Screw dislocation that converts p-type GaN to n-type: Microscopic study on Mg condensation and leakage current in p–n diodes

Abstract

Recent experiments suggest that Mg condensation at threading dislocations induces current leakage, leading to degradation of GaN-based power devices. To investigate this, we perform first-principles total-energy electronic-structure calculations for various Mg and dislocation complexes. We find that threading screw dislocations (TSDs) indeed attract Mg impurities, and that the electronic levels in the energy gap induced by the dislocations are elevated toward the conduction band as the Mg impurity approaches the dislocation line, indicating that the Mg-TSD complex is a donor. The formation of the Mg-TSD complex is unequivocally evidenced by atom probe tomography in which Mg condensation around the [0001] screw dislocation is observed in a p–n diode. These findings provide a picture in which the Mg, being a p-type impurity in GaN, diffuses toward the TSD and then locally forms an n-type region. The appearance of this region along the TSD results in local formation of an n–n junction and leads to an increase in the reverse leakage current.

Correlation between structural properties and nonradiative recombination behaviors of threading dislocations in freestanding GaN substrates grown by hydride vapor phase epitaxy

“…The TDs have deleterious effects on the performance and lifetime of GaN-based devices through mechanisms such as carrier scattering, 9,10 carrier trapping via nonradiative recombination (NRR), [11][12][13][14][15][16] and paths for leakage current. [17][18][19][20][21][22] These different mechanisms are connected by the fact that dislocations act as one-dimensional (1D) charged defects that introduce electronic states into the energy gap. 14,[22][23][24] It is well accepted that the negative effects of dislocations on devices are strongly dependent on their Burgers vector and are susceptible to other dislocation characteristics, such as their line direction, 20 core structure, 20,22,[25][26][27] impurity gettering, 14,22 and surface morphology.…”

“…[17][18][19][20][21][22] These different mechanisms are connected by the fact that dislocations act as one-dimensional (1D) charged defects that introduce electronic states into the energy gap. 14,[22][23][24] It is well accepted that the negative effects of dislocations on devices are strongly dependent on their Burgers vector and are susceptible to other dislocation characteristics, such as their line direction, 20 core structure, 20,22,[25][26][27] impurity gettering, 14,22 and surface morphology. 21 However, there are large discrepancies in the literature regarding the relationship between the dislocation behavior and their structural properties.…”

“…28,29 Others found that the dislocations with the a component (TEDs and TMDs) act as NRR centers, 12,14 whereas the pure TSDs are inactive. 14 terms of leakage current pathways, most groups seem to agree that TSDs, especially those with an open-core, are responsible for the large leakage current under a reverse bias in Schottky barrier diodes (SBDs) 30 and vertical PN diodes, 18,20,22 even though the types of impurities that accumulate at the dislocations are different. When a Na-flux GaN substrate is used, the leakage current is strongly dependent on the growth sector at the initial stage of the Na-flux growth and has a positive correlation with the magnitude of the Burgers vectors, while the effects of oxygen impurities are implied.…”

Comprehensive analysis of current leakage at individual screw and mixed threading dislocations in freestanding GaN substrates

“…19,[37][38][39][40] As an example, Nakano et al reported that a stable atomic core structure of screw dislocations grown under a N-rich environment had no deep states in the bandgap, while a core structure formed under a Ga-rich condition created deep states. 19,37 Belabass et al showed that screw dislocations having non-stoichiometric closed-core configurations with N atoms removed from the core had a metallic-like electronic structure that could provide highly conductive pathways in GaN crystals. 38 Based on these studies, the majority of the b=1c screw TDs in the present work had closed-core structures with no deep states inducing current leakage, while a very few b=1c screw TDs, such as that associated with pit #M2, had an atomically different closed-core structure with harmful deep states.…”

Comprehensive analysis of current leakage at individual screw and mixed threading dislocations in freestanding GaN substrates

“…The inconsistent leakage behavior of TDs observed in these studies strongly suggests that the effects of the TDs on GaN-based power devices are sensitive to the growth conditions 13 and thereby not solely determined by the type of TD. Specifically, factors such as the Burgers vector 5 , core structure 14 , inclination direction 15 , 16 and strain field of the dislocations 17 , as well as point defects interacting with the TDs 4 , 10 , 17 – 19 , may influence the electrical characteristics of these defects. Unfortunately, few studies have examined the detailed relationships between such structural characteristics and the electrical properties of TDs, primarily because of challenges related to quantitative electrical measurements of individual TDs.…”