砷化鎵/砷化鋁異質結構之Γ -Χ 反交叉能隙理論分析

Abstract

本篇論文提出一個新穎的想法來產生微分負阻的機制，並且可用來實現兆赫波震盪器。這個震盪器利用砷化鎵/鋁化鎵異質結構本身的特性—電子可以從 點遷移到X點而不用有聲子的參與。和傳統的Gunn 震盪器比較，此震盪器的震盪頻率不受限於電子—聲子的散射機率。因此，微分負阻的機制主要是由Γ–X耦合的機制來控制，此時反交叉能隙是決定Γ–X耦合強度的一個重要參數。 在本篇研究中所採用的結構是數個量子井和能障長在(001)方向，在此結構中水平方向的對稱性仍然和塊材的對稱性相同。為了考慮Γ–X耦合，必須考慮多能階的特性，此時一般常用的有效質量方程式並不適用，我們採用膺勢法來計算複數能帶圖。 模擬結果顯現出主要是由結構中單位面積的界面數目來決定反交叉能隙的大小，而且即使在很簡單的兩個能障的異質結構中，也可以有很大的反交叉能隙。

This thesis proposes a novel idea to realize the terahertz oscillator, which is based on the mechanism of negative differential resistance. This oscillator utilizes the inherent properties of GaAs/AlAs heterostructures— the possibility of phononless transfer of electrons from Γ–valley to X –valley. Compared to the conventional Gunn oscillator, the frequency of the oscillator could not be limited by electron-phonon scattering rate. Therefore, the effect of negative differential resistance is dominated by the mechanism of Γ–X mixing, where the anticrossing gap is an important parameter to determine the strength of Γ–X mixing effect. The structure of the system in this study consists of several barriers and wells grown on the (001) plane, in which the translational symmetry in the parallel direction is preserved as the bulk. The empirical pseudopotential complex-band structure method is used in order to consider multi-state properties for Γ–X mixing, which cannot be treated by the usual effective-mass theory. The simulation results show that the interface number per unit length dominates the effect of anticrossing gap. Even in a simple GaAs/AlAs double-barrier heterostructure, the anticrossing gap still can be large.