Michal Bockowski

We report room-temperature current-injection electroluminescence (EL) from praseodymium (Pr)-related centers in an ion-implanted gallium nitride (GaN) diode structure. Pr ions were introduced into GaN by ion implantation, and the samples were subsequently subjected to ultrahigh-pressure annealing (UHPA) and metal organic vapor phase epitaxy regrowth to form the p-side layers. Under forward bias, a narrow EL peak at 652 nm was observed. This peak coincides with the Pr-related photoluminescence (PL) peak, indicating that Pr-related centers remain optically active after ion implantation, UHPA, regrowth, and device fabrication and can be electrically excited in the processed GaN structure. Secondary ion mass spectrometry and current density–voltage (J–V) measurements reveal pronounced structural and electrical nonidealities in the fabricated structure, including displacement of the effective junction/interface positions from the target design, impurity redistribution near the regrown region, leakage-related transport, and a large apparent turn-on voltage. The optical data therefore indicate that a Pr-related emission process exists under nonideal current injection, while its efficiency is limited by parasitic current paths, nonradiative losses, and the limited availability of local configurations and defect-related states that enable efficient host-to-Pr energy transfer. The saturation-like behavior of the Pr-related EL, together with the simultaneous increase in near-band-edge and Mg-related emission at higher current densities, suggests that additional current does not proportionally enhance the Pr-related emission but increasingly produces radiative recombination from the GaN host and Mg-related states. These results clarify the excitation and limitation mechanisms of Pr-related EL in ion-implanted GaN diode structures and identify requirements for future electrically driven lanthanoid emitters in GaN.

To fully realize the potential of vertical GaN power devices, proper junction termination methods are needed to reduce the electric field at the device periphery, leading obtaining breakdown voltage values as close as possible to the ideal values. Among these methods, the junction termination extension (JTE), fabricated using selective-area doping using ion implantation or by appropriate shaping of epitaxial p-type layers using plasma etching (step-etched JTE), has proven to be especially effective in obtaining high, robust, non-destructive, avalanche breakdown. However, more research is needed to fully demonstrate the effectiveness and usability of step-etched JTE structures for vertical GaN power devices, especially when supported by measurements of a large, statistically significant number of devices. In this work, we present the fabrication and characterization of 1200 V-class vertical GaN p – i – n diodes with two-zone step-etched JTE structures. By varying the JTE thickness, a high breakdown voltage of over 1700 V and up to 1806 V was obtained, with a termination efficiency up to 91% ideal value. Moreover, an analysis of the measurement of a large number of fabricated devices was also presented, showing low statistical dispersion and tight distribution of breakdown voltage values.

Silicon (Si) is the primary donor dopant in gallium nitride (GaN), introduced through epitaxial growth or ion implantation. However, precise control over Si diffusion remains a critical challenge for high-performance device applications. This study investigates Si diffusion mechanisms in bulk GaN using density functional theory (DFT) calculations, supported by ion implantation (I/I) and ultrahigh-pressure annealing (UHPA) experiments. Vacancy-mediated diffusion pathways were analyzed using the SIESTA code, with minimum energy paths (MEPs) and migration barriers determined via the nudged elastic band (NEB) method. The results indicate that Si diffusion barriers vary with the crystallographic direction, with the lowest barrier of 3.2 eV along [112¯0] and the highest barrier of 9.9 eV along [11¯00], rendering diffusion in this direction highly improbable. Phonon calculations confirm that temperature-induced reductions in effective diffusion barriers are minimal. Experimental validation using SIMS analysis on Si-implanted GaN samples subjected to UHPA (1450 °C, 1 GPa) confirms negligible Si diffusion under these extreme conditions. These findings resolve inconsistencies in prior reports and establish that Si in GaN remains highly stable, ensuring reliable doping profiles for advanced electronic and optoelectronic applications.

In this work, we analyse the reverse current mechanisms in GaN Schottky barrier diodes (SBDs) grown on sapphire and native GaN substrates. For the sapphire-substrate sample, two conduction mechanisms are identified: Poole-Frenkel emission (PFE) and trap-assisted tunneling (TAT), with corresponding trap energy levels of 0.9 eV and 0.29 eV, respectively. In contrast, only PFE is observed in the GaN-substrate sample, with a trap energy of 0.75 eV, suggesting that the presence of TAT is related to the higher dislocation density in structures grown on sapphire substrates. The leakage mechanisms and associated trap energies are extracted by comparing experimental current-voltage (I-V) characteristics with a model that includes thermionic emission and tunneling contributions for different temperatures, from 298K up to 443K.

Annealing behaviors of vacancy-type defects in Mg and N-implanted GaN were studied by positron annihilation. The major defect species in as-implanted samples was identified as Ga-vacancy (VGa)-type defects. For Mg-implanted GaN with sequential N-implantation after annealing above 10000C, the defect species were vacancy clusters such as (VGaVN)3. Due to the downward shift of the Fermi level position resulting from a partial activation of Mg, the charge states of defects tended to become positive. For N-implanted GaN, the size of the vacancy cluster started to decrease above 12000C annealing, which was attributed to recombinations between VNs coupled with VGas and excess N atoms. The impact of sequential N-implantations on vacancies in Mg-implanted GaN was found to be most pronounced when the ratio of the concentration of N to that of Mg was three.

This study presents a computational fluid dynamics (CFD) simulation to investigate fluid flow and heat transfer within the growth zone of gallium nitride crystals synthesized via the alkaline ammonothermal method, with particular emphasis on the influence of seed crystal arrangement and installation geometry. The model analyzes temperature and velocity distributions, highlighting how seed configuration affects turbulent and transitional flow behavior. Key findings demonstrate the effectiveness of CFD in evaluating and optimizing growth zone design. Both simulation and experimental results show that achieving more uniform flow and temperature fields leads to more consistent growth rates and improved structural crystal quality. Furthermore, the study underscores the critical role of installation geometry in shaping flow characteristics such as velocity distribution, temperature gradients, and their transient fluctuations, factors essential for optimizing the ammonothermal crystallization process.

Herein, vertical GaN p–i–n diodes with ultralow bevel mesa (ULBM) termination structures are successfully fabricated using photoresist reflow and single Cl2 ₂ /BCl ₃ /SF ₆ etching step. Using the developed method, bevel mesa structures with slope angles as low as ≈0.8°–1° are fabricated by reducing the GaN etch rate and improving GaN‐to‐photoresist mask etch selectivity through the addition of SF ₆ to the commonly used Cl ₂ /BCl ₃ ‐based plasma. The fabricated diodes with ULBM structures exhibit a low leakage current and repeatable, nondestructive breakdown resulting from avalanche effect, demonstrating termination efficiency of ≈80% with breakdown voltage of about ≈−600 V. This is in contrast to the diodes with nearly vertical or ≈13° bevel mesa structures, for which much lower, often destructive, breakdown voltage (≈ − 350 ÷ −400 V) and higher leakage current are obtained.