| Metadata | Details |
|---|
| Publication Date | 2023-01-01 |
| Journal | Acta Physica Sinica |
| Authors | Qingbin Liu, Yu Cui, Jianchao Guo, Mengyu Ma, Zezhao He |
| Institutions | Hebei Semiconductor Research Institute |
| Citations | 3 |
| Analysis | Full AI Review Included |
- Core Challenge Addressed: Mitigating severe self-heating in GaN High Electron Mobility Transistors (HEMTs) by directly integrating high thermal conductivity polycrystalline diamond (PCD) films.
- Performance Trade-off: Direct PCD growth via Microwave Plasma Chemical Vapor Deposition (MPCVD) significantly degrades GaN electrical properties due to thermal expansion mismatch stress.
- Stress Mechanism: Increasing PCD thickness (9 ”m to 81 ”m) linearly increases the tensile stress applied to the GaN layer, causing lattice distortion (evidenced by increased XRD FWHM002) and mobility loss (up to 25.8%).
- Recoverability Confirmed: Successful chemical stripping of the 81 ”m PCD layer fully restored the GaN materialâs intrinsic properties (Raman peak position, FWHM002, and electron mobility), proving the degradation is recoverable and non-destructive.
- Interface Damage Identified: High-temperature growth causes hydrogen atoms to etch defects in the protective SiNx layer, creating holes that penetrate into the intrinsic GaN layer, which act as localized stress concentration points.
- Mitigation Strategy: The use of a slow cooling rate (~9 °C/min) and optimization of PCD thickness are effective methods for controlling thermal stress and minimizing electrical performance decay while achieving necessary heat dissipation.
| Parameter | Value | Unit | Context |
|---|
| Substrate Material | 2-inch Si-based GaN | N/A | HEMT structure with 30 nm SiNx cap |
| PCD Thickness Range | 9 to 81 | ”m | Samples #1, #2, #3 |
| MPCVD Power | 3500 | W | Diamond growth parameter |
| Growth Temperature | 800 | °C | Diamond growth parameter |
| Carbon Concentration | 6 | % | CH4 in H2 carrier gas |
| Average Growth Rate | 1.50 to 1.16 | ”m/h | Decreases with increasing thickness |
| Cooling Rate | ~9 | °C/min | Implemented to relieve thermal stress |
| Intrinsic Mobility (”intrinsic) | 1588.8 | cm2/(V·s) | Before PCD growth (#3 sample) |
| Mobility After 81 ”m PCD (”epitaxial) | 1178.2 | cm2/(V·s) | Maximum degradation observed |
| Mobility Loss (Δ) | 25.8 | % | For 81 ”m PCD layer |
| Tensile Stress (Ï) on GaN | 0.07 | GPa | Induced by 81 ”m PCD (calculated from Raman shift) |
| Intrinsic FWHM002 (XRD) | 550 | arcsec | Before PCD growth (#3 sample) |
| FWHM002 After 81 ”m PCD | 654 | arcsec | Increased lattice distortion |
| FWHM002 After Stripping | 530 | arcsec | Fully recovered state |
| PCD Raman Peak Position | 1333.0 to 1333.9 | cm-1 | Indicates high crystal quality (low internal compressive stress: 0.57-1.08 GPa) |
| Acid Etching Solution | 3:1 | HF:HNO3 ratio | 50% HF + 50% HNO3 mixture |
| Si Substrate Etching Rate | ~30 | ”m/h | Observed during stripping process |
- SiNx Passivation Layer: A 30 nm SiNx layer was deposited via PECVD onto the GaN HEMT structure to serve as a protective barrier against hydrogen etching during high-temperature diamond growth.
- Diamond Seeding: The SiNx surface was prepared by ultrasonic treatment (20 min) and soaking (20 min) in a 1% diamond slurry (20 nm grit) to create uniform nucleation sites for continuous film growth.
- MPCVD Growth: Polycrystalline diamond films (9, 25, and 81 ”m) were grown using a 3500 W microwave plasma system at 800 °C, utilizing 6% CH4 in H2 gas.
- Thermal Stress Management: A slow cooling rate of approximately 9 °C/min was employed after growth to minimize the thermal stress induced by the mismatch between the PCD and the GaN/Si stack.
- PCD/GaN Separation: The 81 ”m PCD sample was separated using a two-step process:
- Infrared laser cutting was used to thermally separate the edges of the composite material.
- The sample was then immersed in a 3:1 mixture of 50% HF and 50% HNO3 for 3.5 hours to etch the SiNx interface and the Si substrate, allowing the GaN layer to be fully stripped.
- Structural and Electrical Characterization: Raman spectroscopy, XRD (FWHM002), and non-contact Hall measurements were performed on the GaN layer at three stages: intrinsic (before growth), epitaxial (after growth), and exfoliated (after stripping) to quantify stress and performance changes.
- High Power RF/Microwave Amplifiers: The technology is directly applicable to GaN HEMT devices used in high-frequency, high-power applications (e.g., 5G/6G infrastructure, military radar). Direct diamond integration is essential for managing the extreme heat flux generated in the channel region.
- Advanced Thermal Substrates (GaN-on-Diamond): This research informs the fabrication of robust GaN-on-Diamond wafers, providing superior thermal management compared to traditional GaN-on-SiC or GaN-on-Si platforms.
- Stress-Tolerant Device Design: The findings regarding the recoverable nature of electrical degradation due to stress are critical for designing buffer layers and interface engineering strategies that minimize lattice distortion while maximizing thermal coupling.
- High-Reliability Electronics: Understanding the mechanism of interface damage (H etching of SiNx defects) allows for optimization of the SiNx layer thickness and quality, improving the long-term reliability and yield of GaN-on-Diamond devices.
View Original Abstract
Self-heating has become a limited factor for the performance improvement of GaN electronics. Growing polycrystalline diamond directly on GaN material to solve the heating problem of GaN devices has become one of the research highlights. Polycrystalline diamond on Si-based GaN material has the advantages of being close to the channel region and high heat dissipation efficiency. However, there is a problem that the thermal expansion mismatch between polycrystalline diamond and GaN material leads to the deterioration of electrical characteristics of GaN. In this work, we adopt microwave plasma chemical vapor deposition (MPCVD) method to grow polycrystalline diamond on 2-inch Si-based GaN material. The test results show that the polycrystalline diamond is uniform as a whole. The average thickness is in the range of 9-81 ÎŒm. With the thickness of polycrystalline diamond increasing, the XRD (002) diffraction peak FWHM increment and mobility loss gradually increase for the Si-based GaN material. Through laser cutting and acid etching, the Si-based GaN material is successfully stripped from the polycrystalline diamond. It is found that during the process of diamond growth at high temperature, hydrogen atoms etch the defect positions of the silicon nitride epitaxial layer, forming a hole area in the GaN, and the etching depth can reach the intrinsic GaN layer. During the process of cooling, a crack area is formed around the hole area. Raman characteristic peaks, full widths at half maximum of XRD (002) diffraction peaks, and electrical properties of the stripped Si-based GaN materials are all returned to their intrinsic states. The above results show that the thermal expansion mismatch between polycrystalline diamond and Si-based GaN introduces stress into GaN, which leads to lattice distortion of GaN lattice and the degradation of electrical property of GaN material. The degradation of GaN material is recoverable, but not destructive.