Zahia Bougrioua

Cathodoluminescence spectroscopy of epitaxial-lateral-overgrown nonpolar (11-20) and semipolar (11-22) GaN in relation to microstructural characterization

Low temperature spatially resolved cathodoluminescence was carried out on GaN films grown by the epitaxial-lateral-overgrowth (ELO) technique with the nonpolar (11-20) and the semipolar (11-22) orientations on R- and M-sapphires, respectively. Defect related optical transitions were identified and their localization was correlated to different regions of ELO. The sample microstructure was further investigated by plan-view and cross-section transmission electron microscopies. It is shown that the defect related emissions are mainly localized in the seed of the samples, but different defects occur as well in the wings, especially in the case of nonpolar GaN. The structural defect densities are lowest in the overgrown wings of semipolar GaN. In particular, the [0001] wing region of semipolar ELO-GaN is almost defect-free with a cathodoluminescence spectrum dominated by the GaN band-edge emission at 3.476eV.

Microstructural Characterization of Semipolar GaN Templates and Epitaxial-Lateral-Overgrown Films Deposited on M-Plane Sapphire by Metalorganic Vapor Phase Epitaxy

The microstructure of GaN templates and epitaxial lateral overgrown (ELO) films deposited on M-plane sapphire is investigated by transmission electron microscopy. The epitaxial relationship is [1120]sapphire∥[1100]GaN in-plane with a growth plane close to (1122)GaN. The microstructure of the templates is dominated by the presence of basal stacking faults with a density of around 3×105 cm-1. Basal stacking faults are either terminated by partial dislocations or connected by prismatic stacking faults. Perfect dislocations are also observed. ELO films with stripes parallel to [1100]GaN allow us to reduce the density of basal stacking faults in the overgrown regions down to 5×103 cm-1. In these overgrown materials, the density of perfect dislocations is 108 cm-2 while neither partial dislocation nor prismatic stacking faults are observed.

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.

Thermal stability of Pt- and Ni-based Schottky contacts on GaN and Al0.31Ga0.69N

In this work we analyse the performance of Pt- and Ni-based Schottky metallizations on AlxGa1−xN (x = 0, 0.31). An intermediate thin Ti layer is shown to enhance the thermal stability of Pt/Au, and leads to an increase of the Schottky barrier height. Pt/Ti/Au contacts on GaN provide a barrier height of 1.18 ± 0.07 eV, increasing up to 2.0 ± 0.1 eV on Al0.31Ga0.69N. Further improvement of Schottky contacts is achieved by surface passivation with plasma-enhanced chemical vapour deposited SiO2 or SixNy, which reduces the leakage current by two orders of magnitude and structural modifications in the metal due to thermal ageing.

Epitaxial orientation of III-nitrides grown on R-plane sapphire by metal-organic-vapor-phase epitaxy

The absolute epitaxial relationships between III-nitride films and R-plane sapphire are investigated using convergent beam electron diffraction. The resulting relationships are found to be[1−100]III-N‖[11−20]sapphire and [0001]III-N‖[−1101]sapphire, respectively. The O–Al bonds in the sapphire surface are antiparallel to the N-III bonds in the III-N films. High-resolution transmission electron microscopy shows that the AlN nucleation layer is asymmetrically strained. Along its c direction, the AlN layer is in extension whereas it is relaxed perpendicular to c.

Improved AlGaN/GaN high electron mobility transistor using AlN interlayers

This work reports on the effects of AlN interlayers embedded into the GaN semi-insulating buffer of AlGaN/GaN high electron mobility transistors, in comparison with standard heterostructures without AlN interlayers. Detailed optical and structural characterization data are presented, along with computer simulation results. The AlN interlayers generate a compressive strain in the GaN topmost layer, which slightly reduces the total polarization field, but most important, it prevents the AlGaN barrier from plastic relaxation. The final result is an enhanced polarization field with respect to standard heterostructures, providing an increased channel carrier density and pinch-off voltage. Electrical characterization confirms the advantages of using AlN interlayers, reaching maximum drain current density and extrinsic transconductance as high as 1.4 A/mm and 266 mS/mm, respectively, for 0.2-μm gate length.

Material optimisation for AlGaN/GaN HFET applications

An optimisation of some growth parameters for the epitaxy of AlGaN-GaN based heterostructure field effect transistors (HFET) at low pressure in a new 3 * 2 MOVPE reactor is presented. Some possible processes for the growth of semi-insulating buffers have been identified and are described. TEM analysis shows that the insulating character is not due to a high density of dislocations, whereas SIMS analysis shows that classical impurity (Si, O and C) concentrations are in the same range as in conductive undoped layers. Further studies are needed to identify the traps responsible for the compensation of the GaN layers. The properties of the two-dimensional electron gas (2DEG) located at the AlGaN-GaN interface can be tuned by modifying the characteristics of the AlGaN layer and of the insulating buffer. The best mobility (1500 cm 2 V -1 s -1 for n∼6 × 10 12 cm 2 ) is obtained when using a thick buffer layer, whereas the sheet carrier density is found to increase with the Al content in the undoped supply layer and reaches 1.1 x 10 13 cm 2 for a composition of 24%.

Mechanism of mobility increase of the two-dimensional electron gas in AlGaN∕GaN heterostructures under small dose gamma irradiation

The effect of a small dose of gamma irradiation on transport characteristics of the two-dimensional electron gas (2DEG) in AlGaN∕GaN heterostructures was investigated. It is shown that the carrier concentration remains practically unchanged after an irradiation dose of 106rad, while the 2DEG mobility exhibits a considerable increase. The results are explained within a model that takes into account the relaxation of elastic strains and structural-impurity ordering occurring in the barrier layer under irradiation.

Detailed interpretation of electron transport in n-GaN

We report on temperature-dependent differential Hall-effect and resistivity measurements, between 10 and 300 K, on two silicon doped GaN epitaxial layers grown by two different metalorganic chemical vapor deposition sources on sapphire substrates. Reactive ion etching has been used to enable Hall measurements to be taken as a function of film thickness, for 1.62 and 3.92 μm thick films. Temperature-dependent Hall experiments indicate classical donor freeze-out in the doped region, while the depth profile measurements show that in the undoped layer, the Hall electron density passes through a minimum before increasing again at lower temperatures. Such behavior is indicative of impurity conduction in this region. Using a model based upon a doped layer with one type of shallow donor, plus compensating acceptors, in parallel with an interface layer which shows impurity-band conduction, a simultaneous fitting of mobility and carrier concentration has been undertaken to quantify the contribution of different scattering mechanisms, and the densities of the donors and acceptors in the doped region were also found. An important result is that for the carrier concentration fitting, when the temperature dependence of the activation energy is taken into account, the fitted energies are closer to those predicted by free and bound carrier screening. Finally the Fermi level was found to be in the range ∼56–84 meV below the conduction band minimum at room temperature and moved closer to the donor levels at low temperatures.

Reliability of Schottky Contacts on AlGaN

We compare the performance of Pt- and Ni-based Schottky contacts on Al x Ga 1-x N (x = 0, 0.31). Pt/Au contacts on GaN present lower leakage currents than Ni/Au, although they degrade at temperatures as low as 300 °C due to Pt-Au inter-diffusion. An intermediate thin Ti layer is shown to enhance thermal stability and Schottky barrier height. Pt/Ti/Au contacts with a barrier height of (1.18 ± 0.07) eV have been obtained on GaN, increasing up to (2.0 ± 0.1) eV on Al 0.31 Ga 0.69 N. Further improvement of Schottky contacts is achieved by surface passivation with SiO 2 or Si x N y , which reduces leakage current by two orders of magnitude.