The intrinsic free carrier mobility in AlGaN/GaN quantum wells
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The intrinsic free carrier mobility in AlGaN/GaN quantum wells F. Carosella1, M.Germain2 and J.-L. Farvacque1 1 Université des Sciences et Technologies de Lille, CNRS UMR 8008, Bât C6, 59655 Villeneuve d’Ascq Cedex, France 2 IMEC, Microsystems, Components and Packaging, Kapeldreef, 75, B-3000 Leuven, Belgium. ABSTRACT The aim of this contribution is to determine theoretically the maximum mobility that can be expected in AlGaN/GaN quantum wells as soon as the free carriers are only submitted to intrinsic scattering mechanisms associated with phonons and the carrier-carrier interaction. In our model, we consider that the carrier-carrier two bodies collisions do not constitute by themselves a relaxation mechanism since they conserve the momentum and the energy of the electron system. Thus, we assume that the free carriers act only through their contribution to the dynamical dielectric response of the material and, at least, through their collective behavior resulting into plasmons which, when damped, constitute now a real relaxation mechanism. The full scattering strength is connected with the imaginary part of the total reversed dielectric function including the lattice and the free carrier contributions. This approach automatically includes the scattering mechanisms associated with hybrid phonon/plasmon particles. INTRODUCTION AlGaN/GaN heterostructures have been subjected to a lot of experimental and theoretical studies since they offer an unique situation where, due to the spontaneous polarization discontinuity at the heterostructure (0,0,0,1) interface and also to the piezoelectric polarization that appears in the strained AlGaN layer, the interface plane bears a strong positive charge and, as a consequence, attracts free electrons that form a two dimensional (2D) electron gas, without the need of any external doping [1-6]. The total interface polarization charge is so large that one currently obtains 2D electron gas with an areal electron density that can reach more than 2.1013 cm-2. In these circumstances, such quantum wells are expected to have a large conductivity with good carrier mobility (lack of impurity scattering) and are considered as good candidates for power device applications. However, in the current state of the art for the epitaxial growth of such 2D electron gas, the carrier mobility has shown to be a strongly decreasing function of the carrier density [7]. This behavior could be explained by the concept of “interface electrical roughness” [8] associated with the progressive appearance of strain relaxation mechanisms leading to local fluctuations of the interface electrical charge. Since, at the present time, the intrinsic mobility cannot be measured experimentally, the aim of the present paper is to predict theoretically the order of magnitude of the maximum mobility that we may expect in perfect AlGaN/GaN quantum wells. THEORETICAL BACKGROUND Intrinsic scattering mechanisms are essentially associated with acoustic and optical phonons. Similarly to the electron-hole scattering mechanisms
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