The n–Si/p–CVD Diamond Heterojunction

At a Glance

Metadata	Details
Publication Date	2020-08-10
Journal	Materials
Authors	Szymon Łoś, K. Paprocki, Mirosław Szybowicz, K. Fabisiak
Institutions	Poznań University of Technology, Kazimierz Wielki University in Bydgoszcz
Citations	7
Analysis	Full AI Review Included

Executive Summary

This study investigates the electrical conduction mechanisms in undoped polycrystalline n-Si/p-CVD diamond heterojunctions, focusing on the role of structural defects for potential high-performance electronic applications.

Core Mechanism Identified: Electrical conduction in the forward configuration (77-500 K) is primarily limited by hopping through defects, intensified by charge release from the depletion layer (Poole-Frenkel emission).
Novel Modeling: A new I-V-T model (Equation 1) was successfully proposed and fitted (r ~0.9998) to describe the current characteristics, allowing quantification of charge localization strength and material constants.
Low Activation Energy: The conduction mechanism exhibits an extremely low activation energy (T₀ equivalent to 7.5 meV), attributed to space charge accumulation in the depletion layer, enabling current flow at low temperatures/voltages.
Defect Correlation: Raman spectroscopy confirmed high defect concentrations (2.1-5.3 x 10¹⁸ cm^-3). Cathodoluminescence (CL) identified key defects: A-band (2.88 eV, dislocations), vacancy-related states (2.56 eV), and N-aggregates (2.05 eV in DFII sample).
Morphology Impact: The DFII sample (higher methanol concentration, mixed morphology, higher defect density) showed better current conduction, but also exhibited complex current saturation and negative magnification effects due to internal charge repulsion.

Technical Specifications

Parameter	Value	Unit	Context
Material System	n-Si/p-CVD Diamond	Heterojunction	Undoped polycrystalline diamond on n-type Si (100).
Operating Temperature Range	77.4 - 500	K	I-V-T characterization range.
Diamond Layer Thickness	3 - 5	µm	Synthesized film thickness.
DFI Defect Concentration (N_d)	2.1 x 10¹⁸	cm^-3	Derived from Raman FWHM (11.3 cm^-1).
DFII Defect Concentration (N_d)	5.3 x 10¹⁸	cm^-3	Derived from Raman FWHM (15.6 cm^-1).
Conduction Activation Energy (T₀)	7.5	meV	Derived from conductance G temperature dependence.
A-Band Emission Peak	2.88	eV	Cathodoluminescence (CL) peak, attributed to dislocations.
Vacancy-Related Emission Peak	2.56	eV	Cathodoluminescence (CL) peak.
N-Aggregates Emission Peak (DFII)	2.05	eV	Cathodoluminescence (CL) peak in DFII sample.
Model Correlation Coefficient (r)	~0.9998	N/A	Fit of the proposed I-V-T model (Equation 1).
Electron Mobility (µ)	0.21	m²/Vs	Used in calculating carrier concentration (n).

Key Methodologies

The polycrystalline diamond layers were synthesized using Hot Filament Chemical Vapor Deposition (HF CVD) and characterized structurally and electrically.

Synthesis Parameters (HF CVD)

Substrate: n-type (100) silicon wafer.
Pretreatment: Standard procedure including mechanical polishing with diamond paste to enhance nucleation density.
Working Gas Composition: Methanol vapor diluted in hydrogen (CH₃OH/H₂).
- DFI Sample: 1.0 vol.% CH₃OH/H₂ (Resulted in (111) orientation).
- DFII Sample: 4.0 vol.% CH₃OH/H₂ (Resulted in mixed morphology).
Process Conditions: Total pressure 60 mbar; Substrate temperature 1000 K; Gas flow rate 100 sccm.

Characterization Techniques

Structural and Morphological:
- Scanning Electron Microscopy (SEM): Used to observe surface morphology and crystallite orientation.
- Raman Spectroscopy: Recorded at room temperature (488 nm Ar ion laser) to determine structural quality, G-band/D-band ratio, and defect concentration (via FWHM analysis).
Defect Analysis:
- Cathodoluminescence (CL) Spectroscopy: Performed using a modified FE-SEM apparatus (30 kV electron beam) to identify radiative recombination centers (defects) within the diamond bandgap.
Electrical Measurement (I-V-T):
- Setup: Measured current (Keithley 6485 picoammeter) and potential drop (Fluke 8505A DMM) while applying a rectangular voltage wave (4-20 V peak-to-peak, 0.1 Hz frequency).
- Temperature Control: Stabilized using a Mercury controller in an Optistat cryostat (77.4 K to 500 K).
- Contacting: Sample surfaces were metalized with gold electrodes to ensure proper electrical contact.

Commercial Applications

The unique properties of CVD diamond heterojunctions, particularly their stability and wide bandgap, make them suitable for demanding engineering applications.

High-Temperature Electronics: The material’s stability and the demonstrated conduction mechanism (effective down to 77 K and up to 500 K) support the development of electronic devices (diodes, transistors) for extreme thermal environments (e.g., automotive, aerospace, geothermal drilling).
Radiation Detection and Hardness: Diamond’s inherent radiation hardness makes these heterojunctions ideal for sensors and electronics operating in high-radiation fields (e.g., nuclear reactors, particle accelerators, space exploration).
Chemical and Harsh Environment Sensing: The chemical inertness of diamond allows for the creation of robust sensing devices (e.g., chemical sensors, pressure sensors) that maintain performance in corrosive or chemically harsh media.
High-Power RF and Switching Devices: Diamond’s high breakdown voltage and high thermal conductivity are essential for next-generation high-power radio frequency (RF) and switching components where efficient heat dissipation is critical.
Defect-Engineered Devices: The ability to correlate specific synthesis parameters (e.g., methanol ratio) with defect types (N-aggregates, A-band) and electrical response enables precise defect engineering for specialized electronic functions, such as optimized charge storage or emission characteristics.

View Original Abstract

Due to the possible applications, materials with a wide energy gap are becoming objects of interest for researchers and engineers. In this context, the polycrystalline diamond layers grown by CVD methods on silicon substrates seem to be a promising material for engineering sensing devices. The proper tuning of the deposition parameters allows us to develop the diamond layers with varying crystallinity and defect structure, as was shown by SEM and Raman spectroscopy investigations. The cathodoluminescence (CL) spectroscopy revealed defects located just in the middle of the energy gap of diamonds. The current-voltage-temperature, I−V−T characteristics performed in a broad temperature range of 77-500 K yielded useful information about the electrical conduction in this interesting material. The recorded I−V−T in the forward configuration of the n-Si/p-CVD diamond heterojunction indicated hopping trough defects as the primary mechanism limiting conduction properties. The Ohmic character of the carriers flux permitting throughout heterojunction is intensified by charges released from the depletion layer. The magnification amplitude depends on both the defect density and the probability that biasing voltage is higher than the potential barrier binding the charge. In the present work, a simple model is proposed that describes I−V−T characteristics in a wide range of voltage, even where the current saturation effect occurs.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma13163530

References

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