Photoluminescence of Solids

Luminescence in solids is the phenomenon in which electronic states of solids are excited by some energy from an external source and the excitation energy is released as light. When the energy comes from short wavelength light, usually ultraviolet light, the phenomenon is called photoluminescence.

Photoluminescence can be divided into two major types, namely intrinsic luminescence and extrinsic luminescence. In the former there are three kinds, band-to-band luminescence, exciton luminescence and cross-luminescence. The latter consists of two parts, unlocalized and localized. In unlocalized luminescence, excited electrons and holes of the host lattice participate in luminescence processes.

When the luminescence excitation and emission processes are confined to localized centers, this kind of luminescence is called localized luminescence [1].

2.1.1 Intrinsic luminescence

There are three kinds of intrinsic luminescence: band-to-band luminescence, exciton luminescence, and cross luminescence. Here each kind of luminescence will be explained and discussed briefly.

(1) Band-to-band luminescence: Luminescence owing to the band-to-band transition can be observed in very pure crystals at relatively high temperature. For example, an electron in the conduction band recombines with a hole in the valence band.

This has been observed in Si, Ge, and some Ⅲ-Ⅴ compounds such as GaAs.

At low temperature, this luminescence is transformed into exciton luminescence.

From the viewpoint of luminescence application, light emission from light

(2) Exciton luminescence: An exciton is a composite particle of an excited electron and a hole. It moves in a crystal to convey energy and produces luminescence due to the recombination of the electron and the hole. There are two kinds of excitons: the Winnier exciton and the Frenkel exciton. The Winnier exciton model expresses an exciton as composed of an electron in the conduction band and a hole in the valence band bound together by their attractive coulomb interaction. This model works well for inorganic semiconductors such as Ⅲ-Ⅴ and Ⅱ-Ⅳ compounds. The Wannier exciton moves in a crystal but does not contribute to electric conduction because it is electrically neutral. It emits luminescence by the recombination of the electron and hole composing it. This type of exciton is weakly bound, with an average electron-hole distance large in comparison with a lattice constant. Wannier excitons are stable only at relatively low temperatures, where the binding energies of excitons are higher than the thermal energy. At higher temperatures, where the thermal energy is higher, the

excitons are no longer stable and the band-to-band luminescence appears instead.

The Frenkel exciton model is used in cases where the distance between the electron and hole is smaller than the lattice constant. A Frenkel exciton is a tightly bound exciton and essentially an excited state of a single atom. The excitation is localized on or near a single atom because the hole is usually on the same atom as the electron. Typical examples are organic molecular crystals and inorganic complex salts including transition-metal ions. In these materials, the binding within a molecule is strong in comparison with the van der Waals binding between molecules, so that the cxcitons are Frenkel excitons.

(3) Cross-luminescence: Cross-luminescence is produced by the recombination of an electron in the valence band with a hole created in the outermost core band. The luminescence can take place only when the energy difference between the top of

the valence band and that of the outermost core band is smaller than the band gap energy. An Auger process occurs when the hole in the outermost core band is filled nonradiatively with an electron in the valence band and another electron in the conduction band is dropped to the valence band with energy release as luminescence. Therefore, cross-luminescence is observable only in materials satisfying the condition.

2.1.2 Extrinsic luminescence

Luminescence caused by intentionally incorporated impurities, such as metallic impurities or defects, is classified as extrinsic luminescence. Most of the observed types of luminescence that have practical applications belong to this category.

Intentionally incorporated impurities are called activators and materials made luminescent in this way are usually called phosphors. Extrinsic luminescence can be divided into two types: unlocalized and localized. In the unlocalized type, the electrons and holes of the host lattice can participate in the luminescence process. In the case of the localized type, the luminescence excitation and emission processes are confined in a localized luminescence center.

(1) Unlocalized luminescence: In semiconductors, most important impurities are donors and acceptors that dominate semiconductive properties, and these act as luminescence activators. Donor-acceptor pair luminescence is a very important example of the unlocalized extrinsic type. Electrons excited into the conduction band are captured by ionized donors, and the holes in the valence band are captured by ionized acceptors. The emission involves electron transfer between neutral donors and neutral acceptors. Therefore, the emission energy of this luminescence generated on a donor-acceptor pair depends on the distance

Another important luminescence of the unlocalized type is isoelectronic traps. If an impurity atom is introduced and replaces or substitutes for the host atom in a semiconductor, it attracts an electron or a hole because of the difference in electron affinity. Such a substitutional atom is called an isoelectronic trap.

When the electron affinity of the introduced atom is larger than that of the host atom, the substitutional atom can become an electron trap. If an electron is trapped, a hole is attracted through the coulomb interactive force. In this way, a bound exciton is formed and luminescence is produced. In addition to these mentioned above, the unlocalized luminescence is very important in terms of practical applications.

(2) Localized luminescence: Various kinds of metallic impurities intentionally incorporated in the ionic crystals and semiconductors often create efficient localized luminescence centers. In the localized luminescence, the impurity atoms form the emission centers in the semiconductor. These centers are essentially closed and do not couple with the host atoms. Therefore, the luminescence properties are primary decided by the emission centers and the host material almost has no influence on the luminescence. Localized type centers with regard to energy transitions are classified into: allowed-transition type, and forbidden-transition type. Energy levels of localized centers are seen in the energy band scheme in Fig. 2-1. In the case of A, both the ground and the excited states are located in the forbidden gap. In the case B, the states are embedded in or located below the valence band. There are many intermediate cases between A and B. Localized centers can be also excited by the band-to-band transition as shown by C and D in Fig. 2-1. In the case of C, first a hole is captured by the center and then an electron is captured to produce luminescence. In the case of D, the electron is captured first and then the hole.

Fig. 2-1 Energy levels of localized centers and luminescence transition [1].

2.1.3 Classification of luminescent materials

Luminescence is the emission of optical radiation resulting from nonthermal excitation of the energy levels of atoms, molecules, polymers, and crystals.

Luminescent materials can be classified into several broad groups [2, 3]:

(1) Aromatic molecules: These kinds of materials luminesce in the vapor phase, liquid phase, solid phase, and in fluid or rigid solutions. They constitute the large group, and are used widely in luminescent dyes and paints, fabric and paper brighteners, dye laser, etc.

(2) Inorganic crystals: These types of materials include diamond, ruby, alkali halides, zinc sulfide, calcium tungstate and so on. Their emissions are usually efficient, and result from impurity centers or crystal defects. Luminescent inorganic crystals are used for scintillators, luminescent screens, solid state lasers, jewelry, etc.

(3) Noble gases (He, Ne, Ar, Kr, Xe): These materials luminesce in the vapor phase, liquid phase, solid phase, and solutions. They are used in discharge lamps, gas lasers, and scintillators.

(4) Simple inorganic molecules: These molecules usually luminesce in the vapor phases. Some like H2, D2, N2, and Hg, are used in discharge lamps as well as others (N2, I2, and CO2) are applied to gas lasers.

(5) Inorganic ions: These ions notably of the rare earths are used as activators in crystals, glasses, and chelates. Applications include inorganic and glass scintillators, and glass lasers.

(6) Aliphatic molecules (paraffins and cyclohexane): These kinds of molecules are now known to emit in the far UV with a low photon yield.

Whole not exhaustive, this list illustrates the wide range of luminescent materials and their applications.