Point defects in oxide and nitride compounds, alloys and heterostructures

Abstract

In this work, positron annihilation spectroscopy was used for investigating defects in transparent semiconducting oxides and III-nitrides. These semiconductor materials are technologically important for next generation optical devices, space-qualified electronic components as well as for high-power and high-frequency applications. We demonstrate that the presence, concentration and nature of point defects are a subject of growth conditions, doping and post-growth processing in all of the materials in question. In Sb-doped ZnO, the zinc vacancy VZn-associated defects including complexes with oxygen vacancies VO are detected irrespective of the Sb content. Heavy Sb doping results in compensating zinc vacancies dominating the positron data. The compensating zinc vacancies are likely to lead to the significant decrease in the conduction electron concentration at the highest Sb content in thin-film ZnO. The VZn–VO complexes induced by mechanical polishing in ZnO substrates are found to be the dominant defect in bulk material. Removal of parasitic Li and associated polishing-induced damage are most effective under series of high-temperature annealings. In (InxGa1-x)2O3 with graded In/Ga composition, an increase in the indium content leads to suppressed vacancy formation, while Ga-rich regions are characterised with gallium vacancy VGa-like defects. This behaviour is in accordance with In2O3- or Ga2O3-like signal dominating the positron data depending on the alloy composition. Doping (InxGa1-x)2O3 with Si results in a high concentration of indium vacancy VIn-associated point defects. Indium vacancies VIn and vacancy clusters are observed in He+-implanted InxGa1-xN. The changes in the dominant defect type between the InxGa1-xN layers and InxGa1-xN – Si interface suggest redistribution of metal atoms in indium-rich regions or a closer rearrangement of such regions in the vicinity of the thin film – substrate interface triggered by the high-fluence He+-implantation. Experimental methods combined with theoretical modeling enabled defect identification in Be-doped GaN and in N-polar GaN/AlGaN/GaN HEMT-like heterostructures. Substituting BeGa defects are the dominant positron traps in Be-doped GaN. Gallium vacancies VGa emerging after thermal annealing indicate amphoteric behaviour of Be via the BeGa → VGa + Bei mechanism. In heterostructures, the defect state at the bottom AlGaN/GaN interface is attributed to significant nitrogen deficiency in form of nitrogen vacancies VN. This finding implies that in unoptimised N-polar heterostructures excess holes might be driven to the nitrogen vacancies located at the bottom AlGaN/GaN interface. That leads to the charge compensation effect and subsequent light sensitivity and current collapse of the actual devices. Si doping of N-polar heterostructures with graded AlxGa1-xN layer compensates the trapping to the VN due to screening of the built-in electric fields.