View
734
Download
5
Tags:
Embed Size (px)
DESCRIPTION
Plenary lecture of the XIII SBPMat (Brazilian MRS) meeting, given on September 30th, 2014, in João Pessoa (Brazil) by Sir Colin Humphreys, Professor at University of Cambridge (U.K.).
Citation preview
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
COMPANY CONFIDENTIAL 13 April 2023
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
0 5000 10000 15000 20000
-150
-100
-50
0
50
Curv
atu
re/k
m-1
Time (s)
Concave
Convex
0 5000 10000 15000 200000
200
400
600
800
1000
1200
Pro
cess
tem
pera
ture
(T
c)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
Refle
ctance
(a.u
.)
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
5
10
15
20
25
30
35
40
45
50
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
COMPANY CONFIDENTIAL 13 April 2023
the heart of technology
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