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We have performed Shubnikov-de Haas (SdH) and Hall effect measurements to investigate the electronic transport properties of polarisation induced 2DEGs in Al x Ga 1-x N/GaN heterostructures with alloy compositions between x = 0.10 and 0.35 and sheet carrier concentrations of up to n s = 1.05 × 10 13 cm -2 . From SdH measurements of 2DEGs with sheet carrier concentrations of 2.1 × 10 12 and 4.6 × 10 12 cm -2 , effective electron masses were determined to be 0.24 and 0.207m 0 , respectively. In addition, angle resolved SdH measurements were performed to evaluate the effective g-factor from the angle of zero oscillation amplitude. Including the measured electron masses, the g-factors were calculated to be 2.11 and 2.47, respectively. In order to identify the main electron scattering mechanism we determined the ratio between transport- and quantum scattering times as a function of sheet carrier concentration. We observe a significant decrease of the τ t /τ q ratio with increasing 2DEG carrier concentration, indicating a transition from a dominant small angle to large angle scattering mechanism when n s exceeds 7 × 10 12 cm -2 .

The role of hydrogen in gallium nitride was studied on thin films of GaN on sapphire prepared at substrate temperatures in the range of 600 to 1100 °C. By using triethylgallium and ammonia as precursor and hydrogen and/or nitrogen as transport gases, we have observed a strong influence of molecular hydrogen on the deposition rate and the structural properties of epitaxial GaN. By elastic recoil detection analysis and thermal desorption measurements we were able to determine the total concentration of nitrogen, hydrogen and carbon in the bulk material. Isotope substitution of hydrogen by deuterium in the H2 carrier gas did not give rise to a noticeable deuterium incorporation, showing that the sources for hydrogen are the metalorganic precursor, ammonia or reaction products of both. Once incorporated, thermally activated hydrogen effusion from n-type GaN occurs with an activation energy of more than 3.9 eV. With the help of mass spectrometry we established hydrogen effusion from heavily magnesium-doped (2 at%) GaN at temperatures between 600 and 700 °C, which is the temperature range used for acceptor activation.