Y. Cordier

Molecular Beam Epitaxy of Group‐III Nitrides on Silicon Substrates: Growth, Properties and Device Applications

We report on the growth and properties of GaN films grown on Si(111) substrates by molecular beam epitaxy using ammonia. The properties of the layers show that our growth procedure is very efficient in order to overcome the difficulties encountered during the growth of nitrides on silicon substrates: first, no nitridation of the silicon substrate is observed at the interface between the AIN buffer laver and the silicon surface: second. there is no Si autodoping coming from the substrate and resistive undoped GaN layers are obtained; and, also, strain balance engineering allows one to grow thick GaN epilayers (up to 3 mum) without formation of cracks. The optical, structural and electrical properties of these films are studied. In order to evaluate the potentialities of III-V nitrides grown on silicon substrates, we have grown heterostructures to realize light emitting diodes (LEDs), photodetectors and high electron mobility transistors (HEMTs).

InAlAs/InGaAs metamorphic HEMT with high current density and high breakdown voltage

An In/sub 0.3/Al/sub 0.7/As/In/sub 0.3/Ga/sub 0.7/As metamorphic power high electron mobility transistor (HEMT) grown on GaAs has been developed. This structure with 30% indium content presents several advantages over P-HEMT on GaAs and LM-HEMT on InP. A 0.15-/spl mu/m gate length device with a single /spl delta/ doping exhibits a state-of-the-art current gain cut-off frequency F/sub t/ value of 125 GHz at V/sub ds/=1.5 V, an extrinsic transconductance of 650 mS/mm and a current density of 750 mA/mm associated to a high breakdown voltage of -13 V, power measurements performed at 60 GHz demonstrate a maximum output power of 240 mW/mm with 6.4-dB power gain and a power added efficiency (PAE) of 25%. These are the first power results ever reported for any metamorphic HEMT.

Gallium Nitride as an Electromechanical Material

Gallium nitride (GaN) is a wide bandgap semiconductor material and is the most popular material after silicon in the semiconductor industry. The prime movers behind this trend are LEDs, microwave, and more recently, power electronics. New areas of research also include spintronics and nanoribbon transistors, which leverage some of the unique properties of GaN. GaN has electron mobility comparable with silicon, but with a bandgap that is three times larger, making it an excellent candidate for high-power applications and high-temperature operation. The ability to form thin-AlGaN/GaN heterostructures, which exhibit the 2-D electron gas phenomenon leads to high-electron mobility transistors, which exhibit high Johnsons figure of merit. Another interesting direction for GaN research, which is largely unexplored, is GaN-based micromechanical devices or GaN microelectromechanical systems (MEMS). To fully unlock the potential of GaN and realize new advanced all-GaN integrated circuits, it is essential to cointegrate passive devices (such as resonators and filters), sensors (such as temperature and gas sensors), and other more than Moore functional devices with GaN active electronics. Therefore, there is a growing interest in the use of GaN as a mechanical material. This paper reviews the electromechanical, thermal, acoustic, and piezoelectric properties of GaN, and describes the working principle of some of the reported high-performance GaN-based microelectromechanical components. It also provides an outlook for possible research directions in GaN MEMS.

Amplified piezoelectric transduction of nanoscale motion in gallium nitride electromechanical resonators

A fully integrated electromechanical resonator is described that is based on high mobility piezoelectric semiconductors for actuation and detection of nanoscale motion. We employ the two-dimensional electron gas present at an AlGaN/GaN interface and the piezoelectric properties of this heterostructure to demonstrate a resonant high-electron-mobility transistor enabling the detection of strain variation. In this device, we take advantage of the polarization field divergence originated by mechanical flexural modes for generating piezoelectric doping. This enables a modulation of carrier density which results in a large current flow and thus constitutes a motion detector with intrinsic amplification.

Metamorphic In/sub 0.4/Al/sub 0.6/As/In/sub 0.4/Ga/sub 0.6/As HEMTs on GaAs substrate

New In/sub 0.4/Al/sub 0.6/As/In/sub 0.4/Ga/sub 0.6/As metamorphic (MM) high electron mobility transistors (HEMTs) have been successfully fabricated on GaAs substrate with T-shaped gate lengths varying from 0.1 to 0.25 /spl mu/m. The Schottky characteristics are a forward turn-on voltage of 0.7 V and a gate breakdown voltage of -10.5 V. These new MM-HEMTs exhibit typical drain currents of 600 mA/mm and extrinsic transconductance superior to 720 mS/mm. An extrinsic current cutoff frequency f/sub T/ of 195 GHz is achieved with the 0.1-/spl mu/m gate length device. These results are the first reported for In/sub 0.4/Al/sub 0.6/As/In/sub 0.4/Ga/sub 0.6/As MM-HEMTs on GaAs substrate.

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...

GaN transistor characteristics at elevated temperatures

The characteristics of different GaN transistor devices characterized at elevated temperatures for power applications are compared in this paper. High temperature characteristics of GaN metal-oxide-semiconductor field-effect transistors (MOSFETs) and GaN high electron mobility transistors (HEMTs) are reported. For MOSFETs, the transconductance current (gm) increases with temperature, while for HEMTs is reduced. Their specific on resistance (Ron) follows the same trend. Specific contact resistivity (ρc) to implanted Si N+ GaN also diminishes with T, whereas for AlGaN/GaN ρc remains practically constant. We bring a more physical insight into the temperature behavior of these GaN devices by means of physics-based modeling in Sec. VI of this paper. The MOSFET’s field-effect mobility increases with T due to interface trap Coulomb scattering. Analogously, the HEMT’s gm decrease with T is attributed to a significant reduction in the two-dimensional electron gas carrier mobility due to polar-optical-phonon scatte...

Demonstration of AlGaN/GaN High-Electron-Mobility Transistors Grown by Molecular Beam Epitaxy on Si(110)

The growth of AlGaN/GaN-based heterostructure on Si(110) substrates by molecular beam epitaxy using ammonia as the nitrogen precursor is reported. The structural, optical, and electrical properties of such heterostructure are assessed and are quite similar to the ones obtained on Si(111). A 2-D electron gas is formed at the Al0.3Ga0.7N/GaN interface with a sheet carrier density of 9.6 times 1012 cm-2 and a mobility of 1980 cm2/V middots at room temperature. Preliminary results concerning high-electron-mobility-transistor static characteristics are presented and compared with that of devices realized on other orientations of silicon.

Indium content measurements in metamorphic high electron mobility transistor structures by combination of x-ray reciprocal space mapping and transmission electron microscopy

We propose a method to determine the indium concentrations x and y in the InyAl1−yAs/InxGa1−xAs metamorphic structures. This approach is based on the combination of two experimental techniques: (i) reciprocal space mapping (RSM) to determine the average In composition in the InAlAs layers and (ii) transmission electron microscopy (TEM) using the intensity measurements of the chemically sensitive (002) reflection from dark-field images to determine the composition in the InGaAs quantum well. We apply this method to a InyAl1−yAs/InxGa1−xAs metamorphic high electron mobility transistor, with x and y approximately equal to 0.35. Furthermore, we present an original and straightforward way to evaluate experimental errors in the determination of composition and strain with the RSM procedure. The influence of these errors on the TEM results is discussed. For In concentrations in the 30%–40% range, the accuracy of this simple method is about 0.5% on the In composition in the InGaAs quantum well.

MBE grown InAlAs/InGaAs lattice mismatched layers for HEMT application on GaAs substrate

Lattice mismatched high electron mobility transistors with InGaAs and InAlAs buffer layers have been grown by molecular beam epitaxy on GaAs substrate. Surface morphology of epilayers has been investigated using atomic force microscopy. An inverse step layer at the exit of the linear grade has been used to enhance relaxation of strain in heterostructures with indium contents ranging from 30% to 50% and impact of surface roughness on electrical properties has been investigated.