Microstructure and Composition of Focused‐Ion‐Beam‐Deposited Pt Contacts to GaN Nanowires

GaN is a technologically important semiconductor with a wide direct bandgap (3.39 eV), and boasts strong light emission in the blue and UV regions of the electromagnetic spectrum. It finds extensive commercial applications in lasers and light-emitting diodes. Because of its high melting temperature, high breakdown field, and high saturation drift velocity, it is a prime candidate for high-temperature, high-voltage, and high-power optoelectronic-device applications. In recent years, research interest in GaN nanowires has increased significantly because, for sufficiently thin nanowires, quantumconfinement effects may be observed, which may lead to novel behavior and applications. GaN nanowires appear to be especially attractive as low-dimensional high-power blue and UV laser light sources, because it is anticipated that high optical gains and low lasing thresholds will be achievable when the nanowire diameter is smaller than the exciton radius. It has been demonstrated that GaN nanowires possess great potential for photonic-, optoelectronic-, and electronic-device applications. For practical device applications, nanowires have to be controllably assembled, precisely located, and individually contacted in order to build device architectures. This continues to be a formidable technical challenge. Traditional lithographic approaches are hindered by the need to register contacts to individual nanowires, although new methodologies to circumvent this limitation have been proposed. Recently, direct writing techniques using focused ion beams (FIBs) have been used to form interconnects to individual nanowires. To do this, an organoplatinum precursor gas flow was directed at the surface of the sample while the target region was irradiated with an energetic particle beam, which decomposed the gas and deposited Pt over the irradiated region. Decomposition of the precursor and Pt deposition can be achieved using focused beams of ions (IBID-Pt, where IBID = ion-beaminduced deposition) or electrons (EBID-Pt, where EBID = electron-beam-induced deposition). The unusually low resistance or ohmic contacts on n-type GaN nanowires suggest that IBID-Pt contacts are different from conventional thin-film Pt contacts, which generally exhibit a large Schottky barrier on n-GaN. Here we present a complete investigation of the structural and chemical character of FIB-deposited metal contacts on GaN nanowires, in order to understand how composition and microstructure affect the electrical performance. Contact structures have been traditionally studied using cross-sectional transmission electron microscopy (TEM). TEM enables imaging, diffraction, and spectroscopy at nearatomic spatial resolution on the same region of interest, providing a wealth of morphological, structural, and compositional information, so that even the most complex contact structures can be readily understood. Although many preparation techniques are capable of creating cross-sections suitable for TEM imaging, these generally offer little control over the region exposed for TEM observation and are, therefore, not useful for preparing cross-sections of our nanowire circuits. The so-called lift-out technique based on FIB machining gives high positional specificity, so we employed it to prepare cross-sections of individual IBID-Pt contacts for cross-sectional TEM imaging (see Experimental for details). Figures 1a,b are a typical pair of cross-sectional images obtained from the pristine nanowire segment away from the C O M M U N IC A TI O N S