The use of up-converter phosphors in semiconductor light-emitting devices and displays
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/'~.N u p - c o n v e r t e r p h o s p h o r m a k e s u s e of a b s o r p t i o n of a s u c c e s s i v e s e r i e s of low e n e r g y photons so t h at , in a s t e p w i s e f a s h i o n , a final s t a t e is r e a c h e d f r o m w h i c h , by r a d i a t i v e r e c o m b i n a t i o n , a h i g h e r e n e r g y photon c a n be e m i t t e d . In the m o s t g e n e r a l c a s e , the e n e r g y i n t e r v a l s in t h e s u c c e s s i v e e x c i t a t i o n s a r e not the s a m e , so that s e v e r a l d i f f e r e n t e x c i t i n g f r e q u e n c i e s would be n e c e s s a r y in o r d e r to r e a c h the final s t a t e in t he p h o s p h o r . T h e r e do e x i s t , h o w e v e r , a n u m b e r of p h o s p h o r s w h e r e i n the s t e p w i s e e x c i t a t i o n s i n v o l v e e n e r g y i n t e r v a l s which a r e e q u a l o r n e a r l y equal. In t h i s e a s e , only a s i n g l e w a v e l e n g t h is r e q u i r e d f o r e x c i t a t i o n . T h u s , we e n c o u n t e r the a t t r a c t i v e p o s s i b i l i t y of d e v i s i n g a s y s t e m in w h ic h an e f f i c i e n t s e m i c o n d u c t o r s o u r c e of i n f r a r e d r a d i a t i o n can be m a d e to y i e l d v i s i b l e r a d i a t i o n by coupling it with an a p p r o p r i a t e u p c o n v e r t e r p h o s p h o r . * The p a r t i c u l a r type of p h o s p h o r
Th e f e a s i b i l i t y of a d i o d e - p h o s p h o r s y s t e m to obtain v i s i b l e light f r o m an i n f r a r e d s o u r c e w a s f i r s t d e m o n s t r a t e d in L a F 3 : Y b E r , in c o m b i n a t i o n with a diode m a d e f r o m s i l i c o n - d o p e d G a A s . B S i n ce t h e n , v i s i b l e f l u o r e s c e n c e due to i n f r a r e d e x c i t a t i o n f r o m a s e m i c o n d u c t o r s o u r c e has a l s o b e e n r e p o r t e d in BaYFs:Yb and Y3OC17 :Yb with e i t h e r e r b i u m o r h o l m i u m a s the a c t i v a t o r ~ in B a L u F s : Y b E r , a and in Y O C I : Y b E r . 9'1~ In the d i s c u s s i o n w h i ch f o l l o w s , we w i l l f i r s t b r i e f l y d i s c u s s the b a s i c b e h a v i o r of the p h o s p h o r and the diode w h i c h m a k e up t h e t o t a l s y s t e m , and then d i s c u s s the f a c t o r s w h i ch m u s t be c o n s i d e r e d to a c h i e v e d e s i r e d p e r f o r m a n c e of the s y s t e m .
*Weare indebted to R. M. Potter for pointingout to us the potential that these phosphors mighthavein an electrotuminescentdiode-phosphorcombination.
In F i g . 1 is shown a highly s i m p l i f i e d e n e r g y l e v e l s c h e m e in o r d e r to d e m o n s t r a t e what t a k e s p l a c e in the p h o s p h o r . On t h e r i g h t is a set of l e v e l s r e p r e s e n t a t i v e of t h e r a r e - e a r t h ion w h i ch is r e s p o n s i b l e f o r the u p - c o n v e r s i o n ; it m i g h t be E r 3+, f o r e x a m p l e . F o r the c a s e i l l u s t r a t e d , the u p - c o n v e r s i o n would i
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