S. Chenot

Semipolar GaN films on patterned r-plane sapphire obtained by wet chemical etching

It is shown that (112¯2)-oriented GaN films can be achieved from r-sapphire patterned by chemical etching. Growth first occurs selectively from the inclined c-facet of sapphire, leading finally to a fully coalesced layer with (112¯2) orientation. The structural and optical quality of these layers was assessed by x-ray diffraction, cathodoluminescence and photoluminescence measurements. The results clearly show that the quality of (112¯2) GaN on patterned r-sapphire is markedly improved in comparison with (112¯2) GaN on m-sapphire.

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.

The critical role of growth temperature on the structural and electrical properties of AlGaN/GaN high electron mobility transistor heterostructures grown on Si(111)

This work is dedicated to the study of the growth by ammonia source molecular beam epitaxy of AlxGa1−xN/GaN high electron mobility transistors on (111) oriented silicon substrates. The effect of growth conditions on the structural and electrical properties of the heterostructures was investigated. It is shown that even a slight variation in the growth temperature of the thick GaN buffer on AlN/GaN stress mitigating layers has a drastic influence on these properties via a counterintuitive effect on the dislocation density. Both in situ curvature measurements and ex situ transmission electron microscopy and x-ray diffraction experiments indicate that the relaxation rate of the lattice mismatch stress increases with the growth temperature but finally results in a higher dislocations density. Furthermore, a general trend appears between the final wafer curvature at room temperature and the threading dislocation density. Finally, the influence of the dislocation density on the GaN buffer insulating properties ...

Dependence of the Mg-related acceptor ionization energy with the acceptor concentration in p-type GaN layers grown by molecular beam epitaxy

Hall effect and capacitance-voltage C(V) measurements were performed on p-type GaN:Mg layers grown on GaN templates by molecular beam epitaxy with a high range of Mg-doping concentrations. The free hole density and the effective dopant concentration NA−ND as a function of magnesium incorporation measured by secondary ion mass spectroscopy clearly reveal both a magnesium doping efficiency up to 90% and a strong dependence of the acceptor ionization energy Ea with the acceptor concentration NA. These experimental observations highlight an isolated acceptor binding energy of 245±25 meV compatible, at high acceptor concentration, with the achievement of p-type GaN:Mg layers with a hole concentration at room temperature close to 1019 cm−3.

Micro and nano analysis of 0.2 Ω mm Ti/Al/Ni/Au ohmic contact to AlGaN/GaN

As GaN technology continues to gain popularity, it is necessary to control the ohmic contact properties and to improve device consistency across the whole wafer. In this paper, we use a range of submicron characterization tools to understand the conduction mechanisms through the AlGaN/GaN ohmic contact. Our results suggest that there is a direct path for electron flow between the two dimensional electron gas and the contact pad. The estimated area of these highly conductive pillars is around 5% of the total contact area.

Monolithic white light emitting diodes using a (Ga,In)N/GaN multiple quantum well light converter

A monolithic white light emitting diode using a (Ga,In)N/GaN multiple quantum well (MQW) light converter is demonstrated. Blue photons emitted under electrical injection by (Ga,In)N/GaN QWs located inside a GaN p-n junction are partly absorbed by another (Ga,In)N/GaN MQW situated outside the junction which emits yellow-green light. The combination of the blue and yellow-green components results in white light emission.

Blue (Ga,In)N/GaN Light Emitting Diodes on Si(110) Substrate

We have fabricated and characterized blue (Ga,In)N/GaN multiple quantum well light emitting diodes grown on a Si(110) substrate by molecular beam epitaxy. For a 20 mA current, we have found that the operating voltage and the series resistance are as low as 3.5 V and 17 Ω, respectively. A maximum light output power of 72 µW is obtained as measured on the wafer. These characteristics are almost identical to those obtained on a reference sample grown on the commonly used Si(111) orientation.

Polar and semipolar GaN/Al0.5Ga0.5N nanostructures for UV light emitters

AlxGa1−xN-based ultra-violet (UV) light emitting diodes (LEDs) are seen as the best solution for the replacement of traditional mercury lamp technology. By adjusting the Al concentration, a large emission spectrum range from 360 nm (GaN) down to 200 nm (AlN) can be covered. Owing to the large density of defects typically present in AlxGa1−xN materials usually grown on sapphire substrates, LED efficiencies still need to be improved. Taking advantage of the 3D carrier confinement, quantum dots (QDs) are among the solutions currently under investigation to improve the performances of UV LEDs. The objectives of this work are to present and discuss the morphological and optical properties of GaN nanostructures grown by molecular beam epitaxy on the (0 0 0 1) and the (11–22) orientations of Al0.5Ga0.5N. In particular, the dependence of the morphological properties of the nanostructures on the growth conditions and the surface orientation will be presented. The optical characteristics as a function of the nanostructure design (size, shape and dimensionality) will also be shown and discussed. The electroluminescence characteristics of a first series of QD-based GaN/Al0.5Ga0.5N LEDs grown on the polar (0 0 0 1) plane will be investigated.