Ce Sun

Direct Synthesis of van der Waals Solids

The stacking of two-dimensional layered materials, such as semiconducting transition metal dichalcogenides (TMDs), insulating hexagonal boron nitride (hBN), and semimetallic graphene, has been theorized to produce tunable electronic and optoelectronic properties. Here we demonstrate the direct growth of MoS2, WSe2, and hBN on epitaxial graphene to form large-area van der Waals heterostructures. We reveal that the properties of the underlying graphene dictate properties of the heterostructures, where strain, wrinkling, and defects on the surface of graphene act as nucleation centers for lateral growth of the overlayer. Additionally, we show that the direct synthesis of TMDs on epitaxial graphene exhibits atomically sharp interfaces. Finally, we demonstrate that direct growth of MoS2 on epitaxial graphene can lead to a 10(3) improvement in photoresponse compared to MoS2 alone.

Atomic and electronic structure of Lomer dislocations at CdTe bicrystal interface.

Extended defects are of considerable importance in determining the electronic properties of semiconductors, especially in photovoltaics (PVs), due to their effects on electron-hole recombination. We employ model systems to study the effects of dislocations in CdTe by constructing grain boundaries using wafer bonding. Atomic-resolution scanning transmission electron microscopy (STEM) of a [1–10]/(110) 4.8° tilt grain boundary reveals that the interface is composed of three distinct types of Lomer dislocations. Geometrical phase analysis is used to map strain fields, while STEM and density functional theory (DFT) modeling determine the atomic structure at the interface. The electronic structure of the dislocation cores calculated using DFT shows significant mid-gap states and different charge-channeling tendencies. Cl-doping is shown to reduce the midgap states, while maintaining the charge separation effects. This report offers novel avenues for exploring grain boundary effects in CdTe-based solar cells by fabricating controlled bicrystal interfaces and systematic atomic-scale analysis.

Creating a single twin boundary between two CdTe (111) wafers with controlled rotation angle by wafer bonding

The single twin boundary with crystallographic orientation relationship (1¯1¯1¯)//(111) [01¯1]//[011¯] was created by wafer bonding. Electron diffraction patterns and high-resolution transmission electron microscopy images demonstrated the well control of the rotation angle between the bonded pair. At the twin boundary, one unit of wurtzite structure was found between two zinc-blende matrices. High-angle annular dark-field scanning transmission electron microscopy images showed Cd- and Te-terminated for the two bonded portions, respectively. The I-V curve across the twin boundary showed increasingly nonlinear behavior, indicating a potential barrier at the bonded twin boundary.

Leveraging First Principles Modeling and Machine Learning for Microscopy Data Inversion

In microscopy employing electron and x-ray beams, advances in instrumentation and techniques have substantially improved the ability to image, track, and characterize materials at ever-higher resolution and precision. However, determining atomistic arrangements (i.e. configurations) from microscopy data remains a substantial challenge. Whether due to projection of a three-dimensional structure onto one or two dimensions as in pair distribution functions (PDF) and (scanning) transmission electron microscopy (STEM/TEM), or the reduction of a large number of matrix elements into an overall energy-dependent amplitude as in x-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS), the result is that inversion of this mapping is time consuming, imprecise, and sometimes even fruitless, despite a large number of excellent tools which produces characterization data from input atomistic configurations, or further perform fitting to produce configurations from characterization data. The reasons for such difficulties include the extremely high dimensional search space for atomistic configurations, especially for nanostructures and defected or disordered solids.

Creation and analysis of atomic structures for CdTe bi-crystal interfaces by the grain boundary genie

Grain boundaries (GB) in poly-CdTe solar cells play an important role in species diffusion, segregation, defect formation, and carrier recombination. While the creation of specific high-symmetry interfaces can be straight forward, the creation of general GB structures in many material systems is difficult if periodic boundary conditions are to be enforced. Here we describe a novel algorithm and implementation to generate initial general GB structures for CdTe in an automated way, and we investigate some of these structures using density functional theory (DFT). Example structures include those with bi-crystals already fabricated for comparison, and those planning to be investigated in the future.

First principles modeling of grain boundaries in CdTe

A fundamental understanding of the role of vacancies, interstitials, dislocations and grain boundaries on the electronic structure of CdTe may lead to efficiency improvements. Atomistic-level characterization, including microscopy and first principles modeling, is crucial in developing such a fundamental understanding. In the present work, we built atomistic grain boundary and dislocation core models directly from the STEM images using image analysis methods and crystallographic information at the interface. Grain boundaries are modeled using first principles density functional theory (DFT) calculations. Electronic structures of large-scale grain models are also computed with an accurate hybrid functional (HSE06). We report the electronic density of states (DOS) and electrostatic potential profiles of different CdTe grain boundaries to understand charge carrier interactions. Thermodynamics of point defects and pairs of point defects that can exist on or near grain boundaries are studied and pertaining changes in electronic structure are reported. The implications of these electronic structure changes at grain boundaries on photovoltaic performance, and corresponding strategies to improve performance, are discussed.

Atomistic simulations of grain boundaries in CdTe

An improvement in efficiencies of polycrystalline CdTe can possibly be achieved by understanding the role of grain boundaries. Therefore, we systematically studied the atomic and electronic structures of various high angle grain boundaries including asymmetric tilt and twist grain boundaries using empirical potentials and density functional theory (DFT). The density of states analysis revealed that most grain boundaries lead to the formation of midgap states, which can drastically reduce the photovoltaic efficiency. The planar-averaged electrostatic potential analysis indicated attraction for holes around the grain boundary region.

Creating single boundary between two CdTe (111) wafers with controlled orientation by wafer bonding

Zinc-blende (ZB) CdTe has drawn great attention as optoelectronic and solar energy conversion materials since it has a near optimum band gap of 1.6 eV and a absorption coefficient greater than 5x10 5 /cm. CdTe can be either ZB or wurtzite (WZ) structures, resulting in different electronic properties. It has been reported that the twin superlattice with numerous twin boundaries in III-V and II-VI semiconductor nanowires along growth orientation of ZB structures considerably enhance band gap engineering and mechanical behavior in quasi-one-dimensional materials [1]. This opens new possibilities for properties and functionalities at the atomic and quantum scales by controlling the twin boundary and modulating twin densities. Therefore, it is an in-demand and challenging feat to “create” a single twin boundary in a nanowire or bulk material in order to understand the electronic and mechanical characteristics of the III-V and II-VI quantum well or barrier. Wafer bonding, which enables the direct integration of two or more single crystal wafers with controlled surfaces and orientation, is a key technique in creating a single boundary [2]. In this study, we show the creation of a single boundary between two identical CdTe single crystals.