Thierry Taliercio

Influence of electron-phonon interaction on the optical properties of III nitride semiconductors

The electronic band structures of III nitride semiconductors calculated within the adiabatic approximation give essential information about the optical properties of materials. However, atoms of the lattice are not at rest; their displacement away from the equilibrium positions perturbs the periodic potential acting on the electrons in the crystal, leading to an electron-phonon interaction energy. Due to different ways that the lattice vibration perturbs the motions of electrons, there are various types of interaction, such as Frohlich interaction with longitudinal optical phonons, deformation-potential interactions with optical and acoustic phonons and piezoelectric interaction with acoustic phonons. These interactions, especially the Frohlich interaction, which is very strong due to the ionic nature of III nitrides, have a great influence on the optical properties of the III nitride semiconductors. As a result of electron-phonon interaction, several phenomena, such as phonon replicas in the emission spectra, homogeneous broadening of the excitonic line width and the relaxation of hot carriers to the fundamental band edge, which have been observed in GaN and its low dimensional heterostructures, are reviewed.

Optical properties of group-III nitride quantum wells and quantum boxes

We propose an overview of specific optical properties of quantum-size artificial structures made of group-III nitride semiconductors with natural wurtzite symmetry. We consider the cases of quantum wells and of quantum boxes obtained by the Stranski-Krastanov growth mode. We comment on results of continuous-wave and time-resolved optical spectroscopy, in comparison with our envelope-function calculations of excitonic energies and oscillator strengths. The influence on recombination dynamics of internal electric fields and carrier localization is discussed in detail.

Localized surface plasmon resonance frequency tuning in highly doped InAsSb/GaSb one-dimensional nanostructures

We report a detailed analysis of the influence of the doping level and nanoribbon width on the localized surface plasmon resonance (LSPR) by means of reflectance measurements. The plasmonic system, based on one-dimensional periodic gratings of highly Si-doped InAsSb/GaSb semiconductor nanostructures, is fabricated by a simple, accurate and large-area technique fabrication. Increasing the doping level blueshifts the resonance peak while increasing the ribbon width results in a redshift, as confirmed by numerical simulations. This provides an efficient means of fine-tuning the LSPR properties to a target purpose of between 8-20 μm (1250-500 cm(-1)). Finally, we show surface plasmon resonance sensing to absorbing polymer layers. We address values of the quality factor, sensitivity and figure of merit of 16 700 nm RIU(-1) and 2.5, respectively. These results demonstrate Si-doped InAsSb/GaSb to be a low-loss/high sensitive material making it very promising for the development of biosensing devices in the mid-infrared.

All-semiconductor plasmonics for mid-IR applications

We investigate highly doped InAsSb layers grown by molecular beam epitaxy on GaSb substrates. Electrical and optical characterizations demonstrate a metallic behavior with the possibility to control the plasma frequency in the mid-infrared range by adjusting the doping level. Plasmonic resonators based on this new kind of metal can be realized for mid-IR applications. Sub-wavelength periodic arrays are fabricated in the InAsSb layer and localized surface plasmon resonances are observed in reflectance experiments. A perfect control of the doping level and of the geometry of the periodic arrays allows adjusting the frequency of the plasmonic resonances. Our work shows that GaSb-based materials could be the building block for all-semiconductor mid-infrared plasmonic devices.

Direct measurement of the effective infrared dielectric response of a highly doped semiconductor metamaterial

We have investigated the effective dielectric response of a subwavelength grating made of highly doped semiconductors (HDS)xa0excited in reflection, using numerical simulations and spectroscopic measurement. The studied system can exhibit strong localized surface resonances and has, therefore, a great potential for surface-enhanced infrared absorption (SEIRA) spectroscopy application. It consists of a highly doped InAsSb grating deposited on lattice-matched GaSb. The numerical analysis demonstrated that the resonance frequencies can be inferred from the dielectric function of an equivalent homogeneous slab by accounting for the complex reflectivity of the composite layer. Fourier transform infrared reflectivity (FTIR) measurements, analyzed with the Kramers-Kronig conversion technique, were used to deduce the effective response in reflection of the investigated system. From the knowledge of this phenomenological dielectric function, transversal and longitudinal energy-loss functions were extracted and attributed to transverse and longitudinal resonance modes frequencies.

GaSb-based all-semiconductor mid-IR plasmonics

Electrical and optical characterizations of highly-doped InAsSb layers lattice matched to GaSb substrates show the possibility to control their plasma frequency in the mid-infrared range. Reflectance experiments performed on InAsSb sub-wavelength arrays evidence localized surface plasmon resonances which can be modeled by finite difference time domain method. By adjusting the refractive index of the surrounding material and the geometry of the periodic arrays it is possible to control the frequency of the plasmonic resonances. Our results show that GaSb-based materials can be the building block of all-semiconductor mid-infrared plasmonic devices.

Highly doped InAsSb plasmonic arrays for mid-infrared biosensing

We propose 1D periodic, highly doped InAsSb gratings on GaSb substrates as biosensing platforms applicable at the same time for surface plasmon resonance and surface enhanced infrared absorption spectroscopies. Finite-difference time-domain simulations frame array geometries (nanoribbon width and spacing), in relation to the electric field enhancement and the sensitivity on refractive index variations, for a given molecular absorption region in the mid-infrared. These optimized systems achieve sensitivities of ∼ 900 nm RIU−1. A clear red shift of the plasmon resonance as well as the enhancement of an absorption line are presented for 2 nm thin layers. We experimentally analyze the influence of the InAsSb doping level exposing the gratings in reflection measurements to different surrounding environments. A comparison to a gold grating with equivalent optical properties in the mid-infrared is performed. Our simulations and experimental results underline the interest in the alternative plasmonic material InAsSb for highly sensitive plasmonic biosensors for the mid-infrared spectral range.

THz absorbers with highly doped semiconductors based in plasmonic nano-resonators

We demonstrate a periodic array of metallic — insulator-metallic nano-resonators using highly doped InAsSb semiconductors on GaSb substrate that can be described as a high absorber in the infrared region. Our simulation approach permits to obtain analytically the optical properties of the absorption structure with rigorous couple-wave analysis and thus to establish a coherent fabrication geometry for almost total absorption at certain wavelengths. Furthermore, the fabricated nano-resonator arrays by laser lithography and acid wet chemical etching show absorption at certain wavelengths. There is a linear relationship between the peak maximum and the nano-resonator width.

Optimized GaAs High Contrast Grating Design and Fabrication for Mid-infrared Application at 2.3 µm

A tolerant GaAs HCG design has been optimized for a mid-IR VCSEL application through the use of an optimization algorithm. Reflection spectra of an experimental grating are in a correct agreement with the simulations.

Recombination Dynamics in Nitride Quantum Boxes and Quantum Wells for Colors Ranging from the UV to the Red.

ABSTRACT Time-resolved photoluminescence experiments at varying temperature are performed on a series of In x Ga 1-x N/GaN quantum well and quantum box samples of similar compositions (0.15 < x < 0.20). The results are analyzed by using envelope-function calculations of transition energies and oscillator strengths, accounting for internal electric fields. The respective influences of localization and electric fields on radiative and nonradiative lifetimes and on the Stokes shift are deduced. The results indicate that the spatial extension of localization centers is much smaller than the size of the quantum boxes (~10 × 3 nm, typically). The room-temperature radiative efficiency of both quantum well and quantum box samples is enhanced by replacing the topmost GaN barrier by an AlGaN one. INTRODUCTION Despite large densities of nonradiative defects, InGaN/GaN quantum wells (QWs) are known as efficient light-emitting systems [1-3] covering a large part of the visible spectrum. This has been assigned in recent works [4] to carrier localization on deep potential valleys corresponding to the strong potential fluctuations induced by random distribution of indium atoms. The possibility of In-rich nanoclusters induced by de-mixing effects has also been invoked for explaining such localization [4-7]. The complex carrier transfers between these zero-dimensional [7,8] objects and towards nonradiative recombination centers induce complex energy relaxation [9-11] between energy levels below some mobility edge [12]. Whatever their exact origin, these localized potential minima increase the nonradiative lifetime, limited by the escape of carriers towards nonradiative recombination centers, most probably related to dislocations in group-III nitrides grown on sapphire or SiC substrates. Such a localization induces a large red-shift of optical transitions and a strong increase of the radiative lifetime, by expansion of the exciton wave-function in k-space away from the photon wave-vector [13]. Now, it has been also understood recently that internal electric fields of several hundred kV/cm are present along the growth direction of nitride-based QWs, due to the difference of piezoelectric and spontaneous polarizations in the well and barrier materials [14-23]. These electric fields, too, induce a large red-shift of emission lines, due to the quantum-confined Stark effect, and they separate the electron and hole wave-functions so that the radiative lifetime is drastically increased [18,19,22].