Sylvie Contreras

Cyclotron resonance and quantum Hall effect studies of the two-dimensional electron gas confined at the GaN/AlGaN interface

We report on high magnetic fields (up to 40 T) cyclotron resonance, quantum Hall effect and Shubnikov-de-Hass measurements in high frequency transistors based on Si-doped GaN–AlGaN heterojunctions. A simple way of precise modelling of the cyclotron absorption in these heterojunctions is presented. We clearly establish two-dimensional electrons to be the dominant conducting carriers and determine precisely their in-plane effective mass to be 0.230±0.005 of the free electron effective mass. The increase of the effective mass with an increase of two-dimensional carrier density is observed and explained by the nonparabolicity effect.

Electrical transport in n-type 4H silicon carbide

Free electron density and low field electron mobility of 4H–SiC in the temperature range of 35–900 K are examined experimentally and theoretically. Five samples produced by cold-wall atmospheric pressure chemical vapor deposition and doped with nitrogen from 3.5×1015 cm−3 to 7.5×1017 cm−3 are investigated using the electric conductivity and Hall measurements. A complete description of the electron density and mobility is presented taking into account inequivalent positions of cubic and hexagonal donor sites as well as valley-orbit splittings of the donor levels. A good agreement between experiment and theory is achieved for all samples and it is demonstrated that the scattering of electrons by neutral donors is a dominant mode in 4H–SiC at low temperatures. The deformation potential for the intravalley scattering by acoustic phonons and coupling constants for the intervalley scattering by acoustic and optic phonons are determined. The dependence of electron mobility on doping at constant temperatures 77 K...

Activation of aluminum implanted at high doses in 4H–SiC

We report an investigation of the electrical activation of aluminum implanted at high dose in 4H–SiC. We show that at reasonably high temperature implantation and annealing conditions, one activates about 37.5% of the implanted species. Of course, the final (concentration-dependent) activation ratio differs slightly from this average value but varies only between 0.5 and 0.25 when the targeted concentration increases from 3.33×1018 to 1021 cm−3. Provided a standard mobility can be maintained, this results in fairly low sheet resistance. The best (lowest) value obtained in this work is 15 mΩ cm at 700 K (95 mΩ cm at room temperature) for a 190-nm-thick layer implanted with 1021 atoms cm−3. In MESA-etched p–n junctions with a 100 μm diameter, this resulted in a typical on-resistance of 1.5 mΩ cm2, mainly limited by the substrate and n− epitaxial layer.

Reversible optical doping of graphene

The ultimate surface exposure provided by graphene monolayer makes it the ideal sensor platform but also exposes its intrinsic properties to any environmental perturbations. In this work, we demonstrate that the charge carrier density of graphene exfoliated on a SiO2/Si substrate can be finely and reversibly tuned between hole and electron doping with visible photons. This photo-induced doping happens under moderate laser power conditions but is significantly affected by the substrate cleaning method. In particular, it requires hydrophilic substrates and vanishes for suspended graphene. These findings suggest that optically gated graphene devices operating with a sub-second time scale can be envisioned and that Raman spectroscopy is not always as non-invasive as generally assumed.

Unambiguous determination of carrier concentration and mobility for InAs/GaSb superlattice photodiode optimization

In this communication we report on electrical properties of nonintentionally doped (nid) type II InAs/GaSb superlattice grown by molecular beam epitaxy. We present a simple technological process which, thanks to the suppression of substrate, allows direct Hall measurement on superlattice structures grown on conductive GaSb substrate. Two samples were used to characterize the transport: one grown on a semi-insulating GaAs substrate and another grown on n-GaSb substrate where a etch stop layer was inserted to remove the conductive substrate. Mobilities and carrier concentrations have been measured as a function of temperature (77–300 K), and compared with capacitance-voltage characteristic at 80 K of a photodiode comprising a similar nid superlattice.

High sensitivity hall sensors with low thermal drift using AlGaAs/InGaAs/GaAs heterostructures

Abstract We have investigated the interest of AlGaAs/InGaAs/GaAs heterostructures for the realization of Hall effect magnetic sensors with a high sensitivity and a low thermal drift. The physical phenomena responsible for the thermal drift of the Hall sensitivity were reviewed. A set of test devices with well-controlled structure parameters were designed in order to investigate and minimize these phenomena. Experimental data were compared with a physical model of the structure including a self-consistent description of the quantum well. These results were used to optimize the structure design. Hall sensors with much higher performances than conventional GaAs devices were fabricated. A sensitivity of 900 V/A/T with a temperature coefficient of — 160 ppm/°C was obtained.

Electrical transport properties of aluminum-implanted 4H–SiC

The free hole density and low-field mobility of aluminum-doped 4H–SiC were investigated in the temperature range of 100–900K, both, experimentally and theoretically. Experimental data for implanted p-type 4H–SiC were compared with theoretical calculations using parameters determined for high-quality epitaxial layers. The deformation potential for intra- and intervalley scattering by acoustic phonons and the effective coupling constant for intra- and intervalley scattering by nonpolar optical phonons were determined. The detailed analysis of the implanted layers with aluminum-targeted concentration ranging from 3.33×1018to1021cm−3 shows that (i) about half of the implanted atoms are electrically active in the SiC lattice, (ii) a systematic compensation of about 10% of the doping level is induced by the implantation process, (iii) two different ionization energies for the aluminum atoms have to be used. Their origin is discussed in terms of inequivalent hexagonal and cubic lattice sites. Finally, the doping...

Free electron density and mobility in high-quality 4H–SiC

The free electron density and low-field electron mobility of 4H–SiC is examined in the temperature range 35–900 K. In good samples the electron density is constant in the temperature range 300–900 K, which offers interesting possibilities for high temperature sensor applications. On the best sample an experimental electron mobility of 12 400 cm2/V s at 50 K is found. A complete description of the temperature dependence of the electron density and mobility is given. We take into account the effects of the two inequivalent lattice sites as well as the valley–orbit splitting of the ground state at the hexagonal sites. The dependence of room-temperature mobility on electron concentration is established, described theoretically and compared with the results obtained by different authors.

Process-induced strain in silicon-on-insulator materials

We present a detailed investigation of the influence of oxidation and thinning processes on the in-plane stress in silicon-on-insulator materials. Combining double x-ray diffraction, Fourier transformed infrared and micro-Raman spectroscopy, we show that one can separately evaluate the stress present in the silicon over layer, the buried oxide and the underlying (handle) silicon wafer at any time of a device-forming process.

Structural, optical and electrical properties of state of the art cubic SiC films

Abstract The structural, optical and electrical characteristics of commercially available cubic (β)-SiC films grown on 〈001〉 silicon waters were reported. For the structural characterization, combined plane view and cross-section transmission electron spectroscopy observations were made. For the optical investigations, low-temperature photoluminescence (2 K) and room temperature Raman and infrared spectra were measured. For the electrical characterization, Hall effect and resistivity measurements were performed in the temperature range 15–500 K.