Design of Ge-on-Si Photodetectors Applicable to Sensor and Communication Areas

Abstract

Silicon photonics is a technology that uses silicon (Si) substrate to designing photonic devices, so the photonic and electronic devices are able to be fabricated on a chip owing to its CMOS compatibility. In this respect, silicon photonics are attracting great attention in the field of optical interconnects (usually used in data centers), optical transceivers, optical sensors, and so on. In particular, the photodetector design technology is very important in silicon photonics field because the optical receiver is necessary for light signal acquisition. In the past, III-V compounds (GaAs, InGaAs, InP, etc.) have been used to design photodetectors, but they have the disadvantage of being expensive to fabricate because they are difficult to grow or bond on the silicon substrate. Recently, Ge has been spotlighted as a substitute material for III-V compounds since high-purity germanium (Ge) crystals growing technology on silicon was advanced. Ge is a good candidate for designing photodetectors with small size and high-responsivity due to its high absorption coefficient over O-, C-, and L-bands. Especially, Ge-on-Si structures are able to fabricate layer by layer, and they can be applied to various fields because it is easy to be integrated with optical waveguides. In this thesis, Ge-on-Si photodetector designs that can be applied to communications and optical sensors are proposed. Photodetectors were designed by using simulations (FDTD, MODE, and DEVICE solutions of Lumerical inc., Canada) based on finite-difference time-domain (FDTD) method, and their performances were verified by measuring fabricated devices using foundry service (IME A*Star, Singapore) and comparing with other devices. The waveguide integrated Ge-on-Si structure was used for designing a high-speed photodetector. The PIN diode was formed vertically, and the device was made for operating 1.3 um wavelength range to meet zero-dispersion condition. In actual design, light absorption efficiency was 89% to obtain the responsivity of 0.7 A/W or more. Also, the fringing field was analyzed to estimate 3 dB bandwidth considering fabrication limitation of spacing 0.75 um between edges of doping region and Ge layer. It is important because Ge has high absorption coefficient at the wavelength range of 1.3 um, so light penetration depth becomes just a few microns. The designed photodetector was fabricated by foundry service, and the device characteristics was measured with experimental setup that used fiber to grating coupler coupling method. The device showed 3 dB bandwidth of 20.75 GHz at -1 V, and estimation error was smaller than 3 GHz. In addition, dark current and responsivity were measured as 110 nA and 0.704 A/W at the same bias voltage. The Ge-on-Si photodetector was formed as disk-shaped to be applied to refractive-index sensing. In the disk resonator sensor, refractive indices of samples are able to be determined by the resonance wavelength shift that occurs when the sample fluid flows outside the device. The disk-shaped Ge-on-Si photodetector can determine the refractive index in the same way as conventional disk resonators through detecting the absorption of the circularly rotating light inside the Si disk. However, the Ge-on-Si photodetector sensor has a disadvantage of quality factor Q-factor degradation because light absorption acts as a loss factor. In this respect, I proposed a disk-shaped Ge-on-Si photodetector with recess structure on the top of the Si layer to make ring-shaped waveguide along the edge of Si disk. The interaction area was enlarged by recess, so the sensitivity was improved 1.67 times compared to the structure without recess. At the disk radius of 3 um, the proposed structure had Q-factor and sensitivity of 3000 and 50 nm/RIU or more, respectively. The intrinsic limit of detection (ILOD) was 0.95 x 10^(-2) RIU.