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\Friday, February 23, 2024

📅 Daily Questions​

🌜 Last night, after work, I​
🙌 One thing I'm excited about right now is​
🚀 One+ thing I plan to accomplish today is​
  • Organize my obsidian notes
👎 One thing I'm struggling with today is​

📝 Ephemeral Notes​

  • #til2024 Blue LED
    • LED for the longest time only had red and green, more efficient than a traditional light bulb
    • Conductors have empty "seats" in the valence band and can conduct the movement of electrons when electric field is applied
    • Insulators have full "seats" in the valence band and a large band gap between the valence and conduction band so electrons can't jump between the bands nor move in the valence band
    • Semi conductors have smaller band gap so electrons can jump between the valence and conduction band
      • Once the electrons jump, there are space in the valence band to have movement and the few electrons in the conduction band can also move when an electric field is applied
      • Pure semi-conductor like silicon is not useful, but by "doping" the pure silicon with phosphorus (a similar element) it adds one additional valence electron and can jump into the conduction band with small thermal energy as it exists in the donor band which is very close to the conduction band
        • This then can allow for movement of electrons in the conduction band and it is called n-type semi-conductor for negative as it is the electrons which are mobile
        • P-type semi-conductor is made by doping it with boron which has one less electron and sitting above the valence band allowing the electrons to leave behind empty "seats"
      • If you connect the n-type and p-type together you get a diode, the electrons will diffuse between the two semi-conductors and create a depletion region
        • If you connect the battery to this, depending on the polarity it will either have no current or current will flow and electrons will be emitted as a photon
        • The size of the band gap will determine the color of LED
        • Pure-silicon diode will release infrared light as it as a 1.1eV band gap
      • Blue being the higher on the visible light spectrum it required wider band gap
        • These needed clean crystals which is made with a machine called MOCVD
      • MOCVD injects the vapor molecules of the crystal into an oven over a substrate and reacts and forms into thin layers
        • Substrate lattice needs to match with the crystal lattice
        • Art-form to create this
    • One engineer created 3 breakthroughs to create the blue LED
    • Shuji Nakamura at Nichia dedicated his life to producing blue LED
      • The crystal material of choice were zinc selenide (ZnSe) and gallium nitrate (GaN), but GaN was not pursued by scientists at the time
        • There were 3 challenges, the crystal formation was hard because of the lattice incompatibility with the substrate, p-type GaN was not known, output power is not enough for practical use
        • Nakamura decided to pursue the route of GaN because it was a less crowded field
    • Nakamura at first was not able to grow the crystal, so he took apart his MOCVD machine to modify it and did the same routine of modifying and growing every single day for a 1.5 years
      • One day after adding a second nozzle to introduce a downward stream of inert gas to pin down the crystal vapor as it was released into, he was able to finally grow his GaN crystal
      • This second nozzle that he built kept the crystal vapor flow laminar which was causing trouble for other scientists
      • The latest advancement in GaN crystal at the time was the introduction of an aluminum buffer layer to bring the lattice similarity between sapphire substrate and the crystal closer
      • But instead of using a aluminum buffer layer, the downward flow was able to create a buffer layer itself and a higher quality crystal
    • For the second challenge, Nakamura copied a newly discovered approach for p-type GaN which was to dope it with magnesium and then expose it with electron beam
      • However, the process of the exposing to electron beam was inefficient and not scalable. And it was not fully understood why it was necessary in the first place
      • He improved the process with annealing which produced a better product and was scalable
      • Through this process, he also figured out that the hydrogen atoms from the crystal vapor was plugging the holes left behind after doping GaN crystals with Mg. Thus, by heating the material, it releases the hydrogen atoms
    • For the third challenge, the known approach to increase efficiency was to create a well which shrinks the band gap at the depletion region
      • The known material to create this well is the indium gallium nitride, but it was accepted at the time that indium GaN and GaN does not mix
      • By customizing his MOCVD, he brute force his way to mix the indium GaN with GaN and created both wells and "hills" and this
    • In 1992, Shuji Nakamura finally completed the first true blue LED
    • LED lighting became possible by putting a yellow phosphorus layer outside of the blue LED

Notes created today​


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Notes last touched today​


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created: 2023-01-16 11:50