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RAMAN STUDY OF GEOMETRICAL DISORDER AT SEMICONDUCTORS SURFACES
Robert Carles, J. Renucci, A. Zwick, M. Renucci
To cite this version:
Robert Carles, J. Renucci, A. Zwick, M. Renucci. RAMAN STUDY OF GEOMETRICAL DISOR-
DER AT SEMICONDUCTORS SURFACES. Journal de Physique Colloques, 1982, 43 (C9), pp.C9-
363-C9-364. �10.1051/jphyscol:1982970�. �jpa-00222499�
JOURNAL DE PHYSIQUE
Colloque C9, supplement au n°l2, Tome 43, decembre 1982 page C9-363
RAMAN STUDY OF GEOMETRICAL DISORDER AT SEMICONDUCTORS SURFACES
R. C a r l e s , J . B . R e n u c c i , A. Zwick e t M.A. Renucci
Laboratoive de Physique des Solides Associe au CURS,118, Route de Narbonne, 31062 Toulouse Cedex, France
Résumé.- Un désordre de nature géométrique est provoqué à la surface d'échan- tillons par un pollisage mécanique grossier. Son influence sur les pro- priétés vibroniques et électroniques de InSb, InAs et GaSb est discutée.
Abstract.- Geometrical disorder is induced at the surface of samples by rough mechanical polishing. Its influence on both vibrational and electronic proper- ties are reported in InSb, InAs and GaSb.
1.- Introduction.- It is well known that optical properties of crystals (reflectivity, absorption ^luminescence) depend strongly on surface preparation. It is prejudicial when studying bulk properties but provides_, nevertheless, a mean of investigating da- mages at the surfaces. Among other optical techniques Raman spectroscopy is quite suitable to look at these damages since the penetration depth.of the light can be small enough to study the disorder in the vicinity of the surface. Evans and Ushioda
(1) were the first to realize this possibility. They took advantage of the broadening which characterizes the first order Raman lines of polished surfaces to analyse the disorder induced by the polishing process. Recently, Carles et al (2) found that the Raman spectra of pure InAs samples alterated by mechanical polishing display two new bands labelled D.A.T.A. and D.A.L.A. which were interpreted as first order disorder activated processes. The study of these structures enables the characterization of this'so-called "geometrical" disorder.
We present here results relativeto InSb, InAs and GaSb. The influence of the geome- trical disorder on the electronic properties of these compounds is also discussed.
2.- Experiment•- The samples analyzed were single crystals which (100) surfaces were mechanically polished with 0,3 p alumina powder and then rinced in methanol. To avoid scattering from air the spectra were always recorded with the samples inside the vacu- um chamber of the cryostat. They were not analyzed.
3.- Results and discussion.- We display in figures 1 and 2 respectively, the direct traces for InSb and InAs over a wide range of temperature. Arrows indicate phonons frequencies from ref. 3 and 4. We present only the frequency range below the T0(F) and L0(T) which are still present in the spectra and well polarized (2).
In figure 1, besides the 2TA scattering characteristic of crystalline InSb two new bands (D.A.T.A. and D.A.L.A.) appear confirming the results already found in InAs (2).
At room temperature the D.A.L.A. is somewhat obscured by the end of the 2TA scattering (2IA(K) at 121 cm-1). As temperature decreases the second order scattering gets stron- gly attenuated (see the 2TA(X) for example) and the D.A.L.A. band merges around 137 cm"1
at 25 K. The apparent broadening of the low energy tail of the T0(T), as temperature decreases, exemplifies the presence of disorder activated optical processes.
In figures 1 and 2, the intensities of the D.A.T.A. and the D.A.L.A. become stronger, compared to that of the second order scattering, as one goes from 300 K to 25 K. This
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982970
C9-3 64 JOURNAL DE PHYSIQUE
WAVE NUMBER (cm-')
F i g u r e 1.- Evolution with temperature of t h e Raman spectrum of InSb a l t e r a t e d mechanically.
WAVE NUMBER (cm-1) Figure 2.- Same a s f i g . 1 , f o r InAs.
behavior shows t h a t t h e y correspond t o f i r s t o r d e r p r o c e s s e s . I n f i g u r e 2 , t h e i n t e n - s i t y of t h e TO-TA band around 170 cm-1 f o l l o w s t h e expected temperature dependance f o r a s u b s t r a c t i v e combination.
The f r e q u e n c i e s of t h e maxima of t h e D . A . T . A . and t h e D . A . L . A . a r e r e s p e c t i v e l y h a l f t h o s e of t h e 2TA(X) and 2LA(L) i n d i c a t i n g t h a t t h e y correspond t o t r a n s v e r s e and lon- g i t u d i n a l a c o u s t i c a l modes r e s p e c t i v e l y . The D . A . T . A . and t h e D.A.L.A. a p p a r i n g i n both InSb and InAs n e a r phonon f r e q u e n c i e s c h a r a c t e r i s t i c of each compound r u l e o u t t h e e v e n t u a l i t y of I n - I n modes. Also A s - A s o r Sb-Sb modes would l i e o u t s i d e t h e f r e - quency range explored ( 5 ) .
A t t h e lowest t e m p e r a t u r e s t h e shape of t h e D . A . T . A . m i r r o r s t h a t of t h e 2TA s c a t t e - r i n g : same k i n d s of s i n g u l a r i t i e s c h a r a c t e r i s t i c of L , X and K p o i n t s , same r e l a t i v e i n t e n s i t i e s of t h e bands a t t h e s e p o i n t s . The f i r s t a s s e r t i o n a l r e a d y proves t h a t pho-1 nons of t h e whole B r i l l o u i n zone c o n t r i b u t e t o t h e D.A.T.A. i n accordance w i t h t h e breakdown of t h e = 0 s e l e c t i o n r u l e . The second one, t o g e t h e r with t h e f a c t t h a t , i n t h e 1 1 1 - V compounds, t h e 2TA s c a t t e r i n g r e f l e c t s t h e t r a n s v e r s e a c c o u s t i c a l one pho- non d e n s i t y of s t a t e s ( 3 - 4 ) w i t h t h e s c a l e doubled, i m p l i e s t h a t t h e D . A . T . A . t o o images t h i s d e n s i t y of s t a t e s .
From t h e s e f a c t s , C a r l e s e t a l . ( 2 ) proposed t o f i t , over t h e whole a c o u s t i c a l r e g i o n , t h e d i s o r d e r a c t i v a t e d s c a t t e r i n g i n InAs by t h e e x p r e s s i o n :
g l ( u ) i s t h e one phonon d e n s i t y of s t a t e s of InAs convoluted by a g a u s s i a n t o account f o r t h e broadening of t h e s t r u c t u r e s ; ii i s t h e Bose E i n s t e i n f a c t o r ; Cl(w) i s t h e coupling c o n s t a n t of t h e l i g h t with t h e v i b r a t i o n a l modes. The f i t was e x c e l l e n t t a -
