Phillip S. Dobson

SThM Temperature Mapping and Nonlinear Thermal Resistance Evolution With Bias on AlGaN/GaN HEMT Devices

Channel temperature has a strong impact on the performance of a microwave power transistor. In particular, it has a strong influence on the power gain, energetic efficiency, and reliability of the device. The thermal optimization of device geometry is therefore a key issue, together with precise measurements of temperature within the channel area. In this paper, we have used scanning thermal microscopy to perform temperature mapping, at variable dc bias points, on an AlGaN/GaN high-electron mobility transistor made on epilayers grown on silicon carbide substrate. We have analyzed the variation of the thermal resistance values, which are deduced from these measurements, with bias conditions VGS and VDS. The observed nonlinear behavior is found to be in excellent agreement with physical simulations, strongly pointing out the large variability of the extension of the dissipation area with the dc bias conditions

The thermoelectric properties of Ge/SiGe modulation doped superlattices

The thermoelectric and physical properties of superlattices consisting of modulation doped Ge quantum wells inside Si1− y Ge y barriers are presented, which demonstrate enhancements in the thermoelectric figure of merit, ZT, and power factor at room temperature over bulk Ge, Si1− y Ge y , and Si/Ge superlattice materials. Mobility spectrum analysis along with low temperature measurements indicate that the high power factors are dominated by the high electrical conductivity from the modulation doping. Comparison of the results with modelling using the Boltzmann transport equation with scattering parameters obtained from Monte Carlo techniques indicates that a high threading dislocation density is also limiting the performance. The analysis suggests routes to higher thermoelectric performance at room temperature from Si-based materials that can be fabricated using micro- and nano-fabrication techniques.

Screening of biomineralization using microfluidics

Biomineralization is the process where biological systems produce well-defined composite structures such as shell, teeth, and bones. Currently, there is substantial momentum to investigate the processes implicated in biomineralization and to unravel the complex roles of proteins in the control of polymorph switching. An understanding of these processes may have wide-ranging significance in health care applications and in the development of advanced materials. We have demonstrated a microfluidic approach toward these challenges. A reversibly sealed T-junction microfluidic device was fabricated to investigate the influence of extrapallial (EP) fluid proteins in polymorph control of crystal formation in mollusk shells. A range of conditions were investigated on chip, allowing fast screening of various combinations of ion, pH, and protein concentrations. The dynamic formation of crystals was monitored on chip and combined with in situ Raman to reveal the polymorph in real time. To this end, we have demonstrated the unique advantages of this integrated approach in understanding the processes involved in biomineralization and revealing information that is impossible to obtain using traditional methods.

The cross-plane thermoelectric properties of p-Ge/Si0.5Ge0.5 superlattices

The electrical conductivity, Seebeck coefficients, and thermal conductivities of a range of p-type Ge/Si0.5 Ge 0.5 superlattices designed for thermoelectric generation and grown by low energy plasma enhanced chemical vapor deposition have been measured using a range of microfabricated test structures. For samples with barriers around 0.5 nm in thickness, the measured Seebeck coefficients were comparable to bulk p-SiGe at similar doping levels suggesting the holes see the material as a random bulk alloy rather than a superlattice. The Seebeck coefficients for Ge quantum wells of 2.85 ± 0.85 nm increased up to 533 ± 25 μV/K as the doping was reduced. The thermal conductivities are between 4.5 to 6.0 Wm−1K−1 which are lower than comparably doped bulk Si0.3 Ge 0.7 but higher than undoped Si/Ge superlattices. The highest measured figure of merit ZT was 0.080 ± 0.011 obtained for the widest quantum well studied. Analysis suggests that interface roughness is presently limiting the performance and a reduction in the strain between the quantum wells and barriers has the potential to improve the thermoelectric performance.

Ultra-rapid, sustainable and selective synthesis of silicon carbide powders and nanomaterials via microwave heating

Silicon carbide has been synthesised from silicon or silica combined with activated carbon or graphitevia microwave heating over timescales from minutes to seconds without the need for inert atmospheres or subsequent purification. The carbide morphology and phase purity can be controlled by the microwave cavity used and the power applied and hence by the heating rate. Short irradiation times (ca. 5 minutes) in a multimode cavity using activated carbon produce single phase β-SiC nanofibres as small as 5 nm in diameter while large crystallites of α- and β-SiC can obtained in ≪1 minute using high power, single mode cavity microwave techniques.

Microfabricated temperature standard based on Johnson noise measurement for the calibration of micro- and nano-thermometers

An instrument for the definition of absolute temperature on the scale of one micron based on the measurement of Johnson noise in a small metallic resistor has been constructed. The instrument uses conventional monolithic amplifiers and signal processing circuitry and a custom sensor fabricated by bulk micromachining and electron-beam lithography. The instrument is simple to use and accurate to better than 1 K over the range 300–600 K following a single point calibration to ambient temperature. The incorporation of an integrated thermocouple as a transfer standard allows the use of the instrument to define temperatures with a 1 K accuracy and a random error of better than 50 mK in a 1 kHz bandwidth, and permits its operation in electrically noisy environments. Extension to the measurement of temperature on a scale much smaller than 1μm is theoretically straightforward. The instrument has been used to calibrate a nano-thermometer (resistance thermometer atomic force microscopy probe) with applications in th...

High temperature imaging using a thermally compensated cantilever resistive probe for scanning thermal microscopy

The authors have designed and fabricated AFM probes with an integrated resistive temperature sensor and a grooved cantilever structure. The grooved structure compensates for the bilayer thermal bending that normally occurs during scanning thermal microscopy of hot samples. These new probes show reduced bending at high temperatures when compared to commercial, conventional cantilever probes with a similar stiffness. This indicates that the mechanical balance introduced by the grooved structure plays a major role in reducing thermal bending. Successful temperature mapping is demonstrated on an active heater device reaching 108 °C, a sample that would be beyond the imaging capability of conventional probes.

New Methods for Calibrated Scanning Thermal Microscopy (SThM)

A batch microfabricated scanning thermal microscopy (SThM) probe is presented. The sensor, based on a Pd resistance thermometer is shown to be suitable for calibration and stable for very long periods (> 700 hours). A technique for achieving transformer isolation of the SThM probe is described and shown to be a highly effective route to obtaining calibrated SThM scans of electrically sensitive samples as well as those subject to large bias voltages.

Growth and morphology control of carbon nanotubes at the apexes of pyramidal silicon tips

We describe the development of catalysed chemical vapour deposition (cCVD) growth schemes suitable for the production of carbon nanotube atomic force microscopy (CNT-AFM) probes. Growth and sample processing conditions are utilized that both incorporate safety in the process, e.g. the use of ethanol (EtOH) vapour as a carbon feedstock and hydrogen at only 4% (flow proportion), and simplicity, e.g. no catalyst patterning is required. Cobalt is employed as the growth catalyst and thin films of aluminium on silicon as the substrate material. Purpose-fabricated silicon substrates containing large numbers of tip structures are used as models of AFM probes. This enables growth to be carried out on many tips at once, facilitating a thorough investigation of the effect of different growth schemes on yields. cCVD growth schemes are chosen which produce stabilizing high density networks of carbon nanotubes on the sidewalls of the pyramidal tips to aid in anchoring the apex protruding carbon nanotube(s) in place. This results in long-lasting AFM imaging tips. We demonstrate that through rational tailoring of cCVD conditions it is possible to tune the growth conditions such that CNTs which protrude straight from tip apexes can be obtained at yields of greater than or equal to 78%. Application of suitable growth schemes to CNT growth on commercially available AFM probes resulted in CNT-AFM probes which were found to be extremely useful for extended lifetime metrological profiling of complex structures.

Quantification of probe-sample interactions of a scanning thermal microscope using a nanofabricated calibration sample having programmable size.

We report a method for quantifying scanning thermal microscopy (SThM) probe-sample thermal interactions in air using a novel temperature calibration device. This new device has been designed, fabricated and characterised using SThM to provide an accurate and spatially variable temperature distribution that can be used as a temperature reference due to its unique design. The device was characterised by means of a microfabricated SThM probe operating in passive mode. This data was interpreted using a heat transfer model, built to describe the thermal interactions during a SThM thermal scan. This permitted the thermal contact resistance between the SThM tip and the device to be determined as 8.33 × 10(5) K W(-1). It also permitted the probe-sample contact radius to be clarified as being the same size as the probes tip radius of curvature. Finally, the data were used in the construction of a lumped-system steady state model for the SThM probe and its potential applications were addressed.