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Competition between charging and discharging surface reactions as a mechanism for the Fermi-level pinning at
semiconductor surfaces
V.A. Kiselev
To cite this version:
V.A. Kiselev. Competition between charging and discharging surface reactions as a mechanism for the Fermi-level pinning at semiconductor surfaces. Revue de Physique Appliquée, Société française de physique / EDP, 1990, 25 (3), pp.277-286. �10.1051/rphysap:01990002503027700�. �jpa-00246186�
277
Competition
betweencharging
anddischarging
surface reactions as amechanism for the Fermi-level
pinning
at semiconductor surfacesV. A. Kiselev
A.F. loffe Physico-Technical Institute, Academy of Sciences of the U.S.S.R., 194021, Leningrad, U.S.S.R.
(Reçu le 21 avril 1989, révisé le 25 septembre 1989, accepté le 30 novembre 1989)
Résumé. 2014 Un nouveau mécanisme de l’accrochage (« pinning ») du niveau de Fermi à la surface des semiconducteurs est proposé. Il est basé sur l’examen de réactions chimiques électriquement actives qui
crééent ou détruisent la charge de surface. La neutralisation mutuelle des réactions de charge et décharge
conduit à l’accrochage du niveau Fermi. La compensation des écoulements de charges est due à l’inversion du type de conductivité en surface. Dans ce cas le niveau de Fermi est accroché au milieu de la bande interdite.
Les cinétiques normales et anormales de formation de barrières de surface lors de l’adsorption de O, S, Cl, Al, Au, Ag, In, Ga, Mn et Sb sur la surface de GaAs (110) sont interprétées suivant deux modes de chimisorption.
Abstract. 2014 A new mechanism for the Fermi-level pinning at semiconductor surfaces is proposed. It is based
on consideration of electrically active surface chemical reactions which create or destroy the surface charge.
Mutual neutralization of the charging and discharging reactions leads to the Fermi-level pinning. The compensation of charge flows is caused by inversion of the conductivity type at the surface. In this case the Fermi-Level is pinned near mid-gap. The normal and anomalous kinetics of surface-barrier formation for
adsorption of O, S, Cl, Al, Au, Ag, In, Ga, Mn and Sb on GaAs(110) surface are interpreted in a two-mode chemisorption model.
Revue Phys. Appl. 25 (1990) 277-286 MARS 1990,
Classification
Physics Abstracts
73.30 - 68.55 - 73.40N
1. Introduction.
Mechanisms underlying Schottky barrier formation at metal-semiconductor interfaces have been a sub-
ject of much interest over the last few decades [1-4].
Particular attention has been given to (110) surfaces
of III-V compounds. For those, Schottky barrier
formation can be followed from zero band bending,
in case of clean faces, to the ultimate values of the barrier heights characteristic of metal-covered sur-
faces.
As a rule, the ultimate band bending and the tor) are established at coverages of less than 0.1 of a
monolayer, and to a first approximation, do not depend on the chemical nature of the adatoms, including nonmetals.
The most astonishing result is that the pinning position at room temperature does not depend on
the extent of the chemical reaction between the
deposited foreign atoms and the semiconductor surface.
Depending on the reactivity, the surface reactions lead to a wide range of types of interfaces. For
example, a sharp interface is formed and conserved
during deposition of the nonreactive metals. For
moderately reacted interfaces, one finds outdiffusion to the outer surface and segregation of the substrate components, formation of alloys or binary and ternary compounds. The more reactive systems give protective interlayers, etc. [2]. However, despite this variety of products, in almost all those cases foreign
atoms pin the Fermi-level near mid-gap.
The independence of the barrier height on the microscopic-scale chemistry for GaAs stimulated several models of Schottky barrier formation.
e pinning y e me a -m uce gap states (MIGS) [5, 6], the Unified Defect Model
(UDM) [7], and the Effective Work Function Model
(EWFM) [8]. Recently, Mônch proposed a new
mechanism which accounts for the charge transfer
between the virtual gap states of the complex band
structure of GaAs and the adsorbate [9].
It is worth to note that each of the models assumes a certain reaction (or a product of reaction) between
the ’adatoms and the substrate. The MIGS-model
requires a sharp interface between a thick metal
overlayer and a semiconductor, the UDM assumes
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:01990002503027700
creation of native defects with certain positions of
energy levels, the EWFM is valid only in case of a
thick overlayer of a certain chemical composition,
and Mônch’s model is restricted to the initial stages of adsorption. Thus, it should be stated that there is
currently no unified description of Schottky barrier
formation independent of the interfacial chemistry.
In this paper a more general concept is proposed.
It is suggested that surface-charge formation is due to some selected class of chemical reactions
(Sect. 2). The Fermi level gets pinned if those
« surface charging reactions » (SCR’s) stop, for whatever reason, or if two (or more) competing
reactions neutralize each other. The present study is
focused on the most important reactions, that is, the
reactions which involve free carriers from the bulk of the semiconductor.
In sections 3 and 4 a pinning mechanism is pro-
posed which is related to inversion of the conduc-
tivity type within the layer adjacent to the surface. It leads to the pinning in the vicinity of mid-gap.
Also, in section 5, a modification for the conven-
tional pinning mechanism is proposed which arises from the SCR-approach, and a general scheme for
the Fermi-level behaviour is given. Role of the
surface dipole layer is discussed briefly in section 6.
In sections 7 to 9 the general SCR concept is illustrated by consideration of chemisorption of
atoms. In section 7 arguments are provided which
show that two types of chemisorption and conversion of one type into another may lead to band bending
and the Fermi-level pinning. In sections 8 and 9 this two-mode chemisorption model is employed to ex- plain the normal and anomalous pinning in case of 0, S, Cl, Al, Au, Ag, In, Ga, Mn, and Sb adsorption on the GaAs(110) surface.
Preliminary results of the study have been pub-
lished in [10].
2. Surface-Charging Réactions (SCR).
Among a great number of surface reactions there should be a special class of electrically active reac-
tions which create or destroy charge on the surface.
Just the SCR’s determine the variations in the surface barrier height. If the SCR’s stop, or if competition between the charging and discharging
reactions cancels the charge transfer to the surface,
then the barrier height becomes fixed and the Fermi level gets pinned.
The term « SCR » may be used in a very general
sense. It may include adsorption of ions, the
« strong » chemisorption of foreign atoms [11], for-
mation of charged defects by cleavage, sputtering,
electron bombardment, etc. In this sense, the SCR’s may also embrace processes underlying the well-
known models [5-9]. As for the limitations imposed by the models [5-7, 9] on the energy of the surface
pinning levels, those, to a great extent, may be reduced as shown in section 3.
The SCR’s are obviously connected with capture
(or liberation) of charged particles at surfaces.
Those may be free carriers of the bulk of the
semiconductor, charged particles of external flows, ions in gases or liquids, etc. The present paper deals with the first case only, that is, with free electrons and holes in semiconductors. In this case the SCR’s may be referred to as some version of the reduction-
oxydation solid state reactions.
It is clear that those can proceed only if free
carriers are present. Annihilation of the carriers, for
whatever reason, stops the reactions and leads to stabilization of the surface charge, and thus causes
the pinning of the Fermi level. The second reason
for the pinning is a competition between two (or more) SCR’s, namely, between those induced by
electrons and those induced by holes.
A very important question is about the rates of the SCR’s. The reactions may be fast or slow depending
on the density of free carriers and complexity of the chemistry. Another point is that, for a given adsor-
bate-semiconductor pair, certain SCR’s may be endothermic and thus forbidden (those require some
energy supply). However, it seems plausible that any interface system should find (sooner or later) some
allowed SCR which would reduce the energy of free carriers.
It is obvious that the electron-localizing reactions
should proceed in n-type semiconductors, and the
SCR’s capturing holes should take place in p-type materials. Those are different SCR’s even for one
and the same chemical pair. Different are the rates
and products of reactions.
Concluding, the main statement of this section is that any adsorbate may act as amphoteric in the
sense that it may lead to band bending in both n- and p-type semiconductors. Yet, the rates of barrier formation for the two types may be different. For
example, very electronegative elements may be chemisorbed on n-type semiconductors with capture of free electrons. However, to localize holes the
same elements deposited on p-type should take part in a more complicated (subsurface) reaction which may be much slower than chemisorption.
3. Rôle of conductivity inversion.
With growth of the depletion band bending the type of conductivity in the near-surface region tends to change for the opposite one. According to section 2,
this tendency should be followed by a change in the
surface chemistry. The SCR which capture majority
carriers should become less efficient, and there
should appear the second, competing, reaction in- duced by minority carriers. As shown below this
compétition may lead to surface charge stabilization and thus to the Fermi-level pinning.
279
Let Q be the difference between the two-dimen- sional densities of adsorbate-induced states which capture carriers of the alternative signs. To be definite, let the majority carriers be electrons. Then
-eu is the density of surface charge. The time dependence of cr is given by the following equation :
where the first term is the rate of the SCR proceeding
with capture of electrons, and the second one is that of the competing reaction localizing holes. The exponents originate from the expressions for the
densities of carriers (or the flows of carriers) in the region nearest to the surface. Those are controlled
by the electric barrier height, eCPB. Re and Rh depend on many factors - temperature, pressure, coverage, etc., which will be considered either constant or providing dependences much weaker than the exponential dependences on CPB and, thus,
on a - see equation (2).
As will be discussed in section 5, equation (1) is
valid in case the first SCR generates surface states
(of acceptor type) within the lower half of the energy gap. The same mechanism holds for p-type if the surface levels (of donor type) reside in the upper half of the gap. For other positions of the levels within the gap, equation (1) should be modified to incorpo-
rate the conventional pinning mechanism.
The barrier height grows with increasing a accord- ing to the law [12]
where e is the dielectric constant of the semiconduc- tor, and p is the density of space charge below the
surface. It is clear that with increasing a the first
SCR slows down - the first exponent changes from
1 to 10-16 for e OB increasing from 0 to 1 eV. On the contrary, the second reaction accelerates. The sur-
face charge ceases changing (doldt = 0 ) for
gap. At room temperature, the second term in (3)
does not exceed the experimental error (of about
0.1 eV) even if Re and Rh differ by four orders of
magnitude. However, in some cases this term may be larger than that value, that may be regarded as an exception to the rule.
Below, examples will be given for those SCR’s
which require some activation energy. It follows from equation (3) that if Re and/or Rh depend on temperature through an exponentional factor, such
as exp (- E AI kT), then the final result for e CP B does
not change with temperature. However, if the final
pinning is achieved in a time much exceeding the
real times then this is equivalent to the case of the
energy forbidden (endothermic) reactions.
It is interesting that final eOB given by equation (3) may be approached not only from
lower values of the barrier height but from higher
ones as well. This behaviour is expected for the
activation character of the second, the competing,
SCR. In this case the first (fast) reaction leads to an
anomalously high barrier. After that, this inter- mediate barrier relaxes to the normal value (3) due
to the second (slow) SCR. Thus, the curve for
kinetics of barrier formation may be nonmonotonous
(see also Sect. 9).
4. Kinetics of barrier formation.
Equation (1), with account of (2), may be solved at the initial stage of the chemical process when the second term may be neglected. Letting
and choosing a - 1/Z and Ré 1 a - n2 as units of measure
fort and t, respectively, the result of integration
may be presented as follows :
For the integral function in (5), tables are available,
and the asymptotic expression for large cr is checked numerically.
Thus, the first SCR leads to the increase of u close to Jh1ï. This dependence is shown in figure 1 (curve 1). The barrier height, e03A6B, grows as ln t.
Fig. 1. - Dependence of the effective density of the negatively charged surface states without (1) and with (2)
the account of the second SCR.
As a approaches 0"0 defined by equation (3),
solution (5) ceases to be valid and the second SCR should be taken into account. For small deviation ai 1 = 0" - 0" 0’ the following equation can be de-
duced :
The solution is
with t0 = exp (03C320) / (2 03C30). The constant of inte-
gration is chosen such that solutions (5) and (7)
coincide for t to and small 1 aIl. The t-dependence
of 03C30 + 03C31 given by equation (7) is displayed in figure 1 (curve 2). Behaviour of 03C31 at small and large t is given by
The Fermi-level pinning at _ ao corresponds to
mutual neutralization of the SCR’s creating and destroying the surface charge. This point of view
allows to shed light on the relative constancy of the barrier height for the coverage thickness growing
from submonolayer values to those of technological overlayers. Any tendency to change the barrier
height during deposition of foreign atoms (e.g. due
to the screening effects [13]) will be suppressed by switching an appropriate SCR which will restore the barrier height. In the course of the reaction the
density of the surface states able to capture carriers may grow from about 1012 cm- 2, which is character- istic of submonolayer coverages, to about 1014 cm- 2,
which is required [13] for the pinning at thick metal coverages.
5. Two types of pinning.
Thus, two competing processes are responsible for
surface barrier formation. The first one (creative) changes the surface charge and, hence, the barrier
height. The second (destructive) suppresses this variation and leads to fixation of a certain barrier
height. The commonly accepted view is that the second process requires the alignment of the Fermi- level and the surface state energies to neutralize the
charge transfer [1-7]. Therefore, this mechanism will be referred to as an « energy » one.
However, as it is clear from the preceding analysis,
other mechanisms may be proposed for surface
charge stabilization which do not need the energy
alignment. Depending on the microscopic-scale chemistry, one may imagine two possible cases, that
is, (i) the second SCR destroys the surface states borne by the first one, and (ii) the second reaction
creates new states which capture the originally minority carriers and, thus, compensate the contri- bution of majority carriers. These two cases are
embraced by equation (1) and may be referred to as a « density » mechanisms for the Fermi-level pin- ning.
The fundamental difference between the conven-
tional « energy » mechanism and the « density »
ones is illustrated in figures 2a and b. In the first case
(a) the density of the surface states is unlimited, and the Fermi-level becomes fixed due the mentioned energy alignment. This mechanism seems to be
plausible in case the density of surface states (e.g. of
about 1014 CM- 2) is much higher than that required
for the pinning (1012 cm- 2). This is characteristic of clean surfaces (e.g. Si(lll) [14]).
On the contrary, the « density » mechanisms limit the effective density of surface states itself (due to (i) charge destruction and (ii) compensation) see figure 2b. The position of the surface levels is not linked to that of the Fermi level.
The SCR-concept also allows to introduce a new
type of the « energy » pinning which should hold if
Fig. 2. - Types of the Fermi-level pinning at the surface : the « energy » pinning without limitation for the surface state
density (a), the « density » pinning (b), and the « energy » one with fixation of the effective density of the surface states
(c).
281
the surface states do not exist beforehand, but are
borne in the course of deposition of foreign atoms.
Since for any « energy » pinning the majority-carrier
flow turns to zero, the SCR which generates the surface states should stop. Hence, this type of
« energy » pinning also fixes the surface state den-
sity, u, as shown in figure 2c.
In order to include the « energy » mechanism into
consideration, an additional term should be added to the right part of equation (1), which describes the counterflow of majority carriers from the surface to the volume due to liberation (emission) of carriers,
e.g. of electrons from acceptor surface states into the conduction band, that is,
where EF is the Fermi level position in the bulk of the crystal, and E, is the initial position of the
surface level. If EF - E, 1/2 Eg the second term in
(1) is negligible in comparison to term (9). In this
case the surface charge ceases changing for
On the contrary, if EF - ES > 1/2 Eg equation (1) is
true as it is, and the « density » mechanism with
eCP Bo = 1/2 Eg takes place.
Thus for the energy pinning, the flow and coun-
terflow of carriers normal to the surface are deter- mined by a single band, and hence this type of pinning is a « one-band » pinning. For the new type of pinning the flow and counterflow of carriers are
determined by different bands. Hence it is a « two- band » pinning.
The energy and density pinning mechanisms are
special cases of some general scheme. Realization of those depend on the energy positions of the surface levels which are generated by the first - « creative »
- SCR. For simplicity, let it be the only level, Es, of acceptor
(donor)
type in a n- (p-) type semiconductor. Figure 3 exhibits four characteristicregions for Es : I, II, III and IV, within which the Fermi-level at the surface, Eps? behaves differently.
The above analysis predicts the following values for
the barrier height in the E, regions :
The overall dependence of Eps on E, is displayed in figure 3.
Region 1 corresponds to the case of an endother-
mic SCR, e.g. the adsorbate cannot capture the majority carriers because of very low electron affinity
in case of n-type or high ionization energy in case of p-type material. Region II is characterized by barrier heights lower than 1/2 Eg. Those are determined by
the energy mechanism for the pinning. In region III
the density pinning (see Sects. 3 and 4) takes place.
The situation in region IV is contrary to that of region 1. Here, the capture of majority carriers is
never neutralized (very high electron affinity of
adsorbate on n-type semiconductor, or very low ionization energy of adatoms on p-type). The growth
of the depleting band bending, in this case, is limited
by appearance of the surface inversion channel which screens the surface charge.
6. Rôle of dipole layer.
In the preceding section the dependence of e03A6Bo (and Eps) on the initial position of the surface state,
Es, was discussed. However, not less important is
the question about the value of E,. Various adsor- bates are characterized by a wide range of elec-
tronegativities and, at first sight, it seems strange that those create surface levels within region III, or within region II not far from mid-gap (see Fig. 3),
thus providing the mid-gap Fermi-level pinning. In
detail this problem will be analysed in a special publication, and here a brief discussion is given.
The most important aspect of this problem is
formation of the dipole layer on the surface of a semiconductor due to a chemical reaction (including
Fig. 3. - The pinning position, EFS, as a function of the energy, Es, of the surface level in case of a n-type semiconductor
(a), and for p-type (b).
