Abstract
We report the observation of resonant electronic Raman scattering (ERS) originating from Ir4+ ions in bulk β-Ga2O3 crystals grown by the Czochralski method. The observed ERS peak at 5150 cm−1 at room temperature is attributed to an internal transition within the split 2T2g ground state of Ir4+ ions under strong octahedral crystal field conditions and combined action of spin–orbit coupling and low-symmetry field components. The ERS efficiency is found to strongly depend on the photon energy used for optical excitation and exhibits a maximum at about 2.9 eV. In accordance with the linear dependence of the ERS peak intensity on the optical excitation power, the enhancement around 2.9 eV is explained by Raman scattering in resonance with electronic transitions from the Ir4+ ground state 2T2g to the first excited state 4T1g. The optically induced Ir3+/4+ charge transfer is discussed as an alternative, but less likely the origin of the observed enhancement of the ERS efficiency.
Highlights
Monoclinic gallium sesquioxide (β-Ga2O3) is a promising ultra-wide bandgap semiconductor for power electronics and solarblind ultraviolet photodetectors because its large bandgap of ∼4.85 eV1 theoretically enables a high breakdown electric field of up to 8 MV/cm, and it can be n-type doped over a wide conductivity range.[2,3,4] For device fabrication, it is common to grow homoepitaxial layers on β-Ga2O3 substrate wafers obtained from melt-grown single crystals.[5,6,7,8] The growth of the latter crystals is for reasons of scaling-up crystal dimensions preferentially performed by edge-defined film-fed growth[9] or the Czochralski method[10,11] at temperatures above 1800 °C
The electronic Raman scattering (ERS) efficiency is found to strongly depend on the photon energy used for optical excitation and exhibits a maximum at about 2.9 eV
Infrared (IR) absorption spectroscopy combined with secondary ion mass spectrometry (SIMS)[14–16] and electron paramagnetic resonance (EPR) spectroscopy[17] has shown that iridium is present in non-negligible concentrations from a few 1016 up to 1018 cm−3 in melt-grown crystals and acts as a deep donor
Summary
Monoclinic gallium sesquioxide (β-Ga2O3) is a promising ultra-wide bandgap semiconductor for power electronics and solarblind ultraviolet photodetectors because its large bandgap of ∼4.85 eV1 theoretically enables a high breakdown electric field of up to 8 MV/cm, and it can be n-type doped over a wide conductivity range.[2,3,4] For device fabrication, it is common to grow homoepitaxial layers on β-Ga2O3 substrate wafers obtained from melt-grown single crystals.[5,6,7,8] The growth of the latter crystals is for reasons of scaling-up crystal dimensions preferentially performed by edge-defined film-fed growth[9] or the Czochralski method[10,11] at temperatures above 1800 °C At these high temperatures, unintentional incorporation of impurities into the crystal is inevitable, especially of transition metals from the iridium crucibles, Ga2O3 starting material, and thermal insulation. Iridium detection and control of its incorporation in bulk single crystals are important for achieving reproducible electrical properties of the β-Ga2O3 substrates
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