Studies of Self-assembled InAs Quantum Dots/Rings and their Infrared Photodetectors
|關鍵字:||量子點;量子環;紅外線偵測器;化合物半導體;次能階躍遷;光偵測器;quantum dot;quantum ring;infrared detector;compound semiconductor;intersubband transition;photodetector|
□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。
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.
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