High-resolution Photocurrent Research Articles

Electron and Hole g Tensors of Neutral and Charged Excitons in Single Quantum Dots by High-Resolution Photocurrent Spectroscopy

We report a high-resolution photocurrent (PC) spectroscopy of a single self-assembled InAs/GaAs quantum dot (QD) embedded in an n-i-Schottky device with an applied vector magnetic field. The PC spectra of positively charged exciton (X$^+$) and neutral exciton (X$^0$) are obtained by two-color resonant excitation. With an applied magnetic field in Voigt geometry, the double $\Lambda$ energy level structure of X$^+$ and the dark states of X$^0$ are observed in PC spectra clearly. In Faraday geometry, the PC amplitude of X$^+$ decreases and then quenches with the increasing of the magnetic field, which provides a new way to determine the relative sign of the electron and the hole g-factors. With an applied vector magnetic field, the electron and the hole g-factor tensors of X$^+$ and X$^0$ are obtained. The anisotropy of the hole g-factors of both X$^+$ and X$^0$ is larger than that of the electron.

Giant Photocurrent Enhancement by Coulomb Interaction in a Single Quantum Dot for Energy Harvesting

Understanding the carrier excitation and transport processes at the single-charge level plays a key role in quantum-dot-based solar cells and photodetectors. Here, we report on Coulomb-induced giant photocurrent enhancement of positive charged trions (\emph{X$^+$}) in a single self-assembled InAs/GaAs quantum dot embedded in an \emph{n-i-}Schottky device by high-resolution photocurrent (PC) spectroscopy. The Coulomb repulsion between the two holes in the \emph{X$^+$} increases the tunneling rate of the hole, and the remaining hole can be reused as the initial state to regenerate \emph{X$^+$} again. This process brings the PC amplitude of \emph{X$^+$} up to 30 times larger than that of the neutral exciton. The analysis of the hole tunneling time gives the equivalent change of hole tunnel barriers caused by Coulomb interaction between two holes with a value of 8.05 meV during the tunneling process. Our work brings a fundamental understanding of energy conversion for solar cells in nanoscale to improve internal quantum efficiency for energy harvesting.

Coherent photocurrent spectroscopy of single InP-based quantum dots in the telecom band at 1.5 µm

In this work we study the resonant and coherent properties of single InP-based InAs quantum dots, which show an optical emission in the telecom C-band and L-band. High-resolution resonant photocurrent spectroscopy on p–i–n devices reveals narrow linewidths and fully resolved fine structure splittings. We observe Lorentzian line shapes, which allow for the extraction of dephasing times as a function of the applied bias voltage. Coherent ps laser excitation results in pronounced Rabi rotations with increasing pulse area. For π-pulse excitation, we obtain more than 93 % of the theoretically expected photocurrent amplitude. Our results also demonstrate that such state-of-the-art InP-based quantum dots for the telecom band exhibit promising key parameters comparable to well-established InAs/GaAs counterparts.

Electrical control of quantum-dot fine-structure splitting for high-fidelity hole spin initialization

We demonstrate electrical control of the neutral exciton fine-structure splitting in a single InAs/GaAs self-assembled quantum dot by significantly reducing the splitting to near zero through the application of a vertical electric field in the fast electron tunneling regime. This is verified by performing high-resolution photocurrent spectroscopy of the two fine-structure split exciton eigenstates as a function of reverse bias voltage. Using the qubit initialization scheme for a quantum-dot hole spin based on rapid electric-field ionization of a spin-polarized exciton, our results suggest a practical approach towards achieving qubit initialization with near-unity fidelity in the absence of magnetic fields.

Networks of semiconducting SWNTs: contribution of midgap electronic states to the electrical transport.

Single-walled carbon nanotube (SWNT) thin films provide a unique platform for the development of electronic and photonic devices because they combine the advantages of the outstanding physical properties of individual SWNTs with the capabilities of large area thin film manufacturing and patterning technologies. Flexible SWNT thin film based field-effect transistors, sensors, detectors, photovoltaic cells, and light emitting diodes have been already demonstrated, and SWNT thin film transparent, conductive coatings for large area displays and smart windows are under development. While chirally pure SWNTs are not yet commercially available, the marketing of semiconducting (SC) and metallic (MT) SWNTs has facilitated progress toward applications by making available materials of consistent electronic structure. Nevertheless the electrical transport properties of networks of separated SWNTs are inferior to those of individual SWNTs. In particular, for semiconducting SWNTs, which are the subject of this Account, the electrical transport drastically differs from the behavior of traditional semiconductors: for example, the bandgap of germanium (E = 0.66 eV) roughly matches that of individual SC-SWNTs of diameter 1.5 nm, but in the range 300-100 K, the intrinsic carrier concentration in Ge decreases by more than 10 orders of magnitude while the conductivity of a typical SC-SWNT network decreases by less than a factor of 4. Clearly this weak modulation of the conductivity hinders the application of SC-SWNT films as field effect transistors and photodetectors, and it is the purpose of this Account to analyze the mechanism of the electrical transport leading to the unusually weak temperature dependence of the electrical conductivity of such networks. Extrinsic factors such as the contribution of residual amounts of MT-SWNTs arising from incomplete separation and doping of SWNTs are evaluated. However, the observed temperature dependence of the conductivity indicates the presence of midgap electronic states in the semiconducting SWNTs, which provide a source of low-energy excitations, which can contribute to hopping conductance along the nanotubes following fluctuation induced tunneling across the internanotube junctions, which together dominate the low temperature transport and limit the resistivity of the films. At high temperatures, the intrinsic carriers thermally activated across the bandgap as in a traditional semiconductor became available for band transport. The midgap states pin the Fermi level to the middle of the bandgap, and their origin is ascribed to defects in the SWNT walls. The presence of such midgap states has been reported in connection with scanning tunneling spectroscopy experiments, Coulomb blockade observations in low temperature electrical measurements, selective electrochemical deposition imaging, tip-enhanced Raman spectroscopy, high resolution photocurrent spectroscopy, and the modeling of the electronic density of states associated with various defects. Midgap states are present in conventional semiconductors, but what is unusual in the present context is the extent of their contribution to the electrical transport in networks of semiconducting SWNTs. In this Account, we sharpen the focus on the midgap states in SC-SWNTs, their effect on the electronic properties of SC-SWNT networks, and the importance of these effects on efforts to develop electronic and photonic applications of SC-SWNTs.

Correlations between Photovoltaic Characteristics, Adsorption Number, and Regeneration Kinetics in Dye-Sensitized Solar Cells Revealed by Scanning Photocurrent Microscopy.

Newly developed simultaneous scanning photocurrent and luminescence microscopy was applied to ruthenium-based dye-sensitized solar cells (DSCs) comprising a cover glass photoanode with a 100 nm thick TiO2 layer. Using this, we have investigated the lateral variations of several parameters of these DSCs under short-circuit conditions. Simultaneous measurement of photocurrent and luminescence images for the same area of the DSC demonstrated submicrometric lateral resolution of our photocurrent microscopy, which is approximately 10 times better than the resolution of photocurrent microscopy used in past studies. The photovoltaic parameters, such as short-circuit current density, open-circuit voltage, and charge-collection efficiency, were thus evaluated for local (or submicrometric) regions of the DSCs. Furthermore, the photocurrent saturation behavior of the DSCs was examined as a function of the excitation rate and analyzed on the basis of a three-state kinetic model. This protocol allowed for quantification of the dye-adsorption number and dye-regeneration rate constant for any local area of the DSCs. Consequently, the correlations between the dye adsorption number, photovoltaic parameters, and regeneration rate constant, which are difficult to address through examination of the entire cell, were revealed by the "zoom-in" approach utilizing this high-resolution photocurrent microscopy.

High-resolution photocurrent microscopy using near-field cathodoluminescence of quantum dots

We report a fast, versatile photocurrent imaging technique to visualize the local photo response of solar energy devices and optoelectronics using near-field cathodoluminescence (CL) from a homogeneous quantum dot layer. This approach is quantitatively compared with direct measurements of high-resolution Electron Beam Induced Current (EBIC) using a thin film solar cell (n-CdS / p-CdTe). Qualitatively, the observed image contrast is similar, showing strong enhancement of the carrier collection efficiency at the p-n junction and near the grain boundaries. The spatial resolution of the new technique, termed Q-EBIC (EBIC using quantum dots), is determined by the absorption depth of photons. The results demonstrate a new method for high-resolution, sub-wavelength photocurrent imaging measurement relevant for a wide range of applications.

High-resolution photocurrent spectroscopy of the positive trion state in a single quantum dot

We demonstrate high-resolution photocurrent spectroscopy of the positive trion state (X+) in a single InAs/GaAs self-assembled quantum dot (QD). Exploiting a Coulomb-mediated renormalization of the X+ transition energy, our approach allows for a sub-mu eV resolution, which is more than four orders of magnitude higher than that in previous approaches. Furthermore, such high-resolution spectroscopy of X+ permits us to obtain precise values for the X+ permanent dipole moment and polarizability, by fitting a theoretical model to the measured transition energy as a function of electric field. Since optical initialization, manipulation, and readout of a QD spin rely on the trion state as an intermediary state, such high-resolution measurements of the X+ spectrum and its dipole moment are essential for performing high-fidelity quantum computing operations on QD spin qubits.

High-Resolution Photocurrent Imaging of Bulk Heterojunction Solar Cells.

Images obtained from photocurrent scanning of organic bulk heterojunction solar cell devices provide a direct measure of correlation of the morphology to the performance parameters. The peripheral photocurrent induced from light coupled to probe tips in the near-field regime of bulk heterojunction layers permits in situ scanning of active solar cells with asymmetric electrodes. We present a methodology involving a combination of atomic force microscopy, near-field optical microscopy, and near-field photocurrent microscopy to decipher the carrier generation and transport regions in the bulk heterojunction layer. The angular Fourier transformation technique is implemented on these images to rationalize the optimum blend concentration in crystalline and amorphous donor systems and provide insights into the role of the bulk heterojunction morphology.

Thin Au surface plasmon waveguide Schottky detectors on p-Si

Surface plasmon sub-bandgap Schottky detectors based on an asymmetric Au stripe waveguide on p-Si are investigated theoretically and experimentally at free-space wavelengths of λ0 = 1310 and 1550 nm. Au on p-Si produces a low Schottky barrier (0.33 eV), which improves the internal quantum efficiency. Thick and thin Au stripes are compared, with the latter increasing the hot hole emission probability relative to the former, and thus also improving the internal quantum efficiency. Two excitation schemes are considered: end facet illumination which launches surface plasmons on the detector, and top illumination which does not. Both schemes are implemented using a piezoelectric positioner that is programmed to scan the detection area in steps of 100–200 nm, thus enabling the acquisition of high-resolution photocurrent maps. The surface plasmon detectors yield a responsivity of ∼1 mA W−1, ∼2× larger than the same detectors under top illumination, due to the absorption of surface plasmons. We compare the measurements with theoretical results for both excitation schemes and estimate the hot hole attenuation length in our Au stripes to be ∼23 nm.

High-Resolution Photocurrent Mapping of Carbon Nanostructures

The spatial resolution of photocurrent measurements on carbon nanostructures has reached 20 nm, as demonstrated by Hartschuh and co-workers for individual carbon nanotubes in this issue of ACS Nano. In this Perspective, we provide a brief overview of the applications of scanning photocurrent microscopy to various one- and two-dimensional nanostructures and highlight the importance of the optical antenna concept for future studies of the optoelectronic properties of hybrid nanostructures.

Voltage-controlled electron tunneling from a single self-assembled quantum dot embedded in a two-dimensional-electron-gas-based photovoltaic cell

We perform high-resolution photocurrent (PC) spectroscopy to investigate resonantly the neutral exciton ground-state (X0) in a single InAs/GaAs self-assembled quantum dot (QD) embedded in the intrinsic region of an n-i-Schottky photodiode based on a two-dimensional electron gas (2DEG), which was formed from a Si δ-doped GaAs layer. Using such a device, a single-QD PC spectrum of X0 is measured by sweeping the bias-dependent X0 transition energy through that of a fixed narrow-bandwidth laser via the quantum-confined Stark effect (QCSE). By repeating such a measurement for a series of laser energies, a precise relationship between the X0 transition energy and bias voltage is then obtained. Taking into account power broadening of the X0 absorption peak, this allows for high-resolution measurements of the X0 homogeneous linewidth and, hence, the electron tunneling rate. The electron tunneling rate is measured as a function of the vertical electric field and described accurately by a theoretical model, yielding information about the electron confinement energy and QD height. We demonstrate that our devices can operate as 2DEG-based QD photovoltaic cells and conclude by proposing two optical spintronic devices that are now feasible.

High resolution photocurrent imaging by atomic force microscopy on the example of single buried InAs quantum dots

Conductive atomic force microscopy in combination with an optical setup is a useful tool for obtaining spectrally and spatially resolved photocurrent information on photoactive nanostructures. We have demonstrated photocurrent mapping under ambient conditions using sub-surface InAs quantum dots (QDs) as an example, whereby the QDs appear as dark areas in the photocurrent signal. Spectrally resolved measurements of on-dot and off-dot areas show distinct differences, confirming the existence of QDs at the detected position.

Two color, low intensity photocurrent feedback for local photocurrent spectroscopy

In this work, we introduce a two color, low intensity photocurrent feedback method for photocurrent spectroscopy utilizing an atomic force microscope (AFM). In most applications, measurements with weak optical excitations are not feasible with an AFM because the powerful AFM feedback laser severely disturbs the measurements. Therefore, we have developed a feedback system based on the pressure dependent Schottky barrier height at the tip-sample interface. The versatility of the new feedback system is demonstrated by recording high resolution photocurrent spectra on GaAsInAs heterostructures.

High resolution photocurrent‐spectroscopy of a single quantum dot

AbstractWe report on high resolution photocurrent spectroscopy of a single quantum dot. Our attention applies to the two‐level character of a single InGaAs/GaAs quantum dot, defined by the ground state exciton. With high resolution photocurrent spectroscopy we analyze the behaviour of a single quantum dot under cw‐excitation. Experiments include linewidth, powerbroadening and saturation analysis. In the limit of low optical excitation we observe a ground state line width down to 4 µeV. With increasing excitation intensities the line width shows a characteristic power broadening. This effect is a direct consequence of the saturation of the absorption in a two‐level system under conditions of high excitation intensities. Using the high resolution we are able to measure the excitonic fine structure splitting in the ground state. The results of the cw‐excitation experiments indicate as an almost perfect quantum mechanical two‐level system. These experiments are essential for coherent manipulation of single quantum dots, important for possible applications in quantum information processing. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Power broadening of the exciton linewidth in a single InGaAs∕GaAs quantum dot

We use high-resolution photocurrent spectroscopy to investigate the ground state of a single quantum dot. In the limit of low optical excitation power, we observe a ground state linewidth down to 4μeV. With increasing excitation intensities, the linewidth shows a characteristic power broadening. This effect is a direct consequence of the saturation of the absorption in a two-level system under conditions of high excitation intensities. From a comparison of both effects, we conclude that power-dependent dephasing is negligible in our system.

Scanned laser response studies of metal-insulator-silicon solar cells in polycrystalline czochralski silicon

Metal-insulator-silicon (MIS) solar cells with and without grain boundaries were fabricated using polycrystalline Czochralski silicon. Scanned laser spot methods were used to provide high resolution photocurrent and surface reflection images of these cells. Electrically active grain boundaries, revealed by suppressed photocurrent collection, did not correlate perfectly with etc-produced surface features. For the laser wavelength (6328 Å) an effective photocurrent loss of 30–40% occurred over a 16μm width grain boundaries. Comparisons of short-circuit current versus open-circuit voltage over a wide range of irradiance values were made on cells with and without grain boundaries. For a typical polycrystalline cell the air mass one photocurrent loss was 8%, whereas a 3% loss was determined for the laser excitation. The open-circuit voltage reduction was about 25mV, implying an opposing current density associated with grain boundary walls of about 50% of that for the MIS barrier. This work shows the value of combining scanned light spot methods with photovoltaic characterization using cells with and without active grain boundaries. Future extensions should allow studies of detailed grain boundary electrical behavior including procedures to modulate it.