Amorphous Selenium (a-Se) and its Compounds: Photo-induced Metastability and Application in a Novel Gamma Camera
Doctor of Philosophy
ChG amorphous selenium
Archetypal a-GexSe100-x glass compounds
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Despite the large number of successful commercial applications of chalcogenide glasses (ChGs) ranging from memory devices to photonics and medical imaging detectors, the understanding of a fundamental property – photo-induced structural metastability is not yet complete. The inherent trend of amorphous chalcogenides to convert to its crystalline counterpart, can on the one hand, be directly utilized in phase-change memories, while on the other hand can degrade glass properties for applications in sensing. Furthermore, because the structure of amorphous semiconductors is not fixed by thermodynamic equilibrium conditions, its transformation can be triggered by optical/X-ray excitation and influenced by thermal heating or applied electric field. Each case has its own peculiarities of transformation. Thus, to ensure effective application of a given ChG, a solid understanding of the causes for the structural transformation on the microscopic level has to be developed. This thesis is devoted to the study of photo-induced metastability of the most widely used ChG – amorphous selenium (a-Se), and archetypal a-GexSe100-x glass compounds relevant for applications in sensing. We focus on the research of defect creation and defect relaxation processes in these materials since it is hypothesized to be the root cause for structural transformations affecting stability. The detailed experimental and theoretical study is carried out to reveal the pathways from a micro-level, i.e. defect creation/relaxation, to the macro-level photo-induced effects formation, namely photodarkening (PD), photobleaching (PB) and photo-induced crystallization (PC) and their kinetics. The effects of temperature and energy of excitation on PD kinetics are studied in a- Se. Distinctly different PD effects are observed in the case of sub-bandgap and above- bandgap excitation. It is found that above-bandgap excitation causes only transient photodarkening with a temperature independent relaxation time and no significant structural rearrangements. In contrast, sub-bandgap excitation causes both transient and metastable PD effects. Whereas the mechanism responsible for transient PD for sub- bandgap excitation is analogous to that in the case of above-bandgap excitation, metastable PD is controlled by the formation of self-trapped excitons resulting in structural transformations into configuration defects. Subsequent relaxation in the latter case is shown to be a thermally activated process at elevated temperatures; or configuration tunneling below the room temperature. It is confirmed that if left unrestored, metastable PD acts as a precursor for photo-induced crystallization. Detailed investigation of the effects of different substrates and temperature on photo- induced crystallization in a-Se demonstrates that the onset of PC is suppressed by softening of the film-substrate interface and/or by operating at temperatures near the glass transition. Further, photodarkening and photobleaching effects are investigated in a-GexSe100-x as a function of composition across the glass forming region. The Ge:Se ratio is found to play a decisive role in the observed effects. The critical concentration of Ge ≈30% is highlighted to correspond to the crossover from transient PB to a mixture of transient PD and metastable PB. The underlying microscopic mechanism is governed by the availability of lone pair (LP) states at Se atoms. For low-Ge content films (x<20%) LPs are primarily involved in photoexcitation, causing a photodarkening effect (like in a-Se). As Ge concentration reaches 30%, Ge-Se bond breakage becomes prevalent under light exposure, which results in the generation of dangling bonds and the saturation of the available LP states. This is found to be responsible for the transient PB effect. With further increase of Ge %, the amount of homopolar Ge-Ge bonds starts to increase along with the obvious deficiency of lone pairs. This leads to Ge-Ge bond breakage upon excitation, which causes transient photodarkening. In the post-excitation period, however, it initiates transition to a more ordered state with the main feature of the formation of 3D nanostructures. This is reflected in a bandgap increase and an experimentally observed metastable photobleaching effect. Finally, our understanding of a-Se properties allowed us to extend its application in low-energy nuclear medicine devices. A novel fluence integrating gamma camera is proposed, which if commercialized, will be used to guide advanced breast cancer treatment that involves the placement of low-energy radioactive seeds. The feasibility of the proposed approach is confirmed in realistic breast phantom studies. Our results demonstrate the potential of the gamma camera to fulfill the clinical requirements of both spatial resolution and sensitivity.