Unusual Formation of Point-Defect Complexes in the Ultrawide-Band-Gap Semiconductor β−Ga2O3

Abstract

Understanding the unique properties of ultra-wide band gap semiconductors requires detailed information about the exact nature of point defects and their role in determining the properties. Here, we report the first direct microscopic observation of an unusual formation of point defect complexes within the atomic scale structure of β-Ga2O3 using high resolution scanning transmission electron microscopy (STEM). Each complex involves one cation interstitial atom paired with two cation vacancies. These divacancy -interstitial complexes correlate directly with structures obtained by density functional theory, which predicts them to be compensating acceptors in β-Ga2O3. This prediction is confirmed by a comparison between STEM data and deep level optical spectroscopy results, which reveals that these complexes correspond to a deep trap within the band gap, and that the development of the complexes is facilitated by Sn doping through the increase in vacancy concentration. These findings provide new insight on this emerging material's unique response to the incorporation of impurities that can critically influence their properties.

Split Ga vacancies and the unusually strong anisotropy of positron annihilation spectra in β−Ga2O3

“…By comparing to experiments, we show that the colossal signal anisotropy-in particular the differences in the nature of the anisotropy-contains information that can be used for defect identification even when a suitable reference material where the positron annihilation data could be interpreted as originating from the lattice only, typically referred to as "defect-free reference", is missing. We provide evidence of the experimental positron annihilation signals being in many cases dominated by the split Ga vacancies, supporting the recent findings [6,7,9,10]. It is likely that different levels of hydrogenation of these split Ga vacancies determine the level of electrical compensation in n-type β-Ga 2 O 3 .…”

“…However, at this stage we cannot rule out the possible contribution of more complex defects such as split Ga vacancy -O vacancy complexes. The assignment of experimentally observed anisotropy of the Doppler signals to V ic Ga -related defects is also supported by the recent theoretical and experimental work: V ic Ga has been predicted to have the lowest formation energy among clean monovacancy type defects [7], and the existence of V ic Ga was observed with STEM [10]. The fact that these defects could be found by STEM agrees well with the estimate from positron annihilation: in both exper-iments, the split Ga vacancy concentration needs to be at least 10 18 cm −3 .…”

“…Theoretical work suggested already early on that the monovacancy defect of the lowest formation energy in β-Ga 2 O 3 is of a peculiar type [6], where a cation atom neighboring the cation monovacancy relaxes to an interstitial site half-way toward the vacancy [7,8]. These "split" Ga vacancies in which the open volume is split into two parts on either side of the interstitial, have been recently experimentally observed in infrared spectroscopy (hydrogenated form) and scanning transmission electron microscopy (STEM) studies [9,10].…”

“…[25] and the Sn-type split Ga vacancies observed in Ref. [10], where Sn substitutes for the Ga atom relaxing to the interstitial position. This relaxation does not happen when Si substitutes for Ga [6,7], and allows for the formation of regular Ga monovacancies observed in the experiments.…”

“…ic Ga -1H, and V ic Ga -2H. We chose systems that are simple and predicted to be energetically favorable ( [9,10]. At Fermi level close to the conduction band, the charge state of "clean" cation monovacancies is predicted as −3, after passivation with one hydrogen (V ib Ga -1H and V ic Ga -1H) the charge state is predicted as −2, and with two hydrogen atoms (V ib Ga -2H and V ic Ga -2H) the charge state is predicted as −1 [6,7].…”