Absorption of Light

Silicon -- a Material Transparent to Infrared

Covalent bonding between two silicon atoms is visualized as a sharing of electrons supplied by both atoms. The bond comes about because shared electrons orbit around both atoms. This overlap of the bonding orbitals lowers the energy of the system. For the purpose of bonding, the atoms of silicon may be visualized as having 10 inner, closed-shell electrons (which do not participate in bonding), and four outer electrons that do. When a silicon atom is brought together with four other silicon atoms (Fig. 1), it shares its four outer electrons (heavy, curved lines in Fig. 1) with each of the four atoms. These atoms, in turn, share one of their four outer electrons (light, curved lines) wiht the central Si atom. Covalent bonds are quite directional (the four electrons are arranged symmetrically) as illustrated

Figure 1. Covalent bonds of silicon

by the tetrahedral configuration. Again, we emphasize that these lines are just a pictorial representation of a more diffuse electron distribution.

Figure 2 is a two-dimensional representation of the silicon lattice. The covalent bonds are shown by the curved lines connecting the Si atoms. Although this is a line-and-ball representation, it shows that all the outer or valence electrons are tied up in covalent bonds. The inner 10 electrons are tightly bound (binding energies greater than 100 eV) and the outer four form the covalent bonds. Consequently, there are essentially no free electrons available for charge transport.

Figure 2. Two-dimensional representation of the Si lattice for high-purity Si, where all the outer electrons are in covalent bonds.

If we shine light of a fixed energy on the sample, we can break the bonds if the photon energy is greater than the bond energy (about 1.1 eV for Si), which corresponds to the infrared portion of the spectrum. When a bond is broken, the liberated electron is now free to move within the crystal. The empty site, or hole, left by the escaping electron can be occupied by a nearby electron. Consequently, the hole can also migrate through the crystal by exchanges with the bound electrons. Both electrons and holes can trasnport charge leading to a current, a photocurrent.

If the sample is irradiated with low-energy photons which are not energetic enough to break the covalent bonds, the photons will be transmitted through the sample unattenuated (Fig. 3). There is no mechanism for the absorption of light as well as no photocurrent. As the energy of the photons is increased, a threshold is reached where the interaction of light with the Si lattice can break the covalent bonds. In Si this occurs at an energy E_G of about 1.1 eV, at a wavelength of 1.1 micron [from E = 1.24 eV/(m)]. This energy lies in the infrared but close to the visible range.

Figure 3. Photons incident on a silicon sample, with the lower portion displaying the signal from the transmitted photons (not absorbed) and the photocurrent versus photon energy.

As the photon energy is increased above this energy E_G, the number of transmitted photons decreases and the photocurrent increases due to the absorption of light in the bond-breaking process. Each method, transmission or photoconductance, can be used to measure the energy required to free and electron from the covalent bonds.

We have presented the relatively simple case of Si. A compound semiconductor such as a III-V compound, gallium arsenide (GaAs), also forms a lattice with covalent bonding. The similarity to Si can be inferred from Fig. 4 which shows a portion of the periodic table and a representation of the atomic configuration of the electrons.

Figure 4. Portion of the periodic table and representation of the electron distribution around gallium (three outer electrons) and arsenic (five outer electrons).

All five elements have 28 electrons in filled inner shells (K, L, andM shells) and electrons in the outer, less tightly bound shell (N shell). The column III element GA has three outer electrons, and the column V element As has five outer electrons. When these two elements are arranged in a lattice they share outer electrons to form covalent bonds with Ga atoms providing three electrons and As atoms, five electrons. There is, in effect, a Ga sublattice and an As sublattice (Fig.5).

Figure 5. Two-dimensional representation of the gallium-arsenide (GaAs) lattice for high-purity GaAs, showing that all the outer electrons are in covalent bonds.

The compound gallium arsenide has the same response to light that silicon does: transparent to infrared light which is unable to break the electron bonds and opaque to visible light which can break bonds.

Almost all the paint pigments have the same properties as Si and gallium arsenide. They are transparent to infrared light. This transparency to IR occurs because the paint pigments are nearly all oxides (such as titanium white, titanium oxide) or sulfides (such as the red vermilion, mercury sulfide). In pure form, they are insulators or semiconductors with almost no electrons available for light absorption in the IR.

Pigment Response in the Infrared

A particularly convincing demonstration of the transparency of pigments to the infrared is provided in Fig. 6. In this case, (Fig. 6a) an underdrawing is made using charcoal on a white ground. A layer of paint (Fig. 6b) is then applied to the drawing which is viewed with an infrared sensitive video camera. The image, Fig. 6c, of the underdrawing can be seen clearly in the infrared display.

Figure 6. Demonstration of infrared reflectography. a) photograph of charcoal drawing in visible light, b) photograph of overlaid paint layers which hide the charcoal drawing, c) infrared reflectogram revealing the charcoal drawing beneath the paint layer.

Fluorescence with Ultraviolet Light

The absorption of ultraviolet photons causes chemical reactions resulting, in some cases, in the emission of photns in the visible region of the spectrum. The absorption of light photns of high energy and re-emission of photons of lower energy, in the visible region, is commonly called fluorescence.

The fluorescence process sketched in Fig. 7 depicts an incident UV photon interacting with an electron. The photon is absorbed and transfers its energy to the electron raising it to one of the availabe energy levels. This enhanced energy of the electron may be partially lost by heat (process 2 in Fig. 7) and then thte electron can make a transition back to the original state by emitting a photon (process 3 in Fig. 7). The emitted photon is of a lower energy than the incident photon. Aged varnishes have different fluorescent properties than new varnishes and some modern pigments fluoresce differently than traditional pigments. Thus examination of a painting under ultraviolet light can often reveal retouched or restored areas.

Figure 7. Illustration of the fluorescence process: An incident ultraviolet photon is absorbed and transfers its energy to an electron (process 1), the electron loses some of its energy in non-radiative processes, 2, and then the electron makes a transition to the ground state, process 3, emitting a photon with energy in the visible region.

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Last modified 20 December 1999
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