| In this dissertation we focus on the fundamental properties of a two-dimensional electron gas (2DEG) residing at the interface of GaN and AlGaN. We conduct transport experiments over a wide range of temperatures and magnetic fields on 2DEG's characterized by carrier densities, n2 D between 1 and 8 × 1012 cm −2 and mobilities, μ between 15 × 103 and 60 × 103 cm2/Vsec. Our experiments are aimed at understanding electron-phonon interactions, electron scattering from dislocations, non-parabolicity (NP) of the host GaN and cyclotron resonance (CR) of electrons in two-dimensional systems.; Electron-phonon interactions in the AlGaN/GaN structures are explored via mobility measurements at different temperatures. Similar to reports by other authors, we find that near 300K the 2DEG mobility is limited by scattering due to polar optical phonons of GaN. However, at lower temperatures where T-dependence of μ is dominated by acoustic phonons, we find several new and interesting results. Specifically, (1) between ∼30 and 100K, we find that screened deformation and piezoelectric potentials are required to explain electron scattering for n2 D 4 × 1012 cm−2 while a model with unscreened potentials is needed for lower densities. The electron-phonon scattering model does not allow for the usage of both screened and unscreened interactions. Thus, our results highlight the need for improvements to the model in order to account for all the data. (2) For 10 < T < 30K, we report the first observation of a Bloch-Grüneisen (B-G) transition in the mobility of a 2DEG in AlGaN/GaN heterostructures. (3) In the 2< T <10 range, we find a surprising departure of μ from a Bloch-Grüneisen-like T 5 behavior and a cross-over into a regime where the mobility decreases as T → 0. The temperature T0, which defines the onset of this low-T decrease in μ, is found to closely follow the characteristic electron-electron interaction temperature in our samples.; We also investigate the type of scattering processes that dominate our 2DEG at low temperatures, by comparing 4K data on mobility, quantum and CR lifetimes—τt, τq, and τCR, respectively. Using a simple model of density inhomogeneity, we find that τCR and not τq is a proper measure of the inter-event scattering lifetime in our samples. (Abstract shortened by UMI.)... |
