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

Abstract

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

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

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.