Surface Zeta Potential and Diamond Seeding on Gallium Nitride Films

At a Glance

Metadata	Details
Publication Date	2017-10-27
Journal	ACS Omega
Authors	Soumen Mandal, Evan L. H. Thomas, Callum Middleton, Laia Ginés, James T. Griffiths
Institutions	University of Bristol, University of Cambridge
Citations	40
Analysis	Full AI Review Included

Technical Documentation: Direct Diamond Integration on GaN for High-Power Electronics

Prepared for: Engineers and Scientists specializing in GaN HEMT Thermal Management Source Analysis: Surface zeta potential and diamond seeding on gallium nitride films (Mandal et al., 2017) Provided by: 6CCVD - Experts in MPCVD Diamond Solutions

Executive Summary

This research successfully demonstrates a critical advancement in thermal management for Gallium Nitride (GaN) High Electron Mobility Transistors (HEMTs) by achieving direct, high-density diamond growth on both Ga-face and N-face GaN.

Thermal Barrier Elimination: The technique removes the requirement for low-thermal-conductivity adhesion layers (e.g., silicon nitride), enabling direct integration of diamond (up to 2100 W/mK) to replace SiC (360-490 W/mK) substrates.
Optimized Seeding Strategy: Zeta potential measurements confirmed that H-terminated nanodiamond seeds at an electrolyte pH of 8 are optimal for electrostatic self-assembly on the negatively charged GaN surface.
Ultra-High Density Nucleation: The optimized seeding process achieved nucleation densities exceeding 10¹² cm^-2, resulting in fully coalesced, pinhole-free diamond films.
Material Versatility: Successful direct diamond growth was demonstrated on both MOCVD-grown (Ga-face) and MBE-grown (N-face) GaN orientations.
MPCVD Validation: The diamond films were grown using standard Microwave Plasma Chemical Vapor Deposition (MPCVD) parameters (5% CH₄/H₂, 3.5 kW, 40 torr).

Technical Specifications

The following hard data points were extracted from the analysis, detailing material properties and critical process parameters for direct diamond growth on GaN.

Parameter	Value	Unit	Context
Target Thermal Conductivity (Diamond)	Up to 2100	W/mK	Required for high-performance GaN HEMTs
Standard Thermal Conductivity (SiC)	360 - 490	W/mK	Current state-of-the-art substrate
Optimal Seeding pH	8	N/A	Required for H-terminated seed self-assembly
Ga-face GaN Isoelectric Point (IEP)	~5.5	pH	Point where zeta potential is zero
Achieved Seeding Density	> 10¹²	cm^-2	Necessary for fully coalesced, pinhole-free films
Nanodiamond Seed Diameter	~7	nm	Used in the mono-dispersed seeding solution
Diamond Growth Pressure	40	torr	MPCVD process parameter
Microwave Power	3.5	kW	MPCVD process parameter
Methane Concentration	5	%	In H₂ gas mixture
GaN Layer Thickness (MOCVD)	~600	nm	Grown on 2-inch sapphire substrates

Key Methodologies

The experiment relied on precise material preparation and controlled MPCVD growth conditions to achieve high-quality diamond films directly on GaN.

GaN Substrate Preparation:
- Ga-face (0001) GaN layers were grown on 2-inch sapphire via Metal Organic Chemical Vapor Deposition (MOCVD).
- N-face (0001—) GaN layers were grown on 2-inch sapphire via Molecular Beam Epitaxy (MBE).
- N-face wafers underwent a 20-minute surface nitridation step at 750 °C prior to growth.
Zeta Potential Determination:
- Streaming potential measurements were conducted using a Surpass™ 3 electrokinetic analyzer.
- The streaming channel was formed by placing two GaN plates parallel (90-110 µm separation).
- The pH of the 10-3 molar KCl electrolyte was precisely adjusted using 0.1M HCl and 0.1M NaOH solutions.
Nanodiamond Seeding:
- Wafers were seeded using mono-dispersed nanodiamond/H2O solutions in an ultrasonic bath.
- Two types of seeds were tested: commercially available Oxygen-terminated (negative zeta potential) and custom Hydrogen-terminated (positive zeta potential).
- Optimal seeding was achieved using H-terminated seeds at pH 8, maximizing electrostatic attraction to the negative GaN surface.
Diamond Film Growth (MPCVD):
- Seeded 15 x 15 mm samples were placed in a Seki Technotron AX6500X MPCVD system.
- Growth recipe utilized 5% CH4 in H2 at 40 torr pressure and 3.5 kW microwave power.
- Samples were cooled slowly in a hydrogen atmosphere post-growth to mitigate cracking of the sapphire substrate.

6CCVD Solutions & Capabilities

This research validates the critical need for high-quality, thin diamond films to enable next-generation GaN HEMT devices. 6CCVD is uniquely positioned to supply the necessary materials and customization required to replicate and scale this technology.

Applicable Materials for Thermal Management

To achieve the high thermal conductivity (up to 2100 W/mK) required for this application, 6CCVD recommends the following materials:

6CCVD Material	Key Feature	Application Relevance
Optical Grade SCD	Highest purity, lowest defect density (N < 1 ppm).	Ideal for maximizing thermal conductivity (k > 2000 W/mK) and minimizing thermal boundary resistance (TBR).
High-Quality PCD	Excellent thermal properties (k > 1800 W/mK) at large dimensions.	Cost-effective solution for scaling up production to larger wafer sizes (up to 125mm).
Custom Thin Films	SCD/PCD thickness range: 0.1 µm - 500 µm.	Allows precise control over the diamond heat spreader thickness, crucial for optimizing device integration and stress management.

Customization Potential for Advanced Integration

The successful integration of diamond on GaN requires precise material specifications and potential post-processing steps, all of which are core 6CCVD capabilities:

Large Area Substrates: While the paper used 2-inch sapphire, 6CCVD offers Polycrystalline Diamond (PCD) wafers up to 125mm in diameter, enabling industrial scaling of GaN-on-Diamond technology.
Precision Polishing: Achieving low surface roughness is vital for subsequent device layer deposition. 6CCVD guarantees Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD surfaces.
Custom Metalization: For creating ohmic contacts or bonding layers on the diamond surface, 6CCVD provides in-house metalization services, including Au, Pt, Pd, Ti, W, and Cu deposition. This is essential for full device fabrication following the direct diamond growth step.
Custom Dimensions: 6CCVD can provide custom laser cutting and shaping services to produce the specific 15 x 15 mm samples used in this research, or any other required geometry.

Engineering Support

6CCVD’s in-house PhD team specializes in optimizing MPCVD growth recipes and material selection for demanding applications like GaN HEMT thermal management. We can assist researchers and engineers in:

Selecting the optimal diamond grade (SCD vs. PCD) based on required thermal budget and cost constraints.
Consulting on surface termination strategies (H-termination, O-termination) necessary for achieving high nucleation density on specific semiconductor surfaces.
Developing custom metal stacks for low-resistance contacts on the diamond heat spreader.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

The measurement of ζ potential of Ga-face and N-face gallium nitride has been carried out as a function of pH. Both of the faces show negative ζ potential in the pH range 5.5-9. The Ga-face has an isoelectric point at pH 5.5. The N-face shows a more negative ζ potential due to larger concentration of adsorbed oxygen. The ζ potential data clearly showed that H-terminated diamond seed solution at pH 8 will be optimal for the self-assembly of a monolayer of diamond nanoparticles on the GaN surface. The subsequent growth of thin diamond films on GaN seeded with H-terminated diamond seeds produced fully coalesced films, confirming a seeding density in excess of 1011 cm-2. This technique removes the requirement for a low thermal conduction seeding layer like silicon nitride on GaN.

Tech Support

Original Source

DOI: https://doi.org/10.1021/acsomega.7b01069