ICSE2010 Proc. 2010, Melaka, Malaysia

Free Carrier Absorption Loss on p-i-n and n-p-n

Hanim A.R., Hazura H., Mardiana B. and Menon, P.S, Member, IEEE Institute of Microengineering and Nanoelectronics (IMEN)

Universiti Kebangsaan Malaysia (UKM)

43600 UKM Bangi, Selangor, Malaysia

Email: hanim@utem.edu.my

Abstract- The paper reports on the free carrier absorption loss

associated with silicon phase modulator. Two structures are

compared: p-i-n and n-p-n structure. The simulations are realized

utilizing the 2-D semiconductor simulation package SILVACO.

Simulations predict that both structures operate more efficiently at

1.3 µm in terms of free carrier absorption loss. At 1.3 µm, the

calculated free carrier absorption loss for p-i-n structure is 0.1149

dB, while n-p-n structure suffers 0.3956 dB of loss. Structure-wise,

n-p-n silicon phase modulator experience more free carrier

absorption loss compared to p-i-n structure due to extra doping

contact.

I. INTRODUCTION

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 in part to moderate performance at low cost. Silicon

exhibits losses <0.1dB/cm in the infrared (1.3-1.5 um) and

hence potential exists for the fabrication of active and passive

silicon devices at these wavelengths [1].

In 1986, Soref and Bennett first proposed and fabricated the

optical waveguides in silicon. The devices were epitaxially

grown doped silicon-on-silicon [2].

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 [3].

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 [4]. 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 [1].

In this paper, a single mode, two- terminal p-i-n carrier

depletion phase modulator on a silicon substrate is simulated

and compared to the three- terminal n-p-n for wavelength of

1.55µm and 1.3µm respectively. The two-dimensional (2-D)

semiconductor simulation package SILVACO has been

utilized for this purpose. The analyses for absorption loss are

here reported.

II. ABSORPTION LOSS

According to Reed [5], there are three sources of losses in

an optical waveguide namely absorption, scattering, and

radiation. However, for the purpose of this study, absorption

loss is of the primary focus.

Band edge absorption (or interband absorption) and free

carrier absorption are the two main potential sources of

absorption loss. Interband absorption is due to the absorption

of photons with energy greater than the band gap to excite

electrons from the valence band to the conduction band. Thus,

in order to avoid interband absorption, a wavelength selection

must be longer than the absorption edge wavelength of the

material. Since the band edge wavelength of silicon is 1.1um,

for wavelength less than 1.1µm, silicon absorbs very strongly.

The absorption of pure silicon at 1.15µm exhibits loss of

2.83dB/cm while at 1.52µm, the loss is reduced to

0.004dB/cm. Thus, a suitable selection of wavelength will

ensure negligible band edge absorption of semiconductor

waveguide. Therefore, the band edge absorption is negligible

at 1.3 and 1.55µm.

The significant absorption in semiconductor waveguides is

therefore the free carrier absorption. The real and imaginary

refractive indices will be affected by the concentration of free

carriers.

The Drude-Lorenz equations generally described the

e h)

to the a

3 2 2 c3 0 e/m*e

2µe h/ m*h

2µh] (1)

where

e electron charge

c light vacuum

0 vacuum permittivity

h in the vacuum

n refractive index of intrinsic silicon

m*e effective mass of electron

m*h effective mass of electron

µe electron mobilities

µh hole mobilities

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ICSE2010 Proc. 2010, Melaka, Malaysia

Soref and Bennet [6] 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 report:

0

e + h = -[8.8 x 10-22 e + 8.5 x 10 -18 h)0.8] (2)

e h = 8.5 x 10-18 e + 6.0 x 10-18 h (3)

0

e + h = -[6.2 x 10-22 e + 6.0 x 10 -18 h)0.8] (4)

e h = 6.0 x 10-18

e + 4.0 x 10-18

h (5)

where:

e change in refractive index resulting

from change in free electron carrier

concentrations

h change in refractive index resulting

from change in free hole

concentrations

e change in absorption resulting from

change in free electron carrier

concentrations

h change in absorption resulting from

change in free hole carrier

concentrations.

From (2) and (3), it is shown that as the number of injected

carriers decreases, the change in refractive index increases. At

is also shown from (3) and (5) that the change of absorption is

influenced by the change in the silicon refractive index.

(in dB) in

the length of L is given by the following relationship [6,7].

= 10 log {exp[- ]} (6)

where L

III. DEVICE STRUCTURE

In this paper, the free carrier absorption loss of a p-i-n lateral

optical phase modulator is compared to an n-p-n optical phase

modulator. Both are integrated into silicon waveguide. The

cross section of the active of the device is shown in Fig. 1 and

Fig. 2.

7 µm

30 µm

y

Si substrate

n+ p+

metal metal

V

4 µm

4 µm

x

z

Fig. 1. Cross section of p-i-n silicon modulator

7 µmSi substrate

n+

p+

metal metal

V

4 µm

4 µm

30 µm

x

yz

n+

metal

Fig. 2. Cross section of n-p-n silicon modulator

The P+ type region is implanted with 5x10

18 cm

-3 Boron

concentrations while the N+ type region is phosphorus

implanted region with a concentration of 5x1018

cm-3

. Both

structures have a background doping concentrations of 1x

1014

. For both structure, the depth of the implanted region is

N+

+ region. The rib

height and width for both structure is chosen in order to have a

single mode behavior. The rib structure is designed to have 4

µm in height and 4 µm in width. With the chosen doping

concentrations for both structures, the distance of the doped

second structure.

TABLE I

SIMULATION PARAMETERS

Si refractive index 3.475

Si background carrier conc(cm-3) 1x1014

p 2x10-6

n 2x10-6

Temperature (K) 300

Hole conc. of p+(cm-3) 5x1018

Hole conc. of n+(cm-3) 5x1018

Optical power (mW) 1.0

Voltages of 0V to -8V are applied to the structures and the

simulated results are plotted. The simulated parameters are

depicted in Table I.

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ICSE2010 Proc. 2010, Melaka, Malaysia

IV. RESULTS AND DISCUSSION

respectively.

Fig. 3. Change of absorption against applied voltage for p-i-n structure

Voltage(V)

Fig. 4. Change of absorption against applied voltage for n-p-n structure

The simulation results are in good agreement with [2]. For

also can be noted that the more negative the applied voltage is,

-

wavelength were calculated and the free carrier absorption

loss has been determined as in Table II and Table III for both

structure using (6).

TABLE II FREE CARRIER ABSORPTION LOSS FOR p-i-n STRUCTURE

(µm) -1) (dB)

1.3 3.4000e-4

0.7785 0.1149

1.4766e-3

1.55 5.1000e-4

0.6551 0.1451

2.21490e-3

TABLE III

FREE CARRIER ABSORPTION LOSS FOR n-p-n STRUCTURE

-1) (dB)

1.3 5.4399e-4

1.6748 0.3956

2.3625e-3

1.55 7.7999e-4

1.4091 0.4773

3.3875e-3

For both structures, the free carrier absorption loss at

1.55µm is higher than the loss at 1.3µm. Therefore, it can be

deduced that both structures operate more efficiently at 1.3µm.

The losses are less than 1dB/cm, proving that the material loss

in silicon waveguides is very low [2].

The free carrier absorption loss for n-p-n structure is greater

than p-i-n structure. N-p-n structure experiences a trade-off

between modulation efficiency and free carrier absorption

loss. Even though a three- terminal devices offer more

efficient carrier injection/depletion but the main drawback is

extra optical attenuation occurs due to the doping contact at

the rib top [8].

ACKNOWLEDGMENT

The authors would like to thank UTEM for the support and

staffs of Photonic Technology Lab for guidance and co-

operation.

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ICSE2010 Proc. 2010, Melaka, Malaysia

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