Herein, we propose and demonstrate the edge termination for GaN-based one-sided abrupt p–n junctions. The structure is comprised of a combination of a shallow negative bevel mesa and selective-area p-type doping under the mesa. Based on the Technology Computer Aided Design (TCAD) simulation, the maximum electric field at the junction edge is markedly reduced to approximately 1.3 times that of the parallel-plane electric field in the proposed structure, which is almost half of the unimplanted diode. The TCAD simulation also shows that the shallow mesa angle of 6° effectively reduces the optimum acceptor concentration (Na) in the implanted region and enhances the breakdown voltage. The optimum Na value can be covered by the proposed technology based on the Mg-ion implantation and subsequent ultra-high-pressure annealing (UHPA). Using the formation of the shallow bevel mesa, the Mg-ion implantation, and the UHPA process, we experimentally demonstrate the p–n diodes with a breakdown voltage over 600 V, which is in good agreement with the TCAD simulation. The proposed method can be applied to a vertical trench-gate metal-oxide-semiconductor field-effect transistor with a high figure-of-merit.
The key feature for the precise tuning of Vth in GaN-based metal-insulator-semiconductor (MIS) high electron mobility transistors is the control of the positive fixed charge (Qf) at the insulator/III-N interfaces, whose amount is often comparable to the negative surface polarization charge (Qpol−). In order to clarify the origin of Qf, we carried out a comprehensive capacitance-voltage (C-V) characterization of SiO2/AlxGa1–xN/GaN and SiN/AlxGa1–xN/GaN structures with Al composition (x) varying from 0.15 to 0.4. For both types of structures, we observed a significant Vth shift in C-V curves towards the positive gate voltage with increasing x. On the contrary, the Schottky gate structures exhibited Vth shift towards the more negative biases. From the numerical simulations of C-V curves using the Poisson's equation supported by the analytical calculations of Vth, we showed that the Vth shift in the examined MIS structures is due to a significant decrease in the positive Qf with rising x. Finally, we examined this result with respect to various hypotheses developed in the literature to explain the origin of the positive Qf at insulator/III-N interfaces.
P-type doping in selected areas of gallium nitride (GaN) using magnesium (Mg)-ion implantation and subsequent ultra-high-pressure annealing (UHPA) are investigated to improve the performance of vertical GaN power devices. UHPA allows a high-temperature process without decomposition of the GaN surface and virtually complete activation of the implanted Mg ions in GaN. In the present paper, we provide an overview of recent challenges in making UHPA more realistic as an industrial process. Instead of UHPA at more than 1400 °C for a short duration, prolonged UHPA at 1300 °C demonstrates a comparable acceptor activation of Mg-ion-implanted GaN. This can reduce the annealing pressure to approximately 300 MPa and enlarge the processable wafer diameter. The second challenge is controlling the doping profiles in the lateral and vertical directions. We demonstrate fine patterning of the p-type regions, which indicates the limited lateral diffusion of Mg through UHPA. However, controlling the vertical doping profile is challenging. The nitrogen vacancies formed by ion implantation reduce the effective acceptor concentration near the surface, which can be compensated for by sequential nitrogen ion implantation. Defect-assisted Mg diffusion to the deeper region causes a redistribution of the Mg atoms and should be considered in the design of a device. Such anisotropic diffusion of Mg to the c-axis has potential applications in the fabrication of unique vertical device structures such as super junctions.
A nearly-ideal edge termination for GaN p-n junctions was designed and demonstrated using Mg-ions implanted field limiting rings (FLRs). The FLRs were fabricated via the ultra-high-pressure annealing process after implanting Mg-ions into the etched n-type region outside the main p-n junction. The results of the technology computer-aided design simulation indicate that by optimizing the space and width of the rings, the breakdown voltage (BV) can be increased by over 90% of the ideal parallel plane BV (973 V). Accordingly, the fabricated diodes exhibited low leakage current and a BV of 897 V (92% of the ideal BV).
We investigated the excitation intensity (U) dependent photoluminescence (PL), at room temperature (RT), from GaN-based metal-insulator-semiconductor structures under gate bias (V G) from accumulation to deep depletion resulting in variations of the space charge region width. We found that depending on V G , different U-dependencies of the YL band energy position (blueshift or redshift), shape (band enlargement or narrowing) and intensity (signal saturation) can be obtained. In order to explain such an unusual YL behavior, we developed a phenomenological PL model, which is based on the solution of the three-dimensional Poisson's equation, current continuity equations and rate equations, and which takes into account the grain structure of GaN layers and the contribution of interface regions into recombination processes. Our model reproduced well the experimental U-dependencies of the YL band intensity. It also predicts that YL arises from the donor-acceptor pair (DAP) recombination in very limited areas (width of several nanometers) inside the depletion regions related to grain/grain interfaces and external crystal surfaces. On this basis, we showed that V G-controlled Udependencies of the YL peak position and shape, can be well explained if we assume that YL is due to DAP-type transitions, in which the final state consists of the Coulomb interaction and strong interaction between the dipole moment of ionized DAP and the depletion region electric field. This recombination mechanism can play a significant role at RT, but should be negligible at low temperatures, where one can expect the significant reduction of interface barriers under illumination.
The electrical properties of vertical GaN trench MOSFETs without drift layers were evaluated to investigate the effect of nitrogen plasma treatment on the trench sidewalls. It is demonstrated that nitrogen plasma treatment improves the channel property of the vertical GaN trench MOSFET. The possible mechanism of this improvement is the supply of nitrogen atoms from nitrogen plasma treatment to the trench surfaces, and the compensation of the nitrogen vacancies near the trench surfaces by the nitrogen atoms during gate oxide annealing. The temperature dependence and the limiting factors of the channel property are also discussed.
The electrical properties of AlGaN/GaN MOSHFETs with HfO 2 prepared by atomic layer deposition with and w/o oxygen-plasma treatment (further referred to as PHf-MOS and Hf-MOS) were investigated. The sub-threshold slope of the MOSHFETs (350 and 150 mV dec −1 for Hf-MOS and PHf-MOS, respectively) were lower than that for HFET (450 mV dec −1 ), which also correspond with lower leakage current of the MOSHFETs (∼10 −8 A mm −1 at −9 V for PHf-MOS). In addition, the density of the interface states at the oxide/GaN-cap layer near the conduction band edge and mid-gap (∼5×10 12 and 2×10 11 cm −2 eV −1 , respectively) after PHf-MOS was lower than that for Hf-MOS (∼3× 10 13 and 2×10 12 cm −2 eV −1 , respectively). From the x-ray photoemission spectroscopy analysis we observed a shift in the Auger Ga LMM peaks (0.6 eV) and an increase of the intensity area of the Ga-O bond in the Ga2p3 spectrum after the oxygen-plasma treatment, mainly because the GaN-cap layer was oxidized and Ga 2 O 3 was formed.
Vertical GaN junction barrier Schottky (JBS) diodes with superior electrical characteristics and nondestructive breakdown were realized using selective-area p-type doping via Mg ion implantation and subsequent ultra-high-pressure annealing. Mg-ion implantation was performed into a 10 μm thick Si-doped GaN drift layer grown on a free-standing n-type GaN substrate. We fabricated the JBS diodes with different n-type GaN channel widths Ln = 1 and 1.5 μm. The JBS diodes, depending on Ln, exhibited on-resistance ( RON) between 0.57 and 0.67 mΩ cm2, which is a record low value for vertical GaN Schottky barrier diodes (SBDs) and high breakdown (BV) between 660 and 675 V (84.4% of the ideal parallel plane BV). The obtained low RON of JBS diodes can be well explained in terms of the RON model, which includes n-type GaN channel resistance, spreading current effect, and substrate resistance. The reverse leakage current in JBS diodes was relatively low 103–104 times lower than in GaN SBDs. In addition, the JBS diode with lower Ln exhibited the leakage current significantly smaller (up to reverse bias 300 V) than in the JBS diode with large Ln, which was explained in terms of the reduced electric field near the Schottky interface. Furthermore, the JBS diodes showed a very high current density of 5.5 kA/cm2, a low turn-on voltage of 0.74 V, and no destruction against the rapid increase in the reverse current approximately by two orders of magnitude. This work demonstrated that GaN JBS diodes can be strong candidates for low loss power switching applications.
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