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Breakdown characteristics in homoepitaxial GaN p-n junction diodes with p+-n and p-n+ junctions with relatively heavy doping concentrations are systematically investigated. The devices have vertical deep mesa etch termination, which enables uniform (nearly ideal) avalanche breakdown without electric field (E-field) crowding at the device edge. For p+-n junction, breakdown E-field of 3.0, 3.3 and 3.8 MV/cm and breakdown voltage (BV) of 340, 207 and 128 V were achieved at the donor concentrations of 7.5×1016, 1.5×1017, 3.1×1017 cm-3, respectively. For p-n+ junction, breakdown E-field of 3.2, 3.3 and 4.0 MV/cm and BV of 235, 180 and 110 V were achieved at the acceptor concentrations of 1.3×1017, 1.8×1017, 4.1×1017 cm-3, respectively. No significant difference of the breakdown characteristics between n-type and p-type voltage-blocking layers was observed. These results are consistent with numerical simulations using impact ionization coefficients (IICs) in GaN reported in our previous studies.

Deep-ultraviolet (UV) light-emitting diodes (LEDs) based on AlGaN crystals have low light-emission efficiency; therefore, there is a need to improve this light-emission efficiency for a wide range of applications such as water and air sterilizations. UV-light-transparent device structures are considered one of the many solutions toward increasing light output power. To this end, the present study focused on developing a transparent AlGaN-based tunnel junction (TJ) as the anode of a deep-UV LED. Deep-UV LEDs composed of n+/p+-type AlGaN TJs were fabricated under the growth condition that reduced the carrier compensation in the n+-type AlGaN layers. The operating voltage was 10.8 V under the direct current (DC) operation of 63 A/cm2. In addition, magnesium zinc oxide (MgZnO)/Al reflective electrodes were fabricated to enhance the output power of the AlGaN homoepitaxial TJ LED. The output power was 57.3 mW under a DC operation of 63 A/cm2, and it was 1.7 times higher than that realized using the conventional Ti/Al electrodes. The combination of the AlGaN-based TJ and MgZnO/Al reflective contact allows further improvement of the light output power. This study confirms that the AlGaN TJ is a promising UV-transmittance structure that can obtain a high light-extraction efficiency.

This work examined the intentional generation of recombination centers in GaN p– n junctions on freestanding GaN substrates. Irradiation with a 4.2 MeV proton beam was used to create a uniform distribution of vacancies and interstitials across GaN p+/ n− and p−/ n+ junctions through anode electrodes. With increasing proton dose, the effective doping concentrations were found to be reduced. Because the reduction in the doping concentration was much higher than the hydrogen atom concentration, this decrease could not be attributed solely to carrier compensation resulting from interstitial hydrogen atoms. In fact, more than half of the electron and hole compensation was caused by the presence of point defects. These defects evidently served as Shockley–Read–Hall (SRH) recombination centers such that the SRH lifetimes were reduced to several picoseconds from several hundred picoseconds prior to irradiation. The compensation for holes in the p−/ n+ junctions was almost double that for electrons in the p+/ n− junctions. Furthermore, the SRH lifetimes associated with p−/ n+ junctions were shorter than those for p+/ n− junctions for a given proton dose. These differences can be explained by variations in the charge state and/or the formation energy of intrinsic point defects in the p-type and n-type GaN layers. The results of the present work indicate the asymmetry of defect formation in GaN based on the fact that intrinsic point defects in p-type GaN readily compensate for holes.

Defect levels in the vicinity of the Al2O3/p-type GaN interface were characterized using a sub-bandgap-light-assisted capacitance–voltage ( C–V) method. For metal–oxide–semiconductor (MOS) diodes prepared using p-type GaN (p-GaN) and Al2O3 formed by atomic layer deposition, the C–V curves measured in the dark showed capacitance saturation at a negative bias and a large negative voltage shift compared with ideal curves, which implied the effects of donor-like gap states in the vicinity of the Al2O3/p-GaN interface. Upon illumination with monochromated sub-bandgap light with photon energies higher than 2.0 eV under a large positive bias, the subsequently measured C–V curves showed three plateaus. The plateau under the positive bias voltage due to the surface inversion appeared despite the sub-bandgap illumination, which did not appear at 1.8 eV light illumination, indicating the existence of midgap defect levels. Moreover, the other plateaus were attributed to defect levels at 0.60 and 0.7–0.8 eV above the valence band maximum. For a sample whose surface was prepared by photo-electrochemical (PEC) etching to a depth of 16.5 nm, the C–V curve measured in the dark showed a reduced voltage shift compared with the unetched sample. Furthermore, sub-bandgap-light-assisted C–V curves of the sample with PEC etching showed no plateau at a positive bias, which indicated the reduction in the density of the midgap defect states. Possible origins of the detected defect levels are discussed. The obtained results showed that the interface control can improve the properties of p-GaN MOS structures.

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.

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.

We report on homoepitaxial GaN p-n junction diodes with a negative beveled-mesa termination. The electric field distribution in a beveled-mesa was investigated using TCAD simulation, and the devices were designed using currently available GaN growth techniques. Shallow-angle (ca. 10°) negative bevel GaN p-n junction diodes were fabricated with various Mg acceptor concentrations in the p-layers. The suppression of electric field crowding and improvement of the breakdown voltage were observed, as the Mg concentration was decreased. The parallel-plane breakdown field of 2.86 MV/cm was obtained for a device with the breakdown voltage of 425 V.

In this study, we analyzed the slow decay in time-resolved photoluminescence (TR-PL) of n-type GaN homoepitaxial layers with carbon concentrations of (0.26–4.0) × 1016 cm−3. The relative signal intensities of the slow decays to the TR-PL signals at t = 0 s increased almost linearly with increased carbon concentration, suggesting that the carrier recombination process is subjected to the deep level formed by the carbon atoms in GaN. Slow decay curves were calculated based on the rate equations for trapping and emission at the deep level. The experimental carbon concentration dependence of the time constants and the relative signal intensities was reproduced by calculation. TR-PL is a technique used to estimate carbon concentrations in GaN homoepitaxial layers.

Objective This retrospective study examined whether adding the maximum standardized uptake value of a primary tumor to the consolidation-to-tumor ratio from a high-resolution computed tomography scan can improve the predictive accuracy for pathological noninvasive lung cancer and lead to better patient selection for sublobar resection. Methods We included 926 patients with clinical stage IA non–small cell lung cancer. Pathological noninvasive cancer (n = 515) was defined as any case without lymphatic invasion, vascular invasion, or lymph node metastasis. The prediction accuracies of maximum standardized uptake value and consolidation-to-tumor ratio were evaluated using receiver operating characteristic curves and area under the curve. Results For consolidation-to-tumor ratio or maximum standardized uptake value alone, the area under the curves were 0.733 (95% confidence interval, 0.708-0.758) and 0.842 (95% confidence interval, 0.816-0.866), respectively. When the consolidation-to-tumor ratio and maximum standardized uptake value were combined, the area under the curve was 0.854 (95% confidence interval, 0.829-0.876). However, to obtain a predictive specificity of 97%, sensitivity needed to be 42.5% for the consolidation-to-tumor ratio, 38.3% for the maximum standardized uptake value, and 45.0% for these 2 in combination. Conclusions Our results suggest that despite the high area under the curve for maximum standardized uptake value, caution is needed when using maximum standardized uptake value to select candidates for sublobar resection. We found that a low maximum standardized uptake value did not mean the tumor was a pathological noninvasive lung cancer. Therefore, using consolidation-to-tumor ratios from high-resolution computed tomography to decide whether sublobar resection is appropriate for patients with clinical stage IA non–small cell lung cancer is better than using maximum standardized uptake value when setting specificity to a conservative 97% for predicting pathological noninvasive lung cancer.

Defects introduced through fabrication processes of GaN power devices are discussed. The dislocations threading through a freestanding GaN substrate and the upper p-n junction can act as a pathway of current leakage but the number of ‘killer’ dislocations causing significant leakage under a reverse bias is limited. Thus, the electrical property of each dislocation type needs to be studied to the exclusion of factors such as an electric field crowding and growth pits. The carrier compensation due to carbon makes it difficult to control the effective donor concentrations in n-type drift layers of high-voltage devices, so that the growth condition and method are tuned to suppress the carbon incorporation. A formation process of gate oxide produces interface states and traps in the oxide, which can be reduced by a post-deposition annealing. Ion implantation technologies for selective area doping introduce point defects which are transformed into various types of defects through post-implantation annealing, resulting in compensating holes and forming electrically inactive dopants. Ultra-high-pressure annealing at high temperature allows reducing such defects and activating dopants efficiently. To evaluate defects induced via dry etching is challenging because such defects is located near the surface. Methods to characterize etching-induced deep levels are introduced.

Deep levels in GaN are summarized. E1 and E3 electron traps at the respective energies of around EC − 0.25 eV and EC − 0.6 eV have been commonly observed in n-type GaN layers. H1 hole trap at around EV + 0.9 eV is reported in both n-type and p-type GaN layers, and likely associates the yellow luminescence band in a photoluminescence spectrum. The concentration of Ha hole trap at around EV + 0.3 eV in p-type GaN layers is detected with a nearly equal concentration of H1 hole trap. Based on the comparison with the first-principles calculations using hybrid functionals and the photoluminescence studies, the origins of E3, H1 and Ha traps are most likely attributed to 0/- charged state of iron on Ga site (FeGa), 0/− and +/0 charged states of carbon on nitrogen site (CN), respectively. Among deep levels induced by irradiation, EE1 and EE2 at around EC − 0.12 eV and EC − 1 eV correspond to the calculated energy levels of the nitrogen vacancy (VN) and the interstitial (NI). Finally, deep levels compensating carriers in n-type and p-type GaN layers are extracted.

This book focuses on defects in GaN based on the most up-to-date intrinsic material properties, and addresses deep levels and their analytical methods within the wide bandgap of GaN. It demonstrates nanoscale structures of extended defects in GaN using atomic-scale transmission electron microscopy. The identifi cations of their deep levels and extended defect structures are presented by comparing with reported fi rst-principles calculations. It reviews emerging technologies for defect characterizations using atom probe tomography, synchrotron x-ray diffraction topography in wafer scale, and multiphoton-excitation photoluminescence, which allows for the multidirectional characterization of structural defects. Readers will gain insight into:Electrical impacts of defects in GaN-based vertical power devicesPathways to the defect control in the fabrication process of GaN-based electric devicesUp-to-date methods for semiconductor and electronic material defect analysis Characterization of Defects and Deep Levels for GaN Power Devices is an ideal reference for industry materials scientists working in semiconductor materials and devices. It is also suitable for experts in DLTS, TEM, APT, XRDT, and multiphoton-excitation photoluminescence.

Research history of GaN-based devices and defects reducing the performance is reviewed. Vertical GaN power devices are likely sensitive to threading dislocations compared with the lateral devices because the high electric field along with the dislocations are applied. Deep levels formed via point defects compensate carriers in vertical power devices because of the lower carrier concentrations compared with optical devices. The physical properties of GaN are also summarized based on the most reliable experimental data, which is the basis of discussions on characterizations of defects in this book.

Analytical methods for investigating deep levels in GaN are reviewed. Quantitative accuracy of deep-level transient spectroscopy (DLTS) technology is discussed. The isothermal method allows a quick measurement and is useful to characterize trap states for the case that a temperature scan causes reaction of defects or the occupancy condition for each filling pulse needs to be kept constant. The DLTS measurement for hole traps in p-type GaN needs special considerations due to the relatively large ionization energy of magnesium acceptors, where the low-frequency DLTS system is utilized. For investigation of minority carrier traps, DLTS using injection bias pulses for p-n junctions is the most reliable method for the quantitative evaluation. DLTS methods using light filling pulses such as minority carrier transient spectroscopy and optical DLTS are applied for samples without p-n junctions, which needs considerations of light absorption, the minority carrier diffusion length, and the optical capture cross sections of deep levels. Investigation of deep levels far from both band edges requires the approaches without the use of thermal emission process. For this aim, this chapter overviews deep-level optical spectroscopy, photoluminescence involving a time-resolved method, positron annihilation spectroscopy, and an analysis of recombination current in a p-n junction.

In this study, we clarify the impact of annealing pressure and temperature of the ultra-high-pressure annealing (UHPA) as a post-implantation-annealing (PIA) on the acceptor activation for Mg-ions-implanted GaN samples. The pressure to prevent the thermal decomposition is absolutely determined by the equilibrium N 2 partial pressure in the phase diagram of GaN-Ga-N 2 system, so that the pressure below the critical pressure always leads to the serious surface decomposition of GaN. In low temperature cathodoluminescence examinations, the samples processed at 1400°C or above exhibited more intense emissions in the near band edge and donor-acceptor pair band with a suppression of green luminescence related to point defects. Temperature dependent Hall-effect measurement was allowed for the UHPA samples annealed at 1400°C or above, whereas free holes were not observed for the 1300°C-UHPA. Moreover, the acceptor concentrations (N a ) for samples annealed at 1400°C or above were close to Mg concentrations. We thus revealed the key process parameters in UHPA, which is at more than 1400°C under the N 2 overpressure exceeding the equilibrium partial pressure. These findings can play as a key role in the selective area doping using UHPA.

We report density-functional calculations that clarify the atomic and electronic structures of the oxygen vacancy ${V}_{\mathrm{O}}$ in amorphous $({\mathrm{Al}}_{2}{\mathrm{O}}_{3}{)}_{1\ensuremath{-}x}({\mathrm{Si}\mathrm{O}}_{2}{)}_{x}$ mixed oxides, which are promising candidates for the gate insulator in $\mathrm{Ga}\mathrm{N}$ technology. We construct microscopic models of the amorphous structure by the melt-quench scheme and then examine all the possible oxygen vacancies and determine the total-energy-minimized ${V}_{\mathrm{O}}$ structures. We find a clear tendency for ${V}_{\mathrm{O}}$'s formed at oxygen sites surrounded by fewer $\mathrm{Al}$ atoms to have a lower formation energy. More importantly, we find that a ${V}_{\mathrm{O}}$ surrounded by $\mathrm{Si}$ atoms alone does not induce deep levels in the energy gap of $\mathrm{Ga}\mathrm{N}$, whereas ${V}_{\mathrm{O}}$'s surrounded by some $\mathrm{Al}$ atoms induce a deep level. This theoretical finding strongly implies that the majority of ${V}_{\mathrm{O}}$'s in amorphous $({\mathrm{Al}}_{2}{\mathrm{O}}_{3}{)}_{1\ensuremath{-}x}({\mathrm{Si}\mathrm{O}}_{2}{)}_{x}$ are electrically inactive and not very harmful to device operation. Further, we explore the possibilities for structural transformation from the electrically active ${V}_{\mathrm{O}}$ to the electrically inactive ${V}_{\mathrm{O}}$. We identify reaction pathways for such transformations and obtain the corresponding energy barriers. The calculated occurrence rate for these transformations is high enough to ensure that thermal annealing at typical temperatures and times causes conversion of electrically active ${V}_{\mathrm{O}}$'s to inactive ${V}_{\mathrm{O}}$'s, providing a further advantage of $({\mathrm{Al}}_{2}{\mathrm{O}}_{3}{)}_{1\ensuremath{-}x}({\mathrm{Si}\mathrm{O}}_{2}{)}_{x}$ as a gate insulator over other oxides such as ${\mathrm{Si}\mathrm{O}}_{2}$ and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$.

We propose a novel method to extract impact ionization coefficients of electrons and holes using above-and sub-bandgap illuminations for a p−/n+ junction diode. For above-bandgap illumination, the light is absorbed near p-GaN surface. Then, generated minority carriers diffuse and reach the edge of the depletion layer, resulting in an electron-injected photocurrent. On the other hand, for sub-bandgap illumination, the light is selectively absorbed near the p-n junction interface by the Franz-Keldysh effect, resulting in a hole-injected photocurrent. The electron- and hole-initiated multiplication factors are obtained as the ratios of the measured photocurrents to the calculated unmultiplicated photocurrents. By analyzing the electron- and hole-initiated multiplication factors, the impact ionization coefficients of electrons and holes in GaN are extracted separately.

This work examined reverse leakage currents in GaN p-n junctions nearly free of dislocations. Diodes with shallow bevel mesas and breakdown voltages (BVs) in the range of 130–1000 V exhibited avalanche breakdown at the designed voltages. Significant leakage currents were observed in response to reverse bias values far below the BVs and a weak effect of temperature was also evident. The data were explained based on direct band-to-band tunneling (BTBT). The BTBT current was dominant in those devices having BVs of several hundred volts but was far below the detection limit in the case of a 1000 V-class diode.

The ratio of the photoionization cross sections (σno/σpo) of carbon substituting at the nitrogen site [CN (0/−)] in n-type GaN, which is detected as a hole trap H1 (EV+0.85 eV) under sub-bandgap-light irradiation (390 nm), is determined with isothermal capacitance transient spectroscopy (ICTS). The current-injection ICTS and the sub-bandgap-light-excited ICTS were compared for the same p+-n junction diode, whereby the hole occupancy ratio (fT) was obtained. Analysis of the dependence of fT on the temperature gave σno/σpo of 3.0 was then used to estimate the charge state of CN (0/−) under sub-bandgap-light irradiation.

AlGaN-based ultraviolet light-emitting diodes (UV-LEDs) are key components for the inactivation of viruses. Highly efficient and high-power UV-LEDs, capable of inactivating viruses in a short time, are in demand. For this purpose, the growth technologies of n-type AlGaN contact layers were developed from two points of view: first, to decrease the resistivity of n-type Al0.62Ga0.38N by minimizing the electron compensation, resulting in electronic degeneracy with metallic conduction;second, to improve the light emission uniformity in AlGaN multiquantum wells (MQWs) by controlling the morphology of the underlying n-type AlGaN layer to inhibit macrostep formation. A UV-LED module emitting at 275 nm was demonstrated with the developed growth technology, and illuminated with an irradiation power of 2.6 mW cm(-2) on SARS-CoV-2 samples. Over 99.999 % of viruses were inactivated within 5 s owing to the high power of this module.

Reliability tests involving the application of high electrical stresses were employed to assess GaN-based vertical p-n junctions fabricated on freestanding GaN substrates with threading dislocation densities less than 104 cm−2. Electric field crowding at the device edges was eliminated by employing a shallow bevel mesa structure, thus allowing an evaluation of the reliability of the internal p-n junctions. The p-n diodes exhibited reproducible avalanche breakdown characteristics over the temperature range of 25–175 °C. No degradation was observed even during tests in which the devices were held under a reverse bias near the breakdown voltage. Despite this high degree of reliability in response to reverse bias stress, a small number of diodes were degraded during continuous forward current tests, although the majority of diodes remained unchanged. The reverse leakage current exhibited by degraded diodes was increased with an increase in the forward current density within the range of 50–500 A/cm2, while the breakdown voltages were unchanged in response to current stress. The leakage level increased exponentially with an increase in the total amount of injected carriers but eventually plateaued. In the degraded p-n diode, a luminous point in an emission microscope corresponded to one of the threading dislocations observed in the synchrotron x-ray topography, indicating that a specific dislocation played as a leakage path after injecting carriers.

A novel sintered tantalum carbide coating (SinTaC) prepared via a wet ceramic process is proposed as an approach to reducing the production cost and improving the crystal quality of bulk-grown crystals and epitaxially grown films of wide-bandgap semiconductors. Here, we verify the applicability of the SinTaC components as susceptors for chemical vapor deposition (CVD)-SiC and metal-organic chemical vapor deposition (MOCVD)-GaN epitaxial growth in terms of impurity incorporation from the SinTaC layers and also clarify the surface-roughness controllability of SinTaC layers and its advantage in CVD applications. The residual impurity elements in the SinTaC layers were confirmed to not severely incorporate into the CVD-SiC and MOCVD-GaN epilayers grown using the SinTaC susceptors. The quality of the epilayers was also confirmed to be equivalent to that of epilayers grown using conventional susceptors. Furthermore, the surface roughness of the SinTaC components was controllable over a wide range of average roughness (0.4 ≤ Ra ≤ 5 μm) and maximum height roughness (3 ≤ Rz ≤ 36 μm) through simple additional surface treatment procedures, and the surface-roughened SinTaC susceptor fabricated using these procedures was predicted to effectively reduce thermal stress on epi-wafers. These results confirm that SinTaC susceptors are applicable to epitaxial growth processes and are advantageous over conventional susceptor materials for reducing the epi-cost and improving the quality of epi-wafers.

The hysteresis of the capacitance–voltage (C–V) characteristics of an Al2O3/n-GaN metal–insulator–semiconductor structure was evaluated under light (white LED)-irradiation and dark conditions. The hysteresis was not observed under the dark condition but was observed under the light-irradiation condition. The GaN surface was completely depleted in negative bias under the dark condition. The C–V characteristics indicated that the hysteresis is caused by hole trapping under the LED irradiation condition and that the holes are generated in the n-GaN surface by LED irradiation and subsequently injected into the Al2O3 films. When the holes are generated in the depletion region of GaN for any reason, such as a short generation lifetime, they can be trapped in the Al2O3 films.

For the solution to global energy issues, highly-efficient energy conversion using next-generation power devices is required. Gallium nitride (GaN) having superior properties such as high breakdown electric field (2.8–3.75 MV/cm) is a powerful candidate for next-generation high-power semiconductor devices. [1] , [2] The selective area doping makes it possible to precisely engineer high-power devices with complex structures, allowing formation of low-resistivity region for contacting electrodes and optimization of the electric field configuration in the edge termination represented by the field limiting ring (FLR) and in the junction barrier Schottky (JBS) structures, as used in Si and SiC power devices. [3]

In this study, we investigate the sources of unintentional threshold voltage $(V_{\mathrm{TH}})$ lowering in GaN MOSFETs by analyzing a dependence of $V_{\mathrm{TH}}$ on body voltage $(V_{\mathrm{B}})$ . Through the $V_{\mathrm{TH}}-V_{\mathrm{B}}$ characteristics of the MOSFETs, we concluded that the $V_{\mathrm{TH}}$ lowering results from the relatively large amount of positive charge at the MOS interface rather than the donor in the p-type GaN layer.

A valanche multiplication characteristics in GaN p-n junction diodes (PNDs) under high reverse bias conditions were investigated. The GaN-on-GaN PNDs with double-side-depleted shallow bevel termination, which showed low reverse leakage current and excellent avalanche capability, were used for the measurements. Under sub-bandgap light illumination, the photocurrents induced by Franz-Keldysh (FK) effect, which can be well reproduced by the theoretical calculations of the optical absorption, and their avalanche multiplications were observed. The multiplication factors were extracted as the ratios of the experimental photocurrents to the calculated FK-induced photocurrent. Under an assumption of equal impact ionization coefficients of electrons and holes, the electric-field dependence of an impact ionization coefficient in GaN were estimated.

Ion implantation (I/I) and annealing techniques to enable high carrier doping into selected regions are one of important research fields for realization of GaN power devices. However, particularly difficult research challenges are present in Mg-I/I into GaN as p-type doping technique. First problem is related to nitrogen vacancies (VN), crystal defects introduced by Mg-I/I . [1] Second issue is connected to degeneration of GaN surface by pyrolysis reaction during high-temperature annealing process. We examined Mg/N co-implantation into GaN as p-doping in order to compensate of VN defects. Research on ultra-high pressure thermal activation process to maintain equilibrium conditions at high temperature was conducted to avoid degradation of GaN surface. We prepared Mg/N co-implanted GaN-on-GaN samples with 300-nm-deep Mg-box-profile of 1E19 cm-3 and with N-box-profile of various concentrations in the range 0 ~ 1E20 cm-3. The samples were annealed without a protection cap layer at temperature between 1300 °C and 1480 °C under 1 GPa in N2 atmosphere. Samples capped with sputter-AlN were treated by conventional lamp annealing at 1300 °C for a comparison purpose. We obtained the dominant Green Luminescence (GL) emission at 2.3 eV and the recessive Donor Acceptor Pair (DAP) emission at 3.2 eV from a low-temperature cathodoluminescence (LT-CL) spectra in Mg-I/I samples without N co-implantation. This is in good agreement with results published before.[1][2] In addition to this, we found that the Mg/N co-implantation, for optimized N-ion dose, suppresses the GL intensity while maintaining the DAP intensity. Furthermore, in the ultra-high pressure thermal activation process, the DAP intensity markedly increases with anneal temperature and GL intensity suppression is also visible. These results strongly suggest that the origin of GL is related to VN, and the VN defect compensation occurs by N co-implantation. We also compare the AFM images of GaN surface roughness. The ultra-high pressure annealed surfaces were as smooth (RMS = 0.2~0.3 nm) as the as-implanted samples (0.3 nm), whereas surface treated with conventional lamp was rough (1.9 nm). We can, therefore, conclude that Mg/N co-implantation and ultra-high pressure thermal activation process allows activation of the Mg acceptor and recovery of p-GaN crystal quality. Such treatment results in VN defect compensation and a smooth surface of the annealed sample. This work was supported by MEXT “Program for research and development of next-generation semiconductor to realize energy-saving society.” [1] A. Uedono et al., Phys. Status Solidi B 252, No. 12 (2015) [2] K. Kojima et al., Appl. Phys. Express 10, 061002 (2017).

We report homoepitaxial GaN p-n junction diodes with novel beveled-mesa structures. The n-layers and p-layers, the doping concentrations of which are comparable, were prepared. We found that electric field crowding does not occur in the structure using TCAD simulation. The fabricated devices showed the breakdown voltages of 180–480 V, small leakage currents, and excellent avalanche capabilities. The breakdown voltages increased at elevated temperature. At the breakdown, nearly uniform luminescence in the entire p-n junctions was observed in all the devices. These results are strong evidences that the uniform avalanche breakdowns occurred in the devices. We carefully characterized the depletion layer width at the breakdown, and the parallel-plane breakdown electric fields of 2.8-3.5 MV/cm were obtained, which are among the best of the reported non-punch-through GaN vertical devices.

Avalanche multiplication characteristics in a reverse-biased homoepitaxial GaN p–n junction diode are experimentally investigated at 223–373 K by novel photomultiplication measurements utilizing above- and below-bandgap illumination. The device has a non-punch-through one-side abrupt p–-n+ junction structure, in which the depletion layer mainly extends to the p-type region. For above-bandgap illumination, the light is absorbed at the surface p+-layer, and the generated electrons diffuse and reach the depletion layer, resulting in an electron-injected photocurrent. On the other hand, for below-bandgap illumination, the light penetrates a GaN layer and is absorbed owing to the Franz–Keldysh effect in the high electric field region (near the p–n junction interface), resulting in a hole-induced photocurrent. The theoretical (non-multiplicated) photocurrents are calculated elaborately, and the electron- and hole-initiated multiplication factors are extracted as ratios of the experimental data to the calculated values. Through the mathematical analyses of the multiplication factors, the temperature dependences of the impact ionization coefficients of electrons and holes in GaN are extracted and formulated by the Okuto–Crowell model. The ideal breakdown voltage and the critical electric field for GaN p–n junctions of varying doping concentration are simulated using the obtained impact ionization coefficients, and their temperature dependence and conduction-type dependence were discussed. The simulated breakdown characteristics show good agreement with data reported previously, suggesting the high accuracy of the impact ionization coefficients obtained in this study.

N-type GaN epitaxial layers grown via metal organic vapor-phase epitaxy typically exhibit a yellow luminescence (YL) band owing to carbon-related deep levels in the photoluminescence spectra. The decay of YL after pulse excitation involves a long time constant (∼0.2 ms at room temperature), whereas microwave photoconductivity decay (μ-PCD) curves show the corresponding component of the time constant. To clarify the origin of the long decay time, the temperature-dependent time constants of YL decay and μ-PCD curves are analyzed using a numerical model based on rate equations for trapping and emission through a deep level. The characteristics of the decays are well reproduced by a recombination model using a hole trap H1 at an energy of EV + 0.88 eV because of the acceptor-like state of carbon on a nitrogen site (CN) whose electron capture cross section (σn) is estimated to be 3 × 10−21 cm2. The slow decay in μ-PCD signals indicates that the electrons before being captured to H1 traps are free electrons in the conduction band. These findings indicate that the slow recombination process through CN results in tail currents in the turn-off switching periods of devices.