View source for Structure of AlSb(001) and GaSb(001) surfaces under extreme Sb-rich conditions

== Abstract ==

[[File:AlSb_GaSb_Fig_1.gif‎|thumb|right|300px|Fig. 1. 
Reconstruction models proposed for the
AlSb(001) or GaSb(001) surfaces with (4x3) and (4x4) periodicities.
The first two upper layers are shown in a top view. Smaller
white circles represent Sb atoms in the top layer of the underlying
Sb-terminated AlSb(001) or GaSb(001) surface. Larger circles represent
Al or Ga (black) and Sb (white) adatoms. The unit cells are
shown in light blue.]]


We use density-functional theory to study the structure of AlSb(001) and GaSb(001) surfaces. Based on a variety of reconstruction models, we construct surface stability diagrams for AlSb and GaSb under different growth conditions. For AlSb(001), the predictions are in excellent agreement with experimentally observed reconstructions. For GaSb(001), we show that the previously proposed model accounts for the experimentally observed reconstructions under Ga-rich growth conditions but fails to explain the experimental observations under Sb-rich conditions. We propose a model that has a substantially lower surface energy than all (n×5)-like reconstructions proposed previously and that, in addition, leads to a simulated scanning tunneling microscopy image in better agreement with experiment than existing models. However, this model has higher surface energy than some of (4×3)-like reconstructions, models with periodicity that has not been observed. Hence, we conclude that the experimentally observed (1×5) and (2×5) structures on GaSb(001) are kinetically limited rather than at the ground state.
Full article: http://prb.aps.org/abstract/PRB/v76/i20/e205303

== Introduction ==


The surfaces and interfaces of III-V semiconductors constitute
some of the most important components of the semiconductor
industry. For example, III-V heterostructure quantum
wells are key components in a wide range of optical and
high-frequency electronic devices, including field-effect
transistors, resonant tunneling structures, infrared lasers,
and infrared detectors. Many of these devices require extremely
sharp and clean interfaces. For this reason, an understanding
of the atomic-scale morphology of III-V semiconductor
surfaces is critical for controlling the growth and
formation of their interfaces

In this paper, we explore theoretically a large number of
judiciously chosen candidate reconstructions on GaSb(001)
and AlSb(001). We find that as the growth conditions are
varied between Sb-poor and Sb-rich, the predicted sequence
of stable reconstructions for GaSb(001) is exactly analogous
to those of AlSb(001). Experimentally, however, the picture
is more complicated. In the Sb-poor limit, the observed
GaSb(001) reconstruction is indeed analogous to that of
AlSb(001). On the other hand, in the Sb-rich limit, the experimentally
observed reconstructions for GaSb(001) and
AlSb(001) are different. Moreover, in this limit, the predicted
and observed reconstructions are in good agreement only for
AlSb(001), while for GaSb(001) there remains an unresolved
discrepancy between theory and experiment.

== Proposed reconstruction models ==


[[File:AlSb_GaSb_Fig_2.gif‎|thumb|right|300px|Fig. 2. 
Reconstruction models proposed for the
Ga-Sb(001)-(1x5)-like surfaces under the extreme Sb-rich growth
condition. See Fig. 1 for color schemes. Gold circles represent the
second layer Sb adatoms.]]

The basic structural models we considered are taken from
the literature and are shown in Figs. 1 and 2. Surfaces that
satisfy the ECM are generally semiconducting, while those
that do not may be metallic. The degree to which a given
surface satisfies the ECM can be measured by the excess
electron count, which we define here as the difference
between the number of available electrons and the number
required to satisfy the ECM, per (1x1) surface unit cell.

== The possible reconstructions ==

[[File:AlSb_GaSb_Fig_4.gif‎|thumb|right|300px|Fig. 3. 
Reconstruction models with a single substitution
of Sb atoms by Ga atoms.]]

In order to explain the experimentally observed (1x5)
and (2x5) structures on GaSb(001) surface, we studied a
large number of structures based on variations of c(2x10)
and (2x10). We note that c(2x10) violates the ECM substantially
, and substitution of Sb atoms in the top
layer of the underlying Sb-terminated GaSb(001) surface by
Ga atoms can lower the excess electron count. Figure 3
shows the possible reconstructions when a single Sb atom is
replaced by a Ga atom. We use the naming convention of s1x
to denote a “single substitution,” As shown in Table I, all s1x
reconstructions indeed have lower excess electron counts.

== Conclusion ==

We have performed ab initio calculations on the surface
energy and atomic structure of AlSb(001) and GaSb(001)
surfaces with various reconstructions. Surface stability diagrams
for a large number of reconstruction models are constructed
under different growth conditions. For AlSb(001),
we confirmed that the predictions of the currently accepted
models are in good agreement with experimentally observed
reconstructions. For GaSb(001), we showed that previously
proposed model accounts for the experimentally observed
reconstructions under Ga-rich growth conditions but fails to
explain the experimental observations under Sb-rich conditions.
Therefore, we propose s1a-c(2x10) as a better alternative
to existing models for GaSb(001) under extreme Sbrich
growth conditions. Our calculations show that s1a-c(2
x10) has a substantially lower surface energy than all
(nx5)-like reconstructions proposed previously and, in addition,
it leads to a simulated STM image in better agreement
with experiment than existing models. However, s1a-c(2
x10) has higher surface energy than (4x3), a model with
periodicity that has not been observed. Hence, we conclude
that the experimentally observed (1x5) and (2x5) structures
on GaSb(001) are not the ground-state structure but the
kinetically limited ones.


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