GaN-based lateral diode with nanocrystalline diamond passivation layer
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2025-01-01 |
| Journal | Acta Physica Sinica |
| Authors | Zeyang Ren, SONG Songyuan, Tao Zhang, CHEN Heyuan, Yao Li |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research successfully demonstrates the application of a Nanocrystalline Diamond (NCD) passivation layer for thermal management and reliability enhancement in GaN-based lateral Schottky Barrier Diodes (SBDs).
- Core Value Proposition: NCD passivation significantly mitigates thermal accumulation, addressing the low intrinsic thermal conductivity of GaN (200-250 W·m-1·K-1), thereby boosting the maximum operational power density of the device.
- Thermal Performance Achievement: The NCD-passivated SBD (Device A) achieved a thermal failure power density of approximately 7.5 W/mm, a substantial improvement over the conventional SiN-passivated device (Device B), which failed at approximately 4 W/mm.
- Thermal Resistance Improvement: NCD reduced the thermal resistance (Rth) from 8.73 K·mm·W-1 (SiN) to 7.37 K·mm·W-1, resulting in a 5-10 °C lower maximum junction temperature (Tmax) under comparable power output.
- Current Collapse Suppression: The NCD layer provided superior dynamic performance. Under a -20 V DC bias and 2.5 V pulse, the NCD device showed only 2.6% current density degradation, whereas the SiN device degraded by nearly 100%.
- Methodology: NCD films (380-450 nm thick) were grown via Microwave Plasma Chemical Vapor Deposition (MPCVD) at a relatively low temperature (650 °C) on a SiN buffer layer, ensuring compatibility with existing GaN device fabrication processes.
- Significance: This is the first reported application of NCD passivation for thermal management in GaN power diodes (non-HEMT devices), proving the strategy’s potential beyond traditional GaN HEMT structures.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| GaN Substrate | AlGaN/GaN Heterostructure | N/A | Grown on Si substrate |
| NCD Film Thickness | 380-450 | nm | Device A (NCD Passivation) |
| NCD Grain Size | 330-380 | nm | Device A |
| SiN Buffer Layer Thickness | 50 | nm | Used between AlGaN and NCD |
| NCD Growth Temperature | 650 | °C | MPCVD process |
| Thermal Resistance (Rth, NCD) | 7.37 | K·mm·W-1 | Device A |
| Thermal Resistance (Rth, SiN) | 8.73 | K·mm·W-1 | Device B (Conventional) |
| Max Power Density (NCD) | 7.5 | W/mm | Power density before thermal failure (Device A) |
| Max Power Density (SiN) | 4 | W/mm | Power density before thermal failure (Device B) |
| Forward Turn-on Voltage (NCD) | 0.725 | V | Device A |
| Forward Current Density (3 V) | 190 | mA/mm | Device A |
| Current Collapse (NCD) | 2.6 | % degradation | Under -20 V DC bias, 2.5 V pulse |
| Current Collapse (SiN) | 99.9 | % degradation | Under -20 V DC bias, 2.5 V pulse |
| Reverse Breakdown Voltage (SiN) | -488 | V | Device B |
| Reverse Breakdown Voltage (NCD) | -164 | V | Device A (Lower due to etching damage during anode exposure) |
Key Methodologies
Section titled “Key Methodologies”The fabrication process involved MOCVD-grown GaN on Si, followed by specific steps to integrate the NCD layer while protecting the active region.
- Starting Material & Isolation: GaN/AlGaN heterostructure on Si. Mesa isolation performed via Cl2/BCl3-based ICP dry etching.
- Ohmic Cathode Formation: Ti/Al/Ni/Au (20/150/50/100 nm) deposited via Electron Beam Evaporation (EBE), followed by 835 °C anneal in pure N2 for 30 s.
- SiN Buffer Deposition: A 50 nm SiN layer was deposited via Inductively Coupled Plasma Chemical Vapor Deposition (ICP-CVD) at 130 °C to serve as a buffer, protecting the 2DEG channel from high-energy hydrogen plasma during subsequent NCD growth.
- NCD Seeding: NCD seed suspension was applied via spin coating (1000 r/min for 10 s).
- NCD Growth (MPCVD): NCD film was grown using Microwave Plasma CVD (MPCVD) at 650 °C for 15 min.
- Microwave Power: 3.2 kW.
- Chamber Pressure: 135 mbar.
- Gas Flows (Standard Conditions): H2 (300 mL/min), CH4 (12 mL/min), N2 (0.05 mL/min).
- Anode Exposure Etching (Multi-Step): A 100 nm SiN hard mask was deposited and patterned.
- SiN Hard Mask Etch: CF4 ICP etch (300 W/50 W).
- NCD Etch: Pure O2 ICP etch (600 W/300 W) to remove NCD above the anode region.
- SiN Buffer Etch: CF4 ICP etch to expose the AlGaN surface.
- GaN Recess Etch: Cl2/BCl3 ICP etch to form the anode recess, followed by post-etch annealing to repair damage.
- Schottky Anode Formation: Ni/Au (20/200 nm) deposited via EBE.
Commercial Applications
Section titled “Commercial Applications”The integration of high-thermal-conductivity NCD passivation directly onto GaN power devices offers significant advantages for systems requiring high power density and high reliability.
- High-Power Switching and Conversion: Essential for improving the efficiency and reducing the size of power electronics used in data centers, electric vehicle (EV) charging infrastructure, and industrial motor drives.
- RF and Microwave Systems: GaN SBDs and HEMTs used in radar and 5G/6G base stations generate intense localized heat. NCD passivation ensures stable operation and prevents thermal runaway at high frequencies and high power outputs.
- Thermal Management Solutions: The demonstrated low thermal resistance (7.37 K·mm·W-1) makes this technology critical for any GaN device where junction temperature must be tightly controlled to maintain long-term reliability.
- Aerospace and Defense: Applications requiring robust, high-power-density components that must operate reliably under extreme thermal stress.
View Original Abstract
Thermal accumulation under high output power density is one of the key bottlenecks faced by GaN-based power devices. The nanocrystalline diamond (NCD) passivation layer strategy plays a crucial role in improving heat dissipation in high-power GaN devices, while the existing studies focus on GaN-based HEMT. In this study, nanocrystalline diamond films with a thickness of 380-450 nm are grown on Si-based AlGaN/GaN heterostructure materials using a microwave plasma chemical vapor deposition (MPCVD) system. Consequently, lateral Schottky barrier diode devices with NCD passivation are fabricated, and their electrical and thermal properties are investigated. The results show that the DC forward characteristics of the NCD passivated diodes are essentially the same as those of devices without NCD passivation. Moreover, dynamic voltage tests indicate that the NCD passivation layer significantly mitigates current collapse in GaN devices at high frequencies. Under a -20 V DC bias and a pulse voltage of 2.5 V, the current density degradation of NCD passivated devices is only 2.6%, whereas devices without diamond passivation almost completely degrade. Thermal imaging microscopy under varying DC power levels shows that thermal failure occurs at an output power density of approximately 4 W/mm for conventional devices, while NCD passivated devices can reach around 7.5 W/mm. The electrical degradation behaviour of NCD passivated device is also tested under long-time reverse bias. This work demonstrates for the first time the application of nanocrystalline diamond passivation to thermal management of GaN-based power diodes, and clearly demonstrates the potential of this strategy in non-HEMT power device applications.