Computational studies of III-V nitride semiconductors.
Doctoral thesis, UCL (University College London).
GaN and related nitride compounds have found many applications in optoelectronic devices. Point defects introduce energy levels into the band gap and alter the electrical and optical properties of GaN. Previous studies focussed on studying point defects with density functional theory (DFT), periodic boundary conditions (PBC) and relatively inaccurate LDA energy functionals. We aim to improve on the defi�ciencies of this method by implementing a quantum mechanical/molecular mechanical (QM/MM) scheme, which has been specifically designed for the study of point defects; we use a hybrid functional and a formal charge scheme for the MM model. We o�ffer an explanation for why p-type doping is difficult to achieve in GaN; the exothermic formation energies of the Ga interstitial and N vacancy at the VBM are thought to be the main cause. We suggest that the processes responsible for a variety of DLTS signals between 0.18-0.67eV below the CBM may be due to Ga interstitial 3+/2+, 2+/1+ transitions, N interstitials (1+/0) and Ga vacancies (2-/1-, 3-/2-). We attribute the ODMR signal indicating a deep donor state 0.7eV below the conduction band to the N interstitial 0/1- transition. Finally, our results support previous suggestions that Ga vacancies may be the cause of yellow luminescence in GaN. Further refi�nements of the model, especially improving the basis set, are recommended in the future, as well as a more detailed investigation into the causes of discrepancy between our model and PBC calculations. We use the MM model to study the properties of ternary alloys of AlN, GaN and InN, and to �find their thermodynamically stable con�figurations. Our results are in good agreement with PBC DFT calculations. These structures are not observed experimentally; we suggest that this is a growth phenomenon. Our results also support previous �findings that epitaxial strain stabilises highly internally strained alloys.
|Title:||Computational studies of III-V nitride semiconductors|
|Open access status:||An open access version is available from UCL Discovery|
|UCL classification:||UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Chemistry|
Archive Staff Only