Colin HumphreysDepartment of

MaterialsUniversity of

Cambridge, UK

How gallium nitride can save energy, purify water, be used in cancer therapy and improve our health!

XIII Brazilian Materials Research Society Meeting 2014Joao Pessoa, Brazil28 September – 2 October 2014

Acknowledgements

• Cambridge: R A Oliver, M J Kappers, D Zhu, A Phillips, E Thrush, J S Barnard

• Manchester: P Dawson, S Hammersley, D Parris, T J Badcock

• Oxford: D Saxey, A Cerezo, G D W Smith• Imago Scientific Instruments (now

Cameca): P Clifton, D Larson, R Ulfig, T F Kelly

• Brazil in the future?

US DoE Report on GaN LEDs• By 2025 Solid-State Lighting using

GaN-based LEDs could reduce the global amount of electricity used for lighting by 50%

• No other consumer of electricity has such a large energy-savings potential as LED lighting

The Potential of GaN LED Lighting

• Lighting uses one-fifth of all electricity• LEDs are poised to reduce this figure by

50% (d) • Lighting will then use 10% of all electricity • Save 10% of all electricity• In UK, save $3000 million pa electricity

costs– cf Dilnot report on elderly care -- $2500 million

pa•

LEDs• Light emitting diodes (d)• Made from solids (e.g. GaN) that emit

light• LEDs last 100,000 hours (electronics

50,000) • Light bulbs (incandescent) last 1,000

hours• LEDs fail by slow intensity decrease• Light bulbs fail totally and suddenly

Numbers of light bulbs• The average house has:

– 45 light bulbs in the USA– 30 light bulbs in Canada– 25 light bulbs in the UK

• Average use 4 hours/day• If 50 Watt incandescent• Average UK house uses 25x4x50 = 5 kWh electricity per day for

lighting

Wall-plug Efficiency of light sources

Incandescent light bulb = 5%

(15 lm/W)

Fluorescent tube (long) = 25%

(80 lm/W)

Fluorescent lamp (CFL) = 20%

(60 lm/W)

White LEDs (350 mA) = 30%

(100 lm/W)

White LEDs (in lab) = 60%

(200 lm/W)

Sodium lamp (high P) = 40%

(130 lm/W)

Global Impact of LED Lighting

• 560 full-size power plants could close (or not build new)– If 40% of worlds lighting was

LEDs

Why is Gallium Nitride such an Exciting Material?

Main light-emitting semiconductors

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the heart of technology

Growth Facilities at Cambridge

Thomas Swan Scientific 6 × 2”Close Coupled Showerhead (CCS) MOCVD Reactor

AIXTRON 6x2” CCS MOCVD Reactor

•Extensive in-situ characterisation capability on both reactors

• Pyrometry• Wafer bow• Reflectance

8” and 12” MOCVDReactor recently funded

InGaN/GaN quantum-well LED

How to make white light

LED Applications• Billions already used in:

– Displays– Mobile phone backlighting– Flashlights– Interior lighting in cars, aircraft, buses, etc– Front bike lights

• Recent major markets:– Backlighting for LCD screens (in TVs,

computers)– External car lights: headlamps, daytime

running lights

Fremont Street, Las Vegas

1500 feet long

Largest LED display in world – picture continuallychanges

Initial display contained 2.1 million filament light bulbs

New display contains 12.5 million LEDs

16

The InGaN LED mystery• High densities of threading dislocations

(~109 cm-2)

• Threading dislocations act as non-radiative recombination centres (e.g. by cathodoluminescence)

• For efficient light emission, dislocation densities should be less than ~103 cm-2 in GaAs and other semiconductors.

• Some microstructural feature of the InGaN QW appears to localise the carriers preventing them reaching the dislocations.

17

Potential causes of carrier localisation

Uniform quantum well

Compositional variations

Well width variations

Carriers confined in one dimension

Carriers confined in three dimensions?

Indium clusters?

Random alloy fluctuations?

18

Narukawa et al. APL 70 981 (1997)

In-rich clusters: evidence from TEM?

HRTEM image lattice parameter mapping

Cho et al. APL 79 2594 (2001)

Gerthsen et al. PSS (a) 177 145 (2000)

Cheng et al. APL 84 2507 (2004)

Potin et al. J. Crystal Growth 262 145 (2004)

Strain Contrast

Strain contrast and LPMs from our InGaN QWs – “clustering”?

5 nm

InGaN

GaN

GaN

InGaN

GaN

GaN

GaN00020002 dd 1·0

01·02 1·0

41·06 1·08 1·10

(0·00) (0·13) (0·25) (0·37) (0·45) (0·59)(Approximate indium fraction, x)

5 nm

electron beam induced strain

electron beam induced strain

T. M. Smeeton et al., phys. stat. sol (b) 240, p297 (2003)

20

APT imaging of QWs

• Can we detect clustering in blue-emitting and green-emitting QWs?

Green emitting sample

Indium Gallium10 nm

21

Well width variations

• Strong piezoelectric fields in strained QWs

• Monolayer interfacial steps could localise carriers at 300 K– see e.g. Graham et al. (JAP 97 (2005)

103508) which suggests a localisation energy of ca. 50 meV for monolayer steps.

– Some evidence from STEM

STEM-HAADF

FEI Titan image of InGaN/GaN QWs

Recent Growth

5 nm

23

Interface roughness: Isosurfaces

Average rms roughness (upper) = 0.34 nm

Average rms roughness (lower) = 0.18 nm

5 nm

5 nm

Green emitting sample, x = 4%

Upper

Lower

24

A quantum well with a step: use TEM/APT data as input to theory

• A single monolayer island is added to the random quantum well – as seen in the atom probe and TEM data.

nm

25

Electron and hole wavefunctions (1)

• The electron and hole are most likely to be found where the square of the wavefunction is highest.

• The electron and hole are localised at different positions.

• Localisation length: electron ~4 nm, hole ~1 nm

Electron Hole

26

Key points from modelling• Carrier diffusion to dislocations is

prevented even in the absence of gross indium clusters.

• Even in a random InGaN quantum well, areas of higher indium content exist.

• Random alloy fluctuations localise the holes (localisation energy about 20 meV)

• Monolayer steps localise the electrons (localisation energy about 28 meV)

• TEM/APT explain high GaN LED efficiency

What is preventing widespread use of LED lighting in homes and offices?• Problem: Cost

• Low-power LEDs cheap: a few cents

• High-power LEDs for lighting: expensive

• Philips 60 W equivalent LED costs $10

Solving the GaN LED cost problem• All commercial GaN LEDs grown on small-

diameter (2”, 3”, 4”) sapphire or SiC wafers• Reduce costs: grow on large-diameter Si

wafers • Will substantially reduce cost of LEDs • Will enable LED lighting in homes and offices• In UK, save $3 billion pa electricity costs• Close (or not build) 8 large power stations• My group (Dandan Zhu) pioneered growth of

GaN LEDs on 6-inch Silicon

Problems with GaN growth on 6-inch Si• Cracking

–GaN under tensile stress when cooling from growth temperature

• High dislocation density

GaN cracking and stress management• 54% difference in thermal expansion

coefficient between GaN and Si• On cooling from growth temperature GaN

in tension and cracks (GaN in compression on sapphire)

• Can pattern Si substrate to guide the cracks

• We grow on unpatterned substrates and introduce compressive stress layers (AlGaN) to compensate the tensile stress on cooling

UNIVERSITY OF

CAMBRIDGE

Stress control

Control of tensile stress and associated cracking using AlGaN buffer layers

Tensile strain compensation

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Concave

Convex

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Si substrate

AlN

AlGaN

Si-doped GaN

After cooling:

Mg-doped GaN

Si

In-situ annealing:

Si

AlNAlGaN

GaN

AlGaN and GaN growth:AlN growth:

SiAlN

Si

AlNAlGaN

Si-dopedGaN5x InGaN-GaN MQW

Mg-doped GaN

QW and p-GaN growth:

LayTec Epicurve®TT

Curvature during growth of an LED on Si

Problem: High Dislocation Density• 17% lattice mismatch between Si

and GaN, hence high dislocation density

• Reduces the efficiency of the LEDs

• Must use dislocation reduction methods, for example, in-situ SiN mask

Dislocations

GaN

mask

Dislocation reduction using SiNx mask: epitaxial lateral overgrowth (ELOG)

Threading Dislocation Reduction

WBDF TEM image, g = <11-20>, edge + mixed TDs

1st

2nd

3rd

4th2 µm

WBDF TEM image, g = <11-20>, edge + mixed TDs

1st

2nd

3rd

4th2 µm

Scandium nitride interlayer -- dislocation density reduced to ~ 107 cm-2

Multiple SiNx interlayers -- dislocation density reduced from 5 x 109 to 5 x107 cm-2

Processed full LED on 6-inch Si wafer

Full 6” wafer processed on a classical III/V line (in 2009)

Commercial Exploitation• My group set up CamGaN (2010) and Intellec

(2011) to exploit Cambridge GaN on 6” Si LEDs • Plessey acquired both companies in February

2012. Hired 3 post-docs from my group• Plessey is now manufacturing low-cost GaN on

6” Si LEDs at their factory in Plymouth, UK • The first manufacture of LEDs in the UK• Will enable low-cost GaN LED lighting in

homes/offices• Why not have GaN-on-Si production in Brazil?

Phosphor-free LEDs• Eliminate phosphors from GaN LEDs

• White light from mixing blue + green + yellow + red (BGYR) LEDs

–Lighting then use only 5% of all electricity

–LEDs then save 15% of all electricity from power stations. Save UK $5 billion pa

GaN power electronics

• GaN has low power consumption for both lighting and electronics

• Power electronics: replace Si devices by GaN – grow GaN on large-area Si to reduce the cost– Si power electronics for chargers for laptops,

mobile phones, solar cells, electric cars, etc– GaN power electronics 40% more efficient

than Si– Can save 10% of electricity

Energy savings from GaN• Gallium nitride is a key material for saving:

• 10% electricity (low-cost LED lighting )• Extra 5% electricity (LED lighting with RYGB LEDs) • 10% electricity (replacing Si-based power

electronics)

• 25% of total electricity use can be saved by GaN –a key material for energy efficiency (plus 25% CO2 savings)

Purifying Water with Deep-UV Light • 270 nm radiation damages nucleic

acids in DNA, RNA• Bacteria, viruses, unicellular

organisms, cannot reproduce• Fungi, mosquito larvae, etc.,

killed• 270 nm radiation purifies water

AlGaN LEDs for Water Purification• Emission at 270 nm achievable

now• BUT efficiency is much too low for

flowing water – state-of-the-art is about 1%

• Improving efficiency is a major materials challenge

• If we can achieve we will help to solve the major problem in the developing world and save millions of lives

Li-Fi• Major problem: Huge increase in Wi-Fi demand

– 32% pa -- Soon exceed RF (radio frequency) capacity• Use light as carrier instead of radio frequencies• Use LEDs for Wi-Fi, videos, data communication• Light and RF to work together (aircraft,

hospitals)• Li-Fi in every room in house, office, street lights• Li-Fi communication

– LED traffic light to LED car headlamp/daytime RL

Dynamic colour LED lighting• White light from RGYB LEDs• Can do today – expensive – the “green

gap” problem – more research needed• Have tuneable white lighting

– Lighting remote control– Computer controlled circadian corrected

lighting– Mimic sunlight– Mimic daytime variation of natural

lighting

Dynamic lighting for our health• Increasing evidence that circadian

disruption affects health– Hospital patients– Cancer (also LEDs for monitoring X-ray

radiotherapy)– Eating disorders– Depression– Immune deficiencies– Sleepless nights– Productivity at work/school

Overcoming Jet-lag• Don’t sit in hotel room with CFLs

• Walk around the block in natural light

• Resets our internal body clock – Circadian clock – internal biological 24-

hour clock

Summary• Gallium nitride is a key material for saving:• 25% electricity (Lighting and Power

Electronics)• 25% Carbon emissions from power stations

• Millions of lives (UV LEDs for purifying

water)• Solving the coming wi-fi problem with li-fi• Improving cancer therapy• Improving our health, learning and

productivity• Helping manufacturing and job creation

Thank you

• Obrigado

0

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0.5%

Source: Internatinal Energy Agency

2003

Energy consumption: 14 TW

World population: 0.65x1010

•By 2050 the world population will be 1x1010 = minimum need for extra 10 Terawatts per year.

• Biomass is mainly firewood – first to run out

300

200

100

01860 1900 1940 1980 2020 2060 2100

Millions of barrels per day (oil equivalent)

Energy – the 21st century problem

Recent world energy changes

• Demand -- the world’s energy demands are growing more steeply now than at any time in the last 200 years (or ever) – Driven by increase in world’s population– Driven by more cars, planes, mobile

phones, etc.• Supply -- larger than expected shale

gas and oil– New technology enables earlier/deeper

extraction• Still an energy gap in the world• Energy efficiency must be the top

priority

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FEI Titan 80-300

Some Electron Microscopes at Cambridge

Philips CM300FEI Tecnai F20

JEOL 4000 EX

52

Modelling• APT/TEM data used as an input for

theoretical model• A potential energy landscape for a GaN/InGaN quantum well

(QW) has been calculated which includes the following terms:– Band offsets– Spontaneous polarization– Piezoelectric field– Deformation potential

• Both the piezoelectric and deformation terms depend on the strain caused by the random distribution of In atoms.

• A Green's function (continuum) approach used to calculate this local strain.

• A finite difference approach used to solve the Schrödinger equation.

LEDs of all Colours • Made possible by new designed material

– gallium nitride (GaN)InN GaN AlN

Bandgap 0.7eV 3.4eV6.2eVLight IR Near-UV Deep-UV

• Inx Ga1-x N. Vary x. Get light of any colour

• Strong atomic bonds

Cannot grow GaN directly on Si• GaN reacts with Si to form a Ga-Si alloy

and “meltback etching”

• Hence grow an AlN nucleation layer on the Si– Quality of this layer very important for LED

quality– Must optimise– Quality of AlN/Si interface largely determines

the quality of the AlN nucleation layer – hence study

HAADF Imaging - Cs corrected Titan at CCEM

2 nm<110> Si

<11-20> AlN

SixNy ?amorphous layer

Acquisition conditions : Conv. semi-angle : 22 mradDetec. inner angle : ~ 50 mradacquisition time: 25 µs/pixel Image size: 1024 x 1024 pixels

Resolution : ~ 1 Å

Spectrum Imaging - Elemental maps

Si-L23

Al-L23

N-K

HAADF1 nm

SixNy layer from elemental mapsAbsence of detectable O

Interpretation

How can we explain the presence of a continuous amorphous SixNy layer together with an (almost) perfect epitaxial orientation relationship of AlN with the Si substrate ?

Si

Si clean surface

Aluminum

Si

TMA predose

AlN

Si

AlN growthsharp interface

AlNSixNy layer

Si

Growth continuesSi/AlN

interdiffusion

AlN/Si : structure @ low temperature

Si <110>

AlN <11-20>

Al-face polarity

hex

cub

crystallographically sharp interface

AlN buffer grown by MOVPE @ 735 °C

Radtke et al, APL, 2010 and 2012

MOVPE growth of GaN-on-Si LED structure

Total epi thickness ~2.5 μm

p-AlGaN EBL (~20 nm)

Si-doped GaN, ~1.3 µm

Si substrate

AlN ~200 nm

AlGaN buffer, ~0.8 µm

Mg-doped GaN, ~90 nm

SiNx IL

InGaN/GaN MQW

nucleation and growth: T~1000°C

Laytec Epicurve: wafer curvature

AIXTRON Argus: temperature profiler

AIXTRON CCS vertical reactor

New research areas• GaN real-time dose monitoring for

cancer therapy• An implantable GaN neural probe• Optimising light for our health

• Our Cambridge GaN group contains about 30 people

Dynamic lighting for our learning• School experiment – absence and

performance

• Productivity at work

• Incentive for schools and employers (and hospitals and homes)– Need cool white (bluish-white) light for

best exam performance!

Outline of talk• Beyond Graphene: low-dimensional

systems based on graphene and III-Nitrides

• Some recent developments in microscopy– High spatial resolution in imaging– High energy resolution in EELS

• How GaN can help to solve the world’s energy, water, wi-fi, cancer and other problems

• Commercialising low-cost GaN LEDs

Imaging single Si atom impurities in graphene at 60 keV

4-fold: Si substitutes for 2 C atomsCourtesy Wu Zhou

3-fold: Si substitutes for a single C atomCourtesy Matt Chisholm

Can we study the bonding environment of a single atom?

Si atoms in graphene can occupy two different sites (UltraSTEM100 images).

J. Lee, et al. Nature Commun. (2013), courtesy J. Lee and J.-C. Idrobo

Dancing Si atoms

HERMES - Energy resolution

In spectra recorded in 10 s, the energy resolution broadens to 10-12 meV.(It broadens further when we open the slit to get more beam current.)

Nion HERMES at Rutgers U., March 2014, 60 keV, 10 msec acquisition

Vibrational spectra of different materials

LO phonon180 150

C-H stretch 365

Data recorded with ASU HERMES at 60 keV, typically in 10 sec per spectrum.

Most materials with light elements (Z<8) give distinct phonon peaks at ∆E >100 meV. Hydrogen is readily identifiable.

137

collected in “aloof” mode, with probe ~5 nm outside sample

180