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SIMS study of silicon oxynitride prepared by oxidation of silicon-rich silicon nitride layer
Microelectronics Reliability 41 (2001) 2071±2074
Research note
www.elsevier.com/locate/microrel
SIMS study of silicon oxynitride prepared by oxidation of silicon-rich silicon nitride layer M.C. Poon a, Y. Gao a, T.C.W. Kok a, A.M. Myasnikov a, H. Wong b,*
a
Department of Electrical and Electronic Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong b Department of Electronic Engineering, City University of Hong Kong, City U, 83 Tat Chee Avenue, Kowloon, Hong Kong Received 1 March 2001; received in revised form 1 July 2001
Abstract We proposed a novel process for fabrication silicon oxide±oxynitride±oxide structure for ULSI device applications. By deposition of silicon-rich silicon nitride and then following a thermal oxidation process, a good oxynitride layer was obtained. Secondary ion mass spectroscopy (SIMS) study reveals that the hydrogen content of nitride ®lm at the interface can be reduced by more than 40% when compared to stoichiometric nitride. With this method, high nitrogen content oxynitride and smoother oxynitride/oxide interfaces result in the reduction of the interface charge trapping remarkably. Ó 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Crucial reliability challenges for MOS gate dielectrics have now come across because of the increasing memory size and the down scaling device size [1±5]. Particularly, the interface layer is not scalable. In very thin oxide (e.g. 5 nm) the interface region is so large (when compared to the total oxide thickness) that the interface traps can play a dominant role in the tunneling characteristics. Oxide±nitride±oxide (ONO) structure was considered as a good structure and has been used as dielectric in electrically erasable programmable read only memory devices [6,7]. In comparison to thermal oxide, the ONO structure has lower leakage current, higher eective dielectric permittivity, and higher reliability [8±11]. However, these characteristics are still not good enough for the crucial constraints of 1 Gbit ¯ash memory application. The chemical vapor deposition (CVD) silicon nitride ®lm, because of the inherent stain in the networks of the amorphous structure, cannot be of good device quality [12]. Oxynitride by NH3 nitridation would introduce large amount of traps because of the hydrogen incorporation [13±15]. Although N2 O nitridation seems
*
Corresponding author.
to have the advantage of low hydrogen content, the amount of nitrogen incorporation, in the range of 2±3 at.%, is still not large enough to improve the hardness for hot carrier irradiation [16]. This work proposes an alternative method for fabrication high quality oxynitride ®lms. A silicon-rich silicon nitride was ®rst deposited with low-pressure chemical vapor deposition (LPCVD) technique. Oxynitride was then formed by oxidation of silicon-rich silicon nitride. Secondary ion mass spectroscopy (SIMS) and Auger electron spectroscopy measurements were used to study the physical structure and chemical compositions. The experimental details are given in Section 2. Section 3 presents the experimental results obtained from SIMS measurements. Finally, major results of this work will be summarized in Section 4. 2. Sample preparation and measurement Samples with oxide/oxynitride/oxide structure were fabricated on n-type silicon with h1 0 0i orientation and the resistivity is about 4±10 X-cm. A thin thermal oxide was ®rst grown by dry oxidation at 850 of about 100 A silicon nitride or silicon°C, and then a thin (150 A) rich nitride layer was deposited on the thermal oxide using LPCVD. The chemical sources used for the
0026-2714/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 0 1 ) 0 0 2 1 6 - 5
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M.C. Poon et al. / Microelectronics Reliability 41 (2001) 2071±2074
Fig. 1. Auger data for samples 75N. Depth distributions for ratio Si/N and content of oxygen.
nitride layer. For Si-rich sample (85N), the Si/N ratio is as high as 0.95 and decreases with the depth to about 0.75. This observation mainly due to the oxidation of excess silicon on the surface layer and the percentage of nitrogen content is relatively low in this layer. The Si/N when the ratio rises sharply at a depth about 150 A beam sputtered into the nitride/oxide interface. The depth pro®les for oxygen content are also presented at the same plots for comparison. There are two peaks of oxygen concentration representing the surface highoxygen content oxynitride and the bottom oxide layers. The depth of the second maximum is also about 150 A and is in complete agreement with XPS data. Fig. 3 depicts the SIMS oxygen depth pro®le for several samples. As shown in the ®gure, remarkable oxygen tails are found due to the ion recoil as a result of 1-keV Cs ion sputtering. Although the SIMS spectra may not re¯ect the real distribution of atoms and bonds in these samples [6,17], it still provides useful information for the comparative study concern. When comparing with the data measured by AES for the same sample, SIMS value is smaller than that from AES measurements. However, the peak locations have been correlated very well with these measurements. For SIMS measurements, high-oxygen contents on the surface were detected because SIMS has a better resolution than AES in concentration pro®ling. Fig. 4 shows the Si/N ratio from SIMS measurement. The ratio is lower than that from AES results. The bulk nitrogen concentration is as high as 70 at.%. For silicon-rich sample (sample 85N), the bulk nitrogen concentration is signi®cantly lowered to less than 45 at.% as a result of introducing excess silicon atoms. The Si/N ratio falls after reaching the and then rises again at about 180 peak at around 150 A A. The decrease of Si/N in the oxide layer (around 150 is mainly due to low amount of Si atoms and the A) recoiled nitrogen atoms during the 1-keV Cs ion sputtering. When sputtered down to the silicon substrate
Fig. 2. Auger data for samples 85N. Depth distributions for ratio Si/N and content of oxygen.
Fig. 3. SIMS depth distributions of oxygen concentration.
LPCVD are SiCl4 and NH3 . Si/N ratio is about 0.75, 0.85, 0.9 and 0.95 for samples 75N, 85N, 9N and 95N, respectively. The samples were than re-oxidized at 1000 °C for 35 min in dry oxygen ambient. For SIMS depth pro®ling, Riber MIQ256 SIMS instrument was used. The beam source is 1-keV Cs and the beam current is 50 nA. The negative secondary ions were selected with a quadrupole ®lter. AES measurements were made with Riber model LAS-3000 Auger electron spectrometer. The electron beam with energy in the range of 100±3000 eV, with 90° incident angle was used and the re¯ected electrons were measured at 42° with a cylinder mirror analyzer.
3. Results and discussion Figs. 1 and 2 show the Auger results for samples 75N and 85N. For sample 75N, the Si/N is about 0.75 of bulk
M.C. Poon et al. / Microelectronics Reliability 41 (2001) 2071±2074
2073
Fig. 4. SIMS depth distributions of ratio of Si/N.
Fig. 5. SIMS depth distributions of hydrogen concentration.
the recoiled nitrogen atoms almost (beyond 180 A), disappeared and the amount of oxygen atoms decreases also. As a result, the Si/N ratio increases again. The excess silicon will have signi®cant eects on the current conduction of the dielectric ®lms and will participate actively in future oxidation process. The physical con®gurations of the atoms in network are con®ned by the bending and stretching forces. It was found that these constraint forces are a linear function of the average coordination number of the atoms [12]. For silicon oxide, the average coordination number is 2.67 and is optimal because bending forces at oxygen atoms are too weak to function as signi®cant constraints. In silicon nitride the corresponding coordination number is 3.43 and the networks are over-constrained and more defects are resulted. The dierence in the bending constraints also results in the poor Si/Si3 N4 interface. Oxynitride is a good approach to improve the dielectric properties. It can be used for bridging the oxide and nitride and their interface to silicon substrate. The grade interface is more eective in reducing the interface defect density. Fig. 5 displays the hydrogen pro®les of various multilayer dielectrics. SIMS is more sensitive than IR spectroscopy. It is found that large number of hydrogen impurities are introduced by the LPCVD process. The hydrogen concentration is the highest near the surface and decrease gradually and then it increases slightly at the interface. Stoichiometric silicon nitride (sample 75N) has the highest hydrogen content. Silicon-rich samples have lower amount of hydrogen. For Si-rich samples with re-oxidation (samples 85N, 9N and 95N), the hydrogen content decreases by more than 40% when compared to the 75N sample and there is no noticeable hydrogen peak can be found at the interface. The hy is due to the background drogen tail beyond 250 A impurity in the chamber together with the inherent impurity in silicon substrate. Hydrogen was found to be
traps centers under hot electron irradiation [18,19]. The proposed method is an eectively measure to reduced the electronic trapping. With re-oxidation, a multi-layer (oxide/oxynitride/ oxide) structure was formed. SIMS pro®ling reveals that the surface layer is an oxide layer with thickness in the depending on the silicon concentration range of 50±80 A of the as-deposited Si-rich nitride and the center layer is oxynitride. As mentioned earlier, due to its preparation technique and the large coordination number, the quality of LPCVD silicon nitride is poor and the even worst interface to oxide and silicon and hardly make useful in the active device structure. By introducing excess silicon and with re-oxidation, a very good oxynitride can be achieved. The post-CVD oxidation process rehears the structural defects in the CVD nitride. It also grades the oxide/CVD nitride interface by making the oxygen and nitrogen pro®les smoother and releases the interface stress. In addition, it removes the hydrogen atoms which will be trap centers for dielectric ®lm under hot-carrier irradiation. Note that these improvements can only be achieved eectively with the introduction of excess silicon. When compared to the oxynitride prepared by N2 O annealing, the current technique has the advantage of high nitrogen content which leads to a greater hardness for hot-carrier irradiation. It was found ammonia nitridation could induce lot of N±H bonds and OH bonds and many ®xed oxide charges to the nitrided oxide. Although the alternative N2 O nitridation has minimized the hydrogen incorporation, this process still suers from many diculties. The uniformity and reproducibility between wafers and batches are very poor. In addition, thermodynamically, nitridation of oxide is much dicult than silicon oxidation even at high temperature. The nitrogen incorporation is limited to 3 at.% [19]. The nitrogen content is too low to increase the dielectric constant and to improve the dielectric reliability. Our alternative, oxidation
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M.C. Poon et al. / Microelectronics Reliability 41 (2001) 2071±2074
of excess silicon, is a thermodynamically favorable process and has several better results. 4. Conclusions Oxide/Si-rich nitride/oxide (ONO) structures have been fabricated and their properties have been studied in detail. By re-oxidizing the Si-rich nitride layer, SIMS study reveals that the hydrogen content of nitride ®lm and its interface can be reduced by more than 40% when compared to the stoichiometric CVD nitride. Compared to oxynitride prepared by other techniques, this method results in a high nitrogen content oxynitride ®lm and smoother oxynitride/oxide interfaces and the interface charge trapping can be reduced remarkably. Acknowledgements This work was partially supported by the research project no. 7100134 of City U. References [1] Takeda E. Microelectron Reliab 1997;37:985. [2] Cappelletti P. Microelectron Reliab 1998;38:185.
[3] Atitome S, Shirota R, Hemink G, Endoh T, Masuoka F. Proc IEEE 1993;81:776. [4] Park YB, Schroder DK. IEEE Trans Electron Dev 1998; 45:1361. [5] Modelli A. Microelectron Engng 1999;48:403. [6] Fazan PC, Ditali A, Dennison CH, Rhodes HE, Chan HC, Liu YC. J Electrochem Soc 1991;138:2052. [7] Minami S, Kamigaki Y. IEEE Trans Electron Dev 1993;40:2011. [8] Gritsenko VA, Wong H, Xu JB, Kwok RM, Petrenko IP, Zaitsev BA, Morokov YN, Novikov YN. J Appl Phys 1999;86:3234. [9] Gritsebko VA, Shavalgin YG, Pundur PA, Wong H, Lau WM. Microelectron Reliab 1999;39:715. [10] Gritsenko VA, Svitasheva SN, Petrenko IP, Wong H, Xu JB, Wilson IH. J Electrochem Soc 1999;146:780. [11] Gritsenko VA, Petrenko IP, Svitasheva SN, Wong H. Appl Phys Lett 1998;72:462. [12] Lucovsky G, Philips JC. Microelectron Engng 1999;48: 291. [13] Wong H, Cheng YC. J Appl Phys 1993;74:7364. [14] Wong H, Yang BL, Cheng YC. Appl Surf Sci 1993; 72:49. [15] Wong H, Cheng YC. J Appl Phys 1990;67:7132. [16] Hori T, Iwasaki H, Tsuji K. IEEE Trans Electron Dev 1989;ED-36:340. [17] Banerjee I, Kuzminov D. Appl Phys Lett 1993;62:1541. [18] Wong H, Cheng YC. J Appl Phys 1993;74:7364. [19] Hori T, Iwasaki H. Appl Phys Lett 1988;52:736.
Research note
www.elsevier.com/locate/microrel
SIMS study of silicon oxynitride prepared by oxidation of silicon-rich silicon nitride layer M.C. Poon a, Y. Gao a, T.C.W. Kok a, A.M. Myasnikov a, H. Wong b,*
a
Department of Electrical and Electronic Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong b Department of Electronic Engineering, City University of Hong Kong, City U, 83 Tat Chee Avenue, Kowloon, Hong Kong Received 1 March 2001; received in revised form 1 July 2001
Abstract We proposed a novel process for fabrication silicon oxide±oxynitride±oxide structure for ULSI device applications. By deposition of silicon-rich silicon nitride and then following a thermal oxidation process, a good oxynitride layer was obtained. Secondary ion mass spectroscopy (SIMS) study reveals that the hydrogen content of nitride ®lm at the interface can be reduced by more than 40% when compared to stoichiometric nitride. With this method, high nitrogen content oxynitride and smoother oxynitride/oxide interfaces result in the reduction of the interface charge trapping remarkably. Ó 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Crucial reliability challenges for MOS gate dielectrics have now come across because of the increasing memory size and the down scaling device size [1±5]. Particularly, the interface layer is not scalable. In very thin oxide (e.g. 5 nm) the interface region is so large (when compared to the total oxide thickness) that the interface traps can play a dominant role in the tunneling characteristics. Oxide±nitride±oxide (ONO) structure was considered as a good structure and has been used as dielectric in electrically erasable programmable read only memory devices [6,7]. In comparison to thermal oxide, the ONO structure has lower leakage current, higher eective dielectric permittivity, and higher reliability [8±11]. However, these characteristics are still not good enough for the crucial constraints of 1 Gbit ¯ash memory application. The chemical vapor deposition (CVD) silicon nitride ®lm, because of the inherent stain in the networks of the amorphous structure, cannot be of good device quality [12]. Oxynitride by NH3 nitridation would introduce large amount of traps because of the hydrogen incorporation [13±15]. Although N2 O nitridation seems
*
Corresponding author.
to have the advantage of low hydrogen content, the amount of nitrogen incorporation, in the range of 2±3 at.%, is still not large enough to improve the hardness for hot carrier irradiation [16]. This work proposes an alternative method for fabrication high quality oxynitride ®lms. A silicon-rich silicon nitride was ®rst deposited with low-pressure chemical vapor deposition (LPCVD) technique. Oxynitride was then formed by oxidation of silicon-rich silicon nitride. Secondary ion mass spectroscopy (SIMS) and Auger electron spectroscopy measurements were used to study the physical structure and chemical compositions. The experimental details are given in Section 2. Section 3 presents the experimental results obtained from SIMS measurements. Finally, major results of this work will be summarized in Section 4. 2. Sample preparation and measurement Samples with oxide/oxynitride/oxide structure were fabricated on n-type silicon with h1 0 0i orientation and the resistivity is about 4±10 X-cm. A thin thermal oxide was ®rst grown by dry oxidation at 850 of about 100 A silicon nitride or silicon°C, and then a thin (150 A) rich nitride layer was deposited on the thermal oxide using LPCVD. The chemical sources used for the
0026-2714/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 0 1 ) 0 0 2 1 6 - 5
2072
M.C. Poon et al. / Microelectronics Reliability 41 (2001) 2071±2074
Fig. 1. Auger data for samples 75N. Depth distributions for ratio Si/N and content of oxygen.
nitride layer. For Si-rich sample (85N), the Si/N ratio is as high as 0.95 and decreases with the depth to about 0.75. This observation mainly due to the oxidation of excess silicon on the surface layer and the percentage of nitrogen content is relatively low in this layer. The Si/N when the ratio rises sharply at a depth about 150 A beam sputtered into the nitride/oxide interface. The depth pro®les for oxygen content are also presented at the same plots for comparison. There are two peaks of oxygen concentration representing the surface highoxygen content oxynitride and the bottom oxide layers. The depth of the second maximum is also about 150 A and is in complete agreement with XPS data. Fig. 3 depicts the SIMS oxygen depth pro®le for several samples. As shown in the ®gure, remarkable oxygen tails are found due to the ion recoil as a result of 1-keV Cs ion sputtering. Although the SIMS spectra may not re¯ect the real distribution of atoms and bonds in these samples [6,17], it still provides useful information for the comparative study concern. When comparing with the data measured by AES for the same sample, SIMS value is smaller than that from AES measurements. However, the peak locations have been correlated very well with these measurements. For SIMS measurements, high-oxygen contents on the surface were detected because SIMS has a better resolution than AES in concentration pro®ling. Fig. 4 shows the Si/N ratio from SIMS measurement. The ratio is lower than that from AES results. The bulk nitrogen concentration is as high as 70 at.%. For silicon-rich sample (sample 85N), the bulk nitrogen concentration is signi®cantly lowered to less than 45 at.% as a result of introducing excess silicon atoms. The Si/N ratio falls after reaching the and then rises again at about 180 peak at around 150 A A. The decrease of Si/N in the oxide layer (around 150 is mainly due to low amount of Si atoms and the A) recoiled nitrogen atoms during the 1-keV Cs ion sputtering. When sputtered down to the silicon substrate
Fig. 2. Auger data for samples 85N. Depth distributions for ratio Si/N and content of oxygen.
Fig. 3. SIMS depth distributions of oxygen concentration.
LPCVD are SiCl4 and NH3 . Si/N ratio is about 0.75, 0.85, 0.9 and 0.95 for samples 75N, 85N, 9N and 95N, respectively. The samples were than re-oxidized at 1000 °C for 35 min in dry oxygen ambient. For SIMS depth pro®ling, Riber MIQ256 SIMS instrument was used. The beam source is 1-keV Cs and the beam current is 50 nA. The negative secondary ions were selected with a quadrupole ®lter. AES measurements were made with Riber model LAS-3000 Auger electron spectrometer. The electron beam with energy in the range of 100±3000 eV, with 90° incident angle was used and the re¯ected electrons were measured at 42° with a cylinder mirror analyzer.
3. Results and discussion Figs. 1 and 2 show the Auger results for samples 75N and 85N. For sample 75N, the Si/N is about 0.75 of bulk
M.C. Poon et al. / Microelectronics Reliability 41 (2001) 2071±2074
2073
Fig. 4. SIMS depth distributions of ratio of Si/N.
Fig. 5. SIMS depth distributions of hydrogen concentration.
the recoiled nitrogen atoms almost (beyond 180 A), disappeared and the amount of oxygen atoms decreases also. As a result, the Si/N ratio increases again. The excess silicon will have signi®cant eects on the current conduction of the dielectric ®lms and will participate actively in future oxidation process. The physical con®gurations of the atoms in network are con®ned by the bending and stretching forces. It was found that these constraint forces are a linear function of the average coordination number of the atoms [12]. For silicon oxide, the average coordination number is 2.67 and is optimal because bending forces at oxygen atoms are too weak to function as signi®cant constraints. In silicon nitride the corresponding coordination number is 3.43 and the networks are over-constrained and more defects are resulted. The dierence in the bending constraints also results in the poor Si/Si3 N4 interface. Oxynitride is a good approach to improve the dielectric properties. It can be used for bridging the oxide and nitride and their interface to silicon substrate. The grade interface is more eective in reducing the interface defect density. Fig. 5 displays the hydrogen pro®les of various multilayer dielectrics. SIMS is more sensitive than IR spectroscopy. It is found that large number of hydrogen impurities are introduced by the LPCVD process. The hydrogen concentration is the highest near the surface and decrease gradually and then it increases slightly at the interface. Stoichiometric silicon nitride (sample 75N) has the highest hydrogen content. Silicon-rich samples have lower amount of hydrogen. For Si-rich samples with re-oxidation (samples 85N, 9N and 95N), the hydrogen content decreases by more than 40% when compared to the 75N sample and there is no noticeable hydrogen peak can be found at the interface. The hy is due to the background drogen tail beyond 250 A impurity in the chamber together with the inherent impurity in silicon substrate. Hydrogen was found to be
traps centers under hot electron irradiation [18,19]. The proposed method is an eectively measure to reduced the electronic trapping. With re-oxidation, a multi-layer (oxide/oxynitride/ oxide) structure was formed. SIMS pro®ling reveals that the surface layer is an oxide layer with thickness in the depending on the silicon concentration range of 50±80 A of the as-deposited Si-rich nitride and the center layer is oxynitride. As mentioned earlier, due to its preparation technique and the large coordination number, the quality of LPCVD silicon nitride is poor and the even worst interface to oxide and silicon and hardly make useful in the active device structure. By introducing excess silicon and with re-oxidation, a very good oxynitride can be achieved. The post-CVD oxidation process rehears the structural defects in the CVD nitride. It also grades the oxide/CVD nitride interface by making the oxygen and nitrogen pro®les smoother and releases the interface stress. In addition, it removes the hydrogen atoms which will be trap centers for dielectric ®lm under hot-carrier irradiation. Note that these improvements can only be achieved eectively with the introduction of excess silicon. When compared to the oxynitride prepared by N2 O annealing, the current technique has the advantage of high nitrogen content which leads to a greater hardness for hot-carrier irradiation. It was found ammonia nitridation could induce lot of N±H bonds and OH bonds and many ®xed oxide charges to the nitrided oxide. Although the alternative N2 O nitridation has minimized the hydrogen incorporation, this process still suers from many diculties. The uniformity and reproducibility between wafers and batches are very poor. In addition, thermodynamically, nitridation of oxide is much dicult than silicon oxidation even at high temperature. The nitrogen incorporation is limited to 3 at.% [19]. The nitrogen content is too low to increase the dielectric constant and to improve the dielectric reliability. Our alternative, oxidation
2074
M.C. Poon et al. / Microelectronics Reliability 41 (2001) 2071±2074
of excess silicon, is a thermodynamically favorable process and has several better results. 4. Conclusions Oxide/Si-rich nitride/oxide (ONO) structures have been fabricated and their properties have been studied in detail. By re-oxidizing the Si-rich nitride layer, SIMS study reveals that the hydrogen content of nitride ®lm and its interface can be reduced by more than 40% when compared to the stoichiometric CVD nitride. Compared to oxynitride prepared by other techniques, this method results in a high nitrogen content oxynitride ®lm and smoother oxynitride/oxide interfaces and the interface charge trapping can be reduced remarkably. Acknowledgements This work was partially supported by the research project no. 7100134 of City U. References [1] Takeda E. Microelectron Reliab 1997;37:985. [2] Cappelletti P. Microelectron Reliab 1998;38:185.
[3] Atitome S, Shirota R, Hemink G, Endoh T, Masuoka F. Proc IEEE 1993;81:776. [4] Park YB, Schroder DK. IEEE Trans Electron Dev 1998; 45:1361. [5] Modelli A. Microelectron Engng 1999;48:403. [6] Fazan PC, Ditali A, Dennison CH, Rhodes HE, Chan HC, Liu YC. J Electrochem Soc 1991;138:2052. [7] Minami S, Kamigaki Y. IEEE Trans Electron Dev 1993;40:2011. [8] Gritsenko VA, Wong H, Xu JB, Kwok RM, Petrenko IP, Zaitsev BA, Morokov YN, Novikov YN. J Appl Phys 1999;86:3234. [9] Gritsebko VA, Shavalgin YG, Pundur PA, Wong H, Lau WM. Microelectron Reliab 1999;39:715. [10] Gritsenko VA, Svitasheva SN, Petrenko IP, Wong H, Xu JB, Wilson IH. J Electrochem Soc 1999;146:780. [11] Gritsenko VA, Petrenko IP, Svitasheva SN, Wong H. Appl Phys Lett 1998;72:462. [12] Lucovsky G, Philips JC. Microelectron Engng 1999;48: 291. [13] Wong H, Cheng YC. J Appl Phys 1993;74:7364. [14] Wong H, Yang BL, Cheng YC. Appl Surf Sci 1993; 72:49. [15] Wong H, Cheng YC. J Appl Phys 1990;67:7132. [16] Hori T, Iwasaki H, Tsuji K. IEEE Trans Electron Dev 1989;ED-36:340. [17] Banerjee I, Kuzminov D. Appl Phys Lett 1993;62:1541. [18] Wong H, Cheng YC. J Appl Phys 1993;74:7364. [19] Hori T, Iwasaki H. Appl Phys Lett 1988;52:736.