Local Structure around In Atoms in InGaN Single-quantum-well by XAFS In x Ga 1-x N is a key material in high-brightness b l u e / g r e e n l i g h t - e m i t t i n g d i o d e s ( F i g . 1 ) a n d p u r p l i s h - b l u e l a s e r d i o d e s [ 1 ] . A l t h o u g h s u c h d e v i c e s h a v e v e r y h i g h d e n s i t i e s o f t h r e a d i n g dis loc ati ons , the y sho w a hig h qua ntu m effi cie ncy i n c o n t r a s t t o c o n v e n t i o n a l I I I - V a n d I I - V I s e m i c o n d u c t o r - b a s e d d e v i c e s [ 2 ] . I n m o l e fluctuation in InGaN active layers is proposed as its o r i g i n [ 3 ] . I n t h e m o d e l , i n j e c t e d c a r r i e r s a r e lo ca li ze d in to fl uc tu at io n- in du ce d po te nt ia l mi ni ma a n d c a n e f f i c i e n t l y c o n t r i b u t e t o r a d i a t i v e r e c o m b i n a t i o n b e f o r e c a p t u r e d b y d i s l o c a t i o n - a c t i v a t e d n o n - r a d i a t i v e r e c o m b i n a t i o n c e n t e r s , w h i c h i s c o n s i d e r e d t o l e a d t o t h e h i g h q u a n t u m e f f i c i e n c y . T h e r e f o r e , i n o r d e r t o m a k e t h e e m i s s i o n m e c h a n i s m c l e a r i n I n G a N - b a s e d l i g h t - e m i t t i n g d e v i c e s , i t i s i m p o r t a n t t o c l a r i f y l o c a l structures around In atoms in InGaN. E x t e n d e d X - r a y A b s o r p t i o n F i n e S t r u c t u r e ( E X A F S ) i s a p o w e r f u l t o o l t o i n v e s t i g a t e l o c a l structures in thin layers composed of two or more e l e m e n t s . T h e r e a r e s o m e r e p o r t s o n l o c a l structures around In atoms in InGaN by EXAFS [4- 6 ] . T h e v a l u e , h o w e v e r , v a r i e s w i d e l y e v e n f o r a t o m i c d i s t a n c e s m a i n l y d u e t o t h e q u a l i t y o f s a m p l e s . I n t h i s w o r k , E X A F S m e a s u r e m e n t s a r o u n d a n I n K - e d g e w e r e c a r r i e d o u t f o r a h i g h q u a l i t y I n G a N s i n g l e - q u a n t u m - w e l l ( S Q W ) g r e e n light emitting diode. The sample is an In 0.45 Ga 0.55 N SQW of 3 nm t h i c k n e s s , w h i c h w a s g r o w n b y m e t a l l o r g a n i c chemical vapor deposition ( MOCVD ) on a sapphire ( 0 0 0 1 ) s u b s t r a t e . I t c o n s i s t s o f a s a p p h i r e substrate, GaN buffer layer, n-GaN :Si barrier layer ( 4 0 0 0 n m ) , I n 0 . 4 5 G a 0 . 5 5 N S Q W ( 3 n m ) , p - Al 0.2 Ga 0.8 N: Mg bar rie r lay er (10 0 nm ), and p-G aN layer (500 nm ) ( Fig. 2 ). The XAFS measurements w e r e m a d e a t b e a m l i n e B L 0 1 B 1 a n d t h e X A F S d a t a w e r e c o l l e c t e d w i t h a d o u b l e - c r y s t a l m o n o c h r o m a t o r u s i n g S i ( 1 1 1 ) c r y s t a l s [ 7 ] . T h e beam was focused on the sample surface. In K α - f l u o r e s c e n c e e m i s s i o n w a s m e a s u r e d u s i n g a 1 9 e l e m e n t s G e d e t e c t o r . T h e a n g l e b e t w e e n t h e inc ide nt X-r ay bea m and the sam ple pla ne was 2 d e g r e e s . T h e s a m p l e w a s r o t a t e d i n - p l a n e t o remove the Bragg diffraction. I n o r d e r t o a n a l y z e t h e e x p e r i m e n t a l E X A F S d a t a , X A N A D U a n d F E F F 6 . 0 c o d e w e r e u s e d . Fig. 1. High-brightness InGaN blue and green LED. 77 W h a t c a u s e d t h e g r e a t e s t d i f f i c u l t y i n t h e m e a s u r e m e n t i n t h i s s y s t e m w a s a q u i t e s m a l l p h o t o n - c o u n t i n g r a t e i n t h e d e t e c t o r b e c a u s e t h e cap on the InGaN SQW (about 600 nm thick, Fig. 2 ) r e d u c e s t h e i n c o m i n g p h o t o n o f I n - K α . T h e Fourier transforms ( FT ) of the present EXAFS are shown in Fig. 3 . Since the structure of InGaN is a w u r t z i t e t y p e , t h e f i r s t n e a r e s t p e a k i n c l u d e s t h e co nt ri bu ti on of In -N bo nd an d th e se co nd ne ar es t p e a k i n c l u d e s t h a t o f I n - G a a n d I n - I n f o r I n 0 . 4 5 G a 0 . 5 5 N . T o o p t i m i z e t h e v a l u e s o f t h e s t r u c t u r e p a r a m e t e r s , t h e s e t w o p e a k s a r e i n d i v i d u a l l y f i t t e d b y t h e n o n - l i n e a r l e a s t s q u a r e method (curve-fitting) for the k -range of 5.0 ~ 12.5 Å -1 af te r the en er gy sh if t, th e pha se s and am pl it ud es are corrected. First, we analyzed the InN powder a s a s t a n d a r d s a m p l e a n d o b t a i n e d r e a s o n a b l y good results ( r In-N = 2.15 Å, r In-In = 3. 53 Å). Then, w e a p p l i e d t h e e m p i r i c a l p a r a m e t e r s f r o m t h e standard sample to In 0.45 Ga 0.55 N SQW. The results for the interatomic distances are as follows: r In-N is 2.11 Å, r In-Ga is 3.25 Å, and r In-In is 3.31 Å. We had g o o d a g r e e m e n t b e t w e e n t h e t h e o r e t i c a l F T b y c u r v e - f i t t i n g a n d t h e e x p e r i m e n t a l o n e f o r Fig. 2. A schematic view of the structure of the green LED. In 0.45 Ga 0.55 N ( Fig. 3 ). The interatomic distances in t h e I n 0. 45 G a 0. 55 N S Q W a r e s h o r t e r i n c o m p a r i s o n with those in InN. Especially the reduction of In-In dist ance is prom inen t. The pres ent EXAF S give s structural information about the horizontal direction of the SQW surface, because the electric vector of th e in ci de nt X- ra y is po la ri ze d in th e pl an e of th e sam ple sur fac e. It is pos sib le tha t r In-I n has bee n g r e a t l y s h o r t e n e d i n t h e h o r i z o n t a l d i r e c t i o n affected by the atomic spacing of upper and lower G a N i n t h e I n 0 . 4 5 G a 0 . 5 5 N S Q W . C o m p a r i s o n be tw ee n th e me as ur em en ts of th e ho ri zo nt al an d vertical directions to the SQW will be important in the next research project. As for the coordination n u m b e r ( N ) , N I n - N i s 3 . 2 f o r t h e f i r s t p e a k , a n d N In-Ga and N In-In are 11.3 and 1.9 resp ecti vely , for the second peak. The EXAFS results suggest that In ato ms are hom oge neo usl y dis tri but ed and may make an ordered p h a s e i n t h e In 0.45 Ga 0.55 N SQW [7]. How eve r, the re rem ain s a pos sib ili ty tha t the a c t u a l a v e r a g e c o n c e n t r a t i o n o f I n a t o m f o r t h e present sample is lower (x ~ 0.2) than x = 0.45 and a more accurate determination of the local structure is in progress. p-Al 0.2 Ga 0.8 N (100 nm ) In 0.45 Ga 0.55 N (3 nm ) GaN buffer layer sapphire substrate n-GaN (4000 nm ) p-GaN (500 nm ) 78 r (Å) 0 2 4 6 8 0 0.1 0.2 InN exp. | FT (r) | InGaN exp. InGaN fit References [1] S. Nakamura et al. , The Blue Laser Diode, 2 nd ed. (Springer, Berlin, 2000). [2 ] S. F. Ch ic hi bu et al . , In In tr od uc ti on to Ni tr id e S e m i c o n d u c t o r B l u e L a s e r s a n d L i g h t E m i t t i n g D i o d e s , e d s . S . N a k a m u r a a n d S . F . C h i c h i b u ( T a y l o r & Francis, London (2000) 153. [3] S. Chichibu et al. , Appl. Phys. Lett. 69 (1996) 4188. [4 ] N. J. Je ff s et al . , Ma t. Re s. So c. Sy mp . Pr oc . 512 (1998) 519. [ 5 ] S . C . B a y l i s s e t a l . , M a t e r i a l s S c i e n c e a n d Engineering B 59 (1999) 292. [ 6 ] T . M i y a j i m a e t a l . , P h y s . S t a t . S o l . ( b ) 2 2 8 (2001) 45. [ 7 ] S . M a t s u d a , T . M i y a n a g a , T . A z u h a t a , T . Homma, R. Maruko, S. Chichibu, T. Sota, T. Mukai, T. Uruga, H. Tanida, Abstracts of 4 th XAFS Meeting in Japan (2001). F i g . 3 . F o u r i e r t r a n s f o r m s o f I n K - e d g e E X A F S k χ ( k ) f o r I n G a N S Q W ( d a s h e d r e d l i n e , f l u o r e s c e n c e ) a n d I n N p o w d e r ( d a s h - d o t t e d g r e e n l i n e , transmission). Solid blue line is FEFF fit result for InGaN SQW. The energy shift, phases and amplitudes are not corrected in these Fourier transforms. Ta ka fu mi Mi ya na ga a , Ta ka sh i Az uh at a a an d Takashi Mukai b (a) Hirosaki University (b) Nitride Semiconductor Res. Laboratory, Nichia Corporation E-mail: takaf @ cc.hirosaki-u.ac.jp 79
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