Mg diffusion is a common problem in GaN devices with p–n junctions. Although this impurity diffusion is reported to occur through threading dislocations (TDs), no direct evidence has yet been obtained. Therefore, we tried the direct observation of Mg diffusion by atom probe tomography (APT) analysis. The n-type drift layer of the fabricated p–n diode was exposed, and etch pits were formed on the drift layer to identify the TD position. The APT analysis around TDs was carried out by lifting out the drift layer around specific etch pits using a focused ion beam to include TDs. The relationship between the etch pit shape and the TD type was confirmed by cross-sectional scanning transmission electron microscopy observation. The APT analysis of two types of etch pits formed on the mixed dislocations was performed, and Mg diffusion was clearly observed through the mixed dislocations. In this work, we show direct evidence of Mg diffusion via mixed dislocations in GaN p–n diodes and its effect on reverse leakage current.

[1] The elastic moduli of polycrystalline wadsleyite, b-(Mg 0.91 Fe 0.09 ) 2 SiO 4 , were measured up to 470 K by means of the resonant sphere technique. The adiabatic bulk (K S ) and shear (m) moduli were found to be 165.72(6) and 105.43(2) GPa at room temperature. The average slopes (dK S /dT and dm/dT) in the range were determined to be À0.0175(3) and À0.0159(1) GPa/K. We estimated that the P-, S-wave velocity and density jumps for the a-to b-phase transformation at the 410-km depth condition were 9.5, 11.2 and 5.4%, respectively. These results suggest that the olivine component at the depth should be 52 and 42 volume % for P-and S-waves.

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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.

Three-dimensional atom probe (3DAP) equipped with a newly designed preset-type sample stage has been developed. This new type of sample stage can reduce the costs of constructing a 3DAP instrument, and this instrument consists of quite simple components because the complicated mechanism to make any desired adjustment is no longer required to be done in a vacuum. These advantages are expected to lead to the wider distribution of 3DAP.In addition, a wider range of applications is also expected in our equipment because the atoms can be field-evaporated by means of either high-voltage pulses or femto-second laser pulses. In this article, the performance of the pulse voltage mode and pulse laser mode are described and illustrated through the investigation of metals.