Christophe Coinon

Milliwatt-level output power in the sub-terahertz range generated by photomixing in a GaAs photoconductor

It is shown from accurate on-wafer measurement that continuous wave output powers of 1.2 mW at 50 GHz and 0.35 mW at 305 GHz can be generated by photomixing in a low temperature grown GaAs photoconductor using a metallic mirror Fabry-Perot cavity. The output power is improved by a factor of about 100 as compared to the previous works on GaAs photomixers. A satisfactory agreement between the theory and the experiment is obtained in considering both the contribution of the holes and the electrons to the total photocurrent.

Monolithic integration of high electron mobility InAs-based heterostructure on exact (001) Silicon using a GaSb/GaP accommodation layer

We report on the epitaxial growth of high electron mobility AlSb/InAs heterostructure on exactly oriented (001) Si substrate, using a GaP interfacial layer. The growth conditions are first optimized on GaP substrates to achieve the highest electron mobility. The influence of the Sb flux during the early stage of the GaSb buffer layer is particularly emphasized. Using these optimized growth conditions, the AlSb/InAs heterostructure is grown on a GaP/Si template obtained by metal-organic vapor phase epitaxy. An electron mobility as high as 27 800 cm2 V−1 s−1 and 111 000 cm2 V−1 s−1, respectively, at 300 and 77 K is demonstrated.

Inhomogeneous Si-doping of gold-seeded InAs nanowires grown by molecular beam epitaxy

We have investigated in situ Si doping of InAs nanowires grown by molecular beam epitaxy from gold seeds. The effectiveness of n-type doping is confirmed by electrical measurements showing an increase of the electron density with the Si flux. We also observe an increase of the electron density along the nanowires from the tip to the base, attributed to the dopant incorporation on the nanowire facets whereas no detectable incorporation occurs through the seed. Furthermore, the Si incorporation strongly influences the lateral growth of the nanowires without giving rise to significant tapering, revealing the complex interplay between axial and lateral growth.

Nonstoichiometric Low-Temperature Grown GaAs Nanowires

The structural and electronic properties of nonstoichiometric low-temperature grown GaAs nanowire shells have been investigated with scanning tunneling microscopy and spectroscopy, pump-probe reflectivity, and cathodoluminescence measurements. The growth of nonstoichiometric GaAs shells is achieved through the formation of As antisite defects, and to a lower extent, after annealing, As precipitates. Because of the high density of atomic steps on the nanowire sidewalls, the Fermi level is pinned midgap, causing the ionization of the subsurface antisites and the formation of depleted regions around the As precipitates. Controlling their incorporation offers a way to obtain unique electronic and optical properties that depart from the ones found in conventional GaAs nanowires.

Low-Temperature-Grown GaAs Photoconductor with High Dynamic Responsivity in the Millimeter Wave Range

The responsivity of a low-temperature-grown GaAs photoconductor using a metallic mirror Fabry–Perot cavity has been measured up to 48 GHz thanks to an optical mixing experiment. For a bias voltage of 8 V, it reaches 0.18 A/W and is constant over the whole frequency range. Furthermore, an output power Pout of ~100 µW has been measured at 48 GHz with an optical pump Popt of 11.2 mW. The photomixing efficiency, ηph=Pout/Popt2, is improved by a factor of about 60 as compared to the previous works.

InP HBT Thermal Management by Transferring to High Thermal Conductivity Silicon Substrate

We report about the self-heating management of an InP double heterojunction bipolar transistor (DHBT) by the way of the thermal resistance. In order to reduce this latter, an AlInP/GaAsSb DHBT has been transferred on a silicon substrate offering a high thermal conductivity. According to our thermal resistance measurements on a 0.8 × 6 μm2 DHBT, a low thermal resistance of 1625 K/W is obtained, 65 % lower than for the same device fabricated on its own substrate of InP and which exhibited a value of 4452 K/W.

Picosecond carrier lifetime in low-temperature-grown GaAsSb

We study the influence of growth parameters on the properties of low-temperature-grown GaAsSb layers with 15–20% Sb. We demonstrate that a proper choice of growth conditions allows achieving monocrystalline as-grown layers exhibiting carrier lifetime around 1 ps and a resistivity higher than 1 kΩcm. Upon 600 °C annealing, the resistivity strongly decreases, indicating differences with previous observations, which we try to elucidate. The as-grown material properties are promising for THz generation and detection using a wavelength of around 1.05 µm.

High spectral purity microwave and terahertz oscillator

We report on the design of an ultra stable microwave/THz oscillator and on the realization and the characterization of its laser source. The tunable oscillator is expected to show below -150 dB rad2/Hz phase instability at an offset frequency of 10 kHz for a 30 GHz carrier frequency, as well as 18 GHz, 100 GHz, 400 GHz and 1 THz carrier frequencies.

Morphology and valence band offset of GaSb quantum dots grown on GaP(001) and their evolution upon capping

This work presents a detailed study of GaSb quantum dot (QD) epitaxy on (001) GaP substrates by means of molecular beam epitaxy. Despite the large mismatch between GaP and GaSb, we show that in the nucleation-diffusion regime, the QD size distribution follows the predictions of the scaling theory. Scanning transmission electron microscopy analysis of grown QDs reveal that they are plastically relaxed by 60° pairs of misfit dislocations and the valence band offset measured by x-ray photoelectron spectroscopy on such QDs amounts to 0.5 eV. After capping, the QD morphology is strongly modified with a large P/Sb exchange-segregation reaction, which even leads to the formation of core-shell nanostructures. Remarkably the resulting QD layer is coherent to the substrate without any remaining misfit dislocation and exhibits still strong composition modulations.

Impact of P/In flux ratio and epilayer thickness on faceting for nanoscale selective area growth of InP by molecular beam epitaxy

The impact of the P/In flux ratio and the deposited thickness on the faceting of InP nanostructures selectively grown by molecular beam epitaxy (MBE) is reported. Homoepitaxial growth of InP is performed inside 200 nm wide stripe openings oriented either along a [110] or [1-10] azimuth in a 10 nm thick SiO2 film deposited on an InP(001) substrate. When varying the P/In flux ratio, no major shape differences are observed for [1-10]-oriented apertures. On the other hand, the InP nanostructure cross sections strongly evolve for [110]-oriented apertures for which (111)B facets are more prominent and (001) ones shrink for large P/In flux ratio values. These results show that the growth conditions allow tailoring the nanocrystal shape. They are discussed in the framework of the equilibrium crystal shape model using existing theoretical calculations of the surface energies of different low-index InP surfaces as a function of the phosphorus chemical potential, directly related to the P/In ratio. Experimental observations strongly suggest that the relative (111)A surface energy is probably smaller than the calculated value. We also discuss the evolution of the nanostructure shape with the InP-deposited thickness.