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On the determination and control of flats location in liquid-encapsulated Czochralski grown InP wafers
MI'f.IlUlI~S
ELSEVIER
Materials Science and Engineering B28 (1994) 80-83
B
On the determination and control of flats location in liquid-encapsulated Czochralski grown InP wafers M. Favaretto, G.M. Guadalupi, L. Meregalli, B. Molinas*, G. T o l o m i o TEMA V S.p.A., Centro Ricerche Venezia, Via delle Industrie 39, 30175 P. Marghera- Venezia, Italy
Abstract The importance of the distinction between (110) directions on the surface of ( 100)-oriented wafers for epitaxial growth and device fabrication is well known. The distinction is made by means of two flats. The traditional methods used for the accurate determination of flats location on ingots or wafers (with X-ray diffraction) or for the qualitative distinction between the flats (H2SO4 etchant) are compared with alternative procedures found in literature and/or developed in the present work: the use of dislocation-related etch pits (DREPs) revealed by the "Chu et al." etchant and of the brightness and geometry of the faces on the cone of growth. Studies conducted on doped and undoped InP, by using optical and scanning electron microscopy, indicate that (i) concerning the cone of growth, the result differs from that typically used for GaAs and (ii) concerning the "Chu et al." etchant, it originates well-defined pyramids with a rectangular base having its longer side parallel to the [0] i]-[011] axis on side A and rotated 90 ° on side B. It is suggested that DREPs can be used quantitatively for the control of flats location or for the determination of [01 i ] and [0 i 1] directions on wafers without flats.
Keywords: Indium phosphide; Diffraction; Etching; Semiconductors
1. Introduction The presence of anisotropy in many parameters (such as surface migration rate and growth velocity [1]) controlling the epitaxial growth on (100) InP wafers represents one of the examples of the importance of the distinction between [011] and [0 ] 1] directions. The identification of the two directions is essential for the fabrication of some devices. The distinction is carried out by means of two flats: the Primary Flat (or orientation flat, OF ), a (011) face, and the Secondary Flat (or identification flat, IF ), a (0 ] 1) face. The determination of fiats location on (100) InP ingots (without the distinction between them), for the machining of those flats before the cutting of the wafers, is carried out with accuracy by means of X-ray diffraction (XRD). XRD also accurately determines two perpendicular (110) directions on (100) wafers. The distinction between the two flats, or between the [0 ] i] and [0 i 1] directions on wafers without flats, is qualitatively evidenced by means of etch figures (EFs) by using the H2SO 4 etchant [2]. Such etching gives rise to greatly elongated parallel pits on the (100)
*Corresponding author. 0921-5107/94/$7.00 © 1994 - Elsevier Science S.A. All rights reserved
SSDI 0921-5107 (94)00735-7
surface in the [01i] direction [2]; etch patterns are rotated 90 ° on the back surface owing to the rotation of the {111 }facets of In and P on opposite surfaces. In the present work, both methods will be followed and compared with alternative procedures such as the use of dislocation-related etch pits (DREPs) and of the cones of growth.
2. Experimentaldetails The following were taken into account: 2 in wafers from InP single crystals grown in TEMAV-Centro Ricerche Venezia -- undoped InP with carrier concentration (n) of 8 x 1015 cm -3 and mean etch pit density (EPD) of 1 × 10 5 cm-2; S-doped InP with n = 5 . 9 x 1018 cm -3, E P D = 2 x 103 cm 2 and dislocation-free area (DFA) of 7 cm2; Zn-doped InP with n = 3 . 0 x 1018 cm -3, E P D = 5 x 1 0 4 c m - 2 in the periphery and DFA= 12 cm2; semi-insulating (SI) (Fedoped) InP with n = 5.7 x 10 7 c m -3 and EPD < 4 x 104 cm-2), and a wafer from Crismatec (SI (Fe-doped) InP with n = 1.2 x 108 cm -3 and EPD < 5 x 10 4 cm-2). XRD was performed by using a Rigaku goniometer. The H2SO 4 etchant was 1H2SO4:lH202:lH20 [2] for {100} surfaces on as-cut wafers; the best results, in our
M. Favaretto et al.
/
Materials Science and Engineering B28 (1994) 80-83
case, were obtained at room temperature for 20 min. The "Chu et al" etchant was 3HBr:lHNO3 [3] used on polished {100} surfaces at room temperature for 20 s. EFs and DREPs were studied by means of optical microscopy (including Nomarski interference contrast), scanning electron microscopy (SEM) and an image analyzer (Quantimet Q570) operating "on line" with both microscopes. 3. Results and discussion
Clear EFs, covering all the (100) surface of the wafers, each one elongated along the [0i 1]-[011] axis as is indicated schematically in Fig. 1 (feature 1), were created when the U 2 S O 4 etchant was used. The pits are arranged in such a way that [0]1]-[01]] lines can be qualitatively located or drawn on the surface. The EFs rotate 90 ° on the surface (i00) (see feature 2) as was reported in Ref. [2]. Some wafers were then subjected to cleavage. After SEM examination, clear "dove-tail" and V-shaped profiles were found in the intersection of the (100) face with {0 i ]} and {0 i 1} faces, respectively, as is indicated in the Semi M23-93 Standard [4] for the OF and for the IE In addition, the (100) and (]00) surfaces were etched with "Chu et al." etchant and after cleavage all the {100} and {110} surfaces were examined. It was reported, when discovered [3], that the "Chu et al." etchant produces pyramidal pits elongated along one of the (110) directions on the (001) surface of InP. A similar result was already anticipated by Akita et al. [5], who found pits elongated along (110) directions on (100) InP by using HBr:HF and HBr:CHaCOOH as
81
etchants; however, it can be seen from the literature that the definition of the rectangles is higher in the case of the "Chu et al" etchant. In the present work, for all the different materials, it was observed that the "Chu et al" etchant originates well-defined pyramids with a rectangular base having its longer side parallel to the [0 i i ]-[011 ] axis on side A or (100) of the wafers (Fig. 1, feature 3), and having its shorter side parallel to the [0l ]]-[011] axis on side B or (i00)(feature 4). That is to say the rectangles rotate 90 ° following the crystallographic polarity of sphalerite-type crystals. These results indicate that DREPs created with the "Chu et al" etchant are able to differentiate between the [0] ]] and [0 ] 1] directions and between side A and side B. Furthermore, for all the materials, the DREP created on a given side rotates 90 ° with respect to the EF created with the H 2 S O 4 etchant. This general result, which we obtained on comparing DREPs and EFs, is related to a previous result, found for n-type InP [6], reporting that HCl-based etchants produce elongated pits along the [il0] axis on the (001) surface while the pits rotate 90 ° for HBr-based etchants, and that, as a consequence, it has to be assumed that the HBr etchants have a faster etching rate on the {111 } In planes. Often the DREPs exhibited a profile on the ( 0 i l ) face like the one shown in Fig. 2, where ct is the angle between the side wall of the pit and the (100) surface. In Fig. 2, a l = a 2 = ( 9 . 8 + 0 . 2 ) °. Another symmetrical "V" was observed on face (0ii), being ctl(-~a2) estimated at 14 °. The symmetry of this DREP indicates that the direction of the dislocation line is (100). Pits formed on dislocations perpendicular to the surface are center-symmetrical [7]. Other DREPs were observed to have the bottom point of the pyramid shifted with respect to the geometrical center of the base along (001) directions parallel to the (100) face; their associated dislocation lines are inclined to the surface.
i i i i
BB'
\\\
AT-
I.F. I ~ ,'";''i'","'{,
o.F
BE'
Fig. 1. Etch figures with H2SO 4 etchant on (100) and (i 00) faces of InP wafers (features 1 and 2, respectively); DREPs with "Chu et al." etchant (features 3 and 4, respectively).
Fi~. 2. SEM image of a DREP profile ("Chu et al." etchant) on a (011 ) face after cleavage of S-doped InP wafers.
82
M. Favaretto et al.
/
Materials Science and Engineering B28 (1994) 80-83
Fig. 3 shows one flat on a 2 in S-doped InP wafer. All the elongated black spots are DREPs; many of them are following straight lines which are roughly perpendicular to the flat. These DREPs should correspond to dislocations found [8] in liquid-encapsulated Czochralski (LEC) grown ingots which suffer slippage propagating into the ingot interior in (110) directions normal to the growth direction. Resolved components
'=
•
.•
#
,b.
•
t~ •
•
•
•
•
t,
•
411~'
•
•
• J
•
,
•
j
*
• •
o e •
I,
e
•
""
I
-4w" ~ ~O
•
j e
t
¢ *4
•
: a
e
e~,
."
""
•
A
'.~"
O
~
V
:
~B
.
•
e
:
:
"
"
.,
!
Fig. 3. Optical micrograph ( x 100) on a (100) face of a S-doped InP wafer after etching ("Chu et al." etchant).
(in the {111 }/(1 10) systems) of grown-related thermal stresses give rise to the formation and motion of those dislocations. We checked if it is possible to take advantage of the presence of those DREPs on the trace of slip planes, in order to locate the flats. In fact, an isolated DREP, even though it has its sides properly orientated, is not enough for the determination of each (110} direction with accuracy. But hundreds of them, lying along large distances on the same slip plane or on a group of parallel, neighbor planes (()wing to the inhomogeneity of the plastic deformation), can give unexpected, accurate information about the position of the trace of such a plane. We checked the location of the flats, previously determined with the aid of XRD, by using the optical microscope, the image analyzer and the measurement of the angle fl between the slip plane and the normal to the flat. For both the OF orientation, where the standard [4] requires a tolerance of 0.5 °, and for the IF, where 5 ° are required, we found values of fl< 0.5 °. We can therefore state that, when DREPs are present and situated along slip planes (it occurs even for wafers with high DFA), this method can be used quantitatively for the control of flats location or the determination of (110) directions on wafers without flats. It should be pointed out that the H z S O 4 etchant cannot be used for similar purposes because the ElLs, even when created on the perfect lattice, do not present a long-range order. In addition, a rectangular DREP allows the distinction between the [011] and [011] directions. Fig. 4 shows a typical cone of growth of an [100] InP ingot. Two shiny and two matt zones (Fig. 4(b)) were observed in all the materials, and they correspond to shoulders found under In facets and P facets, respectively. The effect can be enhanced with the 1HCI:IHNO3 etchant. Furthermore, a rule for the distinction between the (110) directions is schematically presented in Fig. 4(a): it always happens that (a/b)< 1, where a and b are the bases of the shiny and matt zones. These results differ from those typically used for GaAs ingots: the cone presents a rectangular-like base with its longer side under the Ga facets, and differences in the brightness are not clearly visible.
e~
4.
sh0nv
(b)
ZOnE~
Fig. 4. (a) Schematic view of the cone of growth in LEC-grown [100] InP ingots. (b) Cone of growth in TEMAV's LEC-grown It00] SI (Fe-doped) InP.
Conclusions
Different methods for the determination and control of fiats location, which can help the personnel dealing with the machining and the quality control of wafers, were cross-checked. XRD is always used for the accurate determination of fiats location on ingots or wafers. The qualitative distinction between the two fiats is made by using the H2SO4 etchant. The qualitative dis-
M. Favaretto et al.
/
Materials Science and Engineering B28 (1994) 80-83
tinction between the (110) directions can be made by using the "Chu et al" etchant. In the present work, based on the brightness and geometry of the cone of growth, a rule which also allows the distinction between flats is presented, Furthermore, it is suggested that by means of dislocation-related etch pits it is possible both to make the distinction and to quantitatively determine the location for the flats.
Acknowledgement The present work has been carried out within a contract given to T E M A V SpA -- Porto Marghera (VE) by the Ministero dell'Universith e della Ricerca Scientifica e Tecnologica, Italy, within the "Programma Nazionale di Ricerca per i Materiali Innovativi Avanzati".
83
References [1] R. Bath, M.A. Koza, D.M. Hwang, K. Kash, C. Caneau and R.E. Nahory, J. Cryst. Growth, 110(1991) 353. [2] E.A. Caridi and T.Y. Chang, J. Electrochem. Soc., 131 (1984) 1440. [3] S.N. Chu, C.M. Jodlauk and A.A. Ballman, J. Electrochem. Soc., 129(1982) 352. [4] Semi-International-Standards 1993, Semiconductor Equipment and Materials Int., Mountain View, CA 94043, USA. [5] K. Akita, T. Kusunoki, S. Komiya and T. Kotani, J. Cryst. Growth, 46(1979) 783. [6] D.T.C. Huo, J.D. Wynn, M.F. Yan and D.P. Wilt, J. Electrochem. Soc., 136 (1989) 1804. [7] J.L. Weyher, in S.Mahajan (ed.), Handbook on Semiconductors: Materials, Properties and Preparation, Vol. 3, NorthHolland, Amsterdam, rev. edn., 1994, in press. [8] J. Matsui, 1987 Microsc. Semicond. Mater. Conf., Oxford, Institute of Physics, Conf. Ser., 87, Section 4, p. 249.
ELSEVIER
Materials Science and Engineering B28 (1994) 80-83
B
On the determination and control of flats location in liquid-encapsulated Czochralski grown InP wafers M. Favaretto, G.M. Guadalupi, L. Meregalli, B. Molinas*, G. T o l o m i o TEMA V S.p.A., Centro Ricerche Venezia, Via delle Industrie 39, 30175 P. Marghera- Venezia, Italy
Abstract The importance of the distinction between (110) directions on the surface of ( 100)-oriented wafers for epitaxial growth and device fabrication is well known. The distinction is made by means of two flats. The traditional methods used for the accurate determination of flats location on ingots or wafers (with X-ray diffraction) or for the qualitative distinction between the flats (H2SO4 etchant) are compared with alternative procedures found in literature and/or developed in the present work: the use of dislocation-related etch pits (DREPs) revealed by the "Chu et al." etchant and of the brightness and geometry of the faces on the cone of growth. Studies conducted on doped and undoped InP, by using optical and scanning electron microscopy, indicate that (i) concerning the cone of growth, the result differs from that typically used for GaAs and (ii) concerning the "Chu et al." etchant, it originates well-defined pyramids with a rectangular base having its longer side parallel to the [0] i]-[011] axis on side A and rotated 90 ° on side B. It is suggested that DREPs can be used quantitatively for the control of flats location or for the determination of [01 i ] and [0 i 1] directions on wafers without flats.
Keywords: Indium phosphide; Diffraction; Etching; Semiconductors
1. Introduction The presence of anisotropy in many parameters (such as surface migration rate and growth velocity [1]) controlling the epitaxial growth on (100) InP wafers represents one of the examples of the importance of the distinction between [011] and [0 ] 1] directions. The identification of the two directions is essential for the fabrication of some devices. The distinction is carried out by means of two flats: the Primary Flat (or orientation flat, OF ), a (011) face, and the Secondary Flat (or identification flat, IF ), a (0 ] 1) face. The determination of fiats location on (100) InP ingots (without the distinction between them), for the machining of those flats before the cutting of the wafers, is carried out with accuracy by means of X-ray diffraction (XRD). XRD also accurately determines two perpendicular (110) directions on (100) wafers. The distinction between the two flats, or between the [0 ] i] and [0 i 1] directions on wafers without flats, is qualitatively evidenced by means of etch figures (EFs) by using the H2SO 4 etchant [2]. Such etching gives rise to greatly elongated parallel pits on the (100)
*Corresponding author. 0921-5107/94/$7.00 © 1994 - Elsevier Science S.A. All rights reserved
SSDI 0921-5107 (94)00735-7
surface in the [01i] direction [2]; etch patterns are rotated 90 ° on the back surface owing to the rotation of the {111 }facets of In and P on opposite surfaces. In the present work, both methods will be followed and compared with alternative procedures such as the use of dislocation-related etch pits (DREPs) and of the cones of growth.
2. Experimentaldetails The following were taken into account: 2 in wafers from InP single crystals grown in TEMAV-Centro Ricerche Venezia -- undoped InP with carrier concentration (n) of 8 x 1015 cm -3 and mean etch pit density (EPD) of 1 × 10 5 cm-2; S-doped InP with n = 5 . 9 x 1018 cm -3, E P D = 2 x 103 cm 2 and dislocation-free area (DFA) of 7 cm2; Zn-doped InP with n = 3 . 0 x 1018 cm -3, E P D = 5 x 1 0 4 c m - 2 in the periphery and DFA= 12 cm2; semi-insulating (SI) (Fedoped) InP with n = 5.7 x 10 7 c m -3 and EPD < 4 x 104 cm-2), and a wafer from Crismatec (SI (Fe-doped) InP with n = 1.2 x 108 cm -3 and EPD < 5 x 10 4 cm-2). XRD was performed by using a Rigaku goniometer. The H2SO 4 etchant was 1H2SO4:lH202:lH20 [2] for {100} surfaces on as-cut wafers; the best results, in our
M. Favaretto et al.
/
Materials Science and Engineering B28 (1994) 80-83
case, were obtained at room temperature for 20 min. The "Chu et al" etchant was 3HBr:lHNO3 [3] used on polished {100} surfaces at room temperature for 20 s. EFs and DREPs were studied by means of optical microscopy (including Nomarski interference contrast), scanning electron microscopy (SEM) and an image analyzer (Quantimet Q570) operating "on line" with both microscopes. 3. Results and discussion
Clear EFs, covering all the (100) surface of the wafers, each one elongated along the [0i 1]-[011] axis as is indicated schematically in Fig. 1 (feature 1), were created when the U 2 S O 4 etchant was used. The pits are arranged in such a way that [0]1]-[01]] lines can be qualitatively located or drawn on the surface. The EFs rotate 90 ° on the surface (i00) (see feature 2) as was reported in Ref. [2]. Some wafers were then subjected to cleavage. After SEM examination, clear "dove-tail" and V-shaped profiles were found in the intersection of the (100) face with {0 i ]} and {0 i 1} faces, respectively, as is indicated in the Semi M23-93 Standard [4] for the OF and for the IE In addition, the (100) and (]00) surfaces were etched with "Chu et al." etchant and after cleavage all the {100} and {110} surfaces were examined. It was reported, when discovered [3], that the "Chu et al." etchant produces pyramidal pits elongated along one of the (110) directions on the (001) surface of InP. A similar result was already anticipated by Akita et al. [5], who found pits elongated along (110) directions on (100) InP by using HBr:HF and HBr:CHaCOOH as
81
etchants; however, it can be seen from the literature that the definition of the rectangles is higher in the case of the "Chu et al" etchant. In the present work, for all the different materials, it was observed that the "Chu et al" etchant originates well-defined pyramids with a rectangular base having its longer side parallel to the [0 i i ]-[011 ] axis on side A or (100) of the wafers (Fig. 1, feature 3), and having its shorter side parallel to the [0l ]]-[011] axis on side B or (i00)(feature 4). That is to say the rectangles rotate 90 ° following the crystallographic polarity of sphalerite-type crystals. These results indicate that DREPs created with the "Chu et al" etchant are able to differentiate between the [0] ]] and [0 ] 1] directions and between side A and side B. Furthermore, for all the materials, the DREP created on a given side rotates 90 ° with respect to the EF created with the H 2 S O 4 etchant. This general result, which we obtained on comparing DREPs and EFs, is related to a previous result, found for n-type InP [6], reporting that HCl-based etchants produce elongated pits along the [il0] axis on the (001) surface while the pits rotate 90 ° for HBr-based etchants, and that, as a consequence, it has to be assumed that the HBr etchants have a faster etching rate on the {111 } In planes. Often the DREPs exhibited a profile on the ( 0 i l ) face like the one shown in Fig. 2, where ct is the angle between the side wall of the pit and the (100) surface. In Fig. 2, a l = a 2 = ( 9 . 8 + 0 . 2 ) °. Another symmetrical "V" was observed on face (0ii), being ctl(-~a2) estimated at 14 °. The symmetry of this DREP indicates that the direction of the dislocation line is (100). Pits formed on dislocations perpendicular to the surface are center-symmetrical [7]. Other DREPs were observed to have the bottom point of the pyramid shifted with respect to the geometrical center of the base along (001) directions parallel to the (100) face; their associated dislocation lines are inclined to the surface.
i i i i
BB'
\\\
AT-
I.F. I ~ ,'";''i'","'{,
o.F
BE'
Fig. 1. Etch figures with H2SO 4 etchant on (100) and (i 00) faces of InP wafers (features 1 and 2, respectively); DREPs with "Chu et al." etchant (features 3 and 4, respectively).
Fi~. 2. SEM image of a DREP profile ("Chu et al." etchant) on a (011 ) face after cleavage of S-doped InP wafers.
82
M. Favaretto et al.
/
Materials Science and Engineering B28 (1994) 80-83
Fig. 3 shows one flat on a 2 in S-doped InP wafer. All the elongated black spots are DREPs; many of them are following straight lines which are roughly perpendicular to the flat. These DREPs should correspond to dislocations found [8] in liquid-encapsulated Czochralski (LEC) grown ingots which suffer slippage propagating into the ingot interior in (110) directions normal to the growth direction. Resolved components
'=
•
.•
#
,b.
•
t~ •
•
•
•
•
t,
•
411~'
•
•
• J
•
,
•
j
*
• •
o e •
I,
e
•
""
I
-4w" ~ ~O
•
j e
t
¢ *4
•
: a
e
e~,
."
""
•
A
'.~"
O
~
V
:
~B
.
•
e
:
:
"
"
.,
!
Fig. 3. Optical micrograph ( x 100) on a (100) face of a S-doped InP wafer after etching ("Chu et al." etchant).
(in the {111 }/(1 10) systems) of grown-related thermal stresses give rise to the formation and motion of those dislocations. We checked if it is possible to take advantage of the presence of those DREPs on the trace of slip planes, in order to locate the flats. In fact, an isolated DREP, even though it has its sides properly orientated, is not enough for the determination of each (110} direction with accuracy. But hundreds of them, lying along large distances on the same slip plane or on a group of parallel, neighbor planes (()wing to the inhomogeneity of the plastic deformation), can give unexpected, accurate information about the position of the trace of such a plane. We checked the location of the flats, previously determined with the aid of XRD, by using the optical microscope, the image analyzer and the measurement of the angle fl between the slip plane and the normal to the flat. For both the OF orientation, where the standard [4] requires a tolerance of 0.5 °, and for the IF, where 5 ° are required, we found values of fl< 0.5 °. We can therefore state that, when DREPs are present and situated along slip planes (it occurs even for wafers with high DFA), this method can be used quantitatively for the control of flats location or the determination of (110) directions on wafers without flats. It should be pointed out that the H z S O 4 etchant cannot be used for similar purposes because the ElLs, even when created on the perfect lattice, do not present a long-range order. In addition, a rectangular DREP allows the distinction between the [011] and [011] directions. Fig. 4 shows a typical cone of growth of an [100] InP ingot. Two shiny and two matt zones (Fig. 4(b)) were observed in all the materials, and they correspond to shoulders found under In facets and P facets, respectively. The effect can be enhanced with the 1HCI:IHNO3 etchant. Furthermore, a rule for the distinction between the (110) directions is schematically presented in Fig. 4(a): it always happens that (a/b)< 1, where a and b are the bases of the shiny and matt zones. These results differ from those typically used for GaAs ingots: the cone presents a rectangular-like base with its longer side under the Ga facets, and differences in the brightness are not clearly visible.
e~
4.
sh0nv
(b)
ZOnE~
Fig. 4. (a) Schematic view of the cone of growth in LEC-grown [100] InP ingots. (b) Cone of growth in TEMAV's LEC-grown It00] SI (Fe-doped) InP.
Conclusions
Different methods for the determination and control of fiats location, which can help the personnel dealing with the machining and the quality control of wafers, were cross-checked. XRD is always used for the accurate determination of fiats location on ingots or wafers. The qualitative distinction between the two fiats is made by using the H2SO4 etchant. The qualitative dis-
M. Favaretto et al.
/
Materials Science and Engineering B28 (1994) 80-83
tinction between the (110) directions can be made by using the "Chu et al" etchant. In the present work, based on the brightness and geometry of the cone of growth, a rule which also allows the distinction between flats is presented, Furthermore, it is suggested that by means of dislocation-related etch pits it is possible both to make the distinction and to quantitatively determine the location for the flats.
Acknowledgement The present work has been carried out within a contract given to T E M A V SpA -- Porto Marghera (VE) by the Ministero dell'Universith e della Ricerca Scientifica e Tecnologica, Italy, within the "Programma Nazionale di Ricerca per i Materiali Innovativi Avanzati".
83
References [1] R. Bath, M.A. Koza, D.M. Hwang, K. Kash, C. Caneau and R.E. Nahory, J. Cryst. Growth, 110(1991) 353. [2] E.A. Caridi and T.Y. Chang, J. Electrochem. Soc., 131 (1984) 1440. [3] S.N. Chu, C.M. Jodlauk and A.A. Ballman, J. Electrochem. Soc., 129(1982) 352. [4] Semi-International-Standards 1993, Semiconductor Equipment and Materials Int., Mountain View, CA 94043, USA. [5] K. Akita, T. Kusunoki, S. Komiya and T. Kotani, J. Cryst. Growth, 46(1979) 783. [6] D.T.C. Huo, J.D. Wynn, M.F. Yan and D.P. Wilt, J. Electrochem. Soc., 136 (1989) 1804. [7] J.L. Weyher, in S.Mahajan (ed.), Handbook on Semiconductors: Materials, Properties and Preparation, Vol. 3, NorthHolland, Amsterdam, rev. edn., 1994, in press. [8] J. Matsui, 1987 Microsc. Semicond. Mater. Conf., Oxford, Institute of Physics, Conf. Ser., 87, Section 4, p. 249.