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CONTRIBUTION OF THE DEFECTS TO THE ELECTRONIC PROPERTIES OF THE METAL
SEMICONDUCTOR INTERFACE
P. Langlade, P. Masri
To cite this version:
P. Langlade, P. Masri. CONTRIBUTION OF THE DEFECTS TO THE ELECTRONIC PROPER-
TIES OF THE METAL SEMICONDUCTOR INTERFACE. Journal de Physique Colloques, 1984, 45
(C5), pp.C5-441-C5-447. �10.1051/jphyscol:1984566�. �jpa-00224185�
JOURNAL DE PHYSIQUE
Colloque C5, suppl6ment au n04, T o m e 45, avril 1984 page C5-441
CONTRIBUTION OF THE DEFECTS TO THE ELECTRONIC PROPERTIES OF THE METAL SEMICONDUCTOR INTERFACE
P. ~an~lade' and P. Masri
Laboratoire d'Etudes d e s S u r f a c e s , I n t e r f a c e s e t Composants, U.R. n o 1022
-
associe'e au CNRS, U.S.T.L., Place E. BatailLon, 34060 ~ o n t p e l Z i e r , France Rdsumd
Le contact mdtal-semiconducteur joue un r6le essentiel dans les propridtds des circuits 6lectroniques
i
semiconducteur. Des progr&s rkcents dans ce domaine montrent que les propridt6s klectriques sont contr6ldes par des dtats d'interface, apparaissant d&s les premiers stades de la m6tallisation. Dans ce contexte, nous avons dtudid l'influence des lacunes de surface sur les propridtds de l'interface de fapon self consistante dans le mod&le moldculaire apr&s avoir caractdris; en ddtail la surface idkale. Nous montrons que le barycentre des&tats associds aux lacunes 111 et
V
a tendance 2 se d6placer du bas de la bande interdite vers le haut en fonction de ltionicit6 du compos6. Ce r6sultat est compar6 la relation existant entre l'ancrage du niveau de Fermi par l'absorp- tion m6tallique et l'ionicitd du compos&.Abstract
The metal-semiconductor contact plays an essential role in the properties of semiconductor devices. Recent progress in this field show that the electronic properties are controlled by the interface states, which appear during the first stages of metallization. In this context, we have studied the influence of the (110) surface vacancies on the properties of the interface in a self consistent way within the molecular model framework after the characterization of the ideal (110) surface. We show that the barycenter of the levels associated with the V and 111 vacancies has a tendency to move towards the upper part of the forbidden gap as ionicity of the compound increases. This result can be compared to the existing relation between the pinning of the Fermi level by a metallic adsorption and the compound's ionicity.
Introduction
The origin of the Schottky barrier at the metal/III-V semiconductor interface is of a great importance because its comprehension is bonded to technological applications, the Schottky diode being one of the most elementary of electronic devices. During the last few years, a large effort has been devoted to the research of the metal-semiconductor interface and different models have been proposed (1). The most studied system is those of the metal on the (110) face of the 111-V's because there are no electronic states in the band gap and therefore it is easy to follow the electronic role of the metal on the surface properties. It is now well established that the barrier is controled by
interface states, appearing at the first stages of metallization which are resulting from defects at or near the surface of the semiconductor ; the
"unified model" ( 2 ) has been developped to explain the Fermi level pinning by cation and anion vacancies.
+present address : Laboratoires d'Electronique et de Physique Appliquge, 3 avenue Descartes, 94450 Limeil Brgvannes, France
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984566
C5-442 JOURNAL DE PHYSIQUE
The a i m o f t h i s work i s t o b r i n g a c o n t r i b u t i o n o n t h i s main p o i n t . I n a f i r s t t i m e , we h a v e s t u d i e d t h e i d e a l v a c a n c i e s a t a well c h a r a c t e r i z e d ( 1 1 0 ) z i n c -
b l e n d e s u r f a c e a n d t h e i r i n f l u e n c e on t h e S c h o t t k y b a r r i e r . F o r t h u s , we h a v e c h o o s e n :
-
a p a r a m e t e r which i n t r o d u c e s t h e p e r t u r b a t i o n i n h e r e n t t o t h e s u r f a c e , a s s a y t h e e l e c t r o s t a t i c p o t e n t i a l .- a model which q u a l i t a t i v e l y d e s c r i b e s t h e p h y s i c a l p r o p e r t i e s o f t h e b u l k o f t h e 1 1 1 - V compounds v e r s u s o n e p a r a m e t e r , t h e i o n i c i t y , a s s a y t h e m o l e c u l a r model.
- t o c h a r a c t e r i z e t h e i d e a l s u r f a c e by mean o f a s e l f c o n s i s t e n t c a l c u l a t i o n .
-
t o s t u d y t h e e f f e c t o f p o n c t u a l d e f e c t l i k e v a a a n c i e s on t h e e l e c t r o n i c s t r u c t u r e .E l e c t r o n i c s t r u c t u r e o f t h e i d e a l s u r f a c e
Descfipf !on-of -fhe-b~!k-~-!01ec~1af:~?!0!~~
P r i o r t o i n t r o d u c e t h e v a c a n c i e s a t t h e s u r f a c e , w e m u s t h a v e a c h a r a c t e r i z a t i o n o f t h e i d e a l s u r f a c e . F o r t h u s , we n e e d a p a r a m e t e r w h i c h shows t h e p e r t u r b a t i o n c r e a t e d by t h e s u r f a c e i t - s e l f : w e h a v e c h o o s e n t h e e l e c t r o s t a t i c e n e r g y b e c a u s e i t i s a f u n c t i o n o f t h e i o n i c i t y o f t h e bond a n d a l l o w s u s t o a c o m p a r i s o n b e t w e e n a f a m i l y o f compounds. Then, t h e b u l k p r o p e r t i e s h a v e t o b e d e s c r i b e d i n r e l a t i o n w i t h t h i s p a r a m e t e r : we h a v e d o n e i t w i t h t h e m o l e c u l a r model i n which t h e c r y s t a l i s a c o l l e c t i o n o f m o l e c u l e (Atcm 111-Atcm V ) , d e v e l o p p e d f o r t h e f i r s t time by Coulson and a l . ( 3 ) and m o d i f i e d by t h e t h e o r i c i a n s ' g r o u p o f L i l l e ( 4 , 5 ) .
I n t h e t i g h t b i n d i n g a p p r o x i m a t i o n , t h e h a m i l t o n i a n i s s p e c i f i e d by : - i n t r a - a t o m i c t e r m s aA = <@
I
H(
0'
B AB AB
-
o f f - d i a g o n a l terms 6 = <@AB \ H
1%;
w h e r e 0 a r e t h e a t o m i c o r b i t a l o f t h e A,B atom.
A,B
The r e s u l t i n g m o l e c u l a r o r b i t a l s a r e :
Xis a p a r a m e t e r which t a k e s t h e bond p o l a r i t y i n t o a c c o u n t . S i n c e t h e
a t o m s h a v e a d i f f e r e n t e l e c t r o n e g a t i v i t y , t h e r e i s a n e t c h a r g e on e v e r y atom :
nAO = c h a r g e i n t h e b u l k - c h a r g e o f t h e f r e e atom (N). The n e u t r a l i t y o f t h e c r y s t a l b r i n g s t o : nBO =
-
n A O .W c a n t h e n d e f i n e t h e i o n i c i t y o f t h e bond a s e
-
:The resolution of the Schroedinger equation in a self consistent manner brings to the results of table
I
(6).Table
I
- The different parameters of the molecular model.There is a charge transfer from the atom 111 to the atom V in all the cases in good agreement with several published results (7) and a linear relation exists with the ionicity (Fig. 1).
GaSb GaAs InAs Gap
Nmuer O f rransferr electrons
Figure 1
-
Relation between the net charges and the ionicity A0.576 0.534 0.511 0.497
G B ev -4.21 -3.83 -3.22 -3.77
Therefore, the widths of the "band gap"
in this model follow the real order even if they are to great.
El ev -6.89 -7.83 -7.39 -8.92
Structure at the surface . . .
0 1 0 2 0.3
,, ,,
The creation of the surface gives rise to the disparition of a part of the energe- tic interactions and the electronic structure of the first layers is modified:the net charges are not the same as in the bulk and the resulting potential strengths too. Like it is evident these two quantities are independent, the use of self consistence is necessary. Then we have to fit the molecular model to the case of the surface who there is only two dimensions. The principle of calcula- tion is :
a
-
to cut the crystal in parallel plans oriented (110), identified by the index j ;b - the net charge of the atoms in j plan is noted n
.,
n.
andn Ai - - nAO and nBi = nBO, except for the plans perturbed by $Re stsface.
no A 1,01 1,23 1.34 1.42
"a-u
A
B,,\I
".
2.08 2,9 2.64 3.6
E g = L &
-E I ev 4.16 5.22 4.5 5.97
c
-
the variations of population on a bond are proportional to the variation of the intra-atomic terms :Ealev -2.72 -2.61 -2.89 -2.95
a
nAj = c a(aAj - uBj)d - the intra-atomic terms are expressed in function of the net charges :
fi 0.50 0.56 0.59 0.60
fi Phillips
0.26 0.31 0.36 0.37
a A ev -6.3 -6.73 -5.85 -7.38
JOURNAL DE PHYSIQUE
where the coefficients yAJ,ek are lattice sums, of the form 1 - for the
electrostatic interaction
.
Aj Bke - from the expression of the variations
nAj, nBj, q A j , d B j , we have a set of linear equations, whose dimension is limited by the depth of the perturbation.
The calculations are then self consistent and have been developped in r6f. 6.
In the Table 11, are reported the charges transfers at the surface and in the perturbated plans and the associated energy levels.
Table I1 Gap
InP
Some trends are arising from the results :
a - the perturbation created by the surface runs on two plans and has an oscillatory natur
b
-
the answer of the system is a charge transfer in all the c a s e : + an excess of electrons on the more electronegative atom at the surface (a lack of electrons on the electropositive one)+ a lack of electrons on the more electronegative atom on the first intern plan.
c
-
these transfer lead to a more ionic surface than the bulk d - a true relation exist between the magnitude of the transfer and the ionicity : the more important are the transfers, the weaker the ionicity.1.42 1.58
The intra-atomic terms of the electronegativ atoms at the surface are always weaker than those of the electropositiv one, except for GaSb for which the instability is great.
From a general point of view, our results are in good agreement with experimen- tal observations and theoritical calculation. The interesting point is the rela- tion of the perturbation with the compound ionicity. It is known (8) that the
"covalent reconstruction" produces atom displacements up to three plans in the bulk, little variations of the bond length, while the "ionic one" is only charac- terized by a surface relaxation and more important changes in the bond length.
0.485 0.424
Owing to the fact that there is a change in the Ga 3d and As 3d core level binding energies, Eastman and al. (9) deduce that the GaAs surface is more ionic than the bulk. Therefore, calculations of Pandey (9) support this hypothesis ; they find that the charge transfer from Ga to As at the surface is of order of 0.3 electron, this effect being reduced by surface relaxation. We have neglected this point because the aim of relaxation is to minimize the surface energy and then it is clear that it would reduce the charge transfer at the surface and bring the energy levels closer to the bands without modify the tendency in
-0.475 -0.406
-0.059 -0.044
0.051 0.028
0.9367 1.04
-0.68 -0.32
-0.061 -0.49
0.52 0.36
function of the ionicity.
Charge transfers and energy levels associated to the vacancie
In the molecular model, a vacancie is created by taking out an atom from the lattice and neglecting the induced distorsion and the coupling between the nearest neighbors dangling bonds. The formalism of the calculation is :
a - to create a vacancie at the (110) surface of the 111-V compounds b - to distribute the keeping electronic charge on the dangling bonds, taking the neutrality of the bond and the situation at the ideal surface (more ionic than the bulk) into account.
c - to calculate in a self consistent way the effect on the electronic structure of the neighbors.
We have supposed that the vacancie is created in its bulk charge stage as say the V vacancie corresponds to the disappearing of 5 + n O electrons and a I11 one to those of 3 + n O . Then, 3 + nAs in the case
V
ani) 5 t nBs in the case 111, electrons have to be gistributed in the dangling bonds.The evaluation of the intra-atomic terms is made in the same manner as for the surface : they are linear functions of the net charges on the atoms produced by the vacancie. But the defect has a very low symmetry and the resulting potential is localized around the vacancie. Therefore, for the coulomb-type interactions, which are the proportionality coefficients of the linear relations, it is not possible to use a method based on the periodicity like the k representation in a plan. We have choosen to directly evaluate the interactions between pertur- bated atoms with sums of the form
1 .
The intra-atomic terms are expressed as :r
where : y is the index of the atom natur : A- B
i is the index of the plan to which the y atom belongs
j is the index locating the position of the y atom of the i plan with
~espect to the vacancie.
As for the surface, the charge changes on a bond are a linear function of the intra-atomic terms of the two atoms indebded in the bond. This lead to a set of linear equations.
The results are summarized in Table I11 for the net charges on the atom with respect to their charge level in the case of the ideal surface and in table IV for the electronic levels associated to the dangling bonds.
The remarquable trends are :
a - the perturbation converges quickly and is nearly localized on the nearest neighbors of the vacancie.
b - the answer of the system is a charge transfer, which is function of the situation before the introduction of the defect : for a V vacancie for example, an excess of electrons is distributed. The nearest neighbors I ~ I have before the defect a lack of electrons for those of the surface plan and an excess for those of the first in-plan. After the defect, the atoms at the surface have recovered the distributed excess and those of the first in-plan have lost a part of their charge.
C5-446 JOURNAL DE PHYSIQUE
c - t h e m a g n i t u d e o f t h e t r a n s f e r d e c r e a s e s w i t h t h e i n c r e a s i n g i o n i c i t y . d - t h e s e c h a r g e t r a n s f e r s b r i n g t h e n e i g h b o r o f t h e v a c a n c i e i n a s t a t e n e a r t o t h a t o f t h e b u l k ; t h e y h a v e a t e n d e n c y t o c a n c e l t h e p e r t u r b a t i o n c r e a t e d t h e s u r f a c e .
e - t h e e n e r g e t i c l e v e l s o f t h e d a n g l i n g b o n d s a t t h e s u r f a c e a n d i n t h e f i r s t i n - p l a n a r e a t t h e same p o s i t i o n . T h e r e f o r e , a l l t h e compounds h a v e t h e same b e h a v i o u r , e x c e p t f o r GaSb w h i c h h a s t h e l o w e s t i o n i c i t y and i s a t t h e l o w e r l i m i t o f v a l i d a t i o n o f o u r p a r t i a l l y i o n i c model.
f - t h e m o s t i m p o r t a n t e f f e c t is t h a t t h e b a r y c e n t e r of t h e cx ( V ) a n d a (111) e n e r g y l e v e l s h a s a t e n d e n c y t o m o v e t o w a r d s t h e u p p e r pa%'Af t h e
A s 1
f o r 6 i d d e n g a p a s i o n i c i t y o f t h e compound i n c r e a s e s . T h i s r e s u l t c a n b e compared t o t h e e x i s t i n g r e l a t i o n b e t w e e n t h e p i n n i n g o f t h e F e r m i l e v e l by a m e t a l l i c a d s o r p t i o n and t h e compound's i o n i c i t y ( 2 ) .
Tableau 111-a : C h a r g e nettc sur les atomrs 13ae a l a l a c u n c v
- 0 . 0 0 5 0 . 0 5 7 0 0 - 0 . 0 1 3 - 0 , 0 1 2
Tableau 111-b : cltarge nett, 9.r 10s nLomrs dflr a 1.1 Iar.on- III.
Tableau IV-a : Etats a s s o c i b s aux a t < , m e s p r r L u r i l 6 s par I , ~v c ~ ~ ~
Tableau IV-b : Etats associ6s aux atomes p r r t u r b 6 s par u n e ~ a c u n e I I I
F o r S p i c e r a n d a l . , t h e p o s i t i o n o f t h e p i n n i n g f o r GaSb, GaAs a n d I n P , c h a n g e s from t h e l o w e r p a r t o f t h e g a p t o t h e c u p p e r p a r t f o r I n P , a r o u n d t h e m i d d l e f o r GaAs.
--
e n e V 'BS, I - ~ B S 'AS, ~ - O A S 'A1 , l - O ~ 1 ' ~ 1 , 1 - ~ 0 1 a ~ l , Z-'AI a ~ 2 . 1 - 0 ~ 2 " ~ 2 , l-"&2
T h i s r e s u l t i s i n good a g r e e m e n t w i t h t h e t h e o r i t i c a l c a l c u l a t i o n s i n a more r e f i n e d model o f A . Dow and Smith ( 1 0 ) ; t h e y show t h a t : t h e l e v e l s of t h e v a c a n c i e a r e n o t p e r t u r b a t e d when t h e v a c a n c i e i s moving f r o m t h e b u l k u p t o 2 p l a n s t o t h e s u r f a c e . With t h e i n c r e a s i n g o f i o n i c i t y , t h e a n i o n - l e v e l s h a v e a t e n d e n c y , t o come c l o s e r t o t h e c o n d u c t i o n band w h i l e t h e c a t i o n o n e l e v e l s come c l o s e r t o t h e v a l e n c e b a n d .
I t s e e m s c l e a r t h a t t h e v a c a n c i e s c o u l d f o r m d e e p l e v e l s i n t h e band g a p f o r t h e p a r t i a l l y c o v a l e n t s e m i c o n d u c t o r s a n d t h e n , t h e c o n t r o l o f t h e b a r r i e r S c h o t t k y by t h e s e l e v e l s c o u l d b e e x p l a i n e d ; i t would b e n o t t h e c a s e f o r t h e i o n i c compounds, t h e l e v e l s a s s o c i a t e d t o t h e v a c a n c i e s b e i n g t o o c l o s e r t o t h e b a n d s ; t h e s e compounds would f o l l o w t h e S c h o t t k y t h e o r y .
-.
<:ash -9.14
C a n s 3 3
C o n c l u s i o n
I n c o n c l u s i o n , w e h a v e d e v e l o p p e d a r o u g h b u t p h y s i c a l model w h i c h w e l l d e s c r i b e s t h e i m p o r t a n t p r o p e r t i e s o f t h e c r y s t a l i n f u n c t i o n o f a p a r a m e t e r l i k e i o n i c i t y .
681' .-0.66 -0.93 0. I
. -- - - ---- 0.99 0 . 2 0 -0.3% -0.09 0.95
I n P 0.43 -0.52 0.17 0.92 0.07
-~
I n A s
It a l l o w s u s t o p e r f o r m a n i d e a l s u r f a c e c h a r a c t e r i z a t i o n and t o f o l l o w t h e i n f l u e n c e o f t h e p o n c t u a l d e f e c t l i k e v a c a n c i e . It seems t h a t t h e e x p l a n a t i o n o f t h e F e r m i l e v e l p i n n i n g by v a c a n c i e i n d u c e d s t a t e s i s o n e o f t h e most p o s s i b l e . An e f f o r t t o f i t t h e model t o o t h e r d e f e c t s h a s t o be d o n e f o r a b e t t e r c o m p a r i s o n .
-11,12 -2.91 -3,56 -2,81
-0.71
R e f e r e n c e s
-5,53
- 0 , 7 8
-0.33 -1.81
-0.94 - -
1 F o r a r e v i e w , s e e : BRILLSON L.3., S u r f a c e S c i e n c e R e p o r t s , Vol 2 . , n o 2 ( 1 9 8 2 ) .
2 SPICER W.E., LINDAU I . , SKEATH P . , SU C.Y., 3 . Vacuum S c i . T e c h n o l . 1 7 ( 1 9 8 0 ) 1 0 1 9 .
3 COULSON C.A., KEARSLEY N.3., P r o c . Roy. S o c . ( L o n d o n ) ( 1 9 5 7 ) , A 241, 433.
4 LANNOO M., P h y s . Rev. B 1 0 , no 6 ( 1 9 7 4 ) 2544.
5 LANNOO M . , DECARPIGNY 3 . N . , P h y s . Rev. 0 5 7 0 4 ( 1 9 7 3 ) . 6 LANGLADE P . , Thkse D o c t o r a t M o n t p e l l i e r ( 1 9 8 2 ) .
7 S e e f o r e x a m p l e : DECARPIGNY 3. N . , Thhse D o c t o r a t L i l l e ( 1 9 7 3 ) . 8 KAHN A., CISNEROS G., BONN M., MARK P., S u r f . S c i . 7 1 ( 1 9 7 8 ) 387.
-5,32
-0.52
-0.09 -0.15
0.08
9 EASTMAN D.E. and a l . , P h y s . Rev. L e t t . L 4 5 , 656 ( 1 9 8 0 ) a n d r e f . t h e r e i n . 1 0 SMITH D.L., 3. Vac. S c i . T e c h n o l . 1 7 ( 1 9 8 0 ) 1 0 2 8 .
- 0 , 7 1
0 . 8 9
0 . 8 1
--
0.8
0.97
0.09
0 . 2 7 .-
