Cameras, solar panels, biosensors, and fiber optics are technologies that rely on photodetectors, or sensors that convert light into electricity. With the shrinking size of their component semiconductor chips, photodetectors are becoming more efficient and affordable. However, current materials and manufacturing methods are constraining miniaturization, forcing trade-offs between size and performance.

There are many limitations and drawbacks to the traditional semiconductor chip manufacturing process. The chips are created by growing the semiconductor film over the top of a wafer in such a way that the film’s crystalline structure is in alignment with that of the substrate wafer. This makes it difficult to transfer the film to other substrate materials, reducing its applicability

In addition, the current method of transferring and stacking these films is done through mechanical exfoliation, a process where a piece of tape pulls off the semiconductor film and then transfers it to a new substrate, layer by layer. This process results in multiple non-uniform layers stacked on top of one another, with the imperfections of each layer accumulating in the final product. This process has an impact on the product’s quality and limits the chips’ reproducibility and scalability.

Lastly, certain materials do not function well as extremely thin layers. Silicon remains ubiquitous as the material of choice for semiconductor chips, however, the thinner it gets, the worse it performs as a photonic structure, making it less than ideal in photodetectors. Other materials that perform better than silicon as extremely thin layers still require a certain thickness to interact with light, posing the challenge of identifying optimal photonic materials and their critical thickness to operate in photodetector semiconductor chips.

Manufacturing uniform, extremely thin, high quality photonic semiconductor films of material other than silicon would make semiconductor chips more efficient, applicable, and scalable.

Penn Engineers Deep Jariwala, Assistant Professor in Electrical and Systems Engineering, and Pawan Kumar and Jason Lynch, a postdoctoral fellow and a doctoral student in his lab, led a study published in Nature Nanotechnology that aimed to do just that. Eric Stach, Professor in Materials Science and Engineering, along with his postdoc Surendra Anantharaman, doctoral student Huiqin Zhang and undergraduate student Francisco Barrera also contributed to this work. The collaborative study also included researchers fiber optics at Penn State, AIXTRON, UCLA, the Air Force Research Lab and the Brookhaven National Lab, and was primarily funded by the Army Research Lab. Their paper describes a new method of manufacturing atomically thin superlattices, or semiconductor films, that are highly light emissive.

One-atom-thick materials generally take the form of a lattice, or a layer of geometrically aligned atoms that form a pattern specific to each material. A superlattice is made up of lattices of different materials stacked fiber optics upon one another. Superlattices have completely new optical, chemical and physical properties which make them adaptable for specific applications such as photo optics and other sensors.

The team at Penn Engineering made a superlattice, five atoms thick, of tungsten and sulfur (WS2). After two years of research using simulations that informed us how the superlattice would interact with the environment, we were ready to experimentally build the superlattice,” says Kumar. “Because traditional superlattices are grown on a desired substrate directly, they tend to be millions of atoms thick, and difficult to transfer to other material substrates. We collaborated with industry partners to ensure that our atomically thin superlattices were grown to be scalable and applicable to many different materials.”

They grew monolayers of atoms, or lattices, on a two-inch wafer and then dissolved the substrate, which allows the lattice to be transferred to any desired material, in their case, sapphire. Additionally, their lattice was created with repeating units of atoms aligned in one direction to make the superlattice two-dimensional, compact and efficient.

“Our design is scalable as well,” says Lynch. “We were able to create a superlattice with a surface area measured in centimeters with our method, which is a major improvement compared to the micron scale of silicon superlattices currently being produced. This scalability is possible due to uniform thickness in our superlattices, which makes the manufacturing process simple and repeatable. Scalability is important to be able to place our superlattices on the industry-standard, four-inch chips.”

Source: This news is originally published by scitechdaily

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