披覆銻砷化鎵之砷化銦量子點的能帶結構工程及元件應用

Abstract

本論文旨在探討披覆銻砷化鎵之砷化銦量子點的能帶結構調變工程及其元件應用，主要內容包含兩個主題。在論文的第一個部分，我們利用四種方法去調變披覆銻砷化鎵之砷化銦量子點的能帶結構。首先，我們改變銻砷化鎵披覆層中銻的組成，並由時間解析光譜技術研究載子的動態行為，無論由變功率光激螢光光譜峰值的位移或由較長的時間衰減常數，都證實了在異質接面所形成的第二型能帶結構；不同的載子復合路徑也藉由變溫量測得以區分。第二種方法為熱退火處理。除了明顯的螢光峰值藍位移及光譜線寛縮減外，熱退火所引起的合金混合也造成輻射復合速率的增加及消除在銻砷化鎵披覆層的載子侷限態。同時，在變功率及時間解析螢光的量測中，我們也觀察到由第二型變成第一型的能帶結構變化。第三種方法為披覆層厚度的調變。我們發現發射光的能量和載子復合生命期都與披覆層厚度有密切的關係。理論計算也指出量子侷限和電洞波函數分佈對披覆層厚度的變化相當敏感。同時，銻原子造成的量子點尺寸變化在其光學特性上也扮演相當重要的角色。最後我們採用了銻砷化鋁鎵四元化合物披覆層。由變功率及時間解析螢光量測的結果，披覆銻砷化鎵之砷化銦量子點的第二型能帶結構能藉由加入鋁元素而轉變為第一型能帶結構，而這個能帶結構的轉變也同時從理論計算得到證實。 披覆銻砷化鎵之砷化銦量子點的元件應用為本論文第二部分的重點。首先，我們探討披覆砷化鎵層之砷化銦紅外線量子點光檢測器的光譜響應。相較於傳統的砷化銦紅外線量子點光檢測器，其呈現較窄的光譜線寛和較長的檢測波長，這些變化可歸因於銻砷化鎵披覆層造成之尺寸較大且較均勻的量子點。同時，我們也藉由光激螢光激發光譜得知其光譜響應的躍遷機制來源。另外一個應用為記憶體元件。為了簡化製程手續，我們採用了寛通道平面式柵極電晶體的技術來研究披覆銻砷化鎵之砷化銦量子點的記憶體效應。因此在探討記憶體元件前，我們先介紹俱砷化鎵片電阻之平面式柵極電晶體。在有效控制片電阻的摻雜濃度情況下，理想的電流調變特性和高集極電流之間可取得平衡。其調變開關特性的機制，是藉由操控表面自由電子分佈來空乏元件通道。此外，光電流的量測結果也顯示出該技術有應用於光檢測器的潛力。在對寛通道平面式柵極電晶體了解後，我們展示室溫下使用寛通道平面式柵極電晶體之砷化銦量子點充放電的能力。相較於第一型量子點，俱銻砷化鎵披覆層之第二型量子點呈現出較長的充放電時間。由披覆銻砷化鎵之砷化銦量子點提供之較慢的電荷弛豫和簡化的平面式柵極電晶體技術，揭示該元件俱有實際應用的潛力。

This dissertation is devoted to band structure engineering and device applications of GaAsSb-capped InAs Quantum Dots (QDs). In the first part of this dissertation, four approaches are utilized to tailor the band alignments of GaAsSb-capped InAs QDs. First, carrier dynamics of GaAsSb-capped InAs QDs with different Sb composition were investigated by time-resolved photoluminescence (TRPL). Both the power dependence of PL peak shift and the long decay time constants confirm the type-II band alignment at the heterointerface. Different recombination paths have been clarified by temperature dependent measurements. Second, we study the effects of thermal annealing on the emission properties of type-II GaAsSb-capped InAs QDs. Apart from large blueshifts and a pronounced narrowing of the QD emission peak, the annealing induced alloy intermixing also leads to enhanced radiative recombination rates and reduced localized states in the GaAsSb capping layer (CL). Evidences of the evolution from type-II to type-I band alignments are obtained from time-resolved and power-dependent PL measurements. The third approach is the modulation of the CL thickness. Both the emission energy and the recombination lifetime are found to be correlated with the CL thicknesses. Theoretical calculations indicate that the quantum confinement and the wave function distribution of hole states are sensitive to the GaAsSb CL thickness. The Sb induced change in QD size also plays a role in the optical properties of GaAsSb-capped QDs. The last approach is using the quaternary AlGaAsSb CL. As evidenced from power-dependent and time-resolved PL measurements, the GaAsSb-capped QDs with type-II band alignment can be changed to type-I by adding Al into the GaAsSb CL. The evolution of band alignment with the Al content in the AlGaAsSb CL has also been confirmed by theoretical calculations. The PL thermal stability and the room temperature PL efficiency are also improved by AlGaAsSb capping. Device applications of GaAsSb-capped InAs QDs are demonstrated in the second part of this dissertation. First, spectral responses of GaAsSb-capped InAs/GaAs quantum-dot infrared photodetectors (QDIPs) with different Sb composition are investigated. Compared with the conventional InAs/GaAs QDIPs with wide detection windows, a much narrower spectral width is observed for GaAsSb-capped QDIPs at longer detection wavelengths. The phenomenon is attributed to the larger InAs QDs with improved uniformity resulted from the GaAsSb CL. In this case, lowered energy states and reduced energy difference in-between would result in longer detection wavelengths with narrower spectral widths of GaAsSb-capped QDIPs. By comparing with photoluminescence excitation spectra of the samples, the dominant transition mechanisms for GaAsSb-capped QDIPs are also investigated. The other application is memory device. To further simplify the fabrication procedure, the architecture of wide-channel in-plane gate transistors (IPGTs) is adopted to demonstrate the memory effect of GaAsSb-capped InAs QDs. Hence, we studied IPGTs with an n-GaAs sheet resistance prior to the investigation of memory device. A trade-off between effective current modulation and high saturation drain current is obtained by optimizing the doping density of the sheet resistance. The mechanism responsible for the transistor behaviors of the devices is due to the channel electron depletion related to the population of mobile surface electrons under different gate biases. The photocurrent measurements demonstrate that the IPGT architecture is a feasible approach for the applications of photodetectors. After having the idea of the wide-channel IPGT, we demonstrate room-temperature electron charging/discharging phenomena of InAs QDs using wide-channel IPGTs. The device based on type-II GaAsSb-capped InAs QDs exhibits both the longer charging and discharging times than those of the type-I counterpart with GaAs CL. The slow charge relaxation of GaAsSb-capped InAs QDs and simple architecture of IPGT reveal the potential of this device architecture for practical memory applications.