New forms of silicon with enhanced optoelectronic properties

Carnegie Institution Of Washington, Washington DC

Investigators

Abstract

Nontechnical description: Unlike metals that freely conduct electricity, semiconductors require energy input to promote electrons into conducting states. The minimum input energy, known as the band gap, is either direct or indirect depending on the specific crystalline structure. Silicon, which is essential for the vast majority of modern electronics and solar energy devices, possesses an indirect band gap meaning that light absorption and emission are feeble processes. In contrast, direct band gap materials are effective absorbers and emitters of light. Recent theoretical calculations indicate energetic feasibility for numerous forms of crystalline silicon with direct band gaps, but experimental synthesis pathways are not available. This research team aims to create entirely new forms of crystalline silicon that possess direct band gaps in order to improve optoelectronic properties that impact a range of technologies including solid-state detectors, optical communication and energy conversion devices. Research is focused on novel synthetic pathways that combine high- and low-pressure experiments in order to achieve access to kinetically-stabilized states. More generally, the development of novel synthesis methodologies broadly affects metastable materials beyond silicon. This research project occurs within an educational environment that emphasizes the career development of students and postdoctoral scholars. Technical description: This project utilizes a new approach for metastable materials synthesis through a combined high-pressure / ambient-pressure hybrid method. Numerous metastable silicon allotropes with enhanced optoelectronic properties are predicted to exist within 30 kJ/mol of the ground state, but effective synthesis pathways are lacking. By initiating ambient-pressure chemical synthesis from precursors formed under high-pressure conditions, entirely new synthesis pathways are possible due to the high-energy state of the recovered precursor that is metastable at ambient conditions. This effort explores the depth of realizable materials in silicon and probes the relationships between metastable allotropes and optoelectronic properties in order to create entirely new metastable crystalline allotropes that possess direct or quasidirect band gaps. The research contributes to the longstanding limitations associated with the indirect band gap of the cubic diamond structure. Experiments are conducted at high-pressure conditions in order to optimize single-crystalline growth and at low-pressure conditions to optimize precursor transformations. The intrinsic optical and electronic transport properties of novel silicon allotropes are established experimentally, and the library of potential high-pressure precursor materials is supported by calculations including density functional theory-based structure searching. The overall goal of the project is to produce and characterize new silicon phases exhibiting improved optical activity. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.