GaN Schottky Barrier Research Articles

Lateral polarity control of III-nitride thin film and application in GaN Schottky barrier diode * * Project partially supported by the National Key Research and Development Program of China (No. 2016YFB0400802), the National Natural Science Foundation of China (No. 61704176), and the Open project of Zhejiang Key Laboratory for Advanced Microelectronic Intelligent Systems and Applications (No. ZJUAMIS1704).

N-polar and III-polar GaN and AlN epitaxial thin films grown side by side on single sapphire substrate was reported. Surface morphology, wet etching susceptibility and bi-axial strain conditions were investigated and the polarity control scheme was utilized in the fabrication of Schottky barrier diode where ohmic contact and Schottky contact were deposited on N-polar domains and Ga-polar domains, respectively. The influence of N-polarity on on-state resistivity and I–V characteristic was discussed, demonstrating that lateral polarity structure of GaN and AlN can be widely used in new designs of optoelectronic and electronic devices.

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).

A 1.9 kV/2.61 mΩ·cm2 Lateral GaN Schottky Barrier Diode on Silicon Substrate with Tungsten Anode and Low Turn-On Voltage of 0.35 V

In this letter, we report the achievement of a high-performance lateral GaN Schottky barrier diode (SBD) on a silicon substrate with a low turn- ON voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{ \mathrm{ON}}$ </tex-math></inline-formula> ) of 0.35 V and tungsten (W) as the anode. Non-field-plated lateral GaN SBDs with the anode–cathode distances ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${L}_{\text {AC}}$ </tex-math></inline-formula> ) of 6, 10, 15, 20, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$25~\mu \text{m}$ </tex-math></inline-formula> demonstrate the reverse breakdown voltages of 0.6, 1.1, 1.25, 1.5, and 1.9 kV with the differential specific ON-resistances ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}_{\text{ON}, \text {sp}}$ </tex-math></inline-formula> ) of 0.38, 0.72, 1.23, 1.87, and 2.61 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}\Omega \cdot $ </tex-math></inline-formula> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively. The power figure-of-merit (FOM) is calculated to be <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$ {1}\times 10^{ {3}}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$ {1.7}\times {10}^{ {3}}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$ {1.3}\times {10}^{ {3}}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$ {1.2}\times {10}^{ {3}}$ </tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$ {1.4}\times {10}^{{3}}$ </tex-math></inline-formula> MW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . To the best of our knowledge, this FOM of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${1.7}\times {10}^{{3}}$ </tex-math></inline-formula> MW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is the highest among all the lateral GaN SBDs on a Si substrate. Combined with the ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> current ON/OFF ratio at room temperature, the GaN SBD with the W anode shows a great promise for next-generation power electronics.

Physical model of conductive dislocations in GaN Schottky diodes

The excessive leakage current, commonly observed in GaN Schottky barrier diodes (SBDs), severely degrades device electrical performance and long-term reliability. This leakage current relates to the dislocation-related conductive states as observed by microscopy. Up to now, various transport models have been proposed to explain the leakage current, but none of them can clearly describe in physics the electrically active dislocations. One just equivalently regarded the electric defect as a continuum conductive defect state within the forbidden band, without considering the microscopic electrical properties of the dislocations. Here in this work, on the basis of numerical simulation, we propose a phenomenological model for the electrically active dislocations to explain the leakage conduction of the GaN Schottky diodes, which are fabricated on a freestanding bulk substrate n-GaN wafer with a low dislocation density of about 1.3106 cm-2. In this model, we emphasize that the acceptor-like traps at the core of dislocations could capture electrons from the nearby donor-like traps, resulting in a high Coulomb potential and a decreasing potential at the donor-like sites. In this case, the core of dislocations would be negatively charged, and not favor the electron transport due to a strong Coulomb scattering effect, while the shallow donor-like traps around them can lead to a significant tunneling leakage component. This model is consistent well with the common observation of the localized currents at the edges of the surface V-defects in GaN. The shallow donor-like defects in GaN induced by the substitution of oxygen for nitrogen (ON), rather than the nitrogen vacancies, act as the dominant donor impurities responsible for the significant leakage current, which has a density on the order of 1018 cm-3 and an activation energy of about 47.5 meV, because 1) it has been demonstrated that during the material growth, oxygen diffusion toward the surface pits of dislocations via nitrogen vacancies could produce an exponentially decayed distribution with a density of at least 1017 cm-3, in good agreement with our derivation; 2) by the first principle calculation, the thermal activation energy of the oxygen-related donors is determined to be about 50 meV, which is very close to our derived 47.5 meV. According to this model, we propose that reducing the ON defect density during device growth is a feasible method to suppress the high leakage current in GaN-based SBDs. In addition, this study can also improve our understanding of the leakage current in other GaN-based devices.

Wafer-Level Donor Uniformity Improvement by Substrate Off-Angle Control for Vertical GaN-on-GaN Power Switching Devices

In the mass production of GaN-on-GaN vertical power devices, the wafer-level uniformity of net donor concentration, ${N_{D}- N_{A}}$ , of the n−-drift layer in around $10^{{15}}$ cm−3 is an important factor because it determines the breakdown voltage. A nondestructive simple inspection is also required. In this paper, we demonstrated the wafer-level nondestructive inspection of a GaN Schottky barrier diode epi-layer and improved the wafer-level net donor uniformity by controlling the off-angle of GaN substrates. Epi-structures were grown by metal-organic vapor phase epitaxy on free-standing GaN substrates with various off-angles and deviations. The variation in ${N_{D}- N_{A}}$ was carefully analyzed using non-contact capacitance–voltage measurement and photoluminescence. Silicon and carbon concentrations were confirmed by secondary ion mass spectrometry. We found that the normalized yellow luminescence peak intensity is almost linearly related to the acceptor concentration. A carbon related variation in the acceptor concentration ( ${N_{A}}$ ) resulted in the non-uniformity of ${N_{D}- N_{A}}$ , which is found to be related to the substrate off-angle of the wafer. The ${N_{D} - N_{A}}$ uniformity can be improved by minimizing variation in the off-angle. Criteria of the GaN substrate off-angle deviation for power applications are discussed.

Polarity Control of GaN and Realization of GaN Schottky Barrier Diode Based on Lateral Polarity Structure

This paper reports investigation on a polarity control scheme of GaN thin films and realization of Schottky barrier diode (SBD) fabricated on a lateral-polarity-structure (LPS) GaN without intentionally doping. Specifically, Ga-polar and N-polar GaN were grown simultaneously side by side on sapphire substrate with patterned AlN buffer. Due to the surface energy difference between two polarities, N-polar regions are n-type conductive with rough surface morphology, while Ga-polar regions are semi-insulating with atomic flat surface morphology. Annealing conditions of both ohmic contact and Schottky contact were investigated. Current–voltage (I–V) characteristic revealed that the SBD fabricated on LPS GaN has higher forward current, barrier height closer to 0.7 eV, and ideality factor closer to unity compared to SBD fabricated on conventional undoped GaN. The specific on-state resistance ( ${R}_{{ \mathrm{\scriptscriptstyle ON}}})$ for the SBD based on LPS GaN is 77 $\text{m}\Omega \cdot \text {cm}^{{{{2}}}}$ lower than the SBD fabricated on conventional GaN. With oxide passivation on SBD surface, forward current exceeds 0.2 A at 10 V, while reverse current is less than 10−5 A at −10 V, respectively. The utilization of LPS in SBD demonstrates a promising approach for the development of lateral n+/n− SBD with a simple fabrication scheme.

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.

Facet dependence of leakage current and carrier concentration in m-plane GaN Schottky barrier diode fabricated with MOVPE

In this study, GaN m-plane Schottky barrier diodes fabricated with a metalorganic vapor-phase epitaxy on a GaN substrate were investigated using emission microscope, photoluminescence, and cathodoluminescence. In addition, facet dependence of leakage current under reverse-biased condition was observed. We showed that the leakage-current distribution was caused by the facet dependence of the carrier concentration and oxygen concentration. These results can provide important suggestions for the fabrication of m-plane devices. (a) four-faceted hillocks on m-plane GaN MOVPE sample, facet dependence of (b) leakage current and (c) PL peak intensity of the m-plane GaN Schottky barrier diode.

Wafer-level nondestructive inspection of substrate off-angle and net donor concentration of the n−-drift layer in vertical GaN-on-GaN Schottky diodes

In the mass production of GaN-on-GaN vertical power devices, a nondestructive simple inspection of the net donor concentration (ND − NA) of the n−-drift layer in the range of 1015 cm−3 is required. In this study, we demonstrate the wafer-level nondestructive inspection of GaN Schottky barrier diode epi-structures grown by metal organic vapor phase epitaxy (MOVPE) on free-standing GaN substrates. We found that the normalized yellow (YL) photoluminescence peak intensity of the near band edge (NBE), IYL/INBE, is strongly related to the acceptor concentration NA of the n−-drift layer. This means that the ND − NA of the n−-drift layer can be inspected by photoluminescence measurement at a high speed, when Si concentration is not varying across the GaN wafers. Noncontact capacitance–voltage and secondary ion mass spectrometry measurements were used to investigate the cause of ND − NA variation across the GaN wafers. The discrepancy between C and NA indicates that compensation could be due to another electron trap.

Current Modeling for Accumulation Mode GaN Schottky Barrier MOSFET for Integrated UV Sensors

Current Modeling for Accumulation Mode GaN Schottky Barrier MOSFET for Integrated UV Sensors

Nearly ideal vertical GaN Schottky barrier diodes with ultralow turn-on voltage and on-resistance

Vertical Schottky barrier diodes (SBDs) were fabricated from the metal–organic chemical vapor deposition (MOCVD)-grown GaN epitaxial layer on free-standing GaN substrates. It was found that the quality of GaN drift layers and SBD properties were strongly dependent on the growth rates. The step-flow surface morphology, near-unity ideality factor (n ∼ 1.04), and high Schottky barrier height (∼0.97 eV) were achieved at a relatively low growth rate of 2.61 µm/h. An extremely low turn-on voltage (0.73 V), together with a low on-resistance of 0.72 mΩ·cm2, was obtained.

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.

(Invited) Improved Vertical GaN Diodes with Mg Ion Implanted Junction Termination Extension

GaN is an ideal material for power applications due to its high breakdown field, high switching speed, and low switching losses. The ability to implant and activate dopants, particularly p-type dopants, in GaN still remains a challenge. Dopant implantation and activation is crucial for many devices since it is the most straightforward method of selective lateral doping in GaN. Devices such as the current aperture vertical electron transistor (CAVET), Schottky diodes with guard ring termination or grayscale junction termination extension, and vertical p-i-n diodes will be enhanced and enabled by the ability to selectively dope GaN without the need for etching or regrowth. We have previously reported on the first implanted p-i-n diode fabricated on a bulk GaN substrate. Here, we present improved GaN Schottky barrier diodes (SBDs) and p-i-n diodes with edge termination formed by either a field plate or junction termination extension (JTE) achieved by ion implanted and symmetric multicycle rapid thermal annealing (SMRTA). The devices with JTE exhibited substantially reduced leakage currents and improved turn-on characteristics. This is attributed to the elimination of the plasma process steps associated with the deposition and patterning of the field oxide layer required in a field plate process. The breakdown characteristics were studied by electroluminescence imaging, and is indicative of avalanche behavior. The realization of vertical GaN devices with low reverse leakage and ion implanted termination regions represents a key step for future power electronic devices.

(Invited) Improved Vertical GaN Diodes with Mg Ion Implanted Junction Termination Extension

Improved GaN Schottky barrier diodes (SBDs) and p-i-n diodes with edge termination formed by either a field plate or junction termination extension (JTE) achieved by ion implanted and symmetric multicycle rapid thermal annealing (SMRTA) are demonstrated. The devices with JTE exhibited substantially reduced leakage currents and improved turn-on characteristics. This is attributed to the elimination of the plasma process steps. The breakdown characteristics were studied by electroluminescence imaging, and is indicative of avalanche behavior. The realization of vertical GaN devices with low reverse leakage and ion implanted termination regions represents a key step for future power electronic devices.

Franz–Keldysh effect in n-type GaN Schottky barrier diode under high reverse bias voltage

The photocurrent of GaN vertical Schottky barrier diodes was investigated under sub-bandgap wavelength light irradiation. Under a low reverse bias voltage, the photocurrent is induced by internal photoemission, while under a high reverse bias voltage, the photocurrent increases significantly with the bias voltage. This is due to sub-bandgap optical absorption in a depletion region due to the Franz–Keldysh effect. The voltage and wavelength dependences of the photocurrent are successfully explained quantitatively.

Studies on the Thermal Stability of Ni/n–GaN and Pt/n–GaN Schottky Barrier Diodes

High temperature annealing of Ni and Pt Schottky diodes on n-GaN has been investigated. Ni/n–GaN and Pt/n–GaN Schottky diodes were fabricated using ultra high vacuum and annealed at various temperatures (200 °C−700 °C) using rapid thermal anneal system. It was observed that in case of Ni/GaN diodes, the Schottky barrier height increases for 200 °C and then decreases with increase in annealing temperature. For Pt/GaN diodes, the barrier height found to decrease with increase in annealing temperature. The observed variation of barrier height and ideality factor is discussed on the basis of interfacial chemical reactions between metal and semiconductor.

DLTS analysis of high resistive edge termination technique‐induced defects in GaN‐based Schottky barrier diodes

Efficient edge terminations are fundamental for power electronic devices. High resistive edge termination technique obtained by ion implantation is a possible solution for GaN Schottky barrier diodes (SBD), but may induce defects detrimental to the devices. In this work, two sets of GaN SBDs, with or without a high resistive guard ring edge termination, were investigated by deep levels transient spectroscopy (DLTS). The corresponding DLTS spectra of devices with guard ring were compared to those of as‐fabricated devices (without). Four deep traps at 0.13, 0.26, 0.35, and 0.51 eV were observed below the conduction band in as‐fabricated structures while the ones with guard ring exhibit two additional levels at 0.47 and 0.80 eV and a strong enhancement in DLTS peaks magnitude. Further investigations showed that the additional traps detected in SBD devices with guard ring were consistent with ion implantation‐induced damages. These observations highlight the impact of high resistive edge termination technique on our devices and raise the need of optimization of such crucial structure for the improvement of GaN‐based Schottky barrier diode.

Improved performance in vertical GaN Schottky diode assisted by AlGaN tunneling barrier

In a vertical GaN Schottky barrier diode, the free electron concentration n in the 6-μm-thick drift layer was found to greatly impact the diode reverse leakage current, which increased from 2.1 × 10−7 A to 3.9 × 10−4 A as n increased from 7.5 × 1014 cm−3 to 6.3 × 1015 cm−3 at a reverse bias of 100 V. By capping the drift layer with an ultrathin 5-nm graded AlGaN layer, reverse leakage was reduced by more than three orders of magnitude with the same n in the drift layer. We attribute this to the increased Schottky barrier height with the AlGaN at the surface. Meanwhile, the polarization field within the graded AlGaN effectively shortened the depletion depth, which led to the formation of tunneling current at a relatively small forward bias. The turn-on voltage in the vertical Schottky diodes was reduced from 0.77 V to 0.67 V—an advantage in reducing conduction loss in power switching applications.

High-voltage vertical GaN Schottky diode enabled by low-carbon metal-organic chemical vapor deposition growth

Vertical GaN Schottky barrier diode (SBD) structures were grown by metal-organic chemical vapor deposition on free-standing GaN substrates. The carbon doping effect on SBD performance was studied by adjusting the growth conditions and spanning the carbon doping concentration between ≤3 × 1015 cm−3 and 3 × 1019 cm−3. Using the optimized growth conditions that resulted in the lowest carbon incorporation, a vertical GaN SBD with a 6-μm drift layer was fabricated. A low turn-on voltage of 0.77 V with a breakdown voltage over 800 V was obtained from the device.