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
110 8 5 4 3 2 1 Photon energy (ev) 0.9 0.8 0.7 110 7 110 6 (m -1 ) 110 5 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? 110 4 110 3 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Wavelength (m)
- 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
Photodetection efficiency I R (Responsivity) P I q (Quantum Efficiency) = P h q 1.24 R h [ev] h [ m] Rq[ C] [1/ V] 1.24[ ev ] 1.24 Responsivity (A/W) 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Ideal Photodiode QE = 100% ( = 1) Si Photodiode g 0 200 400 600 800 1000 1200 Wavelength (nm)
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 R? q ( nn) q ( p p) e V V I wd wdqenqhp h 0
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 (Assume n, p are uniform) h wd V P V P I wd wd qe hint = qe hint V 2 h wd h I I q R (Assume dark current is small) e hint V P 2 P h q R G where G int e h V 2 h
h+ e Photoconductor I ph Gain: G e h V 2 Assuming e h, G e V 2 2 = e e V e ; Time for travelling distance V e E v e e ==> electrons circulate many time before recombination With h G ( ) V 2 = 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
d Photoconductors: I n, p + v - w - Very easy to make - arge gain - But slow (speed limited by - Can have significant dark currents
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
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
SiO 2 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
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)
Metal-Semiconductor-Metal (MSM) Ge PD ow responsivity due to metal shadow (surface illuminated type) arge dark current (low schottky barrier, quality of Ge grown on Si) Electrode distance photodetection bandwidth Type Responsivity OE bandwidth Dark current Ge thickness WG MSM 0.14 A/W @ -1V 40 GHz @ -2V 90 ua @ -1V 100 nm Ref) 2010, OE, CMOS-integrated high-speed MSM germanium waveguide photodetector, IBM
Vertical PIN Ge PD Thickness of intrinsic Ge: tradeoff between transit time and junction cap RC time and transit time photodetection bandwidth E-field Type Responsivity OE bandwidth Dark current Ge thickness WG Vertical PIN 0.5 A/W @ -1V 50 GHz @ -1V 50 na @ -1V 400 nm Ref) 2015, JT, High-responsivity low-voltage 28-Gb/s Ge p-i-n photodetector with silicon contacts, IMEC
ateral PIN Ge PD ower minority carrier diffusion length increase photodetection bandwidth Type Responsivity OE bandwidth Dark current Ge thickness WG ateral PIN > 1 A/W @ -1V > 70 GHz @ -1V 100 na @ -1V 500 nm Ref) 2015, OE, High bandwidth, high responsivity waveguide-coupled germanium p-i-n photodiode, IHP
Separate-Absorption-Charge-Multiplication (SACM) PD (Ge/Si APD) Si s low noise property & Ge s strong absorption near 1.55 μm wavelength ow k eff (k ~ 0.09, ratio of ionization coefficients of electrons and holes) high gain-bandwidth products, low noise Need large reverse bias for avalanche high dark current Bias Type Responsivity OE bandwidth Dark current Ge thickness WG SACM APD 22 A/W @ -27V 20 GHz @ -27V 10 μa @ -27V 1 μm Ref) 2013, OFC, High speed waveguide-integrated Ge/Si avalanche photodetector, IME