Short-range Order in Group IV Alloys
Open Access DepositedGroup IV alloys have been long viewed as random solid solutions since 1991 when the concept of SiGeSn alloys was first proposed. However, a fundamental question remains as to how random these alloys truly are. This dissertation theoretically investigates this widely adopted assumption in group IV alloys for the first time. Through advanced atomistic modeling by density functional theory (DFT) and machine-learning potential (MLP), it is discovered that group IV alloys generally exhibit a non-random solid solution behavior through the presence of short-range order (SRO). Remarkably, our study shows SRO is not just a structural sophistication but fundamentally impacts key properties of group IV alloys for opto-electronic applications and enables a new control means to engineer semiconductor alloys. Motivated by our theoretical findings, a center-scale, multidisciplinary investigation has been carried out to experimentally verify the existence, relevance, and controllability of SRO in group IV alloys. In facilitating experimental characterization of SRO, we have developed a new approach that overcomes the insufficient spatial resolution of atom probe tomography to enable its quantification of SRO. Our investigation starts by examining binary group IV alloys (GeSn, SiSn, and GePb). Through DFT-based Monte-Carlo simulation, we find that binary group IV alloys exhibit strong SRO behaviors which are reflected by distinct structural signatures and demonstrated to play a crucial role in modulating the electronic and topological properties. In proceeding to Si-Ge-Sn ternary alloys, we further discover two distinct types of SROs, which carry different energies, distinct degrees of local ordering, and dissimilar electronic structures, can co-exist across a wide range of Si–Ge–Sn compositions. The coexistence of multiple SROs with different band gaps prompts us to propose an SRO-enabled, lattice-matched type I heterostructures which offer great advantages and potential for advanced optical applications due to its inherent lattice match and chemical homogeneity. The discovery of the coexistence of multiple SROs also naturally raises the question regarding the role of SRO distribution on material properties. To address this question, we demonstrate that the SRO distribution in order parameter space indeed carries key structural information that substan- tially contributes to the total energies and direct bandgaps of Si-Ge-Sn alloys. Furthermore, with the development of a highly accurate and efficient neuroevolution machine-learning potential, we also reveal a clear SRO distribution in real space that can potentially impact the optoelectronic properties. To enable an explicit experimental characterization of SRO, we develop a physics-informed, statistical approach, RECOVER (REtrieving Chemical Ordering Via Explicating Radial distribution function), that overcomes the limited resolution in APT to quantify SRO parameters in alloys. The application of RECOVER to measured APT data of GeSn enables a cross validation with extended X-ray absorption fine structure (EXAFS). In particular, both APT and EXAFS unambiguously demonstrate the existence of SRO reflected by first-nearest neighbor depletion and third-nearest neighbor enhancement in GeSn alloys, precisely as predicted by our early study in GeSn. The developed framework of RECOVER is also generally applicable to a wide range of complex alloys, e.g., high-entropy alloys.
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Xiaochen_Jin_draft_dissertation_-_Xiaochen_Jin.pdf | 2025-08-15 | Open Access |
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