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.
Highlights
Gallium oxide (Ga2 O3 ) is an attractive ultrawide-bandgap semiconductor for the use in a variety of applications such as gas sensors, high-power electronics, and deep-ultraviolet (UV) photo-detectors [1,2,3].In recent years, tremendous efforts have been made to improve material quality and device performance.there remain many unsolved problems in the areas of defect reduction, passivation improvement, impurity-doping, device processes, carrier dynamics, etc
Terahertz (THz) emission spectroscopy (TES) and laser THz emission microscopy (LTEM) are emerging technologies that can detect ultrafast photocarrier dynamics and responses in materials and devices [4,5] that are affected by electron mobility, surface potential, defects, and band bending
The wide bandgap semiconductors have narrow depletion layers with large surface potentials, which mean that the THz emission strongly depends on the surface conditions and the dynamic behavior of the excited carriers
Summary
Gallium oxide (Ga2 O3 ) is an attractive ultrawide-bandgap semiconductor for the use in a variety of applications such as gas sensors, high-power electronics, and deep-ultraviolet (UV) photo-detectors [1,2,3]. There remain many unsolved problems in the areas of defect reduction, passivation improvement, impurity-doping, device processes, carrier dynamics, etc. Contributions to resolve these issues require a new type of characterization tool, one that operates in a noncontact, nondestructive manner. The wide bandgap semiconductors have narrow depletion layers with large surface potentials, which mean that the THz emission strongly depends on the surface conditions and the dynamic behavior of the excited carriers. Such information is essential to develop high-performance devices
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