Newswise — The group led by Professor Naoya Shibata of the University of Tokyo, in collaboration with Sony Group Corporation, succeeded in directly observing a two-dimensional electron gas(1) that accumulated at the semiconductor interface.

GaN-based devices are used as highly efficient light-emitting diodes (LEDs) and laser diodes (LDs). Because of their high dielectric breakdown strength and saturated electron velocity, these are expected to be used as next-generation high-frequency devices for communication and as power devices for power conversion(2). In particular, high-electron-mobility transistors (HEMTs) generate a layer of accumulated electrons called a two-dimensional electron gas at the semiconductor interface. Electrons can move at high speeds in this layer, which makes the HEMTs excellent for high-frequency operation. The details of this two-dimensional electron gas are crucial for the performance of semiconductor devices, and they have been estimated via indirect experiments or theoretical calculations. However, direct observation and confirmation of these phenomena are challenging.

In this study, the research group combined magnetic-field-free atomic resolution STEM (MARS)(3) with an newly-developed tilt-scan system(4) and ultrahigh-sensitivity, high-speed division-type detector to directly observe the two-dimensional electron gas that accumulated at the gallium nitride/aluminum indium nitride (GaN/AlInN) heterointerface(5). The group conducted observations using the atomic resolution differential phase contrast (DPC) method(6), an atomic-level electromagnetic field observation technique developed by Professor Shibata and others. They successfully visualized and quantified the two-dimensional electron gas that accumulated in the several-nanometer-wide area of the semiconductor interface. These advances enabled control of the two-dimensional electron gas and are expected to further improve the performance of transistors.

The findings of this study have enabled the development of high-performance high-frequency/power devices that control two-dimensional electron gas, bringing about innovations in interface analysis and control of semiconductor devices. This has also led to groundbreaking measurement technologies that can significantly advance the research and development of cutting-edge materials and devices.

(1) Two-dimensional electron gas

There is a state in which electrons are distributed two-dimensionally in a localized area of a semiconductor. Free electrons with high mobility are spread in an extremely thin layer and are used in high-electron-mobility transistors (HEMTs) and other devices.

(2) Power device

Power devices and power semiconductors are devices and semiconductors that handle high voltages and currents. These are used for power conversions and control, such as high voltage and large current, as well as for power generation, such as motor drives.

(3) Magnetic-field-free Atomic Resolution STEM (MARS)

An electron microscope was developed by this research team in 2019, which can perform measurements in a magnetic field-free environment. For information, view the following press release. “An innovative electron microscope overturning common knowledge of 88 years history” (May 24, 2019 Press Release)

(4) Tilt-scan system

In a typical STEM, an electron probe (thin electron beam that irradiates a sample) is always incident parallel to the optical axis for observation. However, the tilt-scan system is a new scanning method in which the electron probe is intentionally tilted in multiple directions, and where the DPC images are averaged.

(5) Heterointerface

This is the interface between different materials. Typically, it exists in a lattice-matched system or a material system with a similar lattice constant.

(6) Differential phase contrast (DPC) method

When an electromagnetic field exists inside a sample, the electron beam incident on the sample receives a force and slightly changes its trajectory. The DPC method is a technique for detecting this change with a division-type detector and measuring the electromagnetic field at each point on the sample. The resolution is generally determined by the size of the electron probe. Hence, in principle, this method can make electromagnetic field observations at an atomic resolution.

This study was supported by JST under following programs. ERATO program: SHIBATA Ultra-atomic Resolution Electron Microscopy. PRESTO program: Research Area "Precise Arrangement of Atoms and Molecules and Its Properties and Functions", Research Project “Development of Ultra-Low Electron Dose STEM Technique and Structural Analysis of Atomic and Molecular Arrangements in Real Space”

Journal Link: Nature Nanotechnology