High speed Ge waveguide detector for optical communication system Xingjun Wang, Zhijuan Tu and Zhiping Zhou State Key Laboratory of Advanced Optical Communication Systems and Networks, School of Electronics Engineering and Computer Science, Peking University, Beijing, 100871, China Beijing, 12 July, 2013 2013 PKU-NTU Joint Workshop on Silicon Photonics 1
Optical interconnect Optical communication Introduction ti high speed high data volume low cost low power dissipation The photodetector plays an important role in these systems. Conventional systems which are set up with separated devices Silicon photonics http://spm.pku.edu.c 3
Photodetectors t t Conventional Ⅲ-Ⅴ group photodetectors Intel s high performance APD in 2007 have many drawbacks:expensive, poor thermodynamic performance, not compatible with silicon CMOS circuits it. Silicon based photodetectors : small footprint, compatibility with silicon CMOS circuits, low cost, low power consumption. http://spm.pku.edu.c 4
Working principle i Illumination of a reverse-biased p-n junction and resultant drift and diffusion of photo-exited electron-hole pairs. Carriers close to or within the depletion region are swept out by the faster drift process(indicated by thick arrows) and contribute to the photocurrent. Notes: 1.The PD is reverse biased 2.Photocurrent is due to the flow of electrons only The working principle of a PD http://spm.pku.edu.c 5
Characteristics: Quantum Efficiency and Responsivity External Quantum efficiency Number of free EHP generated and collected η= Number of incident photons Or I ph /e e α L η= = TopFEHP (1 e ) P / hυ 0 Responsivity R= ( ) Photocurrent A ( ) Incident Optical Power W 0.4 Responsivity & EQE 0.3 R e eλ λ η η η h υ hc 124 1.24 = = = ( A W ) Responsivity (A/W) 1 0.9 0.8 0.7 0.6 0.5 0.2 0.1 0 Ideal Photodiode QE = 100% ( η = 1) Si Photodiode λ g 0 200 400 600 800 1000 1200 Wavelength (nm) http://spm.pku.edu.c 6
Characteristics: Response Time Three factors determine the fundamental response time limit of the device: The diffusion time of carriers generated outside the depletion region; The drift time of carriers across the depletion region; The junction resistance and capacitance. The measured parameter corresponding to response time is the 3dB bandwidth. 1 1 di f = RC 2 ν 0.45ν 2π RC = 2πε R A f = sat = sat T f = 1 π 3dB f 2 + f 2 T RC d i d i http://spm.pku.edu.c 7
PIN photodiodes: principle Two drawbacks of PN photodiode: Low speed (large C) Low QE (W~1µm <δ) Advantages: Large W is good for QE and speed; Large W is NOT good for speed; Large E is good for speed: V d =µ d E, (a) The schematic structure of an idealized pin photodiode. (b)the net space charge density across the photodiode.(c) The built-in field across the diode.(d) The pin photodiode in photodetection is reversely biased. http://spm.pku.edu.c 8
APDs: principle Two advantages of APD: High speed (no need for preamplifier) Internal gain (Avalanche) (a) A pictorial view of impact ionization process releasing EHPs and the resulting avalanche multiplication.(b) Impact of an energetic conduction electron with crystal vibrations transfers the electron s kinetic energy to a valence electron and thereby excites it to the conduction band. (a) schematic illustration of the structure of an APD biased for avalanche gain. (b) The net space charge density across the photodiode. (c) The field across the diode and the identification of absorption and multiplication regions. http://spm.pku.edu.c 9
Ge on Si Photodetector In order to sensitively detect the near infrared light (such as 1.31μm,1.55μm), researchers are paying attention to the Ge on Si photodetectors. Absorption coefficients for Ge, Si and (In)GaAs. Advantages: Excellent optoelectronic properties High responsivity from visible to nearinfrared wavelengths High bandwidths Compatibility with silicon CMOS circuits Low cost and low power consumption. http://spm.pku.edu.c 11
(a) Normal incident structure (b) Side incident(waveguide integrated) structure Hai Yun Xue, et al, IEEE Electron device letters, 2010 Laurent Vivien et al, OPTICS EXPRESS, 2011 Jian Wang et al, Sensors, 2011 Light: along the waveguide Carrier: perpendicular to the waveguide Christopher T. DeRose et al, Optics Express, 2011. high quantum efficiency high bandwidth 12
Ab butt-coupled waveguide photodetector t t (a) Schematic view of Ge photodetector integrated with SOI waveguide. (b) Cross-section view of the Ge p-i-n region. The photodetector has an active area of only 0.8*10μm 2. The measured optical bandwidths are 32.6, and 36.8 GHz at bias of -1, and -3 V It demonstrates a responsivity of 1.1A/W at a wavelength of 1550 nm. Dazeng Feng et al. Applied Physics Letters 95,(2009) http://spm.pku.edu.c 13
An evanescently-coupled l photodetector t t (b) (a) 3D schematic view of a vertical pin Ge waveguide photodetector integrated on top of an SOI waveguide. (b) Cross-sectional view of the device. It considers the influence of the electrodes. By increasing the thickness of the intrinsic Ge and narrows the width of the electrodes. The detector has an 3dB bandwidth of 36 GHz at the bias of -1V and a responsivity of 0.95 A/W over the wavelength range of 1520nm to 1550nm. Shirong Liao et al. Optics Express 10967 (2011) http://spm.pku.edu.c 14
Our group work http://spm.pku.edu.c 15
iber Leng gth[km] Rate [Mb bit/s] F Bit- 10 12 11 10 10 10 Optical communication 10 9 WDM 8 10 7 10 6 10 5 10 4 10 3 10 400M 32M 100M 100M TDM 800M 2G F-100M F-32M 1.8G 445M F-400M 2.5G F-1.6G DFB Laser 10G 10G 100G 20G FA -10G Coherent +DSP Multilevel 160G 19Ch 100G 10Ch Optical Amplifier 2.5G 48Ch AWG 114G 320Ch 111G 134Ch 160G 160Ch 111G 140Ch 85G 40Ch 100G 10Ch 170G 8Ch 40G 40Ch 10G 80Ch Experimental Commercial H. Wu, CAE 1975 1980 1985 1990 1995 2000 2005 2010 1 16
Coherent optical communication i is basic technology for 100Gbit/s speed Drawbacks: Expensive, Separate device Large size Low efficient SKL lab Advanced Optical Communication Systems and Networks, 17
Si-based monolithic photonic integrated 100Gb/s coherent receivers Device: Coupler PBS Hybrids Detector Modulator 18
100G optical receiver 100 Gb/s coherent receiver High bandwidth (>20GHz) High responsively (0.6 A/W) Low dark current (hundreds of na) 19
III-V group photodetectors: Absorption coefficients for Ge, Si and (In)GaAs not CMOS compatible complicated fabrication very expensive Germanium photodetectors: compatible with the CMOS process high responsivity low cost 20 20
(a) 3D schematic structure of the Ge pin waveguide photodetector integrated on top of an SOI waveguide. (b) Schematic cross-sectional view of the photodetector. The light is evanescently coupled to the overlying Germanium layer. 21 21
Ge Si Ge SiO 2 The field distribution of the photodetector by beam propagation simulation. Almost 95% of the light power is absorbed when the Germanium length reaches about 8 μm. The photodetector is designed to be 1.6 10 μm 2. 23
(a) (b) (a)the top view SEM image of the rib waveguide and the Si pedestal. (b)the top view SEM image of the 1.6 10 μm 2 photodetector after all processing. The combined SiGe buffer layer with the two step growth approach. The final thickness of the Germanium layer was 500nm. 24 24
Dark current (a) (b) A low dark current of 0.66μA at the bias of -1V is achieved. The longer the PD, the larger dark current. 25 25
The germanium photodetector exhibited a high 3-dB bandwidth of 22GHz at the wavelength of 1.55μm. A clear open eye diagram at 30Gb/s was also obtained 27
衷心感谢台大的老师和同学们!!! 欢迎你们明年去北大访问!!! 希望 2014 年到台大交流 也希望能有研究生交流 Taibei, 26 Dec, 2012 28