Electronic Interband Transition And Mobility In Wurtzite Quantum Wells Consisting Of Ternary Mixed Crystals

Wurtzite heterostructures consisting of AlGaN/GaN etc.are typical materials to produce optoelectric devices such as high electron mobility transistors,UV light detectors.They not only have wider band gaps and higher electronic mobility（EM）,but also possess excellent properties of enduring breakdown electric fields,thermal conductivity and radiation resistance.The energy bands,electron（hole）states,phonon modes and transport properties of these systems can be modulated by changing the components of the ternary mixed crystals（TMCs）to meet the requirements of producing high-temperature,high-frequency,radiation resistance and high-power devices.Thus,they have been the focus of theoretical and experimental researchers in the field of semiconductors.The anisotropy,strongly spontaneous and piezoelectric polarization of wurtzite structures result in a high concentration of two dimensional electron gas（2DEG）near the interfaces.The transverse optical（TO）phonons modulated by the components of AlGaN exhibit two mode property,so that the optical phonons of the multilayered heterostructures consisting of AlGaN show special mixed crystal effects.Taking TMC wurtzite quantum wells（QWs）as the object of study in this thesis,the Schrodinger and Poisson equations are solved self-consistently to obtain the electron and hole states.For QWs of MgZnO/ZnO,the Fermi-golden rule is adopted to discuss the interband transition of electrons（holes）.Whereas,for QWs of AlGaN/GaN,the dielectric continuous model and uniaxial model are adopted to obtain the potentials of different kinds of phonons and the interaction between an electron and phonons.The Lei-Ting force balance equation is used to discuss EM under scattering from optical phonons.The main contents and results are summarized as follows:（1）Size and TMC effects on optical absorption of electronic interband transitions in symmetric Mg_xZn_1-xO/ZnO/Mg_xZn_1-xO QWs.The results indicate that the increase of the Mg component in Mg_xZn_1-xO enhances the build-in electric field,which enforces electrons（holes）approaching to the left（right）interface.This induces an exponential decrease of absorption peak due to interband transition and makes a blue shift of the peak.For given components and barrier thickness of QWs,the absorption peak decreases and the peak position shows a red shift with increase of well width.（2）Size and TMC effects on optical absorption of electronic interband transitions in asymmetric Mg_xZn_1-xO/ZnO/Mg)yZn)（1-y）O QWs.The results show that increasing Mg component in left barrier monotonously enhances the build-in electric field in the well,whereas the directions of the build-in fields in left and right barriers will convert.And electrons（holes）tend to the left（right）interface,so as to reduce the overlapping of their wave functions.Thus,the absorption peak decreases rapidly and presents a blue shift with the increment of Mg component or the thickness of the left barrier.Furthermore,the absorption peak is more sensitive to the change of the well width than the left barrier size.The absorption peak reduces sharply and shows a slight red shift with increase of the well width.（3）Effects of two-mode property of TO phonons in AlGaN on optical phonons in Al_xGa_1-xN/GaN/Al_xGa_1-xN QWs.Weight model,single mode model and dielectric functions fitting model are proposed to take the two-mode property of TO phonons into account to solve classical electrostatic equations.It is shown that our numerical curves of TO and longitudinal optical phonon frequencies versus A1 component are almost identical with the experimental data by introducing impurity modes of binary compounds into the modified random-element isodisplacement model.Besides,the electrostatic potentials of four kinds of phonons obtained by weight model are higher than that by the other models.（4）Influences of optical phonons on EM in Al_xGa_1-xN/GaN/Al_xGa_1-xN QWs.For given areal density of 2DEG and at temperature of 300K,the results obtained by the weight model indicate that the total EM decreases to a minimum at first and then increases slowly with A1 component under the influences of the competitions from the eight branches of phonons.As a comparison,the EM is calculated for an Al0.58Ga0.42N/GaN QW and our theoretical result is 1263.0cm2/Ns which is 1.44 times of the experiment value.The further calculation shows that the total EM decreases with the increment of temperature and reduction of well width.Thus,it can be seen that the physical properties such as the intensity of the build-in fields and distribution of 2DEG can be modulated by the TMC components and size of QWs.The above conclusions are expected to give theoretical guidance to improve the opto-electronic properties of materials and design of devices.