Lucia Sorba

Unit Cell Structure of Crystal Polytypes in InAs and InSb Nanowires

The atomic distances in hexagonal polytypes of III-V compound semiconductors differ from the values expected from simply a change of the stacking sequence of (111) lattice planes. While these changes were difficult to quantify so far, we accurately determine the lattice parameters of zinc blende, wurtzite, and 4H polytypes for InAs and InSb nanowires, using X-ray diffraction and transmission electron microscopy. The results are compared to density functional theory calculations. Experiment and theory show that the occurrence of hexagonal bilayers tends to stretch the distances of atomic layers parallel to the c axis and to reduce the in-plane distances compared to those in zinc blende. The change of the lattice parameters scales linearly with the hexagonality of the polytype, defined as the fraction of bilayers with hexagonal character within one unit cell.

Semiconductor nanowires for highly sensitive, room-temperature detection of terahertz quantum cascade laser emission

We report on the development of nanowire-based field-effect transistors operating as high sensitivity terahertz (THz) detectors. By feeding the 1.5u2009THz radiation field of a quantum cascade laser (QCL) at the gate-source electrodes with a wide band dipole antenna, we record a photovoltage signal corresponding to responsivity values >10u2009V/W, with impressive noise equivalent power levels <6u2009×u200910−11u2009W/√Hz at room temperature and a wide modulation bandwidth. The potential scalability to even higher frequencies and the technological feasibility of realizing multi-pixel arrays coupled with QCL sources make the proposed technology highly competitive for a future generation of THz detection systems.

Terahetz detection by heterostructed InAs/InSb nanowire based field effect transistors

Heterostructured InAs/InSb nanowire (Nw) based field effect transistors (FET) have been fabricated and tested as Terahetz radiation detectors. While responsivity and noise equivalent power compare with the ones of InAs nanowire detectors, the presence of small-gap InSb semiconductor gives rise to interesting physical effects such an increase of the detected signal with charge injection through the wire, at odds with standard FET-detectors. Additionally, the photodetected signal voltage changes its sign after a threshold gate bias, which we explain considering surface-related transport and field asymmetries imposed by the use of a lateral gate electrode.

Electronic properties of quantum dot systems realized in semiconductor nanowires

Catalyst-assisted growth of semiconductor nanowires has opened up several new and exciting possibilities for low-dimensional semiconductor structures. The authors review progress on the realization of quantum dots in semiconductor nanowires, and their characterization by transport spectroscopy. Emphasis is placed on the wide range electronic properties exhibited due to flexibility of the growth process in terms of nanostructure composition and size. Particular attention is placed on studies of spin in few-electron quantum dots.

Controlling the diameter distribution and density of InAs nanowires grown by Au-assisted methods

III-V semiconductor nanowires have attracted intensive research interest because of their promising optical and electronic properties that can be manipulated by tailoring nanowire composition and morphology. Therefore, it is crucial to measure and control the diameter distribution of the grown nanowires. In this study, we analyze the diameter distribution of Au-catalyzed InAs nanowires. Au colloidal nanoparticles dispersed on InAs (111) B substrates and nanoparticles obtained by the thermal annealing of Au films were used as catalysts for InAs nanowire growth. The annealing time and temperature, the thickness of the Au film and the colloid sizes were systematically varied not only to understand their influence on nanowire diameter distribution, but also to find the optimal parameters for realizing samples with uniform and controlled diameter distribution. Morphological characterization was performed by scanning electron microscopy measurements and the image analysis was carried out using in-house-developed automated image analysis software to accurately determine the diameter distribution of the nanowires. A description of the image analysis software is also presented. The thermal annealing of films turned out to be the most suitable method for uniformity and density control, while the colloidal nanoparticles yielded narrow and more reproducible diameter distributions.

Detection of a 2.8 THz quantum cascade laser with a semiconductor nanowire field-effect transistor coupled to a bow-tie antenna

The use of a high-electron mobility semiconductor nanowire as transistor channel has recently allowed the extension of the spectral coverage ofu2009THz field-effect transistor detectors up to 1.5u2009THz. In this report, we demonstrate efficient operation of a field-effect transistor detector based on a semiconductor nanowire at a much higher frequency, 2.8u2009THz, with a responsivity ≈5u2009V/W in a bandwidth ≈100u2009kHz, thus proving the full potential of such approach for the detection ofu2009THz quantum cascade lasers. Finally, such au2009THz sensing system is exploited to perform raster scan transmission imaging, with high spatial resolution, signal-to-noise ratio, and acquisition rate.

Electron beam induced current in InSb-InAs nanowire type-III heterostructures

InSb-InAs nanowire heterostructure diodes investigated by electron beam induced current (EBIC) demonstrate an unusual spatial profile where the sign of the EBIC signal changes in the vicinity of the heterointerface. A qualitative explanation confirmed by theoretical calculations is based on the specific band diagram of the structure representing a type-III heterojunction with an accumulation layer in InAs. The sign of the EBIC signal depends on the specific parameters of this layer. In the course of measurements, the diffusion length of holes in InAs and its temperature dependence are also determined.

Sub-wavelength near field imaging techniques at terahertz frequencies

Near-field imaging techniques at terahertz (THz) frequencies are severely restricted by diffraction. To date, different detection schemes have been developed, based either on sub-wavelength metallic apertures or on sharp metallic tips. However high-resolution THz imaging, so far, has been relying predominantly on detection techniques that require either an ultrafast laser or a cryogenically-cooled THz detector, at the expenses of a lack of sensitivity when high resolution levels are needed. Here, we demonstrate two novel near-field THz imaging techniques able to combine strongly sub-wavelength spatial resolution with highly sensitive amplitude and phase detection capability. The first technique exploits an interferometric optical setup based on a THz quantum cascade laser (QCL) and on a near-field probe nanodetector, operating at room temperature. By performing phase-sensitive imaging of THz intensity patterns we demonstrate the potential of our novel architecture for coherent imaging with sub-wavelength spatial resolution improved up to 17 μm. The second technique is a detector-less s-SNOM system, exploiting a THz QCL as source and detector simultaneously. This approach enables amplitude- and phase-sensitive imaging by self-mixing interferometry with spatial resolution of 60-70 nm.

Experimental realization of a Fabry-Perot-type interferometer by co-propagating edge states in the integer and fractional quantum Hall effect regimes

A Fabry-Perot-type interferometer is experimentally realized for electrons in a semiconductor device. Interference conditions are created for co-propagating electrons in quantum Hall edge states, which results in oscillations of the current through the device for integer and fractional filling factors. We find the interference oscillations in transport across the incompressible strips with local filling factors νc = 1, 4/3, 2/3 even at high imbalances, exceeding the spectral gaps. In contrast, there is no sign of the interference in transport across the principal Laughlin νc = 1/3 incompressible strip. This indicates, that even at fractional νc, the interference effects are caused by normal electrons. The oscillations period is determined by the effective interferometer area, which is sensitive to the filling factors because of screening effects.

Transport anisotropy in high mobility In0.75Ga0.25As 2DEGs

Low temperature electron mobility of a two dimensional electron gas formed in a 20 nm thick In0.75Ga0.25As/In0.75Al0.25As Quantum Well grown on a (001) GaAs substrate shows a pronounced difference between the [1–10] and the [110] crystallographic directions. This anisotropy cannot be explained by the the traditional models for the interface roughness scattering. On the contrary, a conduction band energy modulation, correlated to this roughness, is a promising candidate as the mobility limiting mechanism. Using the Landauer approach and calculating the conductances based on the numerical solution of the Schrodinger equation an estimation of the conductance along the two directions can be obtained.