Raman spectroscopy and electrical properties of InAs nanowires with local oxidation enabled by substrate micro-trenches and laser irradiation

Abstract

The thermal gradient along indium-arsenide nanowires was engineered by a combination of fabricated micro-trenches in the supporting substrate and focused laser irradiation. This allowed local control of thermally activated oxidation reactions of the nanowire on the scale of the diffraction limit. The locality of the oxidation was detected by microRaman mapping, and the results were found consistent with numerical simulations of the temperature profile. Applying the technique to nanowires in electrical devices the locally oxidized nanowires remained conducting with a lower conductance as expected for an effectively thinner conducting core.Semiconductor nanowires (NWs) offer a range of properties which makes them attractive to diverse areas of research and technology. The reduced size and high surfaceto-volume ratio make them promising candidates for advanced electronics . Also, the confinement offered by the NW geometry may allow devices providing greatly enhanced thermoelectric properties 7,8 . In order to exploit the potential of semiconductor NWs for devices with improved or new functionalities, methods of locally controlling the properties along the NWs are often required. Such control has been demonstrated either by advanced device architectures incorporating local electrostatic gates 9,10 or by modifying the crystal properties along the wire by changing material composition during or after growth [11][12][13] . An alternative approach which has recently emerged is the possibility of using a high intensity focused laser beam to locally induce chemical changes in the NW 14 . By combination of Raman spectroscopy 15 and transmission electron microscopy 16 it has been established that for the case of InAs the high intensity laser, in the presence of ambient air conditions, promotes the oxidation process As 2 O 3 +2InAs→In 2 O 3 +4As which converts the irradiated parts of the NW surface to crystalline arsenic and polycrystalline indium oxide 16. However, so far, only little is known about the properties of the resulting structures and the spatial resolution that can be achieved has not been analyzed. A similar process occurs for GaAs, and Yazji, S. et al 17 showed that the thermal conductivity of the NW is dramatically decreased upon irradiation, suggesting this method as a way of designing structures with enhanced thermoelectric properties.Here we show that by suspending NWs over microtrenches etched into a SiO 2 substrate the temperature profile of the NW can be locally engineered. By tuning the laser intensity, only the suspended parts of the wires will reach a temperature sufficient to activate the oxidation process.Using Raman spectroscopy we demonstrate the viability of this approach, achieving a spatial resolution of at least 250 nm; the size of the detection area of our Raman setup. Finally, we study the electrical properties of the oxidized NWs and show that they maintain their ability to carry electrical current and also preserve the key semiconductor property of electrostatic tunability using a nearby ...

Morphology and composition of oxidized InAs nanowires studied by combined Raman spectroscopy and transmission electron microscopy

“…The oxidation reaction accelerates with increased temperature, and we attribute the appearance of the arsenic spectra as the result of laser induced heating [30,31], possibly combined with photo-oxidation [28]. The activation of the oxidation reaction can be followed by the power dependence of the relative intensities of the fitted arsenic and InAs Raman peaks presented in figure 1(d) measured, using 520.8 nm laser excitation and 15 min integration time per spectrum, of a NW suspended over a 150 nm deep and 2 μm wide trench [21]. Initially, at the lowest powers, the arsenic peaks are absent, but then irreversibly appear after applying illuminating powers above 120 kW cm −2 .…”

“…Figure 1(b) shows an example of a TEM image of the NWs showing high crystal quality and occasional stacking faults, and figure 1(c) shows typical Raman spectra for the two cases of a pristine InAs NW on Si substrate [21] and an oxidized InAs NW on a TEM membrane. The spectra of the nonoxidized NW is identical to bulk InAs and contain one main peak at ∼216 cm −1 assigned to the InAs transverse optical (TO) phonon mode and a weak side-peak at 237 cm −1 assigned to the longitudinal optical (LO) mode.…”

“…The NWs contain occasional random stacking-faults as seen in figure 1(b). To study the effect of crystal orientation and faceting, branched InAs NWs were grown with a V/III ratio of 40 and corresponding planar growth rate of 0.35 μm h −1 with a substrate temperature of T=425°C, in a two-step method: first, a conventional [ ] 0001 stem was grown and secondly, triggered by introducing a short Ga flux pulse, the catalyst gold droplet moved from the top to a side facet of Samples were prepared for Raman and TEM measurements by ultrasonically suspending NWs from the growth substrate in isopropanol and depositing them either on substrates of 50 nm thick Si 3 N 4 membranes 6 allowing a combination of TEM and Raman analysis, or on Si substrates with a 500 nm silicon oxide cap with or without chemically etched micro-trenches [21]. Figure 1(a) schematically shows the setup and introduces the coordinate system used in the following.…”

“…To this end, locally accelerating the reaction by focused laser irradiation has been demonstrated and can be combined with micro-Raman spectroscopy to provide information about the vibrational modes of individual semiconductor nanowires [8][9][10][11] and also conveying information about the elemental composition [12,13], crystal properties [14], and defects [15] as well as free carrier densities and mobilities [16]. Also, at high intensities, focused laser irradiation has been shown to enable controlled local cutting or chemical modification of InAs and GaAs nanowires by local oxidation and formation of crystalline arsenic [17][18][19][20][21] and enables welding of metal nanowires [22]. This opens the possibility for using focused laser beams for simultaneous micro-Raman characterization and engineering new functionality into nanowire devices.…”

Photon Energy Dependent Micro-Raman Spectroscopy with a Continuum Laser Source

“…Besides the typical photon energy (~ν 4 ) dependence of the Raman signal 1 , many materials 2 – 4 as well as nanomaterials, such as quantum dots 5 , 6 , carbon nanotubes 7 – 11 , semiconductors 12 , semiconducting nanowires 13 , 14 and graphene 15 , 16 show resonances at specific photon energy ranges accompanied by strong enhancement of the Raman signal by several orders of magnitude 2 ,3, 9 , 16 – 19 . Accurate determination of these resonances requires light sources with large tunable energy ranges and narrow emission lines.…”