Modern Physics, Spring 2015

Sajjad Hosseini - May 15th 2015

Introduction

        In our modern physics course, we discussed  on the first two sessions about the invention of blue light-emitting-diodes. We learned how this invention influenced our lives, and we had a look from physics point of view as well. We discussed quite widely about the importance of this invention in a sense that due to its low energy use and high usage time, it is possible for us to enjoy its advantages, like bringing light to places with no power plants or efficient usage of lighting in parking lots, and so on.

However, when it came to physics point of view, we did not go deep that what exactly happened that made us able to create blue LED?

• Was it advancement in laboratory devices?

• Was it finding a totally new material that had not been found before?

• Was it advancement of science?

• Was it even that cost/usability ratio was not high so it was not economical idea to work on?

So in this report, I’m going to analyze what was behind this invention in a precise, concrete, academic way.

My own knowledge and explanation

         I remember while doing research on one of the questions that had been raised:

“Why red and green LEDs were invented before the blue LED?”

I came across an article where someone had replied that the reason behind this is that the “material” that is used to make green and red LEDs had already been found, yet the material for blue LED was actually found, but the important concept was that it was not able to produce blue light to an extent which we could rely on it.

So, I assume that the problem has not been to find a material which is capable of emitting blue light, but rather to be able to extract blue light out of that material reliably.

This explanation is valuable in a sense that I have used my own knowledge without referring to external material, but of course it falls short on explaining what do I actually mean by the terms I have used such as “material” or “reliable” academically. So indeed, I have to provide exact definitions as reliable sources for what I am going to present.

Search results

        In the first reference, the author explains how LEDs actually produce light. As it states, on the contrary to other light sources, LED lighting is of kind “cold”, meaning that the light that is produced is not actually a result of heat but rather through the movement of current which results in the movement of electrons (from the layer that has excessive electrons ) toward holes (layer which lacks electrons) that cancel one another, and generate light in the end. The important concept though, is that it states “The wavelength of the light and hence its color is based on the materials used.”

Further the article explains that until 1950s, the main struggle for creating blue LED was to find the right material, because either the material was theoretically right, but they were not able to grow it bigger than dust particles!, or they could have formed it into crystals, but that was not suitable for the job. Eventually, the key material, gallium nitride, was found, even though in practice it was hard to work on it, because the metallic crystals needed to be of extremely high quality, and several other issues, and in 1986, the first gallium nitride crystal on layer of aluminum nitride on a sapphire substrate was made by Akasaki and Amano. In the following couple of years, they managed to build a p-layer as well. Finally, in 1992, they built their first LED light.

        In the second reference, it states that, the key to build LED is to find the right material to serve as a semiconductor in the diode. What is needed is a band gap that drops a specific amount in voltage and releases a specific wavelength of light when the electron finds its hole.

Figure 1.1, taken from reference 2

Further down, it is stated that the breakthrough of making a blue LED came because of “materials advances” using Gallium Nitride, and by nucleating that material in particular on sapphire substrates, allowing scientists to create the p-n junction which is important for constructing a diode. Also, after experiments, it was found that adding “Indium Nitride” to the Gallium Nitride, allowed the LED to emit blue light with a very satisfactory brightness.

My improved explanation

        Based on my research, now it is safe to believe that the color of the LED is dependent of the material. Also, main problem was finding the right material which could be grown big enough and suitable for the job. In the end, Gallium Nitride was the answer, even though with some difficulties, but because of material advances and the discovery of “adding Indium Nitride”, the result was blue LEDs being produced with good performance.

These indeed raise other questions such as:

• Are the current way of manufacturing blue LEDs is as efficient as possible?

• What was preventing scientists to be able to commercially manufacturing LED appliances such as TVs until just the previous decade?