Vertical GaN Schottky Barrier Diodes Research Articles

Degradation of vertical GaN diodes during proton and xenon-ion irradiation

We investigated the material stability of a vertical GaN Schottky barrier diode (SBD) against proton irradiations by making real-time measurements. The reverse current gradually decreased with increasing proton fluence. The current of the GaN SBD was reduced by 18% after proton irradiations with a displacement-damage dose (D d) of 1012 MeV g−1. We also examined signal and current degradation occurring in a vertical GaN-on-GaN p–n diode (PND) during xenon-ion irradiations. The signal gradually decreased with increasing xenon-ion fluence. Xenon-ion irradiations of D d = 1012 MeV g−1 reduced the collected charge in the PND by 11%. This signal degradation was close to the current degradation in the GaN SBD caused by the proton irradiations. We found that irradiations with D d > ∼1012 MeV g−1 degraded the performance of the GaN devices.

Mg-implanted vertical GaN junction barrier Schottky rectifiers with low on resistance, low turn-on voltage, and nearly ideal nondestructive breakdown voltage

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.

Demonstration of Vertical GaN Schottky Barrier Diode With Robust Electrothermal Ruggedness and Fast Switching Capability by Eutectic Bonding and Laser Lift-Off Techniques

In this letter, we have successfully transferred the 4-inch crack-free GaN films from sapphire substrate to conductive silicon wafer by employing eutectic bonding and laser lift-off (LLO) techniques. The resultant 1-mm2 fully-vertical GaN Schottky barrier diodes (SBDs) exhibit a high current swing of 109, a low ideality factor of 1.03 and a high forward current of 10 A. Meanwhile, a decent breakdown voltage of 312 V is achieved, which is over 3 times higher than that of control device without performing epitaxial lift-off. Most importantly, such rectifiers show significantly enhanced electrothermal ruggedness, achieving a high surge-current density of 2.6 kA/cm2 and a low thermal resistance of 0.77 K.cm2/W. In addition, the excellent power rectification capability with a low reverse recovery time of 14 ns is obtained under high-speed switching condition with a high current ramp rate (di/dt) of 275 A/μs, implying the desired functionality of the LLO-vertical device architecture. These results thus present the great potentials of the substrate-transferred GaN SBDs for high-power and high-efficiency applications.

Physical mechanism of field modulation effects in ion implanted edge termination of vertical GaN Schottky barrier diodes

In this study, the physical properties of F ion-implanted GaN were thoroughly studied, and the related electric-field modulation mechanisms in ion-implanted edge termination were revealed. Transmission electron microscopy results indicate that the ion-implanted region maintains a single-crystal structure even with the implantation of high-energy F ions, indicating that the high resistivity of the edge termination region is not induced by amorphization. Alternately, ion implantation-induced deep levels could compensate the electrons and lead to a highly resistive layer. In addition to the bulk effect, the direct bombardment of high-energy F ions resulted in a rough and nitrogen-deficient surface, which was confirmed via atomic force microscopy (AFM) and X-ray photoelectron spectroscopy. The implanted surface with a large density of nitrogen vacancies can accommodate electrons, and it is more conductive than the bulk in the implanted region, which is validated via spreading resistance profiling and conductive AFM measurements. Under reverse bias, the implanted surface can spread the potential in the lateral direction, whereas the acceptor traps capture electrons acting as space charges, shifting the peak electric field into the bulk region in the vertical direction. As a result, the Schottky barrier diode terminated with high-energy F ion-implanted regions exhibits a breakdown voltage of over 1.2 kV.

Design of bevel junction termination extension structure for high-performance vertical GaN Schottky barrier diode

In this work, a vertical GaN Schottky barrier diode with a bevel junction termination extension termination was designed by Silvaco TCAD. The effect of drift layer doping concentration, p-GaN doping concentration and bevel angle are evaluated extensively. With the optimum parameters, the bevel junction termination extension provides conduction path in the forward bias region to reduce the on-resistance and suppresses the electric field crowding in the reverse bias region. Furthermore, the Schottky contact characteristics are not affected in the relatively low bias region. The bevel junction termination extension is promising to achieve low turn-on voltage, low on-resistance and high breakdown voltage.

Effect of anode area on the sensing mechanism of vertical GaN Schottky barrier diode temperature sensor

Effect of anode area on temperature sensing ability is investigated for a vertical GaN Schottky-barrier-diode sensor. The current-voltage-temperature characteristics are comparable to each other for Schottky barrier diodes with different anode areas, excepting the series resistance. In the sub-threshold region, the contribution of series resistance on the sensitivity can be ignored due to the relatively small current. The sensitivity is dominated by the current density. A large anode area is helpful for enhancing the sensitivity at the same current level. In the fully turn-on region, the contribution of series resistance dominates the sensitivity. Unfortunately, a large series resistance degrades the temperature error and linearity, implying that a larger anode area will help to decrease the series resistance and to improve the sensing ability.

High-Voltage Quasi-Vertical GaN Junction Barrier Schottky Diode With Fast Switching Characteristics

In this letter, we report a quasi-vertical GaN junction barrier Schottky diode on low-cost sapphire substrate. With the high quality GaN epitaxy and selective-area p-islands formed via Magnesium ion implantation at the anode region, reverse leakage in level of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was achieved, as well as a high on/off current ratio of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> and a high breakdown voltage of 838 V. Meanwhile, advantageous characteristics as expected in vertical GaN Schottky barrier diode were realized, including a low turn-on voltage of 0.5 V and fast switching performance under 400 V/10 A operation condition. Along with the improved heat dissipation via substrate thinning and packaging techniques, the diode retains a relatively low thermal resistance, enabling high current rectification level over 60 A, power efficiency up to 98.7 %, while maintaining low case temperatures.

Vertical GaN Schottky barrier diodes with area-selectively deposited p-NiO guard ring termination structure

In this study, the effect of p-NiO guard ring length on the properties of vertical GaN Schottky barrier diode was investigated extensively. The forward biased current-voltage characteristics of all diodes with different guard ring length follow the thermionic emission theory, showing nearly the same ideality factor and Schottky barrier height value. While the series resistance increases by nearly two times with the increasing guard ring lengths because the increasing depletion region at the p-NiO/n-GaN junction compresses the conduction path. Under reverse bias, the breakdown voltage of the SBD without any termination is relatively low due to the electric field crowding. The p-NiO guard ring is effective to enhance the breakdown voltage although the leakage current is comparable. However, the breakdown voltage decreases with the increasing guard ring length, which may be ascribed to the tunneling leakage caused by the imperfect p-NiO/n-GaN interface.

Application of p-type NiO deposited by magnetron reactive sputtering on GaN vertical diodes

In this study, p-NiO deposited by magnetron reactive sputtering was adopted to fabricate NiO/GaN heterojunction and edge termination structure for vertical GaN Schottky barrier diodes. The contact properties of the NiO/GaN heterojunction were evaluated through a vertical pn diode. Turn-on voltage and specific on-resistance are approximately 2.27 V and 1.42 mΩ cm2, respectively. Temperature-dependent current-voltage and low-frequency noise measurements confirm that good interface and thermal stability of NiO/GaN heterojunction are obtained. The leakage current of the NiO pn diode is approximately four orders of magnitude lower than that of Schottky barrier diode, leading to an enhanced breakdown voltage. The p-NiO was selectively deposited as an edge termination structure to alleviate the electric field crowding of Schottky barrier diode, which is beneficial for enhancing the breakdown voltage.

Vertical GaN-Based Temperature Sensor by Using TiN Anode Schottky Barrier Diode

Vertical GaN Schottky barrier diode was fabricated as temperature sensor by using a thermally stable TiN anode. The current-voltage characteristics of the diode measured at various temperature present a zero-temperature coefficient (ZTC) bias point of approximately 0.6 V. At the voltage below the ZTC bias point (sub-threshold region), the forward voltage at a fixed current decreases linearly with the increasing temperature, resulting in a sensitivity of approximately 1.3 mV/K. In the reversely biased region, the leakage current also presents temperature-dependent behavior with a sensitivity of approximately 19.7 mA/K regardless of the bias. Those results can be interpreted by the thermionic emission model.

Charge Transport in Vertical GaN Schottky Barrier Diodes: A Refined Physical Model for Conductive Dislocations

Charge transport mechanisms of forward and reverse leakage currents in vertical GaN Schottky barrier diodes are investigated by measuring the temperature-dependent current–voltage characteristics. The results show that the leakage current is primarily governed by dislocation-associated thermionic field emission (TFE). The primary transport path is the reduced, localized conduction band around the dislocation core rather than the continuum defect states. A refined phenomenological physical model is developed for conductive dislocations in GaN, emphasizing that: 1) surface donors, surrounding the core of dislocations, can significantly shrink the barrier region after ionization, causing severe TFE leakage; 2) the $\text{O}_{N}$ donors likely to be responsible for TFE have a typical density of $\sim 1\,\,\!\times \! \,\,10^{{18}}$ cm $^{-{3}}$ at 300 K and activation energy of 78 meV; and 3) the barrier height at donor sites is ~0.65 eV at 300 K, which is reduced by ~0.4 eV with respect to the dislocation-free region.

Vertical GaN power rectifiers: interface effects and switching performance

The emergence of free-standing GaN substrates enables the development of vertical GaN-on-GaN devices with high-power ratings and high frequencies. The Schottky interface plays an important role in determining the current transport mechanisms and the forward/reverse electrical performance of the vertical GaN Schottky barrier diodes. Moreover, given its direct bandgap and ultrashort minority carrier lifetime, it is of particular interest and importance to evaluate the fast switching performance of the vertical GaN power rectifiers. In this paper, we discuss the technology, physical mechanisms and characterizations of vertical GaN power rectifiers with high-quality interfaces and fast switching performances, including: (a) accurate characterization and carrier transport mechanisms of the Schottky interface; (b) Schottky interface engineering with optimized post-metallization annealing and a tunneling-enhancement layer; (c) fast reverse recovery performance that is characterized by high-speed board-level tests. The investigations and analysis on the interface effects and switching performance of the vertical GaN power rectifiers are valuable for high-efficiency and high-frequency power electronics applications.

Vertical GaN-on-GaN Schottky Barrier Diodes With Multi-Floating Metal Rings

Vertical GaN Schottky barrier diodes (SBDs) with floating metal rings (FMRs) as edge termination structures have been fabricated on bulk GaN substrates. Devices with different FMR geometries were investigated including various numbers of rings and various spacings between rings. These devices have a low Ron of 1.16~1.59 mΩ·cm 2 , a turn-on voltage of 0.96~ 0.94 V, a high on-off ratio of 10 9 , a nearly ideal ideality factor of 1.03~1.09, and a Schottky barrier height of 1.11~1.18 eV at room temperature. These devices have similar forward electrical characteristics, indicating that FMRs don't degrade the device rectifying performance. The ideality factor decreased and the Schottky barrier height increased with increasing temperature from 300 K to 420 K, where the temperature dependencies of the two parameters indicate the inhomogeneity of the metal/semiconductor Schottky interface. In addition, FMRs can improve device breakdown voltages. As the number of FMRs increased from 0 to 20, the reverse breakdown voltage increased from 223 to 289 V. As the spacing between the FMRs increased from 1.5 to 3 μm, the reverse breakdown voltage increased from 233 to 290 V, respectively. These results indicate multiple FMRs with proper spacings can effectively improve breakdown performance without degrading the device forward characteristics. This work represents a useful reference for the FMR termination design for GaN power devices.

High-voltage vertical GaN-on-GaN Schottky barrier diode using fluorine ion implantation treatment

This paper reports on a high-voltage vertical GaN Schottky barrier diode (SBD) using fluorine (F) ion implantation treatment. Compared with the GaN SBD without F implantation, this SBD effectively enhanced the breakdown voltage from 155V to 775V and significantly reduced the reverse leakage current by 105 times. These results indicate that the F-implanted SBD showed improved reverse capability. In addition, a high Ion/Ioff ratio of 108 and high Schottky barrier height of 0.92 eV were also achieved for this diode with F implantation. The influence of F ion implantation in this SBD was also discussed in detail. It was found that F ion implantation to GaN could not only create a high-resistant region as effective edge termination but be employed for adjusting the carrier density of the surface of GaN, which were both helpful to achieve high breakdown voltage and suppress reverse leakage current. This work shows the potential for fabricating high-voltage and low-leakage SBDs using F ion implantation treatment.

Vertical GaN Schottky barrier diodes on Ge-doped free-standing GaN substrates

Vertical GaN Schottky barrier diodes (SBDs) were fabricated on Ge-doped free-standing GaN substrates grown by hydride vapor phase epitaxy. Detailed material characterization was performed, and the results indicate the superiority of Ge doping in the GaN, which contributed to the SBDs with lower stress, fewer defects and higher quality. Based on the capacitance-voltage and current–voltage measurements performed, SBDs achieved together with a low turn-on voltage Von (0.71–0.74 V), high current Ion/Ioff ratio (3.9 × 107–2.9 × 108), high Schottky barrier height (0.96–0.99 eV), and high breakdown voltage Vb (802 V for a 100 μm diameter). This shows that vertical GaN SBDs on the Ge-doped substrates are promising candidates for high power applications.

Vertical and Lateral GaN Power Devices Enabled By Engineered GaN Substrates

GaN has accrued significant research interest as a material platform for next-generation high power devices. This focused research thrust in GaN, fueled by its wide bandgap and high electric field strength, is expected to enable power devices with superior performance compared to the readily available Si power devices. However, the lack of uniform, high quality commercially available GaN substrates has bottlenecked the research progress of GaN high power devices. These substrate issues can be avoided by straying away from native GaN substrates and moving towards cost-effective engineered substrates designated QST™ (Qromis Substrate Technology) [1]. The QST™ substrate is comprised of a core that is thermally matched to GaN, a series of thin films encapsulating the core, and an epi-ready Si (111) surface for GaN epitaxy. Through its coefficient of thermal expansion (CTE) match to GaN, the QST™ substrate enables a material platform for thick, low dislocation density epitaxial layers suitable for high voltage devices. In this work, we present two fabrication processes enabled by QST™ substrates. First, true vertical GaN Schottky barrier diodes (SBDs) were designed from epitaxial GaN layers grown on large area (150 mm) QST™ substrates. The fabrication process is initiated with metal-organic chemical vapor deposition (MOCVD) of epitaxial GaN layers (2 μm GaN nucleation layer, 1.3 μm n+ GaN:Si contact layer with Nd = 5x1018 cm-3, and a 12.3 μm n- GaN drift layer Nd < 1x1016 cm-3) on the engineered QST™ substrate, followed by the deposition of Ni/Au (200 Å/2000 Å) Schottky contacts via a conventional lift-off procedure. The metal contacts and epitaxial GaN were then protected from the remaining etch steps with black wax and loaded metal face down onto a carrier substrate. A backside wet etch was used to selectively remove the QST™ substrate and transition layers. The backside contact was formed with 30 μm of electroplated Cu seeded by Ti/Au (100 Å/1000 Å). Lastly, the black wax was dissolved in xylene, releasing the freestanding vertical GaN device from the carrier substrate. Preliminary current-voltage experiments on true vertical GaN diodes fabricated without the 1.3 μm n+ GaN:Si contact layer (Nd = 5x1018 cm-3), edge termination, or surface passivation show a reverse breakdown voltage of 658 V, corresponding to a 0.58 MV/cm breakdown field. Second, lateral junction field effect transistors (LJFETs) were designed from the following epitaxial GaN layers grown on large area (150 mm) QST™ substrates: 7 μm semi-insulating layer, 1-2 μm n-GaN channel, 0.2 μm p- and 0.2 μm p+ GaN layers, and a 0.2 μm p++ GaN contact cap. To etch the source/drain regions, patterned photoresist on the p++ layer was used as a hard mask for etching approximately 750 nm GaN. An ohmic contact to the n-GaN was formed with Ti/Al/Ni/Au (200 Å/ 1200 Å/ 400 Å/ 800 Å), followed by a 30-second rapid thermal anneal at 850 °C in nitrogen gas. The Pd/Au (200 Å/ 1000 Å) gate metal was deposited on the p-GaN. Preliminary results demonstrate FET modulation. Improvement in the quality of the p layers is expected to improve performance. Moreover, the QST™ substrates provide an economical platform for optimizing epitaxial growth conditions to reach peak device performance. The feasible fabrication processes and promising preliminary results described in this work underline the potential of engineered GaN substrates for producing low cost vertical and lateral power devices. Reference: [1] Travis J. Anderson et al 2017 Appl. Phys. Express 10 126501.

Leakage current reduction of vertical GaN junction barrier Schottky diodes using dual-anode process

The origin of the leakage current of a trench-type vertical GaN diode was discussed. We found that the edge of p-GaN is the main leakage spot. To reduce the reverse leakage current at the edge of p-GaN, a dual-anode process was proposed. As a result, the reverse blocking voltage defined at the leakage current density of 1 mA/cm2 of a vertical GaN junction barrier Schottky (JBS) diode was improved from 780 to 1,190 V, which is the highest value ever reported for vertical GaN Schottky barrier diodes (SBDs).

Vertical GaN Junction Barrier Schottky Rectifiers by Selective Ion Implantation

This letter demonstrates vertical GaN junction barrier Schottky (JBS) rectifiers fabricated with novel ion implantation techniques. We used two different methods to form the lateral p-n grids below the Schottky contact: 1) Mg implantation into n-GaN to form p-wells and 2) Si implantation into p-GaN to form n-wells. Specific differential ON-resistances ( ${R}_{ \mathrm{\scriptscriptstyle ON}}$ ) of 1.5–2.5 $\text{m}\Omega ~\cdot $ cm2 and 7–9 $\text{m}\Omega ~\cdot $ cm2 were obtained in the Mg-implanted and Si-implanted JBS rectifiers, respectively. A breakdown voltage of 500–600 V was achieved in both devices, with a leakage current at high reverse biases at least 100-fold lower than conventional vertical GaN Schottky barrier diodes. The impact of n-well and p-well widths on the ${R}_{ \mathrm{\scriptscriptstyle ON}}$ and BV was investigated. Fast switching capability was also demonstrated. This letter shows the feasibility of forming patterned p-n junctions by novel ion implantation techniques, to enable high-performance vertical GaN power devices.

1.2 kV GaN Schottky barrier diodes on free-standing GaN wafer using a CMOS-compatible contact material

In this paper, we report the formation of vertical GaN Schottky barrier diodes (SBDs) on a 2-in. free-standing (FS) GaN wafer, using CMOS-compatible contact material. By realizing an off-state breakdown voltage VBR of 1200 V and an on-state resistance Ron of 7 mΩ·cm2, the FS-GaN SBDs fabricated in this work achieve a power device figure-of-merit of 2.1 × 108 V2·Ω−1·cm−2 on a high quality GaN wafer. In addition, the fabricated FS-GaN SBDs show the highest Ion/Ioff current ratio of ∼2.3 × 1010 among the GaN SBDs reported in the literature.

GaN Schottky Barrier Diodes on Free-Standing GaN Wafer

In this paper, vertical GaN Schottky barrier diodes (SBDs) have been demonstrated on the freestanding (FS) GaN wafer. Room temperature photoluminescence shows no emission of yellow luminescence, due to the eliminating the point defects (ON, VGa). The average full width half maximum (FWHM) value of ω-scans for (0001) and (10–12) planes is 53 and 103 arcsec, respectively, as measured by high resolution X-ray diffraction; the average dislocation density at the top surface measured by cathodoluminescence is ∼7.7 × 105 cm−2. The vertical GaN SBDs fabricated on this FS-GaN wafer achieve a breakdown voltage of 1200 V, an on-state resistance Ron of 7 mΩ·cm2, and a current on/off ratio Ion/Ioff of ∼2.3 × 1010, which leads to a power device figure-of-merit VBR2/Ron of 2.1 × 108 V2 Ω−1 cm−2. TCAD simulation was employed to study the device breakdown mechanism, and shows the breakdown mechanism is due to the impact ionization at the edge of the Schottky gate.