Mapping of strain in multilayer GaAs/AlAs superlattices from HRTEM micrographs
Introduction
Semiconductor superlattices (SLs) has excellent thermal, optical and electrical properties not found in bulk semiconductors. GaAs/AlAs SLs is an important fundamental tool for developing better thermoelectric energy conversion materials due to its decreased thermal conductivities [1], [2]. Furthermore, the anisotropic thermal properties resulting from both the modified phonon dispersion and increased diffuse scattering of phonons at the interfaces serve as excellent platforms to explore and engineer nanoscale phonon heat conduction [3]. The anisotropy of phonon-plasmon modes in doped semiconductor nanostructures may modify the optical and electronic properties of these structures, thereby raising fundamental and practical importance for the design of quantum-well lasers [4], [5], [6]. In essence, these properties are closely related to the composition, size, shape, structure, strain and other characters of SLs [7]. Additional strain and significant numbers of atomic displacements were introduced by room temperature implantation (N, Si, Zn) into SLs [8]. GaAs SLs grown at a two-temperature growth process has good crystal quality, atomically flat surfaces and small strain [9]. the source mechanism of strain is lattice distortion. Strain state in SLs will be changed with the variation of composition, structure and environmental factors such as heat, electricity and force. Strain state in Ga(P, As)/GaP heterostructures was changed by surface relaxation, although no additional artificial factors were imposed [10]. Strain is one of the most important factors influencing the property and working life of SLs-based device [11], [12]. The composition, structure and strain state of multilayer GaAs/AlAs SLs are very complex. It is necessary and meaningful to reveal the interior and interface strain distribution of multilayer GaAs/AlAs SLs for the application research related to the anisotropic properties of SLs. In this work, high resolution transmission electron microscope (HRTEM) and geometric phase analysis (GPA) were employed to study the strain distribution across the cross-section of multilayer GaAs/AlAs SLs. The purpose of this work is to reveal the strain distribution and its anisotropy, which is helpful for the estimation of anisotropic properties and failure of multilayer GaAs/AlAs SLs-based devices.
Section snippets
Experiments
In this work, the used multilayer GaAs/AlAs SLs grown by molecular beam epitaxy technique for strain mapping is ordered from Institute of Semiconductors, Chinese Academy of Sciences. The structure and nominal layer thicknesses of the SLs are given in Fig. 1. Transmission electron microscope (TEM) sample of the cross-sectional SLs was prepared using mechanical grinding and argon ion milling. The thickness of cross-sectional SLs was milled to about 40 μm by using a series of sandpapers first. The
Results and discussion
Fig. 2 shows the TEM measuring results of the cross-sectional multilayer GaAs/AlAs SLs. A six layers structure can be clearly observed in the HRTEM image of the cross-sectional SLs. However, tiny tilt and distortion at interfaces can be observed between the left GaAs substate and the rightmost GaAs layer. Fig. 2(b) is the Fast Fourier Transform (FFT) diffraction spots image of Fig. 2(a) and the diffracted crystal planes are demarcated in the spots image, which reveals that the crystalline
Conclusions
The GaAs/AlAs SLs had a six layers structure and the structures of all layers were determined to the zinc-blende structure. The farther the interface from the substrate, the larger the strain. The anisotropy of strain distributions along , and directions was obvious. For the direction, the positive strains on the right side of the AlAs film was pretty large in region 3 with a maximum strain of 6.48% and the interface with a large strain had a great possibility of cracking or
CRediT authorship contribution statement
Xu Li: Funding acquisition, Investigation, Project administration. Yanyan Ouyang: Data curation, Formal analysis. Ran Zhang: Conceptualization, Visualization, Resources. Xingfu Tao: Resources, Software, Supervision. Yi Zhang: Supervision, Validation, Visualization. Huifang Gao: Data curation, Formal analysis, Methodology, Resources.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the Youth Programme of the National Natural Science Foundation of China (No. 61904170).
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