By Zhuoying Lin (December 9, 2021)
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.
Diodes conduct electricity, and electricity is everywhere. Whether it is the solar panels that run our houses, or a camera and TV that bring images alive, semiconductor diodes as an electronic device are an integral part of our daily life. In the electronics industry, the emergence of low dimensional semiconductors, such as 2D semiconductors, provide new exciting opportunities as they are well suited for nanotechnology. In the world of science, the tiny size of semiconductor diodes also bears extensive scientific research on their controllable, unique electronic properties. Nevertheless, we can’t optimize a desired electronic effect when the cause is hidden in the crevice. The challenge is, how can we reach the intrinsic performance of a 2D semiconductor when device making generates additional pronounced defects that block our way to the fundamentals?
Conventionally, electrodes in devices are produced by evaporating metal such as gold (Au) atoms that laminate onto a semiconductor surface. This process requires high temperature and inevitably damages the 2D semiconductors, resulting in defective interface and inferior device performance.
Figure 1. Comparison between van der waals contact (left) and evaporated contact (right).
To remove the roadblock, Professor Xiangfeng Duan’s group at UCLA developed the “Van der Waals contact” (vdW-contacted) fabrication method, named after the molecular force that drives the process. It 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 in Nature highlights the vdW-contacted diode produces a higher current and thus 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 electronic devices.”
No longer limited by a defective electrode interface, the research team was able to observe an unprecedented trend in 2D diode performance: current from light (photocurrent) is higher when there are fewer electronic charges present (low doping) in the 2D semiconductor. The presence of photocurrent peak at low doping suggests there’s an optimal number of charges that results in maximal 2D semiconductor diode performance (Figure 2).
Figure 2. 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 question: why does this happen and how does it relate to the 2D material itself?”
To understand the cause and effect, the team set out to measure the lifetime of excited state (also called an “exciton”) that is formed after a 2D semiconductor absorbs light as energy. Exciton lifetimes are a critical fundamental property that governs an electronic 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.
Figure 3. Lifetime of exciton vs. charge density explained by Auger recombination.
The result is electrifying. 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) which also produces the peak photocurrent (Figure 3).
Excitons with shorter lifetime travel shorter distances, and often they are unable to make it to the p-n junction at the middle of the diode; 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. With minimal defects introduced from device making, the underlying reason for a diminished 2D device performance was finally above surface.
In a concerted effort to confirm the low doping resulting in higher photocurrent, the team built a high resolution microscopic photocurrent mapping apparatus for more direct visual representations.
Figure 4. Photocurrent mapping of diodes with low (left) and high (right) charge density.
A side to side comparison of the photocurrent mapping images shows that there is a greater region of photocurrent indicated by the red glow at low doping (-0.4V vs. -4.0V), whereas at high doping it is generated exclusively around the center area of the p-n junction (Figure 4). This verified both qualitatively and quantitatively the team’s interpretation and results.
“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, and 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. They provide important guidance for pushing the performance limit of 2D electronic devices.”
Now excitons, how does it sound that we meet in the middle?
Science writer Zhuoying Lin, zylin@g.ucla.edu, is a first year chemistry graduate student in the UCLA Department of Chemistry & Biochemistry She wishes to thank Drs. Atallah and Chen for their assistance with this article.