The Corpus Callosum

Ultraviolet LEDs

Everyone’s
heard of blue lasers by now.  Some people have them in their
homes.  The reason they are important, is that blue light has
a shorter wavelength than the red lasers that were used in the first CD
and DVD devices.  The shorter wavelength means that the laser
can see smaller dots.  Smaller dots mean that more information
can be packed into the same space.  That means more
information can be put on a DVD is a blue laser is used, compared to a
red laser.

A few years ago, companies started working on ultraviolet
light-emitting diodes and lasers.  Because UV light has even
shorter wavelengths than blue, that leads to a potential for a vast
increase in the amount of information that could be put on a DVD.
  
That’s the good news.  The bad news is that the expensive blue
laser DVD player your just bought is going to be obsolete soon.
 Of course.  

But there is more good news.  UV devices potentially could be
much more efficient.  That does not matter much for DVD
players, but it matters a lot if you intend to use
the device for illumination.  Higher efficiency illumination
could lead to a significant decrease in residential and commercial
power usage.

At first glance, that seems like a nutty idea.  Who wants UV
lights in their home?  Well, nobody.  But it turns
out that UV light can be used to excite phosphors that emit visible
light.

The technical details?  


Previous
attempts to make such devices used gallium nitride.  But it
turns out that plain old zinc oxide works better.
 

href="http://spectrum.ieee.org/mar07/4946">Ultraviolet
lasers and LEDs made from zinc oxide are on their way
IEEE Spectrum
March 2007

…First-generation discs relying on red lasers could store about 5
gigabytes of data, and blue lasers have taken that to 50 GB. But if
disc-player laser wavelengths could be pushed down into the ultraviolet
part of the spectrum, disc densities could be hiked up to as much as
250 GB…

…A practical deep-ultraviolet LED would be especially valuable
because “if you have UV, you can excite anything in the
visible,” says David Look, director of the semiconductor
research center at Wright State University, in Dayton, Ohio.
“You could have pure red, blue, and green phosphors. Then you
excite them in any proportion to get any color.”

The key to making UV-­emitting devices is likely to be zinc
oxide: it’s a better material than gallium nitride for making
these devices, because it naturally emits (and absorbs) at those
wavelengths more efficiently. Plus it is cheap and abundant…

It
turns out that it has been a challenge to find a way to make this work,
to make devices with a useful service lifetime, and to do it cheaply.
 But the IEEE article indicates that there problems are being
overcome.  

The potential payoff:

face="Helvetica, Arial, sans-serif"> href="http://www.jetro.go.jp/en/market/trend/topic/2005_05_led.html">World’s
First Blue LED Made with Zinc Oxide

What’s more, the new LEDs also will deliver 10 times the light-emitting
efficiency. With the way now opened up for low-cost, mass-produced
LEDs, and also laser diodes and other devices using zinc oxide, it is
believed that the LED and laser diode market could expand by as much as
several trillion yen.

LEDs are used as light sources in applications requiring high
luminance, long service life and high efficiency, such as large outdoor
displays, traffic signals, liquid-crystal display backlighting and
illuminations. Laser diodes are used in infrared applications, such as
optical communications devices, and in the red through blue domain they
are used in information read/write light sources.


LED lighting is already promising, already efficient, but expensive.
 It remains to be seen how inexpensively these devices could
be made for illumination purposes, but if they are ten times more
efficient, it almost doesn’t matter what they cost.

Comments

  1. #1 Muatafa Mond, FCD
    March 15, 2007

    The first article doesn’t say much about wavelengths, except that theoretically ZnO could reach down to 200 nm. That would be great, there are a lot of biochemistry applications that need light in the 200 – 300 nm range. Peptides, 215 nm, nucelic acids 260 nm, aromatic proteins 280 nm.

  2. #2 Mustafa Mond, FCD
    March 15, 2007

    �You could have pure red, blue, and green phosphors. Then you excite them in any proportion to get any color.�

    How would that be superior to having red, blue and green LEDs, and being able to light them in any proportion?

    I understand the bit about color balance in today’s white LEDs, but color-adjustable LED arrays is a separate question.

    And thanks for the tip on some fascinating info.

  3. #3 Janne
    March 15, 2007

    Well, for one thing you could have a mix of phosphors in a single LED, creating single units of whatever color you like.

  4. #4 Mustafa Mond, FCD
    March 15, 2007

    Uh? If the phosphors are on a single LED, how do you discreetly control the intensity from each phosphor?

  5. #5 Joseph j7uy5
    March 15, 2007

    If you have all three phosphors in the same unit, you would not be able to vary the color, but you could make the unit any color you like. So in a residential application, you could have a warmer temperature light as opposed to a cold white light. In a restaurant, you could have a slight hue to reflect whatever mod you wanted (e.g. slight pink to enhance skin tones, which some people think is a good thing is restaurants). But the big advantage is in the efficiency. As far as I am concerned, that is the main reason to be interested in this.

  6. #6 Jon H
    March 17, 2007

    I’ve been meaning to get some UV LEDs to illuminate the tank my scorpions live in – they glow green under UV.

  7. #7 portlandengineer
    March 18, 2007

    FYI – there’s also quite a bit of work on inkjet printing of Zinc Oxide. This material can make a decent channel material for FETs.

  8. #8 julia howard
    June 29, 2007

    Is the LCD technology which Sony uses in its Vaio laptops of the ultraviolet kind? If so does the ultraviolet form of LEDs emit ionizing radiation? (This may be an ignorant question as my knowledge of electronics is poor).
    Also would you happen to know whether the source of beta radiation which is used to start the ionization process in Cold Cathode fluorescent lamps emits ionizing radiation as ccfls are used as LCD television backlights?

    Thanking you for your anticipated informed response,

    Julia Howard

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