High Resolution Scanning Transmission Electron Microscopy (S/TEM) Investigation Of Common Defects In Scandium and Aluminum Alloyed β-Ga2O3

Beta gallium oxide (β-Ga2O3) has gained interest in recent years as a candidate for high power electronics and extreme environment applications [1]. An ultra-wide-band gap (UWBG) semiconductor, β-Ga2O3 is capable of being grown into readily available single crystal substrates and bulk crystals using both thin film deposition and melt growth techniques [2]. With a monoclinic structure and space group of C2/m, the highly anisotropic unit cell creates a unique combination of properties. Most important among these is a band gap around 4.8 eV, which is 1.4 eV greater than the most well-known and widely used UWBG semiconductor, GaN [3]. β-Ga2O3’s Baliga Fig. of Merit, which judges viability for application to power electronics, is also nearly four times greater than GaN [1]. These result from an underlying structure of two inequivalently coordinated gallium sites and three inequivalently coordinated oxygen sites existing within the lattice. β-Ga2O3 still faces issues in its comparatively low thermal conductivity, residing an order of magnitude below common UWBG and power semiconductor materials, and an as yet lack of achievable p-type doping [4, 5]. However, the material’s most fundamental constraint is a lack of knowledge about defect formation, behavior, and their impact on properties. Alloying β-Ga2O3 with scandium and aluminum shows promise mainly in its potential for 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 [6, 7]. However, the role these elements play in defect formation and atomic structure is still not well understood. S/TEM provides an ideal tool to investigate this, as information can be collected on length scales spanning from point defects like vacancy complexes to two-dimensional defects such as stacking faults. 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. In this study, we show the prevalence of a nanometer-sized layer of γ-Ga2O3 at the surface of melt-grown bulk single-crystal β-Ga2O3. The γ polymorph of Ga2O3 has previously been identified using atomic resolution S/TEM as a common defect at the surface of β-(AlxGa1-x)2O3 films grown via molecular beam epitaxy [8]. However, nothing in the literature has shown the presence of γ-Ga2O3 as a defect on the surface of melt-grown crystals. The structure of this phase was confirmed using electron diffraction, atomic-resolution S/TEM, and unsupervised machine learning, while the local bonding environment was investigated by studying variations in the Oxygen EELS K edge [9, 10]. Understanding the formation and structure of this defect phase is vital for refinement of processing and growth techniques, while also allowing for the study of the role alloys play in the defect formation process [11]. (a) Annular-dark field S/TEM image of the β/γ interface in a β-(Al0.1Ga0.9)2O3 crystal. (b) EELS oxygen K edge spectra of the two polymorphs taken from a β-(Al0.1Ga0.9)2O3 sample, with arrows indicating a change to the double peak appearance of the fine structure. (a) Bright field TEM image of interfacial region, showing thin layer of γ-Ga2O3 formation. (b) Unsupervised learning algorithm detecting structural clusters in an annular-dark field S/TEM image of the γ/β interface in a β-(Sc0.1Ga0.9)2O3 crystal.