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Lect. 10: Photodetectors
W.-Y. ChoiSi Photonics (2015/2)
Photodetection: Absorption => Current Generation
h Currents
Materials for photodetection: Eg < hVarious methods for generating currents with photo-generated carriers:photoconductors, photodiodes, avalanche photodiodes
Lect. 10: Photodetectors
W.-Y. ChoiSi Photonics (2015/2)
0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8
Wavelength (m)
In0.53Ga0.47As
Ge
Si
In0.7Ga0.3As0.64P0.36
InPGaAs
a-Si:H
12345 0.9 0.8 0.7
1103
1104
1105
1106
1107
1108
Photon energy (eV)
(m-1)
1.0
- Sharp decrease in for >Eg
- Photodetection for indirectbandgap materials?
Lect. 10: Photodetectors
W.-Y. ChoiSi Photonics (2015/2)
- Photodetection for indirect bandgap materials?
E
CB
VB
k–k
Direct Bandgap Eg Photon
Ec
Ev
(a) GaAs (Direct bandgap)
E
k–k
(b) Si (Indirect bandgap)
VB
CB
Ec
Ev
Indirect Bandgap, Eg
Photon
Phonon
Unlike emission, absorption in indirect bandgap semiconductor is highly probable
Lect. 10: Photodetectors
W.-Y. ChoiSi Photonics (2015/2)
Photodetection efficiency
(Responsivity) IRP
0 200 400 600 800 1000 12000
0.10.20.30.40.50.60.70.80.9
1
Wavelength (nm)
Si Photodiode
g
Responsivity (A/W)
Ideal PhotodiodeQE = 100% ( = 1)
(Quantum Efficiency) = I
qP
h
qRh
1.24[eV][ m]
h
[ ] [1/ ]1.24[ ] 1.24
R q C VeV
Lect. 10: Photodetectors
W.-Y. ChoiSi Photonics (2015/2)
Photoconductor
n0, p0
d
w
+ v -
L,
Conductivity: ( : electron, hole mobility)
e h
e h
q n q p
Without light,
I
With light,
0 0, n n n p p p
J EVI wdL
0( ) ( )e hq n n q p p
e hV VI wd wd q n q pL L
?R
Lect. 10: Photodetectors
W.-Y. ChoiSi Photonics (2015/2)
L
n, p
d
w
+ v -I
With light,
0 0
0
, ( ) ( )
e h
e h
n n n p p pq n n q p p
V VI wd wd q n q pL L
int (Assume , are uniform)Pn p n ph wLd
VI wdL
IRP
intqR Gh
inte hP Vwd qh wLd L
int 2= e hPq Vh L
int 2e hq Vh L
(Assume dark current is small)IP
2where e hG VL
Lect. 10: Photodetectors
W.-Y. ChoiSi Photonics (2015/2)
2Gain: e hG VL
Iph
Photoconductore–
h+
With h
2Assuming , e h eG VL
e
e
LVL
==> electrons circulate many time before recombination e
Time for travelling distance L
2 = ( ) ehe h
GL
V
( )( )eh
e h e he h
L L LV E v vL
1 11 1 ( )
e h
e h e h
e h
v vL
2
e
LV
=
e
;Lv
e
LE
Lect. 10: Photodetectors
W.-Y. ChoiSi Photonics (2015/2)
L
n, p
d
w
+ v -I
Photoconductors:
- Very easy to make- Large gain - But slow (speed limited by - Can have significant dark currents
Lect. 10: Photodetectors
W.-Y. ChoiSi Photonics (2015/2)
Faster, less dark-current photodetectors?
B-
h+
p n
As+
e–
E (x)
x0
–Eo
M
Wn–Wp
PN junction in reverse bias
photodiode
- No significant current flow=> small dark currents
- Photo-generated carriers are removedby built-in field in depletion region (space charge region)
Lect. 10: Photodetectors
W.-Y. ChoiSi Photonics (2015/2)
E (x)
x0
–Eo
M
Wn–Wp
- Photo-generated carriers drift into P (holes)and N (electrons) regions generating currents
P N
intPI qh
- One photon creates a pair of electron and hole
- Problem: depletion region is very thin (< 1 m) int is very small
=> Use PIN structure
Lect. 10: Photodetectors
W.-Y. ChoiSi Photonics (2015/2)
p+
i-Si n+
SiO2Electrode
net
–eNa
eNd
x
x
E (x)
R
Eo
E
e–h+
Iph
h > Eg
W
(a)
(b)
(c)
(d )
Vr
Vout
Electrode
PIN Photodiode
Lect. 10: Photodetectors
W.-Y. ChoiSi Photonics (2015/2)
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.
PD with gain?
h+E
šn+ p
e–
Avalanche region
e–
h+
Ec
Ev
EIt is preferred only one type of carrier (either electron or hole) causes impact Ionization
: ratio of ionization coefficients (= hole/electron)
Lect. 10: Photodetectors
W.-Y. ChoiSi Photonics (2015/2)
Metal-Semiconductor-Metal (MSM) Ge PD– Low responsivity due to metal shadow (surface illuminated type)– Large 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
Lect. 10: Photodetectors
W.-Y. ChoiSi Photonics (2015/2)
Vertical PIN Ge PD– Thickness of intrinsic Ge: tradeoff between transit time and junction cap– RC time and transit time photodetection bandwidth
Ref) 2015, JLT, High-responsivity low-voltage 28-Gb/s Ge p-i-n photodetector with silicon contacts, IMEC
Type Responsivity OE bandwidth Dark current Ge thickness
WG Vertical PIN 0.5 A/W @ -1V 50 GHz @ -1V 50 nA @ -1V 400 nm
E-field
Lect. 10: Photodetectors
W.-Y. ChoiSi Photonics (2015/2)
Lateral PIN Ge PD– Lower minority carrier diffusion length increase photodetection bandwidth
Ref) 2015, OE, High bandwidth, high responsivity waveguide-coupled germanium p-i-n photodiode, IHP
Type Responsivity OE bandwidth Dark current Ge thickness
WG Lateral PIN > 1 A/W @ -1V > 70 GHz @ -1V 100 nA @ -1V 500 nm
Lect. 10: Photodetectors
W.-Y. ChoiSi Photonics (2015/2)
Separate-Absorption-Charge-Multiplication (SACM) PD (Ge/Si APD) – Si’s low noise property & Ge’s strong absorption near 1.55 μm wavelength– Low keff (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
Ref) 2013, OFC, High speed waveguide-integrated Ge/Si avalanche photodetector, IME
Type Responsivity OE bandwidth Dark current Ge thickness
WG SACM APD 22 A/W @ -27V 20 GHz @ -27V 10 μA @ -27V 1 μm
Bias ↑