Vacuum-ultraviolet photodetectors
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-11-09 |
| Journal | PhotoniX |
| Authors | Lemin Jia, Wei Zheng, Feng Huang |
| Institutions | Sun Yat-sen University |
| Citations | 190 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis review focuses on the development and performance of filterless Vacuum-Ultraviolet (VUV) photodetectors utilizing Ultra-Wide Bandgap (UWB) semiconductors, highlighting their suitability for harsh environments and high-speed applications.
- Core Advantage: UWB semiconductors (Diamond, AlN, BN) offer inherent VUV selective response and superior radiation hardness compared to traditional Si-based detectors, making them ideal for space exploration and radiation monitoring.
- High-Speed Photovoltaics: Graphene/AlN/GaN heterojunction photovoltaic detectors achieved an ultra-fast rise time of 80 ns and a decay time of 0.4 ms, representing a 104-106 times improvement in speed over currently reported VUV detectors.
- Exceptional Gain: Nanostructured photoconductive detectors based on 2D h-BN and MgO demonstrated External Quantum Efficiencies (EQE) up to 2133% and 1539%, respectively, attributed to persistent photoconductivity gain via surface trap states.
- High Voltage Output: Optimized vertical heterojunction structures achieved an ultra-high open-circuit photovoltage of up to 2.45 V, enhancing signal output and EQE (up to 56.1% at ~45 ns response speed).
- Radiation Tolerance: AlN Metal-Semiconductor-Metal (MSM) detectors showed no significant performance reduction after exposure to 14.4 MeV protons (1 x 1011 p+/cm2), confirming their reliability for space applications (e.g., solar radiometers like LYRA).
- Future Direction: The ultimate goals include achieving efficient VUV dynamic imaging (demonstrated with AlN line arrays) and realizing single-photon detection using high-gain avalanche photodetectors.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| VUV Spectrum Range | 10-200 | nm | Definition of VUV |
| Diamond Bandgap (Eg) | ~5.5 | eV | Corresponds to absorption edge ~225 nm |
| AlN Bandgap (Eg) | ~6.2 | eV | Corresponds to absorption edge ~200 nm |
| h-BN Bandgap (Eg) | 5.955 | eV | 2D material |
| c-BN Bandgap (Eg) | ~6.3 | eV | Ultra-wide bandgap material |
| Max Photoconductive Responsivity (Diamond) | 21.8 | A/W | All-carbon detector, 218 nm, 50 V bias |
| Max Photoconductive EQE (h-BN) | 2133 | % | 160 nm illumination, specific bias |
| Max Photoconductive EQE (MgO) | 1539 | % | 150 nm illumination, specific bias |
| Photovoltaic Open-Circuit Voltage (Max) | 2.45 | V | Graphene heterojunction, 185 nm light |
| Photovoltaic Rise Time (Fastest) | 80 | ns | Graphene/AlN/GaN heterojunction, 193 nm pulse |
| Photovoltaic Decay Time (Fastest) | 0.4 | ms | Graphene/AlN/GaN heterojunction |
| AlN Avalanche Gain (Multiplication Factor) | 1200 | - | 200 nm, -250 V reverse bias |
| AlN Radiation Tolerance (Proton Fluence) | 1 x 1011 | p+/cm2 | 14.4 MeV proton energy |
| c-BN Rejection Ratio (180 nm/250 nm) | > 104 | - | Back-to-back MSM structure |
| AlN Rejection Ratio (VUV/UV-Visible) | > 104 | - | MSM structure |
Key Methodologies
Section titled âKey MethodologiesâThe development of high-performance UWB VUV photodetectors relies heavily on advanced material synthesis and precise device architecture engineering.
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Material Growth Techniques:
- Diamond: Chemical Vapor Deposition (CVD) and High-Pressure High-Temperature (HPHT) methods are used for growing high-quality single-crystal diamond films, often on natural or synthetic Ib substrates.
- AlN: Metal Organic Chemical Vapor Deposition (MOCVD) is used to grow high-quality epitaxial layers on sapphire or SiC substrates. Two-step Physical Vapor Transport (PVT) is used for micro/nanowire growth.
- BN (h-BN/c-BN): Preparation involves mechanical exfoliation, MOCVD, short pulse plasma beam deposition, and ion beam sputtering deposition. Heteroepitaxial growth of c-BN often uses diamond as an intermediate layer on silicon substrates.
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Device Structure Engineering:
- Photoconductive Detectors (MSM): Simple, low-cost planar structures are common. Focus areas include optimizing electrode materials (e.g., Au, In) and designing interdigitated electrode shapes/sizes to improve carrier transmission efficiency.
- Photovoltaic Detectors (Schottky/PIN):
- Single Schottky-barrier: Utilizes p-type doped diamond (boron) in both planar (interdigitated) and vertical (PIM: p-intrinsic-metal) configurations.
- Back-to-back MSM: Used for materials difficult to dope (e.g., AlN, c-BN). This structure provides high sensitivity and easy integration without requiring p-type doping.
- Heterojunctions: Graphene (Gr) is used as a transparent, conductive window layer due to its high VUV transmittance (up to 96%). Structures like p-Gr/AlN/p-GaN are used to suppress thermal diffusion and achieve zero power consumption.
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Performance Enhancement Strategies:
- Gain Mechanism: Photoconductive gain is achieved by utilizing high surface state density (e.g., in 2D h-BN) to capture photogenerated carriers, leading to persistent photoconductivity.
- Response Speed Improvement: For photovoltaic devices, external heating is applied to accelerate the annihilation of trap states, allowing carriers to be collected faster and reducing the decay time (thermally-enhanced response).
- Integration: Fabrication of line array detectors (e.g., p-Gr/AlN/p-Si arrays) on silicon wafers to demonstrate VUV imaging capability.
Commercial Applications
Section titled âCommercial ApplicationsâThe unique properties of UWB semiconductor VUV photodetectors enable critical applications across several high-tech sectors:
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Space Science and Exploration:
- Real-time monitoring of solar activity and space weather (e.g., solar storms, coronal jets).
- On-board radiometers for satellites (e.g., LYRA on PROBA2) requiring extreme radiation hardness and stability.
- Tracking star evolution and cosmic physics research.
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Radiation Monitoring and High-Energy Physics:
- Monitoring high-intensity sources such as synchrotron radiation and Free-Electron Lasers (FELs).
- Particle detection (especially for diamond-based avalanche detectors).
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Electronic Industry and Manufacturing:
- High-resolution semiconductor lithography (e.g., 193 nm, 157 nm excimer lithography, 13.5 nm Extreme Ultraviolet (EUV) lithography).
- Surface technology and process control in vacuum environments.
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Basic Science and Instrumentation:
- Analytical instruments (spectrometers, monochromators) requiring VUV selective response.
- Spectroscopy and photochemistry studies involving VUV light interaction with matter.
- Future development of ultra-fast dynamic imaging systems for celestial activities.
View Original Abstract
Abstract High-performance vacuum-ultraviolet (VUV) photodetectors are of great significance to space science, radiation monitoring, electronic industry and basic science. Due to the absolute advantages in VUV selective response and radiation resistance, ultra-wide bandgap semiconductors such as diamond, BN and AlN attract wide interest from researchers, and thus the researches on VUV photodetectors based on these emerging semiconductor materials have made considerable progress in the past 20 years. This paper takes ultra-wide bandgap semiconductor filterless VUV photodetectors with different working mechanisms as the object and gives a systematic review in the aspects of figures of merit, performance evaluation methods and research progress. These miniaturized and easily-integrated photodetectors with low power consumption are expected to achieve efficient VUV dynamic imaging and single photon detection in the future.