Exciting collaborative effort from the Duan and Caram labs unlocked the secrets approaching the fundamental performance limit of 2D semiconductor diodes for the first time, providing critical design principles for electronic devices.
Electronic devices such as solar cells for accessible and renewable energy production, photodetectors in cameras and light emitting diodes in screens and indoor lighting are an integral part of our daily life. The emergence of low dimensional semiconductors such as 2D semiconductors and other nanomaterials provide new exciting opportunities for the optoelectronic device industry as they are well suited for nanotechnology and their small size results in controllable and unique device and quantum properties. Nevertheless, it has long been a challenge to reach the inherent performance of a 2D semiconductor device when device fabrication often results in pronounced defects.
Conventionally, electrodes in devices are produced by evaporating high energy metal (e.g. gold) atoms onto a semiconductor surface. This process damages the 2D semiconductors, resulting in defects and inferior device performance.
Comparison between van der waals contact (left) and evaporated contact (right).
To overcome this obstacle, Professor Xiangfeng Duan’s group at UCLA developed the “Van der Waals contact” (vdW-contacted) fabrication method which gently places a metal electrode on a semiconductor to produce a pristine and undefective metal-semiconductor interface. “With the traditional evaporated contacts, the interface is really crappy and it breaks the chemical bonds in the [semiconductor] structure. Electrons get stuck in these defects instead of producing useful current,” co-author Dr. Tim Atallah described. “With Van der Waals contacts, we are no longer limited by the crap of the interface so we can actually see how our materials perform.” The team’s most recently published research highlights the vdW-contacted diode produces a higher current and better performance than the traditionally made diode. Co-author Dr. Peng Chen elaborated, “The maximum internal quantum efficiency of our vdW-contacted 2D diode is near 100%, which is the best in the world so far and allows us to fully capture the fundamental physical properties of the 2D optoelectronic devices.” No longer limited by a defective electrode interface, the research team was able to observe an unprecedented trend in 2D diode performance: higher current from light (or “photocurrent”) when there are fewer charges present (called low “doping”) in the 2D semiconductor. The photocurrent peak at low doping suggests there’s an optimal number of charges that results in maximal 2D semiconductor diode performance. 
Performance of vdW-contacted diode and evaporated contact diode
analyzed by plotting generated photocurrent vs. charge density. Atallah remarked, “As we put more and more charges into the device, its performance got worse, which led us to ask why does this happen and how does it relate the 2D material itself.” To understand the reason behind the high (peak) device performance when there are few charges present, the team set out to measure the lifetime of excited state (also called an “exciton”) that is formed after 2D semiconductor absorbs light. Exciton lifetimes are a critical fundamental property of a material governing their optoelectronic device performance: to produce useful current, excitons need to migrate to the middle of the diode (i.e the p-n junction). The longer the lifetime, the more likely for excitons located further from the center to reach the p-n junction. 
Lifetime of exciton vs. charge density explained by Auger recombination. Their results showed that the lifetime of exciton decreases significantly as the number of charges increases. The exciton lifetime reaches a maximum when there are very few charges in the 2D semiconductor (low charge doping): at the same value that produces the peak photocurrent. Excitons with shorter lifetime travel shorter distances, and often are unable to reach the p-n junction; instead they recombine, giving their energy to the extra nearby charges as heat instead of valuable current. This phenomenon is known as Auger recombination; the presence of many extra charges “steal” the exciton’s energy and decrease the photocurrent, which was determined to be the reason for the diminished 2D device performance. In a concerted effort to confirm the results and explanation of the low doping resulting in higher photocurrent, the team built a higher resolution microscopic photocurrent mapping apparatus. 
Photocurrent mapping of diodes with low (left) and high (right) charge density. These new measurements indeed confirmed both qualitatively and quantitatively the interpretation and results previously discussed. A side to side comparison of the photocurrent mapping images shows that there is more and larger region of photocurrent (red glow) at low doping (-0.4V vs. -4.0V), whereas it is generated exclusively around the center area of the p-n junction at high doping. “A key takeaway from our paper is that to make a good 2D semiconductor device, you want to make sure there’s enough doping to create the p-n junction but not too many such that the Auger recombination becomes dominant,” Atallah explained. “The whole project took about two years from conception to publication, while it is a wonderful research and collaboration experience,” Chen concluded. “Our results highlight the impact of the contact interface, exciton diffusion and exciton–charge Auger effects on 2D diode performance, and provide important guidance for pushing the performance limit of 2D optoelectronic devices.”
About the Lead Authors
Lead authors – Professor Xiangfeng Duan, Professor Justin Caram, Dr. Peng Chen, Dr. Timothy Atallah Professor Xiangfeng Duan joined the chemistry and biochemistry faculty in 2008. His group’s research interests include nanoscale materials, devices and their applications in future electronics, energy technologies and biomedical science. Professor Justin Caram joined the chemistry and bIochemistry faculty in July 2017 as an assistant professor. His group studies novel photophysical materials and biological questions, using quantum mechanical approaches to spectroscopy. Dr. Peng Chen is a former postdoctoral scholar in the Duan group. He recently joined the Microelectronics faculty at Southern University of Science and Technology in China as an Assistant Professor. His research interests include nanoelectronics, nano optoelectronics and condensed matter physics. Dr. Timothy (Tim) Atallah, a former postdoctoral scholar in the Caram group, just recently joined the Chemistry & Biochemistry faculty at Denison University as an Assistant Professor where his research focuses on using optical spectroscopy to uncover the fundamental processes which govern next generation semiconductors such as nano- and organic crystals.
By Zhuoying Lin, UCLA Department of Chemistry & Biochemistry, zylin@g.ucla.edu. Lin is a first year chemistry graduate student and science writer. The writer wishes to thank Drs. Atallah and Chen for their assistance with this article.
