Anisotropic surface morphology in a tensile-strained InAlAs layer grown on InP(100) substrates
Introduction
Heteroepitaxial growths of semiconducting materials on lattice-mismatched substrates have been a challenging issue for a long time due to the inevitable formation of interfacial misfit and threading dislocations during strain relaxation [[1], [2], [3], [4], [5]]. When the strain stored in the heteroepitaxial layer is released by the onset of the dislocations beyond a critical thickness, threading segments penetrate through a thin layer, and interfacial misfit segments increase the surface roughness through the formation of cross-hatch patterns that are mostly undesirable for fabricating low dimensional electronic device structures [6,7].
In the heteroepitaxial systems of III–V compound semiconductors (CSs) with a zinc-blende structure and SiGe with a diamond-cubic structure, orthogonal arrays of 60° misfit dislocations lying along two perpendicular ⟨011⟩ in-plane directions are formed at the interfaces [3,7]. In clear contrast to non-polar Si and Ge, polar III-V CSs have two different types of misfit dislocation along directions [011] and . Many previous studies reported that α and β dislocations have different cores of group V and group III atoms, resulting in different dislocation glide velocities. It is well known that the coexistence of the two types of dislocation arrays in heteroepitaxial III–V layers is the direct cause of anisotropic strain relaxation between two orthogonal directions [2,[8], [9], [10]].
Furthermore, the evolution of strain relaxation and cross-hatch morphology in strained InAlAs (or InGaAs) layers have been extensively examined in respect to strain type (tensile or compressive), strain magnitude, layer thickness, and growth temperature [1,2,4,5,[8], [9], [10]]. However, the origin of the anisotropic strain relaxation still remains controversial and less known. For example, it is generally suggested that the strain relaxation of compressive-strained III-V CSs is isotropic, in opposition to the anisotropic relaxation of tensile-strained ones such as InxGa1−xAs/InP (x < 0.53), InyAl1−yAs/InP (y < 0.52) [1,11]. Moreover, crack formation is inherently believed to occur only in tensile-strained layers [1,12,13]. However, some experimental results have claimed anisotropic relaxation and cracking even in compressive-strained III-V layers such as InGaAs/GaAs.
In this study, we found that the surface morphologies of a tensile-strained InAlAs on InP substrates over the cross-hatch arrays along directions and [011] are completely dissimilar due to α and β dislocation. The former is characterized by a big mound-like shape with lower array density. A peculiar phenomenon is that microcracks happen at the top center of every mound-like array along direction . On the other hand, the latter has a sharp ridge shape with a higher array density in the absence of microcracks. Our X-ray spectroscopy analyses further elucidates that preferential surface diffusion of indium atoms along direction [011] causes indium accumulation over the big mound ridges. Based on these experimental observations, we propose that the strain relaxation of the tensile-strained InAlAs layer on InP substrates is undisputedly caused by the formation of interfacial misfit dislocation, localized indium accumulation, and cracking. Indeed, the behaviors of these three phenomena are anisotropic between two orthogonal ⟨011⟩ in-plane directions.
Section snippets
Experimental details
Fabrication of the heteroepitaxial In0.42Al0.58As layers was carried out using a molecular beam epitaxy (MBE, Riber Compact 21T) system. Epi-ready semi-insulating InP(100) substrates with an on-axis orientation were heated up to 470 °C to thermally desorb the surface oxide layer and eventually attain a (2 × 4) surface reconstruction by reflection high-energy electron diffraction (RHEED). During InP buffer layer growth at a substrate temperature of 450 °C, spotty (2 × 4) patterns became streaky,
Results and discussion
Fig. 1(a) shows the AFM image of cross-hatch patterns formed on the partially relaxed In0.42Al0.58As layer. Cross-hatch arrays between two orthogonal in-plane directions display significant differences in array densities and lateral dimensions. The cross-hatch arrays along direction [011] display much higher density than the ones along direction . It is surprising that all the arrays along direction have a mound-like shape with an average height of 9 nm and a width of 3.3 μm, as
Conclusions
In conclusion, we have investigated the anisotropic strain relaxation of a tensile-strained InAlAs layer grown on InP(100) substrates. It was found that the InAlAs layer was relaxed by misfit dislocation formation, preferential surface diffusion, and microcrack formation, which all showed completely anisotropic behaviors. Fig. 4 is a representative schematic depicting the three anisotropic behaviors resulting in imbalanced strain relaxation between two orthogonal directions. More strain
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MEST) (Grant No. 2015004870, 2016910562) and by the KIST Institutional Program and by the Future Semiconductor Device Technology Development Program (10052962) funded by MOTIE (Ministry of Trade, Industry & Energy).
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The first two authors contributed equally to this paper.