Transmission electron microscopy study of novel semiconductor heterostructures and high Tc superconductors

The microstructures of novel semiconductor heterostructures which have applications in optical, electronic and magnetic-opto devices and high Tc superconductors are studied by transmission electron microscopy (TEM). GaN based compounds are the most topical, presently being developed III-V group materials for blue light emitting diodes and blue lasers. The microstructures of GaN grown on different substrates under different growth conditions by molecular beam epitaxy are characterised by various TEM techniques. Zincblende GaN is found to form immediately in the presence of an As flux, while wurtzite GaN forms in the absence of As for the samples grown on (001)GaAs and GaP substrates. Defects in zincblende GaN are mainly planar defects, such as microtwins and stacking faults, while for the wurtzite GaN grown on #left brace#1-bar1-bar1-bar#right brace#B GaAs or GaP substrates, the crystal is of mosaic structure with two types sub-grain boundaries, One is the threading dislocation aligned boundary, in which most dislocations are of mixed-type. The other is the double positioning boundary, one of which has a boundary plane on #left brace#112-bar0#right brace# originating from the single stacking faults on basal plane, and the other one on #left brace#11-bar00#right brace# emanating from the epilayer/substrate interface. They both are formed by island coalescence. Dislocation configurations in a Si/Si_0_._8_8Ge_0_._1_2/(001)Si heterostructure are studied in detail by conventional and high resolution TEM. There are fewer misfit dislocations in the as-grown sample as compared with an annealed sample in which an extensive misfit dislocation network is present. Some dislocations are found to reside in the Si substrate and Si cap with few in the SiGe layer. The exact Burgers vectors of the dislocations in the substrate are determined by g.b analysis combined with large angle convergent beam electron diffraction method and are found to have the same sense of Burgers vector as the misfit dislocations located at the interfaces, A possible misfit dislocation nucleation mechanism is considered. Discussion is also given to the dissociation of misfit dislocations in compressively strained epilayers and an uncommon feature of the two partial dislocations which are associated with two tails. A brief study of MnSb/(001)GaAs grown by hot wall epitaxy is also presented. Three distinct regions were observed, i.e. V-grooves of MnSb within the GaAs substrate, small GaSb particles and MnSb surface islands with two distinct orientations. The nucleation of MnSb islands is discussed. Finally, the cleavage surface structures of high Tc YBa_2Cu_4O_8 particles are studied by high resolution TEM combined with image simulations. The surface of this material is found to terminate with the CuO_2 plane corresponding to a sequence of bulk-Y-CuO_2. There is some evidence of partial oxygen loss from this terminating plane. The different terminating surfaces of (001) cleavage planes of YBa_2Cu_4O_8 and YBa_2Cu_3O_7 are discussed. (author)Available from British Library Document Supply Centre-DSC:D208523 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo