The Undoped Polycrystalline Diamond Film—Electrical Transport Properties
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2021-09-12 |
| Journal | Sensors |
| Authors | Szymon Łoś, K. Fabisiak, K. Paprocki, Mirosław Szybowicz, Anna Dychalska |
| Institutions | Poznań University of Technology, Institute of Mathematics |
| Citations | 10 |
| Analysis | Full AI Review Included |
The Undoped Polycrystalline Diamond Film—Electrical Transport Properties
Section titled “The Undoped Polycrystalline Diamond Film—Electrical Transport Properties”Executive Summary
Section titled “Executive Summary”This research investigates how controlling the synthesis pressure in Hot Filament Chemical Vapor Deposition (HF CVD) tunes the electrical transport properties of undoped polycrystalline diamond films (DF) by altering their surface hydrogenation level and microstructure.
- Core Mechanism: Electrical conduction is governed by two distinct mechanisms: thermally activated band conduction at high temperatures (200-300 K) and Mott Variable Range Hopping (M-VRH) in localized states near the Fermi level at low temperatures (below 200 K).
- Hydrogenation Impact: Increased surface hydrogenation (achieved at lower pressure, 40 hPa) significantly increases surface conductivity and reduces the grain boundary barrier height (ΦGB).
- Activation Energy (Ea): Ea was successfully tuned from a low of 56 meV (highest H content, largest grains) up to 228 meV (lowest H content, smallest grains).
- Localized States Density (N(EF)): The density of localized states decreased by a factor of five, from 4.7 x 1014 eV-1cm-3 to 9.2 x 1013 eV-1cm-3, as hydrogenation increased.
- Structural Correlation: Increasing synthesis pressure (from 40 to 80 hPa) reduced the average grain size (from 66 nm to 35 nm) and decreased the hydrogen concentration (from 26 at.% to 17 at.%).
- Engineering Implication: The degree of hydrogenation is confirmed to be a crucial parameter for precisely tuning the electrical characteristics of diamond layers for device design.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Synthesis Method | HF CVD | - | Undoped polycrystalline DF on n-Si (100) |
| Filament Temperature | 2300 | K | Tungsten filament |
| Substrate Temperature | 1100 | K | Estimated growth temperature |
| Gas Composition (CH4/H2) | 3 / 97 | vol% | Fixed ratio |
| Total Gas Flow Rate | 100 | sccm | Fixed flow rate |
| Film Thickness | 3-4 | µm | Range observed via SEM |
| High T Conduction Range | 200-300 | K | Thermally activated band conduction |
| Low T Conduction Range | 90-200 | K | Mott Variable Range Hopping (M-VRH) |
| Activation Energy (Ea) Range | 56 to 228 | meV | DF40 (low) to DF80 (high) |
| Grain Boundary Barrier (ΦGB) Range | 60 to 257 | meV | DF40 (low) to DF80 (high) |
| N(EF) Range | 9.2 x 1013 to 4.7 x 1014 | eV-1cm-3 | Density of localized states |
| Hopping Distance (R) Range | 1.23 x 10-5 to 4.2 x 10-4 | cm | DF80 (low) to DF40 (high) |
| DF40 Hydrogenation (H) | 26 ± 1 | at.% | Highest H concentration (40 hPa) |
| DF80 Hydrogenation (H) | 17 ± 1 | at.% | Lowest H concentration (80 hPa) |
| DF40 Average Grain Size (L) | 66 ± 1 | nm | Largest crystallite size |
| DF80 Average Grain Size (L) | 35 ± 1 | nm | Smallest crystallite size |
| Diamond Raman Peak | 1331.6 to 1331.9 | cm-1 | Characteristic diamond line |
Key Methodologies
Section titled “Key Methodologies”The study utilized Hot Filament Chemical Vapor Deposition (HF CVD) to synthesize undoped polycrystalline diamond films, followed by comprehensive structural and electrical characterization.
- Synthesis Parameters: Diamond films were grown on n-Si (100) substrates using 3 vol% CH4 in H2. The key variable was the total working gas pressure, set at 40, 60, and 80 hPa, while maintaining a fixed filament temperature (2300 K) and substrate temperature (1100 K).
- Electrode Preparation: Gold (Au) was evaporated onto the diamond surfaces and substrates to create four-probe electrode contacts for electrical measurements.
- Structural Analysis (XRD): X-ray Diffraction (XRD) was performed to identify characteristic diamond reflections ((111), (220), (331)) and calculate the average grain size (L) using the Debye-Scherrer formula.
- Phase and Quality Analysis (Raman): Raman spectroscopy (488 nm excitation) was used to assess diamond quality (FWHM of the 1333 cm-1 peak) and quantify the presence of sp2 amorphous carbon (G-band at 1530 cm-1).
- Hydrogenation Quantification: An empirical formula relating the photoluminescence background slope and the G-band integral intensity was used to estimate the relative hydrogen concentration (H [at.%]) in the amorphous layer surrounding the microcrystallites.
- Electrical Transport (I-V-T): DC conductivity was measured as a function of temperature (90-300 K) under vacuum using an Oxford Optistat cryostat.
- Modeling: Experimental conductivity data were analyzed in two regions:
- High Temperature (200-300 K): Used the standard thermal activation model to extract activation energy (Ea) and grain boundary barrier height (ΦGB).
- Low Temperature (90-200 K): Used the Mott Variable Range Hopping (M-VRH) model to calculate the density of localized states (N(EF)), average hopping distance (R), and hopping energy (W).
Commercial Applications
Section titled “Commercial Applications”The ability to precisely control the electrical transport properties of polycrystalline diamond films through surface hydrogenation makes this technology highly relevant for several advanced engineering sectors.
- Advanced Gas Sensors: Hydrogen-terminated diamond surfaces exhibit negative electron affinity and high sensitivity to gas molecules, making them excellent candidates for highly stable, portable, and low-cost gas sensors.
- High-Power and High-Frequency Electronics: Diamond’s intrinsic properties (wide bandgap, high breakdown voltage, high thermal conductivity) are crucial for developing microelectronic devices capable of operating reliably at high temperatures and in chemically harsh environments.
- Charge Transfer Doping Devices: The mechanism where hydrogen acts as a surface acceptor allows for the creation of surface-channel Metal-Semiconductor Field-Effect Transistors (MESFETs) and other devices relying on controlled surface conductivity.
- Radiation-Hard Electronics: Diamond’s superior radiation hardness makes these films essential for electronics deployed in extreme environments, such as space, nuclear reactors, or high-energy physics experiments.
- Thermistors and Temperature Sensors: The strong, tunable temperature dependence of conductivity (especially the activation energy Ea) suggests potential use in specialized thermistor applications.
View Original Abstract
The polycrystalline diamonds were synthesized on n-type single crystalline Si wafer by Hot Filament CVD method. The structural properties of the obtained diamond films were checked by X-ray diffraction and Raman spectroscopy. The conductivity of n-Si/p-diamond, sandwiched between two electrodes, was measured in the temperature range of 90-300 K in a closed cycle cryostat under vacuum. In the temperature range of (200-300 K), the experimental data of the conductivity were used to obtain the activation energies Ea which comes out to be in the range of 60-228 meV. In the low temperature region i.e., below 200 K, the conductivity increases very slowly with temperature, which indicates that the conduction occurs via Mott variable range hopping in the localized states near Fermi level. The densities of localized states in diamond films were calculated using Mott’s model and were found to be in the range of 9×1013 to 5×1014eV−1cm−3 depending on the diamond’s surface hydrogenation level. The Mott’s model allowed estimating primal parameters like average hopping range and hopping energy. It has been shown that the surface hydrogenation may play a crucial role in tuning transport properties.
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
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