Photoluminescence (PL) spectroscopy is an important method to characterize dopants and defects in gallium oxide. Common features in the PL spectrum include the intrinsic UV band, blue and green bands that involve donor-acceptor pairs, and red emission due to Cr³⁺ impurities. PL mapping with excitation wavelengths ranging from 266 to 532 nm reveals the spatial distribution of these features with micron resolution. Damage due to high-intensity laser pulses results in significant changes in the intensity and energy of the UV band. In Czochralski-grown β-Ga₂O₃:Fe, the Cr³⁺ emission intensity shows striations that are attributed due to inhomogeneities during growth. In addition to defects in the bulk, PL microscopy has revealed several specific defects on the surface. Some of these localized centers are very bright UV emitters. Raman scans of these bright emitters revealed hydrocarbon peaks, which may point toward the origin of the light emission.

An ultra-wide bandgap of 4.8 eV makes b-Ga₂O₃ a promising material for power devices and ultra-violet (UV) optoelectronics such as UV-transparent electrodes and solar-blind photodetectors. It is well-known that the optical absorption of b-Ga₂O₃ is anisotropic, having different threshold energies for different incident linear polarizations. Due to its low symmetry, the polarization of the emitted photoluminescence (PL) of b-Ga₂O₃ should also be polarized; but this phenomenon which could allow distinguishing between point defects based on their structure has received much less attention. Polarized emission has been predicted and measured to be strongly related to self-trapped holes (STHs) involving O displacements, impurities, and doping. The reported typical b-Ga₂O₃ PL is composed of UV, blue, green, and red main emission bands. Previously-reported PL has discussed excitation polarization-dependent PL, but there has been no discussion of polarized PL emission which we have found also to be polarization dependent.

Herein we report the emission polarization dependence of various high-crystalline-quality melt-grown bulk b-Ga₂O₃samples. It was found that (

We also observed polarized emission for Fe-doped bulk b-Ga₂O₃ samples. Both the b-orientation and [102]-orientation showed red PL with different intensities. Whereas UV, blue, and green PL in UID and Sn-doped samples come from band transition recombination, red PL in Fe-doped samples comes from Cr³⁺. Therefore, the causes of this emission polarization dependence are different, potentially caused by orbitals within the Cr³⁺.

Beta gallium oxide (β-Ga₂O₃) has gained interest recently as an attractive candidate for high power electronics and extreme environment applications. Possessing a monoclinic structure (space group C2/m), the anisotropic unit cell creates a unique combination of properties. Most important among these is a band gap around 4.8 eV, 1.4 eV greater than the most widely studied UWBG semiconductor, GaN. This results from the presence of tetrahedral and octahedral gallium sites, as well as three inequivalently coordinated oxygen sites. β-Ga₂O₃ suffers from a low thermal conductivity an order of magnitude below common UWBG and power semiconductor materials, as well as a lack of achievable p-type doping. However, the material’s most fundamental constraint is insufficient knowledge about defect formation, behavior, and their impact on properties. Recent attempts at alloying β-Ga₂O₃ with scandium and aluminum shows promise mainly in increasing β-Ga₂O₃’s band gap, as the monoclinic phases of Sc₂O₃ and Al₂O₃ demonstrate increases up to 5.48 eV and 7.24 eV, respectively. Increasing the band gap via alloying is vital for producing devices in higher critical field applications. However, the role these elements play in defect formation is still not well understood.

Using Scanning/Transmission Electron Microscopy (S/TEM) imaging and spectroscopy, we study how the addition of scandium and aluminum can change the atomic and electronic structure of β-Ga₂O₃. In addition, we identify a nanometer-scale layer of γ-Ga₂O₃ at the surface of a Czochralski-grown single-crystal of β-Ga₂O₃. S/TEM and the suite of tools it provides such as electron energy loss spectroscopy (EELS) and energy dispersive spectroscopy (EDS), also allows for probing local electronic states around defects and general compositional mapping. This provides further information on the environment around defects and their impact on local structure. Using S/TEM imaging we investigate how the γ polymorph of Ga₂O₃ forms as a thin film on the surface of β-Ga₂O_3, while the local bonding environment is uncovered by studying variations in the oxygen EELS K edge. Understanding the formation and structure of this defect phase is vital for improvement of processing and growth techniques, while also allowing for the study of the role alloys play in the defect formation process.

Beta gallium oxide (β-Ga₂O₃) received significant attention recently as a low cost, large area substrate and active layer material for high power, high temperature power electronics and sensing devices. However, growth of β-Ga₂O₃ crystals is difficult due to easily activated a plane and c plane cleavage, the presence of low angle grain boundaries (LAGB), and potential formation of polycrystalline inclusions. In this presentation we report on β-Ga₂O₃ crystals grown by the Edge-defined Film-fed Growth (EFG) technique with an (010) principal face [1]. Two crystals with apparently randomly formed high angle grain boundaries (HAGB) were analyzed by electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI), and cathodoluminescence (CL), with the goal to investigate the LAGBs details and the source of the HAGB formation. We found that LAGBs existed in the region preceding the start of one HAGB formation, and increased misorientation across the LAGB was observed close to the initiation of a new grain.This presentation shows the power of multimodal microscopy characterization, correlating misorientation and variation in optoelectronic properties, with LAGBs and associated dislocations.

[1] Multimodal microscopy of extended defects in β-Ga2O3 (010) EFG crystals. Drew Haven; Helio Moutinho; John S. Mangum; Harvey Guthrey; David Joyce; Andriy Zakutayev; Nancy M. Haegel AIP Advances 13, 075122 (2023)