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Title: Electrical properties of group III nitrides
Author: Lee, Kwon John
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 2001
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Using temperature dependent Hall measurements, electron transport in n-type GaN films grown on sapphire by MOCVD and MBE has been analysed assuming the presence of impurity band conduction. No dependence on growth method or dopant type was observed, but other trends were apparent: (i) the activation energy for the impurity band fell with increased doping; (ii) the temperature of the minimum in the Hall carrier density versus temperature curves increased with doping, but did not depend strongly on the absolute value of mobility; (iii) the ratio of the mobility in the GaN conduction band to that in the impurity band also showed systematic behaviour, possibly arising from structure-related scattering process. Carrier transport in a set of AlGaN/GaN samples from different sources with a range of electron densities and mobilities has been investigated at low temperatures and high magnetic fields. The Shubnikov-de Haas (SdH) oscillations have been analysed to extract the quantum scattering time, and this is compared with the transport lifetime, derived from the low-field mobility. From the relationship between these parameters, it can be concluded that the effects of large-angle scattering, arising from defects such as dislocations and background impurities, dominate most samples, though one sample seems to point to grain boundary scattering. Phonon emission process in an AlGaN/GaN heterostructure with mobility in excess of 25,000 cm2V -1s-1 have also been investigated at low temperatures, where the amplitude of the SdH oscillations has been used as a thermometer for the electron temperature, and has shown that the power input per electron follows a T4.4 dependence. Comparison with numerical modelling indicates that in this sample, electron-acoustic phonon scattering via the screened piezoelectric interaction xis the dominant energy-loss mechanism.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available