Since the discovery of ultraviolet (UV) light, various UV light sources have been developed. Thanks to this development, the applications of UV light have been increasingly expanded. Deep UV (DUV) optical devices are expected to realize various applications, such as air and water purification/sterilization, biomedical research, and semiconductor processes. The current DUV light sources are mainly excimer gas lasers/lamps and mercury vapor lamps. However, these DUV light sources have a lot of

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... advantages such as short lifetime, high cost and use of harmful materials. To overcome these problems, aluminum gallium nitride (AlGaN) based solidstate optical devices are highly expected. Al x Ga 1−x N has a direct bandgap that can be tuned from 3.4 eV to 6.0 eV covering the DUV region by changing the Al composition x. Recently, their crystal qualities have been drastically improved, and consequently the internal quantum efficiencies (IQEs) have reached ∼ 70% at emission wavelengths between 240 nm and 280 nm, including our group. However, despite the high IQEs, the external quantum efficiencies (EQEs) of AlGaN-based light emitting diodes (LEDs) are less than ∼ 10% due to the main problems associated with p-AlGaN. To obtain AlGaN quantum wells (QWs) with higher quality and DUV optical devices with higher emission efficiencies, it is important to understand the optical properties in Al-rich AlGaN QWs. However, their optical properties have not been investigated adequately. This thesis addresses the problems described above. In this thesis, we performed various optical spectroscopy measurements to understand the emission mechanisms in Al-rich AlGaN/AlN QWs. First, we measured photoluminescence (PL) using a Xe * 2 excimer lamp and assessed the temperature dependence of PL in order to investigate the fundamental emission mechanisms under weak excitation condition. As a result, we observed the emission properties derived from the localized excitons based on two types of localization states. In addition, temperature dependent cathodoluminescnece (CL) mapping measurements were performed in order to observe the exciton dynamics directly in Al-rich AlGaN/AlN QWs. From these analyses, we identify that the experimentally observed two types of localization states, that is, shallow and deep localization states originate from the statistically unavoidable alloy disorder effect and 2 ML (±1 ML) fluctuation of the well width, respectively. Moreover, many excitons migrated to deep localization state derived from 2 ML fluctuation and emitted brightly in those regions for narrow QWs. Then, based on the results of the optical properties under weak excitation condition, the optical properties under intermediate and high excitation conditions were discussed. It was found that the excitation power dependence of PL properties could be explained by the abovementioned potential model. Therefore, it is considered that the proposed potential model is ii reasonable. The PL peak shift and its well-width dependence could be reproduced by considering the bandgap renormalization (BGR), the Burstein-Moss (B-M) shift, and the screening effect of the internal electric field. Among them, the major factor for the difference in the PL peak shift due to the well width was found to be the screening effect. Then, we performed TRPL measurements for Al 0.79 Ga 0.21 N/AlN QWs under the selective excitation condition in order to investigate carrier recombination dynamics around the Mott transition. The fast and the slow lifetime components were observed under highly-excited conditions and were attributed to radiative recombination lifetimes of the electron-hole plasma (EHP) and the exciton many-body (EMB) effect, respectively. Additionally, it is also important to obtain the guideline of the optimum QW structures and the design of DUV optical devices with high emission efficiencies. Toward the application of DUV optical devices in particular DUV solid-state lasers, we discussed the optical gain characteristics evaluated by the variable stripe length (VSL) method in Al-rich AlGaN/AlN QWs at RT. Edge PL spectra were much narrower than surface spectrum and their emission intensities were increased exponentially as excitation length was increased. In the result of the well width dependence of VSL measurements in Al 0.79 Ga 0.21 N/AlN MQWs, the largest optical gain of 140 cm −1 was obtained for the QW with L w = 5 nm. Furthermore, comparing the conditions between with and without gain saturation, it was found that the redshift was caused by the gain saturation. The dominant polarization was changed from transverse electric-field (TE) mode to transverse magnetic-field (TM) mode, as Al composition was increased. This result was derived from the turnover of valence band ordering of AlGaN/AlN QWs. Next, to overcome the problems with p-type AlGaN, we propose the DUV optical devices based on AlGaN QWs by electron beam (EB) pumping. Using AlGaN/AlN QWs as a phosphor, high output power and power efficiency are promising, because a QW structure has higher radiative recombination probabilities, lower re-absorption loss than a bulky film, and higher light extract efficiency due to the quantum confinement. We used an EB pumping technique, demonstrating an output of 100 mW from Al-rich AlGaN/AlN QWs emitting at ∼ 240 nm. This achievement is attributed to the carrier confinement within the high-quality quantum wells, as well as the appropriate design of sample structures for EB pumping. Finally, these presented results contribute to the development of DUV optical devices and suggest that EB pumping method overcomes the low EQEs of state-of-the-art DUV LEDs. It should be emphasized that the EB operating conditions in this study are accessible using portable field-emission devices. We therefore believe that EB pumping of QWs is effective for generating UV light, and the present developments form a significant step toward a nextgeneration, compact, high-efficiency DUV light sources. iii