自組式砷化銦量子點/環及其紅外線偵測器之研究

Abstract

本篇博士論文的目的是希望對自組式砷化銦量子點及量子環的磊晶及電子能態特性做一個通盤的瞭解，以便實現長波長量子結構紅外線偵測器並提升其元件特性。 我們研究如何利用分子束磊晶技術來控制砷化銦量子點的面密度及幾何形狀。藉由調變長晶條件及程序，我們已能成長大範圍之不同面密度的量子點，並且控制量子點的形狀變化以獲得其演變至量子環之各種不同形狀的量子結構。對於藉由部分覆蓋及熱退火技術所成長的量子環，我們也探討了造就其形變的機制。再者，透過模擬，我們發現量子環其環狀分佈的位能井迫使電子波函數遠離環心，以致於抬高了電子能階的分佈並縮小其能差。 我們也利用調變激發能量之螢光頻譜來研究量子點的能譜。分析量子點放射光譜隨激發能量改變的情形，我們區分出三個具有不同物理意義的特徵區域。隨激發能量由高至低，它們分別為：連續能態吸收區，離散電子態激發區，以及多聲子共振區。透過自組式量子點特有之沾濕層□wetting layer□與量子點耦合之準連續態，電子之釋能獲得幫助，上述豐富的能譜結果也得以解釋。 □n□畯攽椄膍s藉由調變量子點能障結構的方式來調變量子點紅外線偵測器之偵測波段及極化吸收特性。此研究的結果顯示，透過量子點能障結構的設計，我們能有效操控量子點偵測器之元件特性以符合不同的需求。由此觀念出發，我們設計了一種新的”侷限增強型量子點在井中(confinement-enhanced Dots-in-a-well, CE-DWELL)”之結構，有效加強了DWELL量子點本身在側向上較弱的能障侷限。利用這個新的設計，我們成功製作出高度正向吸收並具8-12微米波段之量子點紅外線偵測器。更重要的是，元件之量子效率及偵測度獲得大幅度的提升。 我們更進一步研究不同CE-DWELL結構之量子點紅外線偵測器。我們發現，用以增強侷限之砷化鋁鎵層的厚度及含鋁成分，對於偵測器元件不論是吸收特性或傳輸特性都有顯著的影響。調整適當之CE-DWELL元件參數，我們成功製作出操作溫度高於200K的8-12微米波段量子點紅外線偵測器。 最後，我們也研究了砷化銦量子環之紅外線偵測器。量子環在側向上較廣的波函數分佈與其較高卻較密的能階分佈造就了量子環偵測器不同於量子點偵測器的特色，例如：寬頻之光電流頻譜、隨溫度變化較為穩定之光響應度、較低之暗電流活化能。藉由一高能障砷化鋁鎵電流阻擋層，量子環本質上較低的載子活化能得以提升，量子環紅外線偵測器的操作溫度也因而提升。

The purpose of this dissertation is to get a comprehensive understanding of the epitaxy growth and electronic properties of the self-assembled InAs quantum dots (QDs) and quantum rings (QRs) to realize long-wavelength quantum structure infrared photodetectors with improved device characteristics. The manipulation of the sheet density and geometry of InAs QDs via fully in situ molecular beam epitaxy growth control were investigated. A wide range of dot densities and the control over the geometric change from QDs, through volcano-shaped structures, to QRs, were achieved. The mechanism of such QR growth by the partial-capping & annealing technique was studied. Moreover, from simulation, it is found the ring-like potential well depletes the wavefunctions out of the ring center and thereby raises the state energies and also narrows the energy separations. The energy spectra of QDs were investigated using the selective excitation photoluminescence technique. Three distinct regions in the emission spectra can be identified as associated with changes in the excitation energy. They can be categorized from high energy to low energy, as continuum absorption, electronic state excitation, and multi-phonon resonance. The special joint density of state tail of the QD that extends from the wetting layer bandedge facilitates carrier relaxation and explains these spectral results. InAs QDs with different confinement barrier schemes were used in quantum-dot infrared photodetectors (QDIPs) for the detection wavelength and polarization absorption tuning. The results show that one can effectively change the device characteristics of QDIPs to fit different application requirement by tailoring the QD confinement schemes. We design a new confinement-enhanced dots-in-a-well (CE-DWELL) structure to enhance the wavefunction confinement of QDs especially in the lateral direction. With this new design, QDIPs with 8-12 um detection and high normal-incident absorption are achieved. More importantly, the device quantum efficiency and detectivity are greatly improved. QDIPs with different CE-DWELL structures were further investigated. The thickness and Al content of the AlGaAs confinement enhancing layers are found of crucial influence on not only the absorption property but also the transport property of the device. With appropriate device parameters of CE-DWELL, we present 8-12 um QDIPs with operation temperatures higher than 200K. Finally, InAs quantum-ring infrared photodetectors (QRIPs) were also studied. The higher but closer electronic state energies as well as the extended wavefunctions of QRs in the lateral directions induce the specific features of QRIPs, such as wider photocurrent spectra, more stable responsivity with temperature change, and lower dark current activation energy. With an Al0.27Ga0.73As current blocking layer, the inherently smaller activation energy of carriers in QRs is effectively compensated and the operating temperature of QRIPs is improved.