Bulk growth of GaP by halogen vapor transport
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ECONOMICALf a b r i c a t i o n of e l e c t r o l u m i n e s c e n t GaP diodes depends on the a v a i l a b i l i t y of s i n g l e - c r y s t a l , l a r g e - a r e a wafers of high quality. The l a r g e a r e a r e q u i r e m e n t is m e t by m a t e r i a l produced by v a p o r growth a n d pulling p r o c e s s e s . In the past the q u a l i t y of v a p o r t r a n s p o r t e d m a t e r i a l grown on GaAs s u b s t r a t e s has b e e n i n f e r i o r to that of solution g r o w n c r y s t a l s . 1 A l a r g e r n u m b e r of defects were found in the v a p o r grown c r y s t a l s and t h e i r p r e s e n c e was a t t r i b u t e d to lattice mismatch. By growing t h i c k e r epitaxlal l a y e r s two o b j e c t i v e s can be met. The d e l e t e r i o u s influence of the s u b s t r a t e is r e d u c e d and the l a y e r can be s l i c e d to y i e l d s e v e r a l GaP wafers for s u b s e q u e n t use a s s u b s t r a t e s . Req u i r e m e n t s for producing t h i c k e r l a y e r s a r e higher growth r a t e s and l o n g e r d e p o s i t i o n t i m e s . The method to be d e s c r i b e d here fulfills both r e q u i r e m e n t s . It depends upon cooling the s u b s t r a t e with a flow of h y d r o gen or n i t r o g e n gas. Substrate cooling has p r e v i o u s l y b e e n used and d e s c r i b e d by B a r k e m a y e r , M c A l e e r , and P o l l a k 2 and by R i c h m a n a n d Tietjen.a B a r k e m a y e r et al. expected to achieve continuous e p i t a x i a l deposition m a i n t a i n e d by slow withdrawal of the s u b s t r a t e h o l d e r . R i c h m a n and T i e t j e n were p r i m a r i l y i n t e r e s t e d in the i n c r e a s e d growth r a t e s obtained by m i n i m i z i n g competitive wall deposition. The p r e s e n t study i n v e s t i g a t e s the l i m i t a t i o n s of bulk growth a s e n v i s i o n e d by B a r k e m a y e r et al. a n d shows that the p r i n c i p a l factor in a c h i e v i n g bulk growth i s the e x t e n s i o n of deposition t i m e s r a t h e r than an i n c r e a s e d growth rate. EXPERIMENTAL The a p p a r a t u s used by the author for epitaxial deposition of GaP by chloride v a p o r t r a n s p o r t has b e e n d e s c r i b e d p r e v i o u s l y . 4 An e s s e n t i a l m o d i f i c a t i o n was made in the c o n s t r u c t i o n of the s u b s t r a t e support. The s u b s t r a t e is now mounted on a graphite block which i s
cooled c o n v e c t i v e l y by an i n t e r n a l flow of n i t r o g e n or hydrogen. Two d e s i g n s have been used. Fig. 1 shows a v e r t i c a l s u b s t r a t e s u s p e n s i o n . The s u b s t r a t e and a t e m p e r a t u r e l e v e l i n g graphite plate a r e held a g a i n s t the cooled s u r f a c e of a 22 m m OD q u a r t z h e a d e r . The top s u r f a c e of the h e a d e r was ground flat to e n s u r e good t h e r m a l contact. The coolant gas is i n t r o d u c e d through the i n t e r i o r 6 m m OD tube. The t h e r m o c
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