Past Events

Abstract
 

After initial work on gallium oxide (Ga₂O₃) in the mid-1900s, the interest to this material was recently renewed on behalf of its potential applications in power electronics and UV devices. Particularly, the monoclinic β-Ga₂O₃ phase attracts interest due to its wide band gap of some 4,8 eV and n-type conductivity, so that very promising MOSFET devices were demonstrated very recently by Higashiwaki et at (2016). However, while the control over the n-type conductivity was quite successfully achieved in the range of 1e16 -1e19 using Si or Sn as dopants, the understanding of deep states is insufficient. In the present talk, upon a short summary of the research status in the field, two new observations will be reported.

Firstly, a prominent interplay between Fe-related and intrinsic defects was observed, as revealed by comparing samples with different impurity contents subjected to proton irradiation tests. Specifically, our experiment resolves a long lasting discussion on whether the dominating deep level state, at E_c-0.7 eV, where E_c is the conduction band minimum, is due to Fe_Ga or Ga-vacancy. Ironically, our data confirm that both defects give rise for the deep levels with very similar activation energies, however and importantly having different capture cross-sections. In order to generalize, three different sorts of samples were studied: bulk crystals, as well as MBE and HVPE films. DLTS and SIMS were used for the sample characterization and theoretical data from the HSE06 screened hybrid functional calculations were used for comparison.

Secondly, by performing the DLTS measurements at elevated temperatures, we have discovered an intriguing phenomenon of field- induced defect generation in otherwise nearly deep level free β-Ga₂O₃ MBE films. Specifically, three new levels at E_c-1.0, E_c-1.6, and E_c-1.8 eV emerged prominently in the sample exposed to ≥ 625 K with the reverse bias voltage applied, clearly observable at the cooling stage of the DLTS measurement. Importantly, the defect generation is found to be a strong function of the electric filed applied, as confirmed by the depth profiling of the corresponding defects.  Importantly, the two deeper states showed significant thermal stability, while the shallow one is metastable and can be removed by anneals with no voltage applied. The nature of these field-induced defects remains unclear but possible options were considered.

In conclusion, Ga₂O₃ is indeed a very interesting semiconductor, having not only its great application potential, but also containing interesting physics to learn.

Anyone interested is welcome to attend.