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6CCVD Research

Orientation-dependent electric transport and band filling in hole co-doped epitaxial diamond films

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2020-06-08
JournalApplied Surface Science
AuthorsErik Piatti, A. Pasquarelli, R. S. Gonnelli
InstitutionsPolytechnic University of Turin, UniversitÀt Ulm
Citations11
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”
  • Co-Doping Strategy: The study successfully employed a co-doping approach combining Boron (B) substitution and Ionic Gating (Electric Double Layer Transistors, EDLTs) to tune the 2D hole gas (2DHG) density in H-terminated epitaxial diamond films.
  • Orientation Dependence: The electrical transport properties are highly sensitive to crystal orientation, specifically (111) versus (110) facets.
  • Capacitance Enhancement (110): B doping led to a five-fold increase in gate capacitance (CG) in (110)-oriented films (CG up to 2.53 ”F cm-2), while CG in (111) films remained largely unaffected.
  • Mobility Suppression (111): B doping severely suppressed hole mobility (”) in (111) films (down to ~3 cm2V-1s-1), suggesting that extrinsic disorder introduced by B dopants dominates scattering on this surface.
  • Density Limit: The maximum total carrier density (n2D) achieved remained below 2.1014 h+cm-2, which is three times lower than the theoretical requirement for high-temperature superconductivity in diamond.
  • IMT Tuning: The co-doping approach tunes the (110) surface toward a frustrated insulator-to-metal transition (IMT) and the (111) surface toward a re-entrant IMT, as determined by the Ioffe-Regel parameter (x).
  • Mechanism: The orientation-dependent CG behavior is directly linked via quantum capacitance (CQ) to the specific energy-dependence of the electronic Density of States (DOS) at the Fermi level (EF) for each surface.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Operating Temperature (T)~240KTransport measurements (above PES glass transition)
Maximum Applied Gate Voltage (VG)-6VLimited by electrolyte electrochemical stability window
Nominal B-doped Layer Thickness2nmEpitaxial thin film growth
H-(111) Gate Capacitance (CG)1.36 ± 0.41”F cm-2H-terminated (111) surface
B-(110) Gate Capacitance (CG)2.53 ± 0.76”F cm-2B-doped (110) surface (Highest measured CG)
Maximum Total Hole Density (n2D)< 2.1014h+cm-2Maximum density achieved in B-doped films
Intrinsic Hole Density (n0) B-(111)1.5 · 1014h+cm-2Hole density at VG = 0
Mobility Range (H-(111))53 to 145cm2V-1s-1H-terminated (111) surface
Mobility Range (B-(111))1.9 to 3.3cm2V-1s-1B-doped (111) surface (Strong suppression)
Helmholtz Layer Capacitance (CH)~22”F cm-2Estimated from CG/DOS fit (Eq. 1)
DFT Energy Cut-off (Wavefunction)65RyDensity Functional Theory calculations
Growth Pressure2kPaMPCVD process
Growth Temperature750°CMPCVD process

Key Methodologies

Section titled “Key Methodologies”
  1. Substrate Preparation: Commercial Ib-type single crystal diamond substrates ((111) and (110) orientation) were cleaned using a multi-step chemical procedure involving sonication (Acetone, Isopropanol) and etching in various hot acid mixtures (e.g., Chromosulphuric acid, HCl:H2O2, H2SO4:H2O2).
  2. Epitaxial Film Growth (MPCVD): Films were grown using Microwave Plasma Chemical Vapor Deposition (MPCVD) at 750 °C and 2 kPa. A 100 nm intrinsic buffer layer was grown first, followed by a nominal 2 nm B-doped layer (d-layer) on half the samples.
  3. H-Termination: The growth process was designed to result in H-termination of all surfaces, enabling surface conductivity.
  4. Device Architecture (EDLT): Samples were incorporated into Electric Double Layer Transistors (EDLTs) using a four-wire measurement setup (Source, Drain, Voltage probes) and a Au side gate.
  5. Electrolyte System: A Polymer-Electrolyte System (PES) ion-gel was used, composed of BEMA oligomer and EMIM-TFSI ionic liquid (7:3 weight ratio), UV-cured in a dry atmosphere.
  6. Transport Measurement: Sheet conductance (σ2D) was measured at T ~ 240 K by applying a small DC current (1 ”A) and monitoring the longitudinal voltage drop.
  7. Capacitance Measurement (DSCC): Gate-induced hole density (Δn2D) and gate capacitance (CG) were determined using Double-Step Chronocoulometry (DSCC) by analyzing the transient gate current (IG) during step-like voltage application.
  8. Theoretical Modeling (DFT): Ab initio Density Functional Theory (DFT) calculations (Jellium model approximation) were performed to determine the electronic bandstructure, Density of States (DOS), and Fermi velocity (v) for the doped (111) and (110) diamond slabs.
  9. Scattering Lifetime Calculation: The charge-carrier scattering lifetime (τ) was extracted by combining the experimentally measured mobility (”) with the theoretically calculated mobility-to-lifetime ratio (”/τ) derived from DFT bandstructure data.

Commercial Applications

Section titled “Commercial Applications”
  • High-Density Field-Effect Transistors (FETs): The demonstrated high gate capacitance (CG > 2.5 ”F cm-2) in B-doped (110) films is critical for scaling down device dimensions and achieving ultra-low power operation in diamond-based EDLTs.
  • Reconfigurable Electronics and Memory: The ability to tune the material state across or near the Insulator-to-Metal Transition (IMT) criterion (Mott-Ioffe-Regel criterion, x=1) allows for the development of novel phase-change memory or reconfigurable switches based on diamond films.
  • High-Power and High-Frequency Devices: Diamond’s intrinsic wide bandgap and high thermal conductivity, coupled with the ability to create highly confined and tunable 2DHGs, are essential for next-generation power electronics and high-frequency RF components operating in harsh environments.
  • Quantum Capacitance Sensors: The strong correlation between CG and the electronic DOS makes these ion-gated structures highly sensitive platforms for detecting changes in surface electronic structure, applicable in advanced quantum sensing and metrology.
  • Superconducting Devices (Future Potential): While current hole densities are insufficient for high-Tc superconductivity, the research provides a pathway for optimizing co-doping and gating techniques, potentially leading to the realization of predicted field-induced superconductivity in diamond surfaces.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2004 - Superconductivity in diamond [Crossref]
  2. 2004 - Dependence of the superconducting transition temperature on the doping level in single-crystalline diamond films [Crossref]
  3. 2005 - Origin of the metallic properties of heavily boron-doped superconducting diamond [Crossref]
  4. 2004 - Superconductivity in diamond thin films well above liquid helium temperature [Crossref]
  5. 2007 - Observation of a superconducting gap in boron-doped diamond by laser-excited photoemission spectroscopy [Crossref]
  6. 2008 - Superconducting diamond: an introduction [Crossref]
  7. 2015 - Signature of high Tc above 25 K in high quality superconducting diamond [Crossref]
  8. 2004 - Three-dimensional MgB2-type superconductivity in hole-doped diamond [Crossref]
  9. 2004 - Superconductivity in boron-doped diamond [Crossref]

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