Highly resistive molecular beam epitaxial GaN layers are characterized by temperature dependent conductivity and Hall effect measurements. Seven n-type GaN samples with room temperature layer resistivity ranging between 8 and 4.2ϫ10 6 ⍀ cm are used in this study. The experimental data are analyzed by considering various transport models such as band and hopping conduction, scattering on charged dislocations and grain boundaries controlled transport. The same defect level of 0.23 eV, attributed to nitrogen vacancy, is found for layers with 300 р3.7ϫ10 3 ⍀ cm. The Hall mobility for two lower resistivity layers is influenced mainly by phonon scattering (H ϳT x , xϭϪ1.4). However, higher resistivity layers show positive mobility power, xϭ0.5-0.9, which can be explained by dominating scattering on charged dislocations. Properties of layers with the highest resistivity ͑1ϫ10 5 and 4.2ϫ10 6 ⍀ cm͒ and extremely low Hall mobility ͑6 and Ͻ0.1 cm 2 V Ϫ1 s Ϫ1 ͒ are consistent with grain boundary controlled transport. The barrier height between grains of 0.11 eV and an average grain size of 200 nm are found. Neither nearest-neighbor or variable range single phonon hopping nor multiphonon hopping can be clearly attributed to the conduction of the layers investigated.

Free-standing highly resistive Fe-doped GaN layers grown by hydride vapor phase epitaxy were characterized by temperature-dependent conductivity and Hall effect measurements. Samples with a room-temperature resistivity of 1.6×107–6×108Ωcm and a Hall mobility of ∼330cm2V−1s−1 showed simple band conduction with the mobility power x=−1.5 and an activation energy 0.58–0.60eV, which can be attributed to a Fe acceptor. Samples with a lower mobility, ⩽10cm2V−1s−1, exhibited an increase of the mobility with temperature. Here, the conduction seems to be strongly influenced by potential barriers at inhomogeneities, with an activation energy of 0.21eV and a barrier height of 0.14–0.18eV. The activation energy 0.36 and 0.40eV, evaluated from the resistivity measurements, does not correspond to that of the Fe acceptor.

Reduction of variable range hopping conduction in low-temperature molecular-beam epitaxy GaAsHeavily carbon-doped In 0.53 Ga 0.47 As on InP (001) substrate grown by solid source molecular beam epitaxy Conductivity, Hall effect as well as ''physical'' and ''geometrical'' magnetoresistances were measured at 290-440 K in molecular-beam epitaxial GaAs layers grown at 200-400°C. The experimental data were analyzed taking into account the combined band and hopping conductance regime. Positive hopping magnetoresistance parameters (⌬/ 0 B 2 ) h Ϸ10 Ϫ4 T Ϫ2 and hopping Hall mobilities lower than 1ϫ10 Ϫ4 m 2 V Ϫ1 s Ϫ1 were determined in the as-grown layers. A transverse-to-longitudinal hopping magnetoresistance ratio of about 2, consistent with hopping transport theories, was obtained. In the annealed layer grown at 200°C ͑J200a͒ the band mobility determined from the geometrical magnetoresitance ͑GMR͒ mobility was found to be significantly higher than the band Hall mobility. It is related to a mixed band conductivity regime with the hole concentration p exceeding the electron one n. The difference between GMR and Hall mobilities decreases with increasing growth temperature as far as a typical single-carrier band conductivity regime (nϾ p) is present in the layer grown at 400°C. In contradiction to the layers grown at higher temperatures, the J200a layer showed the opposite ͑positive͒ sign of the hopping Hall coefficient as well as the largest hopping magnetoresistance parameter (Ϸ3ϫ10 Ϫ2 T Ϫ2 ).

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