Irma Kuljanishvili

Individual GaN Nanowires Exhibit Strong Piezoelectricity in 3D

Semiconductor GaN NWs are promising components in next generation nano- and optoelectronic systems. In addition to their direct band gap, they exhibit piezoelectricity, which renders them particularly attractive in energy harvesting applications for self-powered devices. Nanowires are often considered as one-dimensional nanostructures; however, the electromechanical coupling leads to a third rank tensor that for wurtzite crystals (GaN NWs) possesses three independent coefficients, d(33), d(13), and d(15). Therefore, the full piezoelectric characterization of individual GaN NWs requires application of electric fields in different directions and measurements of associated displacements on the order of several picometers. In this Letter, we present an experimental approach based on scanning probe microscopy to directly quantify the three-dimensional piezoelectric response of individual GaN NWs. Experimental results reveal that GaN NWs exhibit strong piezoelectricity in three dimensions, with up to six times the effect in bulk. Based on finite element modeling, this finding has major implication on the design of energy harvesting systems exhibiting unprecedented levels of power density production. The presented method is applicable to other piezoelectric NW materials as well as wires manufactured along different crystallographic orientations.

Controllable patterning and CVD growth of isolated carbon nanotubes with direct parallel writing of catalyst using dip-pen nanolithography.

We report a process to fabricate carbon nanotubes (CNT) by chemical vapor deposition at predetermined location. This process was enabled by patterning catalyst nanoparticles directly on silicon substrates with nanometer-scale precision using Dip Pen Nanolithography(R) (DPN(R)). A multi-pen writing method was employed to increase the patterning rate. The development of new molecular inks for the deposition of the precursor catalyst resulted in a high yield of isolated carbon nanotubes, ideal for subsequent device fabrication. Here, we demonstrate the advantages of the new method for producing high quality isolated CNT in scalable array geometries.

Scanning-probe spectroscopy of semiconductor donor molecules

Semiconductor devices continue to press into the nanoscale regime, and new applications have been proposed for which a single dopant atom acts as the functional part of the device1,2,3. Moreover, because shallow donors and acceptors are analogous to hydrogen atoms, experiments on small numbers of dopants have the potential to be a testing ground for fundamental questions of atomic and molecular physics4,5. Although dopant properties are well understood with respect to the bulk, the study of configurations of dopants in small numbers is an emerging field6,7. Here we present local capacitance measurements of electrons entering silicon donors in a gallium arsenide heterostructure. To the best of our knowledge, this study is the first example of single-electron capacitance spectroscopy carried out directly with a scanning probe tip8. The precise position with respect to tip voltage of the observed single-electron peaks varies with the location of the probe, reflecting a random distribution of silicon within the donor plane. In addition, three broad capacitance peaks are observed independent of the probe location, indicating clusters of electrons entering the system at approximately the same voltages. These broad peaks are consistent with the addition energy spectrum of donor molecules, effectively formed by nearest-neighbour pairs of silicon donors.

Direct observation of micron-scale ordered structure in a two-dimensional electron system

We have extended the subsurface charge accumulation imaging method to directly resolve the interior structure of a GaAs/AlGaAs two-dimensional electron system in a tunneling geometry. We find that the application of a perpendicular magnetic field can induce surprising density modulations that are not static as a function of the field. Near six and four filled Landau levels, stripelike structures emerge with a characteristic length scale ∼2 μm. Present theories do not account for ordered density modulations on this scale.

Scanning-probe Single-electron Capacitance Spectroscopy

The integration of low-temperature scanning-probe techniques and single-electron capacitance spectroscopy represents a powerful tool to study the electronic quantum structure of small systems - including individual atomic dopants in semiconductors. Here we present a capacitance-based method, known as Subsurface Charge Accumulation (SCA) imaging, which is capable of resolving single-electron charging while achieving sufficient spatial resolution to image individual atomic dopants. The use of a capacitance technique enables observation of subsurface features, such as dopants buried many nanometers beneath the surface of a semiconductor material(1,2,3). In principle, this technique can be applied to any system to resolve electron motion below an insulating surface. As in other electric-field-sensitive scanned-probe techniques(4), the lateral spatial resolution of the measurement depends in part on the radius of curvature of the probe tip. Using tips with a small radius of curvature can enable spatial resolution of a few tens of nanometers. This fine spatial resolution allows investigations of small numbers (down to one) of subsurface dopants(1,2). The charge resolution depends greatly on the sensitivity of the charge detection circuitry; using high electron mobility transistors (HEMT) in such circuits at cryogenic temperatures enables a sensitivity of approximately 0.01 electrons/Hz(½) at 0.3 K(5).

Scanning Charge Accumulation Probe of Semiconductor Donor Molecules

We have developed a scanning probe method that is able to detect individual electrons entering a system of semiconductor donor atoms. We have applied the method to a system of Si donors within a GaAs‐AlGaAs heterostructure sample. The data compare well to a model that considers donor molecules, effectively formed by nearest‐neighbor silicon atoms.

Tunneling images of a 2D electron system in a quantizing magnetic field

Abstract We have applied a scanning probe method, subsurface charge accumulation imaging, to resolve the local structure of the interior of a semiconductor two-dimensional electron system (2DES) in a tunneling geometry. Near magnetic fields corresponding to integer Landau level filling, submicron scale spatial structure in the out-of-phase component of the tunneling signal becomes visible. In the images presented here, the structure repeats itself when the filling factor is changed from ν =6 to 7. Therefore, we believe the images reflect small modulations in the 2DES density caused by the disorder in the sample.