【Domestic News】The Institute of Physics and Chemistry of the Chinese Academy of Sciences has Made a Series of Progress in the Third and Fourth Generation Semiconductor Research of Liquid Metal Direct Printing

  For nearly a century, semiconductors have been playing an extremely important role in promoting modern science, technology and social progress by virtue of their performance advantages and industry-leading role. Up to now, the semiconductor industry is mainly driven by four generations of materials. The first generation, represented by silicon and germanium, began in the 1950s; The second generation, mainly composed of gallium arsenide (GaAs) and indium phosphide (InP), appeared in the 1980s; The third generation is mainly gallium nitride (GaN) and silicon carbide (SiC), which can be traced back to the end of the 20th century; The fourth generation is represented by the rising gallium oxide (Ga₂O₃).

  As the manufacturing industry with the most intensive capital, manpower and technology, the semiconductor industry has so far relied heavily on high temperature treatment, harsh vacuum conditions and water and electricity resources for almost all the classic semiconductor growth technologies, such as molecular beam epitaxy (MBE), pulsed laser deposition (PLD), metal-organic chemical vapor deposition (MOCVD) and atomic layer deposition (ALD). A chip manufacturing process often involves dozens of complex processes, from the initial wafer production to the cutting of the production line and even the final packaging, inspection and testing, any error in the process will lead to the final wafer scrap and subsequent huge losses.

  At present, with the rapid development of information technology, there is an unprecedented demand for high power and high frequency microelectronic devices. Wide band-gap semiconductor materials with inherent performance advantages stand out, represented by gallium nitride (GaN) and silicon carbide (SiC), which are making great achievements in the field of power devices and radio frequency devices by virtue of their significant advantages in reducing energy consumption in power transmission, and become the research focus of the global semi-conductor industry. The emergence of gallium oxide as a new ultra-wide band gap semiconductor material has also brought a new trend. The power devices made from it have the characteristics of high withstand voltage, low loss, high efficiency, small size, etc., and have a wide range of applications. Because of these factors, the third and fourth generation semiconductor technology has become the international hotpot and the commanding point of technological competition among major countries.

  The traditional manufacturing of Ga₂O₃ is usually subject to high equipment cost, complex precursor configuration and low growth rate, which makes it a great challenge to grow Ga₂O₃ materials at large scale, low cost and high efficiency. For semiconductors, in order to realize large-scale applications, the common existence of p-type and n-type is generally required to form p-n junction to manufacture MOS, IGBT and other functional devices. However, at present, Ga₂O₃ only has n-type materials, while p-type Ga₂O₃ materials are extremely deficient, which limits the realization of high-power devices with higher power, stronger heat dissipation and better stability. In response to this situation, the Research Center for Liquid Metals and Cryogenic Biomedical Sciences of the Institute of Physics and Chemistry of the Chinese Academy of Sciences put forward a new solution, and established the one-step low-temperature large-area printing technology of n-type and p-type doped Ga₂O₃ for the first time (Figure 1) and achieved success.

Fig. 1 Principle of n-Ga₂O₃ and p-Ga₂O₃ printing process based on liquid alloy GaInSn and GaInSnCu

  Different from traditional methods, in this new process, the synthesis and doping of semiconductors occur at the same time, which saves the complexity of multi-step doping process, and the whole process is completed at a lower temperature. The team successfully obtained large size and high-quality Cu-doped p-type Ga₂O₃ from liquid metal GaInSnCu alloy surface melt using this process. The field effect transistor (FET) based on the printed p-Ga₂O₃ film exhibits excellent electrical properties, p-conductivity and environmental stability (Fig. 2).

Fig. 2 Electrical performance of fully printed Cu-doped p-type Ga₂O₃ transistor

  In addition, the plasma treatment of printed p-type and n-type Ga₂O₃ further improved the electron concentration on the surface of Ga₂O₃, and successfully realized the good ohmic contact of p-type and n-type Ga₂O₃ without the traditional additional high-temperature annealing treatment. The research shows for the first time the high-performance fully printed Ga₂O₃ p-n homojunction diode (Figure 3), and confirms that it has excellent rectification effect. This revolutionary low-temperature one-step synthesis and doped Ga₂O₃ semiconductor film printing process provides a convenient, efficient and low-cost new strategy for the preparation of Ga₂O₃ semiconductor materials and Ga₂O₃-based electronic functional devices with large size, high quality and controllable scale.

Figure 3 Performance of printed Ga₂O₃ p-n homojunction diode

  The corresponding work has been was published recently, entitled Liquid metal gallium-based printing of Cu-doped p-type Ga₂O₃ semiconductor and Ga₂O₃ homojunction diodes, on Applied Physics Reviews and was selected as the featured article of the journal. The co-first author of the article was Li Qian, an assistant researcher of the Institute of Physics and Chemistry, and Du Ponten, a postdoctor. The corresponding author was Liu Jing.

  Previously, the group also established a limited nitridation reaction principle for printing wide band gap ultra-thin GaN semiconductors at room temperature in a large size, which is different from the traditional high-temperature manufacturing strategy of the third generation semiconductors. It has a wide range of adaptability, and can print GaN two-dimensional thin film materials with thickness ranging from 1 nm to more than 20 nm, and then build transistors.

Figure 4 The inside cover story of the article, and the principle and equipment of the localized nitridation reaction of GaN semiconductors manufactured at room temperature

  As the first experiment in the field, the research team made it possible to print the limited nitridation reaction of liquid metal gallium at room temperature by introducing plasma mediation (Fig. 4), and realized the large-size printing of semiconductor film with the thinnest thickness of 1 nm. The authors defined the new chemical reaction of GaN growth based on this technical path as:

  Just like the iron rule in nature, nitrogen has always been regarded as a classical inert gas. Even at high temperatures, it cannot react directly with gallium. The new discovery has changed the traditional understanding of the academic community, making nitridation reaction of gallium under the room temperature a reality, and realizing the direct printing of transistors with excellent performance without the need for complex precursor configuration and expensive equipment (Figure 5).

  Traditionally, the classical methods for preparing GaN thin films usually require extremely high temperatures, such as MOCVD (about 950 ° C-1050 ° C) and ALD (>250 ° C). At the same time, toxic substances are often unavoidable. There are huge technical challenges to achieve GaN semiconductor thin films with a thickness of 1 nm. The emergence of technology of printing GaN film under room temperature has greatly saved the preparation cost and energy consumption of the third generation semiconductor process, which means that the semiconductor process is expected to usher in a new beginning, which is also of great practical significance for energy conservation and emission reduction in the semiconductor manufacturing industry（Liquid metal printing opening the way for energy conservation in semiconductor manufacturing industry, Frontiers in Energy, 2022）。

Fig. 5 Electrical performance of printed GaN field effect transistor

  The team of the Institute of Physics and Chemistry has been working in the field of liquid metal material science for a long time since the beginning of this century, and has been the first to put forward the concept and technical approach of liquid metal printed electronics and printed semiconductors at home and abroad （Direct writing of electronics based on alloy and metal ink (DREAM Ink): A newly emerging area and its impact on energy, environment and health sciences, Frontiers in Energy, vol.6(4), pp. 311-340, 2012）The corresponding technology is also named DREAM Ink (also known as Dream Ink technology)

  The basic technical route proposed by the research team to manufacture semiconductors such as Ga₂O₃, GaN, In₂O₃, SnO and even more derived materials, diodes, transistors, functional devices and integrated circuits based on liquid metal printing and subsequent processing (such as oxidation, nitridation, ion implantation and more chemical modifications, as well as laser, microwave or plasma assistance) has been confirmed one by one in recent years, The liquid metal electronic manufacturing equipment and electronic material products developed by the research team have also gradually realized large-scale industrial application and popularization.

  In general, the emerging liquid metal printed electronics and semiconductor technology has opened up a new way for the rapid prototyping of the next generation of electronic devices, integrated circuits and even user-side chips, and is expected to inject new vitality into the semiconductor manufacturing industry. In the future, with the continuous expansion of the family of liquid metal printed semiconductor materials and the improvement and enrichment of corresponding manufacturing methods, it will promote the iteration and progress of a new generation of low-cost and energy-saving green manufacturing technology, electronic display, integrated circuit, chip manufacturing, photovoltaic power generation, power devices and new energy vehicles, which is expected to bring important industrial changes.