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The effect of substrate temperature and rf power on the growth rate and the orientation of ZnO thin films prepared by plasma enhanced chemical vapor deposition
November 1994
Materials Letters 21 (1994) 35 l-356
The effect of substrate temperature and rf power on the growth rate and the orientation of ZnO thin films prepared by plasma enhanced chemical vapor deposition Young Jin Kim a, Hyeong Joon Kim b aDepartment ~j,~ater~als Engineering, Kyonggi university, Suwon 44@ 760, Kyonggi-Da, South Korea b Department ojlnorganic Materials Engineering, Seoul National University, Seouf i5i- 742, South Korea Received 5 May 1994; in final form 3 1 August 1994; accepted 1 September I994
Abstract ZnO thin films were deposited on glass and p-Si( 100) wafers using the metalorganic source, diethylzinc (Zn(C2H5)2),and N20 gas by plasma enhanced CVD (PECVD). The growth rate, crystal perfection and c-axis orientation of ZnO thin films deposited under various conditions were determined using SEM, XRD and X-ray rocking curve. The substrate temperature and rfpower density ranged from 100 to 300°C and from 50 W (0.16 W/cm2) to 250 W (0.80 W/cm*), respectively. The degree of the c-axis preferred orientation of ZnO thin films was governed by the rf input power at the same substrate temperature. The growth rate was affected by both substrate temperature and &input power. The activation energies to deposit ZnO thin films at an rf input power of 200 and 250 W were 3.1 and 1.9 kJ/moI, respectively. Even at a low temperature of 2OO”C,a high degree of c-axis preferred orientation of ZnO thin films was achieved at an rfpower of 200 W.
1. Introduction Since zinc oxide (ZnO), which is a well known ntype semiconductor with a band gap of 3.1-3.3 eV, possesses high piezoelectricity as well as high optical transparency in the visible light range, it is widely used as substrate material in surface acoustic wave (SAW) devices and also as window material in opto-electronic devices and solar cells. For these applications, ZnO thin films should be used rather than ZnO ceramics. The typical fabrication methods of ZnO thin films are sputtering and thermal chemical vapor deposition (CVD). However, the former has the disadvantage of film surface bombardment by energetic particles during processing and the latter a problem of high substrate temperature. To overcome these prob0167-577x/94/$07.00
lems, metalorganic PECVD [ 1,2] and photo-CVD [3] have been recently attempted for the low-temperature CVD process. Shiosaki et al. [ 1,2 ] reported that c-axis oriented ZnO thin films were deposited using metalorganic sources by PECVD even at temperatures as low as 200°C but the structural transition and the effects of substrate temperature and rf power on growth rate were not sufficiently investigated. In this paper, we have deposited highly oriented ZnO thin films on glass and p-type Si wafers using diethylzinc and N20 by PECVD at temperatures as low as 200°C. The effect of the substrate temperature and rf input power on the growth rates and orientation of ZnO thin films are investigated and the deposition mechanism and structural change are also discussed.
0 1994 Elsevier Science B.V. All rights reserved
SSD~Ol67-577x(94)00196-0
2. Experimental Diethylzin~ (DEZ, ( C2Hs)2Zn, 99.999%), N20 gas (99.999%) and Ar gas (99.999%) were used as zinc source, oxidizer and carrier gas, respectively. Corning 7059 glass and p-Si ( 100) wafers with a size of 1.5 cmX5 cm were used as substrates. The schematic diagram of PECVD system, which was employed to deposited ZnO thin films, is shown in Fig. 1. DEZ vapor was carried through a manifold to the reactor by Ar gas, while N20 gas was introduced through many holes in the bottom electrode to restrict the prereaction of these gases. A bottle of DE2 was immersed in an ice bath to keep constant at a vapor pressure of 5 mm Hg. For a low Ar gas flow rate and high N20 gas flow rate, most of ZnO thin films were amorphous or crystalline having nonuniform thickness. On the other hand, black powder appears on substrates in high Ar gas flow rate above 350 seem due to lack of oxygen. However, we could experimentally obtain clean and uniform films at the gas flow rate of Ar/N,O=300/25 seem, and thus the gas ratio was fixed at this value in our experiment. The substrate temperature was changed from 100 to 300°C and the rf input power (power density) was varied from 50 W (0.16 W/cmz) to 250 W (0.80 W/cm*). The t-f frequency was 13.56 MHz. Substrates were located at 2 cm away from the center of the holder, which was rotated in 5 rpm to achieve uniform film thickness. The pressure was kept at 0.8 Torr by adjusting a rough valve during deposition. The electrode distance was 3.5 cm. We have determined the orientation and the crystal perfection of the deposited ZnO thin films by
measuring the main peaks of ZnO, ( 10 1), (002) and ( 100)) with X-ray diffractometer (Cu Ka, ,I= 1.5405 A) in a rotating diffraction angle 28 from 25” to 40”. The degree of preferred orientation to the c-axis was evaluated by an X-ray rocking curve with a peak of (OO2), rotating 8 from 0” to 35” at 26~ 34.4”. A scanning electron microscope (25 kV) was employed to observe the morphology of films, and samples were vertically mounted on the holder to examine simultaneously both the surface and the cross section of films. The thickness of samples was measured by a stylus profiler.
3. Results and discussion Fig. 2 shows XRD patterns of as-deposited ZnO films deposited on glass as a function of substrate temperature at different rf input powers. As the rfinput power increased above 200 W, crystalline ZnO films appeared even at temperatures as low as 150°C. For the low rf power range of 50-150 W, as-deposited films were amorphous at substrate temperatures below 2OO”C, but above 200°C they were transformed into the crystalline phase, of which the XRD pattern showed the chara~te~stic peaks of ZnO, (002) and (101) at 28~ 34.4” and 36.3”, respectively. No additional diffraction peaks appeared when the diffraction angles ranged from 25” to 40”. Both therms and plasma energies are available to decompose the reactant gases into atoms and enable atoms to move to their stable sites on the substrate surface. To achieve perfect crystallization and preferred orientation, atoms require enough energy to move on
Ar
Fig. 1. Schematic diagram of the PECVD system.
353
Y. J. Kim, H. J. Kim /Materials Letters 21(1994) 351-356
ZnO/Glass 2oow
40 30
28
40 30
40 30
40
Fig. 2. X-ray diffraction patterns of ZnO films deposited on glass as a function of temperature at different input rf powers: (a) 50 W, (b) 100 W, (c) 150 W, (d) 200 Wand (e) 250 W.
the substrate surface. Therefore, if sufficient rf power is applied, a high degree of crystal perfection can be achieved even at low substrate temperatures. Other investigators have achieved the c-axis orientation of ZnO films at the high temperature range of 300450°C without rf power using atmosphere pressure CVD (APCVD) [4,5,7] or low pressure CVD(LPCVD) [6,8]. But in this experiment, the crystalline ZnO film was deposited at 1SO’%,even if it was consisted of very fine crystallites and its crystal perfection was not good as noted below. This low temperature process could be achieved by adopting PECVD. Since the X-ray rocking curve of the (002) peak represents a Gaussian distribution of the c-axis of crystallites normal to the substrate, we can evaluate the degree of c-axis orientation of ZnO thin films by measuring the standard deviation (CT)of the X-ray rocking curve. Figs. 3 and 4 show the standard deviation of the X-ray rocking curve as a function of substrate temperature and rf input power, respectively. a decreases as the substrate temperature increases, indicating that the higher substrate temperature also provides atoms with enough energy to move and thus produces highly c-axis oriented films. However, at a constant temperature, cr decreases with increasing rf power up to 200 W, but it increases beyond 200 W. Initial decrease of (Tat the low rf power range is due to the better c-axis orientation, while the subsequent increase of ff is believed to originate from the damage
Sub. Temperature(
“C)
Fig. 3. Standard deviation (a) of X-ray rocking curve of (002) peak of ZnO thin film on glass as a function of substrate temperature at a constant rfpower of 200 W.
RF POWER(w) Fig. 4. Standard deviation ( u) of X-ray rocking curve of (002) peak of ZnO thin film on glass as a function of input power at a substrate temperature of 200°C.
of ZnO thin films by highly energetic ions at the high rf power region. The growth rate of ZnO films on Si substrate is shown in Fig. 5 as a function of substrate temperature. The growth behavior of ZnO thin films on glass substrate was the same as that on Si substrate and is not shown here. The temperature dependence of growth rate showed different behavior at each rf power. At high rf powers, 200 and 250 W, the temperature dependence of growth rate was similar: the highest growth rate was shown at the lowest temperature of 100°C. Such abnormally high apparent growth rate is due to the spurious growth of hexago-
Y. J. Kim, H. J. Kim /Materials Letters 21(1994) 351-356
354 300
I
I
I
I
025OW
1
I
l lOOW
200
100
%
i!z,r, 0
, , , ,j ~
200
100
Sub.
300
Temperature(‘C)
Fig. 5. The growth rate of ZnO thin films on p-Si ( I 00) as a function of substrate temperature at different rfpowers.
nal needles out of the growth plane, as shown in Fig. 6a. Lau et al. [6] also observed the spurious growth and explained that it was resulted from excess DEZ complex molecules. DEZ is a typical electron-deficient compound in that the number of low-lying orbitals available for bonding (four) is greater than the number of bonding electron pairs (two). This presence of vacant orbitals available for bonding explains the tendency of DEZ to form complexes with compounds containing hetero atoms with free electron pairs (0, N, P, S, etc). Thus, DEZ readily forms coordination complexes with appropriate electron-donating ligands [ 9 J. DEZ forms complexes at a temperature near ambient or higher with H20, CO2 and N,O. The reaction of DEZ and N,O is [ 6 ] : low temperature ’
(C2H5Mn+N20
“complex” ,
P ?OOT
“complex” -
ZnO~gaseous products .
In this experiment, eventhough extra energy by high rfpower was induced in order to decompose the DEZ complex even at low substrate temperatures, it was impossible to do that because of low total energy. But the spurious growth did not occurred by increasing substrate temperature more than 100°C at the same if power. The high growth rate at the low temperature region decreased to the lowest value with increasing the substrate temperature up to 150°C. Such a decrease in the growth rate @as considered to take place due to
Fig. 6. SEM micrographs ofZn0 thin films: (a) spurious growth surface at low substrate temperature, and well-defined columnar structures with (b) smooth surface morphology and (c) rough surface morphology.
the structural transition from the amorphous phase to crystalline phase. The subsequent gentle increase in the growth rate was due to more decomposition of source materials by higher thermal energy at high substrate temperatures. In this temperature range
Y, J. Kim, H. J. Kim /Materials Letters 21 (1994) 351-356
ZnO thin films had a well-developed columnar structure with smooth surface mo~hology, as shown in Fig. 6b. At an rf power of 150 W, the temperature dependence of growth rate was similar to that at high rfpower. But the lowest growth rate occurred at higher temperature, 200°C than that at high rf power condition. That is to say, the transition temperature, at which the lowest growth rate was observed due to the phase transition, was increased from 150 to 200°C by reducing rf power to 150 W. On the other hand, at low rf power of 50 and 100 W, the growth rate continued to decrease with increasing substrate temperature due to the phase transition from the amorphous to crystalline phase. But the growth rate was expected to increase above 200°C since the crystalline ZnO film was already formed above that temperature. Nevertheless it continuously decreased without an increase. The crystalline film deposited at 200°C had less crystal perfection than that at 250°C as shown in Fig. 6c. Therefore, it required more densification and alignment, which resulted in the decrease of the growth rate. For each rf power, the decrease in the growth rate at 300°C is observed and is believed to originate from gas-phase homogeneous nucleation. Formation of white powder on the reactor wall gave indirect evidence of homogeneous nucleation in the gas phase in this deposition condition, Similar results relating to the transition temperature have been reported by Smith [ 5 ] and Souletie et al. [ 7 1. The former author reported that the growth rate had the lowest point at 375°C. Below the lowest point the deposited thin films had rather random structure with both ( 10 1) and (002) orientations, but above this point the deposited films had the c-axis orientation: the (002) dominant orientation. This transition was also accompanied by a change in the surface morphology. The latter also showed the same results except the lowest growth rate at 350°C. Both results showed higher transition temperatures than those observed in this study. This discrepancy is believed to originate from the different deposition methods: they employed APCVD or LPCVD, while PECVD was used in this experiment. In PECVD, extra plasma energy reduced the transition temperature as well as the deposition temperature. Fig. 7 shows the growth rate of ZnO thin films on p-Si ( 100) as a function of rf-input power at different substrate temperatures. The changes in the growth rate due to structural tran-
I
355
1
0
1
J
I
I
100
200
300
RF Power(W) Fig. 7. The growth rate of ZnO thin fitms on p-Sit 100) as a function of rfinput power at different substrate temperatures,
Temperature 300 I
(‘C)
250
200
I
I
150 I=
1
0
zoow
A 25OW 0
1.5
1.75
2
Temperature(
2.25
2.5
1000/K)
Fig. 8. The Arrhenius plot of growth rate as a function of temperature at rfpowers of 200 and 250 W.
sition are also shown in Fig. 7. The drop of the growth rate at the substrate temperatures of I50 and 200°C occurred at the rf powers of 150 and 200 W, respectively. The structural change of the films occurred at high rf powers at low substrate temperatures, but not at any rf powers at the high substrate temperature. These results are consistent with those from XRD. The Arrhenius plot of the growth rate is shown in Fig. 8 as a function of the growth temperature. The measured activation energies were 3.1 and 1.9 kJ/mol for 200 and 250 W, respectively. These values are
3.56
Y. .I. Kim, H. J. Kim /Materids Letters 21(1994) 351-356
much lower than those reported previously, 48.2 and 67.5 kJ/mol for APCVD [7] and 30.0 kJ/mol for LPCVD [ 81, but these ones are similar to the value 3.4 W/mol for PECVD reported by Shim&u [ 21. The activation energy decreases with increasing the rf input power. It suggests that rfpower affected both decomposition of sources and mobility of atoms on the substrate.
energetic ions. Growth rate was closely related with both the substrate temperature and the rfinput power. Significant changes occurred in the growth rate when the deposited film structurally changed from the amo~hous phase to crystalline phase. The activation energy for reaction was reduced with increasing the input rfpower. References
4. Conclusions c-axis oriented ZnO thin films were deposited even at temperatures as low as 200°C using DE2 and N20 by PECVD. At the low substrate temperature the crystal perfection of ZnO thin films was also significantly affected by the input rf power, The degree of the c-axis orientation of the ZnO thin film was improved with increasing the substrate temperature as well as the input rf power. But at a constant temperature, high rf powers beyond 200 W deteriorate the preferred c-axis orientation of ZnO films due to the damage by highly
[ 11T. Shiosaki and T. Yamamato, Appl. Phys. Letters 39 ( 1981) 399. [2] M. Shimizu, Y. Matsue~, T. Shiosaki and A. Kawabata, J. Cryst. Growth 71 (1985) 209. [3] M. Shimizu, H. Kame, M. Tanizawa, T. Shiosaki and A. Kawabata, J. Cryst. Growth 89 (1988) 365. [4] SK. Ghandhi and R.J. Field, Appl. Phys. Letters 37 (1980) 449. [5] F.T.J. Smith, Appl. Phys. Letters 43 (1983) 1108. [ 61 C.K. Lau, S.K. Tiku and K.M. Lakin, J. Electrochem. SOC. 127 (1980) 1843. [7] P. Souletie and B.W. Wessels, J. Mater. Res. 3 (1988) 740. [8] A.P. Roth and D.F. Williams, J. Eiectrochem. Sot. 128 ( 198 1) 2684. [9] J.G. Nolts, in: Zinc chemicals, eds. M. Farnsworth and C.H. Kline (Zinc Institute Inc., New York, 1973) pp. 212-214.
Materials Letters 21 (1994) 35 l-356
The effect of substrate temperature and rf power on the growth rate and the orientation of ZnO thin films prepared by plasma enhanced chemical vapor deposition Young Jin Kim a, Hyeong Joon Kim b aDepartment ~j,~ater~als Engineering, Kyonggi university, Suwon 44@ 760, Kyonggi-Da, South Korea b Department ojlnorganic Materials Engineering, Seoul National University, Seouf i5i- 742, South Korea Received 5 May 1994; in final form 3 1 August 1994; accepted 1 September I994
Abstract ZnO thin films were deposited on glass and p-Si( 100) wafers using the metalorganic source, diethylzinc (Zn(C2H5)2),and N20 gas by plasma enhanced CVD (PECVD). The growth rate, crystal perfection and c-axis orientation of ZnO thin films deposited under various conditions were determined using SEM, XRD and X-ray rocking curve. The substrate temperature and rfpower density ranged from 100 to 300°C and from 50 W (0.16 W/cm2) to 250 W (0.80 W/cm*), respectively. The degree of the c-axis preferred orientation of ZnO thin films was governed by the rf input power at the same substrate temperature. The growth rate was affected by both substrate temperature and &input power. The activation energies to deposit ZnO thin films at an rf input power of 200 and 250 W were 3.1 and 1.9 kJ/moI, respectively. Even at a low temperature of 2OO”C,a high degree of c-axis preferred orientation of ZnO thin films was achieved at an rfpower of 200 W.
1. Introduction Since zinc oxide (ZnO), which is a well known ntype semiconductor with a band gap of 3.1-3.3 eV, possesses high piezoelectricity as well as high optical transparency in the visible light range, it is widely used as substrate material in surface acoustic wave (SAW) devices and also as window material in opto-electronic devices and solar cells. For these applications, ZnO thin films should be used rather than ZnO ceramics. The typical fabrication methods of ZnO thin films are sputtering and thermal chemical vapor deposition (CVD). However, the former has the disadvantage of film surface bombardment by energetic particles during processing and the latter a problem of high substrate temperature. To overcome these prob0167-577x/94/$07.00
lems, metalorganic PECVD [ 1,2] and photo-CVD [3] have been recently attempted for the low-temperature CVD process. Shiosaki et al. [ 1,2 ] reported that c-axis oriented ZnO thin films were deposited using metalorganic sources by PECVD even at temperatures as low as 200°C but the structural transition and the effects of substrate temperature and rf power on growth rate were not sufficiently investigated. In this paper, we have deposited highly oriented ZnO thin films on glass and p-type Si wafers using diethylzinc and N20 by PECVD at temperatures as low as 200°C. The effect of the substrate temperature and rf input power on the growth rates and orientation of ZnO thin films are investigated and the deposition mechanism and structural change are also discussed.
0 1994 Elsevier Science B.V. All rights reserved
SSD~Ol67-577x(94)00196-0
2. Experimental Diethylzin~ (DEZ, ( C2Hs)2Zn, 99.999%), N20 gas (99.999%) and Ar gas (99.999%) were used as zinc source, oxidizer and carrier gas, respectively. Corning 7059 glass and p-Si ( 100) wafers with a size of 1.5 cmX5 cm were used as substrates. The schematic diagram of PECVD system, which was employed to deposited ZnO thin films, is shown in Fig. 1. DEZ vapor was carried through a manifold to the reactor by Ar gas, while N20 gas was introduced through many holes in the bottom electrode to restrict the prereaction of these gases. A bottle of DE2 was immersed in an ice bath to keep constant at a vapor pressure of 5 mm Hg. For a low Ar gas flow rate and high N20 gas flow rate, most of ZnO thin films were amorphous or crystalline having nonuniform thickness. On the other hand, black powder appears on substrates in high Ar gas flow rate above 350 seem due to lack of oxygen. However, we could experimentally obtain clean and uniform films at the gas flow rate of Ar/N,O=300/25 seem, and thus the gas ratio was fixed at this value in our experiment. The substrate temperature was changed from 100 to 300°C and the rf input power (power density) was varied from 50 W (0.16 W/cmz) to 250 W (0.80 W/cm*). The t-f frequency was 13.56 MHz. Substrates were located at 2 cm away from the center of the holder, which was rotated in 5 rpm to achieve uniform film thickness. The pressure was kept at 0.8 Torr by adjusting a rough valve during deposition. The electrode distance was 3.5 cm. We have determined the orientation and the crystal perfection of the deposited ZnO thin films by
measuring the main peaks of ZnO, ( 10 1), (002) and ( 100)) with X-ray diffractometer (Cu Ka, ,I= 1.5405 A) in a rotating diffraction angle 28 from 25” to 40”. The degree of preferred orientation to the c-axis was evaluated by an X-ray rocking curve with a peak of (OO2), rotating 8 from 0” to 35” at 26~ 34.4”. A scanning electron microscope (25 kV) was employed to observe the morphology of films, and samples were vertically mounted on the holder to examine simultaneously both the surface and the cross section of films. The thickness of samples was measured by a stylus profiler.
3. Results and discussion Fig. 2 shows XRD patterns of as-deposited ZnO films deposited on glass as a function of substrate temperature at different rf input powers. As the rfinput power increased above 200 W, crystalline ZnO films appeared even at temperatures as low as 150°C. For the low rf power range of 50-150 W, as-deposited films were amorphous at substrate temperatures below 2OO”C, but above 200°C they were transformed into the crystalline phase, of which the XRD pattern showed the chara~te~stic peaks of ZnO, (002) and (101) at 28~ 34.4” and 36.3”, respectively. No additional diffraction peaks appeared when the diffraction angles ranged from 25” to 40”. Both therms and plasma energies are available to decompose the reactant gases into atoms and enable atoms to move to their stable sites on the substrate surface. To achieve perfect crystallization and preferred orientation, atoms require enough energy to move on
Ar
Fig. 1. Schematic diagram of the PECVD system.
353
Y. J. Kim, H. J. Kim /Materials Letters 21(1994) 351-356
ZnO/Glass 2oow
40 30
28
40 30
40 30
40
Fig. 2. X-ray diffraction patterns of ZnO films deposited on glass as a function of temperature at different input rf powers: (a) 50 W, (b) 100 W, (c) 150 W, (d) 200 Wand (e) 250 W.
the substrate surface. Therefore, if sufficient rf power is applied, a high degree of crystal perfection can be achieved even at low substrate temperatures. Other investigators have achieved the c-axis orientation of ZnO films at the high temperature range of 300450°C without rf power using atmosphere pressure CVD (APCVD) [4,5,7] or low pressure CVD(LPCVD) [6,8]. But in this experiment, the crystalline ZnO film was deposited at 1SO’%,even if it was consisted of very fine crystallites and its crystal perfection was not good as noted below. This low temperature process could be achieved by adopting PECVD. Since the X-ray rocking curve of the (002) peak represents a Gaussian distribution of the c-axis of crystallites normal to the substrate, we can evaluate the degree of c-axis orientation of ZnO thin films by measuring the standard deviation (CT)of the X-ray rocking curve. Figs. 3 and 4 show the standard deviation of the X-ray rocking curve as a function of substrate temperature and rf input power, respectively. a decreases as the substrate temperature increases, indicating that the higher substrate temperature also provides atoms with enough energy to move and thus produces highly c-axis oriented films. However, at a constant temperature, cr decreases with increasing rf power up to 200 W, but it increases beyond 200 W. Initial decrease of (Tat the low rf power range is due to the better c-axis orientation, while the subsequent increase of ff is believed to originate from the damage
Sub. Temperature(
“C)
Fig. 3. Standard deviation (a) of X-ray rocking curve of (002) peak of ZnO thin film on glass as a function of substrate temperature at a constant rfpower of 200 W.
RF POWER(w) Fig. 4. Standard deviation ( u) of X-ray rocking curve of (002) peak of ZnO thin film on glass as a function of input power at a substrate temperature of 200°C.
of ZnO thin films by highly energetic ions at the high rf power region. The growth rate of ZnO films on Si substrate is shown in Fig. 5 as a function of substrate temperature. The growth behavior of ZnO thin films on glass substrate was the same as that on Si substrate and is not shown here. The temperature dependence of growth rate showed different behavior at each rf power. At high rf powers, 200 and 250 W, the temperature dependence of growth rate was similar: the highest growth rate was shown at the lowest temperature of 100°C. Such abnormally high apparent growth rate is due to the spurious growth of hexago-
Y. J. Kim, H. J. Kim /Materials Letters 21(1994) 351-356
354 300
I
I
I
I
025OW
1
I
l lOOW
200
100
%
i!z,r, 0
, , , ,j ~
200
100
Sub.
300
Temperature(‘C)
Fig. 5. The growth rate of ZnO thin films on p-Si ( I 00) as a function of substrate temperature at different rfpowers.
nal needles out of the growth plane, as shown in Fig. 6a. Lau et al. [6] also observed the spurious growth and explained that it was resulted from excess DEZ complex molecules. DEZ is a typical electron-deficient compound in that the number of low-lying orbitals available for bonding (four) is greater than the number of bonding electron pairs (two). This presence of vacant orbitals available for bonding explains the tendency of DEZ to form complexes with compounds containing hetero atoms with free electron pairs (0, N, P, S, etc). Thus, DEZ readily forms coordination complexes with appropriate electron-donating ligands [ 9 J. DEZ forms complexes at a temperature near ambient or higher with H20, CO2 and N,O. The reaction of DEZ and N,O is [ 6 ] : low temperature ’
(C2H5Mn+N20
“complex” ,
P ?OOT
“complex” -
ZnO~gaseous products .
In this experiment, eventhough extra energy by high rfpower was induced in order to decompose the DEZ complex even at low substrate temperatures, it was impossible to do that because of low total energy. But the spurious growth did not occurred by increasing substrate temperature more than 100°C at the same if power. The high growth rate at the low temperature region decreased to the lowest value with increasing the substrate temperature up to 150°C. Such a decrease in the growth rate @as considered to take place due to
Fig. 6. SEM micrographs ofZn0 thin films: (a) spurious growth surface at low substrate temperature, and well-defined columnar structures with (b) smooth surface morphology and (c) rough surface morphology.
the structural transition from the amorphous phase to crystalline phase. The subsequent gentle increase in the growth rate was due to more decomposition of source materials by higher thermal energy at high substrate temperatures. In this temperature range
Y, J. Kim, H. J. Kim /Materials Letters 21 (1994) 351-356
ZnO thin films had a well-developed columnar structure with smooth surface mo~hology, as shown in Fig. 6b. At an rf power of 150 W, the temperature dependence of growth rate was similar to that at high rfpower. But the lowest growth rate occurred at higher temperature, 200°C than that at high rf power condition. That is to say, the transition temperature, at which the lowest growth rate was observed due to the phase transition, was increased from 150 to 200°C by reducing rf power to 150 W. On the other hand, at low rf power of 50 and 100 W, the growth rate continued to decrease with increasing substrate temperature due to the phase transition from the amorphous to crystalline phase. But the growth rate was expected to increase above 200°C since the crystalline ZnO film was already formed above that temperature. Nevertheless it continuously decreased without an increase. The crystalline film deposited at 200°C had less crystal perfection than that at 250°C as shown in Fig. 6c. Therefore, it required more densification and alignment, which resulted in the decrease of the growth rate. For each rf power, the decrease in the growth rate at 300°C is observed and is believed to originate from gas-phase homogeneous nucleation. Formation of white powder on the reactor wall gave indirect evidence of homogeneous nucleation in the gas phase in this deposition condition, Similar results relating to the transition temperature have been reported by Smith [ 5 ] and Souletie et al. [ 7 1. The former author reported that the growth rate had the lowest point at 375°C. Below the lowest point the deposited thin films had rather random structure with both ( 10 1) and (002) orientations, but above this point the deposited films had the c-axis orientation: the (002) dominant orientation. This transition was also accompanied by a change in the surface morphology. The latter also showed the same results except the lowest growth rate at 350°C. Both results showed higher transition temperatures than those observed in this study. This discrepancy is believed to originate from the different deposition methods: they employed APCVD or LPCVD, while PECVD was used in this experiment. In PECVD, extra plasma energy reduced the transition temperature as well as the deposition temperature. Fig. 7 shows the growth rate of ZnO thin films on p-Si ( 100) as a function of rf-input power at different substrate temperatures. The changes in the growth rate due to structural tran-
I
355
1
0
1
J
I
I
100
200
300
RF Power(W) Fig. 7. The growth rate of ZnO thin fitms on p-Sit 100) as a function of rfinput power at different substrate temperatures,
Temperature 300 I
(‘C)
250
200
I
I
150 I=
1
0
zoow
A 25OW 0
1.5
1.75
2
Temperature(
2.25
2.5
1000/K)
Fig. 8. The Arrhenius plot of growth rate as a function of temperature at rfpowers of 200 and 250 W.
sition are also shown in Fig. 7. The drop of the growth rate at the substrate temperatures of I50 and 200°C occurred at the rf powers of 150 and 200 W, respectively. The structural change of the films occurred at high rf powers at low substrate temperatures, but not at any rf powers at the high substrate temperature. These results are consistent with those from XRD. The Arrhenius plot of the growth rate is shown in Fig. 8 as a function of the growth temperature. The measured activation energies were 3.1 and 1.9 kJ/mol for 200 and 250 W, respectively. These values are
3.56
Y. .I. Kim, H. J. Kim /Materids Letters 21(1994) 351-356
much lower than those reported previously, 48.2 and 67.5 kJ/mol for APCVD [7] and 30.0 kJ/mol for LPCVD [ 81, but these ones are similar to the value 3.4 W/mol for PECVD reported by Shim&u [ 21. The activation energy decreases with increasing the rf input power. It suggests that rfpower affected both decomposition of sources and mobility of atoms on the substrate.
energetic ions. Growth rate was closely related with both the substrate temperature and the rfinput power. Significant changes occurred in the growth rate when the deposited film structurally changed from the amo~hous phase to crystalline phase. The activation energy for reaction was reduced with increasing the input rfpower. References
4. Conclusions c-axis oriented ZnO thin films were deposited even at temperatures as low as 200°C using DE2 and N20 by PECVD. At the low substrate temperature the crystal perfection of ZnO thin films was also significantly affected by the input rf power, The degree of the c-axis orientation of the ZnO thin film was improved with increasing the substrate temperature as well as the input rf power. But at a constant temperature, high rf powers beyond 200 W deteriorate the preferred c-axis orientation of ZnO films due to the damage by highly
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