Universal peroxide formation and Fermi level alignment in semiconductors due to ambient air-induced surface transfer doping

At a Glance

Metadata	Details
Publication Date	2025-09-23
Journal	Journal of Applied Physics
Authors	Shashank Mangu, Vidhya Chakrapani
Institutions	Rensselaer Polytechnic Institute
Analysis	Full AI Review Included

Executive Summary

This research conclusively identifies the dominant electrochemical mechanism responsible for Surface Transfer Doping (STD) and Fermi level (E_F) alignment in semiconductors exposed to humid ambient air.

Universal E_F Alignment: The surface E_F of a wide range of air-exposed semiconductors (including oxides, nitrides, sulfides, and elemental materials) universally aligns to an average value of approximately -5.0 eV relative to the vacuum level.
Dominant Redox Reaction: The alignment is primarily controlled by the two-electron reduction of dissolved oxygen to hydrogen peroxide (H₂O₂): O₂ + 2H⁺ + 2e^- → H₂O₂. This reaction is favored over the full four-electron reduction to water (H₂O) due to kinetic limitations.
Mechanism Confirmation: Systematic titration experiments, coupled with H₂O₂-selective fluorescence detection, confirmed H₂O₂ formation occurs across 20+ semiconductor types, validating the O₂/H₂O₂ redox couple as the universal pinning mechanism.
High-Density Doping: In hydrogen-terminated diamond (H-D), the H₂O₂ reaction drives strong p-type doping (hole accumulation), achieving sheet carrier concentrations up to nearly 10¹⁵ cm^-2 under highly acidic conditions.
Defect Influence: The study highlights that defects (e.g., oxygen vacancies in GaN or V₂O₅) can significantly modulate the STD process, leading to slower kinetics and influencing the direction of charge transfer based on the relative position of midgap states versus the O₂/H₂O₂ redox potential.

Technical Specifications

Parameter	Value	Unit	Context
Universal Surface E_F Alignment	~-5.0	eV	Average E_F position for air-exposed semiconductors (vs. vacuum).
O₂/H₂O₂ Redox Potential (µ_e-,redox)	-4.98 to -5.04	eV	Calculated range based on H₂O₂ concentration (100 nM to 10 µM) at pH 6.
H-D Work Function (Air)	4.9-5.0	eV	Measured by Kelvin probe, consistent with O₂/H₂O₂ pinning.
H-D Sheet Carrier Concentration (Max)	Nearly 10¹⁵	cm^-2	Achieved under highly acidic conditions (pH < 2).
H-D Sheet Carrier Concentration (pH 5)	4.4 x 10¹²	cm^-2	Measured value in acidic solution.
H-D Powder Mean Diameter	0.781	µm	Volume-average particle size.
H-D Powder Specific Surface Area (Est.)	~2.7	m²/g	Estimated for spherical particles of mass density 3.52 g/cm³.
H-D Surface Oxygen Concentration (Post-H₂)	1.6	%	Measured by XPS after hydrogenation.
H₂O₂ Detection Limit (Amplex Red)	10	nM	Typical detection limit for the fluorescent probe used.
H₂O₂ Contribution to STD (Acidic H-D)	71	%	Percentage contribution of O₂/H₂O₂ reaction relative to total O₂/H⁺ consumption.

Key Methodologies

Hydrogenation of Diamond Powder (H-D): Natural diamond powder (0.5-1.0 µm) was hydrogenated in a 1.5 kW ASTeX microwave reactor using pure H₂ plasma (750 W power, 650 °C substrate temperature, 35 torr pressure) for 4 hours, repeated three times to ensure good hydrogen coverage.
Surface Characterization: X-ray Photoelectron Spectroscopy (XPS) was used to confirm surface termination (oxygen concentration reduced from 5.5% to 1.6% after hydrogenation). Raman spectroscopy was used to verify material quality.
Titration Setup: Experiments involved adding 1 g of powder (or 50 mg for other semiconductors) to 20 ml (or 1 ml) of electrolyte (phosphate buffer, unbuffered H₂SO₄, or NaOH). Samples were stirred for 20 minutes.
H₂O₂ Detection (Spectrofluorometry): Hydrogen peroxide concentration was measured using the highly selective Amplex Red fluorescent probe (N-acetyl-3,7-dihydroxyphenoxazine) in the presence of Horseradish Peroxidase (HRP).
- The supernatant was mixed with 50 µM Amplex Red and 2 units/ml HRP, incubated for 30 minutes, and measured using 325 nm laser excitation.
- Fluorescence emission (resorufin) was measured at 585 nm.
Electrochemical Monitoring: Changes in pH (using a pH electrode) and Dissolved Oxygen (DO) concentration (using an electrochemical O₂ probe) were continuously monitored during the titration process to track the consumption or generation of reactants (H⁺, O₂) and determine the direction of charge transfer.

Commercial Applications

The findings regarding universal E_F pinning and the dominant role of peroxide formation are critical for the reliability, stability, and design of semiconductor devices operating in atmospheric environments.

Semiconductor Device Reliability: Understanding the universal E_F alignment at -5.0 eV allows engineers to predict and stabilize the electronic properties of devices (e.g., GaN, ZnO, Si, Ge) when exposed to ambient air, which is crucial for long-term performance.
Surface Transfer Doping (STD) Control: The identification of the O₂/H₂O₂ redox couple enables precise control over surface doping levels. This is vital for creating high-performance p-type surface conductive layers, particularly in materials like hydrogen-terminated diamond (H-D) and carbon nanotubes.
High-Power Electronics (Diamond): The ability to achieve extremely high hole accumulation (up to 10¹⁵ cm^-2) on H-D surfaces under controlled acidic conditions has direct implications for developing high-frequency and high-power diamond-based electronic devices (e.g., FETs).
Catalysis and ROS Generation: The mechanism of H₂O₂ formation on semiconductor surfaces is directly relevant to photocatalysis and electrocatalysis, especially for applications requiring the efficient electrochemical production of hydrogen peroxide or other Reactive Oxygen Species (ROS).
Sensor Technology: The sensitivity of E_F and doping levels to ambient air constituents (O₂, H⁺, H₂O) confirms the fundamental mechanism behind gas and humidity sensors based on materials like carbon nanotubes and various oxides (e.g., V₂O₅).

View Original Abstract

The naturally occurring surface transfer doping phenomenon is a process in which humid atmospheric air exchanges electrons with a semiconductor surface that is in contact with it, thereby modulating the Fermi energy (EF) of the material. In ambient air, the process occurs through an adsorbed water layer on the surface, where an electrochemical reaction serves as an acceptor or donor of electrons in the semiconductor. Despite the broad relevance of this phenomenon for the doping of a wide class of materials, the exact nature of the resulting redox reaction(s) that are operative and responsible for the alignment of EF is not clearly known because of the mixed constituents of atmospheric air. Through systematic titration experiments measuring changes in dissolved oxygen, pH, and reactive intermediates that occur after reaction with diamond and 20 other semiconductor particles of oxides, nitrides, sulfides, and selenides, it is shown that the electrochemical reaction O2+2H++2e−→H2O2 is the primary reaction responsible for surface transfer doping as well as the universal alignment of EF positions of all air-exposed semiconductors to an average value approximately −5.0 eV with respect to the vacuum level. The process appears to occur regardless of the nature of doping, defects, and bandgaps of the materials.

Tech Support

Original Source

DOI: https://doi.org/10.1063/5.0278981

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