Strain contrast of GaN_yAs_1−y(y= 0.029 and 0.045) epitaxial layers on (100) GaAs substrates in annular dark field images

Abstract

The annular dark field (ADF) image contrast of GaNyAs1−y (y = 0.029 and 0.045) epitaxial layers on (100) GaAs substrates was studied with a scanning transmission electron microscope (STEM) as a function of ADF detector inner semi-angles ranging from 28 to 90 mrad. Contradictory to the compositional contrast prediction of ADF-STEM image intensity, the lower average atomic number strained GaNyAs1−y layers were found to be brighter than the higher average atomic number GaAs for an ADF detector semi-angle up to 65 mrad. Multislice simulations reveal that the displacement around substitutional N atoms plays a crucial role in the observed ADF-STEM contrast, while the contribution to the contrast due to misfit strain between GaNyAs1−y and GaAs is small.

Atomic Size Mismatch Strain Induced Reversed ADF-STEM Image Contrast Between dilute Semiconductor Heteroepitaxial Layers and Substrates

“…The other is the atomic size mismatch strain arising from the atomic radius difference between the substitutional N (r N = 0.068 nm) and As host (r As = 0.121 nm) atoms. It has been shown by multislice simulations that the atomic size mismatch strain plays a crucial role in the observed ADF-STEM image contrast, while the contribution to the contrast due to lattice mismatch strain between GaN x As 1-x and GaAs is small [10]. The agreement between the experimental and multislice calculations for GaN x As 1-x (x = 0.045 and 0.029) sample is shown in Fig.…”

Effects of small specimen tilt and probe convergence angle on ADF-STEM image contrast of Si0.8Ge0.2 epitaxial strained layers on (100) Si

“…This difference in image contrast between experiment and simulation may be attributed to factors not included in the model. For example, if the Ge atoms were randomly substituted within the Si lattice, it would be expected that multi-atom clusters of Ge would be present that would lead to potentially large localized strains resulting in increased non-Bragg scattering and thereby increasing the simulated contrast values [8]. In addition, the effects of any inhomogeneous response of the ADF detector were not included [30].…”

“…However, the existence of strain in the films and relaxation of strain at the surfaces of the TEM specimen make interpretation of the measured intensity profile less straightforward. For example, atomic size mismatch strain induced reversed ADF-STEM image contrast between dilute semiconductor heteroepitaxial strained layers and substrates has been reported in GaNAs/GaAs [8,11,12] and SiC/Si systems [9]. In addition, a reduction in the ADF image intensity of InGaAs layers at both interfaces with adjacent GaAs layers due to strain relaxation has also been observed [10].…”

“…For example, CTEM studies of defects (dislocations, twins, stacking faults and cracks) and morphology helped to determine the strain relaxation mechanisms in SiGe/Si, InGaAsP/InP, GaAsN/GaAS films, which then facilitated optimization of growth methodology [2][3][4][5]. In recent years, annular dark field scanning transmission electron microscopy (ADF-STEM) has become a widely used and powerful technique for characterizing strained epitaxial layers due to the fact that ADF-STEM image contrast depends strongly on the atomic number Z of the scattering atoms in a simple Z n form (n¼1.6-1.9), which makes composition variation evident through a change in image intensity [6][7][8][9][10]. However, the existence of strain in the films and relaxation of strain at the surfaces of the TEM specimen make interpretation of the measured intensity profile less straightforward.…”

Convergent beam electron-diffraction investigation of lattice mismatch and static disorder in GaAs/GaAs1−xNx intercalated GaAs/GaAs1−xNx:H heterostructures

“…14 In addition, in this particular stress configuration the m ⊥ values can be retrieved by dividing the perpendicular mismatch measured in the thinned sample by a proper factor (1-ν), 15 where ν=0.31 is the isotropic GaAsN Poisson ratio. 16 While mismatch is detected through the angular shifts of high-angle diffracted beams, static disorder reduces the coherent Bragg diffracted intensity. 18 A quantitative description of static disorder can be obtained by the exponential increase of the extinction distance ξ g -due to the increase of the mean square deviation of atoms from their lattice positions caused by the substitutional impurities 18…”