Precise control of crystal stoichiometry of chalcopyrite compounds using a double-crucible Bridgman crystal growth technique

Zhiqiang Mao, Pennsylvania State University (Principal Investigator)

Jianwei Sun, Tulane University (Co-Investigator)

One central challenge in crystal growth by conventional melt techniques such as Czochralski and Bridgman is that the crystals grown using these techniques are often inhomogeneous and have non-stoichiometric compositions, which often leads to degraded functional properties. This research project aims to address this challenge via developing a novel crystal growth protocol that can precisely control the compositions of single crystals. The candidate materials the PIs proposed for growth are chalcopyrites compounds which are intermetallic chalcogenides and pnictides that exhibit technologically useful functional properties, such as nonlinear optical properties and topological quantum transport. The research team will use a new method called high-pressure double-crucible vertical Bridgman (HP-DCVB) crystal growth technique to grow the chalcopyrite compounds with homogeneous and stoichiometric compositions.

The HP-DCVB crystal growth technique combines high pressure (up to 10 atm) with continuous source material feeding during crystal growth, allowing for precise composition control in grown crystals. Compared with the conventional Bridgman furnace, the HP-DCVB furnace has a distinct advantage in its ability to grow homogeneous crystals with stoichiometric compositions. The HP-DCVB furnace at Penn State is the first of its kind in academia. In addition to crystal growth, the Mao group at Penn State will also characterize the compositions, crystal structures, and optical/electronic properties of the grown crystals using various methods such as energy dispersive spectroscopy, X-ray diffraction, transmission electron microscopy, and optical and transport property measurements. The goal of these experimental efforts is to demonstrate that crystals grown using the HP-DCVB technique have compositions close to stoichiometry, are more homogeneous, and have fewer defects, which improves their functional properties.

In addition to experimental studies, the research team will also perform computational studies to understand the mechanisms behind crystal composition control. First-principles calculations combined with statistical mechanical modeling will be used to evaluate defect densities and their tunability by external control parameters, such as the partial pressure of volatile elements during crystal growth. The Sun group at Tulane will also calculate migration barriers to defects, which can be used as a guidance for post-processing (such as annealing) the grown crystals to further reduce the defects. The results from these computational studies will be compared and validated with the experimental results. These integrated theoretical and experimental efforts are expected to promote chalcogenide/pnictide synthesis science and provide opportunities for workforce training in two important areas, crystal growth and computational physics. Overall, this project has the potential to advance not only the material synthesis science, but also the development of novel materials with applications in energy harvesting and other advanced technologies.