Raman Scattering Matthew Halsall

School of Electrical and Electronic Engineering

The University of Manchester

Outline What is the Raman effect?

What are molecular/lattice vibrations and when do they

demonstrate the Raman effect?

Resonance and surface enhancement

Instrumental issues

The Silicon Raman laser

Materials characterisation examples

C.V.Raman Effect discovered by C.V.

He observed Shifted lines

in the solar spectrum when

light was passed through

certain crystals

Won Nobel Prize in 1930

(first Indian Scientist to do

so)

Inelastic scattering of photons

Experimentally a small shift in energy is observed between the photons of incident and scattered light

Energy difference corresponds to some excitation in the solid

Raman Scattering

ψn+1

ψn Eex

hλi

hλs

Eex= hλi-hλs

Classical Picture In the Classical picture, Light scattering is caused by the

modulation of the susceptibility of a solid by a molecular vibration.

Consider a simple molecule that has no permanent dipole (for simplicity e.g. H2). A light wave with an electric field amplitude F0 and frequency ν0 is incident on it and induces a dipole moment p given by

p= αF0 Cos (ν0 t)

Where αis the polarizability of the molecule

Classical contd.. Now suppose this molecule has a vibrational

“stretch mode” and this has a frequency νk.

Furthermore we suppose that this causes the polarizability to be modulated by an amount αk at the frequency νk

We can now write:-

α=α0 + αk F0 Cos( νk t )

And

p = α0 F0 Cos(ν0 t) +

F0 αk Cos( νk t ) Cos(ν0t)

Using Trig identities we can write p as

p= α0 F0 Cos(ν0 t)+

F0 αk {Cos((ν0 +νk) t ) +Cos((ν0 -νk) t )}

nk =1

2p

K

m/ 2

Classical contd.. In Classical Electromagnetism an oscillating

dipole will radiate EM radiation with a power

given by

Our three terms for polarizability gives us three

scattered components

I =n0

4

12 pe0 c3p

2

n0

4

12 pe0 c3a0

2F0

2Cos2 (n0t) Raleigh

(n0 +nk )4

12 pe0 c3ak

2F0

2Cos2 ((n0 +nk )t) Anti-Stokes Raman

(n0 -nk )4

12 pe0 c3ak

2F0

2Cos2 ((n0 -nk )t) Stokes Raman

Light Scattering… Raleigh Scattering is elastic scattering of light

Frequency of scattered light same as incident

Fundamental ν4 dependence of scattering strength

Explains why the sky is blue!

Raman terms are inelastic scattering

Frequency is either shifter up or down by vibration

“Stokes” is shifted down “Anti-Stokes” is shifted up

typ. 104-107 WEAKER than Raleigh scattering

In visible ν0≈1015, νk≈1012 so Raman spectral line weak

and close in frequency to Raleigh line, this has

implications for experimentation

Raman activity

Consider a simple molecule CO2,There are two ways (“modes”) in which it can vibrate as illustrated below

Called symmetric and anti-symmetric stretch modes

These modes have different νk

Consider αk, if the polarizability is linearly dependent on bond length then for the asymmetric stretch the change in p caused by the change in one bond length is undone by the other- so αk must be zero for this mode

Whereas in the symmetric stretch both bonds increase/decrease in length at the same time and αk has a finite value

Infrared activity Infrared absorption is another technique to look at

molecular vibrations

For a vibrational mode to absorb light it must have a dipole moment associated with the motion

In the case just discussed it should be evident that the asymetric mode will have a dipole moment, whereas the symmetric will not

In General Raman scattering and infrared absorption measure different modes of the molecule and are highly complementary

Molecular/Lattice vibrations More formerly.. Consider a molecule with a total energy E, we can write

Hooke’s Law in this system as

The coefficients D are force constants linking a displacement of atom n in the μcartesian coordinate to that of atom n’ in the ν coordinate- This represents a 3Nx3N matrix where N is the number of atoms in the molecule

The vibrations of this system are found by solving the matrix equation of motion

Where M is a diagonal matrix of the atomic masses and D is the “Dynamic matrix” made up of the Dμν(n,n’) coefficients

This is an eigenvalue equation and diagonalisation of D leads to 3N modes each with a 3N element eigenvector and an associated frequency ω

Dmn (n, ¢n ) =¶2E

¶m(n) ¶n( ¢n )

Mw2e = De

Vibrational Modes 3 Modes correspond to rigid displacements and are

zero frequency

Others (2 or 3 depending on the molecules symmetry)

are rotations

Others may be degenerate because of symmetry.

In general though a Raman spectra of a molecule is

complex and highly specific to that molecule

A note on Units… Spectroscopists cling to old units!

Raman spectra often given in inverse cm or cm-1

For light it is the number of wavelengths that fit in a cm!

i.e. 0.01/λ

As it is the reciprocal of wavelength it is a measure of

energy or frequency of a photon/phonon

To convert

1 cm-1 = 30 GHz

8.025cm-1= 1meV

Examples

In effect these are like fingerprints allowing the identification

of molecules in samples

Widely used in industry and forensic science as such

Selection Rules II In the classical model Raman is caused by a susceptibility

modulated by the vibration

Now we know there may be many such modes we can write the (Stokes) Raman scattering intensity as

Where the sum is over each mode k

Susceptibility is a second Rank tensor and the coefficients αk are also.

As we have seen not all modes are Raman active

I =(n0 -nk )4

12 pe0 c3ak

2F0

2Cos2((n0 -nk)t)k

å

Raman Tensor Using group theory one can deduce the zero or non-zero

nature of the elements of the “Raman Tensor” Rk for the kth

mode, the spontanous scattering efficiency by all modes is

then given by

Where es and ep are the electric field vectors of the stokes

scattered and pump photons respectively

Sµ esRkep

k

å

Raman Tensor Simple example for symmetric stretch of H2 molecule the tensor is

This implies the Scattering only occurs if the electric field of the Laser is aligned along the z axis (the bond direction) and the scattered light has the same polarization

This analysis only tells us if the scattering is possible nothing about its strength (the value of α)

Obviously in chemical samples the molecules will usually be randomly orientated but there may be a difference between the spectrum collected with aligned or crossed polar- chemists can use this to tell the symmetry of a particular mode.

0 0 0

0 0 0

0 0 1

Crystals To solve the vibrational problem for crystals use translational

symmetry. Start with the same Equation for the dynamical matrix linking atoms n and n’ in the primitive unit cell

Where the sum is over all primitive cells and q is a wavevector for the associated phonon mode

Gives rise to similar eigenvalue equation as in molecule case but is a function of q now

This time we have 3N modes, including 3 modes that in the limit of small q tend to zero frequency (acoustic modes)

Dmn (n, ¢n,q) =¶2E

¶m(n) ¶n( ¢n )R

å exp(-iqR)

Phonons and dispersion The plot of the vibrational spectrum of a crystal with q is

known as the phonon dispersion curve

Can be measured by neutron scattering or calculated theoretically

Raman scattering from phonons scattering is widely used to characterise semiconductors

Strain

Composition

Temperature

impurities

Momentum Selection Rule One final selection Rule in Crystals

Momentum must be conserved….

Momentum of a phonon is h/λ

Momentum of photon is h/λ

BUT λ for light is ≈500nm a phonon can have λas small as 1nm! In practice to balance the momentum taken by the scattered photon only very long wavelength phonons are involved (near q=o)

The Raman spectrum and phonon dispersion of crystalline Silicon is shown below

q photon

Example GaAs GaAs has a fcc unit cell with 2 atoms in it this gives 3 optical phonons and 3

acoustic

The optical and acoustic modes are three fold degenerate, the acoustic modes are

zero frequency at the zone centre

The long range coulomb field splits the optic mode into a doubly degenerate

Transverse branch and single longitudinal branch

The Raman tensors for the three modes are (with their polarizations)

If the experimental geometry is z(y,x)-z with the cubic axes being x,y,z which mode will

be raman active?

Using the third tensor the optic mode will be active in this geometry. Since this tensor

has z polarisation, parallel to the incident laser, this must be a longitudinal mode with

the phonon propagating along z

0 0 0

0 0 -k

0 k 0

æ

è

ççç

ö

ø

÷÷÷(x)

0 0 -k

0 0 0

k 0 0

æ

è

ççç

ö

ø

÷÷÷(y)

0 -k 0

k 0 0

0 0 0

æ

è

ççç

ö

ø

÷÷÷(z)

Raman spectrum of GaAs

200 400 600 800 1000

0

20000

40000

60000

80000

100000

120000 LO phonon x 0.1

TO-phonon overtones

2TO TO+LO 2LO

Raman Shift cm-1

Inte

nsity (

A.U

.)

Compare to amorphous materials

Si3N4 is amorphous (as deposited)

No long range order

Raman effect weak

Phonon spectra very broad

Has implications for Raman on-a-chip

Wada et al Journal of non-crystalline solids 43 (1981) 7-15

Quantum Theory.. Classical theory predicts that the stokes and antistokes

scattering intensity ratio is given by

Experimentally it is found to depend strongly on temperature as well

Only Quantum theory can account for this effect

In Quantum theory Raman scattering is viewed as a process that occurs via an intermediate state and results in the annihilation/creation of quanta of vibrational energy

I s

I as

=u0 -uk( )

4

u0 +uk( )4

Quantum theory cont Solving the Time dependent Schrodinger equation for the

transition rate of the molecule + field (to second order) a value for the stokes transition polarizability αfi (proportional to the transition rate)

This matrix element represents the sum over an infinity of quantum states of the system r which mediate the scattering.

The scattering is assumed to proceed by virtual states -this is best illustrated by a figure

a(rs ) fi =1

{f pr r r ps i

wri -w0 - Gr

+f pr r r ps i

wri +w0 + Gr

}r

å

Quantum Picture Raman scattering occurs when a photon is absorbed into a

virtual electronic state then re-emitted in a state with one

less/more vibrational quanta

n,m

3

2

1

Real electronic states

(r)

Virtual states

Vibrational states

Raleigh scattering Anti-stokes Raman Stokes Raman

ν

Stokes/Anti-stokes In the quantum picture the probability of a transition occuring

depends on the occupancy of the vibrational state, which is governed by Bose-Einstein statistics. Hence:-

This depends on temperature and agrees with experiment

This equation can be used to measure the sample temperature remotely

I s

I as

=n0 -nk( )

4

n0 +nk( )4

ehcnk /kt

Raman Cross section How much light is scattered in Raman scattering?

Generally for molecular materials it is easy to define a “Raman cross section” σ assuming randomly orientated molecules we can write the intensity of the Raman scattering Is as

dσ/dΩ is the differential cross section, where Ω is the solid angle and Ωc is the angle collected over. N is the density of molecules and L the interaction length

The values of dσ/dΩ are tabulated for particular vibrations of particular molecules for a given wavelength of excitation

I s =ds

dW

æ

èç

ö

ø÷

Wc

ò dW I 0NL

Example Raman intensity Quick example, for Benzene, 992cm-1 “ring breathing”

mode, differential cross section = 3.65x10-29 cm2/mol sr

at 488nm. N=6.7x1021 mols/cm2, assume L=1cm and

that we collect the full 4π radians

So after traversing 1cm, 3 photons in a million undergo

raman scattering

I s

I p

= 4p ´3.65´10-29 ´ 6.7´1021 ´1= 0.000003

Raleigh versus Raman In general for molecular materials Raleigh scattering may be

typ. 102-104 times stronger than Raman scattering

However, for crystalline materials most modes are forbidden by the momentum selection rule so this ratio can be 106-107

Also many materials absorb/reflect light, e.g. metals and semiconductors. Here the interaction length may be <1μm so the Raman intensity might be a factor of 104 weaker!

So experimentally we need to detect very weak, sharp raman lines close spectrally to intense raleigh/reflected light.

Resonant Raman scattering Consider the Raman cross-section in the Quantum model

What happens if the virtual state actually corresponds to a real quantum state of the system? i.e. ωri=ω0

The first part of this expression can become very large.

This is known at the resonant effect and leads to an increase in the Raman scattering by a factor of up to 106-107 when the laser wavelength corresponds to an electronic transition in the sample

We will see an example in carbon nanotubes later

a(rs ) fi =1

{f pr r r ps i

wri -w0 - Gr

+f pr r r ps i

wri +w0 + Gr

}r

å

Surface Enhanced Raman

Scattering (SERS)

Discovered in 1974 that molecules absorbed onto certain metal surface showed huge enhancements in Raman cross-sections

Enhancements of the order of 106-1014 reported, allowing the obervation of scattering from few or single molecules

Selection rules also relaxed

Raman spectrum of liquid 2-

mercaptoethanol (below) and SERS

spectrum monolayer of same formed

on roughened silver

SERS mechanism Still controversial!

Two effects certainly contribute

Surface plasmon interaction

Greatly intensifies E-field near curved metal surface

As both incoming and outgoing raman photon are enhanced the effect is proportional to E4

Chemical effects

EM alone cannot explain why some molecules show more enhancement than others

Suggestion is that the transfer of charge to metal causes energy levels to be shifted from UV to visible allowing resonant raman scattering to occur

Raman Scattering II –

Instrumentaton and Examples

Instrumentation The challenge in Raman spectroscopy experimentally

is to reject the Raleigh light and detect the much

weaker Raman signal.

Typical figures-

Raman frequency of silicon =520cm-1 (65meV)

Suppose we use light with wavelength 514nm=2.412eV

The photon energy for the stokes raman = 2.412-

0.065=2.347eV = 528.nm

It is also a factor of 104-106 times weaker than the raleigh

scattering

Can we just use a diffraction

grating?

If we use a single diffraction grating to disperse the light we will not be able to see the Raman signal as the scattered light from the raleigh scattering will drown the signal

Even for a perfect diffraction grating the diffracted line from the Rayleigh scattering will drown out the Raman line

Two standard solutions

Use an dielectric filter to remove the Raleigh line

Use a multiple grating spectrometer

Triple Spectrometer

Uses three gratings, the first two filter out the raleigh line

The last disperses the light onto a detector (CCD camera)

Advantages

Can use any light source we want

Disadvasntage

Expensive and complex

Poor through-put

Edge/Notch filters These filters have very strong tranmission dependences on wavelengh

Can be bought of the shelf for a particular laser wavelength

The raleigh line is reflected by the filter

The Raman spectrum can then be measured by a standard spectrometer arrangement

Filter only works for one wavelength

Raman Microscope A raman micrscope uses a microscope to focus the laser source

The light is collected using a microscope objective and analysed using a Raman system

The use of a pinhole in the image plane restricts the volume measured to c. 1μm3

Commercially available and widely used in industry/forensic science etc

Raman Si-Laser Raman scattering in silicon (SOI) waveguides is strong effect due

to strong confinement

Due to crystalline nature of silicon, Raman scattering is narrow linewidth (cf SiO2)

Gain can be estimated using the Einstein relation and known Raman cross section, S, to be

Using known values we get that the gain in a Si waveguide will be of the order of 76 cm/GW

In a silicon wire 200nmx200nm, 1GW/cm2 ≈ 400mW!

gR =8pc2wp

ws

4n2(N +1)DwS

Two photon absorption Main loss mechanism is two particle absorption (TPA)

Process where two sub-band gap photons are absorbed and an electron/hole pair is created. The free carriers (ΔN) then gives rise to free carrier absorption and loss

Two solutions a lateral pn junction to remove the carriers

Pulsed operation

aFCA =1.45´10-17(l /1.55)2 DN

DN = b I p

2t eff / (2hn )

Simple Pulsed excitation

Device Characteristics

Boyraz and Jalali, Opt Express, 12 (2004) 5269

Silicon Nanocrystals

Form thermal oxide on silicon

Implant 7 At% Si at 60keV

Anneal 1000 oC to form Nanocrystals for 1-1000s

Si 60keV

SiO2 (1 μm)

Si substrate

Cs3+ , Nd3+ Er 3+

200keV

0 100 200 300 400 5000

1

23

4

56

7

Depth

SiO2 +NC+RE

Si substrate

At %

Si

Anneal Anneal

X-TEM study

Si-nanocrystals imaged in DF mode using a Philips CM-12 TEM at 120kV and beam

aligned with the <220> zone axis, after Iacona et al, JAP 2000:

• Obtain accurate and repeatable statistical data-set

• Statistical analysis: > 200 Si-NCs counted for each anneal condition, (tA, TA)

• Size distribution described by an asymmetric Gaussian (log-normal)

100nm

100nm

100nm

5s

10s

50s TA = 1100oC

Raman Scattering from small crystallites

In bulk semiconductors, only allowed optical modes are those at the Brillouin

zone (BZ) centre, corresponding to q = 0 due to conservation of momentum.

In SC nanocrystals, spatial confinement leads to a spread in the number of

allowed modes incl. those for q 0 and additional Raman spectra (or a

change in the Raman line-shape) may be observed.

Line-shape can then be described generally by the integral of the standard

Lorentzian function, weighted by the Fourier coefficients of the phonon

confinement function, C(q) over the BZ, as:

Where: (q) is the phonon dispersion relation in the bulk SC and 0 is the

natural line-width

1

2 2 3

0

1( )

BZ

I C q q d q

Accuracy of the model then depends on the choice of an appropriate function

for (q) and C(q)

We chose the analytical form:

For the phonon dispersion relation as it accurately reproduces the dispersion

of optical modes along [001] in bulk Si, after Tripathi et al, i.e:

With 0 (=521cm-1) the phonon frequency at q = 0. q has the units of 2/a,

with a (= 0.543nm) being the lattice spacing in bulk c-Si

2

0 1 0.2q q

0 0.5 1400

500

600

q( )

q

optical modes are represented by a lognormal distribution. as determined

from the TEM images

With and being the only fitting parameters required. Then the Raman line-

shape is described by:

2

2

2

log, , exp

2

qdC q d

11

2 2

00

1, , , , ( )I d C q d q dq

460 480 500 520 5400

0.2

0.4

0.6

0.8

Raman Shift (cm-1)

Norm

ali

zed

Inte

nsit

y (

a.u

)

0.85

0

S4y

10.5P f 1.34 0.331( )

S2y 0.15

9 P f 1.83 0.333( ) 0.15

S3y 0.3

13.5P f 3.5 0.21( ) 0.3

S1y 0.5

16.5P f 4.6 0.22( ) 0.5

3.5 1077

P f 100 0.01( )

10 P f 2 0.3( ) 0.5

12 P f 1.9 0.25( ) 0.3

550450 S4x f 15 S2x f 13 S3x f 11 S1x f 8 f f 16.5 f 18

Si-NC size distribution predicted by Raman

k d 0.452 0.19( )

1.25k d 0.6217 0.2( )

2.7 0.9 k d 1.2306 0.21( ) 2.7 0.1 k d 0.452 0.19( )( )

3.75 0.9 k d 1.5018 0.22( ) 3.75 0.1 k d 0.6217 0.2( )

d d d d S1TEMx

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10

Si-NC diameter (nm)

No

rmali

zed

den

sit

y (

a.u

) TEM

Raman

1100oC,

10s0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10

Si-NC diameter (nm)

No

rma

lize

d d

en

sit

y (

a.u

) TEM

Raman

1050oC,

120s

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10

Si-NC diameter (nm)

No

rma

lize

d d

en

sit

y (

a.u

) TEM

Raman 1

Raman 2

1100oC,

60s0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10

Si-NC diameter (nm)

No

rma

lize

d d

en

sit

y (

a.u

)

TEM

Raman 1

Raman 2

1100oC,

300s1100oC, 300s

1100oC, 60s

1100oC, 10s

1050oC, 120s

Raman Shift cm-1

Fit to Real Raman spectra

I.F.Crowe, Halsall et al Journal of Applied Physics 107, no. 4 (2010)

Graphene

• Single monolayer of Carbon

• First isolated by Andre Geim and Kostya Novoselov in Manchester in

2001

• Remarkable properties

• Zero gap semiconductor

• Mobility >50000 cm2/Vs

• Young modulus in the Tpa range

• Exhibits Room temperature quantum hall effect

• Absorption coefficient = πα where α is the fine structure constant

Raman Scattering from “Graphane” Zone centre optical phonon mode “G” at 1580 cm -1

Defect activate phonon mode at 1350 cm-1

Defect activated mode is enhanced by exposure to

atomic hydrogen

Process is reversible indicating defect is attached hydrogen

atoms

Process is now known to be unstable

-Raman of Materials at high pressures

Diamond anvil cell

Laser excitation Micro-Raman system

Mao-Bell type diamond anvil cell

Diamonds 250 micron culets enable

pressures of up to 100GPa (1 Mbar)

Pressure medium Ethanol/Methanol

or Silicone oil

Stainless steel gaskets, 75-100

micron gasket hole

Raman is good for remote measurements- In our lab we use it to

study the effects of pressure on new materials

Does Graphene stick to Si?

How strong is the adhesion of graphene to silicon?

Load sample of graphene on silicon into Diamond anvil

cell apparatus

• Upto 2GPa (20 kBar) Graphene

strains governed by Silicon

strains

• Above 2GPa the graphene

strains less and is thus slipping

over the silicon

Carbon nanotubes Formed from a Single layer of graphite- Graphene sheet

Define vector Cn =(n,m) unit cells displacement

Cut region of graphene and roll so that Cn forms the equator of the tube

Semiconducting tubes have a band gap ≈2.5/D eV, Diameter D=√Cn.Cn

Cn=(8,0)

Carbon nanotubes CNTs display raman active “breathing modes” corresponding to

the radial expansion/contraction of the tube- the frequency of these is radius dependent

When a laser is used which is resonant with these modes strong scattering from these mode is observed only from nanotubes with the right band gap

100 150 200 250 300 3500

10000

20000

30000

40000

50000

60000

Silicon

(17,2)

(9,7)

(10,3)

Inte

nsity (

a.u

.)

Raman Wavenumber (cm-1)

Elaser=1.5eV

CNTs under pressure Two pressure regimes identifiable

Low pressure, CNTs collapse

symetrically

High pressure, CNTs flatten,

Graphite like behaviour

J.Sandler, M.P.Halsall et al Phys. Rev. B. 67 (2003) art. No. 035417

PRL 92 (2004) 095501 (theory)

Effects of pressure on GaN

GaN is an important Semiconductor used in Blue and

White LEDs

It normally has a wurtzite (hexagonal) crystal structure

Also has a metastable cubic phase which may bave

better properties for Green LEDs

Study of pressure phases important..

“Excimer Lift-off” process (S.J.Henley, University of

Bristol)

193nmArF Excimer

Sapphire

GaN

epoxy

Glass

GaN interface region

evaporated

300 400 500 600 700 800 900 1000

rcm- 1

0

1000

2000

3000

4000

5000

6000

7000GaN Epilayer

300 400 500 600 700 800 900 1000

rcm- 1

0

20000

40000

60000

80000GaN Epilayer

Raman scattering from GaN

12 phonon modes with irreducible

representations

2E2+2B1+2A1+2E1

Raman active modes in

backscattering geometry in

range 200-800 cm-1

A1 (LO) mode at 736cm-1

E2 (high) mode at 567cm-1

E1 (TO) mode at 560cm-1

A1 (TO) mode at 532cm-1

Z||c axis, y and x to plane

x(y-)x

z(y-)z

A1(TO) E1(TO) E2 A1(LO)

A1(LO)

E2

Phonon frequencies dependence on

pressure

Optical Phonons E2 (high) mode

Pressure GPa LO-TO splitting

10 20 30 40 50

600

700

800

900

10 20 30 40 50

550

600

650

700

750

800

GaN epilayer 3 GPa (30kbar)

62 GPa- Rocksalt structure

Rocksalt phase transition

300 400 500 600 700 800 900 1000 0

Raman shift Rcm-1

69GPa

61GPa

51GPa

49GPa

46GPa

42GPa

0GPa

Pseudopotential Calculation

‘PWSCF’ code (www.pwscf.org)

Exchange/correlation energy calculated using Perdew-

Zunger parameterisation of linear density approximation

Pseudopotentials in Troullier-Martin separable form

Non-linear core correction for Ga semi-core 3d levels

potential expanded in plane waves with cut-off 75-80Ryd

Can calculate full band structure/phonon dispersion

curves/total enthalpy as a function of pressure

Predicted phase transition pressure

0 10 20 30 40 50 60 70

-25

-24.9

-24.8

-24.7

-24.6

Enthalpy v Pressure

En

tha

lpy/D

iato

m (

Ryd

be

rgs)

Pressure GPa

LDA model Predicts wurtzite-Rocksalt phase transition 49GPa

MgO-h phase always unstable w.r.t wurtzite or rocksalt structure

wurtzite

rocksalt

MgO-h

Phonon dispersion

0

200

400

600

800

Fre

qu

en

cy

cm

-1

X L

200 400 600 800 1000

0.2

0.4

0.6

0.8

1

Frequency cm-1

Rocksalt 50 GPa

0

200

400

600

800

Wurtzite 43 GPa

Fre

qu

en

cy

cm

-1

L M K

200 400 600 800

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Frequency cm-1

Disorder activation of Raman modes

Rocksalt structure has no raman active modes

disorder activation of modes in rocksalt alloys reported for MgSxSe1-x1 and

KClxBr1-x2

300 400 500 600 700 800 900 1000

GaN Epilayer 51Gpa

Raman shift Rcm-1

Rocksalt

DOS

Data

TO(L) TO(X) LO(X) LO(L)

Ra

ma

n S

hift

1.Huang et al. Appl. Phys. Lett 67 (1995) p3611 2.Nair et al. Phys. Rev. B. 3 (1971)

p3446

45 50 5560 65 70 75

400

500

600

700

800

900

Acknowledgements

University of Manchester: Kostya Novoselov, Iain Crowe, Nic Hylton, Hang Li,

Dr. Uschi Bangert and Reza Jalili-Kashtiban

McMaster: Prof. Andy Knights, Dr. Oksana Hulko

UCL, Tony Kenyon and Maciej Wojdak

Surrey ion beam centre: Prof. Russ Gwilliam

Australian National University: Dr. Simon Ruffell

EPSRC (UK) for financial support

Canadian Institute for Photonic Innovation

Ontario Photonics Consortium

Natural Sciences and Engineering Research Council of Canada

The End