Daniel Hada Harianja
To retain its mainstay status in microelectronics, silicon must undergo improvement for advanced multifunctionality in future electronic devices.
Silicon needs an upgrade. As innovators dream of better devices, they seek more functionalities to be built into their microelectronics. Silicon, the backbone of electronics, cannot fulfill those demands alone.
One upgrade comes in the form of perovskite oxides. Named after the specific crystalline structure of such material, perovskite oxides have for decades captivated scientists with their vast range of electrical, magnetic, and optical properties. The objective, therefore, is to build desired perovskite oxide layers on top of silicon, granting a device of multiple functionalities.
Growing most oxides on top of silicon is difficult to do directly, because silicon is easily oxidized into amorphous forms of itself, which then cannot accommodate the functional oxides. So the scientific community is hard at work to perfect an intermediate layer between the two—something that is sufficiently compatible with silicon and able to act as a template on top of which other oxides can be built.
The Challenge of Growing Perovskite Oxides on Silicon
Zhe Wang, an Applied and Engineering Physics graduate student, is part of that scientific frontier. Wang works in the research group of Darrell Schlom, Materials Science and Engineering. Together with the group, Wang hopes to improve the growing method of SrTiO3, a perovskite and the most widely researched candidate for that middle layer. Specifically, Wang aims to enhance the crystalline quality of the SrTiO3 layer.
“The advantage is that if we can grow these functional material on silicon, we can reach multifunctionality on silicon,” says Wang. “This can be used in future devices, such as smartphones, sensors, antennae, photovoltaic cells, and many others.”
Unlike most oxides, SrTiO3 can be feasibly formed on top of silicon by adjusting the growing conditions. To act as a good template, however, on which other functional materials can be built, the SrTiO3 film must be formed as a single-crystal, which means the layer has a single lattice orientation throughout its crystal structure.
Creating or depositing such a film flawlessly is challenging. “Even though we can achieve single-crystal layers, the crystalline quality is often not very good. It has many defects,” says Wang. “If we grow other functional materials on top of it, the functional materials will also not be perfect, because the underlying layer is not perfect.”
By studying molecular beam epitaxy, one of the most advanced thin-film deposition methods available, Wang hopes to fine-tune the conditions necessary for a good film. This method subjects the deposition process to very low pressures of below 10-8 Torr, which allows for the highest possible purity of the film. To form a layer on silicon, the constituent elements of the layer are separately heated in effusion cells until they sublime into vapor. The vapors, along with a stream of oxygen, then meet on the silicon surface and react to form a film. As the deposition occurs, reflection high-energy electron diffraction is employed to evaluate the crystal growth by firing electrons on the target materials and analyzing its diffraction pattern.
“The parameters [of the process] are complicated to get right,” Wang says. For one, the stoichiometry of the constituent elements of the film must be extremely precise. The temperature must be high enough to allow the film deposition to occur, but not too high that it oxidizes the silicon.
Toward Success, It Takes Collaboration
Despite the challenges, many appreciate the progress in Wang’s work. Within the past year, collaborators from Singapore, Berkeley, and the Netherlands have published separate papers on the properties of other perovskite materials that they have grown atop of Wang’s high quality SrTiO3 films on silicon, including their applications in different microelectronic devices. Wang also plans to try integrating his own perovskite oxides onto his template in the future. It depends, however, on the ability to build good films on top of silicon, and as Wang explains, good films require a good underlying SrTiO3 layer.
It is not simply the cutting-edge tools that boost Wang’s research. “We have a lot of collaboration. We are making the material, but to understand the perfection, performance, and defects at the atomic level, we collaborate with other groups at Cornell.” For instance, the research team of Lena F. Kourkoutis, Applied and Engineering Physics, has used transmission electron microscopy to help with characterizing the interface structure and film quality. Kyle Shen’s research group, Physics, has integrated their angle-resolved photoemission spectroscopy (ARPES) with the molecular beam epitaxy system to study the materials being formed without exposure to air. Other collaborations include research into utilizing density functional theory to predict novel properties of materials. Like silicon, no one researcher can fulfill all those demands alone. Through collaboration, Wang achieves more.
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Once called “the first American university” by educational historian Frederick Rudolph, Cornell University represents a distinctive mix of eminent scholarship and democratic ideals. Adding practical subjects to the classics and admitting qualified students regardless of nationality, race, social circumstance, gender, or religion was quite a departure when Cornell was founded in 1865.
Today’s Cornell reflects this heritage of egalitarian excellence. It is home to the nation’s first colleges devoted to hotel administration, industrial and labor relations, and veterinary medicine. Both a private university and the land-grant institution of New York State, Cornell University is the most educationally diverse member of the Ivy League.
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