Structural and Chemical Analysis of C-BN/Diamond Heterostructures

Inorganic Non-Metallic Materials Information Science Semiconductors and Information Devices Materials Science
C-BN diamond heterostructures chemical vapor deposition electron microscopy

Academic Background

Cubic boron nitride (C-BN) is an ultra-wide bandgap semiconductor material with high thermal conductivity, low dielectric constant, and high breakdown electric field, making it highly promising for applications in high-temperature, high-power electronic devices. However, the synthesis of C-BN still faces numerous challenges, particularly in achieving high-quality single-crystal C-BN films on large-scale substrates. Diamond, due to its small lattice mismatch with C-BN (1.36%), is considered an ideal substrate for the epitaxial growth of C-BN. Nevertheless, the synthesis of C-BN/diamond heterostructures remains in the early stages of development, with many unresolved issues regarding defect reduction and film quality improvement.

This study aims to grow C-BN films on boron-doped diamond substrates using electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR PECVD) and to analyze the morphological features, defect types, and chemical bonding states of the films through techniques such as transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). The research also explores the effects of gas precursor concentration, growth temperature, and substrate cleaning methods on the formation of C-BN phases (cubic or turbostratic), providing important experimental insights for further optimizing C-BN film growth processes.

Source of the Paper

This paper is co-authored by Saurabh Vishwakarma, Avani Patel, Manuel R. Gutierrez, Robert J. Nemanich, and David J. Smith, affiliated with the School for Engineering of Matter, Transport and Energy, the Eyring Materials Center, and the Department of Physics at Arizona State University. The paper was published on April 15, 2025, in the Journal of Applied Physics, titled “Structural and Chemical Analysis of C-BN/Diamond Heterostructures.”

Research Process and Results

1. Experimental Design and Sample Preparation

The study first grew C-BN films on boron-doped single-crystal diamond substrates using ECR PECVD. The substrates were cleaned with hydrogen plasma to remove surface contaminants before growth. Two cleaning methods, C1 and C2, were employed, corresponding to different hydrogen plasma treatment durations and temperatures. Subsequently, C-BN films were grown using gas precursors such as H₂, BF₃, and N₂, with growth temperatures ranging from 735°C to 820°C and a chamber pressure maintained at 1.1×10⁻⁴ Torr.

2. Film Morphology and Structural Analysis

Cross-sectional transmission electron microscopy (TEM) was used to analyze the morphology and structure of the C-BN films in detail. The study found that gas precursor concentration significantly influenced the formation of C-BN phases. Under a H₂/BF₃ ratio of 0.75, cubic-phase C-BN formed at the interface, while a ratio of 1 resulted in predominantly turbostratic (t-BN) films. Additionally, growth temperature significantly affected grain size and defect density. Films grown at 820°C exhibited larger grain sizes and lower defect densities in regions away from the interface.

3. Chemical Bonding State Analysis

Electron energy loss spectroscopy (EELS) was used to further analyze the chemical bonding states of boron, nitrogen, and carbon in the C-BN films. The results showed that sp²-bonded BN dominated the initial growth layers, with sp³-bonded C-BN gradually increasing as growth progressed. Moreover, the diamond substrate exhibited a transition from sp³ to sp² bonding near the interface, likely due to surface disordering caused by the cleaning process or early growth stages.

4. Defect Characteristics and Mechanisms

The study identified twins and stacking faults as the primary defects in C-BN films, particularly near the interface. High-resolution TEM images revealed the formation mechanisms of these defects. Although higher growth temperatures reduced defect densities in regions away from the interface, defect densities near the interface did not significantly decrease. The study also proposed a mechanism explaining the influence of substrate cleaning methods on twin formation, suggesting that hydrogen plasma cleaning-induced surface roughening increased the potential barrier for nitrogen atom migration, thereby promoting twin formation.

Conclusions and Significance

Through detailed TEM and EELS analysis, this study revealed the morphological features, chemical bonding states, and defect formation mechanisms of C-BN/diamond heterostructures. The results demonstrated that gas precursor concentration and growth temperature significantly influence the formation of C-BN phases and film quality, while substrate cleaning methods play a crucial role in defect densities near the interface. These findings provide important experimental insights for further optimizing C-BN film growth processes, particularly in reducing defect densities and improving film quality, with significant scientific and application value.

Research Highlights

  1. Effect of Gas Precursor Concentration on C-BN Phase Formation: The study found that the H₂/BF₃ ratio significantly influences C-BN phase formation, with low H₂ concentration favoring cubic-phase C-BN.
  2. Effect of Growth Temperature on Film Quality: Films grown at 820°C exhibited larger grain sizes and lower defect densities in regions away from the interface.
  3. Effect of Substrate Cleaning Methods on Defect Formation: Hydrogen plasma cleaning-induced surface roughening increased twin formation, revealing the underlying mechanism of interface defects.
  4. EELS Reveals Chemical Bonding States: Using EELS, the study provided a detailed analysis of the chemical bonding states of boron, nitrogen, and carbon in C-BN films, uncovering the transition from sp² to sp³ bonding.

Other Valuable Information

The study also proposed future research directions, including growth experiments at higher temperatures and optimization of substrate cleaning methods to further reduce defect densities in C-BN films. Additionally, the study highlighted the impact of surface roughening on the potential barrier for nitrogen atom migration, offering new perspectives on understanding interface defect formation mechanisms.

Through this in-depth analysis, the synthesis and optimization of C-BN/diamond heterostructures are expected to enable broader applications in high-power electronic devices.