Si-based materials for photonics and electronics/Menu/general/column... · Tutorial - ADOPT 2014 1 Department of Microelectronics and Information Technology KTH, Royal ... ALD Slots

Tutorial - ADOPT 2014 1

Department of Microelectronics and Information Technology

KTH, Royal Institute of Technology

Integrated Devices and Circuits

Royal Institute of Technology (KTH)

Si-based materials for

photonics and electronics

Henry H. Radamson


Outline of this tutorial

* It begins with an introduction why photonics is merging more with electronics.

Shall we have a common roadmap of both electronics and photonics? More than Moore

* Main objective of this tutorial is to present a monolithic solution when Si photonics conqueres, in long term, the

interconnections between circuit-to-cirtuit & chip-to-chip.

How detection and lasing occur in Si-based materials

* The bandstructure of Si-based materials

* Intersubband lasing (Si-based materaisl have indirect bandgap) but transitions occurs within the conduction and

valence band states)

* Interband lasing (make Si-based materials to have direct bandgap )

* Down scaling of CMOS

Which type of strain, which design, and how to maximize strain in channel

* Future materials graphene or two dimentional crystals


IEEE Spectrum Aug. 2002

We need to develop different photonic components beside the CMOS part

Emerging of photonic and electronic world

Electronics

Data processing

Photonics

Data communication


New areas for Si photonics and electronics

Adavantages: Silicon is transparent for light above 1.1µm and has a very high refractive

index, that allows for tight light confinement in a core of about 0.1µm2 and high level of integration. Integrated photonic structures based on SOI wafers can be fabricated using existing microelectronic technology.

Drawbacks: Indirect bandgap, making light generation difficult.


Today’s technology landscape of CMOS

The landscape of the technology is now on nanostructures

The number of transistors in a chip is doubled every two years

5 http://www.legitreviews.com/article/1082/1 http://hexus.net/tech/news

Technology node evolution: non-selective growth to

selective epitaxy, substrate size, metal gate,

silicide. But 22nm node and beyond : 2D to 3D (Tri-

gate) design


The Silicon Photonics Technology Leti – STMicroelectronics, 2010

Silicon photonics roadmap


How about a roadmap for both electronics

and photonics?


Strain engineering and bandstructure of

group IV alloys


Strain and Bandgap Engineering

Lattice mismatch of Si with SiGe, SiC, SiGeC, SiSn, GeSn and GeSnSi

aSn = 0.6489, aGe= 0.5646 & aSi= 0.5431 nm (Sn/Ge~15% , Sn/Si 17% and Ge/Si ~4.2% mismatch) .

Biaxial strain


Strain engineering in group IV alloys

An alloy has different strain and bandgap

The layers can be integrated in advanced photonic device


Strain generation

- Epitaxy: Biaxial strain

(mismatched heterostructures)

-Processing: Biaxial strain

(Silicide & thermal cycling)

Stressor materials: Uniaxial strain

(nitride, SiGe & Si1-yCy)

http://www.tf.uni-kiel.de Liu et al. Appl. Phys. Lett. 87, 011110 2005


Inducing stress of > 1 GPa in pMOSFETs

Tuning SiN stress from highly tensile to compressive stress in LPCVD and PECVD.

The mechanical property of nitride layers is

determined by controlling the gas phase dissociation of Silane, Amonia and gases

in a plasma environment

These nitride layers can be grown on embedded SiGe layers.

The induced stress is additive to the induced SiGe stress.

Arghavani et al. IEEE Electron Device Letters, V27 (2006) 114


Critical thickness of strained-SiGe layers

All strained alloys e.g. SiGe or GeSn are grown in meta-stable region and they relax after a

certain critical thickness.

Radamson et al. Physica Scripta T101 (2002) 42.

X

X

No strain relaxation

is observed for

selectively grown

layers

X

Silicon Substrate

Selective epi Oxide

SiGe


- Low-temperature solid-source MBE/CVD

- Gas precursors: Ge2H6 and SnD4 (expensive and not stable for long time), SnCl4 (cheap and reliable gas source), Si2H6, Si3H8

-Ge growth 300-600 °C

- GeSn growth directly on Si or Ge at low temperatures (e.g. 250-350 °C)

Growth of Ge, GeSi, GeSn and SiGeSn materials

J. Kouvetakis et al, IEEE Photonics Journal, 2, 924, 2010 H. Radamson et al, ECS conference (2012)

A revolutionary developement in production of new gas precursors for CVD

Lower temperatures required for higher Sn content

High quality GeSn(Si) layers have been reported


GeSn alloys’ properties

J. Mathews et al. Appl. Phys. Lett. 97, 221912 (2010) 221912-1


Si Ge

Ge has nearly direct bandgap of 0.85 eV

Bandstructure of Si and Ge


Bandgap of alloys is determined by thw following components:

1. alloying

2. strain (consists of two components: hydrostatic, uniaxial)

Types: Compressive & Tensile (Ge/GeSn & Ge/GeSi)

Designs: Biaxial & Uniaxial (or 2 & 1-D strain)

Four cases can occur:

* Strain compensated (e.g. ternary systems GeSnSi & GeSiC)

* Strain relaxed (Ge or GeSi virtual buffer layer)

(alloying component but defect states within the bandgap)

* Non-strained layers (e.g. ternary systems GeSnSi )

(lattice match e.g. ternary systems GeSnSi)

* Locally strained (Ge, Si & Sn dots)

strained individual dots, relaxed dots in a buried layer

Strain and Bandgap Engineering

http://userweb.eng.gla.ac.uk/douglas.paul/SiGe/split.html


GeSn alloy and its application

Sun et al, Opt Quant Electron (2012) 44:563–573

This provides the possibility for monolithic integration of photonic devices on Si

- Possible material system for photonic

application:

- GeSn alloy system

- GeSnSi alloy system

- strained Ge on GeSn or SiGeSn for direct band

gap (via tensile strain)

- Unstrained Ge (or SiGe) on SiGeSn for

intersubband applications – quantum cascade

lasers (QCLs)

- SiSn alloy on GeSn for communication

wavelengths (1.3 mm – 1.55 mm)

Transition of indirect-bandgap to a direct bandgap with Sn content 6-8%


Decoupling of electronic structure and lattice parameter:

Direct-gap values tunable between 0.8 and 1.4 eV, in GeSiSn alloys epitaxially grown on Ge-buffered Si, as a function of the combined Si + Sn fraction x. Dots are experimental values and solid line is a fit to:

E0(X)=E0Ge+AX+BX2 , A=1.70+0.42, B=-1.62+0.96

V. R. D’Costa, et al., Phys. Rev. Lett. 102, 107403, 2009; J. Kouvetakis, et al., IEEE Photonics Journal, 2, 924, 2010.

Tunable optical gap at a fixed lattice constant - Ge1-x(Si4Sn)x


alloys can be applied as an alternative to other matched

infrared systems e.g.

SiGeSn material may have the same

lattice match but different bandgaps

Si-Ge-Sn ternary system templates for integration of III-V

compounds with Si

A lattice-matched system with tuning bandgap is ideal for multicolor detector

applications like target discrimination, gas leakage detection and enviromental

sensing.


Calculated band diagram of a Ge0.65Si0.15Sn0.20–Ge

strained layer structure showing the indirect to

direct crossover (EcG lower than EcL). Holes are

localized in the lh band due to tensile strain in Ge.

This is of interest for interband quantum well lasers,

Band-edge diagrams for lattice-

matched, SiGeSn–Ge hetero-

structures. Electrons and holes are

confined in Ge layer (type I). This is

of interest for intersubband, quantum

cascade lasers,

Laser design for Ge

G. Sun, et al., Appl. Phys. Lett., 90, 251105, 2007. S.-W. Chang, S. L. Chuang, IEEE J. Quantum Electronics. 43, 249, 2007.


Group IV materials for photonic detectors


Ge is absorbant of light

- High absorption for wavelengths of interest

- CMOS compatible

Si does not absorb/detect and can guide light (λ>1.1um, IR) .

Mario Panicca, Si Photonics, Photodector announcement, Intel devoper forum


PiN Ge detectors

Si Substrate

p-doped Si

p-doped Ge

Intrinsic Ge

n-doped Ge NiGe +

TiW/Al

Contact

NiGe +

TiW/Al

Contact

PIN Ge structure

Ge detectors for 0.86-0.6 eV


Ge/Slot-Waveguide integration Solution

Selectively grown Ge photodetectors on Si

Back coupling as waveguide integration

Waveguide

Stack

High-k

ALD Slots

Incident Light

M. M.Naiini et al, ISDRS 2013


Ge/Slot-Waveguide integration Solution

Simulated waveguide/detector coupling

3% feedback in case of perfect side wall

Negligible feedback due to roughness

0.5 μm

6.8 μm

M. M.Naiini et al, ISDRS 2013


Tensile Ge for NIR detectors

GeSn is defected but Ge cap layer has remarkably higher epi-quality

Strain-relaxed Ge1-xSnx / tensile Ge

The strain in Ge is determined by Sn content in the buffer layer.

HRRLM of a GeSn/Ge structure grown on Si substrate at 290 ºC.

A. Jamshidi,, Surface and Coatings Technology, 2013.


Photodetection (PD)– photoconductive devices

V. R. D’Costa, et al., Semicond. Sci. Technol. 24, 115006, 2009

Ge has a sharp direct gap absorption edge at 0.80 eV (1549 nm) at room

temperature, outside the range of the so-called L-band (1560–1620 nm) utilized

by ultrahigh-speed optical networks. Ge PD response at 1620 nm is just 10% of

that at 1540 nm. This can be overcome by using the relaxed Ge0.98Sn0.02 alloy.


Photodetection – p-i-n diode

J. Mathews, et al., Appl. Phys. Lett. 95, 133506, 2009

… while a p-i-n photodetector based on the same material can cover the

whole O – U range of optical telecommunication bands.

Si photonic technology is leading the way to a new generation of telecommunications


025% strain in Ge: 1.5 µm Ti & annealed @

800 C for 5 min to obtain C54 TiSi2

Liu et al. Appl. Phys. Lett. 87, 011110 2005

Ge p-i-n photodetectors


Lasing and design in group IV materials


0.25% tensile strain, the difference between Γ and L valleys is decreased to 115 meV

Ge: direct gap material at 1.7% tensile strain: Eg=~0.4 eV: λ =3000 nm

Compensate: 7.6×1019/cm3

Jifeng Liu et al. OPTICS EXPRESS, Vol. 15, No. 18, P 11273

Tensile Ge/GeSn systems

Optically pumped

lasing demonstrated


Intersubband laser in regions:

Near infrared (1-2µm) or middle infrared (2-20 µm): Si1-xGex/Si (x=25-50% where

ΔEv is in range of 205-420 meV) is for 5-20 µm laser.

Si-based Intersubband lasers

The emission wavelength depends ΔEv or ΔEc

which controls the subband energy separations.

Three- and two-coupled quatum wells (3CQW and 2CQW)

Wavefunction engineering: barrier height, selecting

well and barrier widths: adjust the subband energies and spatial overlap of wavefunctions into

the neighboring subbands. Carriers are confined in

z-direction and are unconfined in x- and y-direction.

Photon & phonon scattering in 3-subband group IV

laser pumped at subband 3. q32 & q21 are phonon momenta and ħω=phonon energy. The overlap of

wavefunctions has to be engineered that T3>T2

occurs. A desired overlap of wavefunctions 3 & 2 by adjusting the barrier width. The unpolar group IV has

advantage over III-V polar deformation potential.


p-i-p (n-i-n) first, a doped multi-layer Bragg mirror, which may be λ/4

layers of Si and ZnS, on SOI film. The active, undoped three Si CMQW

is grown on that mirror.

Near Infrared lasers

Near-infrared subband laser: deep wells (large band offset which must be above ~1.2 hw (620-1240 meV)

Proposed heterostructures: Si (QW)/ZnS (barrier), Si/SiO2, ...

R. Soref et al., Superlattices and Microstructure, Vol. 23 (1998) 427


Si1−xGex /Si cascade

ex. Si0.75Ge0:25/Si

E(hh1)= 53 meV,

E (hh2)=83 meV,

E(lh1) = 99 meV,

E(lh2)= 142 meV,

E(hh3)=152 meV.

6–8µm wavelength Si0:6Ge0:4


Intersubband lasers (QCLs)

G. Sun et al., Appl. Phys. Lett., 90, 251105, 2007

Strain-free Ge/SiGeSn system for quantum cascade lasers based on the

L-valley intersubband transitions (indirect band gap is irrelevant here,

and using L-valley instead of G may even offer some advantages).


Strained Ge interband lasers

G.-E. Chang, S.-W. Chang, S. L. Chuang, Opt. Express 17, 11246, 2009 S.-W. Chang, S. L. Chuang, IEEE J. Quantum El. 43, 249, 2007,


Strained Ge interband lasers

Surface carrier densities of the G- and L-

conduction valleys as a function of the

injected carrier density. The large density

of states of L-subbands, and four

equivalent L-valleys, lead to a significant accumulation in L-subbands.

Band line-up in Ge/SiGeSn quantum

well structure. Due to tensile strain in

the well, there is only one hole

subband, and it is LH.


Double heterostructure lasers

G. Sun et al Opt Quant Electron (2012) 44:563–573

Cooling is needed


G. Sun et al Opt Quant Electron (2012) 44:563–573

Multi-quantum well lasers


Synthesis of GeSnSi layers

Thermodynamically favorable

for Si and Sn atoms in Ge

H. Radamson et al, ECS conference (2012)

Similar behavior

for P- and B-doped

GeSn


Evolution of CMOS techology


Mark Bohr (Intel Senior Fellow) & Kaizad Mistry (Program manager), Intel’s revolutionary 22 nm technology, Intel 2011

MOSFET’s evolution

The limit is expected to be at the gate length of around 5 nm because of the too huge off-

leakage current in the entire chip

In order to suppress the off-leakage current, Tri-Gate is the most promising

solution


Tri-Gate transistors provide a

steeper sub-threshold slope that reduces leakage current

22 nm Tri-Gate transistors can operate

at lower voltage

with good performance, reducing

active power by >50%

Advantages of Tri-Gate transistors

Mark Bohr (Intel Senior Fellow) & Kaizad Mistry (Program manager), Intel’s revolutionary 22 nm technology, Intel 2011


Sub-22nm technology is 3D growth

45

Nanowires, Si/fins


Downscaling (critical parameters)

The downscaling of MOSFETs improves IDSAT, and decreases CGate and VD per generation node and when

VD is modified then the gate delay and energy-delay product are also improved.

IDSAT = saturation drive

current,

CGate = gate load,

VD = supply voltage.

when the transistor dimensions are scaled down → increased speed and lower power consumption of digital MOS circuits

Power supply voltage (Vdd), Threshold

voltage (Vt) and gate oxide thickness

(tox) vs. CMOS channel length

Danny Rittman et al., http://www.tayden.com/publications/CMOS%20Nanometer%20Designs%20Scaling%20Limited.pdf


Evolution of MOSFETs

SiGe in S/D has

been increased

constantly

Silicides


Mobility of SiGe (some data of biaxial strain)

The improvement of mobility of SiGe layers in biaxial strain is due to decrease of scattering

Mobility values in uniaxial strain is always higher than biaxial strain


Selective epitaxy growth issue (Pattern dependency)

49

SiGe layer profile (thickness, Ge & dopant content) in the openings of the same chip, chip-to-chip and wafer-to-wafer varies due to the exposed Si coverage (size, density) and architecture (oxide & nitride) of the wafer.

Exposed Si coverage in a chip [%]

What is pattern dependency?


New materials and possibilities for channel materials

Ashwin Ramasubramaniam et al, Nano Lett. 2011, 11, 1070–1075

Synthesize two graphene sheets and tune the bandgap


Tuning bandgap of Graphene

Ashwin Ramasubramaniam et al, Nano Lett. 2011, 11, 1070–1075

Graphene as channel material for MOSFETs for 10 nm node?


Single-layer MoS2 is interesting as a semiconducting analogue of graphene, which does not posses a bandgap in its

pristine form. Bandgaps up to 400 meV have been introduced by quantum-mechanical confinement.

Radisavljevic et al, Nature Nanotechnology Vol 6, (2011) 147

Transistors demonstrate current on/off ratio exceeding 1 × 108 and mobility of ~200 cm2 V−1 s−1,

comparable to the mobility achieved in thin Si films or graphene nanoribbons

Alternative materials: GeS, GeSe

New materials and possibilities for channel materials


Moore to year 2035

H. Iwai / Microelectronic Engineering xxx (2009) xxx–xxx


Merging photonics and electronics,

3D design


Detectors, modulators, …

To manufacture laser from group IV material is on-going research: GeSn is a

promising material

(Alt. III-V on Si)

Merging photonics and electronics


I. Jonak-Auer, Proc. of SPIE Vol. 8431, 843115 (2012) SPIE

New integation concept

0.35 µm CMOS

Responsivity value: 0.57A/W at a wavelength of 675nm

One extra mask: PIN is manufactured


57

3%

5% 1%

0%

8%

10%

Our solutions for pattern dependency of selective epitaxy

6%

8% 7%

5%

9%

10%

10%

10%

10%

5%

5%

5%

5%

8%

10%

10%

10%

10%

5%

5%

5%

5%

8%

7

%

7

%

7

%

7

%

7

%

7

% 10%

1) Uniform pattern distribution

2) Insert dummy features

Pattern dependency is caused by

nonuniform gas consumption in

advanced chip design Two solutions to make a uniform

gas consumption over a chip

An example of a test chip with different exposed Si coverages

This mask design will provide

uniform transistor structure


Future 3D integration

Integrated CMOS and laser

R. Soref, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 12, (2006)

3D SOI substrates will be an option for

advanced integration of photonic and electronic circuits


Si photonics has a bright future


Thank you for your attention

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Si-based materials for photonics and electronics/Menu/general/column... · Tutorial - ADOPT 2014 1 Department of Microelectronics and Information Technology KTH, Royal ... ALD Slots