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edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 2 10 MEASURING TEMPERATURE IN GaN HEMTs: AN APPROACH BASED ON RAMAN SPECTROSCOPY Bertrand Boudart and Yannick Guhel Groupe de Recherche en Informatique, Image, Automatique et Instrumentation de Caen, Normandie Université, UNICAEN, ENSICAEN, CNRS, GREYC, France bertrand.boudart@unicaen.fr EDFAAO (2019) 2:10-14 1537-0755/$19.00 ©ASM International ® INTRODUCTION GaN high electron mobility transistors (HEMTs) are excellent candidates for high frequency and high power applications. [1-2] The reduced size and increased power dissipation of these devices result in increasingly higher operating temperatures. Therefore, their electrical pro- perties and reliability can be degraded by self-heating effects. [3-4] It is crucial to accurately determine the self- heating temperature in these components in order to improve reliability and performance by optimizing their design and thermal management. To do so, it is important to estimate the operating temperature of the semiconduc- tor and metal contacts constituting GaN-based HEMTs by using high spatial and thermal resolution techniques. Optical methods such as infrared (IR) thermography, ther- moreflectance, and Raman spectroscopy are the most commonly used techniques for thermally characterizing RF devices. Moreover, these techniques support tempera- ture mapping of the devices. IR thermography is the most widely used technique for measuring the self-heating temperature of electronic devices and integrated circuits. [5] An IR camerawith a 3-10 µm wavelength detects the thermal radiation emitted from the device surface. As the spatial resolution limit is typically related to the wavelength used, this is clearly a strong limitation for this technique. [6] Using a shorter wavelength than IR [visible or ultra- violet (UV)], thermoreflectance and Raman spectroscopy can obtain a higher spatial resolution than IR thermom- etry. The results will also have better thermal resolu- tion. [7-8] Thermometry based on thermoreflectance con- sists of measuring the relative change in the reflectivity of amaterial surface, which is directly proportional to the temperature variation. The proportionality constant is referred toas the thermoreflectance coefficient. Its value is usually between 10 -2 and 10 -5 K -1 and is strongly dependent on the nature of the analyzedmaterial, wavelength of the excitation source, angle of incidence, surface roughness, and composition of the multilayered structure. [9] The thermoreflectance coefficient is higher for ametal surface than for a GaN-based semiconductor surface. This explains why thermoreflectance is typically used to measure the temperatureonmetal surfaces. Nevertheless, this technique has also been used on GaN surfaces, but the information extracted is not as clear with a visible exci- tation source because the subsurface reflections can con- tribute to the total reflectance signal from the AlGaN/GaN structure. Recently, a way to avoid these problems was demonstrated by using a 320-nm UV laser to probe the GaN surface. [10] With this method, the absorption depth is 80 nm, which is less than the typical GaN layer thickness used in GaN-based devices. DEBUT OF RAMAN SPECTROSCOPY The Raman effect was discovered in 1928 by the Indian physicist Sir C.V. Raman who received the Nobel Prize in 1930. [11] It arises when a photon is incident on a molecule and interactswith the electric dipole of themolecule. This process applies to liquids, gases, and solids. When a beam of light illuminates a solid, photons are absorbed by the material and scattered. The vast major- ity of these scattered photons have exactly the same wavelength as the incident photons. This is referred to as Rayleigh scattering. In this scattering process, the incident photon excites an electron into a higher energy level and the electron decays back to the same level from which it started. In this case, it is an elastic scattering. But a small portion, typically around 1 in 10 6 of the scattered photons, has a different wavelength than the incident