Terahertz Emission Spectroscopy and Microscopy on Ultrawide Bandgap Semiconductor β-Ga2O3

Abstract

Although gallium oxide Ga2O3 is attracting much attention as a next-generation ultrawide bandgap semiconductor for various applications, it needs further optical characterization to support its use in higher-performance devices. In the present study, terahertz (THz) emission spectroscopy (TES) and laser THz emission microscopy (LTEM) are applied to Sn-doped, unintentionally doped, and Fe-doped β-Ga2O3 wafers. Femtosecond (fs) laser illumination generated THz waves based on the time derivative of the photocurrent. TES probes the motion of ultrafast photocarriers that are excited into a conduction band, and LTEM visualizes their local spatiotemporal movement at a spatial and temporal resolution of laser beam diameter and a few hundred fs. In contrast, one observes neither photoluminescence nor distinguishable optical absorption for a band-to-band transition for Ga2O3. TES/LTEM thus provides complementary information on, for example, the local mobility, surface potential, defects, band bending, and anisotropic photo-response in a noncontact, nondestructive manner. The results indicated that the band bends downward at the surface of an Fe-doped wafer, unlike with an n-type wafer, and the THz emission intensity is qualitatively proportional to the product of local electron mobility and diffusion potential, and is inversely proportional to penetration depth, all of which have a strong correlation with the quality of the materials and defects/impurities in them.

Heteroepitaxial Growth of Thick α-Ga2O3 Films on Sapphire Substrates by Flow Modulation Epitaxy with Halide Vapor Phase Epitaxy

“…All the α-Ga 2 O 3 films had a high average transmittance of over 90% in the UV–vis wavelength region (300–800 nm), indicating their high transparency in the UV–vis region . In addition, the films had relatively sharp absorption edges around 260 nm, which could be attributed to the absorption of the band gap energy required for the band-to-band transition of α-Ga 2 O 3 . , …”

“…33 In addition, the films had relatively sharp absorption edges around 260 nm, which could be attributed to the absorption of the band gap energy required for the band-to-band transition of α-Ga 2 O 3 . 34,35 By analysis of the plot of optical absorbance data versus energy, the Tauc laws (eq 1) suggest a technique for estimating the band gap. These rules demonstrate that the strength of optical absorption depends on the difference between the photon energy and the band gap, where α is the absorption coefficient, h represents Planck's constant, n denotes the photon frequency, A denotes the slope of the Tauc plot in the linear region, and E g is the band gap.…”

How to use terahertz emission spectroscopy for wide bandgap semiconductor evaluation

“…We have applied TES and LTEM to Si-based materials and devices and proven that one can estimate various parameters, such as surface potential, work function, impurity doping density, defects density in passivation layers, surface state density, and so on, semi-quantitatively and non-contactly by just observing the THz radiation [8][9][10][11][12][13][14][15][16][17]. In the present work, we review the TES and LTEM application to wide bandgap semiconductors [18][19][20][21][22][23][24].…”

Adaptive Nonlinear Phase Compensation in a Femtosecond Hybrid Laser with Varying Pulse Repetition Rate

“…In areas where high-quality laser pulses are desired, such as laser micromachining [1][2][3][4][5][6], ophthalmology [7] and nonlinear microscopy [8][9][10][11], nonlinear optical effects such as self-phase modulation (SPM) often limit the achievable pulse energy of high-energy ultrafast laser systems. The nonlinear optical effects introduce nonlinear spectral and temporal phase to laser pulses.…”