M. Azize

Relaxation mechanisms in metal-organic vapor phase epitaxy grown Al-rich (Al,Ga)N∕GaN heterostructures

The relaxation mechanisms in metal-organic vapor phase epitaxy grown (Al,Ga)N∕GaN heterostructures are studied. The first stage of the relaxation process is a two-dimensional–three-dimensional growth transition with the formation of mesalike islands separated by V-shaped trenches. The tensile stress relief is obtained by an elastic relaxation of the islands edges. In the case of AlN∕GaN, the apexes of the V trenches reach the heterointerface and misfit dislocations are nucleated at the islands coalescence region. These dislocations are a type and glide in the basal plane to promote further relaxation. For (Al,Ga)N∕GaN with an Al concentration below 70%, the apexes of the V trenches do not reach the heterointerface, prohibiting the nucleation of misfit dislocations. For thicker layers, the next stage of the relaxation is the cracking of the films.

Quantum and transport lifetimes of two-dimensional electrons gas in AlGaN/GaN heterostructures

The transport and quantum lifetimes were respectively deduced from low-temperature mobility and Shubnikov–de Haas measurements as a function of carrier density in metal organic vapor phase epitaxy-grown AlGaN∕GaN∕sapphire heterostructures. We show experimentally that the lifetime ratio varies as a bell curve, qualitatively confirming a recent theoretical prediction. However the experimental ratio varied much less than was theoretically predicted: From 9 to 19 for carrier densities in 1–9×1012cm−2 range. Moreover, we show the variation of quantum time with carrier density presents some discrepancy with the theoretical study. We also show that transport to quantum lifetime ratio cannot be used alone as a clear figure of merit from AlGaN∕GaN heterojunctions.

Low electron mobility of field-effect transistor determined by modulated magnetoresistance

Room temperature magnetotransport experiments were carried out on field-effect transistors in magnetic fields up to 10 T. It is shown that measurements of the transistor magnetoresistance and its first derivative with respect to the gate voltage allow the derivation of the electron mobility in the gated part of the transistor channel, while the access/contact resistances and the transistor gate length need not be known. We demonstrate the potential of this method using GaN and Si field-effect transistors and discuss its importance for mobility measurements in transistors with nanometer gate length.

Investigation of AlGaN∕AlN∕GaN heterostructures for magnetic sensor application from liquid helium temperature to 300°C

We report a comparative investigation of the magnetic response of long channel AlGaN∕AlN∕GaN heterostructures (Hall-field effect transistor devices) grown on three different semi-insulating templates on silicon and sapphire. From Hall effect measurements conducted up to 573K (300°C), we find that some of these specific devices can be used as magnetic sensors in a large temperature range (∼600°C) with a magnetic sensitivity close to 60V∕AT and a small thermal drift. On the best sample, between liquid helium temperature and 300°C, the average value of the thermal drift is only −7ppm∕°C.

Low temperature electron mobility and concentration under the gate of AlGaN/GaN field effect transistors

High electron mobility field effect transistors were fabricated on AlGaN∕GaN heterostructures and their magnetoresistance was measured at 4.2K up to 10T with simultaneous modulation of the gate potential. Low and high magnetic field data were used to determine the electron mobility (μ) and concentration (n), respectively, in the gated part of the transistor channel. With these measurements we present a method to determine μ and n under the gate of a transistor, which does not require knowledge of the transistor gate length, access resistance, threshold voltage, or capacitance. We discuss applications of this method for nanometer and ballistic transistors.

Ni∕Al0.2Ga0.8N interfacial reaction and Schottky contact formation using high quality epitaxial layers

The formation of the Ni∕Al0.2Ga0.8N Schottky contacts has been investigated by x-ray photoelectron spectroscopy. In situ scanning tunneling microscopy was used in parallel to investigate the morphology of the Ni covered surface after the last deposition. In the same way, results are presented through two perspectives: the intensity of core-level signals which give information on the growth mode, and the core-level binding energy positions which assess changes in electronic and chemical properties as a function of Ni coverage. Ni deposition on Al0.2Ga0.8N substrates follows the Stranski–Krastanov growth mode. It is suggested that Ni preferably reacts with the contaminants at the surface rather than with the epilayer itself. The Schottky barrier formation is discussed in terms of unified defect and metal-induced gap states models.

Electron Mobility and Concentration on Submicrometer Scale — Investigation of Si and AlGaN/GaN Field Effect Transistors by AC Magnetoresistance Method

Alternative current magnetoresistance (AC MR) method was applied to Si MOSFETs and AlGaN/GaN HEMTs at 4.2 K and 300 K. We show that the AC MR method allows to locally determine the quantum and transport relaxation times under the gate of micrometer and submicometer transistors.

Strain relaxation in (Al,Ga)N/GaN heterostructures

Strain relaxation mechanisms in metal-organic vapour phase epitaxy grown (Al,Ga)N/GaN heterostructures are presented. Relaxation first occurs through a 2D–3D transition. For pure AlN, misfit a-type dislocations are introduced at the coalescence front of growth islands. For (Al,Ga)N (Al concentration≤70%), the second relaxation step is cracking. When cracked, relaxation of the films occurs by the introduction of long and straight a+c-type dislocations and small bowed a-type dislocation half-loops bordering the cracks. These two relaxing features lead for Al0.2Ga0.8N films above 2μm thick to full relaxation.