Abstract
The success of the next generation of (Al)GaN-based power devices depends on the ability to form selective-area p-type regions, which can be accomplished by ion implantation of the prevailing acceptor dopant, Mg. Ion implantation induces lattice damage and creates point defects within the material, which evolve in response to activation annealing. Lattice dilation can be reduced and point defects removed or agglomerated into clusters by annealing [1], however sufficiently extreme annealing conditions degrade or decompose GaN [2]. The p-type activation of Mg-implanted GaN can be further aided by co-implantation with N, which is believed to enhance the removal of implant-induced nitrogen vacancies (VN) in the material that act as donors and compensate free holes [3].
To restore the bulk crystalline quality, lower the concentration of defects, and activate the implanted dopants without degrading the material, specialized annealing techniques are required either to heat very rapidly [4] and/or use ultra-high pressure conditions [5] in order to obtain high-conductivity p-type material, and activation of Mg-implanted material remains an area of active research.
In this study, we report on the micro-structural changes in p-type implanted GaN material after activation-annealing by pulsed gyrotron microwave. MOCVD-grown unintentionally doped (UID) GaN is grown on n+-HVPE GaN, co-implanted with Mg/N to a depth of 250 nm, capped with AlN, and annealed using a gyrotron under an N2 overpressure of 3 MPa. Gyrotron microwave annealing is performed in pulses of 5–8 sec with maximum temperatures between 1350–1450 °C. Detailed optical, structural, and electrical characterization are conducted to better understand the impact on defect microstructure, and to verify p-type conductivity. Mitigation of implant-induced damage to the lattice is further characterized by transmission electron microscopy (TEM) and x-ray diffraction (XRD). We correlate damage removal (by this annealing methodology) with a trend in optical signature through spatially-resolved low-temperature PL. At the optimized anneal condition of 5 pulses at 1450 °C maximum temperature, and 8 s pulse length, the average intensity ratio of the MgGa acceptor-related UVL to the implant-induced donor VN-related GL2 is 6.6 with a standard deviation 66% of the mean across the material, suggesting high and uniform acceptor activation relative to unoptimized beam conditions. We also characterize regions of the sample in which the GL2 luminescence is suppressed and a dominant YL emerges in the 2.2–2.5 eV range, which is attributed to unintentionally introduced CN-ON complexes in GaN grown by MOCVD [5]. For the optimized annealing condition, diode I-V measurements indicate clear p-n diode behavior with threshold voltages between 3.1–3.5 V and Ion/Ioff ratio of up to 106.
This work is funded by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy under the PNDIODES programs directed by Dr. Isik Kizilyalli.
[1] Y. Wang, et al., physica status solidi (b), 257.4, 1900705 (2020)
[2] G. Alfieri, et al. J. Appl. Phys., 123, 205303 (2018)
[3] R. Tanaka et al. Japan. J. Appl. Phys., 59, SGGD02 (2020)
[4] J. D. Greenlee, et al, ECS J. Solid State Sci Technol., 4(9) (2015)
[5] K. Iwata et al, J. Appl. Phys., 127(10) (2020)
[6] V. Meyers et al. in review, J. Appl. Phys. (2021)
Figure 1