Chongfan Technology
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13
2025
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01
High responsiveness, CMOS-compatible near-infrared germanium PIN photodiode
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Although high-performance near-infrared (NIR) detection is crucial for many applications, the current state-of-the-art NIR detectors either have limitations in detecting incident photons (i.e., poor spectral responsivity) or are made from expensive III-V materials that are incompatible with CMOS. Germanium (Ge) is expected to offer high performance, low cost, and scalability simultaneously. More importantly, Ge material is fully compatible with CMOS processes and can, in principle, be processed using existing CMOS chip production lines. In fact, Ge-based photodiodes have already achieved commercial applications in many fields.
According to MEMS Consulting, a research team composed of Aalto University in Finland, ElFys in Finland, and the Physikalisch-Technische Bundesanstalt in Germany has recently developed a CMOS-compatible Ge-based nano-engineered PIN photodiode. This device achieves over 90% external quantum efficiency (EQE) across a wide spectral range (1200 - 1600 nm) at room temperature with zero bias voltage. For example, the responsivity at a wavelength of 1550 nm can reach 1.15 A/W. In addition to having excellent spectral responsivity in the near-infrared band, the device also maintains outstanding performance in the visible and ultraviolet bands (with EQE exceeding 100% below 300 nm), thus achieving an ultra-wide spectral response range. The researchers minimized optical losses using surface nanostructures while employing conformal atomic layer deposition (ALD) of aluminum oxide (Al₂O₃) for surface passivation and replacing traditional PN junctions with dielectric-induced electric fields for carrier collection to minimize electrical losses, ultimately achieving high performance for the device. This research result was published under the title “Near-infrared Germanium PIN-photodiodes with >1A/W responsivity” inLight: Science & Applicationsjournal.
Figure 1 shows a schematic cross-section of the nano-structured Ge photodiode, which combines the nano-structured Ge surface, atomic layer deposited Al₂O₃ surface passivation, and a negatively charged Al₂O₃ induced electric field for carrier separation and hole collection. Unlike the traditional method of forming a PN junction through external doping, this research applies a dielectric layer with fixed charge on top of the Ge nanostructure, thereby avoiding the recombination associated with doping. If the fixed charge has the opposite polarity to the substrate doping, the carriers in the surface layer will begin to deplete, leaving only ionized bulk dopant atoms. This process leads to a change in charge polarity at the dielectric/semiconductor interface, generating a strong electric field at the Ge surface.
As shown in Figure 2, the researchers characterized the ability of the Ge photodiode to convert photon energy into current as a function of wavelength, i.e., spectral responsivity. Experimental results show that this photodiode can detect over 80% of incident photons (EQE>80%) across an ultra-wide spectral range from the ultraviolet band (UV, 200 nm) to the near-infrared band (1700 nm), and even in the ultraviolet band, the EQE of this photodiode exceeds 100%.
In this study, the researchers compared the performance of three types of indium gallium arsenide (InGaAs) photodiodes optimized for different wavelengths (1300 nm, 1550 nm, and 1750 nm), one other high-performance Ge photodiode from another study, and the nano-structured Ge photodiode from this study (as shown in Figure 2). The results showed: (1) Regarding spectral responsivity, the spectral responsivity of the nano-structured Ge photodiode is significantly higher than that of other InGaAs photodiodes and Ge photodiodes. (2) Regarding dark current, the dark current of the nano-structured Ge photodiode is significantly lower than that of other Ge photodiodes; however, due to the higher intrinsic carrier concentration of Ge material, the dark current of the nano-structured Ge photodiode is not comparable to that of InGaAs photodiodes at room temperature. (3) Regarding specific detectivity (D*), the specific detectivity of the nano-structured Ge photodiode at the peak response wavelength (1650 nm) reaches 1.77 × 10¹¹ Jones, significantly better than other Ge photodiodes; however, due to the significant difference in dark current, the specific detectivity of the nano-structured Ge photodiode cannot surpass that of InGaAs devices.
To further investigate the fundamental reasons for the high performance of the nano-structured Ge photodiode, the researchers characterized the spectral reflectivity and spatial reflectivity distribution of the nano-structured photodiode. Figure 3a shows that the reflectivity of the nano-structured surface is very low (<1%) from the ultraviolet to the near-infrared band. Figure 3b shows the spatial reflectivity map, confirming that the nano-structured surface maintains a low reflectivity throughout the active region of the diode. By measuring the spectral responsivity and reflectivity, the internal quantum efficiency (IQE) describing internal recombination losses can be directly determined.
To further clarify the IQE behavior, the researchers simulated the electric field distribution in the nanostructure (as shown in Figure 4a) and the carrier concentration within the nanostructure (as shown in Figure 4c).
In summary, this study developed a novel Ge PIN photodiode for near-infrared detection without the need to form a PN junction and anti-reflective coating. By fabricating prototype devices, the researchers demonstrated the potential of the new Ge PIN photodiode, which achieved record high spectral responsivity, high EQE (ranging from 83% to 125%), and low dark current (reaching 76 μA/cm² at 5 V) across an ultra-wide spectral range (200 nm to 1700 nm). The high performance of this device is primarily attributed to the following aspects: (1) the surface nanostructures effectively reduce reflection losses; (2) the dielectric-induced electric field concentrated near the surface significantly improves internal collection efficiency; (3) high-quality surface passivation is achieved through conformal atomic layer deposited Al₂O₃ coating. The novel Ge photodiode design proposed in this study can be directly used for the fabrication of state-of-the-art Ge photodiodes, providing new solutions for higher performance near-infrared applications.
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