Researchers Enhance Non-Destructive Carrier Lifetime Measurements in SiCs

Transistors composed of Silicon Carbide (SiC) are very attractive for a myriad of applications in the electronics industry. Thanks to its exceptional semiconductor behavior, it has found applications in LEDs and high-voltage devices with low power loss, while operating under 3.3 kV. Engineers are looking for SiCs that can operate at a larger scale, 10kV and up, but in order to do so, one must break their sample, that is, until now.

All of the current SiC devices in production at the moment are unipolar, which is what many researchers believe to be the limiting factor from utilizing them for 10kV and higher applications, such as power generation and distribution systems. Bipolar SiCs are the solution to this bottleneck, as they offer low on-resistance through conductivity modulation. This conductivity modulation effect has a strong direct correlation to charge carrier lifetimes within the SiC, but carrier lifetimes also have a similar relationship to switching losses. Because of this, one must precisely balance the conductivity modulation effect and switching loss with one another, requiring precise measurements of carrier lifetimes.  However, in order to accurately measure this attribute, one must destroy the sample, until now according to a news article from the Nagoya Institute of Technology.

A research team led by Associate Professor Masashi Kato from the Nagoya Institute of Technology has improved one of the known non-destructive carrier lifetime measurement techniques, called Time-Resolved Free-Carrier Absorption with Intersectional Lights (IL-TRFCA). This method consists of an excitation laser and a probe laser plus a detector. The excitation laser created photoexcited carriers and converges on the sample with the probe laser at opposite angles of incidence. The probe laser plus detector is used to obtain the carrier lifetime data, and both laser move in micron-sized steps where the lasers converge now at a point within the sample, giving carrier lifetime data all throughout the surface and within the sample, requiring zero disassembly. The process is shown pictorially in Figure 1 below.

Image Courtesy of Nagola Institute of Technology 

In the above Figure, Professor Kato’s research team made two novel alterations, the adoption of a larger incidence angle of 34o for both lasers, as well as a higher numerical aperture in the objective lens and detector. This larger aperture would make one think that less light is absorbed and thus it is less effective, but as seen in the image above, the detector must be able to focus laser beams that are far apart from one another (even farther if the incidence angle is increased) so it allows for this change to be made without going out of the focusing range of the lens. These changes in conjunction with each other result in an enhanced depth resolution of approximately 10um while being able to measure distributions at a depth of up to 250um according to the abstract of the group’s published research paper.

Dr. Kato talks about his excitement for these advancements, as he comments on the importance of being able to implement SiC technology in power generation systems, "SiC devices can operate with lower power consumption compared with conventional semiconductors, and their commercialization could result in a substantial reduction in energy consumption in power systems throughout the world. In turn, this could alleviate serious environmental threats such as the accumulation of greenhouse gases."

It is interesting to see how currently SiC technology is being limited to our ability to effectively test these devices, not because of design issues, something that most believe is usually the issue. The bipolar SiC testing conundrum truly illustrates how innovative testing systems must be in order to fulfill today’s requirements, a challenge that Dr. Kato and his research group try to solve every day.