COMSOL based simulation of optical response in Si microline array based

broadband photodetector fabricated on Silicon-On-Insulator (SOI) wafers

Vishal Kumar Aggarwal1, Shaili Sett1, Achintya Singha2 , Arup Kumar Raychaudhuri1

1S N Bose National Centre For Basic Sciences, Kolkata2Bose Institute, Kolkata

November 28-29,2019

• Motivation / Science issues

• Pros and cons of work

•Device fabrication process and Electron Microscope

Characterization

•Results and discussion

•COMSOL simulation

• Conclusion

Outline

Motivation / Science issues

Our proposal

➢ We propose to make array of Si microlines (~1 µm wide)fabricated by a top-down process that is compatible with waferlevel processing.

➢ Make Metal-semiconductor-metal (MSM) photodetectorsfrom the Si microlines.

➢ Achieve a large responsivity (R) larger than that available incommercial p-n detectors (0.6 A/W) although less than that ina single Si NW device (104 A/W).

Pros and cons of the work1. Fabrication of array of microlines of Silicon using SOI is a new

technology which is not only compatible with wafer scaleprocessing but also turns out to be an adoptable road map fortranslation.

2. Growth of such type of arrays of microlines (planar) on waferscan be readily coupled to established batch processing tools.

3. The Responsivity (R) is an order higher than the commerciallyavailable 1A/W detectors.

Price paidR will get limited to ~10-20 A/W although much larger than

currently available 0.6 A/W in commercial p-n diode detectors.

120 μm

Device fabrication process20 μm

E-BeamLithography

(EBL)

Au microlines (length ~ 20 μm, breadth ~ 1 μm)

Deposit Cr/Au

The Au lines are used as mask. Thickness (50 nm) is such that the process of etching removes the Si and reaches the bottom SiO2

Si

Au

PMMA coating

PMMA masks the gold electrodes and avoids from any

further etching

PMMA

Device fabrication process (contd..)

6

To make the structure partially suspended and to etch out the SiO2 underneath Si microlines,

we did a wet etching (acid etch) by dipping it in 20%

Hydrofluoric acid (HF) solution.

Remaining SiO2

Silicon

Si microlines on SiO2

Partially suspended Si microlines

Post plasmaEtchingCF4/Ar

SiO2

FESEM image of array of Si microlines

(a) FESEM image of array of Si microlines. (b) Schematic of the partially suspended device created by etching. The etching

progressively removes the oxide under-layer on both sides and left behind a narrow strand that supports the microline.

(c) Magnified view showing partially suspended Si microlines.

0 20 40 60 80 100 1205.0x10

-8

6.0x10-8

7.0x10-8

8.0x10-8

9.0x10-8

1.0x10-7

1.1x10-7

1.2x10-7

1.3x10-7

Cu

rre

nt

(A)

Time(sec)

7.01µW-mm-2

6.32µW-mm-2

4.57µW-mm-2

2.31µW-mm-2

Photo Current vs. Time curve when light is turned ON/OFF

300 400 500 600 700 800 900 1000 1100 1200

0

2

4

6

8

10

12

14

16

18

20

1V

500 mV

100 mV

50mV

Re

sp

on

siv

ity

(A

/W)

Wavelength (nm)

(a) (b)

0 200 400 600 800 10004

8

12

16

20

Re

sp

on

siv

ity

(A

/W)

Bias (mV)

(a) Responsivity curve at different bias over a wavelength range of 400-1100 nm. (b) Peak Responsivity (λ=800nm)

as function of bias.

Responsivity vs. Wavelength

0 2 4 6 8 10 12 14 16 18

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Intensity (W-m-2)

Ph

oto

cu

rre

nt

(µA

)

Power law fit

α = 0.72 (<1)Trap states exists,

depends on generation of electron-hole pairs,

trapping and recombination of

charge carriers in the device.

Photocurrent vs. Intensity curve

As Power increases, Photocurrent increases because of enhancement of electron-hole

pair density..!

0 10 20 30 40 50

0

20

40

60

80

100

120

140

160

1.74x109 W-m

-2

7.38x108 W-m

-2

3.74x108 W-m

-2

Ph

oto

cu

rre

nt

(nA

)

Position (µm)

11

Iph generated at contact region

depends on width of

depletion region and lowering of barrier height

upon illumination.

On approaching the centre of the microline, Iph

decreases, implies characteristic of a

Schottky Photocurrent (SPC)

behavior

Photocurrent at different spots of device with varying intensity

Intensity

(W-m-2)

Iph at centre (nA) Average Iph at contact

(nA)

Fraction of photocurrent

1.74x109 76 125 0.61

7.38x108 33 83 0.40

3.74x108 19 67 0.28

0 10 20 30 40 50

0

20

40

60

80

100

120

140

160

1.74x109 W-m

-2

7.38x108 W-m

-2

3.74x108 W-m

-2

Ph

oto

cu

rre

nt

(nA

)

Position (µm)

COMSOL Simulation

Module Used Semiconductor module coupled with electromagnetic wave

module in frequency domain

Physics we would like to address using COMSOL …??

Partial suspension of Si microline reduces the recombination pathways and increases the carrier

lifetime which ultimately increases the responsivity of detector..!

COMSOL Simulation: Geometry used

Cross-sectional image of device geometry of partially suspended Si microline used for simulation. The width Wof the SiO2 under-layer is a variable parameter in the

simulation

Surface plot of the carrier Recombination rate per unit volume in partially suspended Si microline with

different width

COMSOL Simulation: Results

COMSOL Simulation: Results

Surface plot of the carrier Recombination rate per unit volume in partially suspended Si microline with W=350 nm in sharp anisotropic etch profile and

lateral etch profile

(a) Dependence of the enhancement of photocurrent (over dark current) and (b) carrier

recombination rate Rnon the width W of the underlying SiO2 layer that supports the microline

Conclusion

16

➢ We have developed a simple and useful technique for the fabrication of Silicon microlines from SOI wafer using a top-

down method.

➢ The Responsivity is at least an order higher than commercially available bulk Si detectors in the same spectral

range.

➢ The current in the device is controlled by trap states.

➢ Si microlines, which are partially suspended, prevent recombination of carriers during transit, thereby elongating

its lifetime and has been validated by using COMSOL simulation.

Thank You !!

Prof. Arup K. Raychaudhuri

Ms. Shaili Sett

Acknowledgement

Dr. Achintya Singha