S. Rennesson

Analysis of the AlGaN/GaN vertical bulk current on Si, sapphire, and free-standing GaN substrates

The vertical bulk (drain-bulk) current (Idb) properties of analogous AlGaN/GaN hetero-structures molecular beam epitaxially grown on silicon, sapphire, and free-standing GaN (FS-GaN) have been evaluated in this paper. The experimental Idb (25–300 °C) have been well reproduced with physical models based on a combination of Poole-Frenkel (trap assisted) and hopping (resistive) conduction mechanisms. The thermal activation energies (Ea), the (soft or destructive) vertical breakdown voltage (VB), and the effect of inverting the drain-bulk polarity have also been comparatively investigated. GaN-on-FS-GaN appears to adhere to the resistive mechanism (Ea = 0.35 eV at T = 25–300 °C; VB = 840 V), GaN-on-sapphire follows the trap assisted mechanism (Ea = 2.5 eV at T > 265 °C; VB > 1100 V), and the GaN-on-Si is well reproduced with a combination of the two mechanisms (Ea = 0.35 eV at T > 150 °C; VB = 420 V). Finally, the relationship between the vertical bulk current and the lateral AlGaN/GaN transistor leakage curr...

Complex optical index of single wall carbon nanotube films from the near-infrared to the terahertz spectral range

We retrieve the complex optical index of single-walled carbon nanotube (CNT) films in the 0.6-800 μm spectral range. Results are obtained from a complete set of optical measurements, reflection and transmission, of free-standing CNT films using time domain spectroscopy in the terahertz (THz) and Fourier transform infrared (IR) spectroscopy in the visible-IR. Based on a Drude-Lorentz model, our results reveal a global metallic behavior of the films in the IR, and confirm their high optical index in the THz range.

Power Performance at 40 GHz of AlGaN/GaN High-Electron Mobility Transistors Grown by Molecular Beam Epitaxy on Si(111) Substrate

This letter reports on the demonstration of microwave power performance at 40 GHz on AlGaN/GaN high-electron mobility transistor grown on silicon (111) substrate by molecular beam epitaxy. A maximum dc current density of 1.1 A· mm-1 and a peak extrinsic transconductance of 374 mS · mm-1 are obtained for 75-nm gate length device. At VDS = 25 V, continuous-wave output power density of 2.7 W · mm-1 is achieved at 40 GHz associated with 12.5% power-added efficiency and a linear power gain (G p) of 6.5 dB. The device exhibits an intrinsic current gain cutoff frequency FT of 116 GHz and a maximum oscillation frequency FMAX of 150 GHz. This performance demonstrates the capability of low cost microwave power devices up to Ka-band.

Optimization of

In this paper, we propose to optimize Al0.29Ga0.71N/GaN heterostructures on silicon substrate to obtain high electron mobility transistors featuring high-power/frequency performances. The polarization electric fields are engineered by varying the layer thicknesses of the cap and the barrier, and by changing the type of buffer (GaN or AlGaN). The aim of this paper is to find the best tradeoff between the active layer thickness reduction and the achievement of a reasonable drain current to satisfy the requirements for high performances. The optimum heterostructure device presents an output power density of 1.5 W/mm at 40 GHz, among the best reported on silicon substrate.

{\rm Al}_{0.29}{\rm Ga}_{0.71}{\rm N}/{\rm GaN}

We present a series of microdisk lasers realized within the same GaN-on-Si photonic platform scheme, and operating at room temperature under pulsed optical pumping over a broad spectral range extending over λ = 275 nm–470 nm. The III-nitride microdisks embed either binary GaN/AlN multiple quantum wells (MQWs) for UV operation, or ternary InGaN/GaN MQWs for violet and blue operation. This demonstrates the versatility of this nitride-on-silicon platform, and the realization on this platform of efficient active layers for lasing action over a 200 nm broad UV to visible spectral range. We probe the lasing threshold carrier density over the whole spectral range and found that it is similar whatever the emission wavelength for these Q > 1000 microdisk resonators with a constant material quality until quantum confined Stark effect takes over. The threshold is also found independent of microdisk diameters from 3 to 12 μm, with a β factor intermediate between the one of vertical cavity lasers and the one of small modal volume “thresholdless” lasers.

High Electron Mobility Heterostructures for High-Power/Frequency Performances

We demonstrate intersubband polaritons in the short-infrared wavelength range (λ < 3 μm) relying on the GaN/AlN semiconductor system. The demonstration is given for an intersubband transition centered at λ = 3.07 μm (E = 403 meV). The polaritonic dispersion is measured at room temperature: a Rabi energy of 53 meV (i.e., a minimum splitting of 106 meV), which represents 13.1% of the bare transition, is demonstrated. A metal-insulator-metal resonator architecture is employed, which proves to be efficient even at these short wavelengths.

III-Nitride-on-silicon microdisk lasers from the blue to the deep ultra-violet

Many high-field/attosecond and ultrafast electron diffraction/microscopy experiments on condensed matter require samples in the form of free-standing membranes with nanometer thickness. Here, we report the measurement of the laser-induced damage threshold of 11 different free-standing nanometer-thin membranes of metallic, semiconducting, and insulating materials for 1-ps, 1030-nm laser pulses at 50 kHz repetition rate. We find a laser damage threshold that is very similar to each corresponding bulk material. The measurements also reveal a band gap dependence of the damage threshold as a consequence of different ionization rates. These results establish the suitability of free-standing nanometer membranes for high-field pump-probe experiments.

Short-wave infrared (λ = 3 μm) intersubband polaritons in the GaN/AlN system

III-nitride-on-silicon L3 photonic crystal cavities with resonances down to 315 nm and quality factors (Q) up to 1085 at 337 nm have been demonstrated. The reduction of the quality factor with decreasing wavelength is investigated. Besides the quantum well absorption below 340 nm, a noteworthy contribution is attributed to the residual absorption present in thin AlN layers grown on silicon, as measured by spectroscopic ellipsometry. This residual absorption ultimately limits the Q factor to around 2000 at 300 nm when no active layer is present.

Laser damage of free-standing nanometer membranes

We demonstrate low-loss GaN/AlGaN planar waveguides grown by molecular beam epitaxy on sapphire substrates. By using a proper AlGaN cladding layer and reducing surface roughness we reach <1dB/cm propagation losses at 633nm. These low propagation losses allow an efficient second harmonic generation using modal phase matching between a TM0 pump at 1260nm and a TM2 second harmonic at 630nm. A maximal power conversion of 2% is realized with an efficiency of 0.15%·W-1cm-2. We provide a modelling that demonstrates broadband features of GaN/AlGaN platform by showing second harmonic wavelengths tunability from the visible up to the near-infrared spectral region. We discuss drawbacks of modal phase matching and propose a novel solution which allows a drastic improvement of modal overlaps with the help of a planar polarity inversion. This new approach is compatible with low propagation losses and may allow as high as 100%·W-1cm-2 conversion efficiencies in the future.

Q factor limitation at short wavelength (around 300 nm) in III-nitride-on-silicon photonic crystal cavities

GaN-based power switches are expected to play a key role in uncooled electronics at elevated temperatures. In this paper we explore the thermal activation mechanisms taking place in analogous AlGaN/GaN high electron mobility transistors grown on silicon and sapphire. The on-resistance (α = 1.4/1.8 [Si/sapphire]) and saturation current (α = −1.5/−1.8) temperature coefficients, the thermal activation energies (Ea = 0.02–0.30/0.30 eV), the drain current on/off ratio (α = −1.5–9.1/−9.4), or the thermal impedances (Rth = 76.9/125.8 K/W) were determined and comparatively analyzed by means of physical-based models which include polar-optical phonon scattering, Poole-Frenkel trap assisted and Schottky emission, and the channel self-heating.