Deep level defect states in β-, α-, and ɛ-Ga2O3 crystals and films: Impact on device performance

A review is given of reported trap states in the bandgaps of different polymorphs of the emerging ultrawide bandgap semiconductor Ga2O3. The commonly observed defect levels span the entire bandgap range in the three stable (β) or meta-stable polymorphs (α and ɛ) and are assigned either to impurities such as Fe or to native defects and their complexes. In the latter case, the defects can occur during crystal growth or by exposure to radiation. Such crystalline defects can adversely affect material properties critical to device operation of transistors and photodetectors, including gain, optical output, threshold voltage by reducing carrier mobility, and effective carrier concentration. The trapping effects lead to degraded device operating speed and are characterized by long recovery transients. There is still significant work to be done to correlate experimental results based on deep level transient spectroscopy and related optical spectroscopy techniques to density functional theory and the dominant impurities present in the various synthesis methods to understand the microscopic nature of defects in Ga2O3.