A novel method for depositing semiconductor materials is leading to transistors with exceptional performance by intentionally creating more defects, which is an unexpected outcome. Researchers harnessed these advanced devices to build high-speed logic circuits and a functioning high-resolution inorganic LED display.
A novel method for depositing semiconductor materials is leading to transistors with exceptional performance by intentionally creating more defects, which is an unexpected outcome. Researchers harnessed these advanced devices to build high-speed logic circuits and a functioning high-resolution inorganic LED display.
Traditionally, creating semiconductor devices—key components of modern electronics—involves processing basic materials at elevated temperatures in vacuum environments. This approach inherently limits how efficiently and widely we can manufacture these products.
Methods that use chemical solutions at lower temperatures and atmospheric pressure have been explored as a more efficient and scalable option. However, these techniques typically result in materials with numerous structural defects, leading to subpar performance of the devices produced.
The lab led by Qing Cao, an associate professor of materials science and engineering at The Grainger College of Engineering, University of Illinois Urbana-Champaign, has innovated a technique that produces the highest-performing transistors derived from solution-deposited semiconductors to date. Surprisingly, the research team discovered that the most effective semiconductor material used in this process has a greater number of defects compared to its original form.
“It’s astounding that despite the increased defect levels, the manner in which they form organized defect pairs is responsible for achieving record performances in our materials made via the solution deposition process,” Cao stated. “We extended our research beyond basic materials science and demonstrated that we can build functional circuits and systems, such as displays, setting the stage for wide adoption in many emerging applications that need high-performance electronics across large scales.”
This investigation, recently published in the journal Science Advances, describes a detailed method for producing devices from the ordered defect compound semiconductor CuIn5Se8 via solution deposition. These devices were utilized to create high-speed logic circuits that function in megahertz and a micro-display boasting a resolution of 508 pixels per inch. The transistors in this display powered inorganic micro-LEDs, which are significantly brighter and more durable than the current organic LED standards but necessitate more powerful transistors for pixel activation. Cao believes that this new material and method could be scaled up to support next-generation inorganic micro-LED displays and high-speed printed electronics for applications in healthcare, smart packaging, and the Internet of Things.
The potential of solution deposition
The high-demand conditions of conventional semiconductor fabrication limit the surface area of the materials used. While this limitation is manageable for chips and microelectronics, it is economically unviable for devices needing coordination across large areas, such as electronic displays. Solution deposition, which involves dissolving semiconductors in a liquid and applying them to a substrate, could not only facilitate large-area applications but also streamline processing efficiency.
“The ability of solution deposition to occur at atmospheric pressure and at significantly lower temperatures makes it a favorable alternative to traditional vapor deposition concerning production speed, Costs, and substrate compatibility,” Cao explained.
Nevertheless, vapor deposition techniques have become so advanced that they produce materials with minimal defects, resulting in high-performance devices. Before solution deposition methods can become commercially viable, they must be perfected to produce materials with comparable performance levels.
An improved semiconductor
Cao recalls that copper-indium-selenium materials initially attracted his lab’s attention due to their tunability. By adjusting the exact ratios of each element, they explored a broad design space for creating effective solar cells with a copper-indium-selenium ratio of 0.9:1:2.
“We thought if we had control over the material ratios, could we modify them to create functional semiconductors for electronics rather than just for solar cells?” Cao mused. “We developed a solution deposition method for these materials, experimenting with different proportions until identifying a mixture suitable for electronic applications, specifically a copper-indium-selenium ratio of 1:5:8. This formulation not only surpassed other solution-processable semiconductors but also many used in existing displays.”
The effectiveness of semiconductors is typically measured by charge mobility, which indicates how easily electrons can flow through the material under voltage. The material CuIn5Se8 has a mobility that is 500 times higher than that of the amorphous silicon semiconductors commonly used in large LCD displays, and four times greater than metal oxide semiconductors deployed in advanced organic LED displays.
CuIn5Se8 boasts a charge mobility similar to that of low-temperature polycrystalline silicon, which is utilized in smartphone displays. However, processing polycrystalline silicon usually involves laser annealing, making it challenging to scale and integrate into larger devices. Potentially, solution-deposited CuIn5Se8 could facilitate the production of larger, high-performance displays.
Unexpected increase in defects
Next, the researchers aimed to understand why CuIn5Se8 shows such impressive performance. They consulted Jian-Min Zuo, a professor of materials science and engineering at Grainger Engineering, and a specialist in material characterization.
“Typically, as materials scientists, we believe that higher-performing materials have fewer defects, which was our original assumption,” Cao remarked. “However, after examining the microscopic structure using transmission electron microscopy, Professor Zuo confirmed that not only were there more defects than in the parent compound, but there were probably two types of defects co-existing!”
To resolve this puzzling discrepancy, the researchers worked with theorist André Schleife, an associate professor of materials science and engineering at Grainger Engineering. His group conducted simulations on the new copper-indium-selenium material and discovered that the two defect types in CuIn5Se8 can interact to create a structure known as an ordered defect compound. In these structures, various defects align into a regular pattern and effectively “cancel each other out,” which results in enhanced charge mobility.
A route to printing high-speed electronics and enhancing displays
The researchers showcased the abilities of their process by creating a display using their new defect-tolerant copper-indium-selenium semiconductors alongside gallium nitride micro-LEDs. The CuIn5Se8 material formed the foundation for high-performance transistors operating 8-by-8-micron LED pixels, tightly packed to achieve a resolution of 508 pixels per inch.
“While organic LEDs remain the standard in advanced displays, LEDs based on inorganic materials such as gallium nitride are emerging as faster, brighter, and more energy-efficient alternatives,” said Cao. “Yet, due to their higher brightness, they necessitate powerful electronics to function efficiently, especially when squeezed into smaller formats for high resolution. Our findings demonstrate that our new semiconductor meets this requirement, and we’ve proven its effective manufacturing via solution deposition.”
Besides powering LEDs, these transistors can be integrated into logic circuits, offering significantly better performance than those built with other solution-processable semiconductors. These circuits can function at megahertz speeds with delays as brief as 75 nanoseconds. The compatibility of solution deposition processes with low-cost techniques without compromising performance indicates bright prospects for future printable electronics. They could serve applications like continuous health monitoring, smart packaging with integrated sensing and computing, and affordable Internet of Things devices.
Cao pointed out that while the process is sufficiently developed for commercialization, they are delaying this step until a more environmentally friendly version is established.
“The current process relies on hydrazine, a substance used as rocket fuel,” he noted. “Although it could function in an industrial setting, we aim to refine it using safer chemicals that also minimize our environmental impact.”
