- Email: [email protected]
Low temperature epitaxial growth of CeO2(110) layers on Si(100) using electron beam assisted evaporation
Thin Solid Films 323-3-G (1999) 594-597
Low temperature epitaxial growth of CeOz(110) layers on Si( 100) using electron beam assisted evaporation Tomoyasu Inoue”.*, Yasuhiro Yamamotob, Masataka SatohC
Abstract With the aims of lowering growth temperature and improvement of crystalline quality. the effect of electron-beam assistance is studied in the epitaxial growth of Ce02( 1IO) layers on Si( 100) substrates by electron-beam evaporation in an ultrahigh vacuum. From experiments of evaporauon at positive substrate bia,. it is clarified that electron incidence onto the growing surface ih elrective in the facilitation of the epitaxial growth. Newly developed electron-beam assisted evaporation proves to have much greater effect5 in both the growth temperature lowering and the crystalline quality improvement. The epitauial gro\vth facilitation effect depends on incident electron energy. Optimum electron energy is determined to be around 360 eV, wherein the epitavial temperature is lowered to 710°C. i.e. temperature lowering of more than IOO’C compared \vith the conventional growth method. C 1999 Elsevier Science S.A. All right5 rebervcd. Kqu~ords: Cerium: Oxides:
Evaporation: Silicon
Epitaxy:
1. Introduction Cerium
dioxide
(CeO1) is a promising
insulating
material
with a lot of potential advantages for application to microelectronics. e.g. high dielectric constant of 26 and high chemical stability. Epitaxial Ce02 layers on silicon substrates are of interest both for growing high-quality epitaxial insulators on silicon and for applications to microelectronics such as miniaturized stable capacitors and buffer layers between high-temperature superconductors and Si substrates. In the course of the investigation on the epitaxial growth of CeO! on Si [I-S]. it has been clarified that Ce02 layers grown on Si( 100) substrates have a ( 1 IO) orientation and require a high substrate temperature of -830°C. For silicon-based microelectronic device fabrication, low-temperature processes are strongly desired. In order to lower the epitasial growth tetnperature of CeO?[ 1 IO)/ Sit 100) structure. it is thought that some extrinsic assistance by energetic particles such as ions. electrons and photons. is needed to give sufficient energy for the migration of adsorbed atoms and/or molecules at the growing surface [9.10]. From our first finding of the epitaxial growth temperature lowering by electron-beam evaporation under
w Corresponding :tu~hor. Tel.:
i
Yl-216-297172;
290577.
E+rtrrl n&!w.ts: [email protected] (T. Inoue I
fax:
+ 51-X6-
substrate bias application (bias evaporation) [7.8]. where electrons incident onto the growing surface are effective, we propose evaporation with simultaneous electron-beam itradiation !electron-beam assisted evaporation). This method has advantages over bias evaporation because of the wider variable range and better control of the energy and current density of electrons. In this paper we describe details of the electron incidence effect on facilitating CeOl epitasial growth with electron-beam assisted evaporation.
2. Experimental Silicon( 100) substrate5 with a 2.5” miscut toward the. (I 10) direction 151. This miscut ib optimum for realizing a single-crystalline CeO& 1 10) layer, unless the layer tends to have a double domain structure consisting of a mixture of CeOl;,,:,,, II Si[ ITO] and Ce02!I:,,, [I Si[ I TO]. were chemkally cleaned by repeating four times the following procedure: dipping in a hot ~cqurous solution of HCI and HZ02 aid in diluted hydroRuoric acid. followed by rinsing in deionized water. The base pressure of the grou th chamber was 1.S x IO-’ Torr. CeOI layers were epitaxially grown on Si substrates by electron-beam evaporation of hot-pressed CeO! tablets with 99.999% purity. For control of the stoichiometry of epitaxial layers. the chamber pressure was dynamically controlled at 8 X lo-” Torr by balancing the
00-10-6090/99/S - see front matter *E 1999 Else\ier Science S.-A. All right> rexwed. PII: SOOJO-6090( 99100 120-O
1 graphite healer
of which is anion component and proved to have no effect on crystal growth facilitation [8], Tn order to confirm the further enhancement of the electron irradiation effect. we performed epiraxial growth experiments using electronbeam assisted evaporation varying growth temperature and irradiating election energy. Fig. 2a,b show RHEED patterns of CeOl layers grown at 720°C ai -30 V bias and with NO eV electron-beam assistance, respectively. In Fig. 2a. bright spots of a { 1OOjincidence pattern are clearly seen but weak ring patterns are also seen, indicating lhat the layer is mainly single-crystalline but contains a very small amount of polycrystalhne inclusions. On the other hand.
1
Fig. 2b shows no ring partein and distinct bright spott]
Fig. 1. Schematic diagram an eiectron-beam irradiation
of ekccuun-beam system.
evaporation
equipmenf
with
oxygen gas introduction rate with the pumping rate, so as to oxidize evaporating oxygen-deficient species. The growth rate and layer rhickness were in the ranges of 0.3-0.7 run/s and loo-250 nm, respectiveiy. As ihustrated in Fig. i, charged particles such as ions and secondary electrons? are ejected from the electron-beam evaporation source. Under positive bias conditions, electrons and anions are attracted to the sample surface. For electron assisted evaporation experiments, an electronbeam shower system was constructed using a couple of 0.2 mm-diameter tungsten-rhenium wire filaments, which were located below the sample holder as illustrated in the center part of Fig. 1. The accelerating voltage of electrons was varied up to 500 V. wherein no substrate bias was applied. The electron-beam current can be varied from 0.1 to 4 mA, Epitaxialiy grown layers were characterized by reflection high-energy electron diffraction (RHEED) analy-
patterns, indicating that the layer is completely single-crystalline. Comparing these RHEED patterns, it is clearly understood that the sample grown by electron-beam assisted evaporation has higher crystalline quality than that grown by bias evaporation. This is thought to be due mainly to the difference in the eiectron current density. Sample currents during the epitaxial growth were 0.54 and 1.7 mA in -k 210 V bias evaporation and 210 eV electron-beam assisted evaporation, respectively. Considering that a half of the total current is electronic in bias evaporation [7], the electron current density in electron-beam assisted evaporation is six times higher than that in bias evaporation. Furthermore, in eiectron-beam assisted evaporation, attracting no anion is thought to be favorable, since main components of anions are thought to be O- and 0; which oxidize the silicon surface. The effectiveness of electron assisted evaporation for facilitation of epitasial gromrth is thought to be due not to joule heating, since it is estimated to be only 2 mW/cm’ in
sis and 1S MeV He” Rurherford backscattering spectrometry mw.
3. Resutts and discussion It has been recognized that bias evaporation at positive substrate bias is effective in lowering the epitaxialgrowth temperature of CeQ( 110) layers 171. Only electrons irradiating the growing surface can facilitate the epitaxial growth. The electronic component of the sample current is approximately half of the total current (10-j A), remainder
Fig. L RHEED patterns of the (100) azimuth of CeOL Iayers grown at 720°C; (aj at t 240 V biw and (b) with 240 eV electron-beam assistance.
240 V
300 v
360 V
730°C
420 V
500 v
Single Crystal II
700°C Fig. 3. RHEED
patremr
of CrO:
layers
grown
varymg
710 eV electron assisted evaporation. uhich corresponds to a substrate temperature rise of a few degrees centigrade. Roles of the electrons are thought to be: (1) leading surface atoms and molecules to have sufficient kinetic energy for migration toward their epitasial sites through Coulombic interaction. (2) preventing oxidation of the silicon surface at the early stage of the epitkal growth by inducing dissociation of oxygen atoms adsorbed to outermost silicon atoms. and (3) the enhancement of oxidation of osygendeficient cerium oxides. Here. we discuss the effect of electron energy, since it is thought to be more important parameter than the beam current. Fig. 3 shows RHEED patterns of CeO, layers grown varying growth temperature and electron beam energy. These patterns are arranged vertically and horizontally in order of growth temperature and electron energy, respectively. The broken line indicate the critical tempera-
growth
temperature
and a>Wing
I
I
I
energy.
ture for the epitaxial growth. The region above the broken line corresponds tu single crystal layer growth. where RHEED patterns show bright bpots of (100) azimuth. Conversely. RHEED patterns below the broken line show (1 11)-like spotty patterns with weak rings and reveal no change with sample rotation. indicating that they are (1 1I)-oriented polycrystalline layers. It is clearly seen that lowest epitasial temperature decreases with increasing electron energy up to 36U eV and then it rather increases xvith electron energy increase. Subsidiary experiments proved that 3 keV electron irradiation rather impede the epitasial growth. It is concluded that optimum electron energy exists around 360 eV, where the lowest epitaxial temperature is
r t
15
elecrron
(b)820"C
I
electron energy (eV) Fie. 2. electron
Electron cner_cy.
pcrwrarlonilrpth>Into Si
and CeO:
ss a iuncrion
01
Fi;. 5, RBS spccm~ i&en fnxn CrO, layers groun (a) 31 7W”C eV elccrron-km asGtance. and(h) ;II d2O’C by the wn\entional wn
merhod.
with 240 e\apora-
found at 7 10°C. According to Lineweaver [ i 11, penetration depths of electrons into Si and CeC& having energy of 360 eV are estimated to be 8.9 and 2.9 A, respectively (Fig. 4). Since the depth of maximum energy deposition should be shallower than these values>electrons having this energy are able to efficiently ionize surface atoms and adatoms, resulting in Ihe promotion of adatom migration toward lattice sites through CouIombic interaction between them. Fig. 5 shows RBS spectra taken from the CeO! layer grown ar 730°C with 240 eV-electron-beam assistance (Fig. 5a) and the best sample grown by the conventional evaporation method at 820°C (Fig. 5b) [5j. R&pective layer thicknesses are 150 and 108 nm. Ratios of aligned to random spectra (normalized minimum yields) of these samples (a) and (b) are evaluated to be 2.8 and 5.4% at the surface and 20.3 and 30.5% at the interface, respectively. In spik of the lower growth temperature, the crystalline quality of sample (a) is rather superior to that of sample (bj, especially at the interface. This indicates that the electron incidence is highly effective in facilitating epitaxial growth, especially at the initial stage.
Frotil experiments -varying electron energy in electronbeam as&red evaporation, optimum energy was found to be 360 eV, where the epitaxial growth temperature was successfully lowered to 710°C. i.e. lowering by more than 100°C compared with the conventional gromlh method. Crystalline quality of CeOz layers grown using electronbeam assis&ed evaporation was proved to be superior to those grown using the conventional method at higher growth temperature. For further advances in the epitaxial temperature lowering and crystalline quality improvements. more detailed studies will be needed in conjunction with optimization of electron-beam irradiation parameters such as current densit)i.
References [I] [2] 131
4. Conchsion
[4]
It was clarified that electrons incident on the growing surface greatly facilitate the epitaxial growth of CeO?i 11Oj layers on Si( 100) substrates. We proposed electron-beam assisted evaporation using an additiona electron irradiation system and demonstrated its effectiveness for the facilitation of the CeO? epitaxial growth. This is due mainly to the high eIectron current (lo-” A), which is more than one order of magnitude higher than that in bias evaporation smdied in the previous study.
[S] [6] 171 [S]
T. Inoue. Y, Ycmamoto, S. Koyarna, S. Suzuki, I’. Ueda, App!. Phys. Lert. 56 i 1990) 1332. M E’oshimoto. H. Nzgara, T. Tsukahara H. Koinuma Jpn. J. A@. Pnys. 29 11990) L1199. S. Yaegashi. T. Kurihara, H. Hoshi, H. Hasegawa. Jpn.I. Appl.Pays. 33 i.1994) 270. L. Luo. &,D. Wu, R.C. Dye. R.E. Muenchausoa, S.R. F&on. Y. Coulter. C.J. IMaggiore, T. lnoue, Appl. Phys. Lett. 59 (1991) 1013. T. Inoue, Y. Yamamoto, hl, Satoh. 7. Ohsuna, H. Myoren, T. Yamashita, Prcc. Mater. Res. Sot. 341 (1994) 101. D. Huang. F. Qin. 2. Yao, Z. Ren. L. Lin, TV. Gao, Q. Ren, Appl. Phys. Len. 6? (1Wj 3729, T. lnoue. Y. Yamamoto, M. Satoh. Jpn. I, ?ippl. Phys. 35 (19961 L1665. T. Inone, Y. Yamamoro, M. Satoh. Proc. Mater. Res. Sot. 173 (1997) 321.
191 T. Sugiyama, T. ltoh, J. Electrochem. Sot. L?3 (19%) 6tN. [ 201A. Izumi, K Tsutsui, S. Furukawn, J. Appl. Phys. 75 (1991) [I 13 J.L. Lineweaver, 3. Appl. Phya, 33 (1963) 17%.
1307.
Low temperature epitaxial growth of CeOz(110) layers on Si( 100) using electron beam assisted evaporation Tomoyasu Inoue”.*, Yasuhiro Yamamotob, Masataka SatohC
Abstract With the aims of lowering growth temperature and improvement of crystalline quality. the effect of electron-beam assistance is studied in the epitaxial growth of Ce02( 1IO) layers on Si( 100) substrates by electron-beam evaporation in an ultrahigh vacuum. From experiments of evaporauon at positive substrate bia,. it is clarified that electron incidence onto the growing surface ih elrective in the facilitation of the epitaxial growth. Newly developed electron-beam assisted evaporation proves to have much greater effect5 in both the growth temperature lowering and the crystalline quality improvement. The epitauial gro\vth facilitation effect depends on incident electron energy. Optimum electron energy is determined to be around 360 eV, wherein the epitavial temperature is lowered to 710°C. i.e. temperature lowering of more than IOO’C compared \vith the conventional growth method. C 1999 Elsevier Science S.A. All right5 rebervcd. Kqu~ords: Cerium: Oxides:
Evaporation: Silicon
Epitaxy:
1. Introduction Cerium
dioxide
(CeO1) is a promising
insulating
material
with a lot of potential advantages for application to microelectronics. e.g. high dielectric constant of 26 and high chemical stability. Epitaxial Ce02 layers on silicon substrates are of interest both for growing high-quality epitaxial insulators on silicon and for applications to microelectronics such as miniaturized stable capacitors and buffer layers between high-temperature superconductors and Si substrates. In the course of the investigation on the epitaxial growth of CeO! on Si [I-S]. it has been clarified that Ce02 layers grown on Si( 100) substrates have a ( 1 IO) orientation and require a high substrate temperature of -830°C. For silicon-based microelectronic device fabrication, low-temperature processes are strongly desired. In order to lower the epitasial growth tetnperature of CeO?[ 1 IO)/ Sit 100) structure. it is thought that some extrinsic assistance by energetic particles such as ions. electrons and photons. is needed to give sufficient energy for the migration of adsorbed atoms and/or molecules at the growing surface [9.10]. From our first finding of the epitaxial growth temperature lowering by electron-beam evaporation under
w Corresponding :tu~hor. Tel.:
i
Yl-216-297172;
290577.
E+rtrrl n&!w.ts: [email protected] (T. Inoue I
fax:
+ 51-X6-
substrate bias application (bias evaporation) [7.8]. where electrons incident onto the growing surface are effective, we propose evaporation with simultaneous electron-beam itradiation !electron-beam assisted evaporation). This method has advantages over bias evaporation because of the wider variable range and better control of the energy and current density of electrons. In this paper we describe details of the electron incidence effect on facilitating CeOl epitasial growth with electron-beam assisted evaporation.
2. Experimental Silicon( 100) substrate5 with a 2.5” miscut toward the. (I 10) direction 151. This miscut ib optimum for realizing a single-crystalline CeO& 1 10) layer, unless the layer tends to have a double domain structure consisting of a mixture of CeOl;,,:,,, II Si[ ITO] and Ce02!I:,,, [I Si[ I TO]. were chemkally cleaned by repeating four times the following procedure: dipping in a hot ~cqurous solution of HCI and HZ02 aid in diluted hydroRuoric acid. followed by rinsing in deionized water. The base pressure of the grou th chamber was 1.S x IO-’ Torr. CeOI layers were epitaxially grown on Si substrates by electron-beam evaporation of hot-pressed CeO! tablets with 99.999% purity. For control of the stoichiometry of epitaxial layers. the chamber pressure was dynamically controlled at 8 X lo-” Torr by balancing the
00-10-6090/99/S - see front matter *E 1999 Else\ier Science S.-A. All right> rexwed. PII: SOOJO-6090( 99100 120-O
1 graphite healer
of which is anion component and proved to have no effect on crystal growth facilitation [8], Tn order to confirm the further enhancement of the electron irradiation effect. we performed epiraxial growth experiments using electronbeam assisted evaporation varying growth temperature and irradiating election energy. Fig. 2a,b show RHEED patterns of CeOl layers grown at 720°C ai -30 V bias and with NO eV electron-beam assistance, respectively. In Fig. 2a. bright spots of a { 1OOjincidence pattern are clearly seen but weak ring patterns are also seen, indicating lhat the layer is mainly single-crystalline but contains a very small amount of polycrystalhne inclusions. On the other hand.
1
Fig. 2b shows no ring partein and distinct bright spott]
Fig. 1. Schematic diagram an eiectron-beam irradiation
of ekccuun-beam system.
evaporation
equipmenf
with
oxygen gas introduction rate with the pumping rate, so as to oxidize evaporating oxygen-deficient species. The growth rate and layer rhickness were in the ranges of 0.3-0.7 run/s and loo-250 nm, respectiveiy. As ihustrated in Fig. i, charged particles such as ions and secondary electrons? are ejected from the electron-beam evaporation source. Under positive bias conditions, electrons and anions are attracted to the sample surface. For electron assisted evaporation experiments, an electronbeam shower system was constructed using a couple of 0.2 mm-diameter tungsten-rhenium wire filaments, which were located below the sample holder as illustrated in the center part of Fig. 1. The accelerating voltage of electrons was varied up to 500 V. wherein no substrate bias was applied. The electron-beam current can be varied from 0.1 to 4 mA, Epitaxialiy grown layers were characterized by reflection high-energy electron diffraction (RHEED) analy-
patterns, indicating that the layer is completely single-crystalline. Comparing these RHEED patterns, it is clearly understood that the sample grown by electron-beam assisted evaporation has higher crystalline quality than that grown by bias evaporation. This is thought to be due mainly to the difference in the eiectron current density. Sample currents during the epitaxial growth were 0.54 and 1.7 mA in -k 210 V bias evaporation and 210 eV electron-beam assisted evaporation, respectively. Considering that a half of the total current is electronic in bias evaporation [7], the electron current density in electron-beam assisted evaporation is six times higher than that in bias evaporation. Furthermore, in eiectron-beam assisted evaporation, attracting no anion is thought to be favorable, since main components of anions are thought to be O- and 0; which oxidize the silicon surface. The effectiveness of electron assisted evaporation for facilitation of epitasial gromrth is thought to be due not to joule heating, since it is estimated to be only 2 mW/cm’ in
sis and 1S MeV He” Rurherford backscattering spectrometry mw.
3. Resutts and discussion It has been recognized that bias evaporation at positive substrate bias is effective in lowering the epitaxialgrowth temperature of CeQ( 110) layers 171. Only electrons irradiating the growing surface can facilitate the epitaxial growth. The electronic component of the sample current is approximately half of the total current (10-j A), remainder
Fig. L RHEED patterns of the (100) azimuth of CeOL Iayers grown at 720°C; (aj at t 240 V biw and (b) with 240 eV electron-beam assistance.
240 V
300 v
360 V
730°C
420 V
500 v
Single Crystal II
700°C Fig. 3. RHEED
patremr
of CrO:
layers
grown
varymg
710 eV electron assisted evaporation. uhich corresponds to a substrate temperature rise of a few degrees centigrade. Roles of the electrons are thought to be: (1) leading surface atoms and molecules to have sufficient kinetic energy for migration toward their epitasial sites through Coulombic interaction. (2) preventing oxidation of the silicon surface at the early stage of the epitkal growth by inducing dissociation of oxygen atoms adsorbed to outermost silicon atoms. and (3) the enhancement of oxidation of osygendeficient cerium oxides. Here. we discuss the effect of electron energy, since it is thought to be more important parameter than the beam current. Fig. 3 shows RHEED patterns of CeO, layers grown varying growth temperature and electron beam energy. These patterns are arranged vertically and horizontally in order of growth temperature and electron energy, respectively. The broken line indicate the critical tempera-
growth
temperature
and a>Wing
I
I
I
energy.
ture for the epitaxial growth. The region above the broken line corresponds tu single crystal layer growth. where RHEED patterns show bright bpots of (100) azimuth. Conversely. RHEED patterns below the broken line show (1 11)-like spotty patterns with weak rings and reveal no change with sample rotation. indicating that they are (1 1I)-oriented polycrystalline layers. It is clearly seen that lowest epitasial temperature decreases with increasing electron energy up to 36U eV and then it rather increases xvith electron energy increase. Subsidiary experiments proved that 3 keV electron irradiation rather impede the epitasial growth. It is concluded that optimum electron energy exists around 360 eV, where the lowest epitaxial temperature is
r t
15
elecrron
(b)820"C
I
electron energy (eV) Fie. 2. electron
Electron cner_cy.
pcrwrarlonilrpth>Into Si
and CeO:
ss a iuncrion
01
Fi;. 5, RBS spccm~ i&en fnxn CrO, layers groun (a) 31 7W”C eV elccrron-km asGtance. and(h) ;II d2O’C by the wn\entional wn
merhod.
with 240 e\apora-
found at 7 10°C. According to Lineweaver [ i 11, penetration depths of electrons into Si and CeC& having energy of 360 eV are estimated to be 8.9 and 2.9 A, respectively (Fig. 4). Since the depth of maximum energy deposition should be shallower than these values>electrons having this energy are able to efficiently ionize surface atoms and adatoms, resulting in Ihe promotion of adatom migration toward lattice sites through CouIombic interaction between them. Fig. 5 shows RBS spectra taken from the CeO! layer grown ar 730°C with 240 eV-electron-beam assistance (Fig. 5a) and the best sample grown by the conventional evaporation method at 820°C (Fig. 5b) [5j. R&pective layer thicknesses are 150 and 108 nm. Ratios of aligned to random spectra (normalized minimum yields) of these samples (a) and (b) are evaluated to be 2.8 and 5.4% at the surface and 20.3 and 30.5% at the interface, respectively. In spik of the lower growth temperature, the crystalline quality of sample (a) is rather superior to that of sample (bj, especially at the interface. This indicates that the electron incidence is highly effective in facilitating epitaxial growth, especially at the initial stage.
Frotil experiments -varying electron energy in electronbeam as&red evaporation, optimum energy was found to be 360 eV, where the epitaxial growth temperature was successfully lowered to 710°C. i.e. lowering by more than 100°C compared with the conventional gromlh method. Crystalline quality of CeOz layers grown using electronbeam assis&ed evaporation was proved to be superior to those grown using the conventional method at higher growth temperature. For further advances in the epitaxial temperature lowering and crystalline quality improvements. more detailed studies will be needed in conjunction with optimization of electron-beam irradiation parameters such as current densit)i.
References [I] [2] 131
4. Conchsion
[4]
It was clarified that electrons incident on the growing surface greatly facilitate the epitaxial growth of CeO?i 11Oj layers on Si( 100) substrates. We proposed electron-beam assisted evaporation using an additiona electron irradiation system and demonstrated its effectiveness for the facilitation of the CeO? epitaxial growth. This is due mainly to the high eIectron current (lo-” A), which is more than one order of magnitude higher than that in bias evaporation smdied in the previous study.
[S] [6] 171 [S]
T. Inoue. Y, Ycmamoto, S. Koyarna, S. Suzuki, I’. Ueda, App!. Phys. Lert. 56 i 1990) 1332. M E’oshimoto. H. Nzgara, T. Tsukahara H. Koinuma Jpn. J. A@. Pnys. 29 11990) L1199. S. Yaegashi. T. Kurihara, H. Hoshi, H. Hasegawa. Jpn.I. Appl.Pays. 33 i.1994) 270. L. Luo. &,D. Wu, R.C. Dye. R.E. Muenchausoa, S.R. F&on. Y. Coulter. C.J. IMaggiore, T. lnoue, Appl. Phys. Lett. 59 (1991) 1013. T. Inoue, Y. Yamamoto, hl, Satoh. 7. Ohsuna, H. Myoren, T. Yamashita, Prcc. Mater. Res. Sot. 341 (1994) 101. D. Huang. F. Qin. 2. Yao, Z. Ren. L. Lin, TV. Gao, Q. Ren, Appl. Phys. Len. 6? (1Wj 3729, T. lnoue. Y. Yamamoto, M. Satoh. Jpn. I, ?ippl. Phys. 35 (19961 L1665. T. Inone, Y. Yamamoro, M. Satoh. Proc. Mater. Res. Sot. 173 (1997) 321.
191 T. Sugiyama, T. ltoh, J. Electrochem. Sot. L?3 (19%) 6tN. [ 201A. Izumi, K Tsutsui, S. Furukawn, J. Appl. Phys. 75 (1991) [I 13 J.L. Lineweaver, 3. Appl. Phya, 33 (1963) 17%.
1307.