Photodetector. Prof. Woo-Young Choi. Silicon Photonics (2012/2) Photodetection: Absorption => Current Generation. Currents

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1 Photodetection: Absorption => Current Generation h Currents Materials for photodetection: E g < h Various methods for generating currents with photo-generated carriers: photoconductors, photodiodes, avalanche photodiodes

2 Photon energy (ev) (m -1 ) Si a-si:h Ge In 0.7 Ga 0.3 As 0.64 P 0.36 In 0.53 Ga 0.47 As GaAs InP - Sharp decrease in for >E g - Photodetection for indirect bandgap materials? Wavelength (m)

3 - Photodetection for indirect bandgap materials? E E CB E c CB Indirect Bandgap, E g Direct Bandgap E g Photon E v Photon E c VB VB E v Phonon k k k k (a) GaAs (Direct bandgap) (b) Si (Indirect bandgap) Unlike emission, absorption in indirect bandgap semiconductor is highly probable

4 Photodetection efficiency R (Responsivity) I P (Quantum Efficiency) = R q [ m] h 1.4 I q P h Responsivity (A/W) Ideal Photodiode QE = 100% ( = 1) Si Photodiode g Wavelength (nm)

5 Photoconductor Without light, w d Conductivity: qen qhp ( : electron, hole mobility) J eh, E I V wd n 0, p 0 With light, I + v - n n n, p p p 0 0 q ( nn) q ( p p) e h 0 V V I wd wdqenqhp

6 With light, d nn0 n, p p0 p qe( nn) qh( p0 p) w n, p V V I wd wdqenqhp + v - I P npint and assuming n, p are uniform, h wd I wd V e h P wd q V int = q e h P int V h wd h I q R (assuming dark current is small) e hint P h e hint G int

7 h+ e Photoconductor I ph Gain: G e h V Assuming e h, G e V = e e V e ; V e E Time for travelling distance v e e ==> electrons circulate many time before recombination With h G ( ) V = e h eh eh V ( ) ( ) E v v e h e h e h 1 1 e h ve vh 1 1 ( e h) e h

8 d Photoconductors: I n, p + v - w - Very easy to make - arge gain - But slow (speed limited by and significant dark currents

9 Faster, less dark-current photodetectors? photodiode p n B - h + As + PN junction in reverse bias e - No significant current flow=> small dark currents E (x) W p M 0 W n x - Photo-generated carriers are removed by built-in field in depletion region (space charge region) E o

10 P N - Photo-generated carriers drift into P (holes) and N (electrons) regions generating currents I int P q h E (x) E o W p M 0 W n x - One photon creates a pair of electron and hole - Problem: depletion region is very thin (< 1 m) int is very small => Use PIN structure

11 SiO Electrode p + Electrode (a) i-si n + net en d (b) x PIN Photodiode en a E(x) (c) x E o W h > E g (d) E h+e I ph R V out V r

12 How to realize Si PIN Photodiode? - Does Si absorb light at 1.5m? Photon energy (ev) Thin SiGe layers are already used in Si technology SiGe HBT(Heterojunction Bipolar Transistor) Ge In 0.7 Ga 0.3 As 0.64 P (m -1 ) Si GaAs InP In 0.53 Ga 0.47 As a-si:h Wavelength (m) Ge on Si!

13 Ge on Si PIN Photodiode with Si waveguide - Good quality thick (~ m) Ge layer is required on Si - Defects due to lattice mismatch less damaging for PD than for laser Carriers drift by strong E-field within depletion region SP13: 31GHz Ge n-i-p waveguide photodetectors on Silicon-on-Insulator substrate, Optics Express, Vol. 15, p , 007. SP14: High-speed Ge photodetector monolithically integrated with large cross-section silicon-on-insulator waveguide, Applied Physics etters, Vol. 95, p , 009. SP15: ow Thermal Budget Monolithic Integration of Evanescent-Coupled Ge-on-SOI Photodetector on Si CMOS Platform, IEEE J. of Selected Topics in Quantum Electronics, Vol. 16, p. 106, 010.

14 PD with gain? Avalanche Photodiode (APD) (avalanche: a large mass of snow, ice, earth, rock, or other material in swift motion down a mountainside) Achieve gain by multiplying electrons and/or holes. Impact Ionization: Under high E-field, electrons and holes can have sufficiently high kinetic energies breaking bonds and creating new e-h pairs. E h + e E e It is preferred only one type of carrier (either electron or hole) causes impact E Ionization v E c n + p Avalanche region š h + : ratio of ionization coefficients (= hole/electron)

15 Ge/Si APD - Separate layers for absorption (Ge) and multiplication (Si) - Si is ideal for multiplication since its is very small SP16: Monolithic germanium/silicon avalanche photodiodes with 340 GHz gain bandwidth Product, Nature Photonics, Vol. 3, p. 59, 009.