High-efficiency germanium-tin photodetectors for two micrometer applications

• Main Author: Zhou, Hao
• Other Authors: Tan Chuan Seng
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/155164
• Institution: Nanyang Technological University

The exponentially increasing capacity demand due to the emerging applications of cloud computing, 5G, and big data brings grand challenges for conventional single mode fibers. Owing to the development of low-loss hollow-core photonic bandgap fibers and thulium-doped fiber amplifiers, a novel communication band at 2 µm has been proposed as a promising solution for the increasing capacity boost. To continuously enable the practical applications at 2 µm band, photodetectors operating at such wavelength are pivotal. Si-based GeSn photodetector is a promising candidate for optical transceiver operating at 2 μm due to its complementary metal-oxide-semiconductor (CMOS) compatibility. The group-IV material GeSn has a tunable bandgap which depends on the composition of Sn. The incorporation of Sn not only extends the cutoff wavelength of GeSn photodetector to 3.5 µm, but also enhances the absorption at the existing telecommunication band. This thesis is devoted to the high-efficiency GeSn photodetectors for 2 μm applications. Firstly, the GeSn/Ge multiple-quantum-well (MQW) p-i-n photodiode structure was fabricated for high-detectivity photo detection and effective optical modulation simultaneously. The MQW architecture was designed to reduce the threading dislocation density (TDD) resulted from the lattice mismatch. Due to the low TDD in the pseudomorphic GeSn layers, an ultralow dark current density of 16.3 mA/cm2 at -1 V was achieved. Taking both the low dark current density and high responsivity of 0.307 A/W into consideration, a high specific detectivity of 1.37×1010 cm∙Hz1/2/W was accomplished at 1,550 nm. The achieved detectivity is comparable with commercial Ge photodetectors. The responsivity at 2 µm is 0.0072 A/W which is relatively low since the quantum confinement effect lifts the bandgap in the GeSn layer. Meanwhile, the bias voltage-dependent photo response was investigated from 1,700 to 2,200 nm. The extracted effective absorption coefficient of GeSn/Ge MQW shows the quantum confined Stark effect (QCSE) behavior with electric field-dependent exciton peaks from 0.688 to 0.690 eV. The peak position shifts to lower photon energy with increasing reverse bias voltage, which is one of the characteristics of QCSE. The absorption ratio of 1.81 under -2 V shows early promise for effective optical modulation at 2 µm. In order to enhance the optical response at 2 µm, the photon-trapping structures were incorporated into GeSn-based photodetectors for the first time. The demonstration was realized by a GeSn/Ge MQW p-i-n photodiode on a GeOI architecture. Different from the MQW structure in previous work, the GeSn/Ge MQW structure was optimized with a theoretical direct bandgap of 0.61 eV. A five-fold enhancement in responsivity of 0.11 A/W was achieved for the photon-trapping photodetectors at 2 μm. The photon-trapping structures change the propagation path of normal incident light and guide the lateral propagation in the lateral dimension. Propagation of photons in the larger lateral dimension elongates the effective absorption length, benefiting the enhanced optical absorption. Although the incorporation of photo-trapping microstructure degrades the dark current density, which increases from 31.5 to 45.2 mA/cm2 at -1 V, it benefits an improved 3-dB bandwidth of 2.7 GHz at a bias voltage of -5 V. Resonance-cavity-enhanced GeSn/Ge MQW photodetectors were also designed and fabricated for high-performance photo detection at 2 µm. The resonant cavity sandwiched by metasurface structure and SiO2 layer benefits the multi-pass of incident light in the vertical direction. The corresponding resonance mode profile at 2 μm was simulated using finite-difference time-domain (FDTD) method. The metasurface structure is beneficial for enhanced optical absorption in two parts. On the one hand, it reduces the reflection since the effective refractive index for metasurface structure decreases compared with the plain SiO2 passivation layer. On the other hand, the metasurface structure can be regarded as a two-dimensional grating coupler for the reflected light from the underlying interface. The incorporation of metasurface structure benefits a 10.5 times responsivity of 0.232 A/W at 2 µm. The achieved detectivity at room temperature is 5.34×109 cm∙Hz1/2/W which is the highest in group IV photodetectors operating at 2 µm. Au hole-array and Au-GeSn grating structures were also proposed and incorporated in GeSn metal-semiconductor-metal (MSM) photodetectors for enhanced photo detection at 2 μm. Both plasmonic structures are beneficial for effective optical confinement near the surface due to surface plasmon resonance (SPR), contributing to an enhanced responsivity. The responsivity enhancement for Au hole-array structure is insensitive to the polarization direction, while the enhancement for Au-GeSn grating structure depends on the polarization direction. The responsivity for GeSn photodetector with Au hole-array structure has ~50% enhancement compared with reference photodetector. On the other hand, Au-GeSn grating structure benefits a 3× enhanced responsivity of 0.455 A/W at 1.5V under TM-polarized illumination. This doctoral thesis examines different approaches to enhance the optical response of GeSn photodetectors at 2 μm. The results manifest that GeSn photodetectors have tremendous potentials in 2 μm optical communication and other emerging applications. It is inspiring to integrate GeSn lasers and Ge modulators with the GeSn photodetectors for a fully functional optical interconnect in the future.