Transmission Electron Microscopy of III-V Nanowires and Nanotrees

Abstract: In this work, the morphology and crystal structure of epitaxial semiconductor nanowire structures grown by metal-organic vapour phase epitaxy (MOVPE) are studied by electron microscopy methods. In particular, the three-dimensional structure of nanowires and nanotrees has been characterised by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and multi-slice (MS) simulations. It has been found that the repeated lamellar twinning often observed in III-V <111>B nanowires tends to form a morphology bound by {111} facets only. When this structure is viewed parallel to the twin planes in <-110>, a zig-zag appearance is found as two of the {111} facet types are in the zone. When rotated 30 degrees around the growth axis to a <11-2> direction the edges between the {111} facets align to seemingly flat side facets. Also, the twin planes are clearly distinguishable in <-110> but are less pronounced in <11-2>, due to alignment of the atomic columns in the latter case. Two possible twin types have previously been reported in literature, a rotation twin 60 degrees around the growth axis of the nanowire and a mirror twin 180 degrees over the twin plane. The dominating type in this study, as found from MS-simulations, is the rotation twin type with III-V bonding over the twin plane. Viewing directions of the nanowires non-parallel to the twin plane will show two different crystal orientations in neighbouring twin segments, as a consequence of twinning. These are crystallographically related as they share a common (111) twin plane and can be indexed based on stereographic projections. MS-simulations showed that both twin types would have the same appearance in HRTEM-images of the studied low index zones <-100>, <-1-10>, <1-1-1> and <1-1-2>. The corresponding zone axes in the neighbouring segments were indexed to <1-2-2>, <11-4> and <11-5> in the first three cases. Moreover, the appearance of the nonoverlapping regions in these viewing directions confirmed a suggested three-dimensional model with only {111} facets. To mediate the nanowire growth, Au seed nanoparticles are often used to define the position and diameter of the nanowires. The influence of the interaction between the Au seed nanoparticles and the substrate and nanowire structures during growth was investigated for GaAs. The effects of annealing prior to growth and of different termination procedures during cooling after growth were studied by ex situ EDS-analysis. It was found that the Au seed particles are stable in contact with GaAs nanowires when kept at As-rich conditions during annealing, growth and cooling. However, if Ga-rich conditions are used, alloying between Au and Ga occurs readily and the composition of the seed nanoparticles changes with growth temperature. These observations are consistent with the ternary Au-Ga-As phase diagram where a Au-GaAs-As tie-triangle dominates at As-rich conditions and GaAs alloy tie-lines are found on the Ga-rich side. Therefore, it was concluded that the Au seed particles are solid during growth, in our MOVPE-system, but that the situation can change if alloying is allowed to occur during annealing. For Au-In-As the situation is different as there is no tie-line of Au-InAs and the seed particles will alloy more readily with the substrate. In fact, to stabilise the growth of InAs nanotrees pre-alloyed Au-In seed nanoparticles were used. By sequential nanowire growth using previously grown nanowires as free-standing substrates more complex, so called nanotree structures, can be grown. Due to the similarity of a tree the respective growth sequences are labelled trunk, branch, and leaf. For III-V nanotrees of predominantly zinc blende structure the nanowire growth was found to adopt the <111>B growth directions irrespective of growth sequence. However, a single crystalline transition section was found close to the base of branches and/or leaves nucleating on a twinned region. This section was believed to form during low supersaturation conditions in the initial stage of growth. If nucleation occurred on a single crystalline section, fully single crystalline leaves would form. Similarly, when nucleating at the very base of a branch, fully twinned leaves would grow. Heteroepitaxial growth of InP on GaP resulted in a topotaxial growth behaviour with InP branches or leaves crawling parallel to or spiralling around the previously grown nanowires. Only the top section of the previously grown GaP nanowires would form InP in the <111>B growth direction. This led to the conclusion that the nucleation conditions of the two generations are different. Thanks to the limited diameter of the nanowire structures, HRTEM-images with atomic resolution can be obtained on as grown samples with minimal sample preparation. Based on the appearance of these images, the 3D structure of the nanowires can be characterised using MS-simulations. Also, Moiré patterns and double diffraction of overlapping crystalline materials can be used to determine the relative orientation of sequentially grown nanowire structures as well as neighbouring twin segments.