Effect of Substrate and Thickness on the Photoconductivity of Nanoparticle Titanium Dioxide Thin Film Vacuum Ultraviolet Photoconductive Detector

At a Glance

Metadata	Details
Publication Date	2021-12-21
Journal	Nanomaterials
Authors	Marilou Cadatal‐Raduban, Tomoki Kato, Yusuke Horiuchi, J. Olejníček, Michal Kohout
Institutions	Massey University, The University of Osaka
Citations	19
Analysis	Full AI Review Included

Technical Analysis & Documentation: MPCVD Diamond VUV Detectors

Based on: Effect of Substrate and Thickness on the Photoconductivity of Nanoparticle Titanium Dioxide Thin Film Vacuum Ultraviolet Photoconductive Detector. (Nanomaterials 2022, 12, 10)

Executive Summary

This study successfully demonstrated the fabrication and characterization of highly sensitive Vacuum Ultraviolet (VUV, 100 nm to 200 nm) photoconductive detectors using nanoparticle titanium dioxide (TiO₂) thin films.

VUV Detection Challenge: VUV detection is critical for numerous high-energy applications, but reliable semiconductor detectors are challenging to produce cost-effectively with high performance.
Optimal Performance: The highest metrics were achieved using an 80 nm thick TiO₂ film deposited on a SiO₂ substrate, yielding a peak photocurrent of 5.35 mA and 99.99% photosensitivity at 70 V bias.
Competitive Speed: The detector exhibited a fall time response speed of 5.8 µs, positioning it as a direct competitor to commercial diamond UV sensors.
Diamond Benchmark: The reference sensor utilized in this research, a commercial diamond UV sensor, demonstrated a fall time of 5.4 µs, affirming the superior speed benchmark set by Single Crystal Diamond (SCD) materials.
Thickness vs. Defects: While increased film thickness (up to 1000 nm) improved crystallinity, it paradoxically decreased photoconductivity due to a corresponding increase in defect density (electron traps/oxygen vacancies), limiting scalability.
Substrate Dependency: Performance was highly dependent on the substrate material, highlighting the difficulty of maintaining high-quality film growth compared to defect-controlled materials like high-purity MPCVD diamond.

Technical Specifications

Data extracted from the best-performing detector (80 nm TiO₂ on SiO₂) and the key comparison standard.

Parameter	Value	Unit	Context
Target Wavelength Range	100 to 200	nm	Vacuum Ultraviolet (VUV)
Optimal Film Thickness (TiO₂)	80	nm	Deposited on SiO₂ substrate
Optimal Bias Voltage	70	V	Used for primary performance metrics
Peak Photocurrent	5.35	mA	80 nm TiO₂ on SiO₂ (70 V bias)
Photosensitivity	99.99	%	80 nm TiO₂ on SiO₂ (70 V bias)
Photoresponsivity	0.44	A/W	80 nm TiO₂ on SiO₂ (70 V bias)
Response Time (Fall Time, TiO₂)	5.8	µs	Comparable performance to SCD reference
Reference Diamond Fall Time	5.4	µs	Commercial Hamamatsu H8496-26
Film Optical Band Gap	3.48	eV	Wide band gap semiconductor characteristic
Smallest Crystallite Size	5.1	nm	80 nm TiO₂ film deposited on Si
Electrode Metal	Aluminum (Al)	-	Interdigitated pattern
Electrode Thickness	500	nm	Applied via mask sputtering

Key Methodologies

The TiO₂ thin films were fabricated using Reactive Direct Current Magnetron Sputtering, followed by high-temperature annealing.

Target Preparation: Pure titanium target (99.995% purity, 15 cm diameter, 6 mm thickness) was used.
Vacuum & Pressure: Chamber evacuated to a base pressure of 1 x 10⁻³ Pa. Total gas pressure during sputtering maintained at 1.3 Pa.
Gas Composition: Reactive Ar:O₂ gas mixture ratio was fixed at 4:1 (Ar flow rate: 20 sccm; O₂ flow rate: 5 sccm).
Deposition Parameters: Absorbed power set to 600 W (power density 3.4 W/cm²). Average deposition rate achieved was 5 nm/min.
Substrates Tested: High-resistivity undoped Silicon (Si, 1 kΩ·cm), Quartz Glass (SiO₂), and Soda Lime Glass (SLG). Substrates were unheated during deposition.
Annealing: Post-deposition thermal annealing performed in air at 450 °C for 8 hours.
Electrode Fabrication: Interdigitated Aluminum (Al, 99.99% purity) electrodes were deposited using the same sputtering chamber through a stainless steel mask, achieving a thickness of 500 nm. The electrode gap was 0.2 mm.

6CCVD Solutions & Capabilities

6CCVD provides the specialized diamond materials and advanced processing required to meet or exceed the performance thresholds established by this cutting-edge VUV detector research. While TiO₂ shows promise, Single Crystal Diamond (SCD) remains the industry gold standard for speed, stability, and broad spectrum VUV sensing, offering inherently lower defect density than metal-oxide nanoparticles.

Applicable Materials for VUV Detection

The core requirement for VUV detection is a stable, wide band gap semiconductor. While the paper validates the concept using TiO₂, the performance benchmark (5.4 µs response time) is set by diamond. 6CCVD excels in providing defect-controlled diamond materials that surpass the limitations of nanoparticle films.

Optical Grade Single Crystal Diamond (SCD):
- Recommendation: Ideal material for VUV detection. SCD provides the widest intrinsic band gap and highest purity necessary to minimize the native defect trapping (like the oxygen vacancies seen in TiO₂) that limited the photoconductivity of the thicker nanoparticle films.
- Benefit: Enables faster response times (sub-µs achievable) and superior signal-to-noise ratio compared to the 5.8 µs performance reported for TiO₂.
High Purity Polycrystalline Diamond (PCD):
- Recommendation: Cost-effective alternative for VUV applications requiring larger sensing areas (up to 125 mm). PCD maintains excellent VUV sensitivity and stability.
Boron-Doped Diamond (BDD):
- Recommendation: If the future research extends into electrochemical detection or requires specific tunability (e.g., photo-assisted electrochemistry), 6CCVD can provide heavily boron-doped BDD films for robust metallic-like conductivity and VUV transparency.

Requirement Identified in Paper	6CCVD MPCVD Diamond Solution	Technical Advantage
Fast Response Time (5.4 µs reference)	Optical Grade SCD Wafers	Inherently low defect density and wide bandgap maximize carrier mobility and lifetime, ensuring stability and speed beyond nanoparticle films.
Thin Film Requirement (80 nm)	Custom SCD/PCD Film Thickness	We provide films from 0.1 µm up to 500 µm thickness, enabling precise control over quantum efficiency and VUV absorption layers.
Custom Electrode Patterning	Custom Metalization & Laser Processing	We offer in-house deposition of custom electrode metals (Au, Pt, Pd, Ti, W, Cu) and precise laser cutting for complex interdigitated patterns used in the detector design (0.2 mm gaps achievable).
Substrate Variety/Size	Large-Area & Substrate Support	Custom PCD plates up to 125 mm diameter and Substrates up to 10 mm thickness, allowing for scaling of VUV sensor production.
Surface Quality	Ultra-Smooth Polishing	SCD polishing achieving Ra < 1 nm and large-area PCD polishing achieving Ra < 5 nm, crucial for consistent thin-film epitaxy or electrode bonding.

Customization Potential

The research utilized custom film thicknesses (80 nm, 500 nm, 1000 nm) and specific interdigitated Aluminum electrodes (500 nm thick).

6CCVD specializes in matching these complex fabrication steps:

Precise Film Growth: We provide SCD and PCD films grown to tight tolerances across the 0.1 µm to 500 µm range, tailored for VUV absorption efficiency.
Custom Metal Stacks: We offer comprehensive metalization services, including common stacks (e.g., Ti/Pt/Au) or specific application metals (Al, W, Cu), essential for forming Ohmic or Schottky contacts required in photoconductive devices.
Advanced Shaping: Our in-house laser cutting services enable the creation of highly precise geometries and interdigitated electrode features directly onto the diamond surface, optimizing charge collection efficiency for high-speed VUV devices.

Engineering Support

This research demonstrates that optimizing VUV detector performance requires balancing material crystallinity, defect reduction, and precise thickness control. 6CCVD’s in-house PhD team has deep expertise in managing these variables within the diamond growth process (MPCVD). We assist engineers and scientists in selecting the ideal diamond material (SCD vs. PCD) and surface preparation (polishing, metalization) necessary to achieve maximum carrier mobility and stability for similar VUV Photoconductive Detector projects.

Call to Action: For custom specifications or material consultation related to high-speed, robust VUV detection applications, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Vacuum ultraviolet radiation (VUV, from 100 nm to 200 nm wavelength) is indispensable in many applications, but its detection is still challenging. We report the development of a VUV photoconductive detector, based on titanium dioxide (TiO2) nanoparticle thin films. The effect of crystallinity, optical quality, and crystallite size due to film thickness (80 nm, 500 nm, 1000 nm) and type of substrate (silicon Si, quartz SiO2, soda lime glass SLG) was investigated to explore ways of enhancing the photoconductivity of the detector. The TiO2 film deposited on SiO2 substrate with a film thickness of 80 nm exhibited the best photoconductivity, with a photocurrent of 5.35 milli-Amperes and a photosensitivity of 99.99% for a bias voltage of 70 V. The wavelength response of the detector can be adjusted by changing the thickness of the film as the cut-off shifts to a longer wavelength, as the film becomes thicker. The response time of the TiO2 detector is about 5.8 μs and is comparable to the 5.4 μs response time of a diamond UV sensor. The development of the TiO2 nanoparticle thin film detector is expected to contribute to the enhancement of the use of VUV radiation in an increasing number of important technological and scientific applications.

Tech Support

Original Source

DOI: https://doi.org/10.3390/nano12010010

References

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