Ordered defects enhance solution-deposited semiconductors, enabling larger, high-performance displays

Standard manufacturing techniques for semiconductor devices (the technologies that make electronics possible) involve processing raw materials at high temperatures in vacuum containers. This fundamentally limits manufacturing efficiency and scalability.

Processes based on the deposition of chemical solutions at lower ambient temperatures and pressure have long been sought as a more efficient and scalable alternative, but these processes typically result in materials with a large number of structural defects leading to poor performance. lower parts of the device.

The laboratory of Qing Cao, professor of materials science and engineering at the Grainger College of Engineering at the University of Illinois at Urbana-Champaign, has developed a process to date that produces the highest-performance transistors from semi -conductors deposited in solution. However, the research team was surprised to learn that the best semiconductor for this process has higher defect concentrations than its original material.

“It is remarkable that even though there are more defects, their organization into ordered defect pairs is why our materials have record performance for those made with a solution deposition process,” Cao said.

“We have gone beyond basic materials science and shown that functional circuits and systems such as displays can be built, paving the way for their adoption in many emerging applications requiring high-performance electronics covering large areas .”

The study, published in the journal Scientific advancesdescribes a procedure for manufacturing devices from the ordered defect semiconductor CuIn₅Se₈ prepared by solution deposition.

They were used to form high-speed logic circuits operating in megahertz and a microdisplay with a resolution of 508 pixels per inch. The screen’s transistors powered inorganic micro-LEDs, a brighter, longer-lasting alternative to the current standard of organic LEDs, but requiring much more powerful transistors to drive each pixel.

Cao believes new materials and processes could evolve to support next-generation inorganic micro-LED displays and high-speed printable electronics for healthcare, smart packaging and the Internet of Things.

The promise of the solution deposit

The extreme conditions required for standard semiconductor manufacturing limit the surface areas of the materials processed. While this is acceptable for chips and microelectronics, it is economically prohibitive for applications requiring many devices coordinated and distributed over a large area, such as electronic displays.

Solution deposition, in which semiconductors are dissolved in a liquid and spread onto a target substrate, would not only enable large-area applications but could also make processing more efficient.

“The fact that solution deposition can occur at atmospheric pressure and much lower temperatures makes it a desirable alternative to standard vapor deposition in terms of manufacturing throughput, cost and substrate compatibility,” Cao said.

However, vapor deposition techniques have developed to the point that the processed materials have very few defects, leading to very efficient devices. Before the solution deposition is used in commercial processing, it must be developed to the point where the materials it creates have the same performance levels.

A better semiconductor

Cao recalls that copper-indium-selenium materials first attracted his lab’s attention for their tunability. Changing the exact proportions of each element in the material gave them a vast material design space to make efficient solar cells with a copper-indium-selenium ratio of 0.9:1:2.

“The idea was, ‘We control the proportions of the materials, so can we adjust them to make good semiconductors for electronics instead of good solar cells?'” Cao said.

“We developed a solution deposition process for these materials and experimented with the proportions until we found a material suitable for electronics, having a copper-indium-selenium ratio of 1:5:8. In fact, the combination we found outperformed not only other solution-processable semiconductors, but also most semiconductors currently used in displays.

Semiconductor performance is often quantified by charge mobility, a measure of how easily electrons move through the material when a voltage is applied. Compared to amorphous silicon semiconductors used in large LCD displays, the researchers’ material CuIn₅Se₈ has 500 times greater mobility. Compared to metal oxide semiconductors used in cutting-edge organic LED displays, the mobility of this new material is four times higher.

The mobility of CuIn₅Se₈ is comparable to the low-temperature polycrystalline silicon used in smartphone screens. However, processing polycrystalline silicon requires laser annealing, making it difficult to scale and integrate into larger devices. CuIn deposited in solution₅Se₈ could facilitate larger, high-performance displays.

No more flaws, surprisingly

The researchers’ next step was to understand why CuIn₅Se₈ works so well. They consulted Jian-Min Zuo, professor of materials science and engineering at Grainger Engineering and an expert in materials characterization.

“In general, as materials scientists, we think that better-performing materials have fewer defects, and that’s what we initially expected,” Cao said.

“But then Professor Zuo responded to us after using transmission electron microscopy to observe the microscopic structure. It turned out that there were not only more defects than the parent compound, but probably two types of defects coexisting.”

To resolve this apparent contradiction, the researchers turned to theorist André Schleife, professor of materials science and engineering at Grainger Engineering.

By simulating the new copper-indium-selenium material, Schleife’s group discovered that both types of defects in CuIn₅Se₈ can combine to form a material system called composed of ordered defects. In such systems, different types of material faults organize themselves in a regular pattern and “cancel each other out,” leading to better charge mobility.

A path to printing high-speed electronic components and better displays

The researchers demonstrated the capabilities of their process by using their new defect-tolerant copper-indium-selenium semiconductors to build a display with gallium nitride-based micro-LEDs. The CuIn₅Se₈ This material formed the basis of high-performance transistors that leveraged 8 x 8 micron LED pixels, tightly packed at a resolution of 508 pixels per inch.

“While organic LEDs are the standard for high-performance displays, LEDs based on inorganic substances such as gallium nitride are emerging as a faster, brighter and more energy-efficient alternative,” Cao explained.

“However, because they are brighter, they require high-power electronics to operate and it is particularly difficult to fit them into a smaller footprint for high resolution. We have demonstrated that our new semiconductor is up to the task of the task, and we showed that it could be manufactured efficiently with solution deposition.

In addition to driving LEDs, these transistors can be integrated to form logic circuits, again providing much better performance compared to those built on other processable semiconductors. These circuits can operate at megahertz with a delay of up to 75 nanoseconds.

Compatibility with low-cost solution deposition processes without sacrificing performance holds promise for future printable electronics. They could be used in continuous wellness monitoring, smart packaging with integrated sensing and computing, and affordable Internet of Things devices.

Cao notes that even though the process is developed enough that it can be commercialized, they are waiting until it can be made more environmentally friendly.

“The process is currently based on hydrazine, which is used as rocket fuel,” he explained. “It could be used in an industrial environment, but we first want to modify the process to use chemicals that are safer to work with and have a smaller environmental footprint.”