Terahertz time-domain ellipsometry with high precision for the evaluation of GaN crystals with carrier densities up to 1020\xa0cm\u22123

Verdad C Agulto,Hideaki Kitahara,Yusuke Mori,Toshiyuki Iwamoto,Valynn Katrine Mag-Usara,Masashi Yoshimura,Makoto Nakajima,Kazuhiro Toya,Masayuki Imanishi

doi:10.1038/s41598-021-97253-z

Verdad C Agulto, Hideaki Kitahara + Show 7 more

Open Access

https://doi.org/10.1038/s41598-021-97253-z

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Abstract

Gallium nitride (GaN) is one of the most technologically important semiconductors and a fundamental component in many optoelectronic and power devices. Low-resistivity GaN wafers are in demand and actively being developed to improve the performance of vertical GaN power devices necessary for high-voltage and high-frequency applications. For the development of GaN devices, nondestructive characterization of electrical properties particularly for carrier densities in the order of 1019 cm−3 or higher is highly favorable. In this study, we investigated GaN single crystals with different carrier densities of up to 1020 cm−3 using THz time-domain ellipsometry in reflection configuration. The p- and s-polarized THz waves reflected off the GaN samples are measured and then corrected based on the analysis of multiple waveforms measured with a rotating analyzer. We show that performing such analysis leads to a ten times higher precision than by merely measuring the polarization components. As a result, the carrier density and mobility parameters can be unambiguously determined even at high conductivities.

Highlights

The profound interest in the Gallium nitride (GaN) semiconductor is fueled by its superior properties—wide bandgap, high electron saturation velocity, high breakdown voltage, and high thermal conductivity
This study demonstrates the applicability of THz waves to semiconductors with carrier densities up to the order of 1020 cm−3 and potentially higher using THz time-domain ellipsometry with high precision
THz ellipsometry technology has been combined with external magnetic fields in a phenomenon called the optical Hall effect (OHE), which is analogous to the electric Hall effect but occurring at high frequencies

Summary

Introduction

The profound interest in the GaN semiconductor is fueled by its superior properties—wide bandgap, high electron saturation velocity, high breakdown voltage, and high thermal conductivity. This study demonstrates the applicability of THz waves to semiconductors with carrier densities up to the order of 1020 cm−3 and potentially higher using THz time-domain ellipsometry with high precision. We anticipate this technique to be widely useful in various fields dealing with highly doped semiconductors. THz magneto-optical ellipsometry can provide additional information on free-carrier properties, the use of magnets might not be convenient experimentally In this case when the application of external magnetic fields is not possible, the effective mass and type of free carriers in the material under investigation are assumed to be known. THz time-domain ellipsometry without the use of magnets is sufficiently powerful, while being more compact, to evaluate the carrier density and mobility of GaN crystals, especially those with very high carrier densities

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