A new solution deposition process for semiconductors yields high-performing transistors by introducing more defects, counter-intuitively. Researchers used these devices to construct high-speed logic circuits and an operational high-resolution inorganic LED display.

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

Processes based on deposition from chemical solutions at lower temperatures and ambient pressure have long been pursued as a more efficient and scalable alternative, but such processes usually result in materials with large numbers of structural defects leading to inferior device performance. 

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

"It's remarkable that even though there are more defects, their organisation into ordered defect pairs are the reason our materials have the record-high performances for those made with solution deposition process," says Cao.

"We went further than fundamental materials science and showed that functional circuits and systems like displays can be constructed, paving the road towards their adoption in many emerging applications requiring high-performance electronics covering large areas."

This study, recently published in the journal Science Advances, outlines a procedure for fabricating devices from the ordered defect compound semiconductor CuIn₅Se₈ prepared by solution deposition. They were used to form high-speed logic circuits operating in megahertz and a micro-display with a resolution of 508 pixels per inch.

The transistors in the display drove inorganic micro-LEDs, a brighter and more durable alternative to the current standard of organic LEDs but requiring much more powerful transistors to drive each pixel. Cao believes that the new material and process could scale to support next-generation inorganic micro-LED displays and high-speed printable electronics for healthcare, smart packaging, and internet of things. 

The promise of solution deposition

The extreme conditions required for standard semiconductor manufacturing limit the surface areas of the processed materials. 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 the semiconductors are dissolved in liquid and spread over 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 alone makes it a desirable alternative to standard vapour deposition in terms of manufacturing throughput, cost and substrate compatibility," says Cao.

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

The semiconductor CuIn₅Se₈ is processed into large sheets by solution deposition, in which the material is dissolved in solution then spread over a large area. The process is far more efficient and scalable than standard vapor deposition techniques.

A better semiconductor

Cao recalls that copper-indium-selenium materials first drew the attention of his lab for their tunability. Changing the exact proportions of each element in the material allowed a vast material design space for them to realise effective solar cells with a copper-indium-selenium ratio of 0.9:1:2. 

"The thought was. we have control over the material proportions, so can we adjust them to make good semiconductors for electronics instead of good solar cells?" says Cao.

"We developed a solution deposition process for these materials, and we experimented with the proportions until we found a material good for electronics purposes, which has 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 with charge mobility, a measure of how easily electrons move through the material when voltage is applied. Compared to amorphous silicon semiconductors used in large LCD displays, the researchers' material CuIn5Se8 has a mobility 500 times greater. Compared to metal oxide semiconductors used in state-of-the-art organic LED displays, the new material's mobility is four times greater. 

The mobility of CuIn5Se8 is comparable to low-temperature polycrystalline silicon which is used in smartphone displays. However, polycrystalline silicon processing requires laser annealing, making it difficult to scale up and include in larger devices. Solution-deposited CuIn5Se8 could facilitate larger high-performance displays.

More defects, surprisingly

The researchers' next step was figuring out why CuIn5Se8 performs so well. They consulted Jian-Min Zuo, professor of materials science and engineering in Grainger Engineering and an expert in material characterisation.

"Generally, as material scientists, we think that better performing materials have fewer defects, and that's what we expected initially," says Cao. "But then, Prof Zuo got back 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 likely two types of defects co-existing!"

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

By simulating the new copper-indium-selenium material, Schleife's group found that the two types of defects in CuIn5Se8 can combine to form a material system called an ordered defect compound. In such systems, different types of material defects organise into a regular pattern and 'cancel out', leading an improved charge mobility. 

A path to printing high-speed electronics and higher-performance displays

The researchers demonstrated the capabilities of their process by using their new defect-tolerant copper-indium-selenium semiconductors to construct a display together with gallium nitride based micro-LEDs. The CuIn5Se8 material formed the basis of high-performance transistors which operated 8-by-8-micron LED pixels, closely packed to a resolution of 508 pixels per inch. 

Micro-LED display driven with CuIn₅Se₈ transistors processed by solution deposition. The LEDs are inorganic making them hard to operate without the power available from devices made with the new material.

"While organic LEDs are the standard in high-performance displays, LEDs based on inorganic substances such as gallium nitride are emerging as a faster, higher-brightness, and more energy efficient alternative," says Cao.

"However, since they are brighter, they require high-power electronics to operate and it is especially challenging if we would like to squeeze them within a smaller footprint for high resolution. We demonstrated that our new semiconductor is up to the task, and we've shown that it can be efficiently manufactured with solution deposition." 

In addition to driving LEDs, these transistors can be integrated to form logic circuits, again offering much better performance compared to what are constructed on other solution processable semiconductors. These circuits can operate at megahertz with delay down to 75 nanoseconds.

The compatibility with low-cost solution deposition processes without sacrificing performance is promising for future printable electronics. They could find use in continuous wellness monitoring, smart packing with integrated sensing and computing, and affordable internet of things devices.

Cao notes that while the process is sufficiently developed that it could be commercialised, they are holding off until it can be made more environmentally friendly.

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