Abstract
Photonic integrated circuits (PICs) have become indispensable in modern technology due to their ability to enable scalable, compact, and power-efficient optical systems for applications spanning data communications, sensing, and quantum technologies. The integration of high-performance photodiodes within these circuits is essential for fully realizing their potential. However, research gaps remain with respect to the choice of integration platform. Silicon (Si), the most widely used material in electronics, has limited optical transparency below 1.1 µm, preventing its deployment for visible photonics. Silicon-on-insulator (SOI) has dominated telecommunication PICs but exhibits significant two-photon absorption and free-carrier effects at 1550 nm limiting their usability in applications requiring very low loss. Consequently, alternative low-loss dielectric platforms such as silicon nitride (Si3N4) and tantalum pentoxide (Ta₂O₅) have gained increasing attention due to their wide transparency windows, low propagation loss, and complementary metal–oxide–semiconductor (CMOS) compatibility. Despite their promise, integration of efficient photodiodes with these platforms has not been fully established, leaving critical performance gaps in visible and infrared PICs.
This dissertation addresses these gaps by developing heterogeneously integrated photodiodes on low-loss dielectric waveguides across two wavelength regimes: visible detection using GaAs/AlGaAs heterogeneously bonded to tantalum pentoxide (Ta₂O₅), and 1550 nm detection using InP/InGaAs avalanche photodiodes (APDs) bonded to silicon nitride (Si3N4) waveguides. Together, these efforts demonstrate new material–platform combinations that enable efficient high speed photodetection at visible wavelengths and low optical power detection at 1550 nm.
The first part of the work focuses on the visible spectrum, where applications such as quantum information processing, bio-imaging, and spectroscopy demand integrated photodetectors with high responsivity and low dark current. Tantalum pentoxide (Ta₂O₅), or tantala, offers exceptional transparency into the visible (down to ~ 400 nm), a relatively high refractive index (n ~ 2.1) for tight confinement, and ultra-low propagation loss (as low as 0.1 dB/cm). In this work, GaAs/AlGaAs epitaxial photodetectors were heterogeneously integrated on Ta₂O₅ waveguides using adhesive bonding techniques. The resulting waveguide-coupled devices exhibit efficient coupling from the guided mode into the absorbing GaAs layer, leveraging the strong absorption coefficient of GaAs in the visible regime. Detailed characterization of these photodiodes demonstrated 100 pA dark current, 56% quantum efficiency (QE) between 635 nm and 780 nm wavelengths, and up to 16 GHz bandwidth. Moreover, the photodiodes exhibited data detection capability at 12 Gbit/s. This represents, to our knowledge, the first demonstration of III–V photodiodes for visible-wavelength heterogeneously bonded onto Ta₂O₅ waveguides, filling a major gap in this emerging platform.
The second part of the dissertation targets the 1550 nm wavelength, where avalanche photodiodes (APDs) are indispensable for low-power detection and sensitivity-limited systems. Conventional Si PIN photodiodes are ineffective beyond 1.1 µm, requiring the use of InP/InGaAs heterostructures for 1550 nm detection. A separate absorption, charge, and multiplication (SACM) structure was simulated, and fabricated to optimize dark current, bandwidth, and breakdown characteristics. The layer stack comprised a 1 µm InGaAs absorber, a 70 nm charge layer, a 30 nm grading layer, and a 400 nm InP multiplication region, capped by doped contact layers. The epitaxial films were transferred to a SiN-on-SiO₂ waveguide platform using adhesive bonding, ensuring strong evanescent coupling between the guided SiN mode and the InGaAs absorber. Comprehensive simulation using Ansys Lumerical’s finite difference eigenmode (FDE), eigenmode expansion (EME) and CHARGE solvers predict 80% internal quantum efficiency while experimental characterization validated dark current of 10 nA near breakdown voltage, and 370 fF junction capacitance. At 1550 nm, the integrated SACM APDs exhibited a responsivity of 51 A/W and achieved avalanche gain of 166 near breakdown voltage. The devices showed a 3-dB electrical bandwidth of 4.2 GHz and support data detection capability at 7.5 Gbps, demonstrating the viability of combining SiN waveguides with efficient III–V avalanche photodiodes for the first time.
By bridging these two spectral regimes, this dissertation demonstrates how heterogeneously integrated group III-V photodiodes can augment low-loss dielectric waveguide platforms such as Ta₂O₅ and SiN. The GaAs/AlGaAs–Ta₂O₅ devices address the unmet need for integrated detectors in emerging applications requiring high-speed photonics in the visible, while the InP/InGaAs–SiN APDs establish a pathway for highly sensitive detection at 1550 nm without the limitations of silicon waveguides. These results not only validate new material–platform combinations but also highlight bonding, coupling, and device optimization strategies that can be extended to future broadband photonic integration efforts. Ultimately, this work advances the state of the art in heterogeneous integration by providing a framework for realizing efficient, low-loss, and scalable photodiodes on dielectric PIC platforms, enabling next-generation systems for communications, quantum information, biomedical imaging, energy-efficient optical interconnects that can meet the exponential data movement requirements of the ongoing AI revolution.
In the concluding part of my dissertation and future work, single photon avalanche photodiodes (SPADs) are discussed. The study included the measurement of dark count rate (DCR) – one of the important figures of merit for photodiodes capable of single photon detection. This measurement was aimed at preliminary study to compare the performance of two different material system AlInAsSb and InGaAS to get directions for future research for integrated SPAD development.
