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Floating zone crystallization of silicon
CARRIER EXTRACTION IN GERMANIUM
1059
2. Experimental. I n order t h a t the theory in Section It shall be valid it is essential t h a t the rate of injection of minority carriers from the base connections shall be small compared with the generation rate. This condition can be achieved for near-intrinsic n-type material b y growing a heavily doped, long lifetime n-type section on to one end of t h e crystal. This end is then made positive when the extraction field is applied and the effective injection ratio is then given a p p r o x i m a t e l y by Minority carrier c u r r e n t / T o t a l current = anLp/an+ L#+ where an, an+ and L#, Ln+ are the conductivities and minority carrier diffusion lengths respectively of the near-intrinsic and heavily doped sections.
3. Extraction characteristics. The voltage t o be applied to the specimen to obtain extraction is given a p p r o x i m a t e l y by V = 0.025 (L/Lp) 2 volts where L is the length of the specimen. This equation has been confirmed experim e n t a l l y and it is found t h a t the extraction voltage is largely independent of temperature and background illumination. U n d e r suitable conditions a change in conductiv i t y of a factor 10 can be obtained readily. W h e n the specimelX is e x t r a c te d b y voltage pulses of short duration the time required for extraction and hence the carrier mobility m ay be determined. I t is possible with this technique to e x t e n t drift mobility measurements well into the intrinsic conduction range. The theoretical predictions of S h o c k I e y 1), namely t h a t the carrier group mobility tends to zero in intrinsic material, has been confirmed qua n t i t at i v el y . Acknowledgement i s made to the Chief Scientist, British Ministry of Supply and to the Controller, H. B. M. Stationery Office for permission to publish this paper. British Crown Copyright Reserved. Received 1-7-54. REFERENCES 1) S h o c k 1 e y, W., Electrons and Holes in Semi-Conductors, Van Nostrand, New York, 1950, page 329 et sequi.
Keck, P . H . 1954
Physica X X No. 11 A m s t e r d a m Conference Semiconductors
FLOATING ZONE CRYSTALLIZATION OF SILICON by PAUL H. KECK Signal Corps Engineering Laboratories, Fort Monmouth, New Jersey, U.S.A. The surface tension of molten silicon is 720 dynes/cm 1). This relatively high value makes it possible for a zone of molten silicon to be held stable between two vertically aligned solid silicon rods clamped at both ends thus eliminating a container for the
1060
PAUL H. KECK
m e l t . T h e s h a p e of s u c h a l i q u i d b o d y w h i c h we t e r m e d " f l o a t i n g z o n e " 2) is d e s c r i b e d by Laplace's equation: P = 7 (I/R1 + l/R2) w h e r e p is t h e p r e s s u r e d i f f e r e n c e a c r o s s t h e s u r f a c e m e m b r a n e of t h e l i q u i d a t a n y p o i n t , R 1 a n d R 2 are t h e p r i n c i p a l r a d i i of c u r v a t u r e a t t h a t p o i n t , a n d y is t h e s u r f a c e t e n s i o n of t h e liquid. T h e s o l u t i o n of t h i s e q u a t i o n y i e l d s a f a m i l y of c u r v e s w i t h t h e p r e s s u r e i n s i d e t h e l i q u i d b o d y as a p a r a m e t e r . U s i n g t h e s e c u r v e s t h e s t a b l e s h a p e s of f l o a t i n g z o n e s f o r g i v e n cross s e c t i o n s of t h e s u p p o r t i n g r o d s h a v e b e e n d e t e r m i n e d . T h e r e s u l t s are p u b l i s h e d in a r e c e n t p a p e r ~). I t w a s f o u n d t h a t w i t h silicon l i q u i d z o n e s c a n b e h e l d s t a b l e b e t w e e n solid r o d s of c y l i n d r i c a l s h a p e u p to a p p r o x i m a t e l y 1 c e n t i m e t e r d i a m e t e r . T h e l i q u i d zone c a n b e m a d e t o t r a v e l a l o n g t h e r o d axis, m e l t i n g n e w silicon o n t h e a d v a n c i n g s i d e a n d c r y s t a l l i z i n g in its wake. T h i s m e t h o d of zone m e l t i n g i n a v e r t i c a l d i r e c t i o n r e q u i r e s t h e f o r m a t i o n of a v e r t i c a l t a n g e n t a t t h e l i q u i d solid b o u n d a r y o n t h e s o l i d i f y i n g side of t h e m o v i n g m o l t e n zone if a r e c r y s t a l l i z e d r o d of c o n s t a n t d i a m e t e r is desired. T h e c o r r e s p o n d i n g s h a p e s for m o t i o n of t h e l i q u i d silicon zone e i t h e r u p or d o w n are s h o w n in F i g u r e s 1 a n d 2. SOLID
"'--" 0.79 '
SOLID
4-
LIQUID 0.95
LIQUID 0.66
SOLID
SOLID
"~
0.90 B
A SOLID
~
-h
htCM
A=2.2 B=2.0 C=4.0 D= 1.9
1,74 1.58 3,2 1.5
SOLI:
t LIQUI£
0.6"/"
÷ SOLID C
-
LOWER TANGENCY ALL
D I S T A N C E S IN
0.96
CM.
Fig. 1. S t a b l e S h a p e s of F l o a t i n g Silicon Z o n e s w i t h L o w e r V e r t i c a l T a n g e n c y . I n t h e a p p l i c a t i o n of t h e f l o a t i n g zone m e t h o d , t h e f i r s t a p p r o a c h was to m e l t t h e silicon in a n a t m o s p h e r e of h e l i u m or a r g o n u s i n g r a d i a t i o n h e a t i n g f r o m a t u n g s t e n coil w h i c h e n c i r c l e s t h e i n g o t a l o n g a s h o r t section. T h e a p p a r a t u s , as it was used, is s h o w n s c h e m a t i c a l l y i n F i g u r e 3. T h e h e a t e r e l e m e n t s u r r o u n d i n g t h e silicon rod, in t h e d e s c r i b e d s y s t e m , is a possible s o u r c e for t h e i n t r o d u c t i o n of i m p u r i t i e s , since t h e
1061
FLOATING ZONE CRYSTALLIZATION OF SILICON
vapour pressure of tungsten, at the required high temperatures of 2000°C to 2200°C, is of some concern. For these reasons, an improved apparatus was designed and constructed which completely eliminates the heater element inside the melting chamber. In addition, a number of other i m p r o v e m e n t s were incorporated in the new e q u i p m e n t as a result of the experience with the first apparatus. To avoid placing a heater element inside the melting chamber, heating of the silicon by high frequency induction was chosen, whereby the induction coil is placed concentrically to the silicon rod b u t exterior to the quartz cylinder. A schematic diagram of the apparatus is shown in Figure 4. The melting chamber, A is mounted in a fixed position on a cross-member of the iron frame B. I t consists of '=
.76
-~'
SOLID
_k
~"
LIQ~UID LIQUID
1
1-SOLID
A
SOLID
h CM
A= 1.9 B =2.0 C=2.1
1.50 1.58 1.67
D =2.2 E =4.0
1.74 3.2
B
•
3-
SOLID
C
SOLIDI~2
D
~
E
UPPER TANGENCY
ALL DIMENSIONS IN CM.
Fig. 2. Stable Shapes of Floating Silicon Zones with Upper Vertical Tangency. an exchangeable quartz tube of 250 mm length, held between the water-cooled plates C using rubber gaskets for a v a c u u m - t i g h t seal. The silicon sample is supported by the tungsten fingered chucks D mounted in the water-cooled stainless steel tubes E. These tubes move through v a c u u m - t i g h t "Wilson seals F connected by short metal bellows to the plates C. Both tubes E are supported in brass bearings on the moveable frame G and can be rotated independently by small synchronous gear-head motors H with rubber cog belts. Each tube can be positioned manually, along the rod axis, after loosening a compressed nylon sleeve I. In addition, a fine a d j u s t m e n t of the lower tube can be made vertically by turning the threaded supporting sleeve K. The upper tube E is insulated from the frame by a bakelite spacer, whereas the lower tube has ground connection through the frame.The entire frame G together with the sample holders, is moveable in a vertical direction along the two cylindrical tracks L by means
1062
PAUL
H. KECK
of four ball-bearings E. Suspended weights serve to counterbalance the frame, which is moved b y the hand-wheel N or b y a variable speed motor O up to a rate of six millimeter per minute. The frame, in addition, can be disconnected entirely from the drive system, b y t u r n i n g a knob disengaging a half-nut from the drive shaft, permitring the frame to be pushed m a n u a l l y into a n y desired position. The melting
t?
U 1. 2. 3. 4. 5. 6. 7. 8. 9.
Fig. 3. Schematic drawing of the first floating zone apparatus. Silicon sample 10. Gasket Sample holders 11. Gas connection W a t e r cooled tubes 12. Gas connection Connecting bridge 13. Heater coil 14. Radiation shield T h u m b screws Quartz glass cylinder 15. Liquid zone 16. Insulated connection Base plate 17. Grounded connection Top plate Tight rods
F L O A T I N G Z O N E C R Y S T A L L I Z A T I O N O F SILICON
1063
chamber can be evacuated, through a port in the base plate, connected to a high v a c u u m system. The overall view of the Floating Zone E q u i p m e n t is shown in Figure 5. To heat silicon effectively b y induction requires much higher frequencies t h a n for metal samples. Therefore, a 5 K W induction heater has been especially designed to
U
A. B. C. D. E. F. G.
I
Fig. 4. Schematic diagram of the improved floating zone apparatus. H. Synchronous gear-head motors Melting chamber I. Nylon sleeve Iron frame K. Threaded supporting sleeve Water-cooled plates L. Two cylindrical tracks T u n g s t e n fingered chucks M. Ball bearings Water-cooled stainless steel tubes N. Hand-wheel Wilson seals O. Variable speed motor Moveable frame
1064
PAUL H. KECK
deliver a frequency of a p p r o x i m a t e l y 3 megacycles. Since the resistivity of ultra pure polycrystalline silicon at room t e m p e r a t u r e is high, it is difficult to start induction heating. Preheating of the silicon rod by current passage was found a convenient means to start melting of silicon by induction. Starting with pieces of microcrystalline silicon of at l e a s t 10 m m length, the initial step is to weld together, in succession, several such pieces to form a rod of irregular shape. This is accomplished by adjusting
Fig. 5. Overall view of the floating zone equipment with induction heater. the joint of two aligned pieces in the center of the induction coil and melting the ends together. After an irregular rod of suitable length has been obtained a stable floating zone is produced near one end of the ingot and the liquid zone is pushed slowly along the rod, melting new silicon on the advancing side and crystallizing in its wake. I t can be accomplished either in an upward or downward direction. The process is then repeated, always moving the floating zone in the same direction. During each pass the single crystal areas increase in size and after a n u m b e r of sweeps, single crystals of up to 10 cm length have.been obtained. Both [111] and [100] orientations were
FLOATING ZONE CRYSTALLIZATION OF SILICON
1065
f o u n d t o o c c u r a l o n g t h e r o d axis. Single c r y s t a l s are o b t a i n e d m o r e r e a d i l y if a seed c r y s t a l is u s e d o n o n e side a n d if t h e i n i t i a l f l o a t i n g l i q u i d zone is f o r m e d a t t h e j o i n t w i t h p o l y c r y s t a l l i n e silicon. T h e f l o a t i n g zone is t h e n m a d e t o t r a v e l a l o n g t h e p o l y c r y s t a l l i n e m a s s w h i l e a single c r y s t a l f o r m s f r o m t h e seed. E f f e c t i v e p u r i f i c a t i o n b y successive zone m e l t i n g h a s b e e n o b s e r v e d in m o s t s a m p l e s . Single c r y s t a l s w i t h r e s i s t i v i t i e s u p t o s e v e r a l h u n d r e d o h m - c m h a v e b e e n p r e p a r e d , d i s p l a y i n g w i t h o u t e x c e p t i o n p - t y p e c o n d u c t i o n . W e believe t h a t t h e r e m a i n i n g i m p u r i t y in t h e s e c r y s t a l s is e s s e n t i a l l y B o r o n w h i c h c a n n o t be e f f e c t i v e l y s e g r e g a t e d b y zone m e l t i n g . F o r t h e a p p l i c a t i o n of t h e f l o a t i n g zone m e t h o d t h e silicon m u s t be a v a i l a b l e in t h e s h a p e of r o d s or a t l e a s t as pieces w h i c h c a n b e w e l d e d t o g e t h e r to f o r m a r o d of i r r e g u l a r s h a p e . I n t h e case in w h i c h o n l y p o w d e r is a v a i l a b l e as s t a r t i n g m a t e r i a l , t h e p o w d e r c a n be p r e s s e d h y d r o s t a t i c a l l y i n t o a rod w i t h o u t t h e use of a n y b i n d e r 4). S u c h p r e s s e d r o d s h a v e b e e n s u c c e s s f u l l y t u r n e d i n t o silicon c r y s t a l s b y m e a n s of t h e floating zone equipment. I n c o n c l u s i o n t h r e e s i g n i f i c a n t a d v a n t a g e s of t h e f l o a t i n g zone m e t h o d s h o u l d be emphasized : 1. A v o i d a n c e of i m p u r i t i e s o r i g i n a t i n g f r o m a c o n t a i n e r for t h e m e l t or f r o m a heater element. 2. P o s s i b l e a p p l i c a t i o n to m a t e r i a l s w i t h m e l t i n g p o i n t s e v e n h i g h e r t h a n t h a t of silicon. 3. P r e p a r a t i o n of single c r y s t a l s f r o m r e l a t i v e l y s m a l l q u a n t i t i e s of r a w m a t e r i a l . Received 30-6-54. REFERENCES 1) K e c k , P.H. at,d v a n H o r n , W., Phys. Rev.!Jl (1953) 512. 2) K e c k , P. H. and G o l a y , M. J. E., Phys. Rev. 8.t) (1953) 1297; N e c k , P. H., v a n H o r n , W., S o l e d , J. and M a c D o n a l d , A., Rev. sei. Instr. :~5 (1954) 331; E m e i s , R., Z. Naturforschg. 9a (1954) 67; M fill e r, S., Z. Naturforsehg..t)b (1954) 504. 3) K e c k , P.H., G r e e n , M. and P o l k , M. L.,J. appl. Phys. 24 (1953) 1479. 4) B l a c k b u r n , A. R. and S h e v l i n , T . S . , J . Am. cer. Soe.:14 (1951) 237.
Hrostowski, H.J. T a n e n b a u m , M. 1954
Physica XX No. I 1 Amsterdam Conference Semiconductors
R E C E N T W O R K ON G R O U P III A N T I M O N I D E S A N D ARSENIDES by H. J. HROSTOWSKI and M. TANENBAUM Bell Telephone Laboratories, Murray Hill, N. J., U.S.A. Of t h e m a n y k n o w n s e m i c o n d u c t i n g c o m p o u n d s , t h o s e f o r m e d b y r e a c t i o n of a G r o u p I I I e l e m e n t w i t h o n e of G r o u p V are m o s t closely a k i n t o t h e G r o u p I V s e m i c o n d u c t o r s 1). T h i s p a p e r d e s c r i b e s s o m e of o u r r e c e n t w o r k o n a n u m b e r of t h e s e compounds. T h e a n t i m o n i d e s were p r e p a r e d b y m e l t i n g t o g e t h e r s t o i c h i o m e t r i c a m o u n t s of t h e e l e m e n t s . T h e a r s e n i d e s were p r e p a r e d b y r e a c t i n g t h e c o m p o n e n t s in sealed q u a r t z
1059
2. Experimental. I n order t h a t the theory in Section It shall be valid it is essential t h a t the rate of injection of minority carriers from the base connections shall be small compared with the generation rate. This condition can be achieved for near-intrinsic n-type material b y growing a heavily doped, long lifetime n-type section on to one end of t h e crystal. This end is then made positive when the extraction field is applied and the effective injection ratio is then given a p p r o x i m a t e l y by Minority carrier c u r r e n t / T o t a l current = anLp/an+ L#+ where an, an+ and L#, Ln+ are the conductivities and minority carrier diffusion lengths respectively of the near-intrinsic and heavily doped sections.
3. Extraction characteristics. The voltage t o be applied to the specimen to obtain extraction is given a p p r o x i m a t e l y by V = 0.025 (L/Lp) 2 volts where L is the length of the specimen. This equation has been confirmed experim e n t a l l y and it is found t h a t the extraction voltage is largely independent of temperature and background illumination. U n d e r suitable conditions a change in conductiv i t y of a factor 10 can be obtained readily. W h e n the specimelX is e x t r a c te d b y voltage pulses of short duration the time required for extraction and hence the carrier mobility m ay be determined. I t is possible with this technique to e x t e n t drift mobility measurements well into the intrinsic conduction range. The theoretical predictions of S h o c k I e y 1), namely t h a t the carrier group mobility tends to zero in intrinsic material, has been confirmed qua n t i t at i v el y . Acknowledgement i s made to the Chief Scientist, British Ministry of Supply and to the Controller, H. B. M. Stationery Office for permission to publish this paper. British Crown Copyright Reserved. Received 1-7-54. REFERENCES 1) S h o c k 1 e y, W., Electrons and Holes in Semi-Conductors, Van Nostrand, New York, 1950, page 329 et sequi.
Keck, P . H . 1954
Physica X X No. 11 A m s t e r d a m Conference Semiconductors
FLOATING ZONE CRYSTALLIZATION OF SILICON by PAUL H. KECK Signal Corps Engineering Laboratories, Fort Monmouth, New Jersey, U.S.A. The surface tension of molten silicon is 720 dynes/cm 1). This relatively high value makes it possible for a zone of molten silicon to be held stable between two vertically aligned solid silicon rods clamped at both ends thus eliminating a container for the
1060
PAUL H. KECK
m e l t . T h e s h a p e of s u c h a l i q u i d b o d y w h i c h we t e r m e d " f l o a t i n g z o n e " 2) is d e s c r i b e d by Laplace's equation: P = 7 (I/R1 + l/R2) w h e r e p is t h e p r e s s u r e d i f f e r e n c e a c r o s s t h e s u r f a c e m e m b r a n e of t h e l i q u i d a t a n y p o i n t , R 1 a n d R 2 are t h e p r i n c i p a l r a d i i of c u r v a t u r e a t t h a t p o i n t , a n d y is t h e s u r f a c e t e n s i o n of t h e liquid. T h e s o l u t i o n of t h i s e q u a t i o n y i e l d s a f a m i l y of c u r v e s w i t h t h e p r e s s u r e i n s i d e t h e l i q u i d b o d y as a p a r a m e t e r . U s i n g t h e s e c u r v e s t h e s t a b l e s h a p e s of f l o a t i n g z o n e s f o r g i v e n cross s e c t i o n s of t h e s u p p o r t i n g r o d s h a v e b e e n d e t e r m i n e d . T h e r e s u l t s are p u b l i s h e d in a r e c e n t p a p e r ~). I t w a s f o u n d t h a t w i t h silicon l i q u i d z o n e s c a n b e h e l d s t a b l e b e t w e e n solid r o d s of c y l i n d r i c a l s h a p e u p to a p p r o x i m a t e l y 1 c e n t i m e t e r d i a m e t e r . T h e l i q u i d zone c a n b e m a d e t o t r a v e l a l o n g t h e r o d axis, m e l t i n g n e w silicon o n t h e a d v a n c i n g s i d e a n d c r y s t a l l i z i n g in its wake. T h i s m e t h o d of zone m e l t i n g i n a v e r t i c a l d i r e c t i o n r e q u i r e s t h e f o r m a t i o n of a v e r t i c a l t a n g e n t a t t h e l i q u i d solid b o u n d a r y o n t h e s o l i d i f y i n g side of t h e m o v i n g m o l t e n zone if a r e c r y s t a l l i z e d r o d of c o n s t a n t d i a m e t e r is desired. T h e c o r r e s p o n d i n g s h a p e s for m o t i o n of t h e l i q u i d silicon zone e i t h e r u p or d o w n are s h o w n in F i g u r e s 1 a n d 2. SOLID
"'--" 0.79 '
SOLID
4-
LIQUID 0.95
LIQUID 0.66
SOLID
SOLID
"~
0.90 B
A SOLID
~
-h
htCM
A=2.2 B=2.0 C=4.0 D= 1.9
1,74 1.58 3,2 1.5
SOLI:
t LIQUI£
0.6"/"
÷ SOLID C
-
LOWER TANGENCY ALL
D I S T A N C E S IN
0.96
CM.
Fig. 1. S t a b l e S h a p e s of F l o a t i n g Silicon Z o n e s w i t h L o w e r V e r t i c a l T a n g e n c y . I n t h e a p p l i c a t i o n of t h e f l o a t i n g zone m e t h o d , t h e f i r s t a p p r o a c h was to m e l t t h e silicon in a n a t m o s p h e r e of h e l i u m or a r g o n u s i n g r a d i a t i o n h e a t i n g f r o m a t u n g s t e n coil w h i c h e n c i r c l e s t h e i n g o t a l o n g a s h o r t section. T h e a p p a r a t u s , as it was used, is s h o w n s c h e m a t i c a l l y i n F i g u r e 3. T h e h e a t e r e l e m e n t s u r r o u n d i n g t h e silicon rod, in t h e d e s c r i b e d s y s t e m , is a possible s o u r c e for t h e i n t r o d u c t i o n of i m p u r i t i e s , since t h e
1061
FLOATING ZONE CRYSTALLIZATION OF SILICON
vapour pressure of tungsten, at the required high temperatures of 2000°C to 2200°C, is of some concern. For these reasons, an improved apparatus was designed and constructed which completely eliminates the heater element inside the melting chamber. In addition, a number of other i m p r o v e m e n t s were incorporated in the new e q u i p m e n t as a result of the experience with the first apparatus. To avoid placing a heater element inside the melting chamber, heating of the silicon by high frequency induction was chosen, whereby the induction coil is placed concentrically to the silicon rod b u t exterior to the quartz cylinder. A schematic diagram of the apparatus is shown in Figure 4. The melting chamber, A is mounted in a fixed position on a cross-member of the iron frame B. I t consists of '=
.76
-~'
SOLID
_k
~"
LIQ~UID LIQUID
1
1-SOLID
A
SOLID
h CM
A= 1.9 B =2.0 C=2.1
1.50 1.58 1.67
D =2.2 E =4.0
1.74 3.2
B
•
3-
SOLID
C
SOLIDI~2
D
~
E
UPPER TANGENCY
ALL DIMENSIONS IN CM.
Fig. 2. Stable Shapes of Floating Silicon Zones with Upper Vertical Tangency. an exchangeable quartz tube of 250 mm length, held between the water-cooled plates C using rubber gaskets for a v a c u u m - t i g h t seal. The silicon sample is supported by the tungsten fingered chucks D mounted in the water-cooled stainless steel tubes E. These tubes move through v a c u u m - t i g h t "Wilson seals F connected by short metal bellows to the plates C. Both tubes E are supported in brass bearings on the moveable frame G and can be rotated independently by small synchronous gear-head motors H with rubber cog belts. Each tube can be positioned manually, along the rod axis, after loosening a compressed nylon sleeve I. In addition, a fine a d j u s t m e n t of the lower tube can be made vertically by turning the threaded supporting sleeve K. The upper tube E is insulated from the frame by a bakelite spacer, whereas the lower tube has ground connection through the frame.The entire frame G together with the sample holders, is moveable in a vertical direction along the two cylindrical tracks L by means
1062
PAUL
H. KECK
of four ball-bearings E. Suspended weights serve to counterbalance the frame, which is moved b y the hand-wheel N or b y a variable speed motor O up to a rate of six millimeter per minute. The frame, in addition, can be disconnected entirely from the drive system, b y t u r n i n g a knob disengaging a half-nut from the drive shaft, permitring the frame to be pushed m a n u a l l y into a n y desired position. The melting
t?
U 1. 2. 3. 4. 5. 6. 7. 8. 9.
Fig. 3. Schematic drawing of the first floating zone apparatus. Silicon sample 10. Gasket Sample holders 11. Gas connection W a t e r cooled tubes 12. Gas connection Connecting bridge 13. Heater coil 14. Radiation shield T h u m b screws Quartz glass cylinder 15. Liquid zone 16. Insulated connection Base plate 17. Grounded connection Top plate Tight rods
F L O A T I N G Z O N E C R Y S T A L L I Z A T I O N O F SILICON
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chamber can be evacuated, through a port in the base plate, connected to a high v a c u u m system. The overall view of the Floating Zone E q u i p m e n t is shown in Figure 5. To heat silicon effectively b y induction requires much higher frequencies t h a n for metal samples. Therefore, a 5 K W induction heater has been especially designed to
U
A. B. C. D. E. F. G.
I
Fig. 4. Schematic diagram of the improved floating zone apparatus. H. Synchronous gear-head motors Melting chamber I. Nylon sleeve Iron frame K. Threaded supporting sleeve Water-cooled plates L. Two cylindrical tracks T u n g s t e n fingered chucks M. Ball bearings Water-cooled stainless steel tubes N. Hand-wheel Wilson seals O. Variable speed motor Moveable frame
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PAUL H. KECK
deliver a frequency of a p p r o x i m a t e l y 3 megacycles. Since the resistivity of ultra pure polycrystalline silicon at room t e m p e r a t u r e is high, it is difficult to start induction heating. Preheating of the silicon rod by current passage was found a convenient means to start melting of silicon by induction. Starting with pieces of microcrystalline silicon of at l e a s t 10 m m length, the initial step is to weld together, in succession, several such pieces to form a rod of irregular shape. This is accomplished by adjusting
Fig. 5. Overall view of the floating zone equipment with induction heater. the joint of two aligned pieces in the center of the induction coil and melting the ends together. After an irregular rod of suitable length has been obtained a stable floating zone is produced near one end of the ingot and the liquid zone is pushed slowly along the rod, melting new silicon on the advancing side and crystallizing in its wake. I t can be accomplished either in an upward or downward direction. The process is then repeated, always moving the floating zone in the same direction. During each pass the single crystal areas increase in size and after a n u m b e r of sweeps, single crystals of up to 10 cm length have.been obtained. Both [111] and [100] orientations were
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f o u n d t o o c c u r a l o n g t h e r o d axis. Single c r y s t a l s are o b t a i n e d m o r e r e a d i l y if a seed c r y s t a l is u s e d o n o n e side a n d if t h e i n i t i a l f l o a t i n g l i q u i d zone is f o r m e d a t t h e j o i n t w i t h p o l y c r y s t a l l i n e silicon. T h e f l o a t i n g zone is t h e n m a d e t o t r a v e l a l o n g t h e p o l y c r y s t a l l i n e m a s s w h i l e a single c r y s t a l f o r m s f r o m t h e seed. E f f e c t i v e p u r i f i c a t i o n b y successive zone m e l t i n g h a s b e e n o b s e r v e d in m o s t s a m p l e s . Single c r y s t a l s w i t h r e s i s t i v i t i e s u p t o s e v e r a l h u n d r e d o h m - c m h a v e b e e n p r e p a r e d , d i s p l a y i n g w i t h o u t e x c e p t i o n p - t y p e c o n d u c t i o n . W e believe t h a t t h e r e m a i n i n g i m p u r i t y in t h e s e c r y s t a l s is e s s e n t i a l l y B o r o n w h i c h c a n n o t be e f f e c t i v e l y s e g r e g a t e d b y zone m e l t i n g . F o r t h e a p p l i c a t i o n of t h e f l o a t i n g zone m e t h o d t h e silicon m u s t be a v a i l a b l e in t h e s h a p e of r o d s or a t l e a s t as pieces w h i c h c a n b e w e l d e d t o g e t h e r to f o r m a r o d of i r r e g u l a r s h a p e . I n t h e case in w h i c h o n l y p o w d e r is a v a i l a b l e as s t a r t i n g m a t e r i a l , t h e p o w d e r c a n be p r e s s e d h y d r o s t a t i c a l l y i n t o a rod w i t h o u t t h e use of a n y b i n d e r 4). S u c h p r e s s e d r o d s h a v e b e e n s u c c e s s f u l l y t u r n e d i n t o silicon c r y s t a l s b y m e a n s of t h e floating zone equipment. I n c o n c l u s i o n t h r e e s i g n i f i c a n t a d v a n t a g e s of t h e f l o a t i n g zone m e t h o d s h o u l d be emphasized : 1. A v o i d a n c e of i m p u r i t i e s o r i g i n a t i n g f r o m a c o n t a i n e r for t h e m e l t or f r o m a heater element. 2. P o s s i b l e a p p l i c a t i o n to m a t e r i a l s w i t h m e l t i n g p o i n t s e v e n h i g h e r t h a n t h a t of silicon. 3. P r e p a r a t i o n of single c r y s t a l s f r o m r e l a t i v e l y s m a l l q u a n t i t i e s of r a w m a t e r i a l . Received 30-6-54. REFERENCES 1) K e c k , P.H. at,d v a n H o r n , W., Phys. Rev.!Jl (1953) 512. 2) K e c k , P. H. and G o l a y , M. J. E., Phys. Rev. 8.t) (1953) 1297; N e c k , P. H., v a n H o r n , W., S o l e d , J. and M a c D o n a l d , A., Rev. sei. Instr. :~5 (1954) 331; E m e i s , R., Z. Naturforschg. 9a (1954) 67; M fill e r, S., Z. Naturforsehg..t)b (1954) 504. 3) K e c k , P.H., G r e e n , M. and P o l k , M. L.,J. appl. Phys. 24 (1953) 1479. 4) B l a c k b u r n , A. R. and S h e v l i n , T . S . , J . Am. cer. Soe.:14 (1951) 237.
Hrostowski, H.J. T a n e n b a u m , M. 1954
Physica XX No. I 1 Amsterdam Conference Semiconductors
R E C E N T W O R K ON G R O U P III A N T I M O N I D E S A N D ARSENIDES by H. J. HROSTOWSKI and M. TANENBAUM Bell Telephone Laboratories, Murray Hill, N. J., U.S.A. Of t h e m a n y k n o w n s e m i c o n d u c t i n g c o m p o u n d s , t h o s e f o r m e d b y r e a c t i o n of a G r o u p I I I e l e m e n t w i t h o n e of G r o u p V are m o s t closely a k i n t o t h e G r o u p I V s e m i c o n d u c t o r s 1). T h i s p a p e r d e s c r i b e s s o m e of o u r r e c e n t w o r k o n a n u m b e r of t h e s e compounds. T h e a n t i m o n i d e s were p r e p a r e d b y m e l t i n g t o g e t h e r s t o i c h i o m e t r i c a m o u n t s of t h e e l e m e n t s . T h e a r s e n i d e s were p r e p a r e d b y r e a c t i n g t h e c o m p o n e n t s in sealed q u a r t z