Fundamental investigations on synthesis and characterization of silicon clathrate – a cage-like crystalline silicon allotrope
|Koh, Carolyn A. (Carolyn Ann)
|Collins, Reuben T.
|Includes bibliographical references.
|There is a critical need for developing the next generation electronic and photonic materials. As the second most abundant element in the Earth’s crust, Si has been playing an essential role in nearly all aspects of microelectronics. The development on novel Si-based materials with exceptional properties would revolutionize the electronics industry. Several exotic forms of crystalline Si have been stabilized at ambient temperature and pressure. The unusual caged, open channel, and layered structures found in these novel Si crystalline forms exhibit high potential for fundamentally exciting properties. In this thesis, Si clathrate, a cage-like crystalline Si allotrope, is investigated in detail regarding its defect structure and potentials in optoelectronic applications. Si clathrates have been synthesized in the presence of alkali guest atoms, such as Na, occupying the interstitial sites of the Si lattice. With the guest concentration low enough, Si clathrate can be viewed as a heavily doped semiconductor, and the alkali metal atoms can be viewed as a dopant. In contrast to diamond silicon (d-Si) which has an indirect band gap, theoretical studies have predicted the type II Si clathrate structure to have a direct or nearly direct band gap at about 2.0 eV. Efforts in this work were first focused on understanding alkali metals, like Na, as a dopant in type II Si clathrate structure. A detailed study of the Si defects and Na guest properties was performed through electron paramagnetic resonance (EPR) characterizations on Si clathrate powders with low Na content. An EPR signal associated with paramagnetic defects in the disordered Si phase was identified in the system for the first time. Moreover, along with four known EPR hyperfine lines arising from interactions of the electron with the isolated Na nucleus, the superhyperfine lines were identified, which were associated with the interaction of the donor electrons with 29Si nuclei on the cage. This gives new information on the radius of the Na valence electron wavefunction within the crystal, supporting the view of Na as a shallow donor. A comparison to phosphorus donors in d-Si highlights the potential of Na donors in clathrate as solid-state spin qubits. In addition, the EPR defect structures arising from Na donor pairs and clustered Na provide evidence that the Na distribution is inhomogeneous in clathrate cages. These results give insights into viewing Si clathrate with low Na concentration as an attractive, alternative n-type crystalline Si-based semiconductor. Although Si clathrate can be viewed as a semiconductor, its optical properties have been difficult to verify, especially in the thin film form. Findings in this thesis showed that prior optical measurements and interpretations of results were significantly complicated by the presence of multiple phases in clathrate films. A ubiquitous, amorphous, or disordered Si phase was found mostly on the film surface and grain boundaries that can dominate reported optical properties. For the first time, approaches were demonstrated to isolate the desired type II clathrate phase with sufficiently low Na content in film form, so that the material is a heavily doped semiconductor. With the impurity defect phases removed, the optical properties of the intrinsic clathrate material indicate a much more optically efficient alternative crystalline Si than d-Si that can significantly impact Si-based applications from solar cells to light-emitting diodes (LEDs). In addition to Na, other guest atoms in Si clathrates have been attractive, each of which can be viewed as a dopant at low guest concentration, creating different interstitial defect states in clathrate cages alternative to d-Si, potentially with exciting optoelectronic and spin properties. In this thesis, the thermal diffusion of Li into the guest sites of a nearly empty Si clathrate framework was explored as an approach to forming type II Si-Li clathrate. Li diffusing into the clathrate structure was detected and characterized as a function of diffusion temperature and time. Li atoms were found to be donors in the clathrate lattice. Electrons donated by Li can affect the Si anti-bonding state, causing a frequency reduction of the Si vibrational modes. Moreover, the clathrate structure exhibited reduced structural stability in the presence of Li, converting to polycrystalline or amorphous phases for anneals above 375 oC. EPR studies demonstrated that Li guests in clathrates, instead of simple analogs of Na, were forming pairs with other Li or Na atoms in the cages. Diffusion of Li resulted in a large increase of the free carriers. Techniques developed in this work demonstrated a new method for filling the clathrate cages, and showed the possibility of studying other alternative guests in clathrates. Results of this work also provide insights into using Si-Li clathrate as alternative anode materials for Li-ion batteries. With the unusual, open-cage structure, Si clathrates can help address or mitigate some key issues arising from d-Si applications in energy harvesting and storage. The structural and electronic properties of Si clathrates, along with the potentials of various interstitial impurities/dopants, have not been fully explored. Further developments on Si clathrates and other novel Si allotropes can lead to significant optoelectronic and spin applications.
|Colorado School of Mines. Arthur Lakes Library
|2022 - Mines Theses & Dissertations
|Copyright of the original work is retained by the author.
|electron paramagnetic resonance
|Fundamental investigations on synthesis and characterization of silicon clathrate – a cage-like crystalline silicon allotrope
|Taylor, Philip Craig, 1942-
|Wu, David T.
|Zerpa, Luis E.
|Doctor of Philosophy (Ph.D.)
|Chemical and Biological Engineering
|Colorado School of Mines
|Embargo Expires: 04/22/2024