Electronic Structures and Spin-dependent Transport in Type-II Broken-Gap Quantum Systems
|關鍵字:||碎能隙;中紅外線;能帶結構;自旋;自旋濾波器;自旋正反器;應力;broken gap;MWIR;band structure;spin;spin filter;spin flip-flop;strain|
|摘要:||本論文利用8×8的 Hamiltonian及scattering matrix方法且考慮應力效應下，計算碎能隙量子井的能態結構和對應各量子化能態的波函數機率密度分布，發現電子能態和輕電洞能態有強烈的混成效應，且藉由改變量子井的寬度和應力後發現碎能隙量子井有機會由半導體特性轉變為劣金屬特性。同時成功設計出發光波長在中紅外線波段之量子井結構。當改變材料使其受到一相反應力效應但維持系統為碎能隙結構時，發現在受擴張應力下無法像傳統量子井結構中輕電洞能態成為第一個量子化價帶能階，並且在計算動量矩陣元素後得知此系統對電子躍遷速率沒有明顯改進。但當變化系統為W形和超晶格對稱結構後發現，此系統能有效改進點子躍遷速率，尤其為電子能態至重電洞能態。
In the thesis, we have performed a theoretical investigation of the electronic structures, and optical and spin-dependent transport properties of broken-gap quantum wells. The calculations are based on the 8×8 k．p Hamiltonian and the scattering matrix method, with strain effect taken into account. We found that the electron states and light hole states can be strongly mixing with each other even at zone center while the heavy hole states are decoupled from them. By varying the thicknesses and the stress of the layers, we also found that a phase transition of the system can occur from the semiconducting phase to the semimetallic phase. The active layers for semiconductor lasers emitting in mid IR range were designed using the broken-gap quantum wells. For flexibility in the design and efficient optical transition between electron and light hole states, we use the ternary compounds that make the expitaxial layers tensile-strained. However, it is difficult to pull up the first light hole band above the first valence state by means of the tensile strain for broken gap structures, unlike the case in type-I quantum wells. By comparing the momentum matrix elements of the structures with different stress, we found that the train effect cannot give any significant improvement in the transition efficiency. Fortunately, a significant improvement in transition rate can be achieved in structures consisting of W-shaped quantum wells. A structure of ultra-thin layers (superlattice) has also proposed and the calculated results showed that it can give momentum matrix elements much larger than those of the W-shaped structure. The broken-gap system is a good candidate for spintronics because of the strong spin-orbit interaction. We therefore paid some attention to the spin-dependent transport in the system. The transmission spectra through the broken-gap structures are calculated with the incident electron polarized in various directions and impinging at various angles. It was found that the spin orientation of obliquely incident electrons can be rotated arbitrarily in properly designed asymmetric structures. Our tunneling structure can therefore serve as spin filters and spin flip-flops.