Aaron D. Martinez

Synthesis and optical band gaps of alloyed Si–Ge type II clathrates

Inorganic type II clathrates are low density, semiconducting allotropes of group IV elements with the potential for optoelectronic applications. This class of materials is predicted to have direct or nearly-direct band gaps, and, when Si and Ge are alloyed in the clathrate structure, the band gap is tunable in the range of 0.8–1.8 eV. In this work, we demonstrate for the first time the synthesis of alloyed Si–Ge type II clathrates. Within this alloy system, we find an amorphous region which is likely due to a miscibility gap. The optical absorptance spectra of the crystalline clathrate samples show the predicted band gap tuning with Ge content, and calculations find that the Si type II clathrate has a strong absorption coefficient for the direct interband transition. The findings in this work lay the foundation for the future use of type II clathrates in optoelectronic applications.

Synthesis of Group IV Clathrates for Photovoltaics

Although Si dominates the photovoltaics market, only two forms of Si have been thoroughly considered: amorphous Si and Si in the diamond structure ( d-Si). Silicon can also form in other allotropes, including clathrate structures. Silicon clathrates are inclusion compounds, which consist of an Si framework surrounding templating guest atoms (e.g., Na). After formation of the type II Na 24Si136 clathrate, the guest atoms can be removed (Si136), and the material transitions from degenerate to semiconducting behavior with a 1.9 eV direct band gap. This band gap is tunable in the range of 1.9-0.6 eV by alloying the host framework with Ge, enabling a variety of photovoltaic applications that include thin-film single-junction devices, Si136 top cells on d-Si for all-Si tandem cells, and multijunction cells with varying Si/Ge ratios. In this study, we present electronic structure calculations that show the evolution of the direct transition as a function of Si/Ge ratio across the alloy range. We demonstrate the synthesis of type II Si/Ge clathrates spanning the whole alloy range. We also demonstrate a technique for forming Si clathrate films on d-Si wafers and sapphire substrates.

Efficient route to phase selective synthesis of type II silicon clathrates with low sodium occupancy

Phase selective synthesis of type II silicon clathrates from thermal decomposition of NaSi has previously been limited to small quantities due to the simultaneous formation of competing phases. In this work we show that the local sodium vapor pressure during the NaSi precursor decomposition is a critical parameter for controlling phase selection. We demonstrate synthesis techniques that allow us to tune the local Na vapor pressure, yielding type I or II clathrate products that are ≥90 wt.% phase pure. The “cold plate” reactor design discussed in this work maintains low Na vapor pressure during thermal decomposition of NaSi, thus yielding large scale, phase selective synthesis of type II silicon clathrates. The low Na vapor pressure maintained in this reactor is also shown to efficiently produce low Na (x ~ 1; NaxSi136) Si clathrate through Na sublimation. To further reduce sodium occupancy (x < 1), we demonstrate etching of NaxSi136 in HF/HNO3 solutions, which rapidly yields a clathrate product with reduced x. 29Si NMR and electron spin resonance (ESR) characterization validate the low Na occupancy of Si clathrate synthesized. The acid etch also selectively dissolves the type I silicon clathrate impurity phase, thereby enabling the synthesis of large quantities of phase pure type II silicon clathrate with low Na content.

Self-Organization of Thin Polymer Films Guided by Electrostatic Charges on the Substrate

The self-organization of thin polymer films into functional patterns is important both scientifically and technologically. Electric fields have been exploited as an efficient and powerful means to induce the destabilization and self-organization of soft materials. Previous attention, however, has mainly focused on externally applied electric fields. It is shown herein that the internal electric field is strong enough to guide the self-organization of thin polymer films as well. Patterns of electrostatic charges with micrometer resolution are first introduced on a dielectric substrate. A thin polymer film is then spin-coated onto the topographically flat substrate. Upon thermal annealing, the thin polymer film destabilizes due to a lateral gradient of electrostatic stress and flows away from the electroneutral regime to the charged area, resembling the patterns of charges on the substrate. Theoretical and numerical modeling based on the electrohydrodynamic instability shows excellent agreement with experimental observations both qualitatively and quantitatively. It is also demonstrated that the interplay between charge-driven instability with spinodal dewetting and Rayleigh instabilities can generate finer and hierarchical polymeric patterns that are completely different from the charge patterns preintroduced on the substrate. This study provides direct evidence that the internal electric field caused by charges on the substrate is strong enough to destabilize thin polymeric films and generate patterns. This study also demonstrates new strategies for bottom-up fabrication of structured functional materials.

Synthesis, structure, and optoelectronic properties of II–IV–V2 materials

II–IV–V2 materials offer the promise of enhanced functionality in optoelectronic devices due to their rich ternary chemistry. In this review, we consider the potential for new optoelectronic devices based on nitride, phosphide, and arsenide II–IV–V2 materials. As ternary analogs to the III–V materials, these compounds share many of the attractive features that have made the III–Vs the basis of modern optoelectronic devices (e.g. high mobility, strong optical absorption). Control of cation order parameter in the II–IV–V2 materials can produce significant changes in optoelectronic properties at fixed chemical composition, including decoupling band gap from lattice parameter. Recent progress has begun to resolve outstanding questions concerning the structure, dopability, and optical properties of the II–IV–V2 materials. Remaining research challenges include growth optimization and integration into heterostructures and devices.

Development of ZnSiP

A major technological challenge in photovoltaics is the implementation of a lattice matched optically efficient material to be used in conjunction with silicon for tandem photovoltaics. Detailed balance calculations predict an increase in efficiency of up to 12 percentage points for a tandem cell compared with single junction silicon. Given that the III-V materials currently hold world record efficiencies, both for single and multijunction cells, it would be transformative to develop a material that has similar properties to the III-Vs which is also lattice matched to silicon. The II-IV-V2 chalcopyrites are a promising class of materials that could satisfy these criteria. ZnSiP2 in particular is known to have a bandgap of ~2 eV, a lattice mismatch with silicon of 0.5%, and is earth abundant. Its direct bandgap is symmetry-forbidden. We have grown single crystals of ZnSiP2 by a flux growth technique. Structure and phase purity have been confirmed by X-ray diffraction and transmission electron microscopy. Optical measurements, along with a calculation of the absorption spectrum, confirm the ~2 eV bandgap. Because of its structural similarity to both crystalline silicon and the III-Vs, ZnSiP2 is expected to have good optoelectronic performance.

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Group IV clathrates are a unique class of guest/framework type compounds that are considered potential candidates for a wide range of applications (superconductors to semiconductors). To date, most of the research on group IV clathrates has focused heavily on thermoelectric applications. Recently, these materials have attracted attention as a result of their direct, wide band gaps for possible use in photovoltaic applications. Additionally, framework alloying has been shown to result in tunable band gaps. In this review, we discuss the current work and future opportunities concerning the synthesis and optical characterization of group IV clathrates for optoelectronics applications.

for Si-Based Tandem Solar Cells

ZnSiP2 is a potential optoelectronic material with possible application in lasers, LEDs, photonic integrated circuits, and photovoltaics. The development of ZnSiP2 as a photovoltaic material could address the current technological challenge of implementing a monolithic top cell on silicon for tandem photovoltaics. In this work we present a detailed description of the growth of ZnSiP2 single crystals, which has enabled thorough optoelectronic characterization. A flux growth technique was used, under various conditions, to grow ZnSiP2 single crystals in Zn solution. The results of these growth experiments, along with analysis of previously determined phase diagrams, show that three secondary phases form as a result of the Zn flux growth technique: Zn3P2, Si, and the remaining Zn flux. Potential reasons for the formation of these particular phases are discussed, but their presence is found to be non-detrimental, and they can easily be removed. The resulting single crystals are high purity and enable the characterization of the fundamental optoelectronic properties of ZnSiP2.

Group IV clathrates: synthesis, optoelectonic properties, and photovoltaic applications

ZnSiP2 is a wide band gap material that is lattice matched with Si, offering the potential for Si-based optoelectronic materials and devices, including multijunction photovoltaics. We present a carbon-free chemical vapor deposition process for the growth of both epitaxial and amorphous thin films of ZnSiP2–Si alloys with tunable Si content on Si substrates. Si alloy content is widely tunable across the full composition space in amorphous films. Optical absorption of these films reveals relatively little variation with Si content, despite the fact that ZnSiP2 has a much wider band gap of 2.1 eV. Post-growth crystallization of Si-rich films resulted in epitaxial alignment, as measured by X-ray diffraction and transmission electron microscopy. These films have an optical absorption onset near 1.1 eV, suggesting the possibility of band gap tuning with Si content in crystalline films. The optical absorption is comparably strong to pure ZnSiP2, suggesting a more direct transition than in pure Si.

Single crystal growth and phase stability of photovoltaic grade ZnSiP2 by flux technique

ZnSiP2 demonstrates promising potential as an optically active material on silicon. There has been a longstanding need for wide band gap materials that can be integrated with Si for tandem photovoltaics and other optoelectronic applications. ZnSiP2 is an inexpensive, earth abundant, wide band gap material that is stable and lattice matched with silicon. This conference proceeding summarizes our PV-relevant work on bulk single crystal ZnSiP2, highlighting the key findings and laying the ground work for integration into Si-based tandem devices.