Researchers have discovered how electric charges move across the boundary of two distinct semiconductor materials. By employing scanning ultrafast electron microscopy (SUEM), the research team has captured this brief phenomenon for the first time.
Researchers at UC Santa Barbara have successfully created the first “movie” depicting electric charges as they travel across the interface of two different semiconductor materials. They utilized scanning ultrafast electron (SUEM) techniques from the Bolin Liao lab to directly visualize this fleeting process.
“Numerous textbooks delve into this process based on semiconductor theory,” mentioned Liao, an associate professor in mechanical engineering. “While there are many indirect measurements, being able to see how this really occurs will enable scientists working with semiconductor materials to validate some of these theories and indirect observations,” he added.
This research appears in the Proceedings of the National Academy of Sciences.
‘Hot’ photocarriers
If you’ve ever utilized a solar cell, you’re familiar with the concept of photocarriers: sunlight strikes a semiconductor material, energizing the electrons within, leading to their movement. This movement, along with the separation from their positively charged counterparts called ‘holes,’ generates a current that can be used to power various electronic devices.
However, these photocarriers tend to lose most of their energy within picoseconds (trillionths of a second), meaning that the energy that conventional solar cells can harvest represents just a small portion of those carriers’ initial “hot” energy before they cool down and dissipate the majority of their excess energy as waste heat. Although their hot state has substantial potential for enhancing energy efficiency, it simultaneously poses challenges for the semiconductor material, such as heat that could impact device performance.
Consequently, understanding the behavior of these hot carriers as they traverse different semiconductor materials is crucial, particularly when they cross the interface of two dissimilar materials—known as a heterojunction. In semiconductor technology, heterojunctions play a vital role in influencing charge carrier movement, which is essential for applications ranging from lasers to solar cells, sensors, and photocatalysis.
To illustrate these hot carriers, Liao and his colleagues directed their SUEM at a silicon and germanium heterojunction, created by collaborators at UCLA—this combination of common semiconductor materials shows promise in fields like photovoltaics and telecommunications.
“Our objective is to add temporal resolution to electron microscopy,” Liao stated.
The key aspect of their imaging method is utilizing ultrafast laser pulses as a shutter operating on a picosecond scale while firing an electron beam to scan the surfaces where these hot photocarriers travel, triggered by an optical pump beam. “The events we are discussing occur within the picosecond to nanosecond time frame,” Liao explained.
“What’s particularly thrilling about this research is that we could witness how the charges actually transfer across the junction once generated,” he continued. The resulting images illustrate the diffusion of these photocarriers from one semiconductor material to another.
“When charges are excited in the uniform regions of silicon or germanium, the hot carriers display very high initial speeds due to their elevated temperature,” Liao clarified. “However, if a charge is excited near the junction, some of the carriers become trapped by the junction potential, which causes a slowdown in their movement.” The trapping of hot charges can reduce carrier mobility, potentially hindering the performance of devices designed to separate and collect these charges.
The phenomenon of charge trapping in Si/Ge heterojunctions aligns with semiconductor theory but observing it directly was a striking discovery, Liao noted. “We were surprised to be able to image this effect directly,” he said, adding that this finding may be relevant for semiconductor device designers. “This paper primarily showcases the potential of SUEM to investigate realistic devices.”
This newfound capability to visualize hot photocarrier dynamics at heterojunctions represents a significant advancement in semiconductor research at UC Santa Barbara. The foundational concept was introduced by the late UCSB engineering professor Herb Kroemer, who, in 1957, first proposed the idea of heterostructures in semiconductors, famously stating, “the interface is the device.” This principle laid the groundwork for today’s microchips, computers, and information technology. Kroemer was awarded the 2000 Nobel Prize in Physics for developing semiconductor heterostructures utilized in high-speed and opto-electronics.
