[Show abstract][Hide abstract] ABSTRACT: The application of electrical nanoprobes to measure and characterize nanomaterials has become widely spread. However, the formation of quality electrical contacts using metallic probes on nanostructures has not been directly assessed. We investigate here the electrical behaviour of non-lithographically formed contacts to ZnO nanowires (NWs) and develop a method to reproducibly form Ohmic contacts for accurate electrical measurement of the nanostructures. The contacting method used in this work relies on an electrical feedback mechanism to determine the point of contact to the individual NWs, ensuring minimal compressive strain at the contact. This developed method is compared with the standard tip deflection contacting technique and shows a significant improvement in reproducibility. The effect of excessive compressive strain at the contact was investigated, with a change from rectifying to ohmic I–V behaviour observed as compressive strain at the contact was increased, leading to irreversible changes to the electrical properties of the NW. This work provides an ideal method for forming reproducible non-lithographic nanocontacts to a multitude of nanomaterials.
[Show abstract][Hide abstract] ABSTRACT: Abstract Arrays of CVD catalyst-free ZnO nanowires have been successfully grown without the use of seed layers, using both wet chemical and dry plasma etching methods to alter surface topography. XPS analysis indicates that the NW growth cannot be attributed to a substrate surface chemistry and is therefore directly related to the substrate topography. These nanowires demonstrate structural and optical properties typical of CVD ZnO nanowires. Moreover, the NW arrays exhibit a degree of vertical alignment of less than 20° from the substrate normal. Electrical measurements suggest an improved conduction path through the substrate over seed layer grown nanowires. Furthermore, the etching technique was combined with e-beam lithography to produce high resolution selective area nanowire growth. The ability to pattern uniform nanowires using mature dry etch technology coupled with the increased charge transport through the substrate demonstrates the potential of this technique in the vertical integration of nanowire arrays.
Materials Science and Engineering B 03/2015; 193:41 - 48. DOI:10.1016/j.mseb.2014.11.008 · 2.17 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The surface properties of vertically aligned ZnO nanowires grown by chemical vapour deposition on GaN using a gold layer as a catalyst are investigated by X-ray Photoelectron Spectroscopy as a function of annealing temperature in ultra high vacuum (UHV). The nanowires are 8.5 mum long and 60 nm wide. 87% of the surface carbon content was removed after annealing at 500 °C in UHV. Analysis of the gold intensity suggests diffusion into the nanowires after annealing at 600 °C. Annealing at 300 °C removes surface water contamination and induces a 0.2 eV upward band bending, indicating that adsorbed water molecules act as surface electron donors. The contaminants re-adsorbed after 10 days in UHV and the surface band bending caused by the water removal was reversed. The UHV experiment also affected the nanowires arrangement causing them to bunch together. These results have clear implications for gas sensing applications with ZnO NWs.
[Show abstract][Hide abstract] ABSTRACT: Semiconductor gas sensors based on nanocrystalline SnO2 offer many advantages over current technologies for detecting reducing gases, such as low cost, long lifetime, and high selectivity and sensitivity. However, the local surface properties on the nanoscale of SnO2 nanocrystals are not fully understood, which impedes the exploitation of the full potential of SnO2 for gas sensing applications. In this paper, we present a scanning tunneling microscopy and spectroscopy (STM/STS) study of nanocrystalline SnO2 at room temperature, and under standard sensing conditions at 120°C. STS data indicate that the electronic surface properties change with nanoparticle size, temperature and exposure to gas. The surface density of states in the band gap is shown to increase with temperature while CO exposures induce a large drop in the density of band gap states as the CO molecules react with chemisorbed oxygen species.
International Journal of Nanoscience 11/2011; 03(04n05). DOI:10.1142/S0219581X04002322
[Show abstract][Hide abstract] ABSTRACT: A single-mode buried heterostructure laser has been imaged using Cross-Sectional Scanning Tunneling Microscopy (X-STM). The problem of positioning the tip on the restricted active region on the (110) face has been overcome using combined Scanning Electron Microscopy (SEM). In order to understand the change in the STM scans when biased, particularly the physical change in surface step defects caused by commercial sample preparation, the experimental setup has been modified to allow the sample to be biased. A simpler double quantum well test structure has been biased and it has been demonstrated that it is possible to continue performing STM whilst the device is powered. The change in the relative contrast across the image has been shown to be unaffected by this external bias for the range scanned, as predicted by a fully-coupled Poison drift–diffusion model calculated using Fermi–Dirac statistics.
International Journal of Nanoscience 11/2011; 03(04n05). DOI:10.1142/S0219581X04002334
[Show abstract][Hide abstract] ABSTRACT: Self-assembled ordered arrays of pores are formed when a polymer-solvent solution is deposited in the presence of a humid airflow. These structures can be used as biological scaffolds, photonic bandgap materials and microfluidic beakers. Despite a wealth of material in the published literature regarding the growth of these structures, the dynamics of the process have received little attention from a quantitative perspective. Before the self-assembly mechanism can be understood, it is important to first look at the co-existent driving conditions. Here we develop such a computational model to describe this casting process, which finds excellent agreement with published data. The solvent evaporation profile is found to be near-linear for the majority of the casting process. During this stage a steady-state thermal system exists. The model shows that a humidity threshold exists for the creation of self-assembled structures, with threshold values which find excellent agreement with the literature. Measurement estimates taken of condensate deposition on to the polymer film match the order of magnitude and trend of computational values. Although not given attention in the literature before, slide thickness is shown to be a crucial parameter in this process. The model is able to identify the critical parameters in this system and show which should be controlled and specified to enable experimental results to be repeated. The ability of this model to accurately match experimental results sets it up as the basis for development of a full approach to capture the dynamics of the self-assembly formation process.
[Show abstract][Hide abstract] ABSTRACT: The potential of graphene nanoribbons (GNR's) as molecular-scale sensors is investigated by calculating the electronic properties of the ribbon and the organic molecule ensemble. The organic molecule is assumed to be absorbed at the edge of a zigzag GNR. These nanostructures are described using a single-band tight-binding Hamiltonian. Their transport spectrum and density of states are calculated using the nonequilibrium Green's function formalism. The results show a significant suppression of the density of states (DOS), with a distinct response for the molecule. This may be promising for the prospect of GNR-based single-molecule sensors that might depend on the DOS (e. g., devices that respond to changes in either conductance or electroluminescence). Further, we have investigated the effect of doping on the transport properties of the system. The substitutional boron and nitrogen atoms are located at the center and edge of GNR's. These dopant elements have significant influence on the transport characteristics of the system, particularly doping at the GNR edge.
Physical Review B 01/2011; 83(4):045401. DOI:10.1103/PhysRevB.83.045401 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cross-sectional scanning tunneling microscopy is used to study defects on the surface of semiconductor laser devices. Step defects across the active region caused by the cleave process are identified. Curved blocking layers used in buried heterostructure lasers are shown to induce strain in the layers above them. Devices are also studied whilst powered to look at how the devices change during operation, with a numerical model that confirms the observed behavior. Whilst powered, low-doped blocking layers adjacent to the active region are found to change in real time, with dopant diffusion and the formation of surface states. A tunneling model which allows the inclusion of surface states and tip-induced band bending is applied to analyze the effects on the tunneling current, confirming that the doping concentration is reducing and defect surface states are being formed.
[Show abstract][Hide abstract] ABSTRACT: A technique to generate random fractal aggregates where the fractal dimension is fixed a priori is presented. The algorithm utilizes the box-counting measure of the fractal dimension to determine the number of hypercubes required to encompass the aggregate, on a set of length scales, over which the structure can be defined as fractal. At each length scale the hypercubes required to generate the structure are chosen using a simple random walk which ensures connectivity of the aggregate. The algorithm is highly efficient and overcomes the limitations on the magnitude of the fractal dimension encountered by previous techniques.
[Show abstract][Hide abstract] ABSTRACT: Cross-sectional scanning tunneling microscopy (STM) has been used to study in-operation changes that occur at the active region of clean-cleaved semiconductor laser diodes. A tunneling model that allows the inclusion of tip-induced band bending and surface defect states has been used to study the origin of the surface changes which give rise to the observed modification. Low-doped layers close to the active region are found to undergo both a reduction in doping concentration and an increase in the surface defect state density as the lasers are operated. These changes ultimately lead to device failure. Under different tunneling modes STM can be sensitive to one effect or the other, and the importance of modeling the changes to confirm which are occurring is emphasized.
[Show abstract][Hide abstract] ABSTRACT: We have calculated the electronic structure of ZnO systems doped with Silicon (Si) using generalized gradient approximation. We found that a donor level is formed while Zn is substituted by Si. The variation in band gap is calculated as a function of Si concentration in Zn(1-x)Si(x)O (0 <= x <= 12.5) system and comparisons are made with recently published experimental results. Further, we observed that, substitution of Si at O site forms deep acceptor levels near the top of the valence band, and thereby a weak p-type transformation of the system can be realized.
[Show abstract][Hide abstract] ABSTRACT: We present a scanning tunneling microscopy and spectroscopy (STM–STS) investigation of the effects of ultra high vacuum annealing and oxygen exposure onto ZnO nanoribbons synthesised by chemical vapour deposition. STM imaging revealed a width to height ratio for the nanoribbons between 2:1 and 3:1 and average width and height of 90 and 40 nm, respectively. Imaging before annealing showed the presence of surface contaminants, which were removed after annealing at 800 °C. Analysis of the STS data before annealing shows n-type behaviour with a band gap of 3.4 eV and an upward band bending of 0.9 eV. Annealing up to 700 °C induced a reduction in surface band bending, towards a near flat band behaviour. After the 800 °C anneal the surface electronic properties were significantly altered, with a large increase of tunnelling current at negative sample bias leading to a narrowing of the apparent surface band gap and a mid gap Fermi level. This change was attributed to a loss of surface lattice oxygen and was found to be reversible upon O2 exposure at room temperature. The anneal/O2 exposure process could lead to a way of cleaning the ZnO nanoribbons without using ion bombardment techniques.
Physica Status Solidi (A) Applications and Materials 02/2010; 207(2):282 - 285. DOI:10.1002/pssa.200982485 · 1.62 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present an AFM and STM-STS investigation of the surface of ZnO nanobelts grown by chemical vapour deposition.AFM images showed a type 1 (high aspect ratio) nanobelt lying across a type 2 (low aspect ratio) nanobelt, bending at an angle of 20.9° without breaking.Terraces 10 atomic layer thick were also observed, with step edges running along the  direction.STM images confirmed the AFM results while STS curves and current maps showed higher conductivity for the ZnO nanobelts than for the oxidised silicon surface, as well as an n-type behaviour.
e-Journal of Surface Science and Nanotechnology 04/2009; 7:323-326. DOI:10.1380/ejssnt.2009.323
[Show abstract][Hide abstract] ABSTRACT: Scanning tunnelling microscopy and X-ray Photoelectron Spectroscopy were conducted on magnetron sputtered WO3 thin films, following a sequence of ultra high vacuum anneals from 100 °C to 900 °C. Annealing from 100 °C to 400 °C induced an upward surface band bending of about 0.3 eV, attributed to the oxygen migration from the bulk to the surface, but no changes in the surface topography. Chemical changes occurred from 600 °C to 800 °C, associated with the formation of secondary oxide species. STM imaging showed that the film surface consists of amorphous particles 35 nm in size up to 600 °C, while higher temperatures resulted in an increase in particle size. Crystallisation of the nanoparticles started to occur after annealing at 600 °C. The implications in terms of gas sensing are discussed.
[Show abstract][Hide abstract] ABSTRACT: This paper describes the fabrication of Ni and Ti contacts to single crystal, boron-doped diamond. The electrical performance of metal-diamond contacts has been investigated using current-voltage I(V) characterization of circular transmission line model (CTLM) test structures. X-ray photoelectron spectroscopy (XPS) analysis of Ti/diamond contacts has been performed and is correlated with CTLM results. Post deposition annealing of metal-diamond contacts has a dramatic influence on contact resistivity, with lower resistances observed after annealing at 900°C. Specific contact resistances as low as 9 x 10-5 Ω.cm2 have been obtained. The effect of doping (via epitaxial growth and boron implantation) on metal-diamond contacts is also reported.
Materials Science Forum 01/2009; 615-617:995-998. DOI:10.4028/www.scientific.net/MSF.615-617.995
[Show abstract][Hide abstract] ABSTRACT: Using a near-field scanning optical microscope, near-field photocurrent and topographic imaging has measured the effect on intrinsic electric fields and photocurrent propagation resulting from inserting multi-quantum barrier (MQB) super-lattices into quantum well lasers. Measurements on devices at two different excitation wavelengths have highlighted the sensitivity of the near-field optical technique. Strong correlations were seen in the photocurrent response of the multi-quantum barrier regions when compared with simulations made on the electric field generated within the structure. As a result, photocurrent attenuation was attributed to carrier confinement in these barrier regions when compared to a control sample. The measurements illustrate the effectiveness of the MQB, in addition to the sensitivity and power of the near-field photocurrent technique.
[Show abstract][Hide abstract] ABSTRACT: The silicon carbide (SiC) surface is more complex than that of silicon and can be carbon-terminated or silicon-terminated, and can exist as several reconstructions. Investigations of the surface structure as a function of temperature, under ultrahigh vacuum (UHV) conditions using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED), are presented. The 4H–SiC surface can be passivated using a silicon deposition/evaporation technique to reconstruct the surface. This has a significant effect on the electrical behaviour of metal contacts to the silicon carbide surface, critical in any electronic device. Atomic resolution STM studies of the 4H–SiC surface have revealed step features and micropipe defects in unprecedented detail. STM has also been used to image clusters of metal deposited on the 4H–SiC surface. The effect of annealing on the behaviour of these nickel clusters is also presented. The surface of the silicon carbide is extremely important in the fabrication of silicon carbide electronic devices and this paper presents a discussion of the SiC surface with particular reference to its impact on SiC device applications in power electronics.