Optical Absorption and Structural Order in Sputtered Amorphous Phosphorus

-Optical absorpt ion measurements a re repor ted f o r rf sput tered a-P f i l m s prepared under cond i t i ons o f v a r i a b l e subst rate temperature. Var ia t ions i n t h e o p t i c a l gap w i t h Ts are s i m i l a r t o changes i n t h e Raman s c a t t e r i n g and x-ray d i f f r a c t i o n spectra. The r e s u l t s suggest t h a t i n t e r mediate-range order p lays an important r o l e f o r t h e s t ruc tu ra l , v i b r a t i o n a l and o p t i c a l p roper t ies f o r more ordered a-P f i lms. Photost ructura l e f f e c t s i n As-chalcogenides which e x h i b i t analogous behavior a lso suggest mod i f i ca t ions o f intermedi ate-range order. Introduction.-Recent s tud ies i n t h i n f i l m a-P have emphasized t h a t i n t r i n s i c v a r i a t i o n s i n network s t r u c t u r e may a r i s e w i t h deposi t ion condit ions. Both Raman s c a t t e r i n q and low-angle x-ray d i f f r a c t i o n measurements suggest t h a t s t a t i c and dynamic c o r r e l a t i o n e f f e c t s are present as a consequence o f intermediate-range order, i.e., order beyond t h e f i r s t coord inat ion s h e l l (1,2). Given these s t ruc t u r a l and v i b r a t i o n a l r e s u l t s i t i s o f i n t e r e s t t o i n q u i r e i f t h e e l e c t r o n i c s ta tes o f a-P are a lso in f luenced by s t r u c t u r a l c o r r e l a t i o n s o f in termediate range. I n con t ras t t o s t r u c t u r a l and v i b r a t i o n a l probes, o p t i c a l absorpt ion measurements a re more d i f f i c u l t t o associate w i t h s t r u c t u r a l modi f icat ions. I n addi t ion, e x t r i n s i c e f f e c t s associated w i t h voids o r m ic ros t ruc tu re i n general, may a l s o p l a y a r o l e t h a t requi res considerat ion. I n t h e present study we focus on mod i f i ca t ions o f t h e o p t i c a l spectra i n t h e near gap region on sput tered a-P f i l m s prepared under condi t ions o f r e l a t i v e l y h igh order as deduced from Raman s c a t t e r i n q and x-ray d i f f r a c t i o n measurements. The i n f l u e n c e o f short-range order and f i l m morphology on t h e o p t i c a l spectra w i l l be presented elsewhere. The r e s u l t s discussed below suggest t h a t i n ordered a-P f i l m s intermediate-range order a l s o modi f ies t h e e l e c t r o n i c s ta tes v i a s t r u c t u r a l c o r r e l a t i o n ef fects . The absorpt ion spectra a lso i n d i c a t e t h a t t h e Urbach slope i s not s e n s i t i v e t o i n t e r mediate-range order, but may, i n con t ras t t o e a r l i e r suggestions (3) vary i n e ie mental semiconductors o f s i m i l a r coordinat ion. Experiment.-To sys temat i ca l l y study v a r i a t i o n s i n t h e phys ica l p roper t ies o f a-P f i l m s t h e rf plasma deposi t ion cond i t i ons i n a p lanar diode s p u t t e r i n g system were f i x e d and only the subst rate temperature modified. The temperature o f t h e f i lms was determined d i r e c t l y by a t h i n f i l m Ag-A1 thermocouple. An Ar pressure o f 60 P and a cathode-anode separat ion o f 2" were employed t o thermal ize t h e inc iden t P species and y i e l d ordered f i lms. The t a r g e t employed was a 2" diameter p iece o f p o l y c r y s t a l l i n e black P. Reflectance and t ransmi t tance measurements were performed on a Cary 14 system f o r 0.5-15 p t h i c k f i lms. The absorpt ion coeff i c i e n t and index o f r e f r a c t i o n were determined by s o l v i n g t h e appropr ia te f i l m p l u s subst rate equations (4) us ing a microcomputer. Results and Discussion.-In Fig. 1 t h e o p t i c a l absorpt ion c o e f f i c i e n t o f sput tered a-P f i l m s deposited a t d i f f e r e n t subs t ra te temperatures, Ts, i s shown. The values of Ts = 8, 64, 126 and 180 C f o r f i l m s A, 8, C and D, respect ive ly , were chosen t o y i e l d approximately 60 C i n t e r v a l s i n TS. Although Fig. 1 ind ica tes a r e l a t i v e l y s i m i l a r form f o r each o f t h e f i l m s , t h e s h i f t o f t h e absorpt ion spectra a r e c l e a r l y nonl inear i n TS. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814191 C4-874 JOURNAL DE PHYSIQUE Fiqure 1 a l s o ind ica tes , w i t h t h e exception o f a small deqree o f t a i l i n g lo6, , -