Summary: | 博士 === 國立清華大學 === 電子工程研究所 === 100 === In this thesis, the critical thickness of GaSb QDs is determined to be ~ 2.5 ML by RHEED patterns and AFM measurements. The formation of GaSb QDs under different V/III ratios is investigated. The growth mode of the GaSb QDs would gradually change from IMF mode to the SK mode with decreasing V/III ratios. The different growth temperatures on the GaSb QDs are also investigated. The influence of Sb soaking times that optical property of QDs is improve when the increasing time of Sb soaking is investigated. The results suggest that long soaking time would not only protect GaSb QDs from As-Sb exchange during GaAs capping layer growth but also prevent defect formation in the GaSb/GaAs interfaces, which are advantageous for the fabrication and applications of optical device. The influence of background As on the morphologies of GaSb QDs is investigated. With increasing background As pressures, QD-to-QR transition is observed while similar QD/QR diameters are observed. The As atoms would actually act as a driller to drill down the QDs such that GaSb QRs would be observed with high As background pressures. The STM image of a single GaSb QR shown in this section has revealed that the rings are assembles of even smaller QDs instead of rings with smooth circle surfaces.
GaSb/GaAs QD LED with a single GaSb QD layer is investigated. Significant EL is observed for the device under forward biases, which suggests that pronounced dipole transitions occur at the GaSb/GaAs interfaces. With increasing forward biases, the observed EL peak blue shift confirms that the origin of luminescence is from the type-II GaSb/GaAs QD structures. The linear dependence of PL and EL peaks over the third root of the excitation densities has confirmed that the type-II GaSb/GaAs QDs should be responsible for the luminescence. In the temperature-varying EL measurements lower than 100 K, the device has exhibited a unique optical characteristic of increasing EL intensity and peak blue shift with increasing temperatures. To enhance the device performances, additional carrier confinement schemes are required in the future. The understanding of the operation mechanisms for the device is advantageous for the practical application of type-II LEDs. The 10 K EL spectrums of the device near the turn-on voltage have revealed a dominant luminescence transition from the optical recombination of holes in the LH to HH states with increasing voltages. The large energy separation between HH and LH states suggests that large strain accumulation is observed for the GaSb QDs.
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