Single GaAs Quantum Wires Research Articles

Near-Field Optical Imaging and Spectroscopy of Single GaAs Quantum Wires

Low-dimensional semiconductor structures grown by molecular beam epitaxy on a patterned (311)A GaAs substrate are investigated by near-field spectroscopy at a temperature of 10 K. In particular, the two-dimensional potential profiles of quantum wire and coupled wire-dot structures are determined from photoluminescence (PL) measurements with a spatial resolution of 150 nm. Also presented is an optical method for investigating carrier transport in low-dimensional systems involving performing spatially resolved PL excitation measurements on the wire-dot structure.

Micro-photoluminescence spectroscopy of single (Al, Ga)As quantum wires grown on vicinal GaAs ( [formula omitted]) surfaces

Single AlGaAs and GaAs quantum wires (QWRs) have been investigated by micro-photoluminescence (micro-PL). The micro-PL spectra of the GaAs QWR reveal that the GaAs QWR consists of a number of sharp peaks with a full-width at half-maximum (FWHM) of several hundreds μeV, which is similar to the results of previous reports. In contrast, micro-PL spectra of the AlGaAs QWR exhibit only one peak with a FWHM of ∼1 meV , indicating homogeneous QWR. The CL measurements are consistent with the results of the micro-PL measurements. The different features of the micro-PL spectra of the GaAs and AlGaAs QWR come from fluctuations in the thickness of the QWR. Therefore, the AlGaAs homogeneous QWRs are expected to exhibit intrinsic one-dimensional properties of the QWR and to clarify various properties of one-dimensional systems, such as carrier transportation in the QWRs.

Near-field optical spectroscopy of localized and delocalized excitons in a single GaAs quantum wire

Excitons in a GaAs quantum wire are studied in high-resolution photoluminescence experiments performed at a temperature of about 10 K with a spatial resolution of 150 nm, and a spectral resolution of 100 \ensuremath{\mu}eV. We report an observation of quasi-one-dimensional excitons which are delocalized over a length of up to several microns along the quantum wire. Such excitons give rise to a 10-meV broad luminescence band, representing a superposition of transitions between different delocalized states. In addition, we find a set of sharp luminescence peaks from excitons localized on a sub-150-nm length scale. Theoretical calculations of exciton states in a disordered quasi-one-dimensional potential reproduce the experimental results.

Quantization energy mapping of single V-groove GaAs quantum wires by femtosecond near-field optics

We demonstrate that femtosecond pump–probe spectroscopy in the optical near field is well suited to study the intrinsic properties of single V-groove GaAs quantum wires. Temporally and spatially resolved experiments show that the shape of near-field pump–probe traces sensitively depends on the detuning between the laser photon energy and the lowest exciton resonance of a quantum wire. This detuning dependence allows one to map the quantization energy fluctuations along a single quantum wire with 200 nm spatial resolution. We measure fluctuations of about 12 meV over 2 μm wire length, resulting from wire thickness variations of 1 ML.

Near Field Optical Spectroscopy of Confined Excitons

Two novel approaches for mapping the microscopic carrier dynamics in semiconductor nanostructures in real space and time are discussed. Combining quasi two-color femtosecond pump–probe spectroscopy and near-field microscopy, the temporal evolution of the nonlinear change in reflectivity of a single GaAs quantum wire is imaged with a time resolution of 200 fs and a spatial resolution of 200 nm. The experiments evidence the ultrafast relaxation of carriers into a quantum wire at room temperature and carrier transport along the wire axis. The second approach is based on an optical marker technique using a novel electronically coupled wire–dot sample grown on (311)A GaAs substrates. First low temperature near-field spectra reveal the interesting properties of this nanostructure.

Femtosecond near-field spectroscopy of a single GaAs quantum wire

Quasi-two-color femtosecond pump and probe spectroscopy and near-field scanning optical microscopy are combined to study the carrier dynamics in single semiconductor nanostructures. In temporally, spectrally, and spatially resolved measurements with a time resolution of 200 fs and a spatial resolution of 200 nm, the nonlinear change in reflectivity of a single quantum wire is mapped in real space and time. The experiments show that carrier relaxation in a single quantum wire occurs on a 100 fs time scale at room temperature.

Spatially and temporally resolved near-field spectroscopy of single GaAs quantum wires

Single GaAs quantum wires grown on patterned (311)A GaAs substrates are studied by steady-state and time-resolved near-field optical spectroscopy with a spatial resolution of 250 nm. The photoluminescence and photoluminescence excitation spectra as well as lateral variations of the quasi-one-dimensional confinement potential of the quantum wire are determined by measurements in the temperature range from 10 to 300 K. Diffusive transport of excitons and trapping into the quantum wire are directly monitored in picosecond luminescence experiments, allowing a quantitative measurement of exciton mobility. First near-field pump-probe experiments with femtosecond time resolution demonstrate the potential of this technique for monitoring spatio-temporal dynamics on ultrafast time scales.

Time‐resolved near‐field optics: exciton transport in semiconductor nanostructures

We present a systematic, temperature-dependent study of excitonic real-space transfer into single GaAs quantum wires using time-resolved low-temperature near-field luminescence spectroscopy. Excitons generated by local short pulse optical excitation in a 250 nm spot undergo diffusive transport over a length of several micrometres and are subsequently trapped into the quantum wire by optical phonon emission. The effect of local energy barriers in the vicinity of the quantum wire on the real-space transfer dynamics is monitored directly by mapping the time-resolved quantum wire luminescence. Experiments at variable temperatures are compared to numerical simulations based on drift-diffusive model calculations, and the spatio-temporal evolution of the two-dimensional exciton distribution within the nanostructure is visualized.

Exciton transport into a single GaAs quantum wire studied by picosecond near-field optical spectroscopy

We report a time-resolved near-field luminescence study of excitonic real-space transfer into single GaAs quantum wires. Excitons generated by local optical excitation in a 250 nm spot undergo diffusive transport over a length of several microns and are subsequently trapped into the quantum wire by optical phonon emission. Local energy barriers in the vicinity of the quantum wire, originating from the epitaxial growth mechanism of the nanostructure, directly influence the real-space transfer dynamics and trapping efficiency.

Carrier trapping into single GaAs quantum wires studied by variable temperature near-field spectroscopy

The trapping of carriers into single GaAs quantum wires is studied by near-field luminescence and luminescence excitation spectroscopy with a spatial resolution of 250 nm. The formation of the investigated GaAs wires relies on a new growth mode in the preferential migration of Ga atoms on patterned (3 1 1)A GaAs surfaces. Near-field photoluminescence excitation spectroscopy allows for a separation of quantum wire and well absorption and gives evidence of slight potential barriers in the vicinity of the quantum wire. At room temperature, carriers locally excited in the quantum well undergo real-space transfer over several micrometers, pass the barriers and are trapped into the quantum wire via phonon emission. At temperatures below 20 K, real-space transfer through the barriers is suppressed and only carriers generated in the vicinity of the quantum wire get trapped into bound states.

Near-Field Optical Spectroscopy of Carrier Exchange between Quantum Wells and Single GaAs Quantum Wires

Real-space transfer and trapping of carriers into single GaAs quantum wires on patterned (311)A GaAs surfaces are studied by near-field luminescence and luminescence excitation spectroscopy with a spatial resolution of 250 nm at temperatures of 8 and 300 K. At room temperature, real-space transfer of carriers in continuum states occurs over a length of several micrometers determined by the mobility of holes and the nanosecond carrier lifetimes. At low temperature, local energetic barriers in the vicinity of the wire suppress the capture process.

Real-Space Transfer and Trapping of Carriers into Single GaAs Quantum Wires Studied by Near-Field Optical Spectroscopy

We report the first near-field optical study of single GaAs quantum wires grown on patterned $(311)A$ GaAs surfaces. Spatially resolved optical spectra at a temperature of 10 K give evidence for one-dimensional carrier confinement and subband structure. At 300 K, electron-hole pairs in continuum states undergo diffusive real-space transfer over a length of several microns determined by hole mobility and trapping by optical phonon emission. Optical phonon scattering of carriers in the quantum wire establishes a quasiequilibrium carrier distribution in both wire and continuum states.

Vacuum near-field scanning optical microscope for variable cryogenic temperatures

We describe the design of a novel near-field scanning optical microscope for cryogenic temperatures and operation in vacuum. A helium flow cryostat is used for active temperature control of the sample in the range between 8 and 330 K, while all components of the near-field microscope are kept at room temperature. This design greatly simplifies near-field microscopy at variable sample temperatures and permits large piezoelectric scan ranges of up to 100×100×10 μm 3, the implementation of hardware-linearized piezoelectric scan stages, as well as flexible coarse positioning. First experiments with single GaAs quantum wires demonstrate the excellent performance of this new instrument.

Near-field Optical Spectroscopy of Single GaAs Quantum Wires

The nanoscopic room temperature optical properties of single quantum wires are characterized by a combination of near-field photoluminescence and photoluminescence excitation spectroscopy. Single GaAs quantum wires with a 50 nm lateral dimension are grown at the edge of 15 nm high mesa stripes on patterned GaAs(311) surfaces. Wire formation relies on the preferential migration of Ga atoms from a GaAs layer on the mesa top and bottom towards the sidewall. Spatially resolved photoluminescence spectra separate quantum wire and quantum well emission and image the diffusion of photoexcited carriers into the wires. Photoluminescence excitation spectra give insight into the absorption spectrum of the wires and the spectral position of different interband transitions in the one-dimensional carrier system. They allow the change in local thickness of the GaAs quantum well due to the migration process to be monitored directly with subwavelength spatial resolution. Both the trapping of carriers into the wire and the detrapping of carriers generated within the wire into the surrounding quantum well states are separately resolved.© 1997 John Wiley & Sons, Ltd.

Photoconductivity study of crescent-shaped GaAs/GaAlAs quantum wires grown by flow rate modulation epitaxy

We present photoconductivity data on GaAs quantum wires grown on a V-grooved substrate by flow rate modulation epitaxy. They show that a moderate excitation power density, ∼1 W/cm2, allows the observation of the absorption structure of a single GaAs quantum wire embedded in a p-i-n diode. Furthermore, by increasing the number of active wires inside a diode, the photoconductivity signal is enhanced and additional details of the absorption structure are evidenced. And, finally, a rough quantitative agreement is obtained between the experimental absorption transitions and a simple calculation of the one-dimensional excitonic states using the envelope function approximation.

One-dimensional excitons in V-shaped quantum wires

We report a detailed study of one-dimensional excitons in a planar array of single V-shaped GaAs quantum wires. Two-photon absorption, magnetoluminescence and linear photoluminescence spectroscopy have been used to measure the exciton binding energy, the excited 2p states of the excitons, the higher index transitions and the extension of the confined wavefunctions in the V-shaped region of the quantum wires.