| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 96 === Abstract
This thesis presents the optical characteristics of the Si nanodots together with of Al2O3 and ZnO thin films deposited by atomic layer deposition (ALD). This thesis is divided into five topics. The first topic investigated the carrier traps in Si nanodots. The Si nanodots with different sizes grown on n-type and p-type silicon substrates present various temperature-dependent photoluminescence (PL) characteristics. The hole traps and electron-hole liquid were two carrier competition mechanisms at low temperature, resulting in the significant change of PL intensity with the nanodot size and substrate type. The dominant hole traps in the Si nanodots capture large amount of minority carriers, and thus reduces the PL intensity of the n-type Si substrate at low temperature. As a result, the PL intensity at low temperature decreases dramatically with the size of the Si nanodots. The hole emission from Si nanodots into the n-type Si substrate leads to the significant increase in the PL intensity with temperature at the temperature greater than 40K.
The second topic is the surface passivation of Al2O3 thin films deposited by ALD on Si nanodots. The Al2O3 thin films deposited by ALD on Si nanodots causes the enhancement of PL intensity after the Buffer-Oxide-Etch (BOE) treatment, indicating the excellent surface passivation effect to suppress the nonradiative recombination. The BOE treatment results in the formation of surface defects and remove of a great amount of hole traps in the Si nanodots. For the thermal SiO2/Si nanodot/p-Si samples, the PL vs. temperature characteristics are sensitive to surface defects rather than the hole traps at low temperature. As for the thermal SiO2/Si nanodot/n-Si samples, the PL vs. temperature characteristics are very sensitive to the decrease in concentration of hole traps at low temperature.
The third topic is the efficient n- ZnO/SiO2-Si nanodots-SiO2/p-Si heterojunction photodetector. The n-type ZnO:Al layer acts as the transparent conductive oxide (TCO) and anti-reflection coating (ARC) layer to increase the quantum efficiency of the photodetectors. For samples with the smaller Si nanodots, the hot electrons tunneling through the thicker oxide into ZnO may cause impact ionization in ZnO, resulting in the quantum efficiency of the photodetector greater than 100%. Efficient n-ZnO/SiO2-Si nanodots-SiO2/p-Si heterojunction photodetector with quantum efficiency up to 152% was achieved.
The fourth topic is the efficient n-ZnO/SiO2-Si nanodots-SiO2/p-Si heterojunction light-emitting diodes (LEDs). The n-type ZnO:Al layer acts as the TCO, ARC, and electron injection layer to increase the external quantum efficiency of the Si LEDs. Because of the carrier confinement and surface passivation effects, the Si nanodots surrounded by SiO2 contributes to the increase in the light-emitting efficiency of Si LEDs. Efficient n-ZnO/SiO2-Si nanodots-SiO2/p-Si heterojunction LED with external quantum efficiency up to 3.32×10-4 was achieved at room temperature.
The fifth topic investigated the optical gain in the Si nanodots. Super-linear increase in the PL intensity and the spectral narrowing with the excitation length was observed at room temperature using the variable stripe length (VSL) measurement, which may be attributed to the amplified spontaneous emission in the Si nanodots. The ZnO:Al layer in the n-ZnO/SiO2-Si nanodots-SiO2/p-Si structure enhances the optical gain by decreasing the reflectance of pumping light. Optical gain at the wavelength corresponding to the Si bandgap energy was achieved in the Si nanodots embedded in the SiO2 matrix at room temperature. The optical gain results from the enhancement in spontaneous emission rate caused by carrier localization in Si nanodots as well as the population inversion at high excitation intensity.
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