We report a systematic investigation of the transport properties of highly degenerate electrons in Ge-doped and Si-doped GaN epilayers prepared using the pulsed sputtering deposition (PSD) technique. Secondary-ion mass spectrometry and Hall-effect measurements revealed that the doping efficiency of PSD n-type GaN is close to unity at electron concentrations as high as 5.1 × 1020 cm−3. A record low resistivity for n-type GaN of 0.16 mΩ cm was achieved with an electron mobility of 100 cm2 V−1 s−1 at a carrier concentration of 3.9 × 1020 cm−3. We explain this unusually high electron mobility of PSD n-type GaN within the framework of conventional scattering theory by modifying a parameter related to nonparabolicity of the conduction band. The Ge-doped GaN films show a slightly lower electron mobility compared with Si-doped films with the same carrier concentrations, which is likely a consequence of the formation of a small number of compensation centers. The excellent electrical properties presented in this letter clearly demonstrate the striking advantages of the low-temperature PSD technique for growing high-quality and highly conductive n-type GaN.

The growth of Mg-doped GaN over a wide doping range is demonstrated via pulsed sputtering deposition (PSD). All samples show p-type conductivity without any post-growth annealing, and their room temperature (RT) hole concentration can be controlled for as much as two orders of magnitude from 2.8 × 1016 cm−3 to 2.7 × 1018 cm−3. No apparent structural degradation is observed, even for the most heavily Mg-doped sample with an RT hole concentration of 2.7 × 1018 cm−3. The compensation ratio is lower than 10% for the heavily Mg-doped samples, which explains the high conductivity of PSD-grown p-type GaN. These results indicate the strong potential of PSD for the growth of high-quality p-type GaN and its application to p–n junction devices.

We have characterized highly conductive Si-doped GaN films with a high electron mobility of 112 cm2V−1s−1 at an electron concentration of 2.9 × 1020 cm−3, prepared using pulsed sputtering deposition (PSD). With an increase in the doping concentration, the absorption edge was found to shift toward a higher energy level, owing to the Burstein-Moss effect, thus making this material suitable for the transparent conductive tunneling electrodes of visible and ultraviolet-A light-emitting diodes. The full width at half maximum value of the near-band-edge (NBE) emissions in a photoluminescence spectrum measured at 77 K was as small as 185 meV, even for the sample with the highest electron concentration of 2.9 × 1020 cm−3. Such sharp NBE emissions from PSD-grown heavily Si-doped GaN films can be explained by an analytical model with a low compensation ratio θ of around 0.1, which is consistent with the exceptionally high observed electron mobility. These results indicate the strong potential of the low-temperature PSD growth technique for the formation of high-quality, heavily Si-doped GaN.

We report the characteristics of heavily Si-doped GaN prepared by pulsed sputtering deposition (PSD) and its application as tunneling junction (TJ) contacts for nitride-based light-emitting diodes (LEDs). We determined that the use of PSD allows us to grow extremely heavily Si-doped wurtzite GaN epitaxial layers with [Si] = 1.0 × 1021 cm−3 on commercially available UV-A LED wafers. Then, we processed these samples into LED structures to investigate their device characteristics as TJ-contact LEDs. Compared to commercially available UV-A LEDs with transparent conductive oxide contacts, TJ-contact LEDs with [Si] = 1.0 × 1021 cm−3 at the tunnel-junction interface showed lower differential resistance and, consequently, worked well under a high injection current density of ∼1 kA/cm2 without any degradation. These results indicate that PSD-grown heavily doped n-type GaN is promising for application as a TJ-contact in group III nitride-based optoelectronic devices.