Germanium Quantum Dot Photodetectors

Advancing High-Efficiency, Broadband, and High-Speed Optoelectronics

Germanium quantum dot (Ge QD) photodetectors are a class of optoelectronic devices that exploit quantum confinement effects in Ge QDs to enhance photodetection across a broad spectral range, from visible to near-infrared (NIR) wavelengths. These photodetectors offer significant advantages over bulk Ge and conventional Si-based detectors, including:

Record-High Internal Quantum Efficiency (IQE) – Many of these devices exhibit IQEs well above 100%, sometimes exceeding 2000%, due to charge trapping and photoconductive gain mechanisms.
Broadband Visible-to-Telecom Wavelength Response – The detection range extends from visible wavelengths (~400 nm) to telecom wavelengths (~1550 nm), making them suitable for optical communication, imaging, and sensing applications.
Low Noise and High Signal-to-Noise Ratio (SNR) – Various studies have reported suppressed dark currents and enhanced SNR, making them competitive for low-light applications.
Fast Response Times – The optimization of QD size, active layer thickness, and operating temperature has resulted in response times as low as 10-20 ns, overcoming traditional efficiency-speed trade-offs.
CMOS Compatibility – Ge QDs are compatible with standard silicon fabrication processes, making them viable for large-scale integration in optoelectronic circuits.

This work results from a long standing collaboration with Prof. Alexander Zaslavsky‘s group.

Below are selected papers on high-efficiency, high responsivity, CMOS-compatible germanium quantum dot photodetectors.

Applied Physics Letters, 2021 – Fast and Efficient Germanium Quantum Dot Photodetector with an Ultrathin Active Layer

This work presents a high-performance germanium quantum dot (Ge QD) photodetector with an ultrathin (~13 nm) active layer, achieving internal quantum efficiency (IQE) exceeding 105% for both visible (640 nm) and telecom (1550 nm) wavelengths at low-light conditions (<30 nW). The device exhibits fast response times (<15 ns for both rise and fall times), making it one of the fastest Ge quantum dot-based photodetectors reported to date. By leveraging charge trapping and phonon-assisted tunneling mechanisms, this work overcomes the traditional trade-off between efficiency and speed, offering a practical, CMOS-compatible solution for broadband, high-speed photodetection.

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Applied Physics Letters, 2020 – High-Performance Germanium Quantum Dot Photodetectors: Response to Continuous Wave and Pulsed Excitation

This study demonstrates a highly efficient Ge QD photodetector with IQE up to 2000% across the visible-to-near-infrared (500–800 nm) range. The device exhibits a fast transient response (~20 ns) to pulsed excitation, while a unique optically tunable photoresponse is achieved by superimposing a weak continuous-wave (CW) laser onto a pulsed laser signal. The results provide critical insights into photoexcited carrier generation, charge trapping, and hopping transport mechanisms within the Ge QD matrix, paving the way for high-gain, low-power photodetection.

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Applied Physics Letters, 2018 – Broadband Visible-to-Telecom Wavelength Germanium Quantum Dot Photodetectors

This work extends the spectral range of Ge QD photodetectors to 1550 nm (telecom wavelength) for the first time, demonstrating room-temperature responsivities up to 1.12 A/W and IQE exceeding 1000% at 100 K. By fabricating the photodetectors on germanium substrates, the device achieves a broadband response (400–1550 nm) that surpasses conventional silicon and germanium photodiodes. Noise analysis further confirms a specific detectivity D* of ~10¹² cm Hz^½ W^-1, making these devices ideal candidates for optical communication, imaging, and integrated photonics.

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physics status solidi a, 2017 – Low-Temperature Operation of High-Efficiency Germanium Quantum Dot Photodetectors

This paper investigates the temperature-dependent performance of Ge QD photodetectors, revealing IQE exceeding 22,000% at 100 K due to the saturation of charge trapping mechanisms. Lowering the temperature from 300 K to 100 K enhances the specific detectivity (D*) from 1.2 × 10¹¹ to 2 × 10¹³ cm Hz^½ W^-1, improving signal-to-noise ratio (SNR) by an order of magnitude. These findings demonstrate the potential for low-temperature, ultra-sensitive photodetection for quantum technologies, space applications, and cryogenic imaging.

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Applied Physics Letters, 2016 – Noise Performance of High-Efficiency Germanium Quantum Dot Photodetectors

This study provides a comprehensive noise analysis of Ge QD photodetectors, showing a specific detectivity (D*) of 6 × 10¹² cm Hz^½ W^-1 and a high signal-to-noise ratio (SNR ~ 10⁵). By optimizing the active area using photolithography, the devices achieve suppressed dark current, leading to significantly improved detectivity and lower power requirements. This work establishes Ge QDs as a promising alternative to traditional photodetector materials, particularly for low-light, high-sensitivity applications.

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Journal of Applied Physics, 2012 – Transient Photoresponse and Incident Power Dependence of High-Efficiency Germanium Quantum Dot Photodetectors

This study systematically investigates the time-resolved photoresponse of Ge QD photodetectors, revealing turn-off response times as short as 40 ns. A detailed analysis of photoconduction regimes identifies a transition from trap-limited to hopping-limited conduction around -3V bias. The observed transient current overshoot phenomenon provides key insights into carrier injection and charge transport mechanisms in Ge QD devices. These results form the foundation for subsequent advances in high-speed, high-gain photodetection.

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Applied Physics Letters, 2011 – High-Efficiency Silicon-Compatible Photodetectors Based on Ge Quantum Dots

This pioneering work presents the first demonstration of high-responsivity (4 A/W) Ge QD photodetectors, achieving IQE up to 700% in the 500–900 nm range. The study provides experimental validation of quantum-dot-mediated carrier transport, showing that amorphous Ge QDs embedded in a SiO₂ matrix enhance photocurrent generation. This work laid the groundwork for the development of high-efficiency, silicon-compatible Ge QD photodetectors, proving their feasibility for integrated optoelectronics.

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Main Findings & Impact

Our research on germanium quantum dot (Ge QD) photodetectors has led to significant advancements in high-efficiency, broadband, and high-speed photodetection, surpassing conventional silicon and germanium photodiodes. By embedding Ge QDs in a silicon-compatible dielectric matrix, we have demonstrated devices with internal quantum efficiencies (IQE) exceeding 2000%, rapid response times below 15 ns, and a spectral range spanning visible to near-infrared (NIR) wavelengths, including telecom bands. These breakthroughs mark a major step forward in integrated optoelectronics, quantum-enhanced photodetection, and CMOS-compatible photonics.

A key innovation lies in charge trapping and percolation mechanisms within the Ge QD matrix, which enhance carrier collection efficiency and induce high photoconductive gain. By optimizing the active layer thickness to 13 nm, we have resolved the traditional efficiency-speed trade-off, maintaining high responsivity while significantly improving detection speed. Our findings establish Ge QDs as a viable alternative to III-V and 2D material-based detectors, offering low-cost processing, tunable bandgap properties, and scalable fabrication.

Beyond efficiency and speed, our work explores temperature-dependent effects, showing that cooling to 100 K dramatically enhances performance. At cryogenic temperatures, our devices achieve IQE values exceeding 22,000% and a specific detectivity D* of 2 × 10¹³ cm Hz^½ W^⁻¹, an order of magnitude improvement over room-temperature operation. This enhancement, driven by charge trapping saturation and prolonged carrier lifetimes, makes Ge QD detectors highly promising for quantum sensing, infrared imaging, and low-noise photodetection.

Our approach also prioritizes fabrication simplicity and integration potential. Unlike III-V and 2D-based detectors requiring complex epitaxial growth, our Ge QD photodetectors are fully CMOS-compatible, leveraging low-temperature sputtering and annealing techniques. Their fabrication on both silicon and germanium substrates allows for scalable manufacturing, monolithic photonic integration, and hybrid optoelectronic circuit design, making them ideal for optical interconnects, high-speed imaging, and biomedical sensing.

Future research will focus on extending the spectral response into the mid-infrared (MIR) range (2–5 µm) by integrating GeSn QDs and engineered bandgaps. Additionally, we aim to further improve speed by incorporating plasmonic nanostructures for sub-5 ns response times and developing waveguide-coupled Ge QD photodetectors to optimize their role in silicon photonics and optical computing. Our work not only redefines the potential of Ge-based photodetection but also establishes a foundation for future advances in nanophotonics, quantum optoelectronics, and next-generation high-speed detection systems.

Summary & Future Outlook

Our research on germanium quantum dot (Ge QD) photodetectors has pioneered the development of high-efficiency, broadband, and fast-response photodetection technologies, addressing fundamental challenges in optoelectronics. By leveraging the unique properties of Ge QDs embedded in dielectric matrices, we have demonstrated devices that outperform conventional silicon and germanium photodiodes, offering exceptional internal quantum efficiency (IQE), fast transient response, and extended spectral coverage from the visible (400 nm) to telecom (1550 nm) range.

Through a decade-long evolution, our work has redefined the limits of germanium-based photodetection, achieving unparalleled efficiency, speed, and spectral range. Our advances in quantum dot-mediated charge transport, high-gain photoconduction, and scalable fabrication pave the way for next-generation optoelectronic devices. As we move forward, our research will continue to expand the capabilities of quantum dot photodetectors, enabling transformative applications in optical computing, imaging, and quantum technologies.