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Germanium effect on void defects in Czochralski silicon
Journal of Crystal Growth 243 (2002) 371–374
Priority communication
Germanium effect on void defects in Czochralski silicon Deren Yang*, Xuegong Yu, Xiangyang Ma, Jin Xu, Liben Li, Duanlin Que State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, China Received 15 February 2002; accepted 13 June 2002 Communicated by L.F. Schneemeyer
Abstract The effect of germanium (Ge) on void defects in lightly Ge-doped Czochralski (GCZ) silicon (Si) crystals has been investigated. Three GCZ Si crystals with different Ge concentrations (1015–1018 cm3) and one conventional Czochralski (CZ) Si crystal were grown under almost the same growth conditions. It is found that the density of flow pattern defects (FPDs) related to void defects in the as-grown GCZ Si decreases with the increase of Ge concentration. The voids in the GCZ Si could be eliminated more easily during annealing at the high temperatures of 1050–12001C. It is concluded that Ge can suppress the formation of voids during Si crystal growth. Thus, GCZ Si can be expected to use widely in modern microelectronic industry. r 2002 Elsevier Science B.V. All rights reserved. PACS: 61.72.Cc; 61.72.Ji Keywords: A1. Defects; A2. Single crystal growth; B2. Semiconducting silicon
Si crystals for device applications are usually pulled under the conditions, where vacancies are the prevailing type of intrinsic point defects [1]. During the cooling of the Si crystals from the melting point to room temperature, grown-in voids are formed with densities between 105 and 107/cm3 [2,3]. Voids can be delineated as crystal originated particles (COPs) by treatment in SC1 solution [4], as flow pattern defects (FPDs) by nonagitated Secco etching [5] and as light scattering topography defects (LSTDs) by IR-light scattering [6]. It has been reported that voids can deteriorate *Corresponding author. Tel.: +86-571-8795-1667; fax: +86571-8795-2322. E-mail address: [email protected] (D. Yang).
gate oxide integrity (GOI) yield and enhance leakage current if they are located near the wafer surface [7,8]. Over the years, the nature, the formation behavior, and the techniques to control voids have been studied extensively. Voids are normally of octahedral structure about 100– 300 nm in size as a result of agglomerates of vacancies [9–11]. There are three different methods to control voids: thermally controlled CZ Si crystal growth [12], high-temperature annealing [13], and epitaxial wafers. Recently, nitrogen has been doped into Si crystals to suppress voids [14,15]. However, nitrogen can react with oxygen to generate nitrogen–oxygen complexes as shallow thermal donors, which have the influence on electrical behaviors of Si wafers [16–21]. So far,
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 5 7 2 - 5
D. Yang et al. / Journal of Crystal Growth 243 (2002) 371–374
the relation between point defects, impurities and the formation of voids in CZ Si is still not thoroughly clear. In this letter, an alternative technique to control voids in CZ Si is developed by doping Ge into silicon crystal. It is worthwhile to point out that the tetravalent Ge has an extraordinary high solubility and is not electrically active in silicon crystals. It is verified that Ge doping in Si crystal can suppress the formation of voids, and furthermore, the elimination of grown-in voids in GCZ Si is much easier than that in CZ Si. Moreover, the mechanism for suppression of voids in GCZ Si is discussed. To investigate the effect of Ge on voids, three p-type /1 0 0S GCZ Si crystals with different Ge concentrations and one conventional CZ Si crystal without Ge doping were pulled under almost the same growth conditions. The crystals were of vacancy-type, grown by average pulling rates of 1.3–1.4 mm/min. For the growth of GCZ Si, high purity germanium was put into quartz crucibles with polysilicon nuggets and thus doped into Si ingots. The diameter of the crystals was 125 mm. Wafers sampled from the head and tail segments of the crystals were polished to a thickness of 800 mm. According to the doping weight of Ge, the Ge concentration in the respective head sample of the GCZ1, GCZ2 and GCZ3 Si was estimated to be about B1015, 1016 and 1017 cm3, while, the Ge concentration in the tail sample of each GCZ Si crystal was approximately one order of magnitude higher than that in the head sample. Oxygen concentrations were determined by the Fourier infrared spectroscopy (FTIR) at room temperature with the calibration factor of 3.14 1017 cm2. The initial oxygen concentrations in the head and tail samples of all the crystals were about 1.0–1.1 1018 and 6– 7 1017 cm3, respectively. The samples were etched in non-agitated Secco etchant to observe FPDs by means of an optical microscope. For investigation of annihilation behavior of FPDs, the samples taken from the head segments of the CZ and GCZ1 Si were annealed at 1050–12001C for 2 and 4 h in an Ar ambient, subsequently, the densities of FPDs in the annealed samples were determined.
FPD density (×105/cm3)
372
5
CZ GCZ1 GCZ2 GCZ3
4 3 2 1 0
head segment
tail segment
Fig. 1. FPD densities in as-grown CZ and GCZ Si samples with the different Ge concentrations.
The FPD densities in the as-grown CZ and GCZ Si samples with different Ge concentrations are shown in Fig. 1. It can be seen that the FPD densities in the head samples of the CZ, GCZ1 and GCZ2 Si crystals were about 4.5 105/cm3, while that of the head sample of the GCZ3 with a relatively higher Ge concentration of about 1017/ cm3 was only about 9 104/cm3. For the CZ Si crystal, the FPD density of the tail sample was almost the same as that of the head sample. However, for the GCZ1, GCZ2 and GCZ3 Si crystals, the FPD densities of the tail samples were less than those of the head samples. Furthermore, the FPD densities of the tail samples of the GCZ Si decreased with the increase of Ge concentration. The optical microscope photographs of FPDs in the head samples of the CZ and GCZ3 Si crystals are shown in Fig. 2. It was clear that the FPD density in the GCZ3 was much less than that in the CZ Si. Accordingly, it can be concluded that Ge doping can significantly suppress voids in GCZ Si crystals. The FPD densities of the head samples of the CZ and GCZ1 Si subjected to annealing at 1050– 12001C for 2 and 4 h in a high pure Ar ambient, are shown in Fig. 3. The FPD densities in the asgrown samples of both crystals were similar. After annealing at 10501C for only 2 h, the FPD density in the GCZ1 Si sample reduced significantly, while that in the CZ Si sample almost kept constant. Although the FPD density in the CZ Si sample
D. Yang et al. / Journal of Crystal Growth 243 (2002) 371–374
373
Fig. 2. Optical microscope photographs of FPDs in the head samples of the CZ and GCZ3 Si crystals ( 100).
FPD density (×105/cm3)
6 As-grown
4
CZ annealed for 2h CZ annealed for 4h GCZ1 annealed for 2h GCZ1 annealed for 4h
2
0
1000 1050 1100 1150 1200
temperature ( °C) Fig. 3. FPD densities in the CZ and GCZ1 Si as a function of annealing temperature and time.
decreased to considerable extent after 11501C annealing, it was still a lot higher than that in GCZ1 Si sample. However, after annealing at 12001C, the FPD densities in both of the CZ and GCZ1 Si samples decreased to nearly the same level. It should be pointed out that the prolonged annealing at high temperatures has no notable effect on the annihilation of FPDs because that the FPD densities in the samples annealed for 2 and 4 h at a given temperature only have slight distinction. As mentioned above, it has been proved that the FPDs in the GCZ Si can be annihilated at lower temperature, as compared with those in the CZ Si, which implied that the thermal stability of the voids in the GCZ Si is much poorer. The size effect of Ge atoms is considered to be responsible for suppression of voids in GCZ Si.
( which is larger The radius of Ge atoms is 1.52 A, than that of Si atoms. In general, Ge atoms in Si crystal are at substitutional sites. Therefore, Ge atoms can cause the increase of internal stresses when incorporated into Si lattice. In order to relax the inner stresses, vacancies will combine with the Ge atoms to form complexes Ge–Vn ðnX1Þ or Ge– Om –Vn ðm; nX1Þ prior to vacancy aggregation to form voids during the cooling process of GCZ Si crystal growth. Therefore, the concentration of free vacancies decreases, which leads to suppression of void formation, as verified by the fact that the FPD density decreases with the increase of Ge concentration (see Figs. 1 and 2). Moreover, the Ge concentrations of the tail samples of GCZ Si crystals are higher than those of the head samples, because the segregation coefficient of Ge in Si crystal is much smaller than unity. Therefore, the FPD densities in the tail samples of GCZ Si are less than those in the head samples. Based on our experiments, Ge doping with the concentrations higher than B1017 cm3 has a stronger capability to suppress void defects in GCZ Si (Fig. 1). Furthermore, Ge–Vn or Ge–Om –Vn complexes can enhance oxygen precipitation and thus increase intrinsic gettering (IG) capability of wafer during thermal process. This point has been proved by our experiment, which will be reported elsewhere. On the other hand, because the effective concentration of free vacancies decreases in growing GCZ Si as mentioned above, the formation temperature of voids would reduce, resulting in the decrease of the size of voids. Consequently, the voids in GCZ Si have poorer thermal stability and
374
D. Yang et al. / Journal of Crystal Growth 243 (2002) 371–374
therefore can be eliminated more easily compared with the case of CZ Si crystals (Fig. 3). In conclusion, Ge doping in Si crystals can effectively suppress void defects. It is believed that during the crystal growth the combination of vacancies and doped Ge atoms prior to the vacancy aggregation to form voids, which relaxes the strain originated from the mismatch of Ge atoms in silicon crystal lattice, reduce the concentration of free vacancies significantly. The reduction of free vacancies, on one hand, lead to suppression of grown-in voids; on the other hand, decreases the formation temperature and thus the size of voids, leading to poorer thermal stability of voids as verified by the efficient elimination of FPDs in GCZ Si at relatively lower temperatures. This work is supported by the Natural Science Foundation of China (No. 50032010). The authors would like to thank Mr. Daxi Tian and Mr. Yijun Shen for their helps in crystal growth.
References [1] R. Winkler, M. Sano, J. Electrochem. Soc. 141 (1994) 1389. [2] J. Vanhellemont, S. Senkader, G. Kissinger, V. Higgs, M. Trauwaert, D. Graf, U. Lambert, P. Wagner, J. Crystal Growth 180 (1997) 353.
[3] H. Yamagishi, I. Fusegawa, N. Fujimaki, M. Katayama, Semicond. Sci. Technol. 7 (1992) A135. [4] J. Ryuta, E. Morita, T. Tanaka, Y. Shimanuki, Jpn. J. Appl. Phys. 29 (1990) L1947. [5] H. Bender, J. Vanhellemont, R. Schmolke, Jpn. J. Appl. Phys. 36 (1996) L1217. [6] K. Moriya, T. Ogawa, Jpn. J. Appl. Phys. 22 (1983) 307. [7] J.G. Park, G.S. Lee, K.D. Kwack, J.M. Park, Jpn. J. Appl. Phys. 39 (2000) 197. [8] S. Huth, O. Breitenstein, A. Huber, U. Lambert, J. Appl. Phys. 88 (2000) 4000. [9] M. Itsumi, H. Akiya, T. Ueki, M. Tomita, M. Yamawaki, J. Appl. Phys. 78 (1995) 10. [10] M. Itsumi, H. Akiya, T. Ueki, M. Tomita, M. Yamawaki, Jpn. J. Appl. Phys. 53 (1996) 812. [11] J. Park, J. Park, K. Cho, G. Lee, H. Chung, Proceedings of the Second International Symposium on Advanced Science and Technology of Si Materials, Hawaii, 1996, p. 519. [12] V.V. Voronkov, J. Crystal Growth 59 (1982) 625. [13] W. Wijaranakula, J. Appl. Phys. 75 (1994) 3678. [14] B.M. Park, G.H. Seo, G. Kim, J. Crystal Growth 222 (2001) 74. [15] H. Sawada, K. Kawakami, Phys. Rev. B 62 (2000) 1851. [16] D.R. Yang, J. Lu, L.B. Li, H.N. Yao, D.L. Que, Appl. Phys. Lett. 59 (1991) 1227. [17] C.S. Chen, C.F. Li, H.J. Ye, S.C. Shen, D.R. Yang, J. Appl. Phys. 76 (1994) 3347. [18] D.R. Yang, D.L. Que, K. Sumino, J. Appl. Phys. 77 (1995) 943. [19] D.R. Yang, R.X. Fan, L.B. Li, D.L. Que, K. Sunimo, Appl. Phys. Lett. 68 (1996) 487. [20] D.R. Yang, R.X. Fan, L.B. Li, D.L. Que, K. Sunimo, J. Appl. Phys. 80 (1996) 1493. [21] D.R. Yang, D.L. Que, K. Sunimo, Phys. Stat. Sol. (b) 210 (1998) 295.
Priority communication
Germanium effect on void defects in Czochralski silicon Deren Yang*, Xuegong Yu, Xiangyang Ma, Jin Xu, Liben Li, Duanlin Que State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, China Received 15 February 2002; accepted 13 June 2002 Communicated by L.F. Schneemeyer
Abstract The effect of germanium (Ge) on void defects in lightly Ge-doped Czochralski (GCZ) silicon (Si) crystals has been investigated. Three GCZ Si crystals with different Ge concentrations (1015–1018 cm3) and one conventional Czochralski (CZ) Si crystal were grown under almost the same growth conditions. It is found that the density of flow pattern defects (FPDs) related to void defects in the as-grown GCZ Si decreases with the increase of Ge concentration. The voids in the GCZ Si could be eliminated more easily during annealing at the high temperatures of 1050–12001C. It is concluded that Ge can suppress the formation of voids during Si crystal growth. Thus, GCZ Si can be expected to use widely in modern microelectronic industry. r 2002 Elsevier Science B.V. All rights reserved. PACS: 61.72.Cc; 61.72.Ji Keywords: A1. Defects; A2. Single crystal growth; B2. Semiconducting silicon
Si crystals for device applications are usually pulled under the conditions, where vacancies are the prevailing type of intrinsic point defects [1]. During the cooling of the Si crystals from the melting point to room temperature, grown-in voids are formed with densities between 105 and 107/cm3 [2,3]. Voids can be delineated as crystal originated particles (COPs) by treatment in SC1 solution [4], as flow pattern defects (FPDs) by nonagitated Secco etching [5] and as light scattering topography defects (LSTDs) by IR-light scattering [6]. It has been reported that voids can deteriorate *Corresponding author. Tel.: +86-571-8795-1667; fax: +86571-8795-2322. E-mail address: [email protected] (D. Yang).
gate oxide integrity (GOI) yield and enhance leakage current if they are located near the wafer surface [7,8]. Over the years, the nature, the formation behavior, and the techniques to control voids have been studied extensively. Voids are normally of octahedral structure about 100– 300 nm in size as a result of agglomerates of vacancies [9–11]. There are three different methods to control voids: thermally controlled CZ Si crystal growth [12], high-temperature annealing [13], and epitaxial wafers. Recently, nitrogen has been doped into Si crystals to suppress voids [14,15]. However, nitrogen can react with oxygen to generate nitrogen–oxygen complexes as shallow thermal donors, which have the influence on electrical behaviors of Si wafers [16–21]. So far,
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 5 7 2 - 5
D. Yang et al. / Journal of Crystal Growth 243 (2002) 371–374
the relation between point defects, impurities and the formation of voids in CZ Si is still not thoroughly clear. In this letter, an alternative technique to control voids in CZ Si is developed by doping Ge into silicon crystal. It is worthwhile to point out that the tetravalent Ge has an extraordinary high solubility and is not electrically active in silicon crystals. It is verified that Ge doping in Si crystal can suppress the formation of voids, and furthermore, the elimination of grown-in voids in GCZ Si is much easier than that in CZ Si. Moreover, the mechanism for suppression of voids in GCZ Si is discussed. To investigate the effect of Ge on voids, three p-type /1 0 0S GCZ Si crystals with different Ge concentrations and one conventional CZ Si crystal without Ge doping were pulled under almost the same growth conditions. The crystals were of vacancy-type, grown by average pulling rates of 1.3–1.4 mm/min. For the growth of GCZ Si, high purity germanium was put into quartz crucibles with polysilicon nuggets and thus doped into Si ingots. The diameter of the crystals was 125 mm. Wafers sampled from the head and tail segments of the crystals were polished to a thickness of 800 mm. According to the doping weight of Ge, the Ge concentration in the respective head sample of the GCZ1, GCZ2 and GCZ3 Si was estimated to be about B1015, 1016 and 1017 cm3, while, the Ge concentration in the tail sample of each GCZ Si crystal was approximately one order of magnitude higher than that in the head sample. Oxygen concentrations were determined by the Fourier infrared spectroscopy (FTIR) at room temperature with the calibration factor of 3.14 1017 cm2. The initial oxygen concentrations in the head and tail samples of all the crystals were about 1.0–1.1 1018 and 6– 7 1017 cm3, respectively. The samples were etched in non-agitated Secco etchant to observe FPDs by means of an optical microscope. For investigation of annihilation behavior of FPDs, the samples taken from the head segments of the CZ and GCZ1 Si were annealed at 1050–12001C for 2 and 4 h in an Ar ambient, subsequently, the densities of FPDs in the annealed samples were determined.
FPD density (×105/cm3)
372
5
CZ GCZ1 GCZ2 GCZ3
4 3 2 1 0
head segment
tail segment
Fig. 1. FPD densities in as-grown CZ and GCZ Si samples with the different Ge concentrations.
The FPD densities in the as-grown CZ and GCZ Si samples with different Ge concentrations are shown in Fig. 1. It can be seen that the FPD densities in the head samples of the CZ, GCZ1 and GCZ2 Si crystals were about 4.5 105/cm3, while that of the head sample of the GCZ3 with a relatively higher Ge concentration of about 1017/ cm3 was only about 9 104/cm3. For the CZ Si crystal, the FPD density of the tail sample was almost the same as that of the head sample. However, for the GCZ1, GCZ2 and GCZ3 Si crystals, the FPD densities of the tail samples were less than those of the head samples. Furthermore, the FPD densities of the tail samples of the GCZ Si decreased with the increase of Ge concentration. The optical microscope photographs of FPDs in the head samples of the CZ and GCZ3 Si crystals are shown in Fig. 2. It was clear that the FPD density in the GCZ3 was much less than that in the CZ Si. Accordingly, it can be concluded that Ge doping can significantly suppress voids in GCZ Si crystals. The FPD densities of the head samples of the CZ and GCZ1 Si subjected to annealing at 1050– 12001C for 2 and 4 h in a high pure Ar ambient, are shown in Fig. 3. The FPD densities in the asgrown samples of both crystals were similar. After annealing at 10501C for only 2 h, the FPD density in the GCZ1 Si sample reduced significantly, while that in the CZ Si sample almost kept constant. Although the FPD density in the CZ Si sample
D. Yang et al. / Journal of Crystal Growth 243 (2002) 371–374
373
Fig. 2. Optical microscope photographs of FPDs in the head samples of the CZ and GCZ3 Si crystals ( 100).
FPD density (×105/cm3)
6 As-grown
4
CZ annealed for 2h CZ annealed for 4h GCZ1 annealed for 2h GCZ1 annealed for 4h
2
0
1000 1050 1100 1150 1200
temperature ( °C) Fig. 3. FPD densities in the CZ and GCZ1 Si as a function of annealing temperature and time.
decreased to considerable extent after 11501C annealing, it was still a lot higher than that in GCZ1 Si sample. However, after annealing at 12001C, the FPD densities in both of the CZ and GCZ1 Si samples decreased to nearly the same level. It should be pointed out that the prolonged annealing at high temperatures has no notable effect on the annihilation of FPDs because that the FPD densities in the samples annealed for 2 and 4 h at a given temperature only have slight distinction. As mentioned above, it has been proved that the FPDs in the GCZ Si can be annihilated at lower temperature, as compared with those in the CZ Si, which implied that the thermal stability of the voids in the GCZ Si is much poorer. The size effect of Ge atoms is considered to be responsible for suppression of voids in GCZ Si.
( which is larger The radius of Ge atoms is 1.52 A, than that of Si atoms. In general, Ge atoms in Si crystal are at substitutional sites. Therefore, Ge atoms can cause the increase of internal stresses when incorporated into Si lattice. In order to relax the inner stresses, vacancies will combine with the Ge atoms to form complexes Ge–Vn ðnX1Þ or Ge– Om –Vn ðm; nX1Þ prior to vacancy aggregation to form voids during the cooling process of GCZ Si crystal growth. Therefore, the concentration of free vacancies decreases, which leads to suppression of void formation, as verified by the fact that the FPD density decreases with the increase of Ge concentration (see Figs. 1 and 2). Moreover, the Ge concentrations of the tail samples of GCZ Si crystals are higher than those of the head samples, because the segregation coefficient of Ge in Si crystal is much smaller than unity. Therefore, the FPD densities in the tail samples of GCZ Si are less than those in the head samples. Based on our experiments, Ge doping with the concentrations higher than B1017 cm3 has a stronger capability to suppress void defects in GCZ Si (Fig. 1). Furthermore, Ge–Vn or Ge–Om –Vn complexes can enhance oxygen precipitation and thus increase intrinsic gettering (IG) capability of wafer during thermal process. This point has been proved by our experiment, which will be reported elsewhere. On the other hand, because the effective concentration of free vacancies decreases in growing GCZ Si as mentioned above, the formation temperature of voids would reduce, resulting in the decrease of the size of voids. Consequently, the voids in GCZ Si have poorer thermal stability and
374
D. Yang et al. / Journal of Crystal Growth 243 (2002) 371–374
therefore can be eliminated more easily compared with the case of CZ Si crystals (Fig. 3). In conclusion, Ge doping in Si crystals can effectively suppress void defects. It is believed that during the crystal growth the combination of vacancies and doped Ge atoms prior to the vacancy aggregation to form voids, which relaxes the strain originated from the mismatch of Ge atoms in silicon crystal lattice, reduce the concentration of free vacancies significantly. The reduction of free vacancies, on one hand, lead to suppression of grown-in voids; on the other hand, decreases the formation temperature and thus the size of voids, leading to poorer thermal stability of voids as verified by the efficient elimination of FPDs in GCZ Si at relatively lower temperatures. This work is supported by the Natural Science Foundation of China (No. 50032010). The authors would like to thank Mr. Daxi Tian and Mr. Yijun Shen for their helps in crystal growth.
References [1] R. Winkler, M. Sano, J. Electrochem. Soc. 141 (1994) 1389. [2] J. Vanhellemont, S. Senkader, G. Kissinger, V. Higgs, M. Trauwaert, D. Graf, U. Lambert, P. Wagner, J. Crystal Growth 180 (1997) 353.
[3] H. Yamagishi, I. Fusegawa, N. Fujimaki, M. Katayama, Semicond. Sci. Technol. 7 (1992) A135. [4] J. Ryuta, E. Morita, T. Tanaka, Y. Shimanuki, Jpn. J. Appl. Phys. 29 (1990) L1947. [5] H. Bender, J. Vanhellemont, R. Schmolke, Jpn. J. Appl. Phys. 36 (1996) L1217. [6] K. Moriya, T. Ogawa, Jpn. J. Appl. Phys. 22 (1983) 307. [7] J.G. Park, G.S. Lee, K.D. Kwack, J.M. Park, Jpn. J. Appl. Phys. 39 (2000) 197. [8] S. Huth, O. Breitenstein, A. Huber, U. Lambert, J. Appl. Phys. 88 (2000) 4000. [9] M. Itsumi, H. Akiya, T. Ueki, M. Tomita, M. Yamawaki, J. Appl. Phys. 78 (1995) 10. [10] M. Itsumi, H. Akiya, T. Ueki, M. Tomita, M. Yamawaki, Jpn. J. Appl. Phys. 53 (1996) 812. [11] J. Park, J. Park, K. Cho, G. Lee, H. Chung, Proceedings of the Second International Symposium on Advanced Science and Technology of Si Materials, Hawaii, 1996, p. 519. [12] V.V. Voronkov, J. Crystal Growth 59 (1982) 625. [13] W. Wijaranakula, J. Appl. Phys. 75 (1994) 3678. [14] B.M. Park, G.H. Seo, G. Kim, J. Crystal Growth 222 (2001) 74. [15] H. Sawada, K. Kawakami, Phys. Rev. B 62 (2000) 1851. [16] D.R. Yang, J. Lu, L.B. Li, H.N. Yao, D.L. Que, Appl. Phys. Lett. 59 (1991) 1227. [17] C.S. Chen, C.F. Li, H.J. Ye, S.C. Shen, D.R. Yang, J. Appl. Phys. 76 (1994) 3347. [18] D.R. Yang, D.L. Que, K. Sumino, J. Appl. Phys. 77 (1995) 943. [19] D.R. Yang, R.X. Fan, L.B. Li, D.L. Que, K. Sunimo, Appl. Phys. Lett. 68 (1996) 487. [20] D.R. Yang, R.X. Fan, L.B. Li, D.L. Que, K. Sunimo, J. Appl. Phys. 80 (1996) 1493. [21] D.R. Yang, D.L. Que, K. Sunimo, Phys. Stat. Sol. (b) 210 (1998) 295.