Application-driven synthesis and characterization of hexagonal boron nitride deposited on metals and carbon nanotubes

At a Glance

Metadata	Details
Publication Date	2021-07-02
Journal	2D Materials
Authors	Victoria Chen, Yong-Cheol Shin, Evgeny Mikheev, Qing Lin, Joel Martis
Citations	4
Analysis	Full AI Review Included

Executive Summary

This study details the application-driven synthesis and characterization of hexagonal boron nitride (h-BN) films grown via Low-Pressure Chemical Vapor Deposition (LPCVD) on various substrates, targeting scalable electronic applications.

Superior Film Quality: Monolayer h-BN grown on single crystal Pt(111) exhibited significantly lower surface roughness (0.80 nm RMS) and greater spatial homogeneity compared to films grown on polycrystalline Pt (1.70 nm RMS).
Substrate Reusability: An electrochemical bubbling transfer method was successfully implemented, allowing the expensive Pt growth substrates to be reused for hundreds of growths without measurable degradation in substrate or film quality.
Ultrathin Protection Barrier: Monolayer h-BN (3.33 A thick) was demonstrated as an effective, ultrathin capping layer, protecting monolayer MoS₂ from degradation during high-temperature anneals (up to 450 °C) in H₂/Ar atmosphere.
Direct Conformal Growth: Multilayer h-BN was selectively and conformally deposited directly onto single-walled carbon nanotubes (CNTs), providing a scalable method to cap CNTs and potentially improve device performance without requiring a transfer step.
Tailored Synthesis: The study confirms that substrate crystallinity (e.g., single crystal vs. polycrystalline Pt) strongly influences h-BN film properties (roughness, homogeneity, grain size), enabling selective tailoring of film characteristics for specific applications.

Technical Specifications

Parameter	Value	Unit	Context
h-BN Band Gap	~6	eV	Electrical insulator property
h-BN Thermal Conductivity (In-plane)	>400	W/m^-1K^-1	High thermal dissipation capability
Monolayer h-BN Thickness	3.33	A	Interlayer spacing (ultrathin barrier)
CVD Growth Pressure	~900	mTorr	Low-Pressure CVD condition
RMS Roughness (Single Crystal Pt(111) h-BN)	0.80	nm	Smoother film quality after transfer
RMS Roughness (Polycrystalline Pt h-BN)	1.70	nm	Rougher film quality after transfer
h-BN E’ Raman Peak (Monolayer)	~1370	cm^-1	Characteristic peak position
MoS₂ Protection Temperature (h-BN capped)	Up to 450	°C	Anneal temperature retaining PL spectra
Precursor Decomposition Temperature	100	°C	Ammonia borane heating temperature
Pt Substrate Growth Temperature	1100	°C	LPCVD growth condition
Cu Substrate Growth Temperature	1050	°C	LPCVD growth condition

Key Methodologies

The h-BN films were prepared using Low-Pressure Chemical Vapor Deposition (LPCVD) in a 2” diameter furnace, followed by specialized transfer techniques for characterization.

Precursor Preparation: The air-stable, solid source precursor, ammonia borane (H₃NBH₃), was placed in an ampoule and heated independently to 100 °C, where it decomposed into borazine [(HBNH)₃], polyiminoborane (BHNH), and hydrogen.
CVD Growth Conditions:
- The furnace chamber was maintained at a pressure of ~900 mTorr.
- H₂ gas was used as the carrier gas to diffuse the borazine onto the substrates.
- Metal substrates (Pt, Cu) and CNTs were annealed for 40 minutes at their respective growth temperatures prior to h-BN deposition to remove impurities and smooth the surface.
- Growth temperatures: Pt (polycrystalline and single crystal) and CNTs at 1100 °C; Cu foil at 1050 °C.
Transfer Process (Pt Substrates):
- An electrochemical bubbling method was used at room temperature to delaminate the h-BN/PMMA stack from the Pt.
- The stack was placed in a 1M NaOH solution, with the Pt foil as the negative electrode, generating bubbles at the interface to lift the film. This preserves the expensive Pt substrate for reuse.
Transfer Process (Cu Substrates):
- A standard wet etching method was used to transfer multilayer h-BN from the Cu foil onto the target SiO₂ substrate.
Characterization: Films were primarily transferred onto 300 nm SiO₂/Si substrates for analysis using Atomic Force Microscopy (AFM) for roughness and thickness, Raman spectroscopy (532 nm laser) for film quality, and Transmission Electron Microscopy (TEM) for cross-sectional imaging and crystallinity.

Commercial Applications

The unique combination of electrical insulation, high thermal conductivity, and atomic-scale thickness makes CVD-grown h-BN critical for next-generation electronics and protective coatings.

Application Area	Specific Use Case	h-BN Property Utilized
2D Electronics & Transistor Scaling	Gate dielectric in 2D Field Effect Transistors (FETs).	Ultra-thin (3.33 A), high band gap (~6 eV) insulation, lack of dangling bonds.
Thermal Management	Interlayer dielectric in 3D integrated circuits (ICs).	High in-plane thermal conductivity (>400 W/mK) to spread heat laterally and protect stacked memory layers from logic hot spots.
Contact Engineering	Insulating layer in Metal-Insulator-Semiconductor (MIS) contacts.	Acts as a solid-state barrier to prevent metal reaction and helps depin the Fermi level, improving contact performance.
Protective Coatings & Passivation	Ultrathin barrier protecting air-sensitive 2D materials (e.g., MoS₂, black phosphorus) during high-temperature processing or operation.	High chemical inertness and impermeability to small chemical species; demonstrated protection up to 450 °C.
Non-Volatile Memory	Switching layer in Resistive Random-Access Memory (RRAM) devices.	Defects within the multilayer h-BN film can be utilized to form conductive filaments for switching.
Carbon Nanotube (CNT) Devices	Direct capping/encapsulation of CNTs.	Conformal deposition provides a high-quality gate dielectric or passivation layer directly on the CNT, avoiding performance degradation caused by transfer residues.

View Original Abstract

Hexagonal boron nitride (h-BN) is unique among two-dimensional materials, with a large band gap (~6 eV) and high in-plane thermal conductivity (>400 W m<sup>-1</sup> K<sup>-1</sup>), second only to diamond among electrical insulators. Many studies to date have relied on exfoliated h-BN, however, for large-scale applications the material must be synthesized by methods such as chemical vapor deposition (CVD). Here, we first investigate single-layer h-BN synthesized by CVD on single crystal platinum (Pt), comparing these films with h-BN deposited on more commonly used polycrystalline Pt and Cu. The h-BN film grown on single crystal Pt has the lowest surface roughness and best spatial homogeneity, and our electrochemical transfer process allows the Pt to be reused with no measurable degradation. Additionally, we also demonstrate direct capping of carbon nanotubes (CNTs) with as-grown h-BN, but we find that the direct growth partly degrades the CNT electrical conductivity. On the other hand, we show that transferred monolayer h-BN can serve as an ultrathin barrier which protects MoS2 from damage at high temperatures and discuss other applications that take advantage of the conformal h-BN deposition.

Tech Support

Original Source

DOI: https://doi.org/10.1088/2053-1583/ac10f1

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