Electrical Characteristics of Silicon-On-Insulator

(SOI) Phase Modulator

Hanim Abdul Razak, Hazura Haroon, Mardiana Bidin, Wan Maisarah Mukhtar, ZulAtfyi Fauzan Mohammed

Napiah and P.Susthitha Menon

Institute of Microengineering and Nanoelectronics (IMEN)

Universiti Kebangsaan Malaysia (UKM)

43600 UKM Bangi, Selangor, Malaysia

Email: hanim@utem.edu.my

Abstract- This paper highlights the study of carrier injection

effect on silicon-on-insulator waveguide with trapezoidal cross

section structure. The n-p-n structure will be employed to

study the device performance in terms of modulation efficiency

and absorption loss. The characterization of the proposed

device will be carried out by 2D Silvaco CAD software under

different applied voltages. The device is designed to be

operated at 1.55µm optical wavelength with single mode

behavior. The injection of free carriers into the guiding region

changes the refractive index and the modelling has been

carried out by Atlas from Silvaco to determine the electrical

characteristics. From the simulation, the change of the

refractive index, !" from which we can estimate the phase

shift, !# and the device length that is required for phase shift,

$% are reported. It is predicted that the performance of

trapezoidal cross section waveguide is better than conventional

rib waveguide.

I. INTRODUCTION

Over the years, photonic circuits have been established as

a promising platform for realizing densely integrated optics.

To date, a variety of photonic components that emit, split,

couple, guide and detect light on a chip have been

demonstrated both on silicon and related materials [1-2].

Silicon microelectronic chip for photonic networks offer the

opportunity to overcome the limitations of power and

bandwidth in traditional microprocessor interconnects.

Another reason of the selection of silicon in integrated

optics is due to its moderate performance at low cost.

Silicon exhibits losses <0.1dB/cm in the infrared (1.3-1.5

um) region and hence potential exists for the fabrication of

active and passive silicon devices at these wavelengths [3].

In 1986, Soref and Bennett proposed and fabricated the

optical waveguides in silicon. The devices were epitaxially

grown doped silicon-on-silicon [4]. Silicon high-speed

waveguide-integrated electro-optic modulator is one of the

critical devices for on-chip optical networks. The device

converts data from electrical domain to the optical domain

[5]. Most studies for high speed modulation method in Si or

Si based device are based on free carrier concentration

variations (injection or depletion of free carriers) which are

responsible for local refractive index variations and then

phase modulation of a guided wave traveling through the

active region [6-9]. A change in the refractive

index/absorption can be achieved by injection or depletion

of both electron and holes into the intrinsic region of a

silicon p-i-n diode.

II. THEORY

Soref and Bennet [10] quantified the changes that they had

identified from the literature for both changes in refractive

index and in absorption. The following equations are widely

used in order to evaluate changes due to injection or

depletion of carriers in silicon and hence are utilized in this

research:

!"#$0 %#&'((#)*+

!"#!"e + !"h=-[8.8 x 10-22!$e + 8.5 x 10 -18 (!$h)0.8

]

(1)

!%#!%e + !%h = 8.5 x 10-18!$e + 6.0 x 10-18

!$h (2)

where:

,-e

,-

is changes in refractive index resulting from change in

free electron carrier concentrations

h

,.

is changes in refractive index resulting from change in

free hole concentrations

e

,.

is changes in absorption resulting from change in free

electron carrier concentrations

h is changes in absorption resulting from change in free

hole carrier concentrations.

From (1) and (2), it is shown that the number of injected

carriers is inversely proportional with the change in

refractive index.

Electrical simulations have been performed to the active

region of the phase modulator. The device is operated by

applying an external electrical signal to the electrodes. By

observing the doping concentrations variations at the

waveguide centre, it enables us to work out the refractive

index change for a specific forward bias voltage and

therefore the resulted phase shift, !& of the device, in which

the relationship can be summarized as [11]:

!&'#'()!"*+, (3)

where L /0# "12# 32-4"1#56# "12#*57839"5:'#;5:#<-radian phase

RSM2011 Proc., 2011, Kota Kinabalu, Malaysia

978-1-61284-846-4/11/$26.00 ©2011 IEEE 377

shift, the estimated length will be:

L)'#',+(-" (4)

From eq. (2), (4), we can calculate the absorption loss of the

proposed designs as follows[11,12];

%) = 10 log (exp[-(!%.'*)]) (5)

where L) is the length in the z direction to obtain ) phase

shift modulation. Meanwhile the modulation efficiency can

be predicted by V)L), where V) /0#"12#=53"942#"5#9>1/2=2#9#<#

phase shift and L< is the length required to obtain ) phase

shift. The lower the modulation efficiency, the higher

efficient the device can be achieved.

III. DEVICE STRUCTURE

Fig. 1 illustrates the n+p

+n

+ optical phase modulator. The

trapezoidal waveguide was formed by wet etching process

with slope of 54.74º. Meanwhile, Fig. 2 depicts the cross

section of rib n-p-n silicon modulator.

Fig. 1. Phase modulator with trapezoidal cross section waveguide

Fig. 2. Phase modulator with rib waveguide

The P+

type region is implanted with Boron, meanwhile the

N+

type region is implanted with phosphorus. The values of

concentrations for both were 5x1018

cm-3

. Both structures

have a background doping concentrations of 1x 1014

cm-3

.

Voltages of 0.75, 0.80, 0.85, 0.90, 0.95 and 1.00 Vare

applied to the structures and the simulated results are

plotted. The simulated parameters are depicted in Table 1.

TABLE 1

SIMULATION PARAMETERS

Si refractive index 3.475

Si background carrier conc(cm-3 1x10) 14

? 2x10p-6

@ 2x10n-6

Temperature (K) 300

Hole conc. of p+(cm-3 5x10) 18

Hole conc. of n+(cm-3 5x10) 18

IV. RESULTS AND DISCUSSION

Fig. 3 and Fig. 4 show the effect of voltage (V) to the

refractive index change, A- and absorption loss (dBcm-1

),

respectively. Meanwhile, Fig. 5 illustrates the graphs of

length, BC<D and modulation efficiency, BE<C<D against

voltage (V) for both trapezoidal and rib waveguide.

Fig. 3. Refractive index change against voltage

Fig. 4. Absorption loss against voltage

Fig. 5. Length, modulation efficiency against voltage

0.75µm

RSM2011 Proc., 2011, Kota Kinabalu, Malaysia

378

In general, the refractive index change of the trapezoidal

phase modulator is higher than the rib phase modulator by

12.5%. The higher refractive index change gained by the

trapezoidal phase modulator results in shorter estimated

device and become more efficient device. Nevertheless, the

loss experienced by the trapezoidal phase modulator was

higher than the rib phase modulator due to existence of more

holes and electrons in the device.

In the fabrication process, the trapezoidal cross section

waveguide can be acquired by wet etching process while the

rib waveguide can only be formed by dry etching process.

Results of this work show that the trapezoidal cross section

waveguide can be an alternative to the conventional rib

waveguide in the development of SOI phase modulator.

Future work will focus on implementation of the SOI phase

modulator with trapezoidal cross section waveguide into the

real fabrication for intensity modulator development.

ACKNOWLEDGMENT

The authors would like to thank Universiti Teknikal

Malaysia Melaka (UTeM) for the support. The Ministry of

Higher Education of Malaysia and Universiti Kebangsaan

Malaysia (UKM) is gratefully acknowledged for the grant

under industrial project-2011-015 and UKM-OUP-NBT-27-

119/2011.

REFERENCES

[1] P. S. Menon, K. Kandiah, A. A. Ehsan and S. Shaari, “The

development of a new responsivity prediction model for In(0.53)Ga(0.47)As interdigitated lateral PIN photodiode, Journal of

OpticalCommunications,” vol. 30, no. 1, pp. 2-6. 2009.

[2] P. S. Menon, K. Kandiah, A. A. Ehsan and S. Shaari, 2008. An interdigitated diffusion-based In(0.53)Ga(0.47)As lateral PIN

photodiode. Proceedings of SPIE - The International Society for

Optical Engineering 6838, art. no. 68380C. [3] P.D. Hewitt and G.T. Reed, “ Improving the response of optical phase

modulators in SOI by computer simulation,” Journal of Lightwave

Technology, vol. 18, no.3, pp. 443-450, 2000.[4] R.A. Soref and J.P. Lorenzo, “All-silicon active and passive guided-

F9=2#>5*G5-2-"0#65:#$#%#&'H#9-7#&'I#J*KL#IEEE Journal of Quantum

Electronics, vol. QE-22, no.6, pp.873-879, 1986. [5] K. Preston, S. Manipatruni, A. Gondarenko, C.B. Poitras and M.

Lipson, “Deposited silicon high-speed integrated electro-optic

modulator,” Optics Express, vol. 17, no.7, pp.5118-5124, 2009. [6] D. Marris-Morini, X. Le Roux, L. Vivien, E. Cassan, D. Pascal, M.

Halbwax, S. Main and S. Laval, “Optical modulation by carrier

depletion in a silicon PIN diode,” Optics Express, vol. 14, no. 22, pp.10838-10843, 2006.

[7] Mardiana B., Hazura H., Hanim A.R, Shaari, S., and Abdullah, H.,

“Various Doping Concentration Effect on Silicon-on-Insulator (SOI) Phase Modulator”, Proc. of 1st

[8] Hanim A.R, Hazura H., Mardiana B., and Menon, P.S., “Free Carrier Absorption Loss of p-i-n and NPN silicon phase modulator at

$%&'HJ*# 9-7# $%&'((# J*K# Proc. of 2010 IEEE International

Conference on Semiconductor and Electronics, 28-30 June 2010, Malaysia, pp. 351-354.

. International Conference on

Photonics, 5-7 July 2010, Malaysia, pp.1-3.

[9] S. Shaari, , A.R Hanim, B. Mardiana, H. Hazura, and P.S Menon,

“Modeling and analysis of lateral doping region translation variation on optical modulator performance”, Proc. of the 4th

[10] S. Libertino and A. Sciuto, Electro-Optical Modulators in Silicon, Springer Series in Optical Sciences, 2006.

Asian Physics

Symposium, 23 Dec. 2010, Indonesia, pp. 297-300.

[11] Andrea Irace, Giuseppe Coppola, Mario Iodice, and Antonello Cutolo,

“A high-efficiency silicon optoelectronic modulator based on a Bragg Mirror and integrated in a low-loss silicon on insulator waveguide”,

Proc. Of SPIE, Vol. 3847, 1999.

[10] D. W Zheng, B. T Smith, J. Dong, and Asghari, “ On the effective lifetime of a silicon p-i-n diode optical modulator”, Semicond. Sci.

Technol., Vol. 23, 2008.

[11] R.A Soref and B. R Bennett, “Kramers-Kronig analysis of electro-optical switching in silicon”, Proc. SPIE, vol.704, pp.1622-1631,

2004.

[12] C. Angulos Barrios, V.R Almeida, R.Panepucci,and M. Lipson, “Electrooptic modulation of silicon-on-Insulator submicronmeter-size

waveguide devices”, J. Lightwave Technol., vol.21, no.10, pp. 2332-

2339, 2003.

RSM2011 Proc., 2011, Kota Kinabalu, Malaysia

379