Philipp Mensch

Temperature mapping of operating nanoscale devices by scanning probe thermometry

Imaging temperature fields at the nanoscale is a central challenge in various areas of science and technology. Nanoscopic hotspots, such as those observed in integrated circuits or plasmonic nanostructures, can be used to modify the local properties of matter, govern physical processes, activate chemical reactions and trigger biological mechanisms in living organisms. The development of high-resolution thermometry techniques is essential for understanding local thermal non-equilibrium processes during the operation of numerous nanoscale devices. Here we present a technique to map temperature fields using a scanning thermal microscope. Our method permits the elimination of tip–sample contact-related artefacts, a major hurdle that so far has limited the use of scanning probe microscopy for nanoscale thermometry. We map local Peltier effects at the metal–semiconductor contacts to an indium arsenide nanowire and self-heating of a metal interconnect with 7 mK and sub-10 nm spatial temperature resolution.

Using the Seebeck coefficient to determine charge carrier concentration, mobility, and relaxation time in InAs nanowires

A method for determining charge carrier concentration, mobility, and relaxation time in semiconducting nanowires is presented. The method is based on measuring both the electrical conductivity and the Seebeck coefficient of the nanowire. With knowledge on the bandstructure of the material, Fermi level and charge carrier concentration can be deduced from the Seebeck coefficient. The ratio of measured conductivity and inferred charge carrier concentration then leads to the mobility, and using the Fermi level dependence of mobility one can finally obtain the relaxation time. Using this approach we exemplarily analyze the characteristics of an n-type InAs nanowire.

In situ doping of catalyst-free InAs nanowires

We report on in situ doping of InAs nanowires grown by metal-organic vapor-phase epitaxy without any catalyst particles. The effects of various dopant precursors (Si(2)H(6), H(2)S, DETe, CBr(4)) on the nanowire morphology and the axial and radial growth rates are investigated to select dopants that enable control of the conductivity in a broad range and that concomitantly lead to favorable nanowire growth. In addition, the resistivity of individual wires was measured for different gas-phase concentrations of the dopants selected, and the doping density and mobility were extracted. We find that by using Si(2)H(6) axially and radially uniform doping densities up to 7 × 10(19) cm(-3) can be obtained without affecting the morphology or growth rates. For sulfur-doped InAs nanowires, we find that the distribution coefficient depends on the growth conditions, making S doping more difficult to control than Si doping. Moreover, above a critical sulfur gas-phase concentration, compensation takes place, limiting the maximum doping level to 2 × 10(19) cm(-3). Finally, we extract the specific contact resistivity as a function of doping concentration for Ti and Ni contacts.

Full thermoelectric characterization of InAs nanowires using MEMS heater/sensors.

Precise measurements of a complete set of thermoelectric parameters on a single indium-arsenide nanowire (NW) have been performed using highly sensitive, micro-fabricated sensing devices based on the heater/sensor principle. The devices were fabricated as micro electro-mechanical systems consisting of silicon nitride membranes structured with resistive gold heaters/sensors. Preparation, operation and characterization of the devices are described in detail. Thermal decoupling of the heater/sensor platforms has been optimized reaching thermal conductances as low as 20 nW K(-1) with a measurements sensitivity below 20 nW K(-1). The InAs NWs were characterized in terms of thermal conductance, four-probe electrical conductance and thermopower (Seebeck coefficient), all measured on a single NW. The temperature dependence of the parameters determining the thermoelectric figure-of-merit of an InAs NW was acquired in the range 200-350 K featuring a minor decrease of the thermal conductivity from 2.7 W (m K)(-1) to 2.3 W (m K)(-1).

Kelvin probe force microscopy for local characterisation of active nanoelectronic devices

Summary Frequency modulated Kelvin probe force microscopy (FM-KFM) is the method of choice for high resolution measurements of local surface potentials, yet on coarse topographic structures most researchers revert to amplitude modulated lift-mode techniques for better stability. This approach inevitably translates into lower lateral resolution and pronounced capacitive averaging of the locally measured contact potential difference. Furthermore, local changes in the strength of the electrostatic interaction between tip and surface easily lead to topography crosstalk seen in the surface potential. To take full advantage of the superior resolution of FM-KFM while maintaining robust topography feedback and minimal crosstalk, we introduce a novel FM-KFM controller based on a Kalman filter and direct demodulation of sidebands. We discuss the origin of sidebands in FM-KFM irrespective of the cantilever quality factor and how direct sideband demodulation enables robust amplitude modulated topography feedback. Finally, we demonstrate our single-scan FM-KFM technique on an active nanoelectronic device consisting of a 70 nm diameter InAs nanowire contacted by a pair of 120 nm thick electrodes.

One-dimensional behavior and high thermoelectric power factor in thin indium arsenide nanowires

Electrical conductivity and Seebeck coefficient of quasi-one-dimensional indium arsenide (InAs) nanowires with 20 nm diameter are investigated. The carrier concentration of the passivated nanowires was modulated by a gate electrode. A thermoelectric power factor of 1.7 × 10−3 W/m K2 was measured at room temperature. This value is at least as high as in bulk-InAs and exceeds by far typical values of thicker InAs nanowires with three-dimensional properties. The interpretation of the experimental results in terms of power-factor enhancement by one-dimensionality is supported by model calculations using the Boltzmann transport formalism.

C-V measurements of single vertical nanowire capacitors

The density of interface states, Dit, is important for the device performance in view of the fact that it limits the inverse subthreshold slope in both, MOSFETs and TFETs [1]. This poses particular challenges for nanowire (NW) devices, because the measured Dit is expected to increase due to the extensive processing and the various crystallographic orientations of the surface, which differ from the ideal (100) orientation. For a detailed investigation of the Dit of NWs it is best to analyze single NW MOS capacitors. However, the capacitance of a single NW MOS capacitor lies in the fF regime which is very challenging to measure. To date, very few capacitance measurements on single NWs have been reported, e.g., on lateral devices based on InAs [2], Ge [3], and Si [4]. Dit analysis of NWs has been demonstrated, however, based on capacitance measurements only of large arrays of InAs NWs [5]. In the present work, we report on the capacitance measurement and Dit analysis of vertical silicon MOS capacitors based on single NWs.

Electrical and thermoelectrical properties of gated InAs nanowires

We have investigated the electrical and thermoelectrical properties of 30-nm-thick InAs nanowires in a temperature range between T = 200 K and T = 350 K. Devices were fabricated that allow the measurement of the conductivity and Seebeck coefficient upon the application of a gate voltage. The carrier concentration in the NWs could be varied by two orders of magnitude. The dependence of the Seebeck coefficients measured on the carrier concentration is similar to bulk InAs. A temperature-dependent mobility of IL = 1200-1400 cm 2/Vs of these unpassivated NWs could be determined from both the transistor characteristics and Seebeck coefficient measurements.

III-V semiconductor nanowires for future devices

The monolithic integration of III-V nanowires on silicon by direct epitaxial growth enables new possibilities for the design and fabrication of electronic as well as optoelectronic devices. We demonstrate a new growth technique to directly integrate III-V semiconducting nanowires on silicon using selective area epitaxy within a nanotube template. Thus we achieve small diameter nanowires, controlled doping profiles and sharp heterojunctions essential for future device applications. We experimentally demonstrate vertical tunnel diodes and gate-all-around tunnel FETs based on InAs-Si nanowire heterojunctions. The results indicate the benefits of the InAs-Si material system combining the possibility of achieving high Ion with high Ion/Ioff ratio.

Heat dissipation and thermometry in nanosystems: When interfaces dominate

The technological need for characterization of scaled nano-devices is not paralleled with the availability of methods to measure heat flux and temperature on small scales. To measure local temperature and conductance variation we therefore focus on developing measurement tools. These are based on (A) scanning a thermometer across the sample surface region of interest, so called scanning thermal microscopy (SThM), (B) measuring thermal properties directly through self-heating, and (C) measuring directly the heat-flux through 1D-structures.