GaAs/AlGaAs Chip “Sees” THz Waves

A semiconductor device was fabricated on GaAs/AlGaAs substrate for imaging at THz wavelengths.

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Terahertz (THz) radiation continues to exhibit its value and versatility in medical applications, such as focused treatment of malignant cells. THz techniques are also showing great promise in the area of materials research at the subwavelength electromagnetic (EM) radiation regions, with resolution on the nanometer scale.

In fact, a researcher from the Tokyo Institute of Technology revealed work on cryogenic THz-emission near-field imaging for close scrutiny of semiconductor devices. The research has provided invaluable information on the spatial distribution of electrons injected from an electrode into a semiconductor channel.

The new approach is a departure from existing aperture- and apertureless-type THz imaging methods. The new THz imaging method performs collection of visualization data by scanning an evanescently coupled THz detector closely across a sample surface, compared to existing methods that work in illumination mode. Based on a GaAs/AlGaAs heterostructure chip, the new approach is simpler and more familiar for most users.

The GaAs chip’s aperture and near-field probe are insulated by a 50-nm-thick silicon-dioxide (SiO2) layer. It has a two-dimensional electron gas-layer heterointerface; this layer features electron mobility of 5000 cm2/V-s. Located 60 nm below the surface of the chip, this two-dimensional gas layer acts as a THz detector. Source and drain electrodes extend to the side surfaces of the chip for electrical connections. Detected THz signals are provided as changes in voltage as a function of THz irradiation by the two-dimensional electron gas detector.

The performance of the THz detector was simulated by means of finite-element (FE) simulation software. Devices were compared with aperture alone and aperture with integrated probe, using the same aperture dimensions in both cases. Testing and simulations revealed that chips without probes failed to detect THz signals, while those with probes successfully detected THz radiation, regardless of wavelength. The spatial resolution of the near-field THz imager was found to be 9 µm. Work continues on this THz near-field microscope component, which could prove invaluable for research at the molecular level on materials and biological subjects. 

See “Chip-Based Near-Field Terahertz Microscopy,” IEEE Transactions on Terahertz Science and Technology,” Vol. 6, No. 3, May 2016, p. 356.

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