Structural Investigation of Si-Ge Epitaxial Layers by Convergent Beam Electron Diffraction (CBED)
Date Issued
2014
Date
2014
Author(s)
Chen, Yi-Yen
Abstract
In order to continually downscale the gate size of the transistor, strained Si and Si1-xGex materials have been applied in the modern electronics industry to enhance the carrier mobility of the channel between source and drain. Transmission electron microscopy (TEM) techniques, notably high-resolution TEM (HR-TEM) with geometrical phase analysis (GPA), convergent beam electron diffraction (CBED), nano-beam diffraction (NBD), and electron holography, are effective techniques for strain analysis in the nano-scale range. Of these techniques, CBED has attracted much attention due to its high sensitivity and accuracy in the determination of lattice parameters.
In this study, 4 μm thick Si0.95Ge0.05 and 50 nm thick Si0.8Ge0.2 epitaxial layers were deposited on (001)-orientated p-type Si substrate in an ultra-high vacuum/chemical vapor deposition (UHV/CVD) system. Imaging with bright-field (BF), HR-TEM, corresponding fast Fourier transformation (FFT), scanning transmission electron microscopy (STEM), and energy dispersive spectroscopy (EDS) element line-profiles showed the uniformity of thickness and composition and the excellent crystal quality of the Si0.95Ge0.05 and Si0.8Ge0.2 layers. In the CBED experiment, [430] and 1º tilting toward the [001] direction from the [430] zone axis (the off [430] zone axis) were used on the Si-Ge epitaxial layers with an FEI TECNAI F30 TEM equipped with an energy filter of a 10 eV slit at 300 kV (effective voltage was 298.6 kV) and STEM mode. A good match was achieved with CBED patterns simulated by MacTempas calculating software.
CBED patterns taken close to the Si0.95Ge0.05/Si interface revealed split high order Laue zone (HOLZ) lines and blurred CBED patterns with the on [430] and off [430] zone axes. More HOLZ lines and clearer CBED patterns could be found by off [430] zone axis than by exact [430] zone axis. The off [430] zone axis provides a good method for the reduction of the influences of multi-fringes caused by Kikuchi line pairs and inelastic scattering, and hence the image quality of CBED patterns and spatial resolution can be improved. The spatial resolution of the CBED technique by off [430] zone axis in this study can reach 50 nm, as shown by examination of a 50 nm thick Si0.8Ge0.2 layer.
Using the off [430] zone axis, according to the assumptions of bending of lattice planes along the direction of TEM specimen thickness by -0.03o, 0, and 0.03o and lattice constant changes by 0.549 nm±0.06 nm, the split HOLZ lines in CBED patterns were successfully reproduced by kinematical simulation. From the comparison of split widths of (008), (1-17), (-573), and (-575) lines with the experimental results, especially for the wide splitting of the (008) line, the bending of lattice planes along the direction of TEM specimen thickness is the most likely cause in the case of Si1-xGex layer on Si substrate. Furthermore, at variant specimen thicknesses of 280 nm, 430 nm, and 550 nm, the HOLZ line splitting began at points 170 nm, 255 nm, and 310 nm away from the Si0.95Ge0.05/Si interface on both sides of Si0.95Ge0.05 and Si, respectively, which can be interpreted as the effect of free surface relaxation during the preparation of the thin TEM specimen. From these CBED patterns at 290 nm, 430 nm, and 550 nm thick TEM regions, a 2-D bending map of lattice planes along the direction of the TEM specimen can be reconstructed with high spatial resolution (50 nm) and angle sensitivity (0.02o).
In the strain estimation, the bending of lattice planes along the direction of the TEM specimen can be completely described by the model of distortion strain. From the bending consequences of the lattice plane, measured from the split widths of HOLZ lines, distortion strain has been successfully estimated and used to reconstruct a 2-D distortion strain distribution. Interface distortion strain and free surface relaxation can be clearly observed.
Finally, from the shifts of HOLZ lines, the lattice constants and Ge concentrations in the Si0.95Ge0.05 and Si0.8Ge0.2 layers could be estimated. The lattice constants and Ge concentrations of the Si0.95Ge0.05 and Si0.8Ge0.2 layers were 0.544 nm (5 at%) and 0.548 nm (19 at%), respectively, which were very close to the expected results and much more accurate than the EDS results.
Subjects
矽鍺
收斂電子束繞射
高指數Laue晶帶線
表面應力釋放
線分裂
Type
thesis
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