(Nanowerk Information) UC Santa Barbara researchers have achieved the first-ever “movie” of electrical prices touring throughout the interface of two completely different semiconductor supplies. Utilizing scanning ultrafast electron (SUEM) methods developed within the Bolin Liao lab, the analysis crew has immediately visualized the fleeting phenomenon for the primary time.
“There are a lot of textbooks written about this process from semiconductor theory,” stated Liao, an affiliate professor of mechanical engineering. “There are a lot of indirect measurements.” The flexibility to visualise how this course of really occurs will permit semiconductor supplies scientists to benchmark a few of these theories and oblique measurements, he added.
The analysis is printed within the Proceedings of the Nationwide Academy of Sciences (“Imaging hot photocarrier transfer across a semiconductor heterojunction with ultrafast electron microscopy”).
The ultrafast scanning electron microscope within the Bolin Liao lab. (Picture: Matt Perko)
‘Hot’ photocarriers
For those who’ve ever used a photo voltaic cell, you’ve seen photocarriers in motion: daylight hits a semiconductor materials, thrilling electrons within the materials, which transfer. This motion of electrons, and separation from their opposite-charged ‘holes,’ creates a present that may be harnessed to energy digital gadgets.
Nonetheless, these photocarriers lose most of their vitality inside picoseconds (trillionths of a second) in order that the vitality that standard photovoltaics harvests is however a fraction of the vitality these carriers have of their “hot” state, earlier than they settle down and launch many of the extra vitality as waste warmth. Whereas their scorching state holds a whole lot of potential for issues like vitality effectivity, it additionally presents challenges inside the semiconductor materials, comparable to the warmth that will have an effect on system efficiency.
In consequence, it’s essential to get a good suggestion of how these scorching carriers behave as they transfer by means of completely different semiconductor supplies, and specifically how they transfer throughout the interface of two completely different supplies — the heterojunction. Within the realm of semiconductor supplies, heterojunctions affect the motion of cost carriers for quite a lot of functions, from the creation of lasers to photovoltaics to sensors to photocatalysis.
To visualise these scorching carriers, Liao and his group targeted their SUEM on a heterojunction of silicon and germanium fabricated by collaborators at UCLA, a mix of frequent semiconductor supplies that holds promise in realms comparable to photovoltaics and telecommunications.
“Basically, we’re trying to add time resolution to electron microscopes,” Liao stated.
Key to their imaging approach is their use of ultrafast laser pulses to behave as a picosecond-scale shutter as they hearth an electron beam to scan the floor of the supplies by means of which the new photocarriers journey, excited by an optical pump beam. “What we’re talking about are events happening within this picosecond to nanosecond time window,” Liao stated.
“The really exciting thing about this work is that we were able to visualize how the charges, once generated, actually transfer across the junction,” he continued. The ensuing pictures present these photocarriers as they diffuse from one semiconductor materials to the opposite.
“If you excite charges in the uniform silicon or germanium regions, the hot carriers move very, very fast; they have a very high speed initially because of their high temperature,” Liao defined. “But if you excite a charge near the junction, a fraction of the carriers are actually trapped by the junction potential, which slows them down.” Trapped scorching prices lead to decreased provider mobility, which may negatively have an effect on the efficiency of gadgets that separate and acquire these scorching prices.
This cost trapping in Si/Ge heterojunctions could be understood by semiconductor idea however it was nonetheless hanging to immediately observe it experimentally, famous Liao. “We didn’t expect to be able to image this effect directly,” he stated, including that this phenomenon is likely to be one thing semiconductor system designers might need to tackle. “This paper is really about demonstrating the capability of SUEM to, for example, study realistic devices.”
This new potential to truly see scorching photocarrier exercise at heterojunctions completes a circle in semiconductor analysis at UC Santa Barbara. Pioneered by late UCSB engineering professor Herb Kroemer, who first proposed the function of heterostructures in semiconductors, the idea went on to grow to be the idea of contemporary microchips, computer systems and knowledge know-how. Kroemer acquired the 2000 Nobel Prize in Physics “for developing semiconductor heterostructures used in high-speed and opto-electronics.”
