The three-dimensional imaging of threading dislocations in GaN films was demonstrated using two-photon excitation photoluminescence. The threading dislocations were shown as dark lines. The spatial resolutions near the surface were about 0.32 and 3.2 µm for the in-plane and depth directions, respectively. The threading dislocations with a density less than 108 cm−2 were resolved, although the aberration induced by the refractive index mismatch was observed. The decrease in threading dislocation density was clearly observed by increasing the GaN film thickness. This can be considered a novel method for characterizing threading dislocations in GaN films without any destructive preparations.
Halide vapor phase epitaxy of thick GaN films was demonstrated on ScAlMgO4 (SCAM) substrates, and their self-separation was achieved. The 320-µm-thick GaN film was self-separated from the SCAM substrate during the cooling process after the growth. This separation phenomenon occurred because of both the c-plane cleavability of SCAM and the difference in the thermal-expansion coefficients between GaN and SCAM. The dark-spot densities for the GaN films on the SCAM substrates were approximately 30% lower than those on sapphire substrates. These results indicate that SCAM substrates are promising for fabricating a high-quality freestanding GaN wafer at a low cost.
We have demonstrated a fabrication process for the Ohmic contact on low-doping-density p-type GaN with nitrogen-annealed Mg. An Ohmic contact with a contact resistance of 0.158 Ω cm2 is realized on p−-GaN ([Mg] = 1.3 × 1017 cm−3). The contact resistance of p-type GaN with higher Mg concentration ([Mg]=1.0 × 1019 cm−3) can also be reduced to 2.8 × 10−5 Ω cm2. A localized contact layer is realized without any etching or regrowth damage. The mechanism underlying this reduced contact resistance is studied by scanning transmission electron microscopy with energy dispersive x-ray spectroscopy and secondary ion mass spectrometry, representing a mutual diffusion of Ga and Mg atoms on the interface. Reductions in the barrier height and surface depletion width with the nitrogen-annealed Mg layer are confirmed by XPS and Hall effect measurements qualitatively.
ScAlMgO4 (SCAM) substrates with a small lattice-mismatch to GaN and c-plane cleavability are promising for fabricating high-quality free-standing GaN wafers. To reduce the cost in the fabrication of free-standing GaN wafers, the reuse of a SCAM substrate is demonstrated. By cleaving a SCAM substrate which has been already utilized for the growth of a thick GaN film by halide vapor phase epitaxy, the atomically flat surface can be obtained. The threading dislocation density of a 320 μm thick GaN film grown on this cleaved SCAM substrate is 2.4 × 107 cm−2, which is almost the same as that on a new SCAM substrate. This result indicates that a SCAM substrate can be reused for GaN growth.
Halide vapor phase epitaxy of p-type GaN:Mg films was realized by using solid MgO as the Mg source. The Mg concentration was controlled by supplying HCl gas in a MgO source zone. Mg-related photoluminescence peaks were observed at around 3.3 and 2.9 eV. For a sample with a Mg concentration of 2.8 × 1019 cm−3, the Hall-effect measurement showed p-type conduction with a hole concentration and a hole mobility of 1.3 × 1017 cm−3 and 9.1 cm2 V−1 s−1, respectively, at room temperature. The Mg acceptor level was 232 ± 15 meV, which is in good agreement with the previous report.
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