November 11, 2019 by Ingrid Fadelli
A team of IBM researchers just solved a physics mystery that has been around for approximately 140 years, enabling the extraction of vast amounts of information from semiconductors.
Semiconductors (solid materials whose partial conductivity ranges between that of insulators and excellent conductors) are essential components in most electronic devices today, including of course computers, smartphones, and tablets.
Despite their widespread use, some of their vital physical characteristics—which concern how electrical charges act within them—had for a long time remained a mystery. Past studies were unable to fully uncover their behaviour.
So far, the lack of understanding of these physical properties has limited the development of new and more advanced semiconductors. These include majority and minority carrier properties (e.g. type, density, and mobility), which—if better understood—could allow researchers to govern the performance of semiconductor devices more effectively.
In a recent study, a team led by IBM-based researchers introduced a technique that could help to unlock a host of critical parameters for various majority and minority carriers in semiconductors, including recombination lifetime, diffusion length, and recombination coefficient.
IBM’s concept image of its breakthrough semiconductor solution in action. Image Credit: IBM Research.
The discovery of this technique could help to accelerate the development of new semiconductors to be integrated in both existing and emerging electronic devices.
In an interview with Electronics Point, Oki Gunawan and Doug Bishop, two of the IBM researchers behind the new study, explained the implications of their recent discovery.
Oki Gunawan, the primary investigator for the study, holds a PhD and an M.A. in Electrical Engineering from Princeton University, as well as an M. Eng and B. Eng from Nanyang Technological University in Singapore. He joined IBM’s Thomas J Watson Research Center as a postdoctoral scientist in 2007 and became a research staff member two years later. His research primarily focuses on semiconductor devices and electromagnetism.
Doug Bishop, on the other hand, holds a B.S. from Cornell University and a PhD from the Institute of Energy Conversion at the University of Delaware, where he started investigating new materials for thin-film solar cells. He joined IBM’s T.J. Watson Research Center in 2016, on a quest to continue his research into solar cells. He currently works as a characterisation engineer, focusing on materials that could be used in future technologies, such as AI and quantum computing.
In order to better understand the physics behind semiconductors, researchers would need to uncover the fundamental properties of charge carriers inside these materials. For instance, they would need to find out whether their particles are positive or negative, what their speed is under an applied electric field, and how densely they are packed inside the material.
Back in 1879, Edwin Hall discovered a way to determine these properties, as he realised that a magnetic field can deflect the movement of electric charges inside a conductor and that the amount of deflection can be measured.
This phenomenon, known as the Hall effect, has become one of the most important physics constructs, yet it has so far only allowed researchers to obtain three parameters about majority charge carriers (i.e. dominant carriers) in semiconductors. These parameters are the carriers’ type (positive or negative), density inside the material(d), and mobility (m).
A few decades after Hall’s discovery, physicists realised that they could also perform measurements of the Hall effect with light, in so-called ‘photo-Hall’ experiments. Even in these experiments, however, researchers were unable to extract properties of both carriers inside a given semiconductor material (i.e. majority and minority carriers).
Finding a way to unveil properties of both carriers could ultimately enable the development of new and more advanced types of semiconductors, which could be applied in solar cells and other optoelectronic devices.
In their recent study, Gunawan, Bishop and the rest of their team were able to solve one of the Hall effect’s long-standing mysteries, introducing a new formula that allows researchers to extract information about both majority and minority carriers.
A figure depicting the carrier-resolved photo Hall effect. Image Credit: IBM Research.
Over the past few years, the researchers at IBM have been working on innovative solar cell technology that has achieved world-record efficiency, based on a material called kesterite or CuznSnSSe (CZTS) that is quite abundant on our planet. Some of their collaborators from the Korea Advanced Institute of Science and Technology (KAIST) and the Korea Research Institute of Chemical Technology (KRICT) also approached them with a high performing solar cell material made of perovskite.
“As part of these efforts, we need to know the majority and minority carrier properties in semiconductor simultaneously such as mobility, lifetime and diffusion length at a given light illumination, as these properties impact the solar cell performance,” Gunawan and Bishop explained. “For example, the carrier lifetime and diffusion lengths are ‘indicators’ of how good a material is for fabricating solar cells, as you would want to maximise this number to optimise your solar cell performance and you would need to be able to measure them in the first place.”
However, the inability to simultaneously measure the properties of both majority and minority carriers, which is a long-standing problem in semiconductor physics, had greatly limited the development of new semiconductor-based technologies (such as photovoltaics), both at IBM and at other companies.
“Today, we have both world-class perovskite and kesterite materials in our lab and we were determined to find a way to measure both majority and minority carrier properties simultaneously—in order to solve this long-standing physics problem,” Gunawan and Bishop said.
In their recent study, Gunawan, Bishop and their colleagues introduced a new equation and photo-Hall technique that can be used to obtain information about the properties of both majority and minority carriers under light illumination.
Two centuries advances in semiconductor characterisation. The symbols are: hole and electron mobilities (mP, mN), density (n), photocarrier density (Dn), recombination lifetime (t) and hole/electron/ambipolar diffusion lengths (LD,P, LD,N, LD,am). N is the number of light intensity setting in the experiment. Image Credit: IBM Research.
The new carrier-resolved Photo Hall (CRPH) technique introduced by the researchers can be used to perform Hall measurements with light. Essentially, the light generates electron and hole pairs, allowing one to repeat measurements of conductivity (s) and Hall coefficient (H) as a function of light intensity.
“The new insight is the s-H curve, which reveals new information of the mobility difference between the hole and electron (Dm), specifically by using the new equation that we discovered,” Gunawan and Bishop said. “This is the equation and insight that has been missing for 140 years since the discovery of the original Hall effect, as it can be used to solve the two-carrier transport problem in the photo Hall effect.”
For it to work, the new approach proposed by the researchers requires the collection of high-quality Hall measurements. Recently, IBM has made significant progress in this specific area, as it developed a new a.c. field Hall system known as ‘parallel dipole line’ system.
“This is essentially a unique set of rotating ‘dipole line’ magnets that generates an oscillating magnetic field, which turned out to be the ideal system to perform this CRPH experiment,” Gunawan and Bishop said.
Before this study, information about both minority and majority carriers in a given semiconductor was either unknown or measured individually using different complex systems that are unavailable in most electronics labs. Collecting these measurements was very expensive and often led to non-ideal results, as each measurement technique has different operating points (e.g. light, voltage) and/or sample requirements.
“With our new CRPH technique, we now can extract far richer information from semiconductors, including: majority and minority carrier mobility; photocarrier density; recombination lifetime; diffusion length for electrons, and holes and ambipolar carrier type,” Gunawan and Bishop said. “All of these can be repeated N times the number of light intensity settings in the experiment.”
The key advantage of the new technique devised by Gunawan, Bishop and their colleagues is that it allows researchers to measure properties of both minority and majority carriers at the same time and in the same operating conditions. The measurement can be collected within a cost-effective setup and although collecting it currently takes approximately 20 hours, the process can also be accelerated.
In their study, the researchers successfully applied their technique to a variety of solar absorbers, including high-performance lead-iodide-based perovskites. They found that it enabled simultaneous access to parameters for minority and majority carriers, in a striking and unprecedented way.
“We think the implications of our research are quite profound and fundamental,” Gunawan and Bishop said. “With this discovery, we now have a more complete understanding and a new technology to extract all the relevant information from electronic materials by using three most common excitations in physics: electric field, magnetic field and light.”
Figure illustrating the difference between the Hall effect (1879) and the Carrier-resolved photo-Hall effect (2019). Image Credit: IBM Research.
The new CRPH technique, devised by Gunawan, Bishop, and their colleagues, has helped to solve a physics mystery that has been around for almost two centuries, since physicist Georg Ohm first started experimenting with electric field excitations. Ultimately, this allows researchers to gather a significantly vaster pool of information about the carriers inside semiconductors, enabling the development of more advanced and highly-performing devices.
“In terms of applications, just like the classical ‘Hall effect’ tool is so widely used in physics or semiconductor labs around the world, we envision that our technique could also be widely used and help accelerate R&D effort in electronic materials that need to know the details of the charge carriers inside specific materials,” Gunawan and Bishop said.
“We also plan to improve the tool we developed to make it more suitable for wide-scale adoption by the larger semiconductor research and industry,” Gunawan and Bishop said. “We hope that this new technique will help many semiconductor R&D efforts around the world, just like the ‘classical’ Hall effect tool has been widely used in many labs and industry around the world for over a century.”
The IBM-based researchers would now like to start using their CRPH technique to gather insight about materials that they are currently testing, which are designed to be used in optoelectronics, solar cells, and hardware devices to power artificial intelligence. The team is also exploring new possible areas for research, for instance by trying to identify new high impact materials.
