High-Precision THz Time-Domain Ellipsometry Developed for Wide-Gap Semiconductors

Schematic image of terahertz time domain ellipsometry.Credit: Osaka University

In semiconductor device applications, there is an increasing demand for semiconductors with extremely high carrier concentrations. The parameters of the semiconductor material, namely carrier density and mobility, mainly determine the performance of the device. Therefore, it is important to accurately characterize the carrier density and mobility of semiconductors in the development of device applications.

The use of terahertz waves, or electromagnetic radiation with a wavelength of approximately 300 µm and a frequency of approximately 1 THz, is continually expanding in nondestructive testing of semiconductors. Free carriers in the material absorb THz radiation, so THz waves can be used to estimate the electrical properties of semiconductors.

Researchers at Osaka University have collaborated with Nippon Seimitsu Co., Ltd. to develop a terahertz time domain ellipsometry system (TeraEvaluator®) that extends the range of carrier concentrations that can be measured with terahertz waves to a maximum of 10.20 cm-3 Then, it is potentially increased by improving the accuracy of the optical technique. In THz time domain ellipsometry, a linearly polarized THz pulse is incident on a sample and the field strength of the reflected THz wave is measured as a function of time. Specifically, reflected waves polarized in the parallel direction (NS) And vertical (NS) I am interested in the plane of incidence.

Ratio NS– When NS– The polarization component provides information about the permittivity of the sample and allows evaluation of carrier density and mobility. Therefore, unlike THz time domain spectroscopy, THz time domain ellipsometry does not require reference measurements through apertures or standard mirrors.

TeraEvaluator® employs a polarizer-sample rotation analyzer optical configuration that changes the angle direction of the analyzer from 0 ° to 360 ° in 15 ° increments. This multi-angle technique is used to eliminate systematic errors from the amplitude and phase of the THz electric field detected by the newly adopted analysis in THz time domain ellipsometry. This new correction method improves the accuracy of THz time domain ellipsometry by more than 10 times. As a result, the maximum carrier concentration that can be evaluated is more than two orders of magnitude higher than is normally reported using other THz time domain techniques.

As a demonstration, researchers evaluated gallium nitride (GaN) wide-gap semiconductors, one of the most technically important semiconductors in various power electronics as well as 5G devices. GaN is also a leading candidate for future 6G devices operating above Science report Publications show that precision THz time domain ellipsometry systems are effective in assessing carrier densities up to 10.20-Tentwenty one cm-3 With my boss Accuracy Accuracy was a challenge when using THz waves, especially with very high conductivity. The GaN crystals investigated were manufactured using a crystal growth method called the point seed technology using the Na flux method, which was also developed at Osaka University to produce high-quality GaN crystals.

Precision THz time domain ellipsometry systems are expected to be widely useful for accurate and non-invasive characterization of various semiconductors with very high carrier concentrations.

Reference: “Precise terahertz time domain ellipsometry for evaluating GaN crystals with carrier densities up to 10”20 cm-3September 15, 2021 Science report..

Funding: Economy, Trade and Industry Manufacturing R & D Support Grant Program for SMEs

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