Evolution of Wurtzite Structured GaAs Shells Around InAs Nanowire Cores
© to the authors 2009
Received: 20 January 2009
Accepted: 22 April 2009
Published: 6 May 2009
GaAs was radially deposited on InAs nanowires by metal–organic chemical vapor deposition and resultant nanowire heterostructures were characterized by detailed electron microscopy investigations. The GaAs shells have been grown in wurtzite structure, epitaxially on the wurtzite structured InAs nanowire cores. The fundamental reason of structural evolution in terms of material nucleation and interfacial structure is given.
KeywordsNanowire heterostructures GaAs/InAs Crystal structure
Semiconductor nanowires and their associated heterostructures are ideal candidates to achieve one-dimensional quantum confinement in materials, and thereby they are ideal candidates to explore the physical properties of materials in one-dimension [1, 2]. Promising physical properties and wide variety of applications were demonstrated using these semiconductor nanostructures [1, 2]. Many nanowire based devices have been demonstrated, including nanowire diodes , photodiodes , single-electron transistors , and field-effect transistors [6, 7]. Various mechanisms have been used to synthesize these semiconductor nanowires, such as vapor–liquid–solid (VLS), vapor–solid, oxide-assisted and solution–liquid–solid . Nanowires growth via the VLS mechanism  offers the opportunity to produce axial [10, 11], radial [12, 13], and branched  nanowire heterostructures with control over the nanowire size, shape, and location . As a consequence, VLS mechanism has been the most widely used mechanism for nanowires growth. Radial nanowire heterostructures which consist of core, shell, and multi-shell morphologies, offer the flexibility to tailor the bandgap structure of radial nanowire heterostructures . Such flexibility can be used to tune the desired electrical and optical properties.
Many semiconductors of III–V and II–VI compounds can adopt the polytypism of zinc-blende/wurtzite crystal structures based on their growth conditions and difference in the internal energies of two crystal structures for a specific material [17, 18]. These crystal structures differ by the stacking sequence of their dense atomic planes. Zinc-blende structure has …ABCABC… stacking sequence along 〈111〉 directions, whereas wurtzite structure has …ABABAB… stacking sequence along 〈0001〉 directions. This polytypism gives rise to different band structures depending upon its crystal structure, which, in turn, allows the realization of polytype superlattice structures [19, 20]. Polytypism is often observed when these semiconductors grown in the form of nanowires, especially using Au nanoparticle catalysts. For example, InAs nanowires grown via metal–organic chemical vapor deposition (MOCVD) method generally show wurtzite structure, whereas, GaAs nanowires retain its bulk zinc-blende crystal structure [11, 14]. GaAs is a wide bandgap semiconductor material, and InAs has a narrow bandgap. Since the nanowires can act as ideal one-dimensional materials, novel physical properties can be achieved when InAs nanowires sheathed with GaAs. The resultant GaAs/InAs core/shell nanowire structures can give interesting optical and electronic properties of interest to device applications. Since both the semiconductor materials show different crystal structures when they grow axially, it will be scientifically important and technologically necessary to explore how they behave when they grow laterally.
In this study, we grow InAs/GaAs core–shell structures using MOCVD method. They are characterized by detailed transmission electron microscopy (TEM), in terms of their compositional and structural characteristics.
InAs/GaAs core/shell nanowire heterostructures were grown in a horizontal flow MOCVD reactor at 100 mbar with a growth temperature of 450 °C. Firstly, InAs nanowires were grown for 30 min on a GaAs substrate using Au catalysts with a nominal size of ~30 nm by flowing trimethylindium (TMI) and AsH3. GaAs is then deposited on these nanowires for 30 min by switching off the TMI flow and switching on the trimethylgallium (TMG) flow. Flow rates of TMI, TMG, and AsH3are 1.2 × 10−5, 1.2 × 10−5, and 5.4 × 10−4 mol/min, respectively. The fabricated nanowire heterostructures were characterized by scanning electron microscopy (SEM, JEOL 890) and TEM [Tecnai F20]. TEM specimens were prepared by ultrasonicating the nanowires in ethanol for 10 min followed by dispersal onto holey carbon films.
Results and Discussion
In the case of lateral direction (Fig. 3b) on the other hand, both crystal structures cannot have a lattice registry except for each sixth layer, as shown by the arrows. In fact, lack of this lattice registry would cause high energy heterointerfaces, and it is observed that such energetic conditions would transform the wurtzite structure into zinc-blende structure when the wurtzite nanowires are sheathed with two-dimensional zinc-blende layers . Similarly, in our current study, the normally zinc-blende GaAs structure transforms into a wurtzite structure when brought into contact with the InAs NW side walls, by nucleating epitaxially on the nanowire sidewalls.
We have grown InAs/GaAs core/shell structures using MOCVD method, and the transmission electron microscopy investigations show that both the core and shell contain wurtzite structure. In contrast, when InAs/GaAs heterostructures grow in 〈111〉 or 〈0001〉 axial directions, both the materials can have different crystal structures. This structural difference between both the axial and lateral direction is explained in terms of crystallography and interfacial structure.
The Australian Research Council is acknowledged for the financial support of this project. M. Paladugu acknowledges the support of an International Postgraduate Research Scholarship. The Australian National Fabrication Facility established under the Australian Government’s National Collaborative Research Infrastructure Strategy is gratefully acknowledged for access to the facilities used in this study.
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