Interfacial N Vacancies in GaN/(Al,Ga)N/GaN Heterostructures

We show that N-polar GaN/(Al, Ga)N/GaN heterostructures exhibit significant N deficiency at the bottom (Al, Ga)N/GaN interface, and that these N vacancies are responsible for the trapping of holes observed in unoptimized N-polar GaN/(Al, Ga)N/GaN high electron mobility transistors. We arrive at this conclusion by performing positron annihilation experiments on GaN/(Al, Ga)N/GaN heterostructures of both N and Ga polarity, as well as state-of-the-art theoretical calculations of the positron states and positron-electron annihilation signals. We suggest that the occurrence of high interfacial N vacancy concentrations is a universal property of nitride semiconductor heterostructures at net negative polarization interfaces.Peer reviewe

Ti-Sr antisite : An abundant point defect in SrTiO3

We present a systematic study of the positron lifetime as a function of measurement temperature in strontium titanate ( SrTiO 3) single crystals grown in different conditions and by different synthesis methods. We combine our experimental results with state-of-the-art theoretical calculations of positron annihilation parameters. We find that the essentially omnipresent 180-190ps lifetime component is most likely the Ti Sr antisite defect, possibly coupled with one or more oxygen vacancies, supporting the importance of the Ti Sr antisite related defects in SrTiO 3.Peer reviewe

Split Ga vacancies and the unusually strong anisotropy of positron annihilation spectra in beta-Ga2O3

We report a systematic first-principles study on positron annihilation parameters in the beta-Ga2O3 lattice and Ga monovacancy defects complemented with orientation-dependent experiments of the Doppler broadening of the positron-electron annihilation. We find that both the beta-Ga2O3 lattice and the considered defects exhibit unusually strong anisotropy in their Doppler broadening signals. This anisotropy is associated with low symmetry of the beta-Ga2O3 crystal structure that leads to unusual kind of one-dimensional confinement of positrons even in the delocalized state in the lattice. In particular, the split Ga vacancies recently observed by scanning transmission electron microscopy produce unusually anisotropic positron annihilation signals. We show that in experiments, the positron annihilation signals in beta-Ga2O3 samples seem to be often dominated by split Ga vacancies.Peer reviewe

Point defects in oxide and nitride compounds, alloys and heterostructures

The public defense on 24th April 2020 at 13:00 will be organized via remote technology. Link: https://aalto.zoom.us/j/291206096 Zoom Quick Guide: https://www.aalto.fi/en/services/zoom-quick-guideIn 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

Identification of vacancy defect complexes in transparent semiconducting oxides ZnO, In2O3 and SnO2

Positron annihilation spectroscopy, when combined with supporting high-quality modeling of positron states and annihilation in matter, is a powerful tool for detailed defect identification of vacancy-type defects in semiconductors and oxides. Here we demonstrate that the Doppler broadening of the positron annihilation radiation is a very sensitive means for observing the oxygen environment around cation vacancies, the main open-volume defects trapping positrons in measurements made for transparent semiconducting oxides. Changes in the positron annihilation signal due to external manipulation such as irradiation and annealing can be correlated with the associated changes in the sizes of the detected vacancy clusters. Our examples for ZnO, In2O3 and SnO2 demonstrate that oxygen vacancies in oxides can be detected directly using positron annihilation spectroscopy when they are complexed with cation vacancies.Peer reviewe

Ga vacancies and electrical compensation in β-Ga 2 O 3 thin films studied with positron annihilation spectroscopy

We have applied positron annihilation spectroscopy to study vacancy-type defects in unintentionally doped and Si and Sn doped β-Ga 2 O 3 homoepitaxial thin films grown by metal-organic chemical vapor deposition (MOCVD). We detect Ga vacancy related defects at high concentrations in semi-insulating and highly resistive material, while conductive (ntype) material exhibits very low Ga vacancy concentrations. These findings show that Ga vacancies can act as efficient electrical compensators for n-type conductivity, but their concentrations can be suppressed by controlling the growth environment, leading to efficient n-type doping. We also note the strong anisotropy of the positron annihilation signals and give recommendation for presenting positron data obtained in β-Ga 2 O 3 .Peer reviewe

Amphoteric Be in GaN

We show that Be exhibits amphoteric behavior in GaN, involving switching between substitutional and interstitial positions in the lattice. This behavior is observed through the dominance of BeGa in the positron annihilation signals in Be-doped GaN, while the emergence of VGa at high temperatures is a consequence of the Be impurities being driven to interstitial positions. The similarity of this behavior to that found for Na and Li in ZnO suggests that this could be a universal property of light dopants substituting for heavy cations in compound semiconductors.Peer reviewe