GaN based blue LED Joonas Leppänen Emma Kiljo Jussi Taskinen Niklas Heikkilä Alexander Permogorov 13.5.2016 Photonics School of Electrical Engineering 13.5.2016 Group 3 LED

Content Introduction Theoretical Background Manufacturing Applications Future Prospects Conclusions Photonics Group 3: LED 21.4.2016

Introduction First LEDs from late 50’s to early 60’s Expensive at first, nowadays quite cheap Green and red LEDs easier to create than blue ones Higher energy (gap) needed By combining red, green and blue LEDs, it is possible to create white light Nobel prize for blue LED in 2014 Phosphorus coating of blue-LED also provides white light By replacing incandecent lighting with LEDs, energy can be saved Photonics Group 3: LED 13.5.2016

Theoretical Background The operation princible of LED is based on semiconductor pn- junction Heterojunction structure is a typical structure used in LEDs Confinement of the charge carriers in active layer Reduction of reabsorption Manipulation of bandgaps by alloying compound semiconductors Photonics Group 3: LED 13.5.2016

Theoretical Background The most important materials for blue LEDs are currently GaN based compound semiconductors Zinc compounds and silicon carbide may also be utilized but they are indirect bandgap semiconductors GaN is direct band gap semiconductor, which provides better efficiency. GaN and most substrates have a large lattice mismatch, sapphire is often used as substrate P-type GaN can be produced by doping with Mg or Zn, and n-type by doping with silicon Photonics Group 3: LED 13.5.2016

Manufacturing On foreign substrate MOVPE (Metal-Organic Vapour Phase Epitaxy) Poor quality (TDD = 106 - 109 cm-2) MBE (Molecular Beam Epitaxy) Better quality (TDD = 105 - 106 cm-2)   Pros Cons MBE Control of doping Slower growth Good crystalline quality Expensive Composition at the monolayer level Ultra high vacuum required MOVPE Poisonous precursor gases Faster growth ~ 2µm/h C and H from precursors are incorporated into layers Easier maintenance Widely used in industry Photonics Group 3: LED 13.5.2016

Manufacturing Bulk Growth MOVPE and MBE are not cost effective since they are slow when compared to Czochralski for Si Quality is not that good due to lattice mismatch and other things related to the growth on top of foreign material Low luminous flux Bulk growth of GaN may overcome these issues using GaN seeds which may be grown on foreign substrates Possibility of higher growth rates for cost effective growth Photonics Group 3: LED 13.5.2016

Manufacturing Bulk Growth HVPE (Hydride Vapour Phase Epitaxy) High quality (TDD = 104 - 106 cm-2) Extremely high growth rate (300 µm/h) Can be used to grow GaN seeds on sapphire or silicon, which can be removed by a lift-off technique after the growth Seeded growth is possible but requires high quality seed the quality is seed quality dependent HVPE-GaN can itself be used as a seed Wafer bow/bending causes difficulties for larger wafers Widely used for bulk GaN growth at the moment Process FEOL (Front-End of line) process 1000 ◦C Based on a reaction between GaCl and NH3 to form GaN crystals GaCl is provided via reaction between solid Ga and HCl Photonics Group 3: LED 13.5.2016

Manufacturing Bulk Growth Ammonothermal growth Higher quality (TDD = 103 - 104 cm-2) Difficult to handle safely! Growth rate only 20 µm/h No bending or cracking issues Process BEOL (Back-End of line) process 400 - 500 ◦C 3500 atm pressure Grown from polycrystalline GaN grains in a supercritical ammonia solution Can be grown on HVPE seeds but the quality depends on the seed quality Photonics Group 3: LED 13.5.2016

Manufacturing Bulk Growth Sodium (Na) Flux Extremely high quality (TDD = 102 - 103 cm-2) High growth rate of 100 µm/h No cracking Still on its early stages! Process FEOL process 800 ◦C Grown in Ga-Na melt on top of seed MOCVD-GaN or HVPE-GaN on sapphire, which can be removed afterwards Necking technique decreases the TDD even if the seed is poor quality! Reason behind this are unknown Photonics Group 3: LED 13.5.2016

Why LED chosen Incandescent: 17 lm/W Fluorescent: 90 lm/W LED: up to 425 lm/W Photonics Group 3: LED 13.5.2016

LED in illumination

Blue LED to white light: phosphors Ce3+:YAG Excited by LED radiation Blue + Yellow = White Highest efficiency – 425 lm/W Reasonable Color Rendering Index (CRI)

Blue LED to white light: color mixing Efficiency lower than for phosphors – 300 lm/W Highest CRI Possibility of color changing

Driving Forces of the blue-LED technology Future Prospects Driving Forces of the blue-LED technology US DOE plan that 70 % of public lights will be replaced by LED’s til 2030 Pressure to lower price and increasing efficiency and lumens. Photonics Group 3: LED 13.5.2016

Conclusions First LEDs in the 60’s Blue LEDs enabled white light Current manufacturing methods can be used A need for better quality and more cost-effective wafers Bulk growth may be the answer HVPE+MOVPE-seed is currently used but the TDD must be decreased Na-flux seems most promising in the future but size must be increased Superior lumen per watt production Major possibilities for energy saving Photonics Group 3: LED 21.4.2016

Thanks! Photonics Group 3: LED 21.4.2016