Wide-bandgap semiconductors are fundamental components in many optoelectronic and power devices. The free-carrier properties, i.e., carrier density and mobility, are crucial parameters that determine device performance. This paper presents the characterization of a gallium nitride (GaN) wafer with ~1016 cm-3 carrier concentration using terahertz (THz) time- domain ellipsometry. In addition, THz time-domain spectroscopy of monoclinic beta-gallium oxide (β-Ga2O3) semi- insulating bulk and n-type homoepitaxial film are presented. The free-carrier properties are extracted by theoretically fitting the complex refractive index to the Drude and Drude-Lorentz models. THz time-domain techniques are a practical and powerful tool to nondestructively characterize the free-carrier properties of wide-bandgap semiconductors for device development.
The developments of ultra-high purity material which resist the damage by DUV laser are strongly required. According to the general mechanism of laser-induced damage, some kinds of defects and contaminations on the optical material are very important factors for DUV laser-induced damage. The borate crystal SrB4O7 (SBO) was reported to be a nonlinear optical material with a wide transparency down to 130 nm. In this study, we grew a high-quality SBO single crystal and measured the surface DUV laser-induced damage threshold (LIDT) in several polished conditions.
The SBO crystals grown over 13 days was 60 x 6 x 30 mm3 (a x b x c) without cracks or other defects. Two (020) plates were cut from the SBO crystal, and the both faces of the plates were optically polished. After that, we introduced catalyst-referred etching (CARE) to the one plate in order to atomically produce flat and damage-free SBO surfaces. As a result of the CARE treatment at a removal rate of 364 nm/h, the surface condition changed drastically, and a linear step-and-terrace structure was grew with a step height of 0.2 nm.
The surface LIDT in several polished conditions were measured with a 1-on-1 method at 266 nm (5 ns pulse width). The polarization direction was parallel to the c-axis of the (020) sample. Synthetic silica was also evaluated for comparison. The surface LIDT (17.3 J/cm2) of SBO after optical polishing is 4.3 times that of synthetic silica (4.0 J/cm2). In addition, the surface LIDT (24.1 J/cm2) of CARE-treated SBO is 6.0 times that of synthetic silica. This suggests that CARE-treated SBO crystals are a promising material for optical components in high-power DUV laser systems.
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