THESIS
2003
xiv, 62 leaves : ill. ; 30 cm
Abstract
Even though self-organized semiconductor quantum dots have had a history close to a decade, research on self-organized magnetic quantum dots (QDs) has only been initiated for about a year. The quantum size effects of these magnetic nano-size materials may offer novel physical properties and also may open a new area in spintronics applications. This thesis will present the results of a research initiated in our group on a novel MBE-grown Fe quantum- dot system. The growth of Fe single crystalline thin films by MBE in a growth system which has been heavily used for II-VI growth is not a trivial task since Fe strongly reacts with selenium, and the Fe source is required to heat up to at least 1300℃ to yield a usable growth rate. In fact, in the first few trials of Fe thin film growth using...[
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Even though self-organized semiconductor quantum dots have had a history close to a decade, research on self-organized magnetic quantum dots (QDs) has only been initiated for about a year. The quantum size effects of these magnetic nano-size materials may offer novel physical properties and also may open a new area in spintronics applications. This thesis will present the results of a research initiated in our group on a novel MBE-grown Fe quantum- dot system. The growth of Fe single crystalline thin films by MBE in a growth system which has been heavily used for II-VI growth is not a trivial task since Fe strongly reacts with selenium, and the Fe source is required to heat up to at least 1300℃ to yield a usable growth rate. In fact, in the first few trials of Fe thin film growth using a used effusion cell to store Fe source, we confirmed by both the HRXRD and XPS techniques that the Fe films were seriously contaminated by Se source. After the successful demonstration of pure Fe growth using a new effusion cell together with a prolong high temperature out-gassing of the neighborhood of the cell, we have initiated the fabrication and study of a novel self-organized magnetic quantum dot system. Due to a suitably large lattice-mismatch (about 5 %) between Fe (twice of its lattice constant) and ZnS, one should expect the formation of self-assembled Fe quantum dots if a nanoscale Fe film is deposited on a smoothed ZnS surface. We have carried out the growth of this novel magnetic nanostructure and studied its microstructure as a function of Fe coverage of 0.9 (QD1), 1.6 (QD2) and 3.4 (QD3) ML. These novel Fe quantum-dot systems were grown at 180°C on ZnS/GaP(100) substrates by MBE. Conventional and high-resolution transmission electron microscopy (HRTEM) imaging (both plan-view and cross-section) and electron micro-diffraction techniques have revealed their microstructure. Due to its low Fe coverage, QD1 does not show any Fe finger prints in its electron diffraction, however, its HRTEM plan-view images reveal that Fe QDs do exist in this sample with average diameter around 5 nm and a dot-spacing also around 5 nm. These dots are single crystalline with round bases however they are misaligned with each other (with different azimuthal rotation angles). The cross-section images of QD1 indicate that these QDs are of conical shapes. The resulting electron diffraction patterns of QD2 clearly show Fe characteristic spots embedded in the standard diffraction pattern of the ZnS buffer, indicating the Fe QDs are also single crystalline however they seem to be highly aligned with each other. The HRTEM images of QD2 indicate that the size of these dots is much larger with average diameter around 25 nm and a dot-spacing also around 25 nm. Their shapes are not round and seem to be resulted from coalescence of multi-dots. With the largest Fe coverage among the three samples, QD3 shows cloudy Fe islands with irregular shapes in its plane-view images. Its electron diffraction displays incomplete Fe ring structures indicating the existence of some preferential orientations.
On the other hand, based on the recent successful growth of straight-up ZnSe and ZnS nano-wires, further studies about Fe QDs embedded in the middle of these one dimensional quantum wires are in progress. Recent results of SIMS show that Fe is easier to diffuse into ZnSe than ZnS, thereafter, we carried out single Fe QD growth in the middle of ZnS nano-wires with a structure of Au/ZnS/Fe QD/ZnS/GaAs(n+). TEM studies have already been carried out for the first trial sample. As the growth condition was not controlled well in this sample, the wires are a little bit zip-zap in shape rather than straight-up.
As magnetic properties are hard to be investigated for a single layer of Fe QDs due to its low coverage, multi-layer Fe-QD structure would be a good candidate for us to explore their magnetic properties, this further investigation is underway in our laboratory.
To achieve useful applications from this research in the futufe, the problems about the non-uniform size distributions and largely uncorrelated spatial distributions of these novel Fe quantum dots should be solved first, and more effort should also be devoted to achieve dislocation-free QDs.
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