Electrical engineering material

1. Crystals

The regular surface geometry and the shiny and often colourful appearance have made crystals from the mineral kingdom fascinating objects for everybody. Natural crystals have often been formed at relatively low temperatures by crystallisation from solutions, sometimes in the course of hundreds and thousands of years.

Nowadays, crystals are produced artificially to satisfy the needs of science, technology and jewellery. The ability to grow high quality crystals has become an essential criterium for the competitiveness of nations. Crystal growth specialists have been moved from the periphery to the center of the materials-based technology.

An interdisciplinary crystal growth science has developped with scientific journals, conventions and societies. International networks of crystal growth laboratories and materials science centres have been formed. Crystal laboratories operate in large numbers to satisfy the needs of research and technology for high-quality, tailor-made crystals of all kinds.

"New materials are the lifeblood of solid state research and device technology. Contrary to what many believe, new materials are not usually discovered by device engineers, solid state theorists, or research managers; they are mostly discovered by crystal chemists who are crystal growers. Some physical phenomena are only exhibited in single crystals and can only be studies and understood in single crystals. Thus the crystal grower - especially if he develops a proficiency in relating structure, bonding and other chemo-physical considerations to properties of interest - is in a key position in determining the direction and success of solid state research and - ultimately - technology" (Laudise).

As an introduction to the central topic of crystal growth, preparation and characterisation we answer a few frequently asked questions:  What is a crystal?

 Which quality criteria can be applied to crystals?  What are crystals good for?

Crystals are solids in which the elementary building blocks, the atoms, are arranged regularly in a space lattice with specific geometrical symmetry elements. There is no ideal atomic lattice in nature, and it would be not very useful either. Fig. 1-1 presents common crystal lattice defects /Schilling/. Certain imperfections of the chemical and structural atomic arrangement are essential for the usefulness and value of crystals.

Vacancies, for example, allow atoms to move through the lattice in the course of solid state reactions. Fig. 1-2shows a schematic view of two extreme cases of the microstructure of the growth interface: atomically rough and atomically flat, in terms of a simple cube model of the atoms. Atomically rough interfaces are correlated with many metallic systems wheras atomically flat interfaces usually occur in oxidic systems and are related to macroscopically flat, crystallographically well oriented surfaces or facets. Atomically rough interfaces provide ample sites for the attachment of atoms from the melt during growth which corresponds with relatively small driving forces or small

supercoolings of the interface. Atomic attachments on flat or facetted interfaces are more difficult and require higher driving forces and larger supercoolings.

Fig. 1-1: Defects in crystal lattices

Most solids consist of many single crystals of different orientations which stick together at "grain boundaries". Binding forces are usually weaker at grain boundaries. Therefore, chemical reactions and evaporation processes proceed more easily at these boundaries which makes them visible to the naked eye. Usually, single crystals not only contain point defects but also extended defects, namely dislocations and dislocation networks.

Grain boundaries and subgrain boundaries can easily be recognised by inspection of the crystal surface under varying directions of illumination. Many properties of crystals are influenced by dislocations and subgrain boundaries. These defects contribute to high temperature creep and other mechanical properties. They are usually surrounded by diffusion fields of point defects since they act as sources and sinks for point defects and as nucleations sites for precipitates of all kind. Therefore, subgrain-free and even dislocation-free crystals are essential for solid state research and for many technical applications of crystals. The most radical method to get rid of

dislocations and dislocation networks ist their total elimination by melting and the subsequent growth of crystals without dislocations or with a very low density of dislocations. Although dislocations are thermodynamically not stable they cannot eliminated totally by crystal annealing alone.

The growth of crystals with low dislocation density is a difficult task because dislocations can be easily multiplied by thermal stresses during the cooling-down process of nearly perfect crystals. The handling of nearly perfect ductile crystals at room temperature is equally difficult since dislocation multiplication of such crystals can be initiated even under their own weight and - especially - by mechanical or electric discharge machining.

Fig. 1-2: Surface microstructures

The mosaic structure in the bulk of thick crystals can be studied most easily by using highly penetrating monochromatic gamma rays from cheap neutron activated gold and iridium platelets. Fig. 1-2 shows an example. The initial parts of the CuAu crystal which has been analysed in the example are characterised by a very narrow rocking curve diffraction peak which indicates the high "perfection" in this region with low dislocation density and absence of "mosaic blocks" or subgrain boundaries.

In the course of crystal growth the dislocation density has increased due to imperfect growth procedures until a network of subgrains has been formed. The perfect crystal regions with "Fujiama-type" rocking curves have been replaced by a whole range of "mountain peaks" and, finally, with a "hilly landscape" corresponding to a broad spectrum of subgrains with different orientations.

In research and technology many artificial crystals are required with chemical compositions from all parts of the periodic system with high chemical and - in special cases - even isotopic purity. Roughly speaking, the artificial crystal kingdom can be devided into three sectors. 1. Technical crystals belong to one of the two big sectors of the single crystal market. They are widely present, often in hidden form.

We eat crystals (salt, sugar), we use crystals as clocks in watches and computers (quartz), for information processing and storage (silicon), for switching TV-sets (gallium arsenide), for telecommunication (gallium arsenide) and for transport (turbine blades from nickel-aluminium compounds). Huge salt crystals (CaF2) are used as UV-light lenses in the submicron structuring during electronic device fabrication. Fig. 1-3: 410 MeV Gamma ray diffraction crystal rocking curves

2. Jewellery forms the second big sector of the single crystal market. Verneuil-rubies have been the first artificial crystals which have been growth on an industrial scale to be used in making jewellery and as bearings in mechanical watches "Falckenberg".

Natural crystals are usually much mor expensive than artificial crystals of the same kind. They often can be distinguished only by sophisticated characterisation methods, not obvious for the naked eye. The excessively high costs of certain natural crystals has been an enormous incentive for clever crystal growers to adjust their growth procedures until artificial crystals cannot be distinguished from natural ones in every detail of their microstructure.

3. The market of research crystals is relatively small but extremely diversified. Artificial research crystals of high quality are the basis of solid state research activities. Natural crystals are normally not sufficiently qualified for research purposes. Crystals are also required for modern light and particle scattering and diffraction instruments as monochromators and detectors. A broad range of geometrically well prepared crystals is required for thin film, catalysis and electrochemical studies.

Serious work in solid state and materials science is only possible with the help of specialised and integrated crystal growth laboratories. On the basis of their professional knowledge about crystal growth and characterisation the supply of research groups with research crystals can be optimised. Crystal growth laboratories and crystal companies are part of an international crystal growth community which has develloped in the course of the last few decades into an extremely fruitful network. Many members of this network are on the brink of extermination for demographic reasons. This important basis of all solid state research activities requires nursing to prevent its deterioration.

Finally, a few remarks may be appropriate on the more esoteric parts of crystal utilisation which seem to spread and become quite relevant, economically and spiritually. There are institutions which claim that crystals have magical properties, and more and more people seem to be inclined to believe that crystals have the power to cure sickness by pure contact and to protect against the evil. This confusement is supported by popular TV animated cartoon series in which crystals are used by good and eval characters for conjuring tricks. People of all centuries have believed in the powers which emerge from crystals. Hesiod and Ovid correlate the different aeons with metals of different value. In its famous science fiction story Time Machine H G Wells mentions a rock crystal as an essential part of the machine which is used to reach the year 802 701 in the future.

The topic of crystal growth has gone a long way from alchemistic endeavours to industrial production and even international science policy which is illustrated by crystal growth in orbiting space stations and by megalomaniac plans for crystal growth institutions: NASA and ESA have detected crystal growth as an excuse for having built manned orbiting space stations (Space Lab) and think of crystal growth in "milligravity" as something meaningful. The state of Austria has been suggesting the founding of a big european crystal growth institute "EuroCryst" to improve the industrial and scientific potential of the European Union.

2. Crystal growth

Artificial crystals are usually grown by controlled phase transformations from a disordered "fluid" phase with high atomic mobility. If the raw materials are transformed into a fluid, a gas or a liquid, a virgin state is reached in which all memories of the past are extinguished and the atomic constituents can be mixed perfectly by thermal or enforced convection.

The crystal growth process can be initiated by using a small seed crystal of the same material to define a proper crystallographic orientation and to avoid large supercooling of the fluid phase which could generate uncontrolled nucleation. The degrees of freedom which the growing crystal posesses on a microscopic and macroscopic scale have to be reduced as much as possible by proper design of the growth system. In most cases, crystals are grown in a temperature gradient with superheated fluid and supercooled crystal to define the position and the

geometry of the growth interface. Only a small region of fluid is supercooled close to the growth interface to provide the necessary atomic driving force for crystallisation. Fig. 2-1 and Fig. 2-2 show the two most important growth procedures for bulk crystals: the Bridgman- and the Czochralski-system.

Fig. 2-1

The "Bridgman method" is based on the invention of Tammann in Göttingen early in the century. He used the "gradient-freeze method" as it is called now, in which crystals can be grown by directional solidification in the temperature gradient region of a furnace whose average temperature is decreased gradually. Bridgman has added mechanical movement of the crucible to this Tammann.method. Others, such as Stockbarger, have made important contributions to the Bridgman-method.

The Bridgman-method is cheap and simple, although hampert by the problem of crucible interference with the crystallisation process. In the Czochralski-method the crystal is pulled out of the melt by crystallisation of the upper region of a melt meniscus. The growing crystal is visible and the growth process can be analysed in-situ. The control mechanism which is required for proper shaping of the meniscus makes the method rather expensive.

Artificial crystal growth means: crystallisation controlled in view of the crystal application. The process of controlled crystallisation can be subdivided into seven parts:

1. Suppression of random nucleation in supercooled crystal growth fluids by using a seed crystal with minimal supercooling of its growth interface which is not sufficient for nucleation elsewhere.

2. Shaping the growth interface by using a correponding temperature field which superheats the fluid and supercools the crystal with a proper geometry of the growth interface isotherm.

3. The temperature gradient has to optimised. It has to be large enough to prevent facetting at the interface and constitutional supercooling of the fluid close to the interface. At small temperature gradients the growth rate is limited by the conditions for constitutional supercooling. Too large temperature gradients have to be avoided since they lead to large thermal stresses which induce dislocation multiplication and subgrain-boundary formation in the hot crystal regions.

4. The macroscopic rate of crystallisation follows the movement of the growth isotherm if the atomic transfer rate at the growth interface can keep up with this movement. The atomic transfer limitations set in at growth rates of meters per second. At the usual rates of up to several centimeters per minute the growth rates are only limited by planar interface shape breakdown due to constitutional supercooling. 5. The simplest way of shaping the crystal geometry is by using the Bridgman method. Problems may arise due to crucible contact

with the triple phase boundary. The Czochralski-method avoids this problem by using crucible-free growth out of a melt meniscus. The shape of the meniscus is controlled by the Gauss-Laplace relation between hydrostatic pressure, gas pressure and surface tension due to meniscus curvature. By varying the superheating of the fluid the height of the growth interface changes with corresponding variations of the hydrostatic pressure.

6. The chemical composition of the growing crystal ("stoichiometry") is essentially fixed by the thermodynamic equilibrium conditions of the fluid and crystalline phases. This equilibrium is represented graphically by the phase diagram. Detailed knowledge of the phase diagram is indispensable for the design of any growth process.

Inhomogeneities may arise in closed systems in which the composition of the fluid and crystalline phase are different on a microscopic and a macroscopic scale (macro- and microsegregation phenomena). These problems can be avoided by generating a material feed reservoir with constant composition which is possible by using double crucible, floating liquid zone or hot-wall techniques.

7. Microstructure control is the most difficult task of the crystal growth process. The average concentration and spatial distribution of point defects, of defect aggregates, of unwanted impurities, segregation of additional phases depend on phase relations and the time-temperature history of the crystal in a rather complicated way. Often, the only means of optimising the microstructures is crystal annealing after growth.

8. For the same crystal quality crystal growth can be cheap or expensive, depending on the degree of ingenuity of the crystal grower involved. Realistic evaluation of the quality requirements and experience and discipline of the crystal grower is essential in optimising the costs of crystal growth. The future of whole companies depends on the choice of the least expensive way to generate crystals for the market. Research institutions may have been less careful in the past although the times of excessive spending for crystals of low quality which is especially true for those grown in orbiting laboratories are gone.

3. Sample preparation

The process of sample preparation is manyfold. It starts with the building plan of a material which is laid down graphically in the phase diagram. Fig. 3-1 shows the phase diagram of the Al-Ni system as an example. Such diagrams can be used by those without basic knowledge of or not interested in thermodynamics. It is more difficult to understand solid-gas systems properly without thermodynamics. Fig. 3-2 presents various forms of the Ag-O phase diagram as an example. It is important nowadays for the technical development of high-temperature superconductors but also for many other silver-related materials.

There are many compilations of phase diagrams available in the literature in books, computer compilations and in the internet. They have to be used with care. In most cases it is necessary to consult the original publication to get a feeling of the validity of a diagram. Fig. 3-1, for example, wich has been published by renowned physical metallurgists contains errors on the Ni-rich side: Ni3Al is a peritectic phase which emerges from the liquid mixture during cooling on the Ni-rich side of the eutectic and not - as shown in the diagram - on the Al-rich side. In addition, there is an order disorder transformation in Ni3Al which has not been taken into account in drawing the diagram.

Sample production processes can be split into several interdependent parts: Fig. 3-1: Phase diagram of the binary Al-Ni system

 Raw-material preparation. Production of pure materials, sometimes of isotopic purity. This is mostly done by companies specialising in purification.

 Synthesis of polycrystalline raw material with the proper chemical composition. Cold crucible techniques are quite useful for this step if hot inert crucibles cannot be found.

 Crystal growth

 Post-growth treatment, especially annealing treatment Fig. 3-3 presents an example of the information required for this step in the form of time-temperature-transformation graphs which describe the kinetics of solid state reactions.

 Shaping of crystals by mechanical sawing, spark or electrical discharge erosion or chemical erosion.  Proper use as resarch or construction material in technology and jewellery.

Material and process characterisations are required in each step. Often, the shaping steps and the post-growth treatment are more difficult than the crystal growth process.

Crystal quality depends on its use. Many users of crystals put down excessive quality requirements which often are not justified in view of the way the crystals are used in experiments or devices. The other extreme is not uncommon: the use of unqualified samples for sophisticated experiments.

Quality assessments and quality definitions have to be adjusted properly to the user requirements to avoid expensive over or underratings of the required quality. This problem can be solved only with the help of locally available crystal growth specialists. Therefore, crystal

laboratories are indispensable for solid state research institutions even if part of the samples is purchased on the market. Fig. 3-3: Projection in pressure direction for P(O2) < 1 bar

Raw materials of rather high purity are available on the market. Therefore, trace impurity content determination is usually not difficult except for light impurities like hydrogen, carbon and oxygen. High accuracy is not required. Precision stoichiometry analysis of majority components of non-monocomponent crystals belongs to the most difficult tasks of analytical chemistry because very high precision is required. Chemical analyis with high spatial resolution is a Sisyphos task.

4. Future aspects

There is no tendency in sight of a saturation or decline of bulk crystal requirements in research and technology. Many crystals have not yet been grown because the proper compositions and seeds have not yet been detected. New materials emerge continuously. High-temperature superconductors and quasicrystals are two examples. Most regions of the crystal world are still unknown, even in the natural kingdom. The world of industrial crystals is characterised by an enormous market push for size and quality increase and cost decrease. There is no indication that work is running out for crystal growth laboratories and companies.

5. Remarks and hints for further reading

Bohm, J.:

Steno 1669: unit cell concept

Guglielmi 1688: Korrelation shae - chemical cpecies. 17. Jh. "crystal, cristallisation"

Kepler: atomic models of snow crystals, Abhandlung über "Nix", Haüy 1784, Boyle, Neptunists (water solutions) quarrel with Plutonists (high temperature events) Boyle: immaterial powers emerging from crystals

1900 Gibbs: thermodynamics pioneer

Crystal growth is turning industrial: Verneuil, Djeva, Bitterfeld: Ruby Saphire Tammann Bridgman Czochralski: crystal growth pioneers

Kapitza: turns to horizontal Bridgman-method for crystals which expand during crystallisation. Basically wrong view of the atomic scale crystallisation process Nacken-Kyropoulos 50s: Burton-Cabrera-Frank: Spiral growth, roughning of dislocation lines and surfaces Hartmann-Perdok: PBC Vectors

Chalmers, Rutter, Tiller, Jackson: constitutional supercooling Bohm, J.:

Realstruktur von Kristallen, E Schweizerbart´sche Verlagsbuchhandlg Stuttgart 1995 Brice, J.C.:

Crystal Growth Processes, Blackie, Glasgow 1986. all aspects of crystal growth. See also: H Arend and J Hulliger, Crystal Growth in Science and Technology, eds., Plenum and NATO 1989

Cahn, Robert W. and Eric Lifshin, editors:

Concise Encyclopedia of Materials Characterization, Pergamon Press, Oxford 1993 Chalmers, B.:

Principles of Solidification, Wiley New York 1964 Falckenberg, R. (1978):

The Verneuil Process. In: Crystal Growth, Theory and Techniques. Vol. 2. (Ed: Goodman) Plenum Press, New York, 109-184. In-depth review of the Verneuil Process

Fromm, E. und Gebhardt E., Herausgeber:

Gase und Kohlenstoff in Metallen, Springer, Berlin 1976. Extremely valuable cource of information Hurle, D.T.J. Editor:

Zone melting

From Wikipedia, the free encyclopedia

Liquid moves from left to right during melting in the float-zone crystal growth process

Silicon crystal in the beginning of the growth process

A high-purity (99.999% = 5N)tantalum single crystal, made by the floating zone process (cylindrical object in the center)

Zone melting (or zone refining or floating zone process) is a group of similar methods of purifying crystals, in which a narrow region of a crystal is melted, and this molten zone is moved along the crystal. The molten region melts impure solid at its forward edge and leaves a wake of purer material solidified behind it as it moves through the ingot. The impurities concentrate in the melt, and are moved to one end of the ingot. Zone refining was developed by William Gardner Pfann in Bell Labs as a method to prepare high purity materials, mainlysemiconductors, for manufacturing transistors. Its early use was on germanium for this purpose, but it can be extended to virtually anysolute-solvent system having an appreciable concentration difference between solid and liquid phases at equilibrium.[1] _{This process is also known as the float}

zone process, particularly in semiconductor materials processing.

Contents

[hide]

 1 Process details o 1.1 Heaters

o 1.2 Mathematical expression of impurity concentration

 2 Applications o 2.1 Solar cells  3 Related processes o 3.1 Zone remelting  4 See also  5 References

Process details

[

edit

]

The principle is that the segregation coefficient k (the ratio of an impurity in the solid phase to that in the liquid phase) is usually less than one. Therefore, at the solid/liquid boundary, the impurity atoms will diffuse to the liquid region. Thus, by passing a crystal boule through a thin section of furnace very slowly, such that only a small region of the boule is molten at any time, the impurities will be segregated at the end of the crystal. Because of the lack of impurities in the leftover regions

which solidify, the boule can grow as a perfect single crystal if aseed crystal is placed at the base to initiate a chosen direction of crystal growth. When high purity is required, such as in semiconductor industry, the impure end of the boule is cut off, and the refining is repeated.

In zone refining, solutes are segregated at one end of the ingot in order to purify the remainder, or to concentrate the impurities. In zone leveling, the objective is to distribute solute evenly throughout the purified material, which may be sought in the form of a single crystal. For example, in the preparation of a transistor or diodesemiconductor, an ingot of germanium is first purified by zone refining. Then a small amount of antimony is placed in the molten zone, which is passed through the pure germanium. With the proper choice of rate of heating and other variables, the antimony can be spread evenly through the germanium. This technique is also used for the preparation of silicon for use incomputer chips.

Heaters

[

edit

]

A variety of heaters can be used for zone melting, with their most important characteristic being the ability to form short molten zones that move slowly and uniformly through the ingot. Induction coils, ring-wound resistance heaters, or gas flames are common methods. Another method is to pass an electric current directly through the ingot while it is in a magnetic field, with the resulting magnetomotive force carefully set to be just equal to the weight in order to hold the liquid suspended. Optical heaters using high powered halogen or xenon lamps are used extensively in research facilities particularly for the production of insulators, but their use in industry is limited by the relatively low power of the lamps, which limits the size of crystals produced by this method. Zone melting can be done as a batch process, or it can be done continuously, with fresh impure material being continually added at one end and purer material being removed from the other, with impure zone melt being removed at whatever rate is dictated by the impurity of the feed stock.

Indirect-heating floating zone methods use an induction-heated tungsten ring to heat the ingot radiatively, and are useful

when the ingot is of a high-resistivity semiconductor on which classical induction heating is ineffective.

Mathematical expression of impurity concentration

[

edit

]

When the liquid zone moves by a distance , the number of impurities in the liquid change. Impurities are incorporated in the melting liquid and freezing solid.[2]

: Segregation coefficient : Zone length

: Initial uniform impurity concentration of the rod : Concentration of impurities in the liquid : Number of impurities in the liquid

: Number of impurities in zone when first formed at bottom

The number of impurities in the liquid changes in accordance with the expression below during the movement of the molten zone

Applications

[

edit

]

Solar cells

[

edit

]

In solar cells float zone processing is particularly useful because the single crystal silicon grown has desirable properties. The bulk charge carrier lifetime in float-zone silicon is the highest among various manufacturing processes. Float-zone carrier lifetimes are around 1000 microseconds compared to 20-200 microseconds with Czochralski process, and 1–30 microseconds with cast multi-crystalline silicon. A longer bulk lifetime increases the efficiency of solar cells significantly.

Related processes

[

edit

]

Zone remelting

[

edit

]

Another related process is zone remelting, in which two solutes are distributed through a pure metal. This is important in the manufacture of semiconductors, where two solutes of opposite conductivity type are used. For example, in germanium, pentavalent elements of group V such

as antimony and arsenic produce negative (n-type) conduction and the trivalent elements of group III such

as aluminum and boron produce positive (p-type) conduction. By melting a portion of such an ingot and slowly refreezing it, solutes in the molten region become distributed to form the desired n-p and p-n junctions.

See also

[

edit

]

 Fractional freezing

 Freeze distillation

 Laser-heated pedestal growth

 Wafer (electronics)

References

[

edit

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1. Jump up ^ Float Zone Crystal Growth

2. Jump up ^ James D. Plummer, Michael D. Deal, and Peter B. Griffin, Silicon VLSI Technology, Prentice Hall, 2000, p. 129

 William G. Pfann (1966) Zone Melting, 2nd edition, John Wiley & Sons.

 Hermann Schildknecht (1966) Zone Melting, Verlag Chemie.

 Georg Müller (1988) Crystal growth from the

melt Springer-Verlag, Science 138 pages ISBN 3-540-18603-4, ISBN 978-3-540-18603-8 Categories:  Industrial processes  Semiconductor growth  Chemical processes  Crystals

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Zone-Refining





This method is particularly used when metals are required in high degree of purity. It is based on the

principle that when an impure metal in a molten state is allowed to cool, only the metal crystallizes

while the impurities remain present in the molten state (mass) or melt.



The impure metal converted into a rod which is heated at one end with a circular

heater. A narrow zone of metal is melted. The heater is slowly moved along the rod. The pure

metal recrystallizes out of melt while the impurities remain in the melt which moves along with the

melted zone of the rod with the movement of heater. The process is repeated several times. The end of

rod where impurities have collected is cut off. This method is employed for the purification of

germanium, silicon, gallium etc, which are used in semi – conductors.

Organic luminescent materials

Although the inorganic phosphors are industrially produced in far higher quantities (several hundred tons per year) than

the organic luminescent materials, some types of the latter are becoming more and more important in special fields of

practical application. Paints and dyes for outdoor advertising contain strongly fluorescing organic molecules such as

fluorescein, eosin, rhodamine, and stilbene derivatives. Their main shortcoming is their relatively poor stability in light,

because of which they are used mostly when durability is not required. Organic phosphors are used as optical brighteners

for invisible markers of laundry, banknotes, identity cards, and stamps and for fluorescence microscopy of tissues in

biology and medicine. Their “invisibility” is due to the fact that they absorb practically no visible light. The fluorescence

is excited by invisible ultraviolet radiation (black light).

 QUIZZES

 LISTS





Photoradiation in

gases

,

liquids

, and crystals

When describing chemical principles associated with luminescence, it is useful, at first, to neglect interactions between the

luminescing atoms, molecules, or centres with their environment. In the gas phase these interactions are smaller than they

are in the condensed phase of a liquid or a solid material. The efficiency of luminescence in the gas phase will be far

greater than in the condensed phases because in the latter the energy of the electrons excited by photons or by

chemical-reaction energy can be dissipated as thermal, nonradiative energy by collision of the atoms or by the rotational and

vibrational energy of the molecules. This effect has to be taken into account even more when the radiation of single atoms

is compared with that of multi-atomic molecules. For molecules, radiative (electronic-excitation) energy is internally

converted to vibrational energy; that is, there are radiationless transitions of electrons in atoms. This is the explanation for

the fact that only a relatively small number of compounds are able to exhibit efficient luminescence. In crystals, on the

other hand, the binding forces between the ions or atoms of the lattice are strong compared with the forces acting between

the particles of a liquid, and electron-excitation energy, therefore, is not as easily transformed into vibrational energy, thus

leading to a good efficiency for radiative processes.

Luminescence physics

Mechanism of luminescence

The emission of visible light (that is, light of wavelengths between about 690 nanometres and 400 nanometres,

corresponding to the region between deep red and deep violet) requires

excitation

energies the minimum of which is given

by Einstein’s law stating that the

energy

(E) is equal to Planck’s constant (h) times the frequency of light (ν), or Planck’s

constant times the velocity of light (c) in a vacuum divided by its wavelength (λ); that is,

The energy required for excitation therefore ranges between 40 kilocalories (for red light), about 60 kilocalories (for

yellow light), and about 80 kilocalories (for violet light) per mole of substance. Instead of expressing these energies in

kilocalories, electron volt units (one electron volt = 1.6 × 10

−12

_{erg; the erg is an extremely small unit of energy) may be}

used, and the photon energy thus required in the visible region ranges from 1.8 to 3.1

electron

volts.

The excitation energy is transferred to the electrons responsible for luminescence, which jump from their

ground-state

energy level

to a level of higher energy. The energy levels that electrons can assume are specified by quantum

mechanical laws. The different excitation mechanisms considered below depend on whether or not the excitation of

electrons occurs in single atoms, in single molecules, in combinations of molecules, or in a crystal. They are initiated by

the means of excitation described above: impact of accelerated particles such as electrons, positive ions, or photons. Often,

the excitation energies are considerably higher than those necessary to lift electrons to a radiative level; for example, the

luminescence produced by the phosphor crystals in television screens is excited by cathode-ray electrons with average

energies of 25,000 electron volts. Nevertheless, the colour of the luminescent light is nearly independent of the energy of

the exciting particles, depending chiefly on the excited-state energy level of the crystal centres.

Electrons taking part in the luminescence process are the outermost electrons of atoms or molecules. In

fluorescent lamps

,

for example, a mercury atom is excited by the impact of an electron having an energy of 6.7 electron volts or more, raising

one of the two outermost electrons of the mercury atom in the ground state to a higher level. Upon the electron’s return to

the ground state, an energy difference is emitted as ultraviolet light of a wavelength of 185 nanometres. A radiative

transition between another excited state and the ground-state level of the mercury atom produces the important ultraviolet

emission of 254-nanometre wavelength, which, in turn, can excite other phosphors to emit visible light. (One such

phosphor frequently used is a calcium halophosphate incorporating a heavy-metal activator.)

This 254-nanometre mercury radiation is particularly intensive at low mercury vapour pressures (around 10

−5

_atmosphere)

used in low-pressure discharge lamps. About 60 percent of the input electron energy may thus be transformed into

near-monochromatic ultraviolet light—i.e., ultraviolet light of practically one single wavelength.

Whereas at low pressure there are relatively few collisions of mercury atoms with each other, the collision frequency

increases enormously if mercury gas is excited under high pressure (e.g., eight atmospheres or more). Such excitation

leads not only to collisional de-excitation of excited atoms but also to additional excitation of excited atoms. As a

consequence, the spectrum of the emitted radiation no longer consists of practically one single, sharp spectral

line

at 254

nanometres, but the radiation energy is distributed over various broadened spectral lines corresponding to different

electronic energy levels of the mercury atom, the strongest emissions lying at 303, 313, 334, 366, 405, 436, 546, and 578

nanometres. High-pressure mercury lamps can be used for illumination purposes because the emissions from 405 to 546

nanometres are visible light of bluish green colour; by transforming a part of the mercury line emission to red light by

means of a phosphor, white light is obtained.

When gaseous molecules are excited, their luminescence spectra show broad bands; not only are electrons lifted to levels

of higher energy but vibrational and rotational motions of the atoms as a whole are excited simultaneously. This is because

vibrational and rotational energies of molecules are only about 10

−2

_{and 10}

−4

_{, respectively, those of the electronic transition}

energies, and these many energies can be added to the energy of a single electronic transition, which is represented by a

multitude of slightly different wavelengths making up one band. In larger molecules, several overlapping bands, one for

each kind of electronic transition, can be emitted. Emission from molecules in solution is predominantly bandlike caused

by interactions of a relatively great number of excited molecules with molecules of the solvent. In molecules, as in atoms,

the excited electrons generally are outermost electrons of the molecular orbitals.

The terms fluorescence and phosphorescence can be used here, on the basis not only of the persistence of luminescence

but also of the way in which the luminescence is produced. When an electron is excited to what is called, in

spectroscopy

,

an excited singlet state, the state will have a lifetime of about 10

−8

_{second, from which the excited electron can easily return}

to its ground state (which normally is a singlet state, too), emitting its excitation energy as fluorescence. During this

electronic transition the spin of the electron is not altered; the singlet ground state and the excited singlet state have like

multiplicity (number of subdivisions into which a level can be split). An electron, however, may also be lifted, under

reversal of its spin, to a higher energy level, called an excited triplet state. Singlet ground states and excited triplet states

are levels of different multiplicity. For quantum mechanical reasons, transitions from triplet states to singlet states are

“forbidden,” and, therefore, the lifetime of triplet states is considerably longer than that of singlet states. This means that

luminescence originating in triplet states has a far longer duration than that originating in singlet states: phosphorescence

is observed.

The interactions of a large number of atoms, ions, or molecules are greater still in solution and in solids; to obtain a

narrowing of the spectral band, subzero temperatures (down to that of liquid helium) are applied in order to reduce

vibrational motions. The electronic energy levels of crystals such as zinc sulfide and other host crystals used in phosphors

form bands: in the ground state practically all electrons are to be found on the

valence band

, whereas they reach

the

conduction band

after sufficient excitation. The energy difference between the valence band and the conduction band

corresponds to photons in the ultraviolet or still shorter wavelength region. Additional energy levels are introduced by

activator ions or centres bridging the energy gap between valence band and conduction band, and, when an electron is

transferred from the valence band to such an additional energy level by excitation energy, it can produce visible light on

return to the ground state. A rather close analogy exists between the forbidden transitions of certain excited molecular

electronic states (triplet–singlet, leading to phosphorescence) and the transition of an electron of an inorganic phosphor

kept in a

trap

: traps (certain distortions in the crystal lattice) are places in the crystal lattice where the energy level is lower

than that of the conduction band, and from which the direct return of an electron to the ground state is also forbidden.

When a solid is bombarded by photons or particles, the excitation of the centres can occur directly or by energy transfer. In

the latter case, excited but nonluminescing states are produced at some distance from the centre, with the energy moving

through the crystal in the form of

excitons

(ion-electron pairs) until it approaches a centre where the excitation process can

occur. This energy transfer can also be realized by radiation in inorganic phosphors containing two activators, as well as in

solutions of organic molecules.