Transmission electron microscopic investigations of heteroepitaxial III/V semiconductor thin layer and quantum well structures

The first part of the thesis presents compositional evaluation of ternary quantum well structures grown on GaAs and GaP substrates using the chemical sensitive 002 dark field imaging in Transmission Electron Microscopy (TEM). The composition retrieved by the dark field intensity measurements and the...

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Bibliographic Details
Main Author: Németh Igor
Contributors: Volz, Kerstin (Dr.) (Thesis advisor)
Format: Dissertation
Language:English
Published: Philipps-Universität Marburg 2008
Physik
Subjects:
TEM
Online Access:PDF Full Text
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Summary:The first part of the thesis presents compositional evaluation of ternary quantum well structures grown on GaAs and GaP substrates using the chemical sensitive 002 dark field imaging in Transmission Electron Microscopy (TEM). The composition retrieved by the dark field intensity measurements and the structure factor calculations were compared with compositional values of High Resolution X-Ray Diffraction (HRXRD) measurements and their dynamical simulations. In various ternary material systems, the compositional evaluation of dilute ternary quantum wells are erroneous, when determining the dark field intensities with the virtual crystal approximation. The estimates are more precise when the static atomic displacement (SAD) of the first neighbor atoms of the isovalent impurities are taken into account. The static atomic displacement and the structure factor of the various zincblende materials were calculated with the Valence Force Field (VFF) method. The correctness of the VFF method was proven with ab initio Density Functional Theory (DFT) calculations. A simple expression was proposed to calculate the bond distortions. It was found, that impurities in GaAs and GaP like boron or nitrogen introduce the largest values of static atomic displacement. Impurities which have covalent radii similar to the atoms of the host crystal do not distort the host lattice, hence can be described by the virtual crystal approximation. This is also the case when incorporating impurities above the dilute limit. When calculating the structure factors, the atomic scattering factors are usually taken into account by the isolated atom approximation. The refined structure factor calculations included the recalculation of the atomic scattering factors by DFT. However, the effect of the slight change in the atomic form factors was found to be a secondary factor in comparison to the SADs in the determination of the dark field intensities. The second part of the thesis demonstrates the optimization process of the heteroepitaxial thin-layer growth of GaP on Si substrates by Metal Organic Vapour Phase Epitaxy (MOVPE). This material system could be the link in the transfer of the direct band gap material Ga(NAsP) onto Si substrates. As the lattice mismatch of the GaP/Si material system is low, defects due to lattice relaxation do not occur in thin layers. The main defect types that arise are planar defects: antiphase boundaries (APBs), stacking faults (SFs) and microtwins (MTs). The main objectives of the thesis were the identification, the description of the evolution and the suppression of these defects. To image the antiphase related crystal domains (APDs), Transmission Electron Microscopy (TEM), such as dark field imaging with the 002 and 00-2 reflections was used. The usage of the different {111} reflections was found to be suitable also for imaging antiphasing in thinner TEM sample regions. The efforts to deposit single phase material were made exploiting two phenomena: the evolution of double steps on the Si (001) surface and the self-annihilation of the formed APDs. The Si surface morphology was investigated by Atomic Force Microscopy (AFM). The APD structure of the GaP thin layers was correlated to the monostep morphology of the annealed Si-buffer. Cross sectional and plan view TEM investigations with crystal phase sensitive dark field imaging in comparison with the AFM studies proved the monostep edges of the Si surface to be the origin of APBs. Cross sectional TEM revealed, that small APDs formed also on the terraces of the Si surface, which always self-annihilated. Their possible origin is also discussed. The APBs of the first type penetrated through the whole GaP layer when the growth temperature was low (450°C). At higher growth temperature (575-675°C) they kinked and were laying mainly on the {111} planes. When the lateral separation of the monosteps was lowered by optimizing the annealing conditions of the Si substrate and choosing its right slight misorientation, the APBs showed self annihilation in the first 30-50nm of the layer thickness. Above this layer thickness the GaP material was single phase. The crystal polarity of the non-centrosymmetric GaP crystal was revealed by Convergent Beam Electron Diffraction (CBED). Stacking faults and microtwins were identified in cross sectional and plan view weak beam and high resolution TEM imaging. The stacking faults were found to be originating from laterally confined regions of the heterointerface and laying on the P-terminated {111} planes. Thicker bands of microtwins were also found on the P-terminated {111} planes with a width of up to 1000 nm. The formation of stacking faults was attributed to the lateral growth of the nucleated GaP islands when using continuous nucleation scheme at low temperature. The exact origin of the wide microtwins was not revealed, however both stacking fault - and microtwin - free GaP layers were deposited using Flow Rate Modulated Epitaxy (FME).
DOI:https://doi.org/10.17192/z2008.0142