Research Advances of High Speed Photodetectors(Invited)
Significance With the exponential growth in data transmission,optical interconnection technology has replaced traditional electrical interconnection technology and become the mainstream for low-loss and high-speed transmission over medium and long distances.By utilizing photons for data transfer,optical interconnections offer significant advantages such as large bandwidth and low latency,which are crucial in meeting the demands of modern communication systems.The photodetector,which converts optical signals into electrical signals,is the core component of optical interconnection systems.Rapid advancements in telecommunications and data processing necessitate photodetectors with exceptional speed and efficiency.Among photodetectors,waveguide-coupled designs have garnered significant attention due to their compact size,high bandwidth,and easy integration with other optoelectronic devices.These attributes are critical for the development of integrated photonic circuits,which are essential for applications in data centers,telecommunications,and emerging fields such as quantum computing.Traditional high-performance InGaAs photodetectors have long been the standard in near-infrared communications.Meanwhile,Ge/Si photodetectors are rapidly advancing due to silicon photonics,which enables large-scale,low-cost production compatible with existing semiconductor manufacturing processes.The need for higher data rates and lower power consumption drives the transition from electrical to optical interconnections in communication networks.Photodetectors are crucial in this transition because they directly impact the overall performance of optical communication systems.The integration of photodetectors with waveguides not only enhances the bandwidth but also allows for the development of more compact and efficient photonic devices.This integration is essential for the continued advancement of optical interconnection technologies,expected to play a dominant role in future communication infrastructures.Progress We first introduce the mainstream choices for high-speed photodetectors,primarily including InGaAs photodetectors and Ge/Si photodetectors.Then we discuss the typical structures and research advancements of these two major types of photodetectors.The preferred structure for InGaAs photodetectors is the uni-traveling-carrier(UTC)structure,addressing the slow hole transport issue in Ⅲ-Ⅴ materials compared to the PIN structure.For example,the research group led by Seeds A J at University College London reported a high-speed evanescently coupled photodetector with a 3 dB bandwidth of 110 GHz.Li et al.from the University of Virginia reported a high-speed waveguide-coupled photodetector with a bandwidth exceeding 105 GHz,fabricated using MOCVD on an InP substrate(Fig.1).Additionally,heterogeneous integrations of Ⅲ-Ⅴ photodetectors on silicon,primarily through direct epitaxy and wafer bonding,have received widespread attention.Sun et al.fabricated an improved single-carrier(MUTC)Ⅲ-Ⅴ photodetector on a Si substrate,achieving a responsivity of 0.78 A/W and a 3 dB bandwidth of 28 GHz under a reverse bias voltage of 3 V(Fig.2).For silicon-based group Ⅳ photodetectors,they mainly include all-silicon photodetectors and germanium-silicon photodetectors.Researchers have achieved all-silicon photodetectors by introducing new absorption mechanisms or device structures.Intel reported a waveguide-coupled all-Si photodetector with a 3 dB bandwidth of 15 GHz(Fig.3),achieving a responsivity of 1.6 A/W at a reverse bias voltage of 5.93 V with an active region length of 300 μm.Germanium,similar to silicon,is highly compatible with Si CMOS processes.With advancements in silicon photonics technology,waveguide-coupled Ge/Si photodetectors have become the most widely researched Si-based group Ⅳ photodetectors due to their excellent high-speed performance and mature process flow.Lischke S et al.proposed a lateral Ge/Si photodetector with a 3 dB bandwidth up to 265 GHz by reducing the width of the fin-shaped Ge absorption region(Fig.6),marking the highest bandwidth achieved for Ge/Si photodetectors to date.The width of the fin-shaped Ge absorption region is only 100 nm,effectively reducing carrier transit time and enhancing the high-speed performance of the Ge photodetector.Conclusions and Prospects Photodetectors,as the core components of optical receivers,have gradually matured over the years.Our study introduces the current development of two types of waveguide-coupled photodetectors,includingⅢ-Ⅴ and Ⅳ group photodetectors,each with its advantages and characteristics.Benefiting from Ⅲ-Ⅴ materials'superior light absorption and high electron mobility,InP-based InGaAs photodetectors can achieve ultra-high-speed light detection based on the UTC structure,becoming the most mature photodetectors.On the other hand,Ⅳ group photodetectors,primarily Ge/Si photodetectors,are compatible with Si CMOS processes and possess inherent advantages in integration.With advanced processing technologies on silicon photonics platforms,higher processing accuracy and integration can be achieved.These features enable the fabrication of finer device structures and make ultra-high-speed Ge/Si photodetectors comparable to InP-based high-speed photodetectors.The future development of high-speed photodetectors still faces several challenges.Although Ge/Si and InGaAs photodetectors with a 3 dB bandwidth exceeding 200 GHz have been reported,their responsivity is not satisfactory.Balancing bandwidth improvement while maintaining good responsivity is an important research topic that may require the introduction of process-compatible new materials(high mobility and high absorption coefficient)or novel high-speed device structures.Furthermore,as devices'bandwidth increases,the application scenarios of detectors in microwave photonics links will gradually expand.Unlike the low optical power input and high sensitivity detection in optical communications,this scenario requires high saturation power due to high-power input based on large bandwidth.However,large-bandwidth devices typically employ small-sized structures,making it challenging to produce photodetectors that are both high-speed and high-power-saturation.Therefore,further innovation in carrier extraction and heat dissipation of the devices is necessary.
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