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International audience; Polarity-controlled growth of ZnO by chemical bath deposition provides a method for controlling the crystal orientation of vertical nanorod arrays. The ability to define the morphology and structure of the nanorods is essential to maximizing the performance of optical and electrical devices such as piezoelectric nanogenerators; however, well-defined Schottky contacts to the polar facets of the structures have yet to be explored. In this work, we demonstrate a process to fabricate metal−semiconductor−metal device structures from vertical arrays with Au contacts on the uppermost polar facets of the nanorods and show that the Opolar nanorods (∼0.44 eV) have a greater effective barrier height than the Znpolar nanorods (∼0.37 eV). Oxygen plasma treatment is shown by cathodoluminescence spectroscopy to affect midgap defects associated with radiative emissions, which improves the Schottky contacts from weakly rectifying to strongly rectifying. Interestingly, the plasma treatment is shown to have a much greater effect in reducing the number of carriers in O-polar nanorods through quenching of the donor-type substitutional hydrogen on oxygen sites (HO) when compared to the zinc-vacancy-related hydrogen defect complexes (VZn −nH) in Zn-polar nanorods that evolve to lowercoordinated complexes. The effect on HO in the O-polar nanorods coincides with a large reduction in the visible-range defects, producing a lower conductivity and creating the larger effective barrier heights. This combination can allow radiative losses and charge leakage to be controlled, enhancing devices such as dynamic photodetectors, strain sensors, and light-emitting diodes while showing that the O-polar nanorods can outperform Zn-polar nanorods in such applications.

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.

Manufacturable nanodevices must now be the predominant goal of nanotechnological research to ensure the enhanced properties of nanomaterials can be fully exploited and fulfill the promise that fundamental science has exposed. Here, we test the electrical stability of Au nanocatalyst-ZnO nanowire contacts to determine the limits of the electrical transport properties and the metal-semiconductor interfaces. While the transport properties of as-grown Au nanocatalyst contacts to ZnO nanowires have been well-defined, the stability of the interfaces over lengthy time periods and the electrical limits of the ohmic or Schottky function have not been studied. In this work, we use a recently developed iterative analytical process that directly correlates multiprobe transport measurements with subsequent aberration-corrected scanning transmission electron microscopy to study the electrical, structural, and chemical properties when the nanowires are pushed to their electrical limits and show structural changes occur at the metal-nanowire interface or at the nanowire midshaft. The ohmic contacts exhibit enhanced quantum-mechanical edge-tunneling transport behavior because of additional native semiconductor material at the contact edge due to a strong metal-support interaction. The low-resistance nature of the ohmic contacts leads to catastrophic breakdown at the middle of the nanowire span where the maximum heating effect occurs. Schottky-type Au-nanowire contacts are observed when the nanowires are in the as-grown pristine state and display entirely different breakdown characteristics. The higher-resistance rectifying I-V behavior degrades as the current is increased which leads to a permanent weakening of the rectifying effect and atomic-scale structural changes at the edge of the Au interface where the tunneling current is concentrated. Furthermore, to study modified nanowires such as might be used in devices the nanoscale tunneling path at the interface edge of the ohmic nanowire contacts is removed with a simple etch treatment and the nanowires show similar I-V characteristics during breakdown as the Schottky pristine contacts. Breakdown is shown to occur either at the nanowire midshaft or at the Au contact depending on the initial conductivity of the Au contact interface. These results demonstrate the Au-nanowire structures are capable of withstanding long periods of electrical stress and are stable at high current densities ensuring they are ideal components for nanowire-device designs while providing the flexibility of choosing the electrical transport properties which other Au-nanowire systems cannot presently deliver.

Multi-probe instruments based on scanning tunnelling microscopy (STM) are becoming increasingly common for their ability to perform nano- to atomic-scale investigations of nanostructures, surfaces and in situ reactions. A common configuration is the four-probe STM often coupled with in situ scanning electron microscopy (SEM) that allows precise positioning of the probes onto surfaces and nanostructures enabling electrical and scanning experiments to be performed on highly localised regions of the sample. In this paper, we assess the sensitivity of four-probe STM for in-line resistivity measurements of the bulk ZnO surface. The measurements allow comparisons to established models that are used to relate light plasma treatments (O and H) of the surfaces to the resistivity measurements. The results are correlated to x-ray photoelectron spectroscopy (XPS) and show that four-probe STM can detect changes in surface and bulk conduction mechanisms that are beyond conventional monochromatic XPS.

Selecting the electrical properties of nanomaterials is essential if their potential as manufacturable devices is to be reached. Here, we show that the addition or removal of native semiconductor material at the edge of a nanocontact can be used to determine the electrical transport properties of metal–nanowire interfaces. While the transport properties of as-grown Au nanocatalyst contacts to semiconductor nanowires are well-studied, there are few techniques that have been explored to modify the electrical behavior. In this work, we use an iterative analytical process that directly correlates multiprobe transport measurements with subsequent aberration-corrected scanning transmission electron microscopy to study the effects of chemical processes that create structural changes at the contact interface edge. A strong metal–support interaction that encapsulates the Au nanocontacts over time, adding ZnO material to the edge region, gives rise to ohmic transport behavior due to the enhanced quantum-mechanical tunneling path. Removal of the extraneous material at the Au–nanowire interface eliminates the edge-tunneling path, producing a range of transport behavior that is dependent on the final interface quality. These results demonstrate chemically driven processes that can be factored into nanowire-device design to select the final properties.

ZnO nanowire (NW) arrays were examined with Transmission Electron Microscopy (TEM) in cross-section after preparation by Focused Ion Beam (FIB) milling. This technique revealed that ZnO nanowires grown using a Au catalyzed vapor technique typically have Au particles at the NW tips, and also randomly dispersed across the base crystal growth that joins adjacent NWs. It is shown the adjacent NWs and the combined base growth is one crystal structure which can be used as a back electrical contact making fabrication of vertical array devices possible. However, the base growth displays detrimental features such as embedded Au particles and lattice defects which can affect the electrical output through depletion regions and scattering centers. In an effort to overcome these problems we investigate a growth method that is nucleated through a minor alteration of the a-plane sapphire surface roughness via a weak chemical etch. Observations of various stages of the growth show the growth nucleates as separate nanoislands that grow in c-plane alignment with Sapphire (1-210), and as growth continues these islands meet and form a polycrystalline film. Further growth initiates nanowire growth and the formation of a single crystal base layer and NW structure that can cover several square millimeter’s. This allows high quality arrays that are relatively free from defects to be formed without any metals contamination and ready for further device processing.

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.

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.

Single cell encoding with quantum dots as live cell optical tracers for deriving proliferation parameters has been developed using modelling to investigate cell cycle and proliferative outputs of human osteosarcoma cells undergoing mitotic bypass and endocycle routing. A computer-based simulation of the evolving cell population provides information on the dilution and segregation of nanoparticle dose cell by cell division and allows quantitative assessment of patterns of division, at both single cell and including whole population level cell cycle routing, with no a-priori knowledge of the population proliferation potential. The output therefore provides a unique mitotic distribution function that represents a convolution of cell cycle kinetics (cell division) and the partitioning coefficient for the labelled cell compartment (daughter-daughter inheritance or lineage asymmetry). The current study has shown that the cellular quantum dot fluorescence reduced over time as the particles were diluted by the process of cell division and had the properties of a non-random highly asymmetric event. Asymmetric nanoparticle segregation in the endosomal compartment has major implications on cell-fate determining signaling pathways and could lead to an understanding of the origins of unique proliferation and drug-resistance characteristics within a tumour cell lineage.

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 3.4eV 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 O 2 exposure at room temperature. The anneal/ O 2 exposure process could lead to a way of cleaning the ZnO nanoribbons without using ion bombardment techniques.

This paper reports the investigation of epitaxial graphene growth on 4H-SiC substrates. Growth has been performed under ultra high vacuum (UHV) conditions at temperatures ranging from 1150 to 1250°C, and the formation of the graphene layer has been monitored using X-ray photoelectron spectroscopy (XPS). A gradient of 100°C in temperature was introduced across the sample in order to grow a wide range of thicknesses along the sample. Atomic force microscopy (AFM) of the surface shows that the epitaxial graphene layer follows the topography of the bulk material and introduces very little surface roughness. This paper also reports the electrical characterisation of the graphene layers.

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.

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.

Au/Cr Ohmic contacts on p-type, heavily boron-doped, single-crystal CVD diamond have been studied using X-ray photoelectron spectroscopy (XPS) and electrical measurements. The interaction of chromium, a carbide-forming metal, with the diamond surface is discussed. The Cr/diamond contact formation process has been studied as a function of chromium thickness and post-metal deposition annealing. These data are correlated with specific contact resistances. The effect of annealing on the reaction between chromium and diamond has also been assessed by examining surfaces after removing the metal contact through wet etching. After removal of the metal the surface morphology of the diamond surface preparation was examined using AFM. Finally, new contacts have been fabricated on the reconditioned diamond surface and electrically characterised in order to assess the effect of surface roughness on the contact properties. These studies indicate that a significant interaction between carbon and Cr occurs during annealing of temperatures of greater than 400 °C.

The ability to control the properties of electrical contacts to nanostructures is essential to realize operational nanodevices. Here, we show that the electrical behavior of the nanocontacts between free-standing ZnO nanowires and the catalytic Au particle used for their growth can switch from Schottky to Ohmic depending on the size of the Au particles in relation to the cross-sectional width of the ZnO nanowires. We observe a distinct Schottky to Ohmic transition in transport behavior at an Au to nanowire diameter ratio of 0.6. The current-voltage electrical measurements performed with a multiprobe instrument are explained using 3-D self-consistent electrostatic and transport simulations revealing that tunneling at the contact edge is the dominant carrier transport mechanism for these nanoscale contacts. The results are applicable to other nanowire materials such as Si, GaAs, and InAs when the effects of surface charge and contact size are considered.

Schottky contacts on 4H-SiC surfaces, annealed at high temperature in the presence of a protective carbon cap, are compared to contacts fabricated by the same process, without the protective layer. Annealing at 1600 °C is required to electrically activate implanted dopants, but causes severe surface damage. We show using surface morphology studies, based on atomic force microscopy (AFM), that carbon-cap protected 4H-SiC had a considerably lower surface roughness than surfaces annealed without the protective cap. Nickel diodes fabricated on 4H-SiC samples, after removal of the carbon-cap using oxygen plasma etching, showed similar Schottky I(V) forward bias characteristics to contacts on samples that had not been annealed, and displayed similar reverse blocking voltages. Carbon cap protected samples also displayed lower reverse leakage currents than un-annealed samples. In contrast, contacts fabricated on unprotected surfaces, damaged by annealing at 1600 °C, displayed non-rectifying behaviour.

Atomic force and high resolution scanning tunneling analyses were carried out on nanostructured WO3 films. It turned out that the band gap measured by scanning tunneling spectroscopy at surface is lower than the band gap reported in the literature. This effect is attributed to the high density of surface states in this material, which allows tunneling into these states. Such a high density of surface states pins the Fermi level resulting in modest surface activity at room temperature. Photo activation of WO3 results in unpinning of the Fermi level and thereby in higher chemical activity at surface.

An investigation of the structures and design parameters of 4H–SiC trench inversion-channel MOSFETs using a two-dimensional (2D) numerical device simulation is presented. Material parameters have been adjusted appropriately for the 4H–SiC polytype and a systematic characterisation and optimization of a specific trench MOSFET with a 1.2 kV blocking voltage capability has been performed. Simulations have concentrated on optimizing the p-type doping concentration at the trench bottom, to keep the breakdown electric field in the oxide under its critical value. The trench depth was also examined and optimized to give a better on-state performance. For the MOSFET structure examined, a minimized on-state resistance of 53 mΩ cm2 was obtained.

Cross-sectional scanning tunnelling microscopy (STM) images on a freshly cleaved III–V semiconductor laser facet regularly indicate atomic scale defects, such as steps and vacancies on the non-polar (1 1 0) surface. In this work, studies were made when sub-monolayer quantities of Si were deposited at 280 °C on clean cleaved GaAs(1 1 0). Selective adsorption of Si on atomic defects has been observed with STM imaging of the surface. The simultaneous use of STM and scanning tunnelling spectroscopy (STS) allows the quantification of the Fermi shifts present at atomic steps and vacancies before and after Si adsorption. Localised current–voltage measurements, performed using STS, on the atomic steps and vacancy defect clusters of p-type GaAs(1 1 0) surfaces showed Fermi-level shifts of ∼0.8 and 0.7 eV towards mid-gap position, respectively. However, measurements taken from silicon-coated atomic steps and defect clusters showed that the Fermi-level reverted back towards the ‘ideal’ flat band position by ∼0.4 eV. This behaviour was distinctively different to silicon adsorbed at defect-free surface of GaAs(1 1 0). These results indicated the passivating potential of silicon on the defect sites of GaAs(1 1 0) surface when deposited under these conditions. The ability to measure and control surface properties on the atomic scale is crucial to the success of nano-technology and the STM/STS studies demonstrate the effectiveness of the technique for this role.

In this work, we report on the ability to write features less than 15 nm in size on nanocrystalline SnO 2 by applying negative voltage pulses onto an STM tip. The fact that these features can be erased by scanning with a positive tip bias, and the strong dependence of the apparent height of the features with scanning bias after writing seems to indicate that a degree of charge confinement within the 8 nm nano-crystals is involved.

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.

The electronic changes occurring at the surface of a nano-crystalline SnO 2 sensor upon exposure to O 2 and CH 4 at elevated temperature have been investigated by X-ray photoelectron spectroscopy (XPS). Gas exposures and subsequent XPS scanning were conducted at 120 and 250 °C to simulate the working conditions of the sensor. Exposure to O 2 at 120 °C resulted in a 0.2 eV upward band bending while subsequent exposure to CH 4 resulted in a 0.1 eV downward band bending, as expected from oxidising and reducing gases, respectively. Similar changes in surface band bending were observed at 250 °C, although quantitative analysis suggests that oxygen absorption might be enhanced. The results clearly indicate the electronic nature of the gas sensing mechanism when exposed to O 2 and CH 4 and the very high sensitivity of the sensor.

In this paper, we present a critical study of a multi-quantum barrier (MQB) structure fabricated using the Al0.5In0.5P/Ga0.5In0.5P material system. The structure was optimised theoretically based on a single Γ band model. When an identical structure was placed in a quantum well laser device, no improvement in the threshold current was observed compared to QW laser structure without an MQB. Cross-sectional scanning tunnelling microscopy (STM) was used to assess the structural quality of the MQB structure for the first time. Factors affecting the limited barrier enhancement are discussed in terms of the interfacial quality of the Al0.5In0.5P/Ga0.5In0.5P interface in the superlattice of the MQB and the need to consider multi-valley transport. The paper highlights some of the fundamental problems that must be overcome for MQBs to be a viable method to improve red laser efficiency.

The work presented in this paper is based on the use of scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS) to study the passivation of atomic scale defect-induced surface states on cleaved III–V (1 1 0) surfaces. This is based on the use of thin Si layers deposited in situ on to the atomically clean surface. The simultaneous STM and STS measurements allowed direct correlation of the structural and electronic properties at the nanoscopic level. The preferential adsorption of Si clusters onto surface defects was achieved using elevated temperature growth on the GaAs(1 1 0) substrate. The STS results clearly indicated local electronic passivation of both step defects and vacancy clusters when the interface is formed at 280 °C. This observation was also confirmed on a macroscopic level using X-ray photoelectron spectroscopy (XPS) under identical conditions. The results are interpreted in terms of the surface bonding of Si with the defect sites. Furthermore, this STM/STS study has been extended to real laser devices where comparable defect features are observed. The implications of defect passivation in nanotechnology are also discussed.

The ability to manipulate the electronic properties of semiconductor surfaces and interfaces is of significant technological importance. Band engineering relies on a fundamental understanding of the physics involved in manipulating charge transfer across junctions, at free surfaces, Schottky contacts or heterojunctions. In this paper, the development of new techniques to control and manipulate free surfaces and interface barriers related to the improvement of semiconductor quantum well laser operation are considered. In particular, surface passivation of atomic scale defects on the laser facet using ultra-thin Si layers deposited at 280\r{}C is discussed. The local passivation mechanism was observed directly using a combination of surface science techniques, mainly scanning tunnelling microscopy/spectroscopy (STM/S). Si was found to selectively bond to defects and return the corresponding electronic properties back to the flat band condition. Secondly, heterojunction modification to enhance carrier confinement within the active region of the device is considered using multi-quantum barriers based on theoretical modelling and experimental verification. Limitations of the techniques are also addressed. The use of STM and STS to investigate interface modification on the nanometre scale is highlighted as being relevant to future device fabrication and optimization based on nano-technology.

We demonstrate here a method using a multi-probe UHV instrument to isolate and measure individual metal contacts controllably fabricated on the tips of free standing ZnO nanowires (NWs). The measurements show Au can form reliable Ohmic and rectifying contacts by exercising control over the surface properties. In the as-grown state the Au contacts display low-resistance characteristics which are determined by the adsorbed species and defects on the NW surface. Subjecting the NWs to an oxidising agent (H2O2) increases the surface potential barrier creating more rectifying contacts. These developments are crucial for controllable NW array devices.

In this work, we present the microscopic characterisation of intimate Cu contacts formed on (100) In0.53Ga0.47As surfaces. Previous I(V) measurements indicate that Cu–In0.53Ga0.47As (100) dots formed ohmic contacts. In the present study, the metal-semiconductor interface is examined by a variety of techniques, including transmission electron (TEM) and atomic force microscopies (AFM), Auger electron spectrometry (AES) and X-ray microanalysis (EDX). The configuration of the three intimate contacts is discussed in terms of the interfacial reactions resulting from the differing deposition conditions.

A scanning tunneling microscope (STM) has been used to study the structural formation of silicon overlayers deposited at room temperature on GaAs(110). In addition spectroscopic measurements were obtained simultaneously to reveal electrical properties associated with the interface and overlayer formation. The Si coverage varied in thickness from submonolayer growth up to ∼16 monolayers. The height variations of the STM images indicate that silicon islands did not exceed ∼3 A at either submonolayer growth or thicker coverages. The lack of atomic resolution of the adsorbed silicon suggests that the silicon overlayers are amorphous in nature, confirming soft x-ray photoelectron spectroscopy (SXPS) measurements. The scanning tunneling spectroscopy measurements clearly indicate a Fermi shift of 0.8 eV when silicon was deposited on the clean cleaved surface of p+ GaAs(110). This shift is also observed on regions of the substrate which remained exposed after the silicon deposition. The Fermi shifts and resultant...

The Ga deposition, reduction, and re-evaporation technique commonly used to produce clean n-GaN surfaces and Ag–GaN interface formation on the resultant surface, have been investigated by Soft X-ray Photoelectron Spectroscopy (SXPS) and current–voltage measurements. SXPS studies have indicated that Ga deposition produces a band-bending of ΔEk = + 1.0 eV to higher kinetic energy. Our results show this shift to be a partially reversible process: re-evaporation of the deposited Ga resulted in a Fermi shift of ΔEk = — 0.6 eV to lower energy. Ag deposition did not cause any further Fermi shift, indicating that the Fermi level is pinned (2.2 ± 0.2) eV above the valence band edge, possibly as a consequence of the cleaning procedure itself. Current voltage (I–V) measurements have shown a barrier height of 0.77 eV and an ideality factor of 1.6. Metal induced gap states and the unified defect model are discussed as possible barrier formation mechanisms.

Soft X-ray photoelectron spectroscopy (SXPS) was performed on Si/Ge(111)-c(2 × 8) heterojunctions. The effect of an ordered ultrathin (one monolayer) ionic dipole layer of ZnSe placed at the interface was studied. The formation of the interface was monitored via the evolution of the valence band edges and the movements of core levels. It was found that the ZnSe intralayer dramatically modified the valence band offset of the Si/Ge (111) junction. The valence band offset was increased by 0.57 ± 0.1 eV due to the presence of the ZnSe intralayer. This intralayer-induced modification of the valence band offset is interpreted in terms of the charge transfer at the interface.

The passivation properties of the Si GaAs(110) interface have been studied using scanning tunnelling microscopy/spectroscopy (STM/STS) and soft X-ray photoemission spectroscopy (SXPS). Silicon has been deposited at room temperature and STM images show the sub-monolayer growth of silicon islands on the GaAs substrate. The electrical properties of these islands together with the clean surface have been investigated using scanning tunnelling spectroscopy (STS). The spectroscopy clearly illustrates the difference in electrical properties between atomically flat regions of GaAs as compared to those containing defects or steps, i.e. where surface band bending occurs. We have investigated the use of sub-monolayer Si coverages to modify the electronic structure of the surface. Height variations of 3–4Aacross Si islands and 2Aacross steps on the GaAs surface have also been observed using the STM. STS spectra, collected simultaneously with the STM image, showed the Si to have semiconducting properties differing from that of crystalline Si and the GaAs substrate. Comparisons between the STM and STS results together with SXPS have provided a correlation between the structural, electrical and chemical nature of the Si/GaAs(110) interface.

Cross-sectional scanning tunneling microscopy and spectroscopy have been used to probe the unreconstructed (1 1 0) surface of a commercially available buried heterostructure laser in ultra high vacuum. Complex re-growth above the non-linear blocking layers is shown to induce tensile strain in the device. Spectroscopic measurements show an increase in both the density of filled valence band states and empty conduction band states as a result of the strain, with a particularly large increase at −3.1 V. Current imaging tunneling spectroscopy measurements show an increase in the tunneling current in to and out of the strained regions at both gap voltage polarities, consistent with the spectroscopy. Moving towards tensile strain, InP is known to maintain much the same bandgap, with the split-off level and lower lying states being drawn up towards the valence band edge, consistent with the data.

ZnO is a wide bandgap semiconductor that has many potential applications including solar cell electrodes, transparent thin film transistors and gas/biological sensors. Since the surfaces of ZnO materials have no amorphous or oxidised layers, they are very environmentally sensitive, making control of their semiconductor properties challenging. In particular, the electronic properties of ZnO nanostructures are dominated by surface effects while surface conduction layers have been observed in thin films and bulk crystals. Therefore, the ability to use the ZnO materials in a controlled way depends on the development of simple techniques to modulate their surface electronic properties. Here, we use monochromatic x-ray photoelectron spectroscopy (XPS) to investigate the use of different wet chemical treatments (EtOH, H2O2) to control the electronic properties of ZnO nanowires by modulating the surface depletion region. The valence band and core level XPS spectra are used to explore the relationship between the surface chemistry of the nanowires and the surface band bending.

The adsorption of tellurium on Si(100) has been studied using surface extended X-ray adsorption fine structure (SEXAFS) and X-ray standing wave spectroscopy (XSW). This particular system is of interest due to its potential applicability in the surfactant aided growth of CdHgTeCdTeSi(100) based infra-red detectors. The Te Si (100) structure was generated by depositing a thick layer (∼ 100 A) of CdTe onto a clean Si (2 × 1) double domain surface, and annealing the sample to 350°C. This resulted is a ∼ 1 ML Te terminated surface where the (2 × 1) reconstruction was lost in favour of a (1 × 1) symmetry. X-ray absorption of the Te L3 edge (E = 4341 eV), with a photon energy range of 4440–4700 eV, was probed using a total yield detection scheme. The SEXAFS results indicated that the Te atoms sat in 2-fold bridge sites directly above a fourth layer Si atom. The corresponding bond length was measured to be 2.52 ± 0.05 A . The XSW measurements of the (400) reflection gave a coherent position of 1.63 ± 0.03 A and a coherent fraction of 0.65. This is consistent with the breaking of the SiSi dimers and thus could be an example of the phenomena of adsorbate-induced dereconstruction of the surface. These results are compared with those of Bennet et al. who examined a similar system using soft X-ray photoemission (SXPS) and the STM study of Yoshikawa et al.

Knowing and controlling the resistivity of an individual nanowire (NW) is crucial for the production of new sensors and devices. For ZnO NWs this is poorly understood; a 10(8) variation in resistivity has previously been reported, making the production of reproducible devices almost impossible. Here, we provide accurate resistivity measurements of individual NWs, using a four-probe scanning tunnelling microscope (STM), revealing a dependence on the NW dimensions. To correctly interpret this behaviour, an atomic level transmission electron microscopy technique was employed to study the structural properties of the NWs in relation to three growth techniques: hydrothermal, catalytic and non-catalytic vapour phase. All NWs were found to be defect free and structurally equivalent; those grown with a metallic catalyst were free from Au contamination. The resistivity measurements showed a distinct increase with decreasing NW diameter, independent of growth technique. The increasing resistivity at small NW diameters was attributed to the dominance of surface states removing electrons from the bulk. However, a fundamental variance in resistivity (10(2)) was observed and attributed to changes in occupied surface state density, an effect which is not seen with other NW materials such as Si. This is examined by a model to predict the effect of surface state occupancy on the measured resistivity and is confirmed with measurements after passivating the ZnO surface. Our results provide an understanding of the primary influence of the reactive nature of the surface and its dramatic effect on the electrical properties of ZnO NWs.

Studies of electron tunnelling through internal double barriers produced by n-type (silicon) and p-type (beryllium) delta (δ)-doping of GaAs are reported. Measurements of the current-voltage (I-V), dI/dV and d 2 I/dV 2 characteristics at room temperature, 77 K and 4.2 K were made. The measurements show structure in the dI/dV and d 2 I/dV 2 characteristics attributable to tunnelling into the 2D quantized levels formed in the well region. This is in qualitative agreement with self-consistent calculations of the sub-band structure and I- V characteristics. The results indicate, however, the difficulty of obtaining sufficient control of the concentration and diffusion of the 6-doped layers to use this technique effectively to produce resonant tunnelling structures.

Finite element analysis of the electrostatic field of the electrostatic force microscope (EFM) has been conducted in this paper for various tip-sample distances and non-flat sample surfaces. It is found from the simulation that when the distance between probe tip and sample surface is within 0 nm to 3 nm, the electrostatic field between probe tip and sample surface is smaller compared to that of tip-sample distance above 3 nm. Moreover, non-flat sample surface can only make significant difference on the electrostatic field when the surface roughness is larger than 4 nm.

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 μm 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.

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 Zn1−xSixO ( 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.

X-ray Photoelectron Spectroscopy was 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 to the formation of secondary oxide species. Ag deposition did not induce any band bending, indicating that the Fermi level is pinned. The Ag was entirely removed from the surface after annealing at 600°C. The implications in terms of gas sensing are discussed. [DOI: 10.1380/ejssnt.2009.319]

Scanning tunneling microscopy and spectroscopy (STS) have been used to investigate submonolayer Si depositions on clean cleaved GaAs(110). These studies focus on the effect of cleaving-induced step defects on GaAs(110), the resultant Si overlayer formation, and the electrical properties of the substrate/overlayer system formed at 280 °C. STS measurements at a clean step edge on GaAs(110) show that the Fermi level shifts towards midgap for both n and p type, while measurements performed on Si nanoislands at step edges show that the Fermi level reverts back towards its “ideal” position. Results indicate the passivation properties of Si on GaAs(110) when deposited under these conditions. Implications for use in laser facets are discussed.

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 degrees C to 900 degrees C. Annealing from 100 degrees C to 400 degrees 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 degrees C to 800 degrees 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 degrees C, while higher temperatures resulted in an increase in particle size. Crystallisation of the nanoparticles started to occur after annealing at 600 degrees C. The implications in terms of gas sensing are discussed. (c) 2007 Elsevier B.V. All rights reserved.

Quantum dot fluorophores provide a photo and bio-stable optical marker signal well suited to the tracking of lineage within large cell populations over multiple generations. We have used a Monte Carlo algorithm to model the process of dot partitioning and dilution by cell mitosis. A Genetic Algorithm was used to compare simulated and experiment quantum dot distributions, which shows that the dot fluorescence is divided with a stochastic variation about an asymmetric mean split ratio.

Cross-sectional scanning tunneling microscopy is performed on operating semiconductor quantum well laser devices to reveal real time changes in device structure. Low and nominally doped capping regions adjacent to the quantum well active region are found to heat under normal operating conditions. The increase in anion-vacancy defect formation and the generation of surface states pins the Fermi level at the surface and begins the process of catastrophic optical degradation which eventually destroys the device. The technique has implications for the study of defect generation and in-operation changes in all nanostructures.

Interfaces between semiconductors and insulators, metals or other semiconductors play a crucial role in the behaviour of solid state electronic devices. It is therefore no surprise that techniques to accurately control the electronic properties of such interfaces has long been an ambitious goal. Here we consider some of the key contributions to this area over several decades, concentrating in particular on the role of dipole layers at semiconductor interfaces. We consider the current state of our understanding with respect to the fabrication of microelectronic and optical devices. Furthermore, we present views on some of the future challenges particularly in high performance optical devices and nanostructures.

We collected high resolution scanning tunneling (STM) images on thin films of WO/sub 3/. This provided information about the morphology and the nanostructure of the samples. The surface gap was then determined through scanning tunneling spectroscopy (STS) and was found to be much narrower than the bulk gap determined via photoluminescence. This effect is a consequence of the high density of surface states in this material. Pinning of the Fermi level due to these states is decreased when the sample is illuminated at low temperature, whereas at high temperature the effect is masked by the high density of free carriers.