Transparent Conducting Oxide Electro-Optic Modulators: A...

http://photonics.ee.auth.gr

G. Sinatkas1, D. C. Zografopoulos2, A. Pitilakis1, R. Beccherelli2, E. E. Kriezis1

1Dept. of Electrical & Computer Engineering, Aristotle University of Thessaloniki, Greece

2Istituto per la Microelettronica e Microsistemi, Consiglio Nazionalle delle Ricerche (CNR-IMM), Rome, Italy

18th European Conference on Integrated Optics 18 – 20 May, 2016 – Warsaw, Poland

SESSION #6 - ACTIVE DEVICES

Transparent Conducting Oxide Electro-Optic Modulators:

A Comprehensive Study based on the Drift-Diffusion Semiconductor Model

ECIO 2016 – Warsaw • Georgios Sinatkas, Dept. of ECE, AUTH – Greece

Presentation Outline

2

Introduction

Transparent Conducting Oxides (TCOs)

TCO-based Electro-Optic Modulation

Motivation & Main Objectives

Multiphysics Modeling Framework

Solid-State Physics

Electromagnetic Modeling

SOI-based TCO Modulators

Silicon Rib Platform

Silicon Slot Platform

Summary & Conclusions


Introduction

Transparent Conducting Oxide Electro-Optic Modulation

3


Transparent Conducting Oxides

Introduction

4

Material n (cm-3) λp (μm)

Ag 5.86x1022 0.14

ITO 6.17x1020 1.55

n-InSb 4.00x1018 14.0



Introduction

5

Material n (cm-3) λp (μm)

Ag 5.86x1022 0.14

ITO 6.17x1020 1.55

n-InSb 4.00x1018 14.0


ITO @ 1.55 μm


Introduction

6


Principle of TCO-based Electro-Optic Modulation

Switch between states of low and high mode loss by modulating the free-carrier concentration in the TCO

ENZ Effect: The guided modes polarized normally to the TCO layer suffer from increased losses (discontinuity of the electric field normal component)

ITO @ 1.55 μm


Introduction

7




ENZ Effect: The guided modes polarized normally to the TCO layer suffer from increased losses (discontinuity of the electric field normal component) Low loss (ON state) Dielectric region, non = 1019 cm-3

ITO @ 1.55 μm


Introduction

8

ON




ENZ Effect: The guided modes polarized normally to the TCO layer suffer from increased losses (discontinuity of the electric field normal component) Low loss (ON state) Dielectric region, non = 1019 cm-3 High Loss (OFF state) ENZ region, noff ~ 6x1020 cm-3

ITO @ 1.55 μm


Introduction

9

ON

OFF




ENZ Effect: The guided modes polarized normally to the TCO layer suffer from increased losses (discontinuity of the electric field normal component) Low loss (ON state) Dielectric region, non = 1019 cm-3 High Loss (OFF state) ENZ region, noff ~ 6x1020 cm-3

ITO @ 1.55 μm


Introduction

10

How can we control the TCO free-carrier concentration after fabrication? Using an external electric field attract/repel carriers forming accumulation/depletion regions

ON

OFF


Motivation

Addressable and versatile TCO properties

Simplifications in describing semiconductor physics employing averaging or approximate techniques

Inaccuracy in bridging the gap between solid-state and wave physics

Understudied silicon-photonic platforms comprising TCOs

Main Objectives

Establish a comprehensive, multiphysics modeling framework for TCO-based devices

Design novel, high-performing, SOI-based TCO modulators comprising ITO

Motivation & Main Objectives

Introduction

11



Solid-State & Wave Physics

12


13

Schematic Diagram



14

Schematic Diagram



15

Schematic Diagram



16

Schematic Diagram


Finite Element Method (FEM) Platform– COMSOL Multiphysics® Software


Semiconductor Materials: Drift-Diffusion (DD) Model

Charge Conservation Law (Poisson Equation)

Current Conservation Laws

17

Solid-State Physics






DD Current Expressions

18

Solid-State Physics


Fermi-Dirac Carrier Distributions Essential for degenerately doped semiconductors such as TCOs



Insulating Materials Laplace Equation




19

Solid-State Physics





Insulating Materials Laplace Equation

Boundary Conditions Semiconductor/Insulator Interfaces

Voltage Source Contacts




20

Solid-State Physics



Ideal Ohmic


21

The n-Si/HfO2/ITO Junction



22



ND, Si = 1018 cm-3 increase

silicon conductivity

ND, ITO = 1019 cm-3 maintain

low wave losses (low loss ITO

dielectric behavior)

Positive bias Accumulation Layers

Negative bias Depletion Layers


23



ND, Si = 1018 cm-3 increase

silicon conductivity

ND, ITO = 1019 cm-3 maintain

low wave losses (low loss ITO

dielectric behavior)

Positive bias Accumulation Layers

Negative bias Depletion Layers

Va > Vth = 3V ENZ region

EO Modulation: Toggle between 0 & >3V

nEZ = 6.17x1020

Vth = 3V


24

Schematic Diagram


Finite Element Method (FEM) Platform

Model for ITO Drude-like permittivity expression

[Kulkarni & Knickerbocker, J. Vac. Sci. Technol. A, 1996]

Model for Silicon [Soref & Bennett, IEEE J. Quantum Electron., 1987] [Nedeljkovic et al., IEEE Photon. J., 2011]


25

Schematic Diagram


Finite Element Method (FEM) Platform



Si-rib & Si-slot Platforms

26


SOI Platforms

27

Silicon Rib (Si-rib)



SOI Platforms

28




SOI Platforms

29




SOI Platforms

30



Silicon Slot (Si-slot)


SOI Platforms – Degrees of Freedom

31



Silicon Slot (Si-slot)

Electrostatic Study

Rib Silicon access for biasing

5nm HfO2 layer

1018 cm-3 n-Si doping

Wave Study

1019 cm-3 ITO doping

Geometry Effect

Eigenmode Solver


Guided Modes & Polarization

32

TE

IL 0.02 dB/μm

IL 0.01 dB/μm

TM


Si-slot Pros & Cons

Confinement Field Enhancement Fabrication

Challenges

ND,ITO=1019 cm-3

Performance

Silicon Rib TM < Silicon Rib TE < Silicon Slot


Geometric Effect – Primary & Secondary Parameters

33


Si-Rib TE

Primary w

Secondary h

Si-Rib TM

Primary h

Secondary w

Si-Slot

Primary w, g

Secondary h

w

w

g

h

Only the field components normal to ITO undergo the ENZ effect

TE

TM


Two-Step Design Algorithm

34

Step 1

1D Cut Line Studies (Parallel to Polarization Vector)

Primary Parameters

Step 2

2D Cross-Sectional Studies (Fixed Primary Parameters) Secondary Parameters


Modulation Performance Evaluation Criteria


TE mode – 1D: Silicon width, w

35

w




36

w


Ex Horizontal Cut



37

1D Electrostatics

w




38

1D Electrostatics

w 1D Eigenmode Solver


|Ex|



39

1D Electrostatics

w 1D Eigenmode Solver

0 V

5 V

Vth


|Ex|



40

1D Electrostatics

w w

1D Eigenmode Solver

ER 0.40 dB/μm

w = 180nm


IL 0.03 dB/μm

Same Trend

|Ex|


TE mode – 2D: Silicon height, h

41




2D Electrostatics


180nm


2D Eigenmode Solver


43

2D Electrostatics


180nm


2D Eigenmode Solver


44

2D Electrostatics


180nm


2D Eigenmode Solver


45

2D Electrostatics


180nm

ER 0.30 dB/μm

IL 0.03 dB/μm

Voff = 4V

h = 220nm

Complies with standard TE Si-photonic

waveguide dimensions (e.g. 400x220nm2)


TM mode – 1D: Silicon height, h

46

1D Electrostatics

h

1D Eigenmode Solver

Vth

ER 0.30 dB/μm

h = 200nm


IL 0.03 dB/μm

Same Trend

|Ey|

h

200nm


2D Eigenmode Solver

TM mode – 2D: Silicon width, w

47

2D Electrostatics


w

ER 0.25 dB/μm

IL 0.02 dB/μm

Increasing w values:

Low IL maintenance

Higher ER Shorter device lengths, L

Weak effect on energy consumption

Voff = 4V

w = 400nm

Reduced device length

200nm

w


Silicon slot – 1D: Silicon width, w

48


w




Ex Horizontal Cut

w



50

1D Electrostatics


w



1D Electrostatics

1D Eigenmode Solver


|Ex|

w



52

1D Electrostatics

1D Eigenmode Solver

0 V

5 V

Vth


|Ex|

w



53

1D Electrostatics

1D Eigenmode Solver

w

ER 1.60 dB/μm

w = 180nm


IL 0.01 dB/μm

Same Trend

|Ex|

w


Silicon slot – 1D: Slot width, g

54

1D Electrostatics

1D Eigenmode Solver

ER 1.60 dB/μm

w = 180nm


IL 0.01 dB/μm

Opposite Trend!

g

g = 20nm

g

20nm

Vth


Silicon Slot – 2D: Silicon height, h

55


Fix w=180nm & g=20nm 180nm

20nm



56

2D Electrostatics


180nm

20nm


2D Eigenmode Solver


57

2D Electrostatics


ER 1.10 dB/μm

IL 0.01 dB/μm

Increasing h values:

No impact on IL

Higher ER Shorter device lengths, L

Weak effect on energy consumption

Voff = 4V

h = 240nm

Minimal energy consumption

180nm

20nm


Si-rib & Si-slot – Design & Performance Comparison

58

SOI Platforms



Tunable NIR properties by free-carrier concentration changes

Promising for applications in photonics (properties altering from dielectric to metallic)

ENZ effect in the NIR Electro-Optic modulators


Rigorous model developed instead of approximate techniques by integrating Solid-State physics and

Wave physics on a unified FEM platform

SOI-based TCO Electro-Optic Modulators

Si-rib Waveguide engineered for weak waveguiding (FoM not applicable)

TE-mode 0.30 (0.40) dB/μm ER & 0.03 dB/μm IL [0.11 & 0.045 dB/μm, Vasudev et al., Opt. Express, 2013]

TM-mode 0.25 (0.30) dB/μm ER & 0.02 dB/μm IL

Si-slot Waveguide engineered for strong waveguiding (FoM applicable)

Slot mode 1.10 (1.60) dB/μm ER & 0.01 dB/μm IL

What about bandwidth?

10% – 90% rise-time estimated around 2ps bandwidth exceeds 100GHz!

59

Summary & Conclusions


60

Thank you!

[email protected]



G. Sinatkas1, D. C. Zografopoulos2, A. Pitilakis1, R. Beccherelli2, E. E. Kriezis1

1Dept. of Electrical & Computer Engineering, Aristotle University of Thessaloniki, Greece

2Istituto per la Microelettronica e Microsistemi, Consiglio Nazionalle delle Ricerche (CNR-IMM), Rome, Italy

Transparent Conducting Oxide Electro-Optic Modulators:

A Comprehensive Study based on the Drift-Diffusion Semiconductor Model

Backup Material


62

Temporal Response – Silicon Slot

tr ~ 2ps 10%

90%

Backup Material

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