ADF-STEM Imaging of Strained GaN 0.045 As 0.955 Epitaxial Layers on (100) GaAs Substrates
- PDF / 3,343,585 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 99 Downloads / 238 Views
0982-KK01-03
ADF-STEM Imaging of Strained GaN0.045As0.955 Epitaxial Layers on (100) GaAs Substrates X. Wu1, M.D. Robertson2, J.A. Gupta1, J.-M. Baribeau1, J.C. Bennett2, M. Kawasaki3, and T. Aoki3 1 Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, ON K1A0R6, Canada 2 Department of Physics, Acadia University, Wolfville, NS B4P2R6, Canada 3 JEOL USA, Peabody, MA, MA 01960
ABSTRACT The annular dark field (ADF) image contrast of a 0.92% tensile strained GaN0.045As0.955 layer on GaAs substrate was studied with a scanning transmission electron microscope (STEM) as a function of ADF detector inner semi-angles ranging from 28 mrad to 90 mrad. The GaN0.045As0.955 layers were brighter than the surrounding GaAs for the ADF detector semi-angle up to 65 mrad, and the measured contrast decreased with increasing ADF detector inner semiangle. For a 37 nm thick specimen, the GaN0.045As0.955 intensity is about 13% higher than that of GaAs for the 28 mrad ADF detector inner semi-angle. Multislice simulations show that the displacement around substitutional N atoms plays an important role in the observed ADF-STEM contrast, while the contribution to the contrast due to misfit strain between GaN0.045As0.955 and GaAs is small. INTRODUCTION An ADF-STEM image is formed by scanning an electron beam across the surface of a sample and collecting the scattered electrons using an annular-shaped detector located in the diffraction plane of the microscope. If the ADF detector inner angle is sufficient large, the ADF image is called a high-angle ADF (HAADF) image or Z-contrast image because the image contrast is sensitive to the variation in chemical composition i.e., the atomic number Z. The Zcontrast imaging technique has been successfully applied to problems ranging from the determination of dislocation core structures in semiconductor interfaces [1], to the observation of individual dopant atoms [2], and to the determination of the atomic configuration of ultradispersed catalysts [3]. In addition, the contrast of an ADF-STEM image has been shown to be dependent on strain, for example due to point defects and dislocations. A study of B-doped layers in Si revealed significantly stronger contrast having opposite sign relative to simple atomic number contrast (Zcontrast) predictions [4]. It was suggested that misfitting substitutional B atoms act as point defect sites in a Si matrix which enhance scattering to high angles via a static Debye-Waller effect [4]. Furthermore, the effects of strain on the ADF-STEM image intensity at the interfaces of c-Si/a-SiO2 [5] and c-Si/a-Si [6] have been reported. It was found that there was a bright band of contrast at the interface and the extra contrast in the ADF-STEM images could be attributed to the strain present at the interface [5]. In the c-Si/a-Si interface, the sign of the strain contrast was observed to be dependent on specimen thickness [6]. When the sample was less than 10 nm thick the interface contrast was negative but reversed to positive contrast at greater
Data Loading...
