Tohoku University, Sumitomo Electric Industries (Sumitomo Electric Industries), and Japan Synchrotron Radiation Research Center (JASRI) announced on October 29 to solve the “current collapse phenomenon” that is the problem of the high-speed transistor “GaN-HEMT”. We have developed an “operator nanospatio-temporal decomposition X-ray absorption spectroscope” with a high-precision spatiotemporal resolution of 1 trillionth of a second of the electronic state under device operating conditions and 100 nm, and GaN under operating conditions. -Announced that it succeeded in direct observation of HEMT.

As a result, the spatiotemporal behavior of “surface electron capture”, which is one of the factors that determine the characteristics of GaN-HEMT, was elucidated for the first time, and in addition to elucidating the detailed mechanism, the mechanism of improving the characteristics by the surface protective film Was announced at the same time as it was revealed.

This result is a result of industry-government-academia collaborative research between the research team of Associate Professor Hirokazu Fukidome of the Research Institute of Electrical Communication, Tohoku University, and Sumitomo Kiko, a high-intensity optical science research center that operates the large-scale radiation facility SPring-8. thing. Details were published in “Applied Physics Letters” published by the American Institute of Physics.

GaN-HEMT, a transistor that uses the interface between GaN and AlGaN, which is a mixed crystal of AlN and GaN, as an electron transport layer, is a promising ultra that operates in the X band (several GHz) and millimeter wave band (several tens of GHz). It is a transistor for high-speed communication.

At the interface between GaN and AlGaN grown on it, electrons are two-dimensionally confined and are characterized by high-speed operation. Furthermore, GaN has a large bandgap (energy band in which electrons cannot exist), so it is possible to increase the output. Since GaN-HEMT has such excellent characteristics, it is expected as one of the key devices for next-generation communication technology.

Sumitomo Electric has put GaN-HEMT into practical use and currently boasts the largest market share in the world. The company is currently conducting joint research with Tohoku University on electronic devices that use two-dimensional electronic systems such as GaN-HEMT and graphene, which is a two-dimensional crystal of carbon. Although GaN-HEMTs have been put into practical use, they need to be further improved in performance, and it is necessary to solve problems that have not yet been sufficiently solved. The biggest issue is the “current collapse phenomenon”.

The current collapse phenomenon is a phenomenon in which the output current fluctuates or decreases with time when a high output operation is attempted. One of the major causes of the current collapse phenomenon is electron capture on the surface of the device. Surface electron capture is not only industrially important, but also a core research topic in surface physics that emerged during the invention of the transistor.

It is difficult to elucidate the mechanism of surface electron capture involved in the current collapse phenomenon of GaN-HEMT because the macroscopic electrical measurement evaluation method has been used to elucidate the operating mechanism of semiconductor devices. .. Local information cannot be obtained by this evaluation method. In order to elucidate the mechanism of surface electron capture of GaN-HEMT this time, it is necessary to obtain local information.

Then, the arrow of white feathers stood out in the “operand nano X-ray absorption spectroscopic device” installed in the “soft X-ray solid-state spectroscopy beamline” installed in the large synchrotron radiation facility SPring-8. Operand nano X-ray absorption spectroscopy is a spectroscopy performed under device operation by giving X-ray absorption spectroscopy a high microscopic function of 100 nm or less using a photoemission electron microscope (PEEM). This method is most suitable for observing the electronic and chemical states of the device surface.

In fact, the device has the ability to observe the surface state of a moving device with high spatial resolution. That is the spatial resolution. So far, we have a track record of successfully observing the microscopic electronic state of graphene transistors and GaN-HEMTs to which voltage is applied.

However, this device does not have sufficient time resolution, and so far it has been limited to measurement only under steady-state voltage, and has a problem that measurement under actual high-frequency operation is difficult. In order to elucidate the mechanism of surface electron capture of GaN-HEMT, it was necessary to improve the time resolution.

Therefore, this time, the joint research team has started research using “operand nano spatiotemporal decomposition X-ray absorption spectroscopy” that enhances the time resolution of the device. Then, a system utilizing the pulse property of synchrotron radiation X-rays was constructed, and the operand nano X-ray absorption spectroscope was equipped with a high temporal resolution in addition to a high spatial resolution. Was successfully developed.

The maximum spatial resolution of the device is 100 nm or less, which is the same as before, but the maximum time resolution is 100 ps or less. Another major feature of this device is that it can observe the electronic state under voltage application, and has added functions and improved performance, such as being able to evaluate electrical characteristics at the same time as spectroscopic measurement.

  • Operand nano spatiotemporal decomposition X-ray absorption spectrometer

    Outline of measurement of operand nano spatiotemporal decomposition X-ray absorption spectroscopy developed this time (Source: Tohoku University press release PDF)

From the experimental results on the surface electron capture of GaN-HEMT using this device, it was found that the measurement spectrum was weakened only in the vicinity of the gate electrode immediately after the applied voltage was turned off.

  • Operand nano spatiotemporal decomposition X-ray absorption spectrometer

    Outline of the results of this operand nano-spatiotemporal decomposition X-ray absorption spectroscopy experiment. (a) Observation sample image. (b) Position dependence of the spectrum immediately before the applied voltage is cut off. Dependence of each spectrum after (c) 1 μs, (d) 27 μs, and (e) 34 μs. It can be seen that only the spectrum of Region II (drain side near the gate electrode) in (c) is weakened (Source: Tohoku University Press Release PDF).

The strength of this spectrum is determined by the degree of covalent bonding of surface Ga atoms. What can be said from the weakening of this spectrum is “Ga.+ + e– – It means that an electrochemical reaction such as “= Ga” occurs only in the vicinity of the gate electrode immediately after the applied voltage is turned off.

From the results of this observation, the collaborative research team has succeeded in proposing a new mechanism that explains the spatiotemporal behavior of surface electron capture. The content is that surface electron capture occurs only near the gate electrode immediately after the voltage is applied, and electrons hopping away from the gate electrode over time.

  • Operand nano spatiotemporal decomposition X-ray absorption spectrometer

    Mechanism for explaining the spatiotemporal behavior of surface electron capture devised from the results of this research (Source: Tohoku University Press Release PDF)

It is difficult to propose a mechanism to explain such spatiotemporal behavior only by observation using conventional electrical methods. It is said that this mechanism can explain the difference between the current collapse phenomenon under DC voltage and high frequency voltage well, and it is useful for designing for stable operation under high frequency operation conditions.

Based on the results obtained this time, the joint research team is currently conducting research on device modeling that takes into account the current collapse phenomenon. He said that he is developing a method for modeling a black box-like model based on solid-state physics with the help of machine learning.

By making it a white box, it will be possible to translate what was previously called tacit knowledge or know-how possessed by device developers into explicit knowledge, that is, verbalization. This verbalization will allow device developers and physical property researchers to discuss on the same table, and is expected to foster the soil for new devices.