Strain And Polarization Induced Band Engineering And The Transport Dynamics Of Semiconductors

Studying the basic science in semiconductors, providing theoretical guideline and experimental basis for advancing material and device performance, promoting the mass application of energy-related devices to alleviate the energy crisis and to achieve sustainable development, is the ultimate goal of the energy-related scientific research. In this thesis, based on the concepts of band engineering and electrothermal dynamic transport theory, some basic problems in the energy-related topics such as optoelectronics, thermoelectrics and photovoltaics are studied from both theoretical and experimental point of view. The major results and conclusions are summarized as follows:1. The mechanisms of strain and polarization induced modification of the energy band structure and corresponding optical properties of Ⅲ-nitride semiconductors are systematically studied.(1) The modification of valence band structure by strain in GaN is carefully investigated based on the well-known k-p perturbation theory. The polar c-plane of GaN is predicted to show optical anisotropy under anisotropic strain. By introducing asymmetric in-plane strain using a well-designed uniaxial stress device, for the first time we observed evident anisotropic photoluminescence in conventional c-plane GaN, which validates the theoretical prediction. By further tuning the strain applied, the degree of optical anisotropy can be well controlled.(2) Single-phase m-plane GaN and InN thin films are successfully grown by using metal organic chemical vapor deposition (MOCVD) system. Both structural and optical anisotropies of the non-polar plane are observed. The correlation between the polarized photoluminescence and the splitting of their band structure is discussed based on the k·p perturbation theory.(3) The evolution of the unique valence band structure of AIN under strain is systematically studied. It is found that the two topmost valence subbands exchange their band characteristics at the degenerate point where εzz=0.98%and εxx=εyy=-1.70%. The symmetry order of its valence subbands{VB1, VB2, VB3} changes as expected from{Г7, Г9, Г7} to{Г9, Г7, Г7} across this critical point, which provides theoretical basis for band structure modification of this material.2. The effects of substrate, strain and composition on the Al(Ga, In)N alloy system with different growth planes (polar and non-polar plane) are comprehensively investigated. Optimized strategies for enhancing the ultraviolet emission efficiencies are outlined from the band engineering point of view:(1) for AlGaN and AlInN alloys pseudomorphically grown on c-plane GaN and AIN templates, it is found that AlGaN and AlInN alloys grown on AIN template show wider optimal windows than that grown on GaN template.(2) c-plane AlxGa1-xN modified by uniaxial strain shows more advantages over biaxial-strained AlxGa1-xN. This is due to the relatively more flexible tuning range and the advantage of obtaining pure linear polarization, which can be utilized to design polarized emission devices. For m-plane AlxGa1-xN, there are always in-plane polarized emissions under both biaxial and uniaxial strain conditions, thus, it is more likely to obtain high surface emission efficiency.(3) For AlxGa1-xN films pseudomorphically grown on AlyGa1-yN templates (x<y), the film/template Al-content combinations with y>-0.03+1.19x-0.06x2(0<x<y<1) for the c-plane case and all y:x (0<x<y<1) combinations for the m-plane case, are particularly suitable for obtaining efficient UV light emissions.3. Aiming at thermoelectric applications,(1) The thermal conductivity of high-quality InN is determined to be120W/mK and the effects of point defects generated by high-energy particle irradiation on the thermal conductivity and Seebeck coefficient are examined. It is shown that irradiation can be used to modulate the thermal and thermoelectric properties of InN by controlling point defect concentrations. The thermoelectric figure of merit of InN was found to be insensitive to irradiation. When the InN is alloyed with GaN, its thermal conductivities are further reduced due to alloy scattering of phonons, which makes this material system a promising candidates for thermoelectric applications.(2) Using Si P-N junction as a prototype system, the electrothermally driven current vortices are predicted in inhomogeneous bipolar semiconductors for the first time. It is found that the effective thermopower can be significantly modified by the current vortices. Joule heating arising from the vortices reduces the thermal conductivity by an amount comparable to the electronic thermal conductivity.4. Aiming at photovoltaic applications,(1) the electrothermal dynamics of free charge carriers in semiconductor nanowires under local carrier modulation (in the scanning photocurrent microscopy configuration) are comprehensively modeled. The simulation reveals and predicts important effects that are previously not recognized or appreciated and evaluates the validity of hypotheses that are routinely assumed in analyzing experiments of scanning photocurrent microscopy. The limitation as well as potential of the scanning current techniques in nanowire characterization are assessed.(2) The mechanism of a newly observed efficient photovoltaic effect which occurs in ferroelectrics with periodic domain structures is elucidated using BiFeO3as a model system. Under sufficiently strong illumination, domain walls function as nanoscale generators of the photovoltaic current. In open circuit, photovoltages for periodically ordered domain walls are additive and voltages much larger than the band gap can be generated. The internal quantum efficiency for individual domain walls can be surprisingly high, approaching10%for above band-gap photons. Although we have found the effect in BiFeO3films, it should occur in any system with a similar periodic potential.