Ultraviolet reflectance of Al2O3, SiO2 and BeO

Solid State C o m m u n i c a t i o n s Vol. 2, pp. 269-272, 1964. P e r g a m o n P r e s s , Inc. P r i n t e d in the United States.

U L T R A V I O L E T R E F L E C T A N C E OF A1203, SiO 2 AND BeO Eugene Loh Hughes R e s e a r c h L a b o r a t o r i e s , Malibu, C a l i f o r n i a (Received 8 J u l y 1964 by E. Burstein)

The r o o m t e m p e r a t u r e u l t r a v i o l e t r e f l e c t a n c e s p e c t r a of flux grown r e d r u b y (A1203 w i t h - 4 p e r cent Cr203), flux grown b e r y l l i a (BeO) and natural quartz (SiO 2) have been m e a s u r e d on n a t u r a l f a c e s of the c r y s t a l s f r o m 6 to 14 eV with an angle of incidence of 45 °. The s p e c t r a show e x c i t o n - l i k e p e a k s at 9.1, 10.4 and 10.4 eV f o r A1203, BeO and SiO2, r e s p e c t i v e l y , followed by b r o a d s t r u c t u r e s p r e s u m a b l y due to interband t r a n s i t i o n s . Striking s i m i l a r i t i e s between the s p e c t r a of A1203 and MgO a r e o b s e r v e d .

THE u l t r a v i o l e t s p e c t r a of s e m i c o n d u c t o r s and i n s u l a t o r s have been i n t e r p r e t e d 1 in t e r m s of t h r e e p r i n c i p a l m e c h a n i s m s : exciton t r a n s i t i o n s which r e s u l t in s h a r p peaks, interband t r a n s i t i o n s which m a n i f e s t t h e m s e l v e s e i t h e r as b r o a d p e a k s or as a b s o r p t i o n e d g e s of v a r i o u s types, and p l a s m a r e s o n a n c e s that show up as m a r k e d dec r e a s e s in r e f l e c t i v i t y with i n c r e a s i n g photon e n e r g y . Sharp exciton p e a k s p r e d o m i n a t e in the s p e c t r a of the alkali halides while interband t r a n s i t i o n s a r e the dominant f e a t u r e s of the s p e c t r a of group IV and I I I - V s e m i c o n d u c t o r s . In c a s e s w h e r e t h e o r e t i c a l band s t r u c t u r e c a l c u lations a r e available, m a n y of the p e r t i n e n t band p a r a m e t e r s can be deduced f r o m u l t r a v i o l e t s p e c t r a , and e v e n f o r other m a t e r i a l s qualitative conclusions c o n c e r n i n g t h e i r e l e c t r o n i c e n e r g y Ievel s t r u c t u r e can be d r a w n f r o m c o m p a r i s o n with the m o r e c o m p l e t e l y studied s e m i c o n d u c t o r s and i n s u l a t o r s . In this note we wish to r e p o r t p r e l i m i n a r y m e a s u r e m e n t s of the r e f l e c t i v i t y of s i n g l e c r y s t a l s of BeO, A1203, and SiO 2 in the r a n g e of photon e n e r g i e s f r o m 5 to 14 eV. A M c P h e r s o n Model 225 v a c u u m u l t r a v i o l e t m o n o c h r o m a t o r , with a 1 m concave g r a t i n g blazed at 1500A and an ac h y d r o g e n d i s c h a r g e s o u r c e was u s e d for t h e s e m e a s u r e m e n t s , which w e r e m a d e at a 45 ° angle of incidence. Incident and r e f l e c t e d r a d i a t i o n w e r e m o n i t o r e d with f l u o r e s c e n t sodium s a l i c y late and an EMI 6256S phototube. In o r d e r to avoid s u r f a c e d a m a g e that might r e s u l t f r o m m e c h a n i c a l polishing, c r y s t a l s with well d e v e l o p e d n a t u r a l f a c e s w e r e s e l e c t e d 269

f o r the m e a s u r e m e n t s . Flux grown BeO c r y s t a l s w e r e used; one s a m p l e was p y r a m i d a l in shape with a well developed (0001) s u r f a c e as the b a s a l plane, while the o t h e r had a p r i s m a t i c s e c t i o n with good (1010)planes (which contain the c - a x i s ) . The A1203 s a m p l e was a f l u x - g r o w n r u b y p l a t e l e t with (0001) s u r f a c e s . The SiO 2 was a c r y s t a l of natural Brazilian quartz. A reflection spectrum f r o m a c l e a v a g e plane of a Norton MgO c r y s t a l 2 was r u n f o r c o m p a r i s o n with the t h r e e m a t e r i a l s studied. The r e f l e c t i o n s p e c t r u m of the r e d r u b y p l a t e let is shown in Fig. 1 along with that of MgO. T h e r e a r e s t r i k i n g s i m i l a r i t i e s between these two s p e c t r a : both d i s p l a y a p r o m i n e n t low e n e r g y r e f l e c t a n c e p e a k followed by a b r o a d hump with four m o r e weak p e a k s s u p e r p o s e d . C o m p a r i s o n with s p e c t r a of o t h e r m a t e r i a l s leads to the int e r p r e t a t i o n that the f i r s t p e a k is an exciton t r a n s i t i o n and the b r o a d s t r u c t u r e is p r o b a b l y due to i n t e r b a n d t r a n s i t i o n s . The r a t i o s of the e n e r g i e s of the exciton p e a k and the four i n t e r b a n d p e a k s in the two m a t e r i a l s is v e r y close to 1.2 f o r all of the o b s e r v e d f e a t u r e s . In this c o n n e c tion it is w o r t h noting that the coordination of the m e t a l ions is e s s e n t i a l l y the s a m e in both of these m a t e r i a l s : e x a c t l y o c t a h e d r a l in MgO, which has the NaC1 lattice, and a p p r o x i m a t e l y o c t a h e d r a l in A1203. The s q u a r e of the r a t i o of the interionic d i s t a n c e in A1203 to that in MgO is (1.21) -1. It will be i n t e r e s t i n g to see whether the A1203 s p e c t r u m will have another p e a k at 16.2 eV, c o r r e s p o n d i n g to the 13.5 eV p e a k in MgO. The s i m i l a r i t y in the s p e c t r a of these two m a t e r i a l s p a r a l l e l s the o b s e r v a t i o n s that I I I - V

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UV REFLECTANCE OF A1203, SiO 2 AND BeO

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UV R E F L E C T A N C E OF A1203, SiO 2 AND BeO

and II-VI s e m i c o n d u c t o r s with z i n c - b l e n d e or w u r t z i t e s t r u c t u r e s show s t r i k i n g s i m i l a r i t i e s . 3 It is not c l e a r why the exciton p e a k in A1203 is so weak c o m p a r e d to that in MgO. This m a y be an i n t r i n s i c p r o p e r t y of the A1203 lattice, or it m a y be connected with p e r t u r b i n g e f f e c t s due to the high Cr +++ ion c o n c e n t r a t i o n in the r u b y s a m p l e used. SiO 2 d i s p l a y s a s h a r p low e n e r g y p e a k at 10.1 eV (Fig. 2), but the high e n e r g y s t r u c t u r e is different f r o m that of A1203 and MgO. The silicon ions a r e t e t r a g o n a l l y c o o r d i n a t e d in this compound, with only four n e a r e s t O = n e i g h b o r s (as c o m p a r e d to six in o c t a h e d r a l coordination). The photon e n e r g y of the exciton p e a k i n c r e a s e s as the interionic d i s t a n c e 4 d e c r e a s e s in the sequence MgO, A1203, SiO 2. The BeO s p e c t r u m a l s o shows a low e n e r g y exciton p e a k (at 10.3 eV) followed by b r o a d e r bands indicative of interband t r a n s i t i o n s . The dip in the r e f l e c t i v i t y o b s e r v e d in BeO i m m e d i a t e l y below the f i r s t exciton p e a k cannot be a t t r i b u t e d to a p l a s m a r e s o n a n c e b e c a u s e of its location below the band edge. T h e r e is no e v i dence in our data of sufficient o s c i l l a t o r s t r e n g t h below the photon e n e r g y of the dip to account for so m a r k e d a f e a t u r e . The index of r e f r a c t i o n of BeO and the angles between s a m p l e f a c e s a r e such that at 45 ° angle of incidence the light that e n t e r s the c r y s t a l can be totally i n t e r n a l l y r e f l e c t e d and e m e r g e p a r a l l e l to the r e f l e c t e d b e a m , thus giving the a p p e a r a n c e of an a n o m a l o u s l y high

r e f l e c t i v i t y at photon e n e r g i e s w h e r e the s a m p l e is nonabsorbing. 5 If the BeO s p e c t r u m is c o m p a r e d with that of L i F 6 and diamond, 7 which a r e m e m b e r s of the s a m e i s o e l e c t r o n i c s e r i e s of compounds, it a p p e a r s that BeO is m o r e like LiF, w h e r e exciton s t r u c t u r e p r e d o m i n a t e s , than it is like diamond w h e r e interband t r a n s i t i o n s dominate. The r e f l e c t a n c e s p e c t r u m of BN c r y s t a l , the other m e m b e r of the i s o e l e c t r i c s e r i e s containing diamond, should r e s e m b l e 8 that of diamond r a t h e r than that of LiF. The intrinsic band gap in ionic crystals such as the oxides reported in this note usually is identifiable as a shoulder in the energy dependence of E2, the imaginary part of the dielectric con~ stant. This can only be crudely estimated from our reflectivity spectra and for the three materials reported in this note, we obtain: A1203, i0 eV; SiO2, I i eV; BeO, i i . 2 eV. Better estimates of these values and other electronic band parameters can be obtained from either thin film absorption or normal incidence reflection, coupled with a Kramers-Kronig analysis to determine E1 and E2. The author w i s h e s to thank F . H . A n d r e s f o r the flux grown r e d r u b y and a p y r a m i d a l BeO single c r y s t a l , S.A. A u s t e r m a n of A t o m i c s I n t e r national f o r the p r i s m a t i c BeO single c r y s t a l , and M. H i m b e r f o r the natural quartz c r y s t a l s . The author is m u c h indebted to G.S. P i c u s for f r e quent d i s c u s s i o n s and r e v i s i o n of the m a n u s c r i p t .

References 1. See f o r e x a m p l e P H I L I P P H . R . and E H R E N R E I C H H., P h y s . Rev. 131, 2016 (1963) or P H I L L I P S J. C., Ibid. 133, A452 (1964). 2. R E I L I N G G . H . and HENSLEY E . B . ,

271

P h y s . Rev. 112, 1106 (1958).

3. See CARDONA M. and GREENAWAY D. L., Phys. Rev. 131, 98 (1963); PHILIPP H.R. and EHRENREICH H., Ibid. 129, 1550 (1963), and r ~ e n c e s therein. 4. F o r e x a m p l e , PAULING L . , N a t u r e of the C h e m i c a l Bond, (2nd E d . ) . Cornell U n i v e r s i t y P r e s s (1960). 5. H o w e v e r the dip has a l s o been o b s e r v e d in n e a r - n o r m a l incidence m e a s u r e m e n t s with p o l a r i z e d light. 6. WALKER W . C . , J. Opt. Soc. A m e r . 52, 223 (1962); MILGRAM A. and PARKER GIVENS M., P h y s . Rev. 125, 1506 (1962). - 7. WALKER W. C. and OSANTOWSKI J . , P h y s . Rev. 134, A153 (1964); P H I L I P P H . R . and T A F T E . A . , Ibid. 127, 159 (1962).

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8. BN, the remaining member of the isoelectronic series containing diamond, would be expected to have a diamond-like spectrum, since theoretical band structure calculations 9 indicate the bonding is predominantly homopolar. 9. KLEINMAN L. and PHILLIPS J . C . , Phys. Rev. 117, 460 (1960). Die Reflexionsspektren von Rubin (A1203 mit 4% Cr203) , Beryllia (BeO) und nat~irlichem Quarz (SiO2) wurden im ultravioletten Bereich von 6 his 14 eV bei Zimmertemperatur an gewachsenen Kristalloberfl~chen gemessen. Die Spektren zeigen scharfe Maxima bei 9.1 eV (f~ir A1203), bei 10.4 eV (f~r BeO) und bei 10.4 eV (fUr SIO2), die als Exzitonlinien gedeutet werden, gefolgt von breiten Banden, wahrscheinlich als Band-Band Uberg~nge erzeugt. Es werden lJbereinstimmungen zwischen den Spektren von A1203 und MgO beobachtet.