Published on October 13, 2014
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.
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