ELECTRONIC STRUCTURE OF AMORPHOUS Si3N4

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ELECTRONIC STRUCTURE OF AMORPHOUS Si3N4

I. Brytov, E. Obolenskii, Yu. Romashchenko, V. Gritsenko

To cite this version:

I. Brytov, E. Obolenskii, Yu. Romashchenko, V. Gritsenko. ELECTRONIC STRUCTURE OF AMORPHOUS Si3N4. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-887-C2-890.

�10.1051/jphyscol:19842203�. �jpa-00223880�

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JOURNAL DE PHYSIQUE

Colloque C2, suppl6ment au n02, Tome 45, f6vrier 1984 page C2-887

ELECTRONIC STRUCTURE OF AMORPHOUS Si3N4

I.A. Brytov, E.A. Obolenskii, Yu.N. Romashchenko and V.A. Gritsenko

VNIINauekprybor, Leningrad, U. S.S. R

R6sumS - On a effect& llinterprGtation des Btats electroniques dans la bande de valence et la bande de conduction du Si3N4 amorphe et on a Btabli le diagramme des 6nergies d'aprss les spectres d'Bmission et le rendement quantique de SiK -,

SiL2 3 -, NK en combinaison avec les donnees de la spectrosco- pie des photo6lectrons de rayons X des couches internes et de la bande de valence.

Abstract

-

There is performed interpretation of electronic states $n valence and conduction bands of amorphous Si3N4 and constructed an energy diagram of electronic levels using X- ray SiK

-,

^SiLg3

-,

^NK

-

emission and quantum yield spectra in combination with the XPS data of inner levels.

The main structural unit of Si3N4 is tetrahedron SiN4/3

,

i.e. atom of Si is surrounded by four a t o m of N, atom of N couples three tetrahedrons.

In the present work there were studied f i l m of a-Si3Nq deposited on substrates of saphire ( Chi1203 ) or silicon single crystals, film thickness was equal to 800 nm and 85-150 nm,respectively.

X-ray photoelectron spectra (XPS) were obtained at the HP 5950A spectrometer. NK* -and SiL2,3

-

X-ray emission spectra (XES) were obtained using electron excitation at the RSM-500 spectrometer-mo- nochromator /I/. K o c -and K

p-

XES of Si from a-SigQ deposited on

X-A1203 substrate were studied at the SAW-1 spectrometer using fluorescent radiation /2/. Information about vacant states was ob- tained with the method of X-ray photoeffect quantum yield spectra (XYS) /3/.

Energy resolution depending on the spectrum part under study is not worse than 0.5 eV for SIK; 0.5 eV for NK; 0.2 eV for SiLz,3 (XYS);

0.4 eV for SIL 2 , ~ (XES)

.

Pig.1 shows XES and XYS of Si and N combined in common energy scale as well as XPS of a-Si3 N 4 valence band.

3ne can perform identification of spectra maxima basing upon the dipole selection rules for X-ray transitions and calculations of Si Nq electronic structure /4/.

In all the spectra obtained in experiment (Pig.1) maxima C correspond to the group of MO the main contribution to which is brought in by 2s-A0 of nitrogen with an admixture of 3s-and 3p-A0 of Si;maxima B and B t correspond to 6'-bonds due to interaction of 3s-and

3ps-A0 of Si with 2pg-A0 of nitrogen; maximum A indicates the presence

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842203

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C2-888 JOURNAL DE PHYSIQUE

of S i 3 p ~

-

^N 2p&.rrteraction w i t h an admixture of 39, d

-

^AO ^{of S i}

(maximum A of SiL2.3 ) and f i n a l l y t h e m a i n maximum A o f NKoc -band m d a high-energy e x t e n t i o n on it correspond t o non-bonding MO con- s i s t i n g mainly of 2pn -A0 o f n i t r o g e n and s p l i t t e d a c c o r d i n g t o c a l - c u l a t i o n / 4 / i n b two components.

Fig.1

-

X-ray s p e c t r a o f a-SijN4 (

-

f o r d e p o s i t e d on S i ,

- - -

f o r d e p o s i t e d on ^o(-A1203). ( 1 )

-

^SiL2,a^;^{( 2 )}

-

^SiK; ^{( 3 )}

-

^NK;

(41

- -

XPS of valence band;

- - -

t o t a l d e n s i t y of s t a t e s /4/.

Bonding energy Eb f o r : S i 2p = 101.6 eV; N I s = 397.6 eV; S i I s = 1842.1 eV.

SiL 2,3 -XES a t E=1 0207 eV h a s maximwn A t r e l a t i v e i n t e n s i t y o f which i s one t e n t h s of maximum A i n t e n s i t y . Nature o f t h i s maximum c a n ' t be e x p l a i n e d w i t h i n t h e l i m i t s o f c a l c u l a t i o n method f o r e l e c t r o n i c s t r u c t u r e of K - and 4

-

Si3N4 used i n /4/ and may be connected w i t h t h e s e l f - a b s o r p t i o n e f f e c t . However, we f e e l as more t r u s t w o r t h y t h e assumption t h a t maximum A' corresponds t o occupied s t a t e s gene- t i c a l l y connected w i t h 3 s, d- s t a t e s o f S i s p l i t t e d o f f towards t h i s p a r t of t h e band. Our experimental d a t a (SiLz.3

-

XES and SiNx

-

XYS) a r e i n f a v o u r of t h i s assumption.

A s i t i s seen i n Fig. 2 w i t h t h e r e d u c t i o n o f x, ^i.e. w i t h t h e i n t r o d u c t i o n o f s u r p l u s s u p e r s t o i c h i o m e t r i c s i l i c o n i n a-SisNy t h e f o l l o w i n g changes o f S i L z , s - s p e c t r a t a k e place: r a t i o A/B of maxima i n t e n s i t i e s i s reduced from 1.1 d o n t o 0.9; i n t e n s i t y o f mx3mum A t d r o p s and f o r S a x w i t h minimum x i t p r a c t i c a l l y i s n o t n o t a b l e o v e r background; i n i t i a l a b s o r p t i o n range i s s h i f t e d towards t h e l o w e r e n e r g i e s and f o r SFNx w i t h minimum x i n SiLza-XYS t h e r e ap- p e a r s a s t r u c t u r e c o i n c i d f i g in i t s e n e r g e t i c p o s i t i o n w i t h maximum A * o f a- Si3N4; energy of maximum "bn i s p r a c t i c a l l y mchanged and maximum "eU i s consequently s h i f t e d towards t h e lower e n e r g i e s from 110.5 eV t o 108.5 eV. In t h i s c a s e a quantum y i e l d v a l u e ( ?f ) in maximum "bn o f SiL2,3 -XYS and i t s c o n t r a s t a r e b o t h reduced o n l y by

-

^4a which c a n f t r e s u l t i n such a c o n s i d e r a b l e d i f f e r e n c e f o r s e l f - a b s o r p t i o n v a l u e in short-wave p a r t o f Si-XPS which i s observed

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in experiment.

Fig. 2

-

SiLL3-XES and XYS i n SiNx. S ~ H ~ / N H ~ =1:1; 1:2; 1:5; 1:IO.

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According t o c a l c u l a t i o n s /4,5/ t h e main c o n t r i b u t i o n t o t h e d e n s i t y of v a l e n c e s t a t e s i s g i v e n by s t a t e s o f S i and b a s i n g upon t h e d i - p o l e s e l e c t i o n r u l e s f o r X-ray t r a n s i t i o n s we s u g g e s t t h e f o l l o w i n g way of i d e n t i f i c a t i o n f o x SiLo,3, K

-

^{and NK} ^-XYS m a x i m a : maxima "a"

r e p r e s e n t t h e l e v e l shaped by h y b r i d i z e d vacant s t a t e s 3d-A0 o f S i and 2p-A0 of N; t h e main c o n t r i b u t i o n t o "btl l e v e l i s brought i n by vacant 3s-A0 o f S i w i t h an admixture o f 2p-A0 o f N ; l e v e l s "c, dn a r e connected i n t h e main w i t h c o n t r i b u t i o n o f vacant 3p -A0 o f S i and 2p-A0 of N t d t h an admixture of 3d-A0 o f S i ; and f i n a l l y l e v e l s t t e , f w c o n s i s t of c o n t r i b u t i o n s o f 3d-A0 o f S i and 2p-A0 o f N.

To c o n s t r u c t an energy diagram of d i e l e c t r i c u s i n g X-ray s p e c t r a l da- t a one should know t h e following: p o s i t i o n o f valence band t o p (Evb) r e l a t i v e t o combined i n common energy s c a l e X-ray s p e c t r a ; pho t o i o n i - z a t i o n t h r e s h o l d v a l u e (Eth) which d e t e r m i n e s p o s i t i o n o f Ejb r e l a - t i v e t o vacuum l e v e l Ev; e l e c t r o n a f f i n i t y v a l u e ( X 1, i.e. p o s i t i o n of conduction band bottom r e l a t i v e t o vacuum l e v e l ( E c ) ; in t h i s c a s e t h e f o r b i d d e n band width ( A Ef) i s determined by e q u a t i o n

4 Ef = E t h

- ^{X '}

Knowing v a l u e Eb of mR- and S i 2p

-

l e v e l s (Fig. 1 ) one can f i n d u s i n g X-ray s p e c t r a a p o s i t i o n o f v a l e n c e band t o p r e l a t i v e t o t h e Permi l e v e l E V ~ = 2.2 eV and t a k t n g i n t o account r e s u l t s of determi- n a t i o n of E v ~ from XPS (1.7 eV) one can determine mean v a l u e Evp = 2.0 eV. We c o n s i d e r 6.8 and 2.0 eV as r e l i a b l e v a l u e s of Ev and E,

,

r e s p e c t i v e l y / 6 / .

It seems a d v i s a b l e t o t a k e as a width of f o r b i d d e n b a d o f a-Si3N4 a v a l u e 4 Ef =

-

Ê ^=4.8 êV ⁽^{where Ev}ândEc a r e independent on matrix elementE8escrfbing t r a n s i t i o n from v a l e n c e band t o conduction band) coincLding w i t h v a l u e A Ef = 4.8 ~ 0 . 2 eV o b t a i n e d in t o t a l cur- r e n t s p e c t r o s c o p y experiments w i t h a-Si3N4/6/,

With knowledge of v a l e n c e band t o p p o s i t i o n r e l a t i v e t o vacuum l e v e l (Ev = -6.8 eV) m d r e l a t i v e t o Fermi l e v e l o f an e l e c t r o n s p e c t r o - m e t e r (EvF = 2 0 0 eV) one can determine p o s i t i o n o f Fermi l e v e l of

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C2-890 JOURNAL D E PHYSIQUE

t h e e l e c t r o n s p e c t r o m e t e r r e l a t i v e t o vacuum l e v e l ₍EF = -4.8 eV) and t h e r e f o r e i o n i z a t i o n e n e r g i e s of l e v e l s Ei = Eb + 4.8 eve

Fig, 3 shows energy d i a g a m s o f a-Si3N4levels r e l a t i v e t o vacuum l e - v e l ss w e l l as t r a n s i t i o n s corresponding t o t h e main maxima o f X-ray

s p e c t r a and ELS. A s i t i s seen i n F i g s , l , 3 m a x i m u m A t (SiLz3-X~S) and maximum "av (SiL2,3-, SiK

-,

^NK-XYS) l o c a t e d below t h e bottom o f conduction band by 1.7 a s d 0.4eV, r e s p e c t i v e l y a r e i n f o r b i d d e n band.

E n e r g i e s o f ELS maxima corresponding t o t r a n s i t i o n s from i n n e r l e - v e l s ( S i 2p- and N I s ) and from valence band t o e x c i t e d vacant s t a t e s a r e in good agreement w i t h p o s i t i o n s o f a b s o r p t i o n m a x i m a o f corresponding X-ray s p e c t r a w i t h t h e e x c e p t i o n o f t r a n s i t i o n s 101.0 ( 8 1 2 ~ ) ; 396.8 (N I s ) and 3.2, 4.6, 6,8 eV (from valence band) w i t h t h e final l e v e l s l o c a t e d in forbidden band above t h e t o p o f v a l e n c e band by 1.2

-

1.6 eV. Authors of work /7/ c o n s i d e r these l e vels as corresponding t o broken Si-N bonds.

References

1. LUKIRSKII A.Po, BRYTOV I.A., KOMYAK M , I . , Apparatura i metody rentgenovskogo a n a l i z a , Leningrad,Mashinostroenie 2 (1967) 4.

2. BRYTOV I.A., OBOLENSKII EoAo, GOLDENBERG IVI0S., RABINOVICH LOG., ANTOEVA TOM., PTE f (1983) 2880

3. LUKIRSKII AoPo, BRYTOV I o A o , EHPT

&

(1964) 43.

4. SHANG-YULE FBN, CHING W.Y,, Phys. Rev, B.

a

(1981) 5454.

5. ROBERTSON J,, P h i l , Mago B 44 (1981) 217.

6 , KOMOLOV S.N., FTT

23

(1981) 827.

7. LIESKE N., HEZEL R., Thin S o l i d Films 6J (1 979) 217.

ELECTRONIC STRUCTURE OF AMORPHOUS Si3N4

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