Nanowire growth DOI: 10.1002/smll.200700222 Novel Growth Phenomena Observed in Axial InAs/GaAs Nanowire Heterostructures** Mohanchand Paladugu, Jin Zou,* Ya-Nan Guo, Graeme J. Auchterlonie, Hannah J. Joyce, Qiang Gao, H. Hoe Tan, Chennupati Jagadish,* and Yong Kim Semiconductor nanowires have many potential applications in nanoelectronic and nano-optoelectronic devices owing to their unique physical properties, [1] which have drawn exten- sive research attention in the past decade. The growth of semiconductor nanowire heterostructures has enabled the demonstration of single-nanowire devices, such as field- effect transistors, [2] light-emitting diodes [3] and nanowire res- onant tunneling diodes. [4] Nanowire heterostructures of III– V materials are of particular interest for their optoelectronic applications. So far, III–V nanowire axial heterostructures of GaP/GaAs, [5] InP/InAs, [6] and their related ternary alloys [7] have been studied systematically to understand their growth behavior, hetero-interfacial structure and chemistry, and their properties. As a key semiconductor het- erostructure system for optoelectronic applications, [8] two- dimensional (2D) In x Ga 1x As/GaAs (x  1) quantum-well heterostructures have been studied extensively in the past. [9] In comparison, 1D InAs/GaAs nanowire heterostructures have been less studied [10] after the first report by Hiruma et al. about a decade ago, [11] in spite of their promising phys- ical properties and potential optoelectronic applications, as in the case of their 2D counterparts. The vapor–liquid–solid (VLS) mechanism [12] has been a widely used mechanism for the growth of semiconductor nanowires and their heterostructures. [5] In typical VLS growth, nanosized metal-alloy liquid droplets form and then catalyze nanowire growth, so that the nanowires and their associated axial heterostructures have metal particles at their growth front. [5] During VLS growth of nanowire heteroACHTUNGTRENNUNGstructures, changes in nanowire growth directions have been occasionally observed, for example, in the case of InAs/InP nanowire heterostructures. [13] Since such a change is often accompanied by changes in the physical properties of the nanowire, it is scientifically important and technologi- cally necessary to understand the driving force behind these changes. We use transmission electron microscopy (TEM) to study the changes in the InAs growth direction when it is grown on GaAs nanowires, and we account for this phe- nomenon in terms of the fundamental growth mechanism. This change in the growth direction ultimately leads to the failure of InAs axial growth on the GaAs nanowires. The growth of InAs/GaAs nanowire heterostructures was catalyzed by Au particles with a nominal size of  30 nm in a horizontal-flow metal-organic chemical vapor deposition (MOCVD) reactor at 100 mbar with a growth temperature of 450 8C. The detailed process and growth pa- rameters for the nanowire growth can be found in Ref. [14]. Initially, GaAs nanowires were grown on a {111}B (= (  1  1  1)) GaAs substrate for 30 min by flowing trimethylgallium (TMG) and AsH 3 at flow rates of 1.210 5 and 5.4 10 4 molmin 1 , respectively. To study the initial growth be- havior of InAs on GaAs nanowires, InAs nanowire sections were grown for only 1 min on the GaAs nanowires by switching off the TMG flow and switching on a trimethylin- dium (TMI) flow at 1.210 5 molmin 1 while maintaining the AsH 3 flow rate. The fabricated nanowire heterostructures were charac- terized by scanning electron microscopy (SEM, JEOL 890) and TEM (Tecnai F30 and Tecnai F20 equipped with scan- ning transmission electron microscopy (STEM) and energy dispersive spectroscopy (EDS) facilities). TEM specimens were prepared by ultrasonicating the nanowires in ethanol for 10 min followed by dispersal onto holey carbon films. Figure 1a is an SEM image showing the typical mor- phology of the InAs/GaAs nanowire heterostructures. SEM investigations showed that a) the diameters of the nanowires varied between 30–90 nm and b) thinner nanowires tend to bend more than the thicker ones. In general, the nanowire diameter is close to the size of the catalyst, that is, the Au nanoparticles. The nanowire growth was initiated by  30 nm Au particles, while the electron microscopy analysis showed a significant amount of thicker nanowires, with di- ameters up to 90 nm, probably due to the coalescence of Au particles during heating in the growth chamber. In addition, SEM studies also showed that all of the nanowires are free- standing with tapered bodies and thick tip portions (top re- gions), with some tips indicated by the arrows. However, it is difficult to clarify the structure and chemistry of the tip region of the nanowires by SEM investigations. To under- stand their detailed structural characteristics, TEM investi- gations were carried out on a larger number of nanowires and a typical example is shown in Figure 1b. From this image, two nanowires with different diameters can be clear- ly seen. In the case of the thinner nanowire, the Au particle has moved away from the tip region down the nanowire. In [*] M. Paladugu, Prof. J. Zou, Y.-N. Guo School of Engineering The University of Queensland, Brisbane (Australia) Fax: (+ 61)733-654-422 E-mail: j.zou@uq.edu.au H. J. Joyce, Dr. Q. Gao, Dr. H. Hoe Tan, Prof. C. Jagadish Department of Electronic Material Engineering Research School of Physical Sciences and Engineering The Australian National University Canberra (Australia) Fax: (+ 61)261-250-511 E-mail: Chennupati.jagadish@anu.edu.au Prof. J. Zou, G. J. Auchterlonie Centre for Microscopy and Microanalysis The University of Queensland, Brisbane (Australia) Prof. Y. Kim Department of Physics Dong-A University, Busan (Korea) [**] This research is supported by the Australian Research Council. M. Paladugu acknowledges the support of an International Post- graduate Research Scholarship. small 2007 , 3, No. 11, 1873 – 1877 # 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1873