S.P. Wilks

Nano-crystalline SnO2 gas sensor response to O2 and CH4 at elevated temperature investigated by XPS

Abstract 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.

Comparative surface studies on wet and dry sacrificial thermal oxidation on silicon carbide

Abstract A comparative study on the effect of wet and dry thermal oxidation on 4H-silicon carbide (SiC) and on sacrificial silicon (Si) thermal oxidation on 4H-SiC surface has been conducted using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The AFM images show the formation of ‘nano-islands’ of varying density on the SiC surface after the removal of thermal oxide using hydrofluoric (HF) acid etch. These nano-islands are resistant to HF acid and have been previously linked to residual carbon [1] , [2] , [3] resulting from the oxidation process. This paper presents the use of a sacrificial silicon oxidation (SSO) step as a form of surface preparation that gives a reproducible clean SiC surface. XPS results show a slight electrical shift in binding energy between the wet and dry thermal oxidation on the standard SiC surface, while the surface produced by the SSO technique shows a minimal shift.

Ultra-high resolution imaging of DNA and nucleosomes using non-contact atomic force microscopy

Visualisation of nano‐scale biomolecules aids understanding and development in molecular biology and nanotechnology. Detailed structure of nucleosomes adsorbed to mica has been captured in the absence of chemical‐anchoring techniques, demonstrating the usefulness of non‐contact atomic force microscopy (NC‐AFM) for ultra‐high resolution biomolecular imaging. NC‐AFM offers significant advantages in terms of resolution, speed and ease of sample preparation when compared to techniques such as cryo‐electron microscopy and X‐ray crystallography. In the absence of chemical modification, detailed structure of DNA deposited on a gold substrate was observed for the first time using NC‐AFM, opening up possibilities for investigating the electrical properties of unmodified DNA.

Wavelength-tunable and high-temperature lasing in ZnMgO nanoneedles

Zn1−xMgxO nanoneedles were prepared by an ion-beam technique on Zn1−xMgxO thin films with Mg contents of up to 21at.%. The photoluminescence emission energies of the Zn1−xMgxO nanoneedles measured at room temperature increased monotonically with Mg contents and it reached 3.6eV when x=0.21. Random laser action was observed in the Zn1−xMgxO nanoneedles with x⩽0.1 at temperature ranging from 300to470K under 355nm optical excitation. The characteristic temperature of the Zn1−xMgxO nanoneedles was determined to be 84K. The high-temperature lasing of the Zn1−xMgxO nanoneedles are attributed to the high crystal quality of the nanoneedles, enhancement of oscillator strength in nanostructures, and a self-compensation mechanism in random laser cavities.

Modeling multiple quantum barrier effects and reduced electron leakage in red-emitting laser diodes

Severe electron leakage impedes the full exploitation of AlGaInP laser diodes in the 630nm regime. Such thermally activated currents are attributed to inherently small conduction band offsets and intervalley transfer between the Γ and X conduction band minima. To negate the detrimental effect of these two intrinsic material issues a theoretical model is proposed. A multi-quantum-barrier (MQB) structure able to inhibit both Γ- and X-band transmissions is inserted in the p-doped region adjacent to the active region of the device, allowing a greater percentage of injected electrons to be reflected back within the active region. The design of the MQB follows a strict optimization procedure that takes into account fluctuations of superlattice layer width and composition. This model is used in conjunction with a dual conduction band drift-diffusion simulator to enable the design of the MQB at an operating voltage and hence account for nonlinear charge distribution across it. Initial results indicate strong agre...

Improvements to the Schottky barrier heights of intimate metal-InGaAs contacts by low temperature metallisation

One challenge in the processing of In 53 Ga 47 As/InP(100) heterostructures is the enhancement of the low Schottky barrier heights (Φ b ) typically exhibited by metal-nIn 53 Ga 47 As(100) contacts. Previous studies of the metallisation of chemically etched In 53 Ga 47 As surfaces have reported increases in Φ b if metallisation is performed at 77 K. This study extends this concept to probe the idealised case of the formation of metal contacts (Au-, Cu- and In-) to atomically clean nIn 53 Ga 47 As/InP(/I) at 80 K and room temperature (294 K). The in-situ current-voltage measurements of intimate Au and Cu contacts formed at both temperatures exhibited ohmic characteristics. However, intimate In contacts formed at 80 and 294 K showed diodic behaviour (Φ b 0.45 and 0.30 eV, respectively), with an improvement in the rectification when formed at 80 K. Conversely, In contacts to etched surfaces were ohmic. It is proposed that this behaviour is linked to the alteration of the structure and nature of the interface as a result of the various fabrication techniques. These results indicate the potential to select Φ b on In 53 Ga 47 As via a combination of cryogenic processing and surface preparation.

Ellipsometric and MEIS Studies of 4H-SiC/Si/SiO2 and 4H-SiC/SiO2 Interfaces for MOS Devices

The high density of interface states of thermally grown oxides on silicon carbide has prompted research into alternative oxidation methods and post oxidation anneals. One such alternative is oxidation of a deposited sacrificial silicon layer. A recent variation of this technique is a partial oxidation of the deposited Si layer, so that a thin Si layer remains between the SiO2 and SiC layers. If the SiO2/Si interface has lower interface state densities than the SiO2/SiC interface, the SiO2/Si/SiC hetero-structure could yield improved channel mobilities in MOS devices. Moreover, by correct optimization of the MOSFET device structure, breakdown can be designed to occur in the bulk SiC layer, thus maintaining a high blocking voltage. Post oxidation annealing in N2O is another technique often used to reduce interface state densities. However, little is known about the chemical and physical nature of these N2O oxidized dielectrics. Ellipsometric and Medium Energy Ion Scattering (MEIS) investigations of conventional SiO2/SiC interfaces compared with SiO2/Si/SiC hetero-junction structures and N2O oxidized samples are reported.

The electronic passivation properties of Si nanoislands on GaAs(110) step defects induced by cleaving

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.

An investigation of the properties of intimate InInxGa1-xAs(100) interfaces formed at room and cryogenic temperatures

Abstract The electrical, chemical and structural properties of the interfaces formed at room and low temperatures, between In and atomically clean In 53 Ga 47 As/InP(100) have been studied. Current-voltage measurements indicate that diodes formed at 80 K exhibit significantly higher Schottky barriers (φ b = 0.45 eV) than diodes formed at 294 K (φ b = 0.30 eV). The reactions occurring during the formation of In In 53 Ga 47 As/InP(100) interfaces at room and low temperatures have been investigated using Soft X-Ray photoemission spectroscopy. Our results show that metallisation at room temperature results in a predominantly three dimensional mode of growth, accompanied by the out-diffusion of As. Low temperature (125 K) metallisation appears to reduce clustering and inhibit As out-diffusion. Examination of the resulting interfaces by transmission electron microscopy confirms the more uniform nature of the metal layers formed at low temperature. These observations, in conjunction with the barrier heights measured by the I-V technique, are discussed in the context of currently supported models of Schottky barrier formation.

Is your laboratory a turn-off?

Generations of students have discovered, too often, that laboratory experience is the least satisfactory aspect of reading engineering. It is a regular comment from employers of engineering graduates that new employees are deficient in basic laboratory skills, yet the problem persists and unquestionably leaves serious expositors of the subject with a feeling that more should be done in this area. Some three years ago the authors decided a serious effort was needed to address these issues in our own department. The vehicle chosen was the Level 1 circuit design course. This paper will describe the strategy adopted to overcome the perceived weaknesses of our previous laboratory course as measured by a quality audit of student reactions.