Characterization of Gallium Arsenide by Magneto-optical

14 Characterization of Gallium Arsenide by Magneto-optical Photoluminescent Spectroscopy

Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: January 28, 1986 | doi: 10.1021/bk-1986-0295.ch014

D. C. Reynolds Air Force Wright Aeronautical Laboratories, AADR, Wright-Patterson Air Force Base,

OH 45433

Identification of residual and doped donors have been identified in expitaxial GaAs using the photoluminescence technique in the presence of applied magnetic fields. Transitions occur between excited initial and final states of the neutral-donor-bound-exciton complexes. The magnetic field compresses the wave function which sharpens the optical transitions. The magnetic field also separates the different donors when viewed from the neutral-donor-bound-exciton transitions. These two effects make possible the identification of donors when the donor concentration is in the mid 1015cm-3 range. R e f l e c t i o n , e m i s s i o n and a b s o r p t i o n i n s o l i d s has l o n g been s t u d i e d . I n t e n s e p h o t o l u m i n e s c e n c e i s o b s e r v e d i n many s e m i c o n d u c t o r s a t low temperatures. When s p e c t r a l l y a n a l y z e d , t h i s p h o t o l u m i n e s c e n c e p r o v i d e s an e x t e n s i v e s o u r c e o f e x p e r i m e n t a l d a t a which c o n t r i b u t e s t o the u l t i m a t e i d e n t i f i c a t i o n o f t h e e l e c t r o n i c s t a t e s o f i m p u r i t i e s and d e f e c t s i n t h e s e s e m i c o n d u c t o r s . Many sharp l i n e s appear i n such s p e c t r a , p a r t i c u l a r l y from bound e x c i t o n s , which p r o v i d e a " f i n g e r p r i n t " o f t h e i m p u r i t i e s and d e f e c t s which a r e p r e s e n t i n the s e m i c o n d u c t o r l a t t i c e . The e x c i t o n i s t h e probe i n t h i s c a s e , becoming bound t o v a r i o u s i m p u r i t i e s , d e f e c t s , and complexes and t h e subsequent decay from t h e bound s t a t e y i e l d s i n f o r m a t i o n c o n c e r n i n g t h e c e n t e r t o which i t was b o u n d . The e f f e c t i v e mass l i k e donors i n many I I I - V b i n a r y and i n s e v e r a l o f t h e I I I - V t e r n a r y systems a r e s h a l l o w . The c h e m i c a l s h i f t s and c e n t r a l c e l l c o r r e c t i o n s a r e s m a l l , t h e r e f o r e the energy s e p a r a t i o n o f donors r e s u l t i n g from d i f f e r e n t i m p u r i t i e s or host d e f e c t s i s s m a l l . T h i s r e q u i r e s low r e s i d u a l c o n c e n t r a t i o n s to p r e v e n t c o n c e n t r a t i o n b r o a d e n i n g and merging o f t h e i m p u r i t y l e v e l s with the conduction band. C o n t r o l l e d doping e x p e r i m e n t s w i t h known donors must a l s o be i n t h e low c o n c e n t r a t i o n range t o p e r m i t the i d e n t i f i c a t i o n o f s p e c i f i c d o n o r s . The b i n d i n g e n e r g i e s o f a c c e p t o r s a r e i n g e n e r a l l a r g e r than d o n o r s . T h i s makes the e x p e r i m e n t a l c h a r a c t e r i z a t i o n o f a c c e p t o r s e a s i e r than t h a t f o r d o n o r s .

This chapter not subject to U.S. copyright. Published 1986, American Chemical Society Casper; Microelectronics Processing: Inorganic Materials Characterization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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The b e h a v i o r o f these s h a r p l i n e t r a n s i t i o n s i n p e r t u r b i n g magnetic and s t r a i n f i e l d s make i t p o s s i b l e t o d i f f e r e n t i a t e between s i m p l e s u b s t i t u t i o n a l donors and a c c e p t o r s and complexes composed o f comb i n a t i o n s o f i m p u r i t i e s and o r d e f e c t s .


Exciton Spectra I n t r i n s i c E x c i t o n s . The i n t r i n s i c fundamental gap e x c i t o n i n s e m i c o n d u c t o r s i s a h y d r o g e n i c a l l y bound h o l e - e l e c t r o n p a i r , t h e h o l e b e i n g d e r i v e d from the t o p v a l e n c e band and the e l e c t r o n from the bottom c o n d u c t i o n b a n d . I t i s a normal mode o f the c r y s t a l c r e a t e d by an o p t i c a l e x c i t a t i o n wave, and i t s wave f u n c t i o n s a r e a n a l o g o u s t o those o f the B l o c k wave s t a t e s o f f r e e e l e c t r o n s and h o l e s . When most s e m i c o n d u c t o r s a r e o p t i c a l l y e x c i t e d a t low t e m p e r a t u r e s i t i s the i n t r i n s i c e x c i t o n s t h a t a r e e x c i t e d . The energy o f t h e ground and e x c i t e d s t a t e s o f t h e e x c i t o n l i e below t h e band gap energy o f the s e m i c o n d u c t o r . Hence, the e x c i t o n s t r u c t u r e must f i r s t be d e t e r m i n e d i n o r d e r t o determine the band gap e n e r g y . The e x c i t o n b i n d i n g energy c a n be d e t e r m i n e d from s p e c t r a l a n a l y s i s o f i t s h y d r o g e n i c ground and e x c i t e d s t a t e t r a n s i t i o n s . P r e c i s e bandgap e n e r g i e s c a n be d e t e r m i n e d by a d d i n g the e x c i t o n b i n d i n g energy t o the e x p e r i m e n t a l l y measured photon energy o f t h e ground s t a t e transition. E x t r i n s i c E x c i t o n s . Bound e x c i t o n complexes o r i m p u r i t y e x c i t o n complexes a r e e x t r i n s i c p r o p e r t i e s o f m a t e r i a l s . These complexes are observed as sharp l i n e o p t i c a l t r a n s i t i o n s i n both phot ol umi ne s cence and a b s o r p t i o n . The bound complex i s formed by b i n d i n g a f r e e e x c i t o n t o a c h e m i c a l i m p u r i t y atom ( i o n ) , c o m p l e x , o r a h o s t l a t tice defect. The b i n d i n g energy o f t h e e x c i t o n t o t h e i m p u r i t y o r d e f e c t i s g e n e r a l l y weak compared t o the f r e e e x c i t o n b i n d i n g energy. The r e s u l t i n g complex i s m o l e c u l a r - l i k e (analogous t o the hydrogen m o l e c u l e o r m o l e c u l e - i o n ) and bound e x c i t o n s have many s p e c t r a l p r o p e r t i e s w h i c h a r e a n a l o g o u s t o those o f s i m p l e d i a t o m i c molecules. The c e n t e r t o which the f r e e e x c i t o n s a r e bound c a n be e i t h e r n e u t r a l donor and a c c e p t o r c e n t e r s o r i o n i z e d donor and acceptor centers. The e m i s s i o n o r a b s o r p t i o n e n e r g i e s o f t h e s e bound e x c i t o n t r a n s i t i o n s a r e always below those of the c o r r e s p o n d i n g f r e e e x c i t o n s , due t o the m o l e c u l a r b i n d i n g e n e r g y . The s h a r p s p e c t r a l l i n e s o f bound e x c i t o n complexes c a n be very intense (large o s c i l l a t o r strength). The l i n e i n t e n s i t i e s w i l l , i n g e n e r a l , depend on t h e c o n c e n t r a t i o n s o f i m p u r i t i e s a n d / o r d e f e c t s p r e s e n t i n the s a m p l e . The t h e o r y o f " i m p u r i t y " o r d e f e c t a b s o r p t i o n i n t e n s i t i e s i n s e m i c o n d u c t o r s has been s t u d i e d by Rashba (_1) . By use o f t h e Fredholm method, he f i n d s t h a t i f t h e a b s o r p t i o n t r a n s i t i o n o c c u r s a t k=0 and i f t h e d i s c r e t e l e v e l a s s o c i a t e d w i t h t h e i m p u r i t y a p p r o a c h e s the c o n d u c t i o n b a n d , t h e i n t e n s i t y o f the a b s o r p t i o n l i n e increases. The e x p l a n a t i o n o f f e r e d f o r t h i s i n t e n s i t y b e h a v i o r i s t h a t the o p t i c a l e x c i t a t i o n i s not l o c a l i z e d i n t h e i m p u r i t y b u t encompasses a number o f n e i g h b o r i n g l a t t i c e p o i n t s o f the h o s t crystal. H e n c e , i n the a b s o r p t i o n p r o c e s s , l i g h t i s a b s o r b e d by the e n t i r e r e g i o n o f t h e c r y s t a l c o n s i s t i n g o f t h e i m p u r i t y and i t s surroundings.

Casper; Microelectronics Processing: Inorganic Materials Characterization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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MICROELECTRONICS PROCESSING: INORGANIC MATERIALS CHARACTERIZATION

I n an a t t a c k on t h e p a r t i c u l a r p r o b l e m o f e x c i t o n s w h i c h a r e w e a k l y bound t o l o c a l i z e d " i m p u r i t i e s " , R a s h b a and G u r g e n i s h v i l i (2) d e r i v e d the f o l l o w i n g r e l a t i o n between the o s c i l l a t o r s t r e n g t h of the bound e x c i t o n and the o s c i l l a t o r s t r e n g t h o f the i n t r i n s i c excitons £ » u s i n g the e f f e c t i v e - m a s s approximation F

-(E

3 / 2

/|E|) f (1) o ' ' ex 2 2/3 w h e r e E = ( 2 h /m)(*jr/fi ) , E i s the b i n d i n g energy of the e x c i t o n t o t h e i m p u r i t y , m i s t n e e f f e c t i v e mass o f t h e i n t r i n s i c e x c i t o n , and fig i s t h e v o l u m e o f t h e u n i t c e l l . I t h a s b e e n s h o w n i n some m a t e r i a l s t h a t F , e x c e e d s f by more than four orders of magnitude. An i n s p e c t i o n or E q u a t i o n ? r e v e a l s t h a t , as the i n t r i n s i c e x c i t o n becomes more t i g h t l y bound t o the a s s o c i a t e d c e n t e r , the o s c i l l a t o r s t r e n g t h , and h e n c e the i n t e n s i t y o f t h e e x c i t o n c o m p l e x l i n e , s h o u l d d e c r e a s e as ( l / E ) * * ' . I n m a g n e t i c f i e l d s , b o u n d e x c i t o n s h a v e u n i q u e Zeeman s p e c t r a l c h a r a c t e r i s t i c s , from which i t i s p o s s i b l e to i d e n t i f y the types of c e n t e r s t o w h i c h the f r e e e x c i t o n s a r e bound. Bound e x c i t o n s p e c t r o s c o p y i s a v e r y p o w e r f u l a n a l y t i c a l t o o l f o r t h e s t u d y and i d e n t i f i c a t i o n o f i m p u r i t i e s and d e f e c t s i n s e m i c o n d u c t o r m a t e r i a l s . d

Q

X


2

Magneto-Optical Spectroscopy

Techniques

M a g n e t o - o p t i c a l s p e c t r o s c o p y techniques have been a p p l i e d to the c h a r a c t e r i z a t i o n o f GaAs w h i c h i s a p o t e n t i a l l y i m p o r t a n t m a t e r i a l f o r many t e c h n o l o g i c a l a p p l i c a t i o n s . H i g h q u a l i t y m a t e r i a l i s r e q u i r e d f o r many o f t h e s e a p p l i c a t i o n s . To i m p r o v e t h e q u a l i t y r e q u i r e s a knowledge of the r e s i d u a l i m p u r i t i e s i n undoped m a t e r i a l . The a c c e p t o r s , h a v i n g r e l a t i v e l y l a r g e b i n d i n g e n e r g i e s a s c o m p a r e d t o t h e d o n o r s (~30 v s . 5.7 m e V ) , c a n be i d e n t i f i e d q u i t e e a s i l y . The s h a l l o w h y d r o g e n i c d o n o r s , o n t h e o t h e r h a n d , h a v e s m a l l b i n d i n g e n e r g i e s and a l s o have s m a l l c e n t r a l - c e l l c o r r e c t i o n s . T h i s makes the r e s o l u t i o n of d i f f e r e n t donors r e s u l t i n g from d i f f e r e n t c h e m i c a l impurities d i f f i c u l t to achieve. The e a r l y e x p e r i m e n t s f r o m w h i c h d i f f e r e n t c h e m i c a l donors were i d e n t i f i e d employed h i g h - r e s o l u t i o n F o u r i e r - t r a n s f o r m i n f r a r e d magnetospectroscopy (FTIR) which used the modulated p h o t o c o n d u c t i v i t y d e t e c t i o n technique to monitor the 1S-2P_^ t r a n s i t i o n i n a f i x e d m a g n e t i c f i e l d . FTIR S t u d i e s . T h e s e s t u d i e s w e r e m o t i v a t e d by b o t h f u n d a m e n t a l a n d t e c h n o l o g i c a l i n t e r e s t s . F u n d a m e n t a l l y t h e r e was a d e s i r e t o d e t e r m i n e how e f f e c t i v e l y t h e s e i m p u r i t y s t a t e s c o u l d be t r e a t e d by e f f e c t i v e mass t h e o r y . A l s o t h e e f f e c t o f l o c a l p o t e n t i a l s due t o the d i f f e r e n t c o r e c o n f i g u r a t i o n s of d i f f e r e n t c h e m i c a l i m p u r i t i e s was o f p a r t i c u l a r i n t e r e s t . I t i s t h e s e c h e m i c a l s h i f t s t h a t r e q u i r e c o r r e c t i o n s t o b e made t o t h e e f f e c t i v e m a s s t h e o r y . These s h i f t s a l s o make p o s s i b l e t h e i d e n t i f i c a t i o n o f d i f f e r e n t c h e m i c a l species which i s of great t e c h n o l o g i c a l i n t e r e s t . Considering III-V s e m i c o n d u c t o r s , d o n o r s a r e i n t r o d u c e d by s u b s t i t u t i n g g r o u p IV atoms on t h e g r o u p I I I s i t e o r s u b s t i t u t i n g g r o u p V I atoms on t h e g r o u p V site. I n a s i m i l a r manner a c c e p t o r s a r e i n t r o d u c e d by s u b s t i t u t i n g g r o u p I I atoms on t h e g r o u p I I I s i t e o r by s u b s t i t u t i n g g r o u p IV atoms on the g r o u p V s i t e . The s l i g h t e n e r g y s h i f t ( c e n t r a l c e l l


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c o r r e c t i o n ) r e s u l t i n g from i n t r o d u c i n g d i f f e r e n t c h e m i c a l donors o r a c c e p t o r s makes p o s s i b l e the i d e n t i f i c a t i o n o f the s u b s t i t u t e d atom. The FTIR experiment can be e i t h e r a t r a n s m i s s i o n experiment o r a p h o t o c o n d u c t i v e e x p e r i m e n t . P h o t o c o n d u c t i v e d e t e c t i o n i s much more s e n s i t i v e t h a n the t r a n s m i s s i o n experiment w h i l e s t i l l r e t a i n i n g the h i g h r e s o l u t i o n f e a t u r e o f the t e c h n i q u e . I t was shown by S t i l l m a n (_3) t h a t p h o t o c o n d u c t i v e d e t e c t i o n a l l o w s one t o observe t r a n s i t i o n s between bound i m p u r i t y s t a t e s . The e x c i t e d c a r r i e r can absorb a phonon and make a t r a n s i t i o n t o a c o n d u c t i o n band s t a t e . E f f e c t i v e Mass Model. The e f f e c t i v e mass d e s c r i p t i o n o f a n i m p u r i t y (donor) s t a t e r e q u i r e s t h a t t h e e l e c t r o n o r b i t extend over many l a t t i c e d i s t a n c e s . T h i s p e r m i t s the use o f the m a c r o s c o p i c d i e l e c t r i c c o n s t a n t t o d e s c r i b e the e l e c t r o n i c m o t i o n . W i t h these c o n d i t i o n s s a t i s f i e d , the i m p u r i t y energy s t a t e can be approximated by the h y d r o g e n i c form w i t h the e l e c t r o n mass r e p l a c e d by an e f f e c t i v e mass m* 2

E = -R*/m (2) * where R i s an e f f e c t i v e Rydoerg which i s r e l a t e d t o the hydrogen Rydberg by the f o l l o w i n g e x p r e s s i o n m

R* = h m* e / e o 4

2

2

tf

(3)

I t i s c l e a r t h a t the e f f e c t i v e mass model must be m o d i f i e d i n o r d e r to account f o r d i f f e r e n t c h e m i c a l donors. The FTIR experiment m o n i t o r s the n = 1 t o n = 2 t r a n s i t i o n . For s h a l l o w donors i n many of the I I I - V compounds the 2s and 2p s t a t e s a r e degenerate f o r a l l p r a c t i c a l p u r p o s e s . The r e s o l u t i o n i s improved by a p p l y i n g a magn e t i c f i e l d . The magnetic f i e l d s e p a r a t e s the o r b i t a l a n g u l a r momentum s t a t e s . The magnetic f i e l d a l s o compresses the wave f u n c t i o n which sharpens the t r a n s i t i o n s , i t a l s o s e p a r a t e s t r a n s i t i o n s due t o d i f f e r e n t c h e m i c a l i m p u r i t i e s (h). H i g h R e s o l u t i o n Photoluminescence S t u d i e s . More r e c e n t l y , s h a l l o w r e s i d u a l donors have been i d e n t i f i e d i n h i g h - p u r i t y VPE GaAs u s i n g h i g h - r e s o l u t i o n photoluminescence s p e c t r o s c o p y (_5, 6 ) . The o p t i c a l t r a n s i t i o n s t h a t were used t o i d e n t i f y the r e s i d u a l donors r e s u l t from the c o l l a p s e o f n e u t r a l - d o n o r - b o u n d e x c i t o n s . The decay o f an e x c i t o n bound t o a donor ( a c c e p t o r ) may l e a v e the donor ( a c c e p t o r ) i n an e x c i t e d s t a t e . T h i s was f i r s t p o i n t e d out by Thomas and H o p f i e l d ( 7 ) . They observed t r a n s i t i o n s i n CdS t h a t were c h a r a c t e r i z e d by l a r g e magnetic f i e l d s p l i t t i n g s and n e g a t i v e d i a m a g n e t i c s h i f t s which they t e n t a t i v e l y i d e n t i f i e d w i t h t r a n s i t i o n s o f t h i s t y p e . E x c i t e d - s t a t e t r a n s i t i o n s o f t h i s type were l a t e r i d e n t i f i e d i n GaP, ( 8 ) , CdSe, ( 9 , K>) CdS, (11) ZnO, (12) and ZnSe (13). Residu a l donors have s u b s e q u e n t l y been r e s o l v e d i n GaAs by Almassy e t a l . (5) from o p t i c a l t r a n s i t i o n s r e s u l t i n g from the c o l l a p s e o f an e x c i t o n bound t o an e x c i t e d donor s t a t e , l e a v i n g a n e x c i t e d t e r m i n a l s t a t e . The t e r m i n a l s t a t e o f t h i s t r a n s i t i o n , from w h i c h c h e m i c a l i d e n t i f i c a t i o n s a r e made, i s the n = 2 s t a t e . When the t e r m i n a l s t a t e i s a 2s s t a t e , then the c e n t r a l - c e l l c o r r e c t i o n t o t h i s s t a t e i s assumed t o be 1/8 o f i t s v a l u e f o r the Is s t a t e . I n t r a n s i t i o n s of t h i s type i t i s p o s s i b l e t o observe 7/8 o f the t o t a l c e n t r a l - c e l l



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c o r r e c t i o n . When t h e t e r m i n a l s t a t e i s a 2p s t a t e t h e f u l l c e n t r a l c e l l c o r r e c t i o n i s observed. E m i s s i o n l i n e s have been observed on t h e h i g h - e n e r g y s i d e o f the n e u t r a l - d o n o r many m a t e r i a l s , CdTe, ( 1 4 ) GaAs, (^5, J_6) CdS, (17) and ZnSe ( 1 3 ) . These t r a n s i t i o n s were i n t e r p r e t e d as e x c i t e d s t a t e s o f t h e D*,X but w i t h very l i t t l e d e t a i l as to the nature of the e x c i t e d s t a t e s . Guillaume and L a v a l l a r d ( 1 8 ) proposed a r i g i d - r o t a t o r model t o e x p l a i n t h e s e e x c i t e d s t a t e s i n CdTe. I n t h i s model t h e h o l e i s e x c i t e d t o r o t a t e around t h e f i x e d donor analogous t o r o t a t i o n o f d i a t o m i c m o l e c u l e s . T h i s model had d i f f i c u l t y i n p r e d i c t i n g t h e o b s e r v e d e n e r g i e s f o r the e x c i t e d - s t a t e t r a n s i t i o n s . A n o n - r i g i d - r o t a t o r model was subs e q u e n t l y proposed by Ruhle and K l i n g e n s t e i n , (19) w h i c h was s u c c e s s f u l i n p r e d i c t i n g t h e e x c i t e d - s t a t e e n e r g i e s i n InP and GaAs. I t was t h e c o l l a p s e o f e x c i t o n s bound t o t h e n = 2 r o t a t i o n a l s t a t e o f the donors from w h i c h t h e r e s i d u a l donors were r e s o l v e d i n R e f e r e n c e 5. I t has been observed t h a t i n many c r y s t a l s t h e D*,X s t a t e a s w e l l as t h e f i r s t n o n - r i g i d - r o t a t i o n a l s t a t e a r e broadened w h i l e o t h e r b o u n d - e x c i t o n s t a t e s a r e n o t . H e r z b e r g and S p i n k s (20) have o b s e r v e d b r o a d e n i n g o f r o t a t i o n a l l i n e s from d i a t o m i c m o l e c u l e s and have a l s o o b s e r v e d t h a t t h i s b r o a d e n i n g d e c r e a s e d w i t h i n c r e a s i n g r o t a t i o n a l quantum number. I t has been proposed ( 2 1 ) t h a t an analogous b r o a d e n i n g mechanism i s r e s p o n s i b l e f o r t h e b r o a d e n i n g o f the E r , X t r a n s i t i o n a s w e l l a s t h e t r a n s i t i o n from t h e f i r s t nonr i g i d - r o t a t i o n a l s t a t e . I t was observed t h a t when t h e ground s t a t e was broadened t h e e x c i t e d - ( n = 2) s t a t e t r a n s i t i o n s were a l s o broadened. The energy s e p a r a t i o n between t h e D ,X s t a t e and t h e f i r s t r o t a t i o n a l s t a t e i n GaAs i n 50yV. I f one a t t e m p t s t o r e s o l v e r e s i d u a l donors from t h e e x c i t e d - s t a t e ( n = 2) t r a n s i t i o n s o f these two s t a t e s , t h e combined l i n e b r o a d e n i n g and s m a l l energy s e p a r a t i o n r e n d e r i t i m p o s s i b l e i n most c a s e s . Almassy e t a l . ( 5 ) c i r c u m v e n t e d t h i s problem by o b s e r v i n g t h e n = 2 s t a t e s r e s u l t i n g from t h e c o l l a p s e o f t h e e x c i t o n bound t o t h e second n o n - r i g i d - r o t a t i o n a l s t a t e o f t h e donor, w h i c h i s n o t broadened. T h i s has t h e advantage t h a t t h e experiment i s done i n z e r o magnetic f i e l d and t h e r e f o r e the donor e n e r g i e s a r e d i r e c t l y measured. The scheme has t h e d i s advantage t h a t t h e i n t e n s i t y o f t h e n = 2 s t a t e a s s o c i a t e d w i t h t h e second n o n - r i g i d - r o t a t i o n a l s t a t e i s c o n s i d e r a b l y l e s s than t h e i n t e n s i t y o f the n = 2 s t a t e a s s o c i a t e d w i t h the f i r s t n o n - r i g i d rotational state. I t would be d e s i r a b l e t o i d e n t i f y t h e r e s i d u a l donors i n GaAs from t h e t r a n s i t i o n i n v o l v i n g an e x c i t o n bound t o t h e f i r s t nonr i g i d - r o t a t i o n a l s t a t e . The t e r m i n a l s t a t e c o n s i s t s o f t h e e x c i t e d s t a t e ( n = 2) o f t h e e l e c t r o n on t h e donor. The o b s e r v a t i o n o f d i f f e r e n t r e s i d u a l donor s p e c i e s from t h i s t r a n s i t i o n i s made p o s s i b l e by p e r f o r m i n g t h e experiment i n a magnetic f i e l d . The magnetic f i e l d produces two e f f e c t s : ( a ) I t s e p a r a t e s o u t s t a t e s w i t h d i f f e r e n t o r b i t a l a n g u l a r momentum and ( b ) i t compresses t h e wave f u n c t i o n which sharpens t h e l i n e s and s e p a r a t e s t h e donors. I n t h e f i n a l s t a t e t h e t r a n s i t i o n can t e r m i n a t e i n e i t h e r t h e 2s o r 2p s t a t e . From p a r i t y arguments i t c a n be shown t h a t t h e i n i t i a l s t a t e o f t h e D*,X t r a n s i t i o n has odd p a r i t y . The 2s f i n a l s t a t e i n t h i s t r a n s i t i o n w i l l have even p a r i t y whereas t h e 2p f i n a l s t a t e w i l l have odd p a r i t y . The p r e f e r e n t i a l t r a n s i t i o n , t h e r e f o r e , from t h e D ,X i n i t i a l s t a t e w i l l be t o t h e 2s f i n a l s t a t e . By s i m i l a r


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a r g u m e n t i t c a n b e shown t h a t t h e i n i t i a l s t a t e o f t h e e x c i t o n bound to t h e f i r s t r o t a t i o n a l s t a t e o f t h e n e u t r a l donor w i l l have even parity. The p r e f e r e n t i a l t r a n s i t i o n f o r t h i s s t a t e t h e n w i l l be t o t h e 2p f i n a l s t a t e h a v i n g o d d p a r i t y . T h e i n t e n s i t i e s o f t h e s e t r a n s i t i o n s were o b s e r v e d b y Dean e t a l . ( 2 2 ) f o r t h e c a s e o f ZnTe. They showed t h a t when t h e e x c i t o n c o l l a p s e d f r o m t h e f i r s t r o t a t i o n a l s t a t e t h e t e r m i n a l s t a t e i n t e n s i t y r a t i o 2p/2s = 20. They f u r t h e r s h o w e d t h a t w h e n t h e e x c i t o n c o l l a p s e d f r o m t h e D*,X s t a t e t h e t e r m i n a l s t a t e i n t e n s i t y 2 p / 2 s = 1/5. I t c a n t h e r e f o r e b e c o n c l u d e d t h a t i n t h e c a s e o f GaAs t h e 2 p f i n a l s t a t e s a r e p r e d o m i n a t e l y a s s o c i a t e d w i t h e x c i t o n s bound t o t h e f i r s t n o n - r i g i d - r o t a t i o n a l s t a t e , and t h e 2s f i n a l s t a t e s a r e p r e d o m i n a t e l y a s s o c i a t e d w i t h t h e lf*,X s t a t e . T r a n s i t i o n f r o m N e u t r a l Donor S t a t e s . When a n e x c i t o n d e c a y s f r o m t h e D*,X s t a t e i n a m a g n e t i c f i e l d t h e e x c i t e d 2 s a n d 2p t e r m i n a l states are separated. T h e 2 p a n g u l a r momentum s t a t e s a r e a l s o separated. The i n i t i a l s t a t e o f t h e complex c o n s i s t s o f two p a i r e d e l e c t r o n s a n d one u n p a i r e d h o l e a s shown i n F i g u r e ( 1 ) . The unpaired hole w i l l s p l i t i n a magnetic f i e l d . The f i n a l s t a t e o f the complex c o n s i s t s o f one u n p a i r e d e l e c t r o n e i t h e r i n t h e ground state o r i n an excited state. The t r a n s i t i o n s o f i n t e r e s t i n t h i s paper a r e those a s s o c i a t e d w i t h the t e r m i n a l e l e c t r o n i n t h e n = 2 state. T h e i n s e t i n F i g u r e ( 1 ) i s a d e n s i t o m e t e r t r a c e o f t h e D ,X state and i t s associated non-rigid-rotational states. The i n i t i a l s t a t e o f t h e t r a n s i t i o n c a n o r i g i n a t e f r o m D*,X o r a n y o f i t s r o tational states. T h e t r a n s i t i o n o f i n t e r e s t i s t h e one whose i n i t i a l state i s the f i r s t non-rigid-rotational state. The s p e c i f i c t r a n s i t i o n r e s u l t s from t h e c o l l a p s e o f a n e x c i t o n bound t o t h e f i r s t n o n - r i g i d - r o t a t i o n a l s t a t e a n d t e r m i n a t i n g i n t h e 2p s t a t e o f the e l e c t r o n on t h e n e u t r a l donor. The i n i t i a l s t a t e o f t h e complex s p l i t s i n t o a q u a r t e t a n d t h e f i n a l 2p s t a t e s p l i t s i n t o t h r e e widely separated s t a t e s 2 p ^ , 2 p , and 2p_.. I n t h i s t r a n s i t i o n a n e g a t i v e d i a m a g n e t i c s h i f t o c c u r s s o t h a t t h e l o w e s t - e n e r g y 2p state r e s u l t s i n the highest-energy o p t i c a l t r a n s i t i o n . This i s the 2p_^ s t a t e w h i c h h a s t h e h i g h e s t i n t e n s i t y o f t h e p - s t a t e transitions. The m a g n e t i c f i e l d s p l i t t i n g o f t h i s s t a t e a t 40 kG i s shown in Figure (2). The i n s e t o f F i g u r e ( 2 ) shows t h e z e r o - f i e l d t r a c e i n t h e n = 2 s p e c t r a l r e g i o n o f t h e n e u t r a l - d o n o r - bound e x c i t o n . Here i t i s seen from t h e c o l l a p s e o f t h e e x c i t o n bound t o t h e second, t h i r d , and fourth n o n - r i g i d - r o t a t i o n a l s t a t e s that t h e c r y s t a l contains r e s i d u a l S i and S donors. The s p l i t t i n g o f t h e 2p_^ s t a t e shows components o f t h e s e t w o d o n o r s a s marked i n F i g u r e (2). The h i g h e s t - i n t e n s i t y t r a n s i t i o n s a r e t h e s p i n - c o n s e r v i n g transitions. The l i n e s a r e sharp a n dw e l l r e s o l v e d . The donors could n o t be r e s o l v e d from t h i s t r a n s i t i o n i n z e r o magnetic f i e l d due t o b r o a d e n e d l i n e s a n d t h e n e a r p r o x i m i t y o f t h e p r i n c i p a l donor - bound e x c i t o n . Much c a n be g a i n e d by u s i n g a m a g n e t i c f i e l d with the photoluminescence i d e n t i f i c a t i o n o f shallow donors. +

Q

I t was shown b y F e t t e r m a n e t a l . ( 4 ) t h a t t h e s e p a r a t i o n b e tween d i f f e r e n t c h e m i c a l d o n o r s i n c r e a s e d m o n o t o n i c a l l y w i t h magnet i c f i e l d strength. I n t h e i r case they were a n a l y z i n g t h e donors by t h e F T I R t e c h n i q u e . S i n c e t h e e x c i t o n i s v e r y l o o s e l y bound t o the donor i n t h e p h o t o l u m i n e s c e n c e scheme, a s i m i l a r r e s p o n s e m i g h t


246



r

I.5I4I0D ,X 0

F i g u r e 1. Schematic r e p r e s e n t a t i o n o f r a d i a t i v e r e c o m b i n a t i o n o f an e x c i t o n bound t o a n e u t r a l donor where t h e f i n a l s t a t e i s t h e donor i n t h e ground o r i n t h e e x c i t e d c o n f i g u r a t i o n . The i n s e t shows t h e i n i t i a l s t a t e o f t h e n e u t r a l - d o n o r - b o u n d e x c i t o n i n t h e ground and s e v e r a l e x c i t e d r o t a t i o n a l s t a t e s . (Reproduced w i t h p e r m i s s i o n from R e f . 24. C o p y r i g h t 1983 American P h y s i c a l S o c i e t y . )



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ENERGY (eV) F i g u r e 2. M a g n e t i c f i e l d c o m p o n e n t s o f t h e 2 p ^ s t a t e a t 4 0 k G r e s u l t i n g f r o m t h e c o l l a p s e o f a n e x c i t o n bound t o t h e f i r s t r o t a t i o n a l s t a t e o f t h e n e u t r a l donor f o r both s u l f u r a n d s i l i c o n . The i n s e t s h o w s t h e same t w o d o n o r s i n z e r o m a g n e t i c f i e l d r e s u l t i n g f r o m t h e e x c i t o n bound t o t h e s e c o n d , t h i r d , a n d f o u r t h r o t a t i o n a l s t a t e s o f these n e u t r a l donors. (Reproduced w i t h perm i s s i o n f r o m R e f . 24. C o p y r i g h t 1983 A m e r i c a n P h y s i c a l Society.)

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be e x p e c t e d . The p e r t u r b a t i o n t h e o r y of Fetterman e t a l . r e s u l t e d i n the f o l l o w i n g e x p r e s s i o n f o r the magnetic f i e l d s e p a r a t i o n of donors 1 and 2: A

1,2

( B )

K

= l, IXi

s

where K. ^ i s an a d j u s t a b l e parameter independent of magnetic f i e l d . X, (0) ^ * l °f e f f e c t i v e - m a s s envelope f u n c t i o n f o r an e l e c t r o n i n the donor ground s t a t e a t the o r i g i n i n the presence of a magnetic f i e l d . T h i s f u n c t i o n i s magnetic f i e l d dependent. X can be c a l c u l a t e d from e f f e c t i v e - m a s s t h e o r y . The s o l i d l i n e i n F i g u r e (3) i s a t h e o r e t i c a l p l o t of \x ( 0 ) | as a f u n c t i o n of magnetic f i e l d t a k e n from Cabib e t a l . f23). I n the same f i g u r e i s plotted A ( s i l i c o n , s u l f u r donor s e p a r a t i o n ) by a d j u s t i n g the r i g h t - h a n d s c a l e t o p l a c e the p o i n t a t 40 kG c l o s e to the t h e o r e t i c a l c u r v e . W i t h the use of u n i t K the r e m a i n i n g e x p e r i m e n t a l p o i n t s f a l l as shown i n F i g u r e (_3). The f i t i s r e a s o n a b l y good and shows t h a t the p e r t u r b a t i o n t h e o r y i s a l s o a p p l i c a b l e when an e x c i t o n i s l o o s e l y bound t o the donor. I t i s e v i d e n t t h a t a t h i g h e r magnetic f i e l d s i n c r e a s e d donor s e p a r a t i o n w i l l o c c u r . A g e o m e t r i c c o n s t r u c t i o n was used t o a n a l y z e the magnetic f i e l d s p l i t t i n g of the 2p_^ s t a t e s of the S i and S donors whose d e n s i t o m e t e r t r a c e s are shown i n F i g u r e ( 2 ) . S i n c e the e x c i t o n s are v e r y l o o s e l y bound t o the donors i t i s r e a s o n a b l e t o assume t h a t the e l e c t r o n g v a l u e i n these bound s t a t e s w i l l be e s s e n t i a l l y the same as the f r e e - e l e c t r o n g v a l u e . U s i n g an e l e c t r o n g v a l u e of g = -0.55, the nomogram a t 40 kG shown i n F i g u r e (4) was p r o duce!. From the nomogram a h o l e K v a l u e o f 0.65 i s measured. I n F i g u r e 5 the d e v i a t i o n from the c e n t e r of mass of the M Q t r a n s i t i o n f o r the s i l i c o n donor as a f u n c t i o n of magnetic f i e l d i s p l o t t e d . An e f f e c t i v e g v a l u e o f 1.1 i s o b t a i n e d from t h i s s p l i t t i n g . T h i s i s c l o s e t o the sum o f the magnitudes of the e l e c t r o n and h o l e g v a l u e s w h i c h i s e x p e c t e d f o r t h i s s p l i t t i n g . An i d e n t i c a l s p l i t t i n g i s o b t a i n e d f o r the s u l f u r donor. x

t

ie

v a

u e

t n e


l g

I d e n t i f i c a t i o n of Donors i n Doped GaAs M a g n e t o - o p t i c a l p h o t o l u m i n e s c e n t s p e c t r o s c o p y was used t o i d e n t i f y the donors i n two samples of S i doped GaAs. The S i c o n c e n t r a t i o n i n sample A as determined by C-V p r o f i l i n g was N = 5X10 cm""3. The t o t a l donor c o n c e n t r a t i o n i n sample B was = 3X10^^cm"^ w i t h a c a r r i e r m o b i l i t y = 45,000 cm /V-sec a t 77 K determined from H a l l e f f e c t measurements. The donors were i d e n t i f i e d from the c o l l a p s e of e x c i t o n s bound t o n e u t r a l donors. The o p t i c a l t r a n s i t i o n s i n v o l v e the c o l l a p s e of e x c i t o n complexes whose i n i t i a l s t a t e c o n s i s t s o f an e x c i t o n bound to the f i r s t n o n r i g i d r o t a t i o n a l s t a t e of the n e u t r a l donor and whose t e r m i n a l s t a t e c o n s i s t s of the 2p_j s t a t e of the n e u t r a l d o n o r . The most i n t e n s e t r a n s i t i o n s i n t h i s p r o c e s s a r e the s p i n c o n s e r v i n g t r a n s i t i o n s . The measurements were made i n an a p p l i e d magnetic f i e l d of 36kG. The t r a n s i t i o n s t o the 2 p , s t a t e a r e shown i n F i g u r e 6. The dashed c u r v e f o r sample A snows the s p i n c o n s e r v i n g t r a n s i t i o n s f o r the S i donor i n t h i s S i doped D

2



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H (kG) F i g u r e 3. S e p a r a t i o n o f t h e S i a n d S d o n o r s a s a f u n c t i o n o f m a g n e t i c f i e l d i s shown by t h e c l o s e d c i r c l e s . The s o l i d l i n e i s the t h e o r e t i c a l curve o f I X i ( 0 ) | f u n c t i o n o f magnetic field. T h e z e r o - f i e l d v a l u e o f A . „ i s 0.43 cm"" . ( R e p r o d u c e d w i t h p e r m i s s i o n from Ref. 24. C o p y r i g h t *ft83 American P h y s i c a l Society.) 2

a

s

a

g

S

1

F i g u r e 4. Nomogram f o r i n t e r p r e t a t i o n o f t h e 2 p _ i m a g n e t i c f i e l d components o f the S i a n d S donors. The f r e e - e l e c t r o n g v a l u e i s a s s u m e d , a h o l e k v a l u e o f k = 0.65 i s t h e n m e a s u r e d . (Reproduced w i t h p e r m i s s i o n f r o m R e f . 24. C o p y r i g h t 1983 A m e r i c a n P h y s i c a l Society.)



250


-160

1

0

' 1 1 10 20 30 MAGNETIC FIELD (kG)

L 40

F i g u r e 5. D e v i a t i o n from t h e c e n t e r o f mass i s p l o t t e d a s a f u n c t i o n o f magnetic f i e l d f o r t h e S i donor. Reproduced w i t h p e r m i s s i o n from Ref. 24. C o p y r i g h t 1983 American P h y s i c a l S o c i e t y . )

I 1.51092

I

I

i

1.51112 ENERGY (eV)

I

I

1.51132

F i g u r e 6. S h a l l o w donors i d e n t i f i e d i n two d i f f e r e n t GaAs samples from t h e 2p_^ t r a n s i t i o n . The dashed c u r v e shows t h e S i donor i n sample A, t h e s o l i d c u r v e shows t h e S i donor as w e l l as t h e r e s i d u a l S donor i n sample B.


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sample. The s o l i d curve f o r sample B , a l s o S i doped shows the S i donor w i t h the w e l l r e s o l v e d S r e s i d u a l donor a l s o p r e s e n t . This demonstrates t h a t the t e c h n i q u e i s c a p a b l e o f i d e n t i f y i n g donors a t r e l a t i v e l y high concentrations (5X10 c m " ) . These r e s u l t s c o u l d not have been a c h i e v e d i n z e r o magnetic f i e l d . The m a g n e t o - o p t i c a l s p e c t r o s c o p y t e c h n i q u e has c e r t a i n advantages over FTIR: (1) I t can p e n e t r a t e l a y e r s such as a c t i v e l a y e r s as w e l l as l a y e r s a s s o c i a t e d w i t h some h e t e r o s t r u c t u r e s , (2) i t does not r e q u i r e c o n t a c t s , (3) donors and a c c e p t o r s can be i d e n t i f i e d s i m u l t a n e o u s l y p e r m i t t i n g an e s t i m a t e o f sample compensation, and (4) s i n c e i t samples a v e r y t h i n l a y e r o f m a t e r i a l i t can be used i n p r o f i l i n g l a y e r s . These two t e c h n i q u e s p r o v i d e methods f o r i d e n t i f y i n g very shallow l e v e l s i n semiconductors. 1 5

3


Conclusions I n o r d e r to make judgement about the q u a l i t y o f an e p i t a x i a l l a y e r , o r any semiconductor m a t e r i a l , i t i s n e c e s s a r y to c h a r a c t e r i z e t h a t m a t e r i a l . The c h a r a c t e r i z a t i o n may take many f o r m s , depending on the i n t e n d e d end p r o d u c t o f the m a t e r i a l . I n t h i s paper o p t i c a l c h a r a c t e r i z a t i o n has been emphasized, f o c u s i n g on the magnetoo p t i c a l photoluminescent spectroscopy technique f o r i d e n t i f y i n g impurities. The e a r l y experiments from w h i c h d i f f e r e n t c h e m i c a l donors were i d e n t i f i e d i n m a t e r i a l s l i k e GaAs employed the FTIR technique. Both o f t h e s e t e c h n i q u e s a r e v e r y u s e f u l f o r i d e n t i f y i n g i m p u r i t i e s however, i n t h i s p a p e r , i t i s p e r t i n e n t t o p o i n t out some o f the advantages o f the m a g n e t o - o p t i c a l s p e c t r o s c o p y t e c h n i q u e : (1) I t can p e n e t r a t e l a y e r s such as a c t i v e l a y e r s as w e l l as l a y e r s a s s o c i a t e d w i t h some h e t e r o s t r u c t u r e s , (2) I t does not r e q u i r e c o n t a c t s , (3) donors and a c c e p t o r s can be i d e n t i f i e d s i m u l t a n e o u s l y p e r m i t t i n g an e s t i m a t e o f sample compensation, and (4) s i n c e i t samples a v e r y t h i n l a y e r o f m a t e r i a l i t can be used i n p r o f i l i n g layers.

Literature Cited 1. Rashba, E. I., Opt. Spektrosk. 1957, 2,508. 2. Rashba, E. I. and Gurgenishvili, G. E., Fiz. Tverd. Tela 1962, 4,1029 (English trans.: Sov. Phys. Solid State 1962, 4 759. 3. Stillman, G. E.; Wolfe, C. M. and Korn, D. M., Proc. Int. Conf. Physics of Semicond. (Warsaw), 1972, p. 863. 4. Fetterman, H. R.; Larsen, D. M.; Stillman, G. E.; Tannenwalk, P. E. and Waldman, J., Phys. Rev. Lett. 1971, 26,975. 5. Almassy, R. J.; Reynolds, D. C.; Litton, C. W.; Bajaj, K. K. and G. L. McCoy, Solid State Commun. 1981, 38,1053. 6. Reynolds, D. C.; Litton, C. W.; Smith, E. B.; Yu, P. W. and Bajaj, K. K., Solid State Commun. 1982, 42,827. 7. Thomas, D. G. and Hopfield, J . J., Phys. Rev. 1962, 128,2135. 8. Dean, P. J.; Cuthbert, J . D.; Thomas, D. G., and Lynch, R. T., Phys. Rev. Lett. 1967, 18,122. 9. Reynolds. D. C.; Litton, C. W. and Collins, T. C., Phys. Rev. 156,3 (1967); 156,881 (1967). 10. Reynolds, D. C.; Litton, C. W. and Collins, T. C., Phys. Rev. 1969, 177,1161. 11. Reynolds, D. C.; Litton, C. W. and Collins, T. C., Phys. Rev. 1968, 174,845.



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12. Reynolds, D. C. and Collins, T. C., Phys. Rev. 1969, 185,1099. 13. Merz, J . L . ; Kukimoto, H.; Nassau, K. and Shiever, J. W., Phys. Rev. 1972, B6,545. 14. Hiesinger, P.; Suga, S; Willmann, F. and Dreybrodt, W., Phys. Status Solidi 1975, B67,64. 15. White, A. M.; Dean, P. J.; Taylor, L. L . ; Clarke, P. C.; Ashen, P. J . and Mullin, J . B. J . Phys. 1972, C5,1727. 16. White, A. M.; Dean, P. J . and Day, B. J . Phys. 1974, C7,1400. 17. Henry, C. H. and Nassau, K., Phys. Rev. 1970, B2,977. 18. Benoit a' la Guillaume, C. and Lavallard, P., Phys. Status Solidi 1975, B70,K143. 19. Ruhle, W. and Klingenstein, W., Phys. Rev. 1978, B18,7011. 20. Herzberg, G. and Spinks, J . W. T., Proc. R. Soc. London Ser. A, 1934, Ser. a 147,434. 21. Reynolds. D. C.; Langer, D. W.; Litton, C. W.; McCoy, G. L. and Bajaj, K. K., Solid State Commun. 1983, 46,473. 22. Dean, P. J., Herbert, D. C. and Lahee, A. M., J . Phys. 1980, C13,5071. 23. Cabib, D.; Fabri, E. and Fiorio, G., Nuovo Cimento 10B, 185(1972). 24. 24. Reynolds, D. C.; Bajaj, K. K.; Litton, C. W. and Smith, E. B., Phys. Rev. 1983, 28B,3380. RECEIVED

November 8, 1985


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