| Summary: | 博士 === 國立中央大學 === 電機工程學系 === 103 === This thesis investigates a cooperative mechanism that involves the participation and interaction of Si, Ge and oxygen interstitials for precise numbering and placing Ge quantum dots (QDs). First, using the method, selective oxidation of Si atoms then formation of SiO2 and separation of Ge atoms during high-temperature oxidation of SiGe layer, we are able to fabricate Ge QDs embedded SiO2. During high-temperature oxidation or annealing, we further observe the interaction of Si, Ge, and oxygen interstitials to facilitate the aggregation, growth and migration and even changing crystal morphologies of the Ge nanocrystallites. There are three key Factors： Whether the presence of the Si-containing layer (or source of Si) adjacent to Ge nanocrystallites, whether the presence of sufficient oxygen interstitials, and the local oxidation of Si-containing layer adjacent to Ge nanocrystallites by the Ge acting as a catalyst. We observe the interaction of competition, destruction, and reconstruction between Si, Ge, and oxygen interstitials at the surface of Ge nanocrystallites by the designed experiments. If we take advantage of the interaction, we would be able to precisely control the number, placement and crystal mophology of Ge nanocrystallites, furthur control the size and crystal morphology, ane even lead Ge nanocrystallites migratation to specified place by controlling the place of Si-conaiting layer (or source of Si interstitials).
The interation of Si, Ge and oxygen interstitials is described briefly below：if the Si-containing layers (or the source of Si) are present near as-formed Ge nanocrystallites by oxidizing SiGe, the Si-containing layers would be decomposited and release Si interstitials by Ge acting as a catalys under thermal annealing in an oxidizing ambient. Because of their affinity for Ge, the Si interstitials first migrate to the surface of the Ge nanocrystallites where they facilitate the creation of voids in front of Ge nanocrystallites via the following reaction: Si(interstitial) + SiO2(s) 2SiO(g). The voids thus created, facilitate the growth and migration of the Ge nanocrystallites. Furthermore, SiO migrate behind the QD and react with O interstitials to re-form the SiO2. The volume of formation SiO2(g) is 2.2 times the original volume of Si interstitials. The stress of volum expansion is released by pushing Ge nanocrystallites into forward void. Thus, when the both sufficient Si and oxygen interstitials are supplied, Ge nanocrystallites would be able to continually migrate towards the source of Si interstitials by unceasingly destruction and construction.
In particular, Si interstitials is released by Ge-catalyzed local oxidation and decomposition of the Si-containing layer (Si3N4 layer or Si layer). Thus, except Ge nanocrystallites and Si-containing layer, sufficient O2 which is supplied at the Ge nanocrystallite/Si3N4 layer interface is another key factor for facilitating local oxidation. The oxygen concentration is significantly reduced because of the Ge’s low affinity for oxygen and the low segregation coefficient of oxygen in Ge. Hence, the high-content or high-density of Ge nanocrystallites embedd SiO2 retard the diffusion of extern oxygen, and cause the insufficient concentration of oxygen at the Ge nanocrystallites/buffer Si3N4 layer interface, and result in Ge nanocrystallites can not catalyze the local decomposition and oxidation of Si-containing layer (Si3N4 layer or Si layer). Thus, there is neither releasing Si nor inducing the growth and migration of high-density Ge nanocrystallites.
For overcoming this issue, we design and insert a low Ge-content poly-SiGe layers between high G-content poly-SiGe layers and buffer Si3N4. Thus, this method not only induce a series reaction by the oxygen effectively diffusing to Ge nanocrystallites/Si3N4 interface, but also form large Ge nanocrystallites by oxidizing high-content Ge poly-SiGe for effective controlling the size of Ge nanocrystallites.
Based on the above mechanism, we oxidize the pre-filled SiGe in nanotrenches and nanocavities, and successfully form spherical Ge QD. Furthermore, we precisely control the position and number of Ge nanocrystallites by tailoring the sizes and geometries of pre-patterned nanotrenches or nanocavities. If the radius (r) of the incircle of nanocavity is more than 15 nm, Ge QDs are placed at the corner or side of nanocavities. If the radius (r) of the corresponding incircle of nanocavity is less than 15 nm, Ge atoms are led the cavity center and form a single Ge QD. Using this method, we further demonstrate quantum tunneling diode and observe clear Coulomb staircases and differential conductance oscillations at 77K
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