AlGaN基光偏振调制和深紫外发光机理研究

AlGaN alloys/quantum wells are ideal materials for short-wavelength deep-ultraviolet (UV) light emitting diodes (LEDs) and photodetectors. However, all of the reported external quantum efficiency of these devices is less than 1%, which extremely restricts their application. Here, we will aim at the low light emission efficiency and power of AlGaN-based UV-LEDs, considering some important experimental results, such as AlGaN containing nanoscale compositional inhomogeneities, light emission dominated by the localized states, and enhancing of the polarization switch value by strains and confinement, and investigate the disordered distribution of Ga atoms using powerful first-principles calculations, and determine the crystal microstructures related to the atomic-size compositional inhomogeneity of AlGaN alloys/quantum wells by means of the structural optimization. Moreover, considering the disordered distribution of Ga atoms, strains, polarizations and confinement, we will investigate the modulating of the valence-band top of AlGaN quantum wells with high Al content by several-atom Ga-N clusters/chains and strains, and change the valence-band order with the HH band, substituting the CH band, as the valence-band top. The polarization switch value is thus improved. Hence, the polarization is transformed from TM into TE and the light emission efficiency of the desirable TE polarization is enhanced. The present investigation will offer the important theoretical foundation for deep understanding of the optical properties of AlGaN alloys, designing and optimizing of the related device structures.

AlGaN是制备短波长深紫外发光和探测器件的理想材料。但迄今报道的AlGaN基紫外发光器件的外量子效率很低(<1%)，极大地限制了器件的应用。本项目将瞄准AlGaN基紫外发光器件的发光效率和光功率低这一核心问题，基于AlGaN 形成纳米尺度的成分不均匀合金、局域态主导其发光过程及应变和受限可以提高偏振开关值等实验事实，借助第一性原理，研究AlGaN合金/量子阱中Ga原子的无序分布，优化结构，确定与AlGaN中原子尺度成分不均匀相对应的晶体微结构。考虑到Ga原子无序分布、应变、极化和受限等因素，进一步研究小Ga-N团簇/短链和应变对AlGaN(特别是高Al组分)量子阱价带顶的调控，调整价带顶能带次序，使之成为重空穴带而非晶体场劈裂空穴带，提高偏振开关值，实现TM到TE偏振模式的转变，改进E⊥c模式的出光效率。为深入理解AlGaN光学性质、设计和优化器件结构、改进外量子效率等提供重要理论依据。

本项目主要解决了3个关键问题：（1）实现了高铝AlGaN的 偏振模式的转变；（2）解决了高铝AlGaN的p型掺杂难题；（3）极大提高了二维InSe的空穴迁移率约2-3个数量级。.基于AlGaN 形成纳米尺度的成分不均匀合金、局域态主导其发光过程及应变和受限可以提高偏振开关值等实验事实，借助强有力的第一性原理计算，澄清了这些因素对高铝AlGaN 的价带结构和光偏振的调控机理。研究表明：（1）采用纳米尺度的(AlN)m/(GaN)n(m>n) 超晶格替换高铝AlGaN合金，可以实现能量由高到低依次为重空穴、轻空穴、晶体场劈裂空穴的价带排序，价带顶（重空穴带）具有类px和py特性，并且在超晶格中高度局域，从而保证了其带间光跃迁是横向（TE, E⊥c）光偏振；（2）具有E⊥c偏振的带边光吸收主导带边光跃迁，表明了超晶格中确实实现了TE光偏振；（3）重空穴在平行于晶体c 轴方向具有非常大的有效质量，这对于增强AlGaN基发光二极管（LED）和激光二极管（LD）的光发射效率非常有利；（4）对于(AlN)m/(GaN)n超晶格中实现价带反转和TE偏振的物理起因进行了深入分析，断言薄的GaN层可以显著影响晶格常数和内部结构参量，从而有效的调制高铝AlGaN合金的电子结构和光学性质。.研究了高铝AlGaN 的p型掺杂这一世界性难题。通过在(AlN)m/(GaN)n的超晶格结构的GaN 层中掺入Mg受主，可以有效的降低其受主电离能，增强其p型电导，Mg替GaN单层中的Ga 原子具有最低的形成能和最小的受主电离能，随着掺杂位置从里面的GaN层移到外面的AlN 层，受主电离能EA逐渐增加，而且EA随着GaN单层数的增加逐渐下降（趋近于GaN的0.2 eV）。结果表明可以用MgGa δ-掺杂的(AlN)m/(GaN)n超晶格来代替传统的高铝AlGaN无序合金实现p型掺杂。该理论结果最近被武汉大学刘昌课题组的实验完美验证。.此外，基于优势互补的新思想，考虑到InSe单层的高电子迁移率和黑磷（BP）的高空穴迁移率，通过构建新颖的InSe/BP异质结，我们发现可以显著提高InSe单层的空穴迁移率（从40到104 cm2V-1s-1），给出了一个提高二维材料迁移率的普适图像/方法。