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Harvesting hybrid mechanical and solar ambient energy with one small device remains a challenge. Here, we report on producing electric current using a Schottky type metal-oxide-semiconductor struct...

Regions of single-layer- and bilayer graphene on the surface of thermally processed 4H-SiC substrates are studied using Kelvin probe force microscopy and Raman spectroscopy. We establish experimentally the key parameters of the adopted graphene growth technique which enables the fraction of bilayer graphene to be reduced to a minimum, while samples with a fraction of single-layer graphene as high as 95% are obtained.

The structural and electronic characteristics of epitaxial graphene films grown by thermal decomposition of the Si face of a semi-insulating 4H-SiC substrate in an argon environment have been studied by a large set of analytical techniques. It is shown that the results of a complex study make it possible to optimize the growth parameters and develop a reliable technology for the growth of high-quality single-layer graphene films. The charge-carrier concentration in the graphene layer was within 7 × 1011–1 × 1012 cm–2, and the maximum mobility of electrons at room temperature approached 6000 cm2/(V s).

The electrical properties of GaAs nanowires grown on a 6H-SiC (0001) substrate covered with graphene single layers and bilayers are studied. The nanowires are grown by molecular-beam epitaxy, with gold as a catalyst. The electrical properties are studied by measuring and analyzing the current–voltage characteristics of single nanowires vertically grown on a substrate. Numerical simulation of the experimental current–voltage curves revealed the presence of a ~0.6-V-high Schottky barrier between the nanowires and graphene. The appearance of the barrier is due to the formation of excess arsenic at the nanowire/graphene interface.

Piezoresistance effect of the InxGa1 –xAs nanowires was studied by scanning probe microscopy. The effect was investigated by measuring of the I–V curves with simultaneous bending of the nanowire by the probe. A modulation of the conductivity of In0.85Ga0.15As NWs induced by a mechanical stress exceeding three orders of magnitude was observed. For the explanation a model based on the surface states is presented. Such an observation opens new ways for the design of electromechanical switches with record parameters.

The structural, chemical, and electronic properties of epitaxial graphene films grown by thermal decomposition of the Si-face of a semi-insulating 6H-SiC substrate in an argon environment are studied by Raman spectroscopy, atomic-force microscopy, the low-energy electron diffraction method, X-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy and X-ray absorption spectroscopy at the carbon K edge. It is shown that the results of a systematic integrated study make it possible to optimize the growth parameters and develop a reliable technology for the growth of high-quality single-layer graphene films with a small fraction of bilayer graphene inclusions.

AbstractThe properties of the hybrid energy sources “piezoelectric nanogenerator-solar cell” based on GaAs nanowires with the wurtzite crystal structure were investigated. Measurements were performed by the bending of the nanowire by the probe of the atomic force microscope with simultaneous recording of short circuit current in dark and illuminated conditions. We show that a piezoelectric current pulse of ~10 pA arises in the “nanowire-probe” circuit during the deformation of nanowire by the AFM probe. Under laser illumination, the value of the pulse increases by two orders of magnitude as a result of the piezophototronic effect. Deformation of the nanowire boosts the photocurrent by 40% up to 0.5 nA.

In this work, the stability of short-wavelength spatial modes in quarter-disk cavities was studied. It was found that in such resonators, short-wave modes (with wavelengths λ smaller than the radius R of the quarter-disk λ<0.1 lack total internal reflection modes.

Research regarding ways to increase solar cell efficiency is in high demand. Mechanical deformation of a nanowire (NW) solar cell can improve its efficiency. Here, the effect of uniaxial compression on GaAs nanowire solar cells was studied via conductive atomic force microscopy (C-AFM) supported by numerical simulation. C-AFM I&ndash, V curves were measured for wurtzite p-GaAs NW grown on p-Si substrate. Numerical simulations were performed considering piezoresistance and piezoelectric effects. Solar cell efficiency reduction of 50% under a &minus, 0.5% strain was observed. The analysis demonstrated the presence of an additional fixed electrical charge at the NW/substrate interface, which was induced due to mismatch between the crystal lattices, thereby affecting the efficiency. Additionally, numerical simulations regarding the p-n GaAs NW solar cell under uniaxial compression were performed, showing that solar efficiency could be controlled by mechanical deformation and configuration of the wurtzite and zinc blende p-n segments in the NW. The relative solar efficiency was shown to be increased by 6.3% under &minus, 0.75% uniaxial compression. These findings demonstrate a way to increase efficiency of GaAs NW-based solar cells via uniaxial mechanical compression.

Efficiency of in situ AlGaAs and GaP and ex situ nitride surface passivation of p+ GaAs nanowires was studied. The efficiency was estimated by comparing of the photoluminescence intensity of the passivated nanowires with the unpassivated nanowire. The AlGaAs and nitride passivation lead to the increasing of the PL intensity by three orders of magnitude while the GaP passivation increases PL intensity only by one order. Photodegradation of the passivated NWs under intensive laser illumination was observed. AlGaAs, GaP and nitride passivated NWs photodegrade after one-minute exposure under laser power densities of 500, 300 and 30 kW/cm2, respectively.

Optomechanical resonators formed by the surface of an emitting semiconductor laser and the reflective surface of the probe of scanning probe microscope (SPM) were studied. The use of scanning probe microscopy techniques allowed studying the properties of such optomechanical resonators in details. The effect of optical cooling of an SPM probe in an optomechanical resonator was investigated by measuring the spectra of thermal vibrations of an SPM probe. Minimal achievable temperatures of SPM probe cooling were experimentally established.

Energy harvesting is an area that presents the greatest potential for powering wireless low-energy electronics. Development of semiconductor-based energy harvesting is of interest. Herein we focus on studying two different energy harvesting mechanisms in indium phosphide. Piezoelectric harvesting was checked by bending vertical InP nanowires with conductive atomic force microscope (AFM) probe and simultaneously detecting current passing through the probe. No current pulses were observed that we associate with surface trapping of piezoinduced carriers. Triboelectric generation was studied by creating frictional contact between AFM tip and metal-dielectric-semiconductor (MIS) interface formed by InP layer covered with native oxide. Current peaks of 320 pA amplitude corresponding to the current density around 2.3 kA/m2 were observed under sliding reciprocating motion of the tip during AFM scanning. Achieved current density was higher than in polymer- and silicon-based triboelectric structures. The open-circuit voltage value exceeded 15 mV and the output electric power was 35 W/m2.

Usually electromagnetic modes inside a laser resonator are a matter of the theoretical studies. In a sense we manage "to have a look into a whispering gallery mode (WGM) resonator" and observe how the resonator modes arrange in reality. The picture occurs to be quite different from the commonly used Bessel modes in a disk resonator. A chance to explore optical modes inside a resonator appears in a WGM laser with a cleaved cavity. The flat laser facet gives an opportunity to study both far and near field patterns formed by different modes. In this research we use a high resolution technique of detection of laser emission based on an atomic force microscope, which allowed us to visualize even high Q modes normally sealed inside the resonator. This information was completed with spatially resolved emission spectra and far-field patterns measured using an infrared camera. The analysis of the obtained results using both wave and geometrical optics approaches and finite elements simulations showed that emission of the studied devices is governed by a few low order optical modes experiencing a small number of reflections from the resonator walls. These modes can be considered as counter propagating Gaussian beams and their interference at the laser facet was also observed in the experiment. This work showed that, contrary to conventional ridge or surface emitting lasers, in such deformed disk resonators outputs of different optical modes are spatially separated and can be studied individually along the cleaved facet of the laser.

Surface nitridation by hydrazine-sulfide solution, which is known to produce surface passivation of GaAs crystals, was applied to GaAs nanowires (NWs). We studied the effect of nitridation on conductivity and microphotoluminescence (μ-PL) of individual GaAs NWs using conductive atomic force microscopy (CAFM) and confocal luminescent microscopy (CLM), respectively. Nitridation is found to produce an essential increase in the NW conductivity and the μ-PL intensity as well evidence of surface passivation. Estimations show that the nitride passivation reduces the surface state density by a factor of 6, which is of the same order as that found for GaAs/AlGaAs nanowires. The effects of the nitride passivation are also stable under atmospheric ambient conditions for six months.

We experimentally and theoretically examined the mode structure of half-disk lasers fabricated by cleaving whispering gallery mode (WGM)-disk lasers. The WGM lasers with a radii R of 50–150 μm emitting near 2.2 μm were fabricated using the GaSb-based quantum well structure [1]. The cleavage usually passes not through the centre of the disk with the deviation δ/R up to 20 % (Fig. 1a). Recently we have demonstrated that in the half-disk lasers the output beam is formed by optical modes located at a distance of 5–20 μm from the disk periphery [2], whereas in the full disk the light propagates along the disk wall, much closer to its edges.

In this manuscript, we present the study of elastic properties of InP nanowires with help of scanning probe microscope in advanced PeakForce Tapping® regime. The measuring method was developed in order to investigate the Young's modulus of these cone-shaped structures with significant accuracy. The difference in InP elasticity for wurtzite phase and zinc- blende phase was revealed. It was shown that elastic modulus of InP nanowires significantly increases from 60 GPa to more than 100 GPa when diameter of a nanowire is reduced below 50 nm. The core-shell model for InP nanowire was used for the explanation of this effect.

Scanning probe microscopy allows to study surface processes in semiconductor heterostructures. This work shows the investigation of topography and surface potential distribution in AlGaAs/GaAs p-n diode after applied reverse voltage. Formation of oxide on the surface in the area of p-n junction was found due to increasing of the applied reverse voltage. Formed oxide increases a breakdown voltage by 20%.

This work presents the results of an investigation of thin layers of MoSe2 on graphene by Scanning Probe Microscopy (SPM) methods. Dependences of the surface potential and work function on the number of monolayers of the structure are presented. The dependence of the surface potential on air humidity is shown. The band structure and doping of the MoSe2 monolayer on graphene was determined. These data can be important for detecting of the number of MoSe2 layers and for designing nanodevices, because the surface potential has a strong influence on the operation of such devices.

By means of atomic force microscopy (AFM) the spatial mapping of the laser intensity was performed on the cleavage of the whispering gallery modes (WGM) half-disk laser. The study was carried out in the near- and far-field regime. It showed a strong spatial divergence of different modes in the laser.

We demonstrate the effect of infrared (IR) illumination on mechanical resonance frequency of scanning probe microscope (SPM) probe. The analytical estimations permit to relate the observed effect with the SPM-probe heating by IR-radiation. This effect can be used for the SPM mapping of IR-radiation spatial distribution on the emitting surfaces. The proposed method demonstrates high sensitivity to the infrared radiation and lateral resolution in the sub-micron range. It was applied to visualize complex spatial structure of lasing modes on the mirror of whispering-gallery mode GaSb/GaAlAsSb/GaInAsSb laser.

Optical and electrical properties of MoSe2 monolayer transferred to graphene/SiC substrate are studied by scanning probe microscopy and photoluminescence spectroscopy. By Kelvin probe microscopy it was measured a work function of the MoSe2 monolayer (4.3 eV) and it was revealed the n-type of doping. With the increasing of MoSe2 layers quantity the work function also increases which leads to the reducing of the Schottky barrier height between the SPM probe and the layers. Monolayer graphene quenches photoluminescence of the MoSe2 monolayer while the bilayer graphene does not perturb the photoluminescence.

An operating semiconductor laser has been studied using a scanning probe microscope. A shift of the resonance frequency of probe that is due to its heating by laser radiation has been analyzed. The observed shift is proportional to the absorbed radiation and can be used to measure the laser near field or its output power. A periodical dependence of the measured signal has been observed as a function of distance between the probe and the surface of the laser due to the interference of the outgoing and cantilever-reflected waves. Due to the multiple reflections resulting in the interference, the light absorption by the probe cantilever is greatly enhanced compared with a single pass case. Interaction of infrared emission of a diode laser with different probes has been studied.

In this work we demonstrate the possibility of studying the current-voltage characteristics for single vertically standing semiconductor nanowires on standard AFM equipped by current measuring module in PeakForce Tapping mode. On the basis of research of eight different samples of p-doped GaAs nanowires grown on different GaAs substrates, peculiar electrical effects were revealed. It was found how covering of substrate surface by SiOx layer increases the current, as well as phosphorous passivation of the grown nanowires. Elimination of the Schottky barrier between golden cap and the top parts of nanowires was observed. It was additionally studied that charge accumulation on the shell of single nanowires affects its resistivity and causes the hysteresis loops on I-V curves.

The control of nanowire-based device performance requires knowledge about the transport of charge carriers and its limiting factors. We present the experimental and modeled results of a study of electrical properties of GaAs nanowires (NWs), considering their native oxide cover. Measurements of individual vertical NWs were performed by conductive atomic force microscopy (C-AFM). Experimental C-AFM observations with numerical simulations revealed the complex resistive behavior of NWs. A hysteresis of current-voltage characteristics of the p-doped NWs as-grown on substrates with different types of doping was registered. The emergence of hysteresis was explained by the trapping of majority carriers in the surface oxide layer near the reverse-biased barriers under the source-drain current. It was found that the accumulation of charge increases the current for highly doped p+-NWs on n+-substrates, while for moderately doped p-NWs on p+-substrates, charge accumulation decreases the current due to blocking of the conductive channel of NWs.

In this paper a probe of an atomic force microscope (AFM) served as a detector of optical radiation. We investigated absorption of optical radiation in the visible range by commercially available Si and Si3N4 probes. It has been shown that the radiation in the far field is mainly absorbed in the probe tip. By scanning a laser beam with a diameter of ~ 0.7 microns by the AFM probe, a lateral resolution of ~ 1.5 microns was demonstrated. Numerical modeling of evanescent wave absorption by AFM probes showed enhancement of the absorption in the probes as compared with a flat surface. A Si3N4 probe more effectively absorbs the evanescent radiation.

Novel scanning probe microscopy method of near field imaging of laser radiation is proposed. The method providing a submicron spatial resolution is based on detection of a shift of the probe resonance related to its heating by absorbed radiation. The method has been realized with a conventional silicon probe and has been employed for visualization of infrared emission from a half-disk semiconductor whispering gallery mode laser. Keywords— scanning probe microscopy; near field; laser radiation

In this work a method is proposed for measurements of current-voltage (I-V) characteristics for individual semiconductor nanowhiskers (NWs) standing on the growth surface. It is shown that conductive probe of atomic force microscope (AFM) can be placed at the top of nanowhiskers with a great precision and used as a stable local electrode. For that, a fixation of NWs is necessary to avoid lateral bending of NWs under pressure of AFM probe. In the present work we have studied current-voltage characteristics of NWs with radial heterostructure GaAs/AlGaAs grown under Au droplets on GaAs substrate. For fixation, NWs were partially overgrown with GaAs layer. The current-voltage characteristics measurements on the fixed NWs permitted to investigate the Si-doping of GaAlAs/GaAs NWs. They were found to be Schottky barrier p-type curves, what shows that Si-atoms incorporate into GaAs NWs with a formation of acceptor centres contrary to the case of bulk GaAs. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We propose an experimental method of near field optical imaging by scanning probe microscopy in which the probe itself serves as an infrared photodetector. The method providing a submicron spatial resolution is based on detection of a shift of the probe resonance related to its heating by absorbed IR radiation. The method does not require an apertured probe and can be realized with a conventional silicon probe used in atomic force microscopy. The method has been employed for visualization of infrared emission from a half-disk semiconductor whispering gallery mode laser.

Current–voltage (I–V) characteristics of vertical p-GaAs nanowires (NWs) covered by different surface passivation materials were experimentally measured by conductive atomic force microscopy (C-AFM). The obtained I–V curves for individual NWs with a diameter of 100 nm covered with AlGaAs, GaN, GaP or InP shell layers were compared to analyse the influence of surface passivation on the density of surface states and choose the most beneficial passivating material for technological applications. We have found the absence of a Schottky barrier between the golden catalytic cap on the top of a NW and the nanowire situated below and covered with an ultrathin GaP passivating layer. It was suggested that passivating material can arrange the heterostructure configuration with the GaAs NW near the Au cap. The latter mechanism was proposed to explain a strong energy barrier found in nanowires covered with InP passivation. AlGaAs passivation affected the forward threshold voltage of nanowires for NWs, which was measured simultaneously with the resistivity of each individual vertical structure from an array by means of AFM in the regime of measuring the I–V curves and onefold calculations. We made an attempt to develop the methodology of measurement and characterization of electric properties of passivated NWs.