Growth and surface structrue of hydrogen terminal diamond thin films

At a Glance

Metadata	Details
Publication Date	2024-01-01
Journal	Acta Physica Sinica
Authors	Meng-Yu Ma, Cui Yu, Ze-Zhao He, Jian-Chao Guo, Qingbin Liu
Institutions	Hebei Semiconductor Research Institute, China Electronics Technology Group Corporation
Citations	1
Analysis	Full AI Review Included

Executive Summary

This study details the optimization of homoepitaxial growth of high-purity, hydrogen-terminated (H-terminated) diamond thin films using Microwave Plasma Chemical Vapor Deposition (MPCVD) to enhance electrical performance for high-power devices.

Purity Achievement: Epitaxial layers achieved ultra-high purity, with nitrogen (N) concentration confirmed by SIMS to be less than 1x10¹⁶ atom/cm³, significantly reducing defect scattering compared to the N-doped substrate.
Growth Mode Control: Methane (CH₄) concentration was identified as the critical factor controlling the growth mode, balancing growth and etching effects.
Optimal Surface Quality: A CH₄ concentration of 4% resulted in two-dimensional planar growth, yielding the smoothest surface with an RMS roughness of only 0.225 nm (10 µm x 10 µm scan).
Enhanced Conductivity: The optimized film exhibited P-type conductivity with a high hole mobility of 207 cm²/(V·s) and a low square resistance of 4981 Ω/square.
Surface Structure: LEED and XPS confirmed that the surface transitioned from an oxygen-terminated (1x1: O) structure to the desired H-terminated (2x1: H) reconstruction after the short growth period.
Growth Rate Dependence: Growth rate increased significantly with CH₄ concentration, ranging from 23 nm/min (3% CH₄) to 81 nm/min (5% CH₄).

Technical Specifications

Parameter	Value	Unit	Context
Hole Mobility (Best)	207	cm²/(V·s)	4% CH₄ sample (Sample 2)
Square Resistance (Best)	4981	Ω/square	4% CH₄ sample (Sample 2)
Epitaxial N Concentration	< 1x10¹⁶	atom/cm³	SIMS detection limit
Film Thickness Range	230 to 810	nm	Dependent on CH₄ concentration
Growth Rate (5% CH₄)	81	nm/min	Highest rate observed
Growth Rate (4% CH₄)	59	nm/min	Optimal electrical performance
Growth Rate (3% CH₄)	23	nm/min	Lowest rate observed
Surface Roughness (RMS, Best)	0.225	nm	4% CH₄ sample (10 µm x 10 µm scan)
Surface Roughness (RMS, Worst)	2.823	nm	3% CH₄ sample (Step-flow/etching)
Growth Temperature	860	°C	Constant for all samples
Microwave Power	3500	W	Constant for all samples (6 kW system)
CH₄ Concentration Tested	3, 4, 5	%	Relative to H₂ flow
Substrate Type	CVD Diamond (001)	N/A	1.5° off-angle, N-doped
Substrate N Concentration	2x10¹⁷-5x10¹⁷	atom/cm³	Prior to epitaxy
Final Surface Termination	(2x1: H)	N/A	Confirmed by LEED

Key Methodologies

The diamond thin films were grown using a 6 kW MPCVD system (2.45 GHz) following a precise process flow:

Substrate Preparation:
- Substrates were 10 mm x 10 mm x 0.5 mm N-doped CVD diamond (001) with a 1.5° off-angle.
- Cleaning involved high-temperature boiling in mixed acid (H₂SO₄:HNO₃ = 3:1) for 20 minutes, followed by standard solvent cleaning (ethanol, acetone, deionized water).
Pre-Etching/Cleaning:
- Samples were loaded into the MPCVD chamber and pumped to 5.0x10^-6 mbar.
- High-purity H₂ gas (200 sccm) was introduced and excited into plasma.
- H₂ or H₂/O₂ plasma pre-etching was performed to remove surface contaminants (e.g., alcohol, dust) and relieve stress from mechanical polishing.
Homoepitaxial Growth:
- Growth time was fixed at 10 minutes for all samples.
- Fixed Parameters: Microwave Power (3500 W), Temperature (860 °C).
- Variable Parameter: Methane (CH₄) concentration was varied at 3%, 4%, and 5% relative to the H₂ flow (~190 sccm).
Growth Mode Observations:
- 5% CH₄: Growth dominated etching, leading to localized overgrowth and high roughness (Island Growth).
- 4% CH₄: Growth balanced etching, resulting in smooth, two-dimensional planar growth.
- 3% CH₄: Etching dominated growth, resulting in step-flow morphology with significant etching pits/troughs.
Characterization Techniques:
- Structural/Morphology: Optical Microscopy (OM), Atomic Force Microscopy (AFM).
- Purity/Thickness: Secondary Ion Mass Spectrometry (SIMS) for N concentration gradient and film thickness.
- Surface Chemistry/Structure: Low-Energy Electron Diffraction (LEED) and X-ray Photoelectron Spectroscopy (XPS) confirmed the (2x1: H) termination and P-type characteristics (low O/N ratio).
- Electrical: Hall measurement system for mobility, square resistance, and carrier density.

Commercial Applications

The successful fabrication of high-mobility, ultra-smooth, H-terminated diamond films provides a critical foundation for next-generation semiconductor devices, particularly in areas demanding high power and high frequency:

High-Power RF Electronics: Used in the fabrication of Diamond Field-Effect Transistors (FETs) and High Electron Mobility Transistors (HEMTs) for 5G/6G infrastructure and radar systems, leveraging diamond’s high breakdown field (>1 MV/mm).
Power Switching Devices: Development of high-efficiency diamond power diodes and switches for electric vehicles, smart grids, and industrial power conversion, benefiting from the high carrier mobility and thermal stability.
Thermal Management Substrates: The high thermal conductivity of the high-purity epitaxial layer ensures efficient heat dissipation when integrated into complex microelectronic circuits.
Sensors and Detectors: The wide bandgap (5.5 eV) and stable surface termination are valuable for UV detectors and harsh environment sensors.

View Original Abstract

The conductivity of hydrogen-terminated diamond is a limiting factor in its application in field-effect transistor devices. The traditional preparation process hinders the improvement of the electrical properties of hydrogen-terminated diamond due to impurity elements in the diamond bulk and surface damage caused by processing near the diamond surface. To overcome this, researchers have explored the epitaxial growth of a high-purity and flat-surfaced diamond thin film on a diamond substrate. However, this approach still faces challenges in film characterization and achieving high surface smoothness. In this study, microwave plasma chemical vapor deposition technology is used to epitaxially grow a sub-micron thick diamond film on a nitrogen-doping chemical vapor deposition diamond substrate of 10 mm × 10 mm × 0.5 mm in size. The influence of methane concentration on the growth and conductivity of diamond film is investigated. The test results reveal that the growth thickness of the diamond film ranges from 230 to 810 nm, and the nitrogen concentration in the epitaxial layer is lower than 1×1016 atom/cm3. Three growth modes are observed for the homoepitaxial growth of the diamond thin film under different methane concentrations. A methane concentration of 4% enables two-dimensional planar growth of diamond, resulting in a smooth and flat surface with a roughness of 0.225 nm (10 μm×10 μm). The formation of different surface morphologies is attributed to the growing process and etching process of diamond. Surface low-energy electron diffraction testing indicates that the surface of the diamond film undergoes a structural transition from oxygen terminal (1×1: O) to hydrogen terminal (2×1: H) when grown for a short period of time. X-ray photoelectron spectroscopy analysis reveals an extremely low ratio of oxygen element to nitrogen element, giving the grown diamond film P-type conductivity characteristics. The Hall test results demonstrate that the hydrogen-terminated diamond film grown with a methane concentration of 4% exhibits the highest conductivity, with a square resistance of 4981 Ω/square and a hole mobility of 207 cm2/(V·s). This enhanced conductivity can be attributed to the lower defect density observed under these specific conditions. The findings of this study effectively improve the electrical properties of hydrogen-terminated diamond, and contribute to the development and practical application of high-power diamond devices.

Tech Support

Original Source

DOI: https://doi.org/10.7498/aps.73.20240053