Kensuke Ogawa

GROWTH AND OPTICAL PROPERTIES OF NANOMETER-SCALE GAAS AND INAS WHISKERS

The growth process, crystal structure, and optical properties of ultrathin GaAs and InAs wires (whiskers) as thin as 15–40 nm and about 2 μm long are reviewed and discussed. Experimental results for growing whiskers using Au as a growth catalyst during metalorganic vapor phase epitaxy (MOVPE) and the shape and growth direction of whiskers provide new insight into growth control of GaAs and InAs whiskers. The crystal structure of whiskers, Au behavior during MOVPE, and their growth mechanism are reviewed and discussed on the basis of transmission electron microscopic analysis. The photoluminescence spectra of GaAs wires are compared with those of a GaAs epitaxial layer, and the effect of surface treatment on the luminescence peak energy shift is discussed. The time dependent photoluminescence of GaAs wires is also discussed. The application of GaAs whiskers to light emitting devices is reviewed because a semiconductor wire structure employing quantum size effects is a very important element of electronic a...

GaAs p‐n junction formed in quantum wire crystals

A p‐n junction is formed for the first time in a cross‐sectional area of a GaAs wire crystal with a diameter of about 100 nm. Ultrafine cylindrical growth by metalorganic vapor phase epitaxy is employed for the fabrication. Current‐voltage and capacitance‐voltage characteristics confirm the formation of the p‐n junction in a narrow area at the midpoint of a wire crystal. Intensive light emission by current injection is observed at 77 K and even at room temperature. These results suggest that ultrafine optoelectronic devices with quantum‐size p‐n junction are possible.

GaAs free‐standing quantum‐size wires

Ultrathin GaAs wires as thin as 15–40 nm and about 2 μm long have been grown on a GaAs substrate by metal‐organic vapor‐phase epitaxy. The wires, which consist of whiskers, are grown between 380 and 550 °C using trimethylgallium and arsine (AsH3) as source materials. It is found that the wire growth direction is parallel to the [111] arsenic dangling‐bond direction and can be perfectly controlled by the crystallographic orientation of the GaAs substrate surface. From transmission electron microscopic analysis it is revealed that the crystal structure of the wire coincides with the zinc‐blende type for the growth temperature range of 460–500 °C, but it changes to the wurtzite type at 420 °C and temperatures higher than 500 °C. It is also found that the wires have a twin‐type structure around the [111] growth axis for zinc blende and [0001] growth axis for wurtzite. Photoluminescence study of these wires shows that the luminescence peak energy shifts to a higher energy as the wire width decreases from 100 t...

Quantum size microcrystals grown using organometallic vapor phase epitaxy

Needle‐shaped quantum size microcrystals as thin as 10 nm have been selectively grown by employing reduced pressure organometallic vapor phase epitaxy using trimethylgallium and arsine as source materials. The microcrystals grown within a SiO2 window area have their growth axes along the [111] direction. Transmission electron diffraction analysis shows that the crystal structure of microcrystals is consistent with the zinc‐blende structure of GaAs. The mechanism for growing the needle‐shaped crystals is similar to a vapor‐liquid‐solid (VLS) equilibrium phase growth model. From photoluminescence measurements at 4.2 K, it is found that the microcrystals show a very distinct spectra for free exciton and neutral acceptor‐bound exciton recombinations, meaning good crystal quality.

Excitonic polaritons in quantum‐confined systems and applications to optoelectronic devices

An excitonic polariton is a complex quasiparticle that consists of a photon and an exciton. Excitonic polaritons have recently been shown to exist in quantum‐confined systems such as GaAs quantum wells. Based on the coherent coupling between the charged electron (hole) and light, the quantum‐confined excitonic polariton has the characteristics of large coherence length and large phase modulation under electric fields. Furthermore, because of the inherent large refractive index, the spatial shape of the guided mode of the excitonic polariton transmitted in such quantum‐confined waveguides is expected to be squeezed significantly. We discuss the characteristics of such excitonic polaritons and possible applications to ultrasmall optoelectronic devices.

Optical characterization of GaAs quantum wire microcrystals

Abstract Optical properties of quantum-size GaAs wire crystals grown by organometallic vapor-phase epitaxy (OMVPE) are measured. The typical size of the wire-shaped microcrystals is 1–5 μm long and 10–200 nm wide. Photoluminescence measurements at 4 K reveal spectral features dominated by free carrier to acceptor impurity recombination. A free exciton recombination line is also observed and is more intense relative to other features than that observed from a conventional OMVPE epitaxially grown layer. Small spectral shifts (0.5 meV) of the free exciton and the acceptor-bound exciton recombination lines are considered to be due to the effects of quantum confinement on the energy levels of the system.

Time-of-flight measurement of excitonic polaritons in a GaAs/AlGaAs quantum well

We report the first experimental observation of excitonic polaritons in a GaAs/AlGaAs quantum well with incident light propagating parallel to the quantum well layer. We have built a time‐of‐flight measurement system which allows us to investigate low‐temperature optical properties of semiconductor waveguides with picosecond time resolution. This system has been used to measure propagation delay time of an incident light pulse transmitted through the quantum well. The delay time increases drastically near the photon energies resonant to the optical absorption lines of the quantum well excitons. This behavior shows that the group velocity of the light pulse decreases as a result of the formation of quantum well excitonic polaritons. The group velocity drops to 7×104 m/s at a heavy‐hole exciton absorption line at 6.0 K.

Ultrafast characterization of an in‐plane gate transistor integrated with photoconductive switches

An in‐plane gate field‐effect transistor is characterized by ultrafast electro‐optic sampling. The transistor is monolithically integrated with photoconductive switches in coplanar waveguide and <0.5 ps measurement time resolution is achieved. The gate‐drain capacitance of the transistor is obtained as 1.8 fF at zero drain voltage from displacement current transients. The gate‐drain capacitance is dominated by parasitic capacitance and the intrinsic gate‐drain capacitance is estimated as less than 0.2 fF.

Monolithically-integrated optoelectronic circuit for ultrafast sampling of a dual-gate field-effect transistor

An integrated optoelectronic circuit for ultrafast sampling of multi-terminal devices is described. This is achieved using optimized photoconductive switches fabricated from low-temperature-grown GaAs, monolithic integration of the device with the sampling circuit, control of the electromagnetic modes propagating on the coplanar waveguide using microfabricated airbridges, and discrimination of guided and freely-propagating modes using a novel electrooptic sampling method. As an example, the scattering parameters associated with the propagation of a picosecond pulse through one of the gates of a dual-gate heterojunction field-effect transistor are obtained at frequencies up to 300 GHz. The inter-gate capacitance is determined by measuring the electromagnetic transient coupled between the gates.

Spectral and temporal features of photoluminescence of gallium arsenide quantum-wire crystals

Abstract Optical properties of GaAs quantum-wire crystals are investigated by means of time-resolved photoluminescence measurements. Recombination dynamics of carriers in the quantum-wire crystals are characterized on the basis of a rate equation which includes surface recombination and effect of depletion layer. Surface treatment with sulphur solution decreases significantly the surface charge density and surface recombination. Reduction in radiative life time is observed after the surface treatment. This implies that the modification of spatial profile of carrier wave functions is caused by a change in band bending at the surface.