【Domestic News】A Research Group of Wuhan University Publish A Review Paper on Wide Bandgap Semiconductor Thermoreflectance

  Casmita News: Recently, the Research Group led by Yuan Chao from the Institute of Technology Sciences, Wuhan University has published an article entitled “A review of thermoreflectance techniques for characterizing wide band gap semiconductors ‘thermal properties and devices’ temperatures” in Journal of Applied Physics, an international authoritative journal, in which they discussed the application of thermoreflectance techniques in wide band gap semiconductor materials and devices.

  As wide band gap and ultra-wide band gap semiconductor devices have increasing power, the heat dissipation of devices is becoming a huge challenge in the industrial community. The thermophysical properties of semiconductor materials are the most direct parameters reflecting the heat dissipation capacity of devices, and the junction temperature of devices is a key parameter to evaluate thermal reliability and service life. Therefore, the detection of thermophysical properties and junction temperature has become essential to the research and development and production of wide bandgap semiconductor devices.Wide bandgap semiconductor devices are usually of thin film heterostructures, and the film size ranges from tens of nanometers to several microns (as shown in Figure 1). Therefore, nano-micron resolution is a must for the thermophysical properties detection technology. Traditional detection methods, such as steady-state hot plate method, transient hot wire method and laser flash method, cannot meet the resolution requirement. Although the 3-omega method meets the requirement, complex micro-machining is needed on the material surface, so that the testing process is too complicated and the material surface must be of high quality. On the other hand, wide bandgap semiconductor devices are of so small channel size (sub-micron) and often operate at high frequency (GHz) that high spatial resolution and high time resolution are essential to the junction temperature detection method.

Figure 1: Several typical structures of wide band gap devices: (a) GaN High Electron Mobility Transistor (GaN HEMT); (b) Gallium Oxide Field Effect Transistor (β-Ga₂O₃ FET). The above typical structures indicate the presence of numerous micro-nano structures and heterogeneous interfaces in the devices.

  Recent decades have witnessed that based on the principle of thermoreflectance, a variety of pump-probe thermorefletance techniques have been developed in the world, realizing the nano-micron resolution detection capability and widely used in the thermal property detection of wide bandgap semiconductor materials. Based on the same principle, a transient thermoreflectance imaging technique has been developed at the same time in the world, realizing the temperature measurement ability of nanosecond time resolution and nanometer spatial resolution and also widely used in the steady and transient junction temperature detection of wide band gap semiconductor devices.This paper primarily introduces the phenomenon and principles of thermoreflectance, and on this basis, summarizes and discusses various pump-probe thermoreflectance techniques, including time-domain thermoreflectance method and frequency-domain thermoreflectance method, transient thermoreflectance method and steady-state thermoreflectance method.It also summarizes how these methods are applied to the detection of common wide bandgap semiconductor materials, including GaN-based structure, β-Ga₂O₃-based structure, diamond film, alloy materials (such as scandium-doped aluminum nitride ScAlN and aluminum-doped gallium nitride AlGaN) and wide bandgap two-dimensional materials (such as hexagonal boron nitride h-BN), and presented the reported values of thermal properties of all materials (refer to Figure 2 for some results and refer to the paper for the detailed results).

  This paper lays also great emphasis on the characteristics comparison between different pump-detection thermoreflectance techniques. Among all the methods, the time domain thermoreflectance method is the earliest one, and it is widely adopted at present. Frequency domain thermoreflectance method and transient thermoreflectance method are gradually recognized and widely adopted for they have similar resolution and detection accuracy to the time domain thermoreflectance method. Attention should be paid to the transient thermoreflectance method (as shown in Figure 3). Compared with the time-domain thermoreflectance method, this method has greatly reduced the construction cost, and features faster detection and analysis and simpler procedures. Therefore, it has great potential in semiconductor production lines. In addition, this paper also summarizes and discusses the thermoreflectance imaging technique and its application in temperature measurement of wide bandgap devices.

Figure 2: Reported Thermal Conductivity of Gallium Nitride Thin Films; The reported thermophysical properties (thermal conductivity and interfacial thermal resistance) of gallium nitride heterostructure, gallium oxide heterostructure, diamond film and wide band gap alloy materials are also presented in details.

Figure 3: Schematic Diagram of Traditional Transient Thermoreflectance (TTR) System.

  Conventional pump-probe thermoreflectance and thermoreflectance imaging techniques rely metal films to detect. The pump-detection thermoreflectance technique requires a layer of thin metal film (such as gold and aluminum) on the material surface before detection, which may damage the material. Therefore, it belongs to destructive detection. For the thermoreflectance imaging technique, temperature detection is concentrated in the metal electrodes of devices instead of the device channel. As a result, the real device junction temperature is often underestimated.This paper has introduced the improvements that some scholars (including Yuan Chao) have made in traditional pump-probe thermoreflectance techniques in recent years, and has developed the transducer-less pump-probe thermoreflectance technique without metal coating, so as to realize nondestructive testing of gallium nitride epitaxy, silicon and other materials, to provide rapid feedback for material research and development, and to improve research and development and production efficiency and reduce costs. It is expected to provide real-time monitoring for semiconductor production lines, so as to make it possible “to produce while monitoring and regulating”. In addition, it has introduced the direct junction temperature measurement technique of thermoreflectance channel and its application in GaN HEMTs devices.

Figure 4: Schematic Diagram of Transient Thermoreflectance Method (TTR) System Without Metal Coating

Details of the paper: The first author and corresponding author is Professor Chao Yuan, and the co-authors are Dr. Riley Hanus from Georgia Institute of Technology and Professor Samuel Graham from the University of Maryland.

Brief Introduction to the Corresponding Author

  Researcher Yuan Chao has been engaged in thermal characterization and thermal management of wide band gap semiconductors for a long time. He has joined in wide band gap research teams of famous universities in Britain and America for scientific research. He owns technical advantages and scientific research characteristics in the fields of thin film scale thermoreflectance characterization method, phonon heat transport theory, and (ultra-) wide band gap semiconductor device design, and is committed to developing semiconductor nondestructive thermal detection equipment. At present, he is undertaking several scientific research projects for major strategic needs at the national/provincial/international levels, and has published many papers in influential journals (including Materials Today Physics, Communications Physics, APPL. Phys.Lett., etc.). In addition, he has established long-term cooperation with well-known semiconductor integrated circuit enterprises and institutions at home and abroad. Homepage of the research group.

http://jszy.whu.edu.cn/yuanchao