rXXXX American Chemical Society A dx.doi.org/10.1021/jp2047823 | J. Phys. Chem. C XXXX, XXX, 000–000 ARTICLE pubs.acs.org/JPCC Characterizing Atomic Composition and Dopant Distribution in Wide Band Gap Semiconductor Nanowires Using Laser-Assisted Atom Probe Tomography Ravi Agrawal, † Rodrigo A. Bernal, † Dieter Isheim, ‡ and Horacio D. Espinosa* ,† † Department of Mechanical Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3111, United States ‡ Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, Illinois 60208-3108, United States b S Supporting Information O ne-dimensional semiconducting nanostructures are envi- sioned as building blocks for emerging high-performance piezoelectric and optoelectronic devices. 1 The exceptionally low defect density, dense columnar morphology, and high light- extraction efficiency of arrayed GaN nanowire LEDs make such technology a compelling new approach to high efficiency solid- state lighting. 2 Use of semiconductor nanoscale morphologies (nanowires, nanobelts, etc.) is also the path being taken for further miniaturization of CMOS devices. The techniques to grow such nanostructures in a controlled manner based on vaporÀ liquidÀsolid (VLS) growth, solidÀliquidÀsolid (SLS) growth, and molecular beam epitaxy (MBE) are quite developed. 3À6 However, to exploit these nanoscale components to their full potential, appropriate characterization of their properties, in- cluding local composition and structure, is necessary. It is well- known that the electronic properties of semiconducting materials can be greatly influenced by the presence of intentional or un- intentional impurities arising from the growth processes. In bulk materials and epitaxial films, techniques such as Hall measure- ments and capacitanceÀvoltage (CÀV) profiles are used to measure free carrier concentration n and infer the doping level (using foreknowledge of dopant ionization). Secondary ion mass spectroscopy (SIMS) is often used to directly measure atomic concentrations of dopants and impurities. Because of the small dimensions and 3D morphology often encountered in nanos- tructures, traditional Hall and CÀV methods may be extremely difficult or impossible to apply. A common technique to estimate n and μ (electron mobility) in semiconductor nanowires (NWs) relies on measuring the behavior of NW field effect transistor (FET) test structures and fitting the experimental results to device simulations. This approach is hampered by a number of factors including the ele- ctrical contact resistivity, the nature of the gate (e.g., back-gated, top-gated, conformally gated, use of a gate oxide layer, or use of a Schottky gated structure) and its influence on parasitic capaci- tance, the true cross-sectional morphology and dimensions of the NW, etc. The complex 3D nature of the device geometry generally necessitates the use of full 3D numerical Poisson and drift-diffusion simulation codes to accurately infer n and μ. 7,8 Nonetheless, depending on the level of approximation and accuracy of the modeling used, variations in fitted values of μ may be rather high; values ranging from 2 to 600 cm 2 V À1 s À1 have been reported. 9 Moreover, other parasitic effects that are difficult to quantify (such as long-lived traps in gate oxides) may Received: May 23, 2011 Revised: July 21, 2011 ABSTRACT: Characterization of atomic composition and spatially resolved dopant distribution in wide band gap semi- conducting nanowires is critical for their applications in next- generation nanoelectronic and optoelectronic devices. We have applied laser-assisted atom probe tomography to measure the spatially resolved composition of wide band gap semiconduct- ing undoped GaN nanowires and Mg-doped GaN nanowires. Stoichiometric evaporation of individual GaN nanowires was achieved, and optimal experimental conditions to characterize the concentration and spatial distribution of the dopant in the Mg:GaN nanowire samples were established. Extremely mild operating conditions, with laser pulse energy as low as 3 pJ, are required to avoid preferential loss of nitrogen and achieve stoichiometric evaporation. The role of nanowire morphology in the selection of optimal experimental conditions is discussed in the context of thermal transport within the nanowire under a heat load imposed by the pulsing laser. The results of this work are expected to help guide similar atom probe tomography studies of related wide band gap IIIÀV semiconductor alloys, which will facilitate a better understanding of material response and will help develop structureÀproperty relationships.