray was used in the measurement.
[l] P. K. Boyer, G. A. Roche, W. H. Ritchie and G. J. Collins, Appl. Phys. Lett. 40 (I 982) 716. 121 R. Solanki and G. J. Collins, Appl. Phys. Lett. 42 (1983) 662. [3] R. Solanki, W. H. Ritchie and G. J. Collins, Appl. Phys. Lett. 43 (1983) 454. [4] K. Tokita, F. Okada and H. Segawa, Rev. Laser Eng. 23 ( 1995) 355 [in Japanese]. [S] K. Tokita and F. Okada, J. Appl. Phys., to be published. [61 T. Hirai, T. Goto, H. Matsuhashi, S. Tanimoto and Y. Tarui, Japan. J. Appl. Phys. 32 (1993) 4078. [7] M. Otsu, H. Funakubo, K. Shinozaki and N. Mizutani, in: Advanced Materials ‘93, l/B: Trans. Mat. Res. Sot. Japan., Vol. 14 B (Elsevier, Amsterdam, 1994) p. 42.
V. LASER PROCESS/APPLICATIONS