# In pursuit of a blue LED Bashny.Net

Recently, Habré and Giktayms there were many informative articles about LED lamp , their circuitry and production . Development of the main components - the blue LED - took a quarter century, and the authors of the most successful technologies have been awarded this fall Nobel Prize . I would like to highlight this story from the physics and explain why the way to the blue diode was so long and thorny.

Introduction
Habré has talked in detail about based on semiconductor electronics and about, How does the LED . Briefly recall the main points. If you apply for a pn junction forward voltage, electrons from the n-region and holes from the p-region will move towards each other and recombine, emitting energy in the form of photons.

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Now look at the band diagram. The applied voltage crosses the electrons in the conduction band (respectively, holes in the valence band). Met, they recombine.

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It can be seen that the energy of the emitted photons is approximately equal to the width of the band gap. Actually, it also determines the wavelength of light and color. Thus, the energy of the photons of blue light more than red - so for blue LEDs need a semiconductor with a wide band gap. Historically, these semiconductors are called wide band
Generally speaking, the semiconductors in the world is not so much, and their basic properties are well understood. Very instructive here is the schedule (Alferov in his Nobel lecture, calls it "world map" semiconductors):

Horizontal here postponed lattice constant of a semiconductor - roughly the distance between two neighboring atoms in the crystal (to her we shall return later). Vertical - bandgap in electron volts (eV). To have an idea, one sees from photons with an energy of 1.8 eV (wavelength 700 nm, red) to 3.1 eV (400 nm, violet). We are interested in the blue-violet region, with a margin of about 2.6-3.3 eV it.

Digression: electron volts According to the definition, energy of 1 eV is sufficient to reduce the potential of an electron at 1 volt. Conversely, if we increase the potential of the electron at 1 Volt (say, after he could run from "-" to "+" odnovoltovoy batteries), he will receive the energy of 1 eV. Thus, if the band gap of the semiconductor is 3 eV (violet LED), then to throw an electron in the conduction band, you can increase its capacity at 3 V. In general, this is determined by the operating voltage on the LED, reaching as much as 3-3.5 In for blue / violet diodes.

As we can see, in the blue-violet region fall only three semiconductor: SiC, ZnSe and GaN. Historically, in the order they appeared on "LED" arena.

1. Silicon carbide (SiC)
Silicon carbide is remarkable in that it can form a huge amount of crystalline modifications. Already in the 50s is allowed to create structures with different band gaps - and thus generate radiation in different parts of the visible spectrum. After the red and yellow LEDs first blue LED was developed in 1969. In the '80s they became commercially available.

But for all the technological progress efficiency devices does not exceed 0.03%. The reason was fundamental. To understand it, we have to go a little further into physics.

##### acceptor in GaN h5> Another difficulty was the p-doping of gallium nitride. In the early stages as a acceptor (material for p-doping) used magnesium. However, the resulting material does not behave as a p-type semiconductor. There was some unknown factor that prevented Magnesium plays the role of acceptor.

The easiest way to determine the degree of doping - is to measure the resistance of the material. At the p- and n-doped semiconductors is low due to the excess carriers (electrons or holes in the n- to p-type). In a pure semiconductor free carriers is not, so the resistance is high. Blockquote> Notable next episode. Akasaka was able to show that irradiation GaN low-energy electron beam solves the problem: magnesium starts to behave as an acceptor . However, in the production of material again ceased to be conductive. This occurred during the annealing
Nakamura noted that the p-conductivity disappears at an annealing temperature above 400 ° C. At the same temperature to begin to dissociate the ammonium nitrogen and hydrogen. And what if the problem had something to do with hydrogen? Detailed studies have shown that Nakamura was right: atomic hydrogen penetrates the structure, was associated with magnesium and shared with him their electron without giving play the role of acceptor. Annealing in nitrogen instead of ammonium immediately solved the problem:

The upper curve: during annealing in Ammonium above 400 ° C the resistance of the material increases dramatically. In nitrogen (lower line) does not. Source. i>

Later it was found that annealing in nitrogen not only solves the problem of ammonium - it is able to completely replace the electron beam. That is, instead of what he was doing Akasaka, you can simply anneal in nitrogen material: it will destroy the connection with hydrogen and magnesium to make p-conductive material. Technologically, it was extremely important: processing of large crystals of electron beam - it is not cheap and not quick.

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