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	Czech Academy of Sciences, Institute of Physics, Czech Academy of Sciences, Institute of Plasma Physics
Citations	19
Analysis	Full AI Review Included

Effect of Substrate and Thickness on TiO₂ VUV Photoconductive Detectors

Executive Summary

This study reports the development and optimization of a room-temperature Vacuum Ultraviolet (VUV) photoconductive detector utilizing nanoparticle Titanium Dioxide (TiO₂) thin films.

Core Achievement: Demonstrated a cost-effective TiO₂ detector capable of sensing VUV radiation (100 nm to 200 nm) with performance metrics competitive with high-cost materials like diamond.
Optimal Configuration: The best performance was achieved using an 80 nm thick TiO₂ film deposited on a Quartz (SiO₂) substrate.
Key Performance Metric: This optimal detector exhibited an extremely high photosensitivity of 99.99% at 70 V bias voltage.
Response Speed: The fall time was measured at 5.8 µs, which is comparable to the 5.4 µs response time of a commercial diamond UV sensor used as a reference.
Thickness Trade-off: Increasing film thickness (500 nm, 1000 nm) led to improved crystallinity but simultaneously increased defect density (electron traps), resulting in a net decrease in photocurrent and overall performance.
Wavelength Tuning: The cut-off wavelength of the detector can be adjusted from 280 nm (80 nm film) up to 320 nm (1000 nm film) by controlling the film thickness.

Technical Specifications

The following specifications detail the performance of the optimized 80 nm TiO₂ thin film detector on the SiO₂ substrate, measured at 70 V bias voltage.

Parameter	Value	Unit	Context
Target Wavelength Range	100 to 200	nm	Vacuum Ultraviolet (VUV)
Optimal Film Thickness	80	nm	Deposited on SiO₂
Optimal Bias Voltage	70	V	For photosensitivity calculation
Photocurrent (I_photo)	5.35	mA	80 nm TiO₂ on SiO₂
Dark Current (I_dark)	8.83 x 10^-6	mA	80 nm TiO₂ on SiO₂
Photosensitivity (S)	99.99	%	(I_photo - I_dark) / I_dark
Photoresponsivity (R_x)	0.44	A/W	At 160 nm wavelength
Detector Fall Time (TiO₂/SiO₂)	5.8	µs	Time to decay to 50% intensity
Diamond Sensor Fall Time (Ref)	5.4	µs	Commercial benchmark
Optical Band Gap (80 nm films)	3.48	eV	Estimated via Tauc plots
Crystallite Size (80 nm TiO₂/SiO₂)	11	nm	Anatase phase, preferred (004) orientation
Cut-off Wavelength (80 nm film)	280	nm	Shifts to 320 nm for 1000 nm film

Key Methodologies

The TiO₂ thin films were fabricated using reactive direct current (DC) magnetron sputtering, followed by thermal annealing.

1. Thin Film Deposition (TiO₂)

Technique: Reactive DC Magnetron Sputtering.
Target Material: Pure Titanium (99.995% purity).
Substrates Tested: High resistivity Si (1 kΩ·cm), SiO₂ glass, and Soda Lime Glass (SLG).
Substrate Temperature: Unheated.
Base Pressure: 1 x 10^-3 Pa (prior to deposition).
Working Gas Mixture: Argon (Ar) and Oxygen (O₂).
Gas Flow Ratio (Ar:O₂): 4:1 (20 sccm Ar, 5 sccm O₂).
Total Gas Pressure: 1.3 Pa.
Power Input: 600 W (3.4 W/cm² power density).
Deposition Rate: 5 nm/min.
Thicknesses Fabricated: 80 nm, 500 nm, and 1000 nm.

2. Post-Processing and Electrode Fabrication

Annealing: Films were annealed in air for 8 hours at 450 °C to induce crystallization (anatase phase).
Electrode Material: Aluminum (Al, 99.99% purity).
Electrode Thickness: 500 nm.
Electrode Geometry: Interdigitated pattern (0.2 mm gap, 7.8 mm length).

3. Characterization Techniques

Crystallinity: Grazing Incidence X-ray Diffractometry (GIXRD) using Cu Kα radiation.
Optical Properties: Double-beam UV-visible-near infrared spectrophotometry (Transmission spectra, Tauc plots).
Defect Analysis: Time-independent and time-resolved Photoluminescence (PL) spectroscopy (Excitation at 290 nm).
Photoconductivity: Measured I-V characteristics under VUV illumination (160 nm peak wavelength lamp) and darkness using an ultra-high-resistance electrometer.
Wavelength Response: Measured using a D₂ lamp (400 nm down to 115 nm) coupled with a vacuum ultraviolet electroscope.
Time Response: Measured using a VUV flash lamp (170 nm) and oscilloscope, processed with a 2.5 MHz low-pass filter.

Commercial Applications

The development of high-speed, high-sensitivity, and cost-effective TiO₂ VUV photoconductive detectors is critical for applications requiring reliable detection of high-energy photons, particularly as an alternative to expensive diamond or complex III-V nitride sensors.

Semiconductor Manufacturing:
- Monitoring VUV sources used for optical cleaning of semiconductor substrates.
- Process control in VUV lithography and etching systems.
Sterilization and Medical Devices:
- Real-time monitoring and quality control of VUV germicidal lamps (e.g., 172 nm Xe₂ excimer lamps) used for sterilization of medical apparatus and surfaces.
Photochemical Processing:
- Detection and control of high-energy VUV light sources used in industrial photochemical reactions and material surface treatments.
Scientific Instrumentation:
- Detectors for VUV spectroscopy (e.g., molecular spectroscopy, gas chromatography).
- Plasma diagnostics in fusion research or low-temperature plasma systems where VUV emission is prevalent.
Space and Atmospheric Science:
- Sensors for detecting VUV radiation in space environments or upper atmospheric studies, benefiting from the robust chemical stability of TiO₂.

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

2014 - Vacuum ultraviolet field emission lamp utilizing KMgF3 thin film phosphor [Crossref]
2014 - Vacuum Ultraviolet Field Emission Lamp Consisting of Neodymium Ion Doped Lutetium Fluoride Thin Film as Phosphor
2014 - Perovskite fluoride crystals as light emitting materials in vacuum ultraviolet region [Crossref]
2013 - Pr or Ce-doped, fast-response and low-afterglow cross-section-enhanced scintillator with 6Li for down-scattered neutron originated from laser fusion [Crossref]
2013 - Scintillation properties of SrF2 and SrF2-Ce3+ crystals [Crossref]
2010 - Nd3+:LaF3 as a step-wise excited scintillator for femtosecond ultraviolet pulses [Crossref]
2021 - Picosecond UV emissions of hydrothermal grown Fe3+-doped ZnO microrods [Crossref]
2012 - Fabrication of In-Doped ZnO Scintillator Mounted on a Vacuum Flange [Crossref]
2020 - X-ray radiation excited ultralong (>20,000 s) intrinsic phosphorescence in aluminum nitride single-crystal scintillators [Crossref]
2020 - Vacuum-ultraviolet photodetectors [Crossref]