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 Why are blue diamonds and LEDs colored? (doped semiconductors)									« »
 Blue diamonds
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For the preface to the Color website:

[Start with a photograph out of doors -- for example, a red-haired person in a partly sunny landscape with lightning and a rainbow].

Color "colors" our whole lives. Our sense of the world we live in comes first from what we see. What we see is colored; and the colors of our world affect us deeply. The sky is firstly blue sky, the grass green grass, a red head a red head. We value color so highly that, for thousands of years, we have tried to make our lives more colorful, by coloring our clothes, our homes and even our faces. Today, for the first time, we are able to color anything in any way we want; and that remarkable achievement is one consequence of the Industrial Revolution of the past two hundred years.

What is color and what makes some things colored and some not? How are we to describe and understand such a powerful feature of our life? Color is fairly complex. There is not one source of color but rather three different sources of color. Color always involves three things — light (obviously), our eyes (which is how we detect the light) and the object (which we will identify as colored). The color of our object can have three different origins which we can label as absorption, emission and scattering. In our picture, the green of the grass and the blue of the jeans are due to absorption. The lightning is due to emission. And the blue of the sky and the colors of the rainbow are due to scattering. Each different type of color —absorption, emission and scattering —occurs naturally — grass, lightning and rainbows without the participation of man. Man, in making his life so much more colorful, has simply exploited these natural effects.

  • In absorption, light — sunlight which is white light — strikes an object and part of the light may be absorbed by the object. The light we see coming from that object is the light which was not absorbed by the object. We see the "not-absorbed" light as the color of the object. If no light is absorbed, the object appears to be colorless. Vegetation, such as the grass in our picture, absorbs all the light except green light and that absorption, perhaps the most important process on earth, drives photosynthesis and makes life on earth possible. The paints and dyes produced by man as artificial colorants appear colored for the same reason that grass appears colored: they absorb some of the light that strikes them. The jeans in the picture show man as a color maker, in this case not a very good colormaker. Over time the jeans fade.

  • In emission, the object makes colored light and "throws" it at us. It actually does no"make" light because it cannot "make" energy. We can only transform energy from one form into another. (This ia fundamental law of physics, based on experience: "Matter/energy can neither be created nor destroyed".)

  

The Hope Diamond contains boron and will conduct electricity. Since the acceptor-level energy is so small, even the thermal energy at room temperature can produce this change, and the resulting holes in the valence band can now move in the presence of an electric field.

The Hope Diamond -- the world's largest deep blue diamond -- is more than a billion years old. It is a doped semiconductor, formed deep within the Earth and carried by a volcanic eruption to the surface in what is now Golconda, India. Since the Hope Diamond was found in the early 1600s, it has crossed oceans and continents and passed from kings to commoners. It has been stolen and recovered, sold and resold, cut and recut. In the early 1800's, it was sold to King George IV of England. At the king's death (1830) his debts were so enormous that the blue diamond was likely sold through private channels. By 1839, the gem entered the well known collection of Henry Philip Hope, the man from whom the diamond takes its name.

Light energy can be absorbed in or emitted from a band-gap semiconductor if an added substance forms an impurity level within the gap. A diamond crystal is composed only of carbon atoms, each of which has four valence electrons in its outermost shell. In a blue diamond, a few carbon atoms out of a million have been replaced by nitrogen atoms, each containing five valence electrons. The structure of the diamond is not significantly perturbed, but the extra electrons enter a "donor level," so called because, with the absorption of energy, these electrons can be donated to the empty conduction band.

 
      

Electrons can be donated to the empty conduction band. The valence band is completely filled. The donor level is broadened by a number of factors, including thermal vibrations, as at the right of this figure. The resulting absorption at the blue end of the spectrum leads to a yellow color seen in both natural and synthetic nitrogen-containing diamonds.
 

Synthetic diamond crystals (3 mm across) grown at General Electric. The clear diamond is pure, the blue contains a boron acceptor, and the yellow contains a nitrogen donor.
 

 

g

  

Boron has one less electron than carbon, and the presence of a few borons per million carbons in diamond leads to a hole level in the band gap. This is called an "acceptor" level since it can accept an electron from the full valence band.
 

The nitrogen donor level energy in diamond is rather large, about 4 eV. For various complex reasons (including thermal vibrations), the nitrogen level is actually broadened into a band as shown above left. Light quanta with any energy above 2.2 eV can excite the extra electrons into the conduction band as shown by the arrows in the figure. This results in the absorption of blue and violet light and leads to the yellow color of nitrogen-containing diamonds. The 2.2. eV donor energy is large by comparison with the energy of thermal excitation at room temperature, hence yellow nitrogen-doped diamonds remain insulators. A much rarer green color can result from a higher nitrogen content of about 1 atom per 1000 atoms of carbon than the 1 in 100,000 concentration that produces yellow. At even higher nitrogen concentrations, the donor level broadens additionally, so that all visible light can be absorbed, resulting in a black color.

The blue color shown in the synthetic diamond is the result of absorption at the red end of the spectrum. This occurs in the presence of a boron acceptor, which causes a deficiency of one electron per added trivalent boron atom. The resulting energy in the band gap is shown below left, where the missing electron (or hole) is shown as an open circle on the boron level. The boron acceptor energy is only 0.4 eV, so any energy light can be absorbed during excitation from various levels within the valence band. Like the nitrogen donor, the boron acceptor band is broadened, and the absorption tapers off throughout the visible light energies. At a level of one or a few boron atoms for every million carbon atoms, an attractive blue color results. Natural diamonds of this color are rare and highly priced, such as the Hope diamond.

 

Some materials containing both donors and acceptors, can absorb ultraviolet or electrical energy to produce the transition a.
 

      

Inside a fluorescent lamp, phosphor powders are used as a coating in fluorescent lamps to convert the plentiful ultraviolet light produced by the mercury arc into visible light, particularly into red light so as to produce a "warmer" light approximating daylight.
 

 

 

Other materials

Some materials containing both donors and acceptors (as in the figure at left), can absorb ultraviolet or electrical energy to produce the transition a. If the return path proceeds via f, g, and d, then light may be emitted corresponding to the energy release from g; this is then fluorescence or electroluminescense, respectively. The former occurs in "phosphor" powders, for example zinc sulfide ZnS containing Cu and other additives. Phosphors are also used inside a television screen, activated by a stream of electrons (cathode rays). Electroluminescence can use a similar powder deposited onto a metallic plate and covered with a transparent conducting electrode to produce lighting panels, often used for nightlights.

Some phosphors contain impurities which form "trapping" levels, as at the right of the figure at left. When an electron falls into a trap, as by process j, in this figure, it can only be released when additional energy is added to permit process k and subsequent light emission, say f, g, and d. If the trap level is close to the conduction band, then even room temperature may be able to supply the required energy slowly, this resulting in phosphorescence. If the required energy is a little larger, then infrared light may permit the escape, so that higher-energy visible light is produced in an activated infrared-detecting screen.

Light-emitting diodes

Finally, there is injection luminescence in a crystal containing a junction between differently doped semiconducting regions. An electric current now produces recombination between electrons and holes in the junction region, giving light from light-emitting diodes (LEDs, usually red), widely used on display devices in electronic equipment. With a suitable geometry, the emitted light can be coherent in the similarly operated semiconductor lasers.

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