[ APPLIED PHYSICS LETTERS ] High-speed Ge photodetector monolithically integrated with large cross silicon-on-insulator waveguide Dazeng Feng, Shirong Liao, Roshanak Shafiiha. etc
Contents 1. Introduction 2. Background 3. Process & Results 4. Conclusion 2
Introduction System degradation in IC Core increase in IC Pin number of package High power consumption I/O bandwidth limitation Copper wires reaching physical limits ~10 Gbps or higher becoming challenging Distance/speed tradeoff shortens lengths Why Photonics? Consuming an enormous amount of power Alternative : Transmit data over optical fiber Much further reach at any given speed Multiple signals can travel on one fiber Thin & light = easy cable management Silicon photonics Optical waveguide Broaden bandwidth Low power consumption Cost effective 3
Introduction Silicon Photonics laser Photo detector modulator MUX / DEMUX No EMI High frequency response low signal distortion in optical link Low cost optical communications in and around future PCs, servers and consumer devices 4
Introduction Why do we need photodetectors with large cross-section SOI waveguides? Submicron waveguides High fiber coupling loss High speed Ge photodetector efficiently butt-coupled with large cross-section SOI waveguide in which the Ge p-i-n junction is placed horizontal direction High polarization dependent loss Large waveguide birefringence Phase noise 5
Background Ge-Si photodetector in silicon photonics Responsivity (A/W) 1.0 Quantum Efficiency = 1 Germanium 0.5 InGaAs 0.1 Silicon < absorption coefficient vs. wavelength for various materials > 500 1000 1500 Wavelength (nm) < resposivity vs. wavelength for various materials > 6
Background Butt-coupling & tapering waveguide Transmission line mismatching between tx,rx module and waveguides Coupling loss, reflection loss and polarization variation Tapering waveguide 7
Background p-i-n junction With bias Vr Vr E E o ( Vr V o) W W Response time depends on transit time Increasing W increasing QE But decreasing of speed of response t drift W V d 8
Process & Results Process Light propagates form ridge SOI to PD region Light butt-coupled & absorbed in Ge region Horizontal p-i-n configuration vary narrow intrinsic Ge region Reducing the transit time Transit time(drift time) =, W : intrinsic layer width, : drift velocity Transit time limited bandwidth =., : intrinsic-ge layer width, : saturation drift velocity Side wall doping to the Ge minimize the free carrier absorption loss Quantum efficiency Generating photo-current Response velocity 9
Process & Results Process Photo detector fabrication Silicon recess region was formed by etching the silicon layer above the BOX layer Ge buffer layer was selectively grown Ge film was intentionally over grown Boron, phosphorus Forming a horizontal p-i-n junction and p-type & n-type Ohmic contact areas Thinned down and planarized with a chemicalmechanical polishing(cmp) step Ge waveguides, and the silicon horizontal tapers were formed by the etching step Ti/Al metal stack was deposited and patterned to form p-type and n-type metal contacts Oxide and nitride films were deposited as waveguide cladding and passivation layers 10
Process & Results Results < Measured resposivity of TE and TM polarization light over 1260 to 1640 nm > < Measured frequency response for a device with 0.8um X 10um active area> Satisfactory operation over a 300nm wide wavelength range Very low polarization dependent responsivity η η Increasing wavelength Decreasing absorption of Ge Roll-off responsivity after 1560nm As the drift velocity already reaches the saturation velocity in Ge at -1V bias, high optical bandwidth is achieved at this relative small voltage 11
Conclusion Efficiently butt-coupled with a large cross-section SOI Ge p-i-n junction is placed in the horizontal direction to enable very high speed operation Ge-Si photodetector Active area : 0.8X10, Greater than 32GHz optical bandwidth Responsivity of 1.1 A/W at a wavelength of 1550nm 12
Thank you for listening Q&A 13