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金氧半穿隧光偵測器及
發光二極體的增進
Enhancement of Metal-OxideSemiconductor Tunneling Photodetectors
and Light Emitting Diode
•指導教授：劉致為 博士
•學生：梁啟源
•台灣大學光電工程學研究所
1
Outline
Introduction ……………………………………..3
NMOS photodetector…………………………..4
SOI NMOS photodetector……………………..9
Metal-HfO2-Silicon LED …………….............32
Surface plasmon enhanced transmission..34
Summary………………………………………..40
2
Introduction
• The electro-optical products may be one of
the killer applications in the future Si market.
• The worldwide revenue of the optical
semiconductor is ~6% (~10 B) of the total
semiconductor revenue.
• The ITRS has predicted that the
incorporation of electro-optical components
into CMOS compatible process is needed in
2004 to achieve System-on-a-Chip (SOC).
• Si-based optoelectronics
- low cost, high reliability, VLSI compatible
3
NMOS detector I-V characteristic
1E-4
0.24 mW/cm
Gate Current (A)
1E-5
0.71 mW/cm
photo current
1E-6
1.77 mW/cm
• Al gate
• The dark and photocurrents are
relatively constant
in the log scale at
large gate bias.
2
2
2
1E-7
1E-8
1E-9
1E-10
Al or ITO gate
Ultrathin SiO2
1E-11
dark current
1E-12
1E-13
1E-14
-3
•
p-type or n-type siliocn wafer
-2
-1
0
1
2
3
Ilight intensity  => Iph 
4
Gate Voltage (V)
4
NMOS detector operation principle
•
Positive gate
bias
•
Deep depletion
•
Soft-pinning of
Vox
5
Vox pinning
5
Oxide voltage drop
Si surface potential
4
Voltage ( V )
3
•
Simulation results
•
Vg falls on Si
substrate.
•
Soft-pinning of Vox
2
1
0
-1
-2
-3
-4
-4
-3
-2
-1
0
1
2
3
4
Gate Voltage ( V )
6
Minority carrier distribution
Dn
n p
x 2

n p  n p0
n
 G ( x)  0
x  L , n p  n p0
x  W , np  0
n p ( x)  n p 0  A exp(  x)  C1 exp( 
C1 
x
x
)  C 2 exp(  )
Ln
Ln
1

1  exp 2(W  L) Ln 
 
W 2L
L 
(W  2 L) 
 A exp( W   )  exp(  L  )   nn 0 exp

Ln Ln
Ln 
Ln 
 
C2 
1

1  exp 2(W  L) Ln 
 
W
L 2W 
W


A
exp(


W

)

exp(


L


)

n
exp
 

 p0
L
L
L
L
n
n
n 
n
 
7
Minority carrier distribution
Excess carriers generated before 46um could contribute to
electron density distribution
total current.
J diff  qDn
~46 um
8
1.5x10
n p
x
x W
W
J drift  q  G ( x)dx 
8
1.0x10
0
q  0 1  exp(  W ) 
7
5.0x10

0.0
0
100
200
300
400
500
600
( J dr  J diff ) / q
Pop / Ah
  dr   diff
 0.031  0.904  0.94
depth (um)
8
SOI MOS Photodetector
• Responsivity and Bandwidth
• Previously reported tunneling MOS
photodetectors (PD) couldn’t
promote it’s speed to GHz => far
from application
• The absorption length of 850 nm
lightwave in Si (~16 um) is much
larger than the Si depletion width.
9
hv
Vg
oxide
Depletion
width
Silicon substrate
• Photo-generated
carriers in the bulk
neutral region.
•
=> collected by
gate electrode
through the slow
diffusion process.
• The large diffusion
current limits the
device bandwidth to
hundreds MHz.
10
Simulation Details
• Proposed MOS/SOI structure carried out by ISE.
11
Simulation Details
• Thick buried oxide => stops the
diffusion current from substrate
• thin absorption layer => makes
sure that the device is fully-depleted
during operation.
• The absorption region and ground
electrode are separated by oxide and
connected by buffer layer.
• The grid structure of Al gate
electrode => allows the light directly
exposures on the absorption region.
12
Results and Discussion
• The diffusion current is eliminated in SOI-MOS PD to
increase speed => total photocurrent is reduced
• Thicker absorption region increase the responsivity but
reduces bandwidth.
0
Bulk-MOS PD
SOI-MOS PD
2
Current Density ( A/cm )
1x10
Photocurrent
-4
1x10
-8
1x10
Dark current
-12
1x10
-16
10
-6
-5
-4
-3
-2
-1
0
1
2
3
4
Gate Voltage ( V )
13
Results and Discussion
• Under inversion bias (Vg > 0), most voltage drops falls
on Si substrate and the depletion width increases with Vg.
=> Tunneling MOS diode is deeply depleted under
inversion bias.
5
oxide voltage drop
Silicon surface potential
buried oxide
4
Voltage (V)
3
2
1
0
-1
-2
-3
-4
-2
0
2
4
Gate voltage (V)
14
Results and Discussion
• SOI-MOS devices with different absorption layer thickness.
• Thicker absorption layers => higher responsivity
=> bandwidth decreases from 30 GHz to 0.9 GHz.
• The absorption region outside the depletion region
produces the diffusion current and reduces the bandwidth.
Quantum Efficiency
linear scale (a. u.)
Active region 2  m
0.9 GHz
Active region 1  m
17 GHz
Active region 0.5  m
30 GHz
10M
10G
1G
100M
Frequency ( Hz )
100G
15
Results and Discussion
band energy (eV)
• Band diagrams of the SOI-MOS PDs with different buffer
layer doping along the route of hole current.
• A energy barrier for hole is observed for device with 1016
buffer layer doping.
1
Ec
0
Ev
-1
-2
-3
16
10
18
10
20
10
Carrier Transport Path ( a. u. )
16
Results and Discussion
• The photocurrent of device with 1020 cm-3 buffer layer
doping rises and falls quickest.
• SOI-MOS PD generates more photocurrent than the bulk
devices, unlike the result in DC condition.
Bulk-MOS PD
20
-3
Buffer layer 10 cm
18
-3
Buffer layer 10 cm
16
-3
Buffer layer 10 cm
5 ps Optical impulse
Photocurrent ( 10
-16
A)
12
8
4
0
0
10
20
Time ( ps )
30
40
17
Analytical Model
• Depletion region
hv
Vn
Vp
V>0
Electron-hole
pairs
V
Vp: hole saturation
velocity
N (t ) 
Vn
Vn: electron
saturation velocity
P
W
[exp(  vn t )  exp(  W )][u (t )  u (t  t 2 )] ; t 2 
hv
vn
N(t)
Vp
V
P(t)
P(t ) 
P
W
[1  exp(  W   v p t )][u (t )  u (t  t3 )] ; t3 
hv
vp
q
J (t ) 
[vn N (t )  v p P(t )]
Wd
18
Analytical Model
• Fourier transform :
 e
N (t )  N ( ) 
[
h
 Wd
i Wd vn
(e
j
i Wd v p
  (1  e
P(t )  P( ) 
[
h
j
i Wd vn  Wd
 1) (1  e
)

]
j   v n
i Wd v p  Wd
) (1  e
)

]
 v p  j
q
J ( )  [vn N ( )  v p P( )]
Wd
• Considering depletion capacitance Cd and series
resistance Rs :
q vn N ( )  v p P( )
J ( ) 
[
]
Wd
1  j Cd Rs
19
Analytical Model
• Because of symmetry, we can just analyze right half side of the
device, and transform it into equalized circuit.
Vg
Vg
Wd
C3
C2
C1
R1
R2
R3
• There are three currents with different capacitance and resistance. The
total frequency response was formed by linearly adding currents together.
q vn N ( )  v p P( )
J ( )   [
]
1  j Ci Ri
i 1 Wd
3
20
Analytical Model
W3
W2
W1
• Buffer layer of 1016 doping
is depleted and holes from
center are blocked by
depletion region.
• Only holes generated in
the outside part could
transport to ground contact.
=> lower responsivity
 (1e16) 
W1
1
 
W1  W2  W3
5
21
Analytical Model
• The parameter values used in the analytical model are
given as followed.
Wd
absorption thickness
5x10-5 cm
vn
electron saturation velocity
1x107 cm/s
vp
hole saturation velocity
6x106 cm/s
2C1  C2  C3
R201
R181
R161
R202
R182
R162
R203
R183
R163
capacitance
A
 Si
0.2 f F
Wd
resistance for 1E20 doping
l
 

A
240
3k
60k
2.4k
30k
600k

4.8k
60k
1.2M
absorption coefficient
382 cm-1

quantum efficiency
0.024

light intensity
1x10-6 W/cm2
h
light energy
1.459 eV
For 850nm light
22
Analytical Model
• The analytical model precisely fits the simulation results.
20
Quantum Efficiency
linear scale (a. u.)
10
18
16
DC
10
10
20
10
frequency (Hz)
10m
10
18
10
16
10
10k 10M 10G
simulation
analytical formula
1G
10G
Frequency ( Hz )
100G
23
Bandwidth of SOI PD
For tsi>0.5um ， diffusion process ↑；bandwidth↓
For tsi>0.5um ，capacitance↑；bandwidth ↓
11
10
10
Bandwidth (Hz)
10
9
10
8
10
7
10
6
10
0.1
1
absorption thickness (m)
10
24
Bandwidth of SOI PD
As tsi ↑，diffusion length↑ ，bandwidth↓
ISE simulation
analytic model
9
Bandwidth (Hz)
10
t diff
L2

2D
1

2f 3dB
8
10
t
drift
 t diff
  RC 
2
7
10
0
5
10
15
Device Thickness ( m )
20
25
2
Bragg reflector
2
i
2


cos(
n
h
cos

)

sin(
n
h
cos

)
n
n 

0 n n
pn
0 n n
mn  

2

2

 ip sin(
nn hn cos  n )
cos( nn hn cos  n ) 
n


0
0
pn 
n
cos  n
n
pn 
n
cos  n
n
Transform matrix
of layer n
for TE wave
for TM wave
m12 
m
M N  m1m2 ......mN 1mN   11

m21 m22 
r
t
(m11  m12 p f ) p0  (m21  m22 p f )
(m11  m12 p f ) p0  (m21  m22 p f )
2 p0
(m11  m12 p f ) p0  (m21  m22 p f )
reflectance
transmittance
26
Bragg reflector
Reflectivity has period relation with silicon
thickness
0.7
0.6
Reflectivity
0.5
445, 560, 675....... 
0.4
850
(2, 2.5, 3......)
nsi
0.3
0.2
0.1
0.0
TM SOI
TE
TM wo BOX
TE
400
500
600
700
800
Si layer thickness (nm)
27
Bragg reflector
Reflectivity has minimum values at
850nm and 1300nm.
Reflectivity
1.0
0.8
850  2.5  560nsi ( )
0.6
1020  2  560nsi ( )
0.4
1300 1.5  560nsi ( )
0.2
0.0
700
800
900 1000 1100 1200 1300 1400 1500 1600
wavelength (nm)
28
Bragg reflector
When incident angle is smaller than 40o,
reflectivity of SOI device is smaller than bulk one.
1.0
TM with BOX
TE
TM without BOX
TE
Reflectivity
0.8
0.6
0.4
0.2
0.0
0
20
40
60
80
100
Angle (degree)
29
Bragg reflector
The TEM and thickness of SOI MOS PD
Tox (2nm) << 850nm, gate oxide was neglected.
SiO2 2nm
Si 0.56um
SiO2 0.25um
30
Bragg reflector
The measured photocurrent of ~30o incident
light is 80% higher than normal incident one.
Gate current (A)
1E-6
1E-7
1E-8
dark current
normal 0.28 mA/W
0
~30
0.52 mA/W
1E-9
-4
-2
0
2
4
Gate voltage (V)
31
Metal-Insulator-silicon tunneling LED
Al
SiO2
p-Si
h
Light emitted from
electron hole
recombination at
accumulation
EF

VGS < 0
32
Metal-Insulator-silicon tunneling LED
-6
Efficiency (10 )
The light efficiency of metal-HfO2-silicon LED is 4
times larger than MOS LED.
2.0
k↑

capacitance( t ox ) ↑
charge (Q=CV) ↑
recombination ↑
efficiency ↑
HfO2
1.5
1.0
0.5
SiO2
0.0
0
20
40
60
80
Gate Current (mA)
100
33
Surface plasmon Enhanced transmission
Surface plasmon at metal-dielectric
interface resonance with TM light.
dielectric
2
1
kz
kx
metal
34
Surface plasmon Enhanced transmission
14
Dispersion relation
dispersion of Al/SiO2
light line
Si emitting energy
12
 m d
 k sp
c m  d

hw (eV)
10
8
6
4
2
Drude model
0
0.00
0.02
0.04
kx (1/nm)
0.06
0.08
 m/  1   p2 /( 2  c2 )
 m//   p2c /  ( 2  c2 )
35
Surface plasmon Enhanced transmission


c
sin  0
2
SP
Lig
ht
lin
e
1
SP right at light line
imaginary kz
kx
k
kx 

c
sin  0  k x 
x
 m d
 k sp
c m  d

Radiative
resonance
36
Surface plasmon Enhanced transmission
T. W. Ebbesen, Nature (London) 391, 667 (1998)
single subwavelength
hole
I  (d /  ) 4
transmission increase
larger than area
37
Surface plasmon Enhanced transmission
max (i, j ) 
a0
i2  j2
 m d
m  d
The periodicity of the array
determines the position of the
peaks, independent of metal
hole diameter and film
thickness.
38
Surface plasmon Enhanced transmission
air
Aluminum
Oxide
silicon
0
 Al ox
  Al si

c  Al   ox
c  Al   si

 1 d1
2
2nm
d2
3
 1/
 1/   2
 2 1
k x (d 2 ) 
( /
)2 /
c  2 1  1 1  1
1
 1/
2 d 2

a=316nm, r=50nm, and h=100nm.
39
Summary
•Photodetectors using NMOS tunneling
structures are demonstrated.
• The novel SOI-MOS PDs can reach high
bandwidth (22 GHz) and are fully compatible
with ULSI technology.
• The device structure could be optimized with
Bragg reflector by tuning the doping and
thickness of epi-layers.
40
Summary
•The light emission from Al/HfO2/p-Si
tunneling diodes has an enhanced intensity
because of the high dielectric constant of HfO2
to increase the hole concentration at Si/HfO2
interface
•Surface plasmon could enhance the
transmission through hole array on metal film.
The size of hole array is predicted.
41