Impact of Nitrogen, Boron and Phosphorus Impurities on the Electronic Structure of Diamond Probed by X-ray Spectroscopies

At a Glance

Metadata	Details
Publication Date	2021-03-09
Journal	C – Journal of Carbon Research
Authors	Sneha Choudhury, Ronny Golnak, Christian Schulz, Klaus Lieutenant, N. Tranchant
Institutions	Centre National de la Recherche Scientifique, Freie Universität Berlin
Citations	4
Analysis	Full AI Review Included

Executive Summary

This study utilized soft X-ray spectroscopies (XAS, XES, XPS) to comprehensively characterize the occupied and unoccupied electronic states created by boron (B), phosphorus (P), and nitrogen (N) impurities in single-crystal diamond (SCD).

Electronic Tuning Confirmed: Doping successfully introduced new electronic states near the Conduction Band Minimum (CBM) and Valence Band Maximum (VBM), confirming the ability to tune diamond’s electronic properties for applications like electron emission.
Core Exciton Suppression: Impurity incorporation (B, P, N) significantly reduced the intensity and broadened the sharp diamond core exciton peak (289.3 eV), particularly in P-SCD, indicating strong sensitivity to lattice disruption.
Bulk vs. Surface Sensitivity: By comparing bulk-sensitive PFY-XAS and XES with surface-sensitive TEY-XAS and XPS, the study differentiated between bulk defects and surface modifications.
Bulk Defect States: PFY-XAS revealed new unoccupied states in B-SCD (284.1 eV, related to B-induced defects) and enhanced bulk sp² and C-H incorporation in P-SCD, likely due to the large size of the P atom causing lattice strain.
Surface Reconstruction: XPS analysis of the valence band showed that occupied surface electronic states were strongly affected by doping, with evidence of surface reconstruction observed in both B-SCD and P-SCD.
Bulk Occupied States Stability: XES demonstrated that the bulk occupied electronic states (valence band) were not significantly altered by the impurity concentrations investigated, confirming the localized nature of the doping effects.

Technical Specifications

Parameter	Value	Unit	Context
Boron Concentration ([B])	2.7 x 10²⁰ (~1500)	atoms/cm³ (ppm)	B-SCD sample (SIMS plateau)
Phosphorus Concentration ([P])	8 x 10¹⁹ (~400)	atoms/cm³ (ppm)	P-SCD epilayer (SIMS plateau)
Nitrogen Concentration ([N])	4.9 x 10¹⁸ (~30)	atoms/cm³ (ppm)	N-SCD epilayer (SIMS plateau)
Diamond Bandgap (Measured)	5.5	eV	Derived from VBM (XES onset, 284.0 eV) and CBM (XAS, 289.5 eV)
Core Excitonic Peak Energy	289.3	eV	Feature III in C K-edge XAS
B-induced Defect State Energy	284.1	eV	New unoccupied state (Feature IV) observed in PFY-XAS (B-SCD)
P-SCD Growth Temperature	~1000	°C	Ellipsoidal MPCVD reactor
N-SCD Growth Temperature	~900	°C	MPCVD reactor
P/C Source Gas Ratio (P-SCD)	20	%	Used Trimethylphosphine (P(CH₃)₃) source gas
N₂ Source Gas Ratio (N-SCD)	1	vol%	Used in H₂ (95 vol%) and CH₄ (4 vol%) mixture
TEY Probing Depth	<1	nm	Highly surface sensitive (Inelastic Mean Free Path)
PFY Probing Depth	<60	nm	Bulk sensitive (X-ray attenuation length)

Key Methodologies

The electronic structure analysis relied on synchrotron-based soft X-ray spectroscopies, complemented by detailed material characterization using Secondary Ion Mass Spectrometry (SIMS) and Differential Interference Contrast (DIC) microscopy.

1. Diamond Synthesis (MPCVD)

P-SCD Growth: Conducted on <111> substrates using an ellipsoidal Microwave Plasma Chemical Vapor Deposition (MPCVD) reactor. Growth parameters included a temperature of ~1000 °C, 2.1 kW microwave power, and a high P/C ratio of 20% using diluted trimethylphosphine.
N-SCD Growth: Grown as an epilayer on <100> substrates. Growth utilized a gas mixture containing 1 vol% N₂, 4 vol% CH₄, and 95 vol% H₂ at 100 mbar pressure and ~900 °C.
Sample Preparation: All samples underwent thorough cleaning (3:1 sulfuric:nitric acid mixture at 250 °C) followed by surface hydrogenation in H₂ plasma at 750 °C.

2. Spectroscopic Characterization

Technique	Detection Mode	Sensitivity	Probed States	Key Findings
XAS	Total Electron Yield (TEY)	Surface (<1 nm)	Unoccupied	Highly sensitive to surface sp² bonds and C-H termination.
XAS	Partial Fluorescence Yield (PFY)	Bulk (<60 nm)	Unoccupied	Revealed B- and P-induced defect states within the bulk bandgap.
XES	Photon-out	Bulk	Occupied (Valence Band)	Confirmed bulk valence band structure was largely unaffected by doping concentration.
XPS	Electron-out	Surface (<1 nm)	Occupied (Valence Band)	Showed strong modification of surface electronic states, indicating reconstruction and potential donor states (P-SCD).
RPES	Resonant Photoemission	Surface	Occupied (Valence Band)	Used at 299 eV excitation to enhance carbon cross-section and probe resonant features.

Commercial Applications

The ability to precisely tune the electronic structure of diamond via controlled doping, as characterized in this study, is fundamental to several high-performance engineering sectors:

Electron Emission Technology: The tuning of electron emission properties, particularly the creation of shallow donor levels (P-doping) and the modification of surface states (XPS findings), is crucial for developing high-efficiency cold cathode emitters and vacuum electronic devices.
High Power RF and Switching: Controlled p-type (B) and n-type (P, N) doping is necessary for fabricating functional diamond p-n junctions, MOSFETs, and Schottky diodes capable of operating under extreme voltage, temperature, and radiation conditions.
Photoelectrochemical Energy Conversion: The introduction of sub-bandgap absorption states (observed in B-SCD and P-SCD PFY-XAS) enables diamond to absorb visible light, significantly improving its efficiency as a photoelectrode for solar-driven chemical processes, such as CO₂ reduction and water splitting.
Advanced Sensor Technology: Understanding how impurities affect the core exciton and band edges is vital for optimizing diamond-based UV and radiation detectors, which rely on the material’s intrinsic wide bandgap properties.
Boron-Doped Diamond (BDD) Electrodes: The detailed characterization of B-induced defects and surface reconstruction provides critical data for optimizing BDD electrodes used in industrial electrochemistry, wastewater treatment, and ozone generation.

View Original Abstract

Doping diamond with boron, nitrogen or phosphorus enables a fine tuning of its electronic properties, which is particularly relevant for applications involving electron emission. However, the chemical nature of the doping sites and its correlation with electron emission properties remain to be clarified. In this work, we applied soft X-ray spectroscopy techniques to probe occupied and unoccupied electronic states in undoped, boron-, phosphorus- and nitrogen-containing single crystal diamonds. X-ray absorption, X-ray emission and X-ray photoemission spectroscopies, performed at the carbon K-edge, provide a full picture of new electronic states created by impurities in diamond. The different probing depths of fluorescence- and electron-based detection techniques enable a comparison between surface and bulk contributions.

Tech Support

Original Source

DOI: https://doi.org/10.3390/c7010028

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