Pressure Effect On Screened Exciton In Strained GaN/AlGaN Quantum Well

In recent decades,the low-dimensional structures made of wurtzite and zinc-blende group-â…¢nitride semiconductors such as A1N,GaN and InN were paid much attention owing to their promising application in light-emitting diodes,laser diodes and so on.In particular,different lattice structures,different grown axes,the polarized charges induced by lattice mismatch present at the interface and the strong built-in electric field induced by the charges consequentially influence the properties of excitons in strained nitride quantum wells(QWs).Furthennore,the investigation of pressure effect on the screened excitons in strained nitride QWs becomes more significant due to the modulation to the physical properties of materials by pressure.In this thesis,the eigenequations for electrons and holes in QWs with finite barriers under the influence of screening effect induced by the electron-hole gas are solved firstly to obtain the eigenfunctions and their corresponding eigenvalues.Then, in the framework of effective mass and single-band approximation,a variational method combined with a self-consistent procedure is adopted to discuss the binding energies of excitons in strained wurtzite and zinc-blende GaN/Al_xGa_1-xN QWs.By considering the hydrostatic pressure effect and screening due to the electron-hole gas, the binding energies of excitons in wurtzite([0001J-oriented) and zinc-blende([001]-and [111]-oriented) GaN/A1_xGa_1-xN QWs are investigated,respectively.Finally,the interaction between electrons(holes) and confined bulk longitudinal optical phonons and interface optical phonons is considered to calculate the binding energies of screened excitons in wurtzite strained QWs,and the pressure effect is also discussed.For the eigenequations of electrons and holes,the numerical results are computed by solving self-consistently the Poisson equation and Schrodinger equation. The result shows that the built-in electric field separates electrons and holes to move towards the opposite barriers respectively,whereas the presence of the electron-hole gas will screen the field and inforce electrons and holes to move towards the center of the wells.Furthermore,the results about excitons in wurtzite or zinc-blende strained QWs indicate that the binding energies nearly linearly increase with pressure even under consideration of the modulation to strain by pressure.It is also found that the increase percentage of the binding energy with pressure is influenced by the electron-hole density due to the pressure influence on the screening and exclusion effects.Under consideration of the interaction between excitons and optical phonons, the results show that the negative contribution from phonons to the binding energy increases first,then reaches a maximum and decreases finally as the electron-hole density increases.The phononic contribution decreases to be zero at some large electron-hole density where the exciton collapses,It is also found that the contributions from confined and interface optical phonons to the binding energies of excitons linearly increase with pressure and the increase percentages are lowered as the electron-hole density increases.