| Compared with conventional semiconductors,wide bandgap semiconductors present better electronic transport,optoelectronic,and electro-optical properties.Among them,the ternary mixed crystal semiconductor quantum wells(QWs)consisting of ?-? and special ?-? compounds represented by ZnO and GaN are capable of forming high concentration two-dimensional electron gas near the hetero-interfaces to enhance effectively the electronic mobility,response rate and frequency of the materials and related devices.Furthermore,this kind of QWs is also widely applied as the functional units in short wavelength devices especially with ultraviolet spectra,due to the characteristic of tunable large bandgaps.Exploring their fundamental physical properties in theory not only can explain and predict the experimental phenomena,but also can instruct the research and design for novel materials and devices.Meanwhile,the related theory can be improved as well.In this thesis,based on Loudon uniaxial crystal,medium and dielectriccontinuum model,the basic properties of acoustic phonons in AlGaN/GaN asymmetric QWs as examples are discussed first.Then,MgZnO/ZnO QWs are chosen as an example,the electron states,optical phonons,related electron-phonon interactions and the electronic mobility are investigated with consideration of mixed-phase effects,the properties for optical absorption of electron inter-subband transitions are also given.Our main contents in this thesis and corresponding results are summarized as follows(1)Acoustic phonons in AlxGa1-xN/GaN/AlyGa1-yN asymmetric QWs.At the limit condition of binary compounds,the dispersion relations of the phonons approach to four boundary lines which possess the gradients approaching to the group velocities of related bulk materials.Based on different boundary lines,the phonons can be classified into propagation,confined,and half space modes,respectively.The phonon modes with adjacent frequencies interact and hybridize with each other,resulting in the appearances of interface modes within a certain region of frequencies According to the cut-off conditions and intersections between dispersion relations and boundary lines,the cut-off frequencies and wave vectors for the phonon modes are obtained,that is to say the existing and transforming conditions of phonon modes The results of binary compounds can be generalized to the ternary ones.It can be found that the effects of ternary mixed crystals can induce the appearance of cut-off frequencies and wave vectors dependent on the dispersion relations within the confined positions of the phonon modes,the boundary lines approached by the dispersion relations can appear as well(2)Optical phonons and related interactions with electrons in MgxZn1-xO/ZnO symmetric QWs.Compared with wurtzite,as MgxZn1-xO is rocksalt phase for QWs,the interface and confined modes are confined more strongly in the well layers and weaker in the barrier layers,respectively.With increase of Mg composition,the electronic mobility influenced by optical phonons exhibit a minimum and a maximum in wurtzite and rocksalt phases of MgZnO,respectively.Based on the discussion of the size effect,a thicker well is more beneficial to obtain higher electronic mobility.(3)Optical absorption of electron inter-subband transitions in MgxZn1-xO/ZnO/MgyZn1-yO asymmetric QWs.For two side barriers with a pure phase,only an absorption spectrum with a single peak can be observed.For a given Mg composition for right barrier in mixed phases and the left barrier in wurtzite phase,the clearly double absorption peaks can be observed.With Mg composition for left barrier increasing into mixed phases,three and even four absorption peaks can be found.With a left barrier in rocksalt phase,although double peaks should exist in theory,but only one peak is detectable due to the other one is too weak.Our results show that the influence from ternary mixed crystal effects should be considered as the primary factors for the multi-color photo detectors. |
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