Effect of annealing on the depth profile of hole concentration in (Ga,Mn)As

The effect of annealing at 250 C on the carrier depth profile, Mn distribution, electrical conductivity, and Curie temperature of (Ga,Mn)As layers with thicknesses > 200 nm, grown by molecular-beam epitaxy at low temperatures, is studied by a variety of analytical methods. The vertical gradient in hole concentration, revealed by electrochemical capacitance-voltage profiling, is shown to play a key role in the understanding of conductivity and magnetization data. The gradient, basically already present in as-grown samples, is strongly influenced by post-growth annealing. From secondary ion mass spectroscopy it can be concluded that, at least in thick layers, the change in carrier depth profile and thus in conductivity is not primarily due to out-diffusion of Mn interstitials during annealing. Two alternative possible models are discussed.Comment: 8 pages, 8 figures, to appear in Phys. Rev.

Dispersion force for materials relevant for micro and nanodevices fabrication

The dispersion (van der Waals and Casimir) force between two semi-spaces are calculated using the Lifshitz theory for different materials relevant for micro and nanodevices fabrication, namely, gold, silicon, gallium arsenide, diamond and two types of diamond-like carbon (DLC), silicon carbide, silicon nitride and silicon dioxide. The calculations were performed using recent experimental optical data available in the literature, usually ranging from the far infrared up to the extreme ultraviolet bands of the electromagnetic spectrum. The results are presented in the form of a correction factor to the Casimir force predicted between perfect conductors, for the separation between the semi-spaces varying from 1 nanometre up to 1 micrometre. The relative importance of the contributions to the dispersion force of the optical properties in different spectral ranges is analyzed. The role of the temperature for semiconductors and insulators is also addressed. The results are meant to be useful for the estimation of the impact of the Casimir and van der Waals forces on the operational parameters of micro and nanodevices

Fabrication and characterisation of nanocrystalline graphite MEMS resonators using a geometric design to control buckling

The simulation, fabrication and characterisation of nanographite MEMS resonators is reported in this paper. The deposition of nanographite is achieved using plasma-enhanced chemical vapour deposition directly onto numerous substrates such as commercial silicon wafers. As a result, many of the reliability issues of devices based on transferred graphene are avoided. The fabrication of the resonators is presented along with a simple undercutting method to overcome buckling, by changing the effective stress of the structure from 436 MPa compressive, to 13 MPa tensile. The characterisation of the resonators using electrostatic actuation and laser Doppler vibrometry is reported, demonstrating resonator frequencies from 5–640 kHz and quality factor above 1819 in vacuum obtained

Phase optimization of thermally actuated piezoresistive resonant MEMS cantilever sensors

The asymmetric resonance response in thermally actuated piezoresistive cantilever sensors causes a need for optimization, taking parasitic actuation–sensing effects into account. In this work, two compensation methods based on Wheatstone bridge (WB) input voltage (VWB_in) adjustment and reference circuit involvement were developed and investigated to diminish those unwanted coupling influences. In the first approach, VWB_in was increased, resulting in a higher current flowing through the WB piezoresistors as well as a temperature gradient reduction between the thermal actuator (heating resistor: HR) and the WB, which can consequently minimize the parasitic coupling. Nevertheless, increasing VWB_in (e.g., from 1 to 3.3&thinsp;V) may also yield an unwanted increase in power consumption by more than 10 times. Therefore, a second compensation method was considered: i.e., a reference electronic circuit is integrated with the cantilever sensor. Here, an electronic reference circuit was developed, which mimics the frequency behavior of the parasitic coupling. By subtracting the output of this circuit from the output of the cantilever, the resonance response can thus be improved. Both simulated and measured data show optimized amplitude and phase characteristics around resonant frequencies of 190.17 and 202.32&thinsp;kHz, respectively. With this phase optimization in place, a phase-locked-loop (PLL) based system can be used to track the resonant frequency in real time, even under changing conditions of temperature (T) and relative humidity (RH), respectively. Finally, it is expected to enhance the sensitivity of such piezoresistive electro-thermal cantilever sensors under loading with any target analytes (e.g., particulate matter, gas, and humidity).</p

Partially integrated cantilever-based airborne nanoparticle detector for continuous carbon aerosol mass concentration monitoring

The performance of a low-cost partially integrated <i>can</i>tilever-based airborne nanoparticle (NP) detec<i>tor</i> (CANTOR-1) is evaluated in terms of its real-time measurement and robustness. The device is used for direct reading of exposure to airborne carbon engineered nanoparticles (ENPs) in indoor workplaces. As the main components, a miniaturized electrostatic aerosol sampler and a piezoresistive resonant silicon cantilever mass sensor are employed to collect the ENPs from the air stream to the cantilever surfaces and to measure their mass concentration, respectively. Moreover, to realize a real-time measurement, a frequency tracking system based on a phase-locked loop (PLL) is built and integrated into the device. Long-term ENP exposure and a wet ultrasonic cleaning method are demonstrated to estimate the limitation and extend the operating lifetime of the developed device, respectively. By means of the device calibrations performed with a standard ENP monitoring instrument of a fast mobility particle sizer (FMPS, TSI 3091), a measurement precision of ENP mass concentrations of < 55% and a limit of detection (LOD) of < 25 μg m⁻³ are obtained

Size-selective electrostatic sampling and removal of nanoparticles on silicon cantilever sensors for air-quality monitoring

This paper reports the size-selective sampling and removal of airborne nanoparticles (NP) using a silicon resonant micro cantilever mass balance combined with a miniaturized electrostatic precipitator (ESP). This allows to design an overnight-refreshable, pocket-sized, autarkic monitor for NP mass concentration and size in the range below 300 nm

Self-exciting and self-sensing resonant cantilever sensors for improved monitoring of airborne nanoparticles exposure

Self-exciting and self-sensing resonant cantilever sensors for airborne nanoparticles (NPs) monitoring are investigated. A fabricated self-sensing piezoresistive cantilever sensor is actuated using a piezo stack and operated in the second resonant mode. It is able to detect a resonant frequency shift of 33.58 Hz that corresponds to an airborne carbon NPs mass of 1.03 ng deposited during a 15-min sampling. The quality factor is 2100 resulting in a mass resolution of the sensor of 9.8 pg. In order to realize a portable airborne NPs monitoring device, thermally excited silicon cantilevers vibrating in different modes are proposed. The integration designs of heating resistors, sensing piezoresistors and electrostatic NPs sampling are further analyzed using finite-element modeling (FEM). An electrode for electrostatic NPs sampling is placed on the free-end of the cantilever. The results indicate that a self-sensing silicon electrothermal cantilever can fulfill the requirements of portable monitoring of airborne NPs exposure