【作者】 傅德颐；

【导师】 张荣； 吴军桥；

【作者基本信息】 南京大学 ， 微电子学与固体电子学， 2012， 博士

【摘要】 研究半导体材料中的基本科学问题,为提升材料和器件性能提供理论指导和实验依据,推动实现基于半导体材料的能源相关器件的大规模应用,最终缓解能源紧张局面,实现可持续发展,是当前能源相关科学研究的价值和目标所在。本论文以能源研究为切入点,以半导体能带工程和动力学输运理论为方法论,针对光电器件、热电器件以及光伏器件这三个当前能源相关研究的热点领域中的一些基本问题,从理论和实验两方面展开了材料基础研究和器件应用的探索。本论文得到的主要成果和结论如下：1.系统地研究了应变和极化对Ⅲ族氮化物的能带结构及其发光性质的调制机理：(1)采用k·p微扰理论详细计算并预言了GaN的极性c面在非对称应变作用下的各向异性发光。通过利用单轴应力仪施加面内非对称应变,首次在传统c面GaN薄膜中观察到了面内各向异性光致发光,印证了理论。进而通过定量控制应变,实现了对光学各向异性的人为调控。(2)采用金属有机物化学气相沉积(MOCVD)系统制备出单一取向的m面GaN和InN薄膜,发现了非极性面薄膜的面内晶体结构和光学各向异性,基于k·p微扰理论探讨了其偏振光致发光特性与能带分裂的内在关联。(3)系统研究了AlN独特的价带结构在应变作用下的演化过程,发现其价带第一、二子带在应变为εx=0.98%和εxx=εyy=-1.70%时发生能量简并,同时二者的能带性质发生互换。在该临界点处,价带的三个子带{VB1,VB2,VB3}按照能量从高到低的顺序对称性由{Γ7,Γ9,Γ7}转变为{Γ9,Γ7,Γ7},从而为AlN的能带改性提供了理论依据。2.理论上系统研究了生长衬底、应变和组分等对不同生长面(极性面和非极性面)Al(Ga, In)N合金体系的能带及其紫外发光性质的调制规律,从能带工程的角度提出了实现紫外光高效发射的优化方案：(1)对于赝晶生长的c面AlGaN和AlInN合金来说,A1N较之GaN为更加合适的生长模板,因为前者对应更宽的紫外发射(组分)窗口。(2)c面A1GaN合金受到面内单轴应变时比受到双轴应变时具有更优的表面紫外发光效率。这是因为前者具有更宽的发射窗口,而且可以获得线性偏振发光,因此可以用来设计偏振发光器件。对于m面A1GaN合金来说,不论受到单轴应变还是双轴应变都能获得面内偏振发射,因此具有高的紫外发光效率。(3)当AlxGa1-xN沿c面赝晶生长于AlyGa1_yN(x<y)模板上时,调节二者的A1组分到y>-0.03+1.79x-0.06x2(0<x<y<1)的区域,可以获得高效紫外光面发射；而当沿m面生长时,在全组分范围内都具有很高的表面出光效率。3.针对热电应用,(1)测量得到了高质量InN薄膜的热导率为120W/mK,并研究了通过高能粒子束辐照产生的点缺陷对InN热导率和Seebeck系数的影响。结果表明InN的热电品质因子对高能辐照并不敏感,而且可以通过控制点缺陷浓度来调制材料的热学和热电性质。当InN与GaN形成合金后,材料热导率得以进一步降低,从而使其成为潜在的热电材料。(2)采用电热动力学输运模型首次模拟并揭示了非均匀、双极性结构中(以典型的P-N结为例)电热驱动的涡旋电流。进一步分析发现该涡旋电流对体系的Seebeck系数将产生显著影响,由电流流动产生的焦耳热将有效降低系统的热导率,相应的减小量可与电子热导率相比拟。4.针对光伏应用,(1)系统模拟了半导体纳米线在载流子受到局域调制时的电热动力学输运过程,揭示并预言了之前并未被充分认识或受到重视的重要效应,对分析扫描光电流谱实验时通常所作假设的合理性进行了重新评估,为采用扫描电流技术表征纳米线中的载流子输运特性提供了理论指导。(2)在具有周期性畴结构的铁电材料BiFeO3薄膜中观测到了高效光伏效应,并采用微观输运模型全面阐释了这一效应的形成机制。在足够强的光照下,光伏电流产生于纳米尺度的铁电畴壁。开路状态下,周期排列的畴壁处的光生电压具有可叠加性,总的开路电压远远超过了材料的带隙电压。对于高于带隙能量的入射光子来说,单个畴壁的内量子转化效率高达10%。虽然该效应是在BiFeO3薄膜中发现的,但是这类效应也应该存在于具有类似周期性电势的材料和结构之中。更多还原

【Abstract】 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.更多还原