Finding Invisible Hot Spots with Molybdenum Disulfide Films

High power electronic devices are extremely important in electric vehicles and for renewable energy conversion. These devices must be constructed from specific materials that won’t breakdown if you place a large voltage across or large amount of current through the device (most common electronics would burn out instantly at high power). A subset of materials called ultrawide band gap (UWBG) semiconductors are often used in power electronics devices. However, even these UWBG materials can have limitations – heat doesn’t flow easily through the materials so they develop “hot spots” that can eventually cause the device to fail. We know that the hot spots occur, but we don’t always have the right tools to find out where along the device they tend to occur. This is because high-resolution thermal imaging techniques often use light, and the ultrawide band gap material doesn’t absorb the typical types of light used (the energy of the light is much smaller than the energy of the band gap).

To circumvent this problem, we coated the UWBG device with a material that does absorb the light. First, we deposited a film of molybdenum disulfide (MoS₂) on top of UWBG materials gallium nitride (GaN) and aluminum gallium nitride (AlGaN). Then, we turned on the device. MoS₂ absorbs the heat from the layer below, developing a similar thermal profile as the UWBG device. Finally, we used a technique called thermoreflectance imaging (TTI) to scan a green laser across the device. In TTI, a light detector measures how much green light is reflected from the MoS₂ surface. Because MoS₂ reflects a different amount of light at different temperatures, we are able to visualize a temperature change across the surface of the device. Thus, the combination of the MoS₂ coating with TTI imaging allows us to identify the exact location of the hot spot in the UWBG device.

Check out the research article:
R.C. Hanus, S.V. Rangnekar, E. Heller, M.C. Hersam, A. Kahn, S. Graham. “Thermoreflectance imaging of (ultra)wide bandgap devices with MoS₂ enhancement coatings.” ACS Applied Materials and Interfaces 13, 42195–42204 (2021).