Geometry and electronic structure of GaAs„001…„2 4… reconstructions
W. G. Schmidt* and F. Bechstedt
Friedrich-Schiller-Universita ¨t, Institut fu ¨r Festko ¨rpertheorie und Theoretische Optik, Max-Wien-Platz 1, 07743 Jena, Germany
Received 23 May 1996
Structural and electronic properties of the As-rich GaAs00124 reconstructions are investigated by
means of converged first-principles total-energy calculations. For an As coverage of =3/4, we find the
two-dimer 2 phase to be energetically preferred over the three-dimer phase. As the As chemical potential
decreases, the phase of GaAs001 represents the ground state of the surface. All geometries are character-
ized by similar structural elements as As dimers with a length of about 2.5 Å, dimer vacancies, and a nearly
planar configuration of the threefold-coordinated second-layer Ga atoms leading to a steepening of the dimer
block. Consequently, the resulting electronic properties also have similar features. The surface band structures
are dominated by filled As-dimer states and empty Ga dangling bonds close to the bulk valence- and
conduction-band edge, respectively. The measured Fermi-level pinning cannot be related to intrinsic surface
states. The calculated surface states and ionization energies support the 2 structure as the surface geometry
for an As coverage of =3/4. S0163-18299604248-8
I. INTRODUCTION
The GaAs001 surface is one of the most studied polar
semiconductor surfaces and has attracted much interest of
both experimentalists and theoreticians because of its impor-
tance for the growth of multilayer device structures. As-rich
24/c 28 reconstructions, in which the surface is
mainly terminated by various arrangements of As dimers are
most important in molecular-beam epitaxy MBE of GaAs-
based layered structures.
Three different phases of the 2 4 surface have been
identified by reflection high-energy electron-diffraction
experiments.
1–3
The , , and phases correspond to a
characteristic ratio of fractional order intensities. On grounds
of a comparison with scanning tunneling microscopy STM
images, Hashizume et al.
2,3
concluded that all three phases
have the same outermost surface layer of the unit cell, which
consists of two As dimers and two dimer vacancies. The
phase is described by the two-dimer model of Farrel and
Palmstro ” m
1
with a significant relaxation of the second-layer
Ga atoms cf. Fig. 1. The phase corresponds to the two-
dimer model originally introduced by Chadi,
4
which is char-
acterized by an additional As dimer in the third layer. Ac-
cording to the notation of Northrup and Froyen,
5
this
structure will be called 2 in the following cf. Fig. 1. The
phase was found to be a mixture of the phase and the
c 44 phase with the surface As coverage varying depend-
ing on the actual growth conditions. On the other hand, three
and two As dimers in the outermost surface layer in 2 4
unit cells are seemingly observed by STM Refs. 6–8 and
explained by the and structures shown in Fig. 1. Both
the three-dimer structure and the two-dimer 2 structure
correspond to the same As coverage of =3/4. Ab initio
calculations by Ohno
9
and Northrup and Froyen
5
came to
opposite conclusions concerning their energetical stability.
Also the atomic structure of the phase is not clear.
Northrup and Froyen
5
observed a dimerization of the second-
layer Ga atoms in agreement with the early prediction by
Chadi,
4
but in contrast to Ohno,
9
who ruled out a Ga-Ga
bonding for the phase. There is also a lack of consensus on
other structural details. Recent STM studies
8
indicated an
apparent twisting of the As dimers in the structure. How-
ever, symmetric dimers are found to be in best agreement
with x-ray photoelectron diffraction XPD data.
10
A rather
wide range of As-As dimer bond lengths is reported. XPD
studies
10
indicate a dimer length of 2.2 Å, energy-dependent
photoelectron diffraction experiments
11
determined a value
of 2.4 Å, and secondary-ion mass spectrometry investiga-
tions
12
found the As dimer atoms to be 2.73 Å apart. An
early theoretical study
13
considering As-As dimers in 2 1
surface unit cells predicts a dimer length of 2.52 Å. Recent
ab initio calculations
14,15
state values of 2.39 and 2.60 Å,
respectively. Other structural parameters as interplanar dis-
tances and relaxations in deeper layers are even less well
known than the precise dimer length and symmetry.
Relatively little is known about the electronic structure of
the GaAs001 surface. The 2 4 reconstruction models con-
sidered in this work satisfy the electron counting rule, i.e.,
the dangling bonds are filled on surface anions and are empty
at surface cations.
16
The surface band structure should be
semiconducting rather than metallic since there exists no
partly occupied surface state. However, work function
measurements
17
on p - and n -type samples indicate a Fermi-
level pinning. Irrespective of the type of reconstruction the
Fermi level lies about 0.5 0.1 eV above the valence-band
maximum VBM.
18
Defects, in particular kinks, were made
responsible for the band bending.
19
In contrast to the con-
stant Fermi-level position for the different phases, a small
dependence of the ionization energy on the reconstruction
model was observed.
20
It assumes its maximum for the
phase of the GaAs00124 reconstructions. The occupied
surface bands of the 24 reconstruction have been mapped
by angle-resolved photoemission spectroscopy ARPES.
21,22
Larsen et al. found weakly dispersive states with sp
z
char-
acter in the energy range 0.5–1.6 eV below the VBM and a
nearly dispersionless state near -3 eV. By applying the scat-
tering theoretical method based on an empirical tight-binding
TB scheme, these features were attributed to asymmetric
PHYSICAL REVIEW B 15 DECEMBER 1996-I VOLUME 54, NUMBER 23
54 0163-1829/96/5423/167427/$10.00 16 742 © 1996 The American Physical Society