Epitaxy of III-V semiconductor nanowires towards optoelectronic devices

GaAs and InP based nanowires were grown epitaxially on GaAs or InP (111)B substrates by MOCVD via VLS mechanism. In this paper, I will give an overview of nanowire research activities in our group. © 2009 IEEE.

Growth behavior of epitaxial semiconductor axial nanowire heterostructures

In this paper, we demonstrate the key issues of axial nanowire heterostructures, such as, the fundamental criteria for formation and failure of axial nanowire heterostructures via vapor-liquid-solid mechanism and lateral misfit strain relaxation in these structures. We show the failure of axial nanowire heterostructures by growing InAs axially on GaAs nanowires, and the lateral misfit strain relaxation by axial growth of GaSb on GaAs nanowires. © 2008 IEEE.

Nature of heterointerfaces in GaAs/InAs and InAs/GaAs axial nanowire heterostructures

The structural and compositional characteristics of heterointerfaces of Au-catalyzed GaAs/InAs and InAs/GaAs axial nanowire heterostructures were comprehensively investigated by transmission electron microscopy. It has been found that the GaAs/InAs interface is not sharp and contains an InGaAs transition segment, and in contrast, the InAs/GaAs interface is atomically sharp. This difference in the nature of heterointerfaces can be attributed to the difference in the affinity of the group III elements with the catalyst material. © 2008 American Institute of Physics.

Direct observation of charge-carrier heating at WZ-ZB InP nanowire heterojunctions.

We have investigated the dynamics of hot charge carriers in InP nanowire ensembles containing a range of densities of zinc-blende inclusions along the otherwise wurtzite nanowires. From time-dependent photoluminescence spectra, we extract the temperature of the charge carriers as a function of time after nonresonant excitation. We find that charge-carrier temperature initially decreases rapidly with time in accordance with efficient heat transfer to lattice vibrations. However, cooling rates are subsequently slowed and are significantly lower for nanowires containing a higher density of stacking faults. We conclude that the transfer of charges across the type II interface is followed by release of additional energy to the lattice, which raises the phonon bath temperature above equilibrium and impedes the carrier cooling occurring through interaction with such phonons. These results demonstrate that type II heterointerfaces in semiconductor nanowires can sustain a hot charge-carrier distribution over an extended time period. In photovoltaic applications, such heterointerfaces may hence both reduce recombination rates and limit energy losses by allowing hot-carrier harvesting.

Semiconductor laser with a biconical waveguide

The propagation losses in the fundamental mode of a bicone made of highly reflecting metal or a dielectric of large refraction were approximately estimated using Leontovich's boundary condition. A 400-fold concentration of the energy flux density lias been obtained in a cross section which is much smaller than λ. Here, the losses are 2.5% at λ = 550 nm in an Ag bicone and 12% in a semiconductor bicone with a band gap of ≈1 eV for hv larger than the band gap. The excitation efficiency of a bicone has been estimated. While not too large, it can be increased significantly using the method proposed in the present paper. The application of the optical bicone for the multiplication of a semiconductor-laser frequency is discussed. The results obtained are also of use in scanning near-field optical microscopy and in experiments on focusing laser pulses of ultrahigh power. © 2000 Plenum/Kluwer Publishing Corporation.

Role of a high gain of the medium in superradiance generation and in observation of coherent effects in semiconductor lasers

A theoretical model of superradiant pulse generation in semiconductor laser structures is developed. It is shown that a high optical gain of the medium can overcome phase relaxation and results in a built-up superradiant state (macroscopic dipole) in an assembly of electron - hole pairs on a time scale much longer than the characteristic polarisation relaxation time T2. A criterion of the superradiance generation is the condition acmT2 > 1, where α is the gain coefficient and cm is the speed of light in the medium. The theoretical model describes both qualitatively and quantitatively the author's own experimental results.

Superfluorescence in semiconductor lasers

An analysis is made of the conditions for the generation of superfluorescence pulses in an inverted medium of electron-hole pairs in a semiconductor. It is shown that strong optical amplification in laser semiconductor amplifiers characterised by αL ≫ 1 leads to suppression of phase re-laxation of the medium during the initial stages of evolution of superfluorescence and to formation of a macroscopic dipole from electron-hole pairs. Cooperative emission of radiation in this system results in generation of a powerful ultrashort pulse of the optical gain, which interacts coherently with the semiconductor medium. It is shown that coherent pulsations of the optical field, observed earlier by the author in Q-switched semiconductor lasers, are the result of superfluorescence and of the coherent interaction between the optical field and the medium.