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.).

1. How gallium nitride can save energy,
purify water, be used in cancer
therapy and improve our health!
Colin Humphreys
Department of Materials
University of Cambridge, UK
XIII Brazilian Materials Research Society Meeting 2014
Joao Pessoa, Brazil
28 September – 2 October 2014

2. 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?

3. 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

4. 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
•

5. 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

6. 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

7. 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)

8. Global Impact of LED Lighting
• 560 full-size power plants could close (or
not build new)
– If 40% of worlds lighting was LEDs

9. Why is Gallium Nitride such
an Exciting Material?

10. Main light-emitting semiconductors

11. Growth Facilities at Cambridge
Thomas Swan Scientific 6 × 2”
Close Coupled Showerhead
(CCS) MOCVD Reactor
•Extensive in-situ characterisation
capability on both reactors
• Pyrometry
• Wafer bow
• Reflectance
the heart of technology
8” and 12” MOCVD
Reactor recently funded
AIXTRON 6x2” CCS MOCVD Reactor
COMPANY CONFIDENTIAL 13 October, 2014

12. InGaN/GaN quantum-well LED

13. How to make white light

14. 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

15. Fremont Street, Las Vegas
1500 feet long
Largest LED display in
world – picture continually
changes
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.
16

17. 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. In-rich clusters: evidence from TEM?
18
Narukawa et al. APL 70 981 (1997)
Cho et al. APL 79 2594 (2001)
HRTEM image lattice parameter mapping
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

19. Strain contrast and LPMs from our InGaN QWs – “clustering”?
5 nm
GaN
InGaN
GaN
GaN
InGaN
GaN
GaN
0002 0002 d  d 1·00 1·02 1·04 1·06 1·08 1·10
(Approximate indium (0·00) (0·13) (0·25) (0·37) (0·45) (0·59)
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. 20
APT imaging of QWs
Green emitting sample 10 nm Indium Gallium
• Can we detect clustering in blue-emitting and
green-emitting QWs?

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
21
suggests a localisation energy of ca. 50 meV for monolayer
steps.
– Some evidence from STEM
STEM-HAADF

22. FEI Titan image of InGaN/GaN QWs
Recent Growth
5 nm

23. 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. 24
A quantum well with a step: use
TEM/APT data as input to theory
nm
• A single monolayer island is added to the
random quantum well – as seen in the atom
probe and TEM data.

25. 25
Electron and hole wavefunctions (1)
Electron Hole
• 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

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
26

27. 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

28. 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

29. Problems with GaN growth on 6-inch Si
• Cracking
– GaN under tensile stress when cooling
from growth temperature
• High dislocation density

30. 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

31. UNIVERSITY OF
CAMBRIDGE
Stress control
Control of tensile stress and associated
cracking using AlGaN buffer layers
Tensile strain compensation

32. Curvature during growth of an LED on Si
0.40
0.35
0.30
0.25
Reflectance 0.20
0.15
(0.10
a.u.)
0.05
0 5000 10000 15000 20000
0 5000 10000 15000 20000
1200
1000
800
600
400
200
50
0
-50
-100
-150
Curvature/km-1
Time (s)
Concave
Convex
0
Process temperature (Tc)
0.00
LayTec Epicurve®TT
QW and p-GaN growth:
AlGAalNN garnodw Gtha:N growth:
After cooling:
In-situ annealing:
Mg-doped GaN
Si-doped GaN
Si-dopedGaN
AlGaN
AlN
GaN
AlGaN
AlGaN
AlN
Si substrate
AlN
AlN
Si
Si
Si
Si
5x InGaN-GaN
MQW
Mg-doped GaN

33. 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

34. Dislocation reduction using SiNx mask:
epitaxial lateral overgrowth (ELOG)
Dislocations
GaN
mask

35. Threading Dislocation Reduction
2 μm 4th
WBDF TEM image, g = <11-20>, edge + mixed TDs
3rd
2nd
1st
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

36. Processed full LED on 6-inch Si wafer
Full 6” wafer processed on a classical III/V line (in 2009)

37. 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?

38. 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

39. 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

40. 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)

41. 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

42. 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

43. 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

44. 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

45. 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

46. 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

47. 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

48. Thank you
• Obrigado

49. Energy – the 21st century problem
50
45
40
35
30
25
20
15
10
5
0
Oil
Coal
Gas
Fission
0.5%
Biomass
Hydroelectric
Solar, wind, geothermal
Source: Internatinal Energy Agency
2003
Energy consumption: 14 TW
World population: 0.65x1010
30
0
20
0
10
0
Millions of barrels per day (oil equivalent)
1860 1900 1940 1980 2020 2060
2100
•By 2050 the world population will be 1x1010 = minimum
need for extra 10 Terawatts per year.
0
• Biomass is mainly firewood – first to run out

50. 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

51. Some Electron Microscopes at Cambridge
FEI Titan 80-300
Philips CM300
the heart of technology
FEI Tecnai F20
JEOL 4000 EX
COMPANY CONFIDENTIAL 13 October, 2014

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.
52

53. LEDs of all Colours
• Made possible by new designed material –
gallium nitride (GaN)
InN GaN AlN
Bandgap 0.7eV 3.4eV
6.2eV
Light IR Near-UV Deep-UV
• Inx Ga1-x N. Vary x. Get light of any colour
• Strong atomic bonds

54. 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

55. HAADF Imaging - Cs corrected Titan at CCEM
2 nm
<11-20> AlN
<110> Si
SixNy ?
amorphous layer
Acquisition conditions :
Conv. semi-angle : 22 mrad
Detec. inner angle : ~ 50 mrad
acquisition time: 25 μs/pixel
Image size: 1024 x 1024 pixels
Resolution : ~ 1 Å

56. Spectrum Imaging - Elemental maps
1 nm HAADF
Si-L23
N-K
Al-L23
SixNy layer from elemental maps
Absence of detectable O

57. 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 clean
surface
Si
TMA predose
Aluminum
Si
AlN growth
sharp interface
AlN
Si
Growth continues
Si/AlN
interdiffusion
AlN
SixNy layer
Si

58. AlN/Si : structure @ low temperature
AlN <11-20>
Si <110>
Al-face polarity
hex
cub
crystallographically
sharp interface
AlN buffer grown by MOVPE @ 735
°C
Radtke et al, APL, 2010 and 2012

59. MOVPE growth of GaN-on-Si LED structure
Total epi thickness
~2.5 μm
Mg-doped GaN, ~90 nm
p-AlGaN EBL (~20 nm)
Si-doped GaN, ~1.3 μm
AlGaN buffer, ~0.8 μm
AlN ~200 nm
Si substrate
InGaN/
GaN
MQW
SiNx IL
nucleation and growth:
T~1000°C
Laytec Epicurve: wafer curvature
AIXTRON Argus: temperature profiler
AIXTRON CCS vertical reactor

60. 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

61. 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!

62. 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

63. Imaging single Si atom impurities in graphene at 60 keV
Si atoms in graphene can occupy two different sites (UltraSTEM100 images).
4-fold: Si substitutes for 2 C atoms
Courtesy Wu Zhou
3-fold: Si substitutes for a single C atom
Courtesy Matt Chisholm
Can we study the bonding environment of a single atom?

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

65. HERMES - Energy resolution
Nion HERMES at Rutgers U., March 2014, 60 keV, 10 msec acquisition
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.)

66. TiHx
(x~2)
Vibrational spectra of different materials
collected in “aloof” mode,
with probe ~5 nm outside sample
Epoxy resin
Intensity Intensity
0
50 100 150 200 250 300 350 400 450
Energy loss / meV
0
LO phonon
180
180
150
C-H stretch
365
137
Most materials with light elements (Z<8) give distinct phonon peaks at ΔE >100 meV.
Hydrogen is readily identifiable.
Data recorded with ASU HERMES at 60 keV, typically in 10 sec per spectrum.