Control of Semiconductor Interfaces. Proceedings of the First International Symposium, on Control of Semiconductor Interfaces, Karuizawa, Japan, 8–12 November, 1993

568 Pages·1994·44.914 MB·English

by  I. Ohdomari, M. Oshima and A. Hiraki (Eds.)

Control of
Semiconductor
nterfoces
Proceedings of the First International Symposium
on Control of Semiconductor Interfaces,
Karuizawa, Japan, 8-12 November, 1993
Edited by
I. Ohdomari
Waseda University, Tol<yo, Japan
M. Oshima
NTT Laboratories, Tol<yo, Japan
A. Hiraki
Osal<a University, Osal<a, Japan
1994
ELSEVIER
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PREFACE
This volume is a collection of selected papers which were presented at the First International
Symposium on the Control of Semiconductor Interfaces (ISCSI-1) held at Karuizawa, in the
vicinity of Tokyo, from November 8-12, 1993.
Let me explain briefly how the Symposium originated. In April 1989, the Board of Education
of Japan (MONBUSHO) sponsored a three-year research project entitled "Metal-
Semiconductor Interfaces: a key problem for development of ultimate micro-electronics",
chaired by myself, with a budget equivalent to 6 miUion US dollars. The two factors which
motivated this project were the importance of the study, from both scientific and technological
viewpoints, and a number of excellent research activities in Japan.
Worldwide appreciation of these activities led to the successful 2nd International Conference
on the Formation of Semiconductor Interfaces (ICFSI-2) in 1988, chaired by myself, at
Takarazuka near Osaka. This succeeded the 1st conference (ICFSI-1) which was held in
Marseille in 1985.
Approximately 50 university people from both theoretical and experimental areas were
involved in the "Metal-Semiconductor Interface" project. One year later, in 1990, the Japan
Society of Promotion of Science (GAKUSHIN) decided, because of the technological
importance of the project, to support research in this field for a lengthy period of time for both
universities and companies by encouraging technical meetings, conferences and symposiums
with an annual support of 10 thousand US dollars. This Symposium was the first international
technical meeting supported by the GAKUSHIN and around 160 scientists, including 35 from
overseas, participated herein.
The main body of the Symposium was formed by invited, oral and poster papers and two
interesting rump sessions.
The Symposium was successful from many aspects. The quality of the presentations was very
good and they induced many active discussions among the participants. Friendships were
successfully formed through both these discussions and chats at the Symposium, especially
during the rump sessions held in the evening, with very effective and helpful wine services,
and also during the social activities. These social events between the organizing committee
and the overseas participants and also the invited speakers included a welcome-reception, a
banquet, and a get-acquainted party at the "Hiraki bar" every night. The participants also
enjoyed the Karuizawa area because of its colorful beauty and oriental quietness in early
November.
VI
The friendship, as I stressed both in my opening and closing address, is a valuable by-product
of this Symposium and it will work strongly for the continuation of future ISCSIs. The next
one will be held at the same place and about the same time in 1996.
Penultimately, the success of the Symposium was greatly dependent on the efforts of the
organizing, steering and program committees, and also the financial support of approximately
15 organizations.
Finally, I, not as a chairman but personally, would like to heartily thank Professor Jim Mayer.
Indeed, Jim guided me into the wonderful world of Metal-Semiconductor Interface through
the very strange finding of Au-Si interface reaction with fascinating RBS technique almost 25
years ago at Caltech as his deep but humorous invited talk at the Symposium demonstrated.
Without Jim, this Symposium could never have existed!
Akio Hiraki
Chairman of ISCSI-1
vil
First International Symposium on Control
of Semiconductor Interfaces
Organizing Committee
C ha i r m an A. Hiraki (Univ. of Osaka)
V i ce C ha i r m an Y. Yasuda (Univ. d Nagoya)
S ec r β t a r y T. Katoda (Univ. of Tokyo)
Comm i -t - t ee Υ· Murayama (Univ. of Tpyo)
Μ. Fujimoto (ΝΓΓ)
Κ. Arai (Electrotechnical Lab.)
T. Gonda (Univ. rf Osaka)
K. Hará (Nippondenso)
T. Hattori (Musashi Inst, Tech.)
M, Kashiwagi (Toshiba)
M. Katayama (Shin-Etsu Handotai)
A. Kinbara (Univ. of Tokyo)
Η. Kotani (Mitsubishi Elect.)
S. Kuwahara (NISSIN Elect.)
K. Masumoto (Univ. of TohAu)
S. Mnagawa (Hitachi)
E. Morita (Sony)
Y. Nannichi (Univ. Tsukuba)
T. Nishino (Kobe Univ.)
I. Ohdomari (Waseda Univ.)
0. Ohtsuki (Fujitsu)
S. Okabayashi (NEC)
S. Oono (Oki)
M. Saga (Fuji Elect.)
K. Tada (Sumitomo Elect.)
K. Takahashi (Tokyo Inst, oi Tech.)
Μ -Mano (KYOCERA)
Τ. Tamagawa (ULVAC Japan)
Y. Tañida (Fuji Xerox)
Μ Ikiiguchi (MKKCKYCDO)
K. Terakura (Univ. of Tokyo)
Μ. Toyama (Kawasaki Steel)
Τ. Yamashita (Univ. d Hddtaido)
T. Yonezawa (Keio Univ.)
Steering Committee
C h a i r m an Y. Yasuda (Univ. of Nagoya)
V i ce C ha i r m an T. Katoda (Univ. of Tokyo)
S ec r e ta ry T. Itoh (Univ. oí Osaka)
T. CMíumura (Tdíyo Metrc^litan Univ.)
H. Watanabe (Matsushita Elect.)
VIH
Comm i "t t ee Μ DesHer (ADCTRON)
T. Kantíjyo (Oki)
H. Kawai (Sony)
H. Kotani (Mitsubishi Elect.)
Μ. Konagai (Tokyo Inst, of Tech.)
S. Minagawa (Hitachi)
Μ Mizuta (NEC)
Y. Nanishi (ΝΊΤ)
I. CÄidomari (Waseda Univ.)
Μ Ofehima (ΝΤΓ)
0. Ueda (Fujitsu)
S. Zaima (Univ. Nagoya)
Program Committee
C ha i r m an 1. Ohdomari (Waseda Univ.)
V i ce C ha i ri l an Μ Oshima (ΝΤΓ)
Comm i i: - t ee H. Fukuda (Oki)
T. Fukui (Univ. of Hokkaido)
Η. Hayashi (Sumitomo Elect.)
H. Ishiwara (Tokyo Inst, of Tech.)
H. Ito (Toshiba)
M. I \^ (JEOL)
M. Iwami (dcayama Univ.)
J. Kasahara (Sony)
N. Kawai (Hitachi)
H. Kawarada (Waseda Univ.)
E. Morita (Mitsubishi Materials)
M. Murakami (Kyoto Univ.)
Y. Nishioka (Texas Instruments, Japan)
M. Niwa (Matsushita Elect.)
T. Ohba (Fujitsu)
K. Oura (Univ. of Osaka)
H. Oyanagi (Electrotechnical Lab.)
K. Saiki (Univ. of Tokyo)
H. Shigekawa (Univ. rf Tsukuba)
M. Tabe (ΝΤΓ)
W. Tanaka (LUVAC-PHI)
N. Tanaka (Univ. of Nagoya)
M. Tkiiguchi (MKKOKYCDO)
T. Tomita (Sharp)
K. Tsubouchi (Tohdiu Univ.)
T. Yamaguchi (Sanyo Elect.)
S ec r e - ta ry K. Hirose (NEC)
T. Ueno (Tokyo Univ. of Agri. and Tech.)
CONTROL OF SEMICONDUCTOR INTERFACES
I. Ohdomari. Μ. Oshima and A. Hiraki (editors)
1994 Elsevier Science B.V.
Understanding semiconductor interfaces
Walter Α. Harrison
Applied Physics Department, Stanford University
Stanford, CA 94305 USA
A tutorial review of the electronic structure of semiconductors and their interfaces is given, providing an
elementary predictive representation of electronic states in heterojunction systems. The bulk bands remain
meaningful even in thin layers and provide bulk states or exponentially decaying states, depending upon energy.
Band discontinuities are obtained by aligning hybrid energies on the two sides, shifting the energy bands with
them. For interfaces with metals the Fermi energy replaces the hybrid energy. Quantum-well states themselves
are best obtained plane-by-plane through the interface using a simple recursion relation. The effects of strained
layers are also discussed. For polar interfaces, distinction is made between charged interfaces, those with a
dipole layer, and neutral interfaces. The effects of dipole layers on band discontinuities are estimated; it is seen
how they may be used to tune the band discontinuities.
1. ELECTRONIC STRUCTURE a composite state (more usually called a molecular
orbital in molecules, or a Bloch state in solids)
We begin at the most elementary quantum- which is written uilsi> + U2ls2> . This is called
mechanical level in order to be sure that the central tight-binding theory of the electronic states in
concepts and terms are clearly stated. This part multiatom systems. The second essential concept is
follows our more extensive text on the electronic that these coefficients ui and U2 are to be thought
structure of solids [1]. We treat the simplest of as amplitudes , since the probability of finding
systems which illustrate the formation of energy the electron on the first atom is proportional to ui^
bands and the behavior of band states at interfaces. just as the probability of finding a photon in a
We also discuss the nature of the particular bands in particular place is proportional to the intensity of the
tetrahedral semiconductors. It will then be possible light, proportional to the square of the light
to understand the interfaces in semiconductor amplitude at that point. It follows that if we choose
systems without carrying out the detailed
ui^ + U2^ = 1, the probabilities of being on each
calculations
atom will equal ui^ and U2^ . The energy of such a
composite state is then given by
1.1. Elementary introduction
The most essential concept of quantum
ε = ui^Esi -f 2 uiU2Vssa + U2^es2, (1)
mechanics, for our purposes, is that electrons
occupy states of particular energy. In a lithium
where Zs\ and es2 are the energies for an electron
atom, the electron of highest energy is in a state
on each of the two atoms, in this case equal
which we label as ls> and it requires 5.4 eV to
energies, and Vssa is called the coupling between
remove the elecüOn from the atom; thus we give
the two state, arising because the two atomic states
that electron's energy as Es = -5,4 eV, We may
overlap each other. It is not a coupling between
think of the "s" as standing for spherical (though the
electrons since we are talking about a single
origin of the term in spectroscopy was not that),
electron. The lowest-energy state for an electron is
since the state is spherically symmetric. If two
found by minimizing Eq. 1 with respect to ui and
lithium atoms come together in a molecule, an
U2 , subject to the condition ui^ + U2^ = 1. Because
electron might be in the state lsi> on the first atom,
VssG ^urns out to be negative, the lowest energy
or the state Is2> on the second atom or it might be in
State occurs for ui = U2 = 1/V2 and has energy, states, with now k = nK/((N+l)d) and 0 < η < Ν .
It can be confirmed that the energies of these Ν
obtained from Eq. 1 of Es +Vssa · If Esi were
states are exactly given by Eq. 2.
different from £§2. the same analysis would give an
This leads to an important conclusion that
energy (esi + es2)/2- V (esi-es2)^/4+Vssa^ · extends to semiconductors with surfaces or very thin
In either case the state is called a "bonding state". layers of semiconductors. The energy bands of a
For LÍ2 it is occupied by an electron of spin up and crystal, represented here by Eq. 2, may remain
one of spin down and the energy gain, 2Vssa » completely valid in a thin layer or near a surface, it
comparison to the isolated atoms is what holds the is only the allowed wavenumbers which are
atoms together in the molecule. The state of highest changed. We note that the wavenumbers differ
energy for our case is 65 -Vssa · That state is called between the finite chain and the ring discussed
an antibonding state. In fact any state uilsi> + above. Of course if effects of a surface, such as
U2ls2> can be written as a combination of the surface potentials, extend through the film, the
bonding and antibonding states, which are the two energies themselves could be affected, but that will
molecular orbitals arising from the two atomic not usually be a large effect.
states.
1.2. Bonds and bands in semiconductors
We may immediately generalize this system to
In a semiconductor such as silicon the atomic p-
a chain of Ν lithium atoms to define the energy
states are important, as well as the s-states, and two
bands for such a many-atom system. Each of the
electrons occupy p-states in the free atom. If silicon
Ν composite states can be written Zj Ujlsj> . If this
atoms were brought together so that there was a
chain of atoms were bent into a ring (called periodic
small coupling between adjacent atoms, the s-states
boundary conditions), the minimization of the
would be broadened into a band, as for lithium, and
energy leads to states with Uj = V1/N exp(ikdj),
the three p-states per atom would also be broadened
where d is the spacing between atoms and the
into bands, one third full; it would be a metal. What
wavenumber characterizing the state is given by k happens instead in real silicon can be described as
= 2Kn/(Nd) with η an integer -N/2 < η < N/2 . the formation of bonds between adjacent silicon
The Ν atomic states have been broadened out into atoms just as between lithium atoms as described
a band of Ν states just as the pair of lithium states above.
was split into an antibonding and a bonding state. This can be understood [1] by forming four
The energies of these states are immediately composite sp-states on each atom, called sp^-
obtained from the generalization of Eq. 1 as hybrids, each of which is one quarter s-state and
three quarters p-state, with each hybrid oriented
ek = es + 2VsscjCos(kd) . (2) toward a neighboring atom. Then bonds are formed
between the hybrids directed at each other. There
are just enough electrons to place two in each bond
It is interesting that if Vssa were about
state, leaving the corresponding antibonding states
-n^/(2md^), which will turn out to be approximately
empty. Finally, there is coupling between
the case, the energies for small k are equal to εο + neighboring bonding states which broadens these
ífi k^/2m , just as for free electrons. The electrons bond states into bands just as the coupling between
can indeed propagate through the chain almost as if lithium s-states broadened them into bands. Since
free, by virtue of this coupling between adjacent there were just enough electrons to fill the bond
atoms. states, there will be just enough elecüOns to fill the
Standing wave states, Uj = V2/N sin(kdj), are bonding bands, called valence bands. The
antibonding states are similarly broadened into
valid alternative composite states. In fact if we do
conduction bands, which are unoccupied in the pure
not bend the chain of atoms into a circle but let j =
material at low temperature.
1 and j = Ν be end atoms, these remain valid
2. BAND DISCONTINUIΉES
We found in Section 1.1 that the energy bands
for a finite chain remained the same but the allowed
wavenumbers changed. This is true also in a thin
CondTiciion Bands slab of semiconductor where the bands, as in Fig. 1,
τ remain appropriate but only wavenumbers are
allowed which provide an integral number of half-
wavelengths through the slab, kL = ηπ . (The
equation for k which we obtained for the chain, k =
nπ/((N+l)d), would suggest that the thickness L
should be taken as that measured between planes
Valence Bands
one interlayer spacing beyond the surface atomic
planes, and more complete analysis supports that.)
Similarly, at a heterojunction [these interfaces,
in which the crystal structure continues though the
Figure 1. A part of the energy bands of silicon
interface, only the atomic species changing, are the
(schematic) for wavenumbers k parallel to [100].
only interfaces we shall consider here] between two
The energy gap Eg between the conduction and
semiconductors, the bands, as in Fig. 1, remain
valence bands is indicated.
appropriate on each side. Further, having referenced
the bands to the free-atom p-states, we would have a
way of specifying the relative energies of the two
The bands themselves, in this tight-binding
sets of bands (the band line-up) and the resulting
theory, are given by formulae similar to Eq. 2, but
difference in Ey on the two sides (the valence-band
more complicated because they involve four atomic
discontinuity ). Although this method, given for
states per atom and couplings between each kind of
example in Ref. 1, predicts rather well the observed
state. Figure 1 shows a part of such a set of energy
valence-band discontinuities, there can be important
bands for silicon. The most important state energy
corrections to it. We discuss here only the intrinsic
is the highest level in the occupied valence band,
corrections, not those due to defects in the structure.
which turns out to be given by [1]
4V ρρσ SVppTt 2.1 The origin of the discontinuities
Ev = Ep - 2 (3) The first of these corrections can be easily
understood by imagining a heterojunction between
two semiconductors which differ only in that the
in tight-binding theory. Note in particular that it is
atomic energies. Es and Ep, were higher on the right
given relative to the atomic p-states. The
conduction-band minimum lies above Ey by the by an energy Δ . Since the couplings appearing in
energy gap. Eg. Because these bands are more Eq. 3 were assumed to be the same on both sides,
contorted the minimum may occur away from k = 0, we would predict that Ev on the right would be Δ
as it does in silicon, but not in GaAs. higher than that on the left. However, just this
In a polar semiconductor, such as GaAs, the situation could be accomplished by applying an
energies of the s- and p-states on the Ga differ from electric field inside the semiconductor which
those on the As, and Eq. 3 for the valence-band shifted the atomic levels on the two sides relative to
maximum takes a more complicated form, as did the each other. In that case we know that the bonds
bond energies given at the beginning of Section 1.1 would polarize to reduce the net field by a factor
for Esi ^ Zsl · However, the valence bands remain equal to the reciprocal of the dielectric constant,
full and the conduction bands empty. 1/12 for silicon. Thus our prediction would have
overestimated the band discontinuity by a factor of
order 12. This point was made early on by Tersoff Table 1
[2] who defined "neutrality levels" within the band <8h> - Ev in eV^ from Ref. 3, for a selection of
structure which are brought into register by this semiconductors.
dielectric relaxation. A subsequent tight-binding
analysis [3] showed that in that context the
neutrality levels on each side were the hybrid Si -0.03 AIP 0.76
energies on the two sides, equal to zJA + 3ερ/4 ; Ge -0.32 AlAs 0.46
for a compound semiconductor the average for the Sn -0.33 AlSb 0.23
two constituent atoms is to be taken. Thus a table
can be given of the average hybrid energy <eh> , GaN 2.55 InN 2.59
relative to the valence-band maximum in each GaP 0.66 InP 0.77
semiconductor, as in Table 1. Then by matching the GaAs 0.34 InAs 0.47
hybrid energies on the two sides, the valence-band GaSb 0.14 InSb 0.28
discontinuity is directly predicted.
There is an important lesson to be learned from ZnSe 1.69 CdSe 1.71
this history. The fact that a theory - in this case ZnTe 1.40 CdTe 1.43
referring the bands to the free-atom levels -
describes quite well the observed offsets does not
mean that it is right. There has been enough described by the macroscopic constant. Further, the
variation in the experimentally determined off-sets macroscopic dielectric constant is really just the sum
that a considerable number of independent theories of the effects of the polarizability of individual
could be proposed to explain the line-ups, and each, bonds and the concept of the screening arising at the
even if it was fundamentally in error, gave heterojunction from the interface bonds is a
predictions within the experimental error of those perfectly reasonable one.
observed. In fact, when the predictions from Table 1 These screening shifts are frequently interpreted
are compared with those based upon atomic term in terms of "gap states", which extend deep into the
values, the agreement with available experiments is semiconductor, as suggested by Heine [4] some time
not conspicuously better. In addition we shall ago. We would argue on the other hand that if we
discuss in Section 3 effects at polar interfaces which are to distinguish such gap states from the direct
can shift the predicted, and experimental, line-ups. interface dipole itself, only the interface dipole is
We would argue however, that the neutrality-point significant.
picture, upon which Table 1 is based, correctly Another complication arises if there is a large
describes the basic physics of the interface and lattice mismatch between the lattices on opposite
should be the starting point for making estimates, or sides of a heterojunction. The lattice spacing
for understanding more complicated effects on the parallel to the interface is forced to be in register
valence-band discontinuities. except for thick-enough layers that misfit
dislocations appear to relieve the strain. Then the
One might have concern that use of a
corresponding stretching of the material of smaller
macroscopic dielectric constant to describe the
natural lattice distance parallel to the interface will
response of such local fields of atomic origin is
cause, as predicted by Poisson's ratio, a contraction
unjustified. That appears not a warranted concern.
perpendicular to the interface. Such a tetragonal
In a more complete analysis one might Fourier-
distortion of one lattice unquestionably modifies the
transform this step-like potential and screen each
band structure associated with that side. In
component by the appropriate wavenumber-
particular it splits the three valence bands which are
dependent dielectric constant. Then the offset some
degenerate at k = 0, as seen in Fig. 1, to a doubly
distance from the heterojunction would be
degenerate set and a nondegenerate band. It is clear
determined entirely by the long-wavelcngth
that the valence-band discontinuity is modified, and
components, for which the screening is well