Exciton Optical Absorption In ZnO/Zn_1-xMg_xO Asymmetric Double Quantum Wells

There are mixed phases of wurtzite and zinc-blende in ternary mixed crystals?TMCs?Zn_1-xMg_xO with Mg component in the range of 0.37<x<0.62 for ZnO/Zn_1-xMg_xO double quantum wells.This thesis focuses on the characteristics of exciton optical absorption with mixed phase structure in the region.Taking into account of the built-in electric field induced by spontaneous polarization and piezoelectric polarization and the Hartree potential,the optical absorption coefficients?OACs?of exciton transitions are calculated by numerical method for the barrier materials are wurtzite and zinc-blende structures,respectively.The OACs of the two kinds of structures are fitted by the weight model set by us to obtain the properties of exciton optical absorptions,and the effects of size and TMCs are discussed furthermore.Based on the review of related properties of semiconductors with ZnO as a host in this thesis,and the development status and existing problems of exciton optical absorptions in single well,double wells and multi wells materials of ZnO/Zn_1-xMg_xO are emphasized.Aimed at the related problems,the electron?hole?states are obtained by a self-consistent solution of schr?dinger equation and Poisson equation within the effective mass approximation.Furthermore,the different energy levels of exciton are obtained by combining the numerical computation with variational process.Using the density matrix approach,the OACs of the exciton interstate transitions from the basic stats to the fist excited?the first kind?,from the first excited state to the second excited state?the second kind?and from the basic state to the to the second excited state?the third kind?are obtained.Consequently,according to the cumulative integral of Gaussian normal distribution function,a weight model is established,and the characteristics of OACs in mixed phase structures are discussed.The results show that the energy levels of electrons?holes?decrease?increase?due to the Hartree potential and the built-in electric field to force an electron?hole?moving near the left?right?and resulting the increase?decrease?of the energy levels of electrons?holes?.For the second and third kinds of exciton transitions mentioned above,in the double quantum wells that barriers are zinc-blende structures with same width of the left and right wells,an exciton transforms from indirect to direct with increase of the intermediate barrier thickness.A mergence of optical absorption peaks for the third kind of exciton transition occurs for a thicker intermediate barrier.However,the exciton remains indirect when the width of left well is larger than that of right well,and the above phenomenon disappears.It indicates that the existence of asymmetric factors can keep the exciton more stable.In addition,for the two kinds of ADQW structures,the OACs occur red shifts with increasing the well width and occur blue shifts as Mg component increases under the influence of built-in electric fields.In order to verify the feasibility of the theory,the exciton interband transition energies are also computed in the thesis,the result agrees well with the experiment.The results obtained here can provide theoretical guidance to improve the performance of optoelectronic devices consisting of this kind of double quantum wells.