Electroluminescence radiation. Physics of p-n junction injection

Luminescence excited by an electric field

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Description

Electroluminescence is luminescence excited by an electric field. It is observed in gases and solids. With electroluminescence, the atoms (molecules) of a substance pass into an excited state as a result of the occurrence of some form of electric discharge in it. Of the various types of electroluminescence in solids, injection and pre-breakdown electroluminescence are the most important. Injection electroluminescence is characteristic of the pn junction in some semiconductors, such as SiC or GaP, in a constant electric field included in the forward direction. Excess holes are injected into the n-region, and electrons are injected into the p-region (or both into a thin layer between the p- and n-regions). The luminescence arises from the recombination of electrons and holes in the p-n layer.

Pre-breakdown electroluminescence is observed, for example, in powdered ZnS activated by Cu, Al, etc., placed in a dielectric between the plates of a capacitor, to which an alternating voltage of audio frequency is applied. At the maximum voltage on the capacitor plates, processes close to electrical breakdown occur in the phosphor: a strong electric field is concentrated at the edges of the phosphor particles, which accelerates free electrons. These electrons can ionize atoms; the holes formed are captured by luminescence centers, on which electrons recombine when the field direction changes.

Timing

Initiation time (log to -3 to -1);

Lifetime (log tc from -1 to 9);

Degradation time (log td -6 to -3);

Optimal development time (log tk 0 to 6).

Diagram:

Technical realizations of the effect

Option 1:

In reality, it is an ordinary mains screwdriver-probe inserted into the mains socket to check for voltage.

Electroluminescence in a gas indicator

Rice. one

Designations:

3 - fluorescent tube of arbitrary shape;

Option 2: Solid State Implementation in a p-n Electroluminescence Semiconductor

Really - a standard LED used for light indication of inclusion in modern electronic household appliances.

Solid-state implementation in the p-n junction of electroluminescence

Rice. 2

Designations:

3 - p-n transition;

4 - flux of luminescent radiation;

U - variable EMF voltage.

Applying an effect

It is observed in semiconductor substances and crystal phosphors, the atoms (or molecules) of which pass into an excited state under the influence of a passed electric current or an applied electric field.

Mechanism

Electroluminescence is the result of the radiative recombination of electrons and holes in a semiconductor. Excited electrons give up their energy in the form of photons. Before recombination, electrons and holes are separated - either by activating the material to form a p-n junction (in semiconductor electroluminescent illuminators such as LED) - or by excitation by high-energy electrons (the latter are accelerated by a strong electric field) - in crystal phosphors of electroluminescent panels.

Electroluminescent materials

Typically, electroluminescent panels are produced in the form of thin films of organic or inorganic materials. In the case of the use of crystal phosphors, the color of the glow is determined by an impurity - an activator. Structurally, the electroluminescent panel is a flat capacitor. Electroluminescent panels require a sufficiently high voltage supply (60 - 600 volts); for this, as a rule, a voltage converter is built into the device with electroluminescent backlight.

Examples of thin film electroluminescent materials:

• Powdered zinc sulfide activated with copper or silver (blue-green glow);
• Zinc sulfide activated with manganese - yellow-orange glow;
• Semiconductors III-V InP, GaAs, GaN (LEDs).

Application

Electroluminescent illuminators (panels, wires, etc.) are widely used in consumer electronics and lighting engineering, in particular, for illuminating liquid crystal displays, illuminating instrument scales and film keyboards, decorating buildings and landscapes, etc.

Electroluminescent graphic and character-synthesizing displays are produced for military and industrial applications. These displays are characterized by high image quality and relatively low sensitivity to temperature conditions.

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Literature

• Gershun A.L.,.// Encyclopedic Dictionary of Brockhaus and Efron: in 86 volumes (82 volumes and 4 additional). - St. Petersburg. , 1890-1907.

Links

• (unavailable link - story , copy)

An excerpt characterizing Electroluminescence

Ministry of Higher Education of Ukraine

National Technical University of Ukraine

"Kyiv Polytechnic Institute"

Abstract on the topic:

Luminescence

electroluminescence

Completed by: 2nd year student

PSF PM-91 Milokosty A. A.

Checked by: Nikitin A.K.

Plan:

1. Introduction__________________________________3

2. Classification of luminescence phenomena _______4

3. Types of luminescence ____________________________5

4. Physical characteristics of luminescence___7

5. Kinetics of luminescence ____________________7

6. Luminescent substances __________________ 9

7. Methods of research ____________________________________11

8. Phosphors________________________________11

9. List of used literature __________14

Introduction

Luminescence - radiation, which is an excess over the thermal radiation of the body at a given temperature and having a duration significantly exceeding the period of light waves. The first part of this definition was proposed by E. Wiedomann and separates luminescence from equilibrium thermal radiation. The second part - a sign of duration - was introduced by S. I. Vavilov in order to separate luminescence from other phenomena of secondary luminescence - reflection and scattering of light, as well as from stimulated emission, bremsstrahlung of charged particles.

For the occurrence of luminescence, therefore, some source of energy is required, different from the equilibrium internal energy of a given body, corresponding to its temperature. To maintain stationary luminescence, this source must be external. Non-stationary luminescence can occur during the transition of the body to an equilibrium state after preliminary excitation (luminescence decay). As follows from the definition itself, the concept of luminescence refers not to individual radiating atoms or molecules, but also to their aggregates - bodies. The elementary acts of excitation of molecules and emission of light can be the same in the case of thermal radiation and luminescence. The difference consists only in the relative number of certain energy transitions. It also follows from the definition of luminescence that this concept is applicable only to bodies having a certain temperature. In the case of a strong deviation from thermal equilibrium, it makes no sense to speak of thermal equilibrium or luminescence.

The sign of duration is of great practical importance and makes it possible to distinguish luminescence from other nonequilibrium processes. In particular, he played an important role in the history of the discovery of the Vavilov-Cherenkov phenomenon, making it possible to establish that the observed glow cannot be attributed to luminescence. The question of the theoretical substantiation of the Vavilov criterion was considered by B.I. Stepanov and B. A. Afanasevich. According to them, for the classification of secondary luminescence, the existence or absence of intermediate processes between the absorption of energy that excites luminescence and the emission of secondary luminescence (for example, transitions between electronic levels, changes in vibrational energy, etc.) is of great importance. Such intermediate processes are characteristic of luminescence (in particular, they take place during non-optical excitation of luminescence).

Classification of luminescence phenomena

According to the type of excitation, there are: ionoluminescence, candoluminescence, cathodoluminescence, radioluminescence, X-ray luminescence, electroluminescence, photoluminescence, chemiluminescence, triboluminescence. According to the duration of luminescence, fluorescence (short glow) and phosphorescence (long glow) are distinguished. Now these concepts have retained only a conditional and qualitative meaning, since it is impossible to indicate any boundaries between them. Sometimes fluorescence is understood as spontaneous luminescence, and phosphorescence as stimulated luminescence (see below).

The most rational classification of luminescence phenomena, based on the characteristics of the mechanism of elementary processes, was first proposed by Vavilov, who distinguished between spontaneous, forced, and recombination luminescence processes. Subsequently, resistive luminescence was also isolated.

Types of luminescence

1) resonant luminescence(more commonly called resonant fluorescence ) observed in atomic vapors (mercury, sodium, etc.) in some simple molecules and, sometimes, in more complex systems. The emission is spontaneous and occurs from the same energy level that is achieved by absorbing the energy of the exciting light. As the vapor density increases, resonant luminescence transforms into resonant scattering.

In all cases, this type of luminescence should not be related to luminescence and should be called resonant scattering.

2) Spontaneous luminescence involves a transition (radiative or, more commonly, non-radiative) to the energy level from which radiation occurs. This type of luminescence is typical for complex molecules in vapors and solutions, and for impurity centers in solids. A special case is represented by luminescence due to transitions from exciton states.

3) Metastable or stimulated luminescence is characterized by a transition to a metastable level that occurs after energy absorption and a subsequent transition to the radiation level as a result of the communication of vibrational energy (due to the internal energy of the body) or an additional quantum of light, for example, infrared. An example of this type of luminescence is the phosphorescence of organic substances, in which the lower triplet level of organic molecules is metastable. At the same time, in many cases, two bands of luminescence duration are observed: long-wavelength, corresponding to the spontaneous transition T-S 0 and then (slow fluorescence or β-band), and short-wavelength, coinciding in spectrum with fluorescence and corresponding to the forced transition T-S 1 and then spontaneous transition s 1 -s 0 (phosphorescence or α-band).

4) Recombination luminescence occurs as a result of the reunification of particles separated by the absorption of exciting energy. In gases, recombination of radicals or ions can occur, resulting in a molecule in an excited state. The subsequent transition to the ground state may be accompanied by luminescence. In solid crystalline bodies, recombination luminescence arises as a result of the appearance of nonequilibrium charge carriers (electrons or holes) under the action of some energy source. A distinction is made between recombination luminescence during “zone-zone” transitions and luminescence of defective or impurity centers (the so-called. luminescence centers). In all cases, the luminescence process may include the capture of carriers in traps with their subsequent release by thermal or optical means, i.e., include an elementary process characteristic of metastable luminescence. In the case of luminescence centers, recombination consists in the capture of holes to the ground level of the center and electrons to the excited level. Radiation occurs as a result of the transition of the center from the excited state to the ground state. Recombination luminescence is observed in crystal phosphors and typical semiconductors such as germanium and silicon. Regardless of the mechanism of the elementary process leading to luminescence, radiation, in the final case, occurs during a spontaneous transition from one energy state to another. If this transition is allowed, then dipole radiation takes place. In the case of forbidden transitions, the radiation can correspond to both an electric and a magnetic dipole, an electric quadrupole, and so on.

Physical characteristics of luminescence

Like any radiation, luminescence is characterized by a spectrum (spectral density of the radiant flux) and a state of polarization. The study of luminescence spectra and the factors affecting them is part of spectroscopy.

Along with these general characteristics, there are specific ones for luminescence. Luminescence intensity in itself is rarely of interest. Instead, the value of the ratio of radiated energy to absorbed energy is introduced, called luminescence output. In most cases, output is defined under stationary conditions as the ratio of radiated and absorbed power. In the case of photoluminescence, the concept of quantum yield is introduced and the yield spectrum is considered, i.e. the dependence of the output on the frequency of the exciting light; and the polarization spectrum, the dependence of the degree of polarization on the frequency of the exciting light. In addition, the polarization of luminescence is characterized by polarization diagrams, the form of which is related to the orientation and multipole nature of elementary emitting and absorbing systems.

Luminescence kinetics, in particular, the shape of the rise curve after the excitation is turned on and the luminescence decay curve after it is turned off, and the dependence of the kinetics on various factors: temperature, intensity of the exciting source, etc., are important characteristics of luminescence. The kinetics of luminescence depends to a large extent on the type of elementary process, although it is not uniquely determined by it. The damping of spontaneous luminescence with a quantum yield close to unity always occurs according to the exponential law: I(t)=I 0 exp(-l/τ), where τ characterizes the average lifetime of the excited state, i.e., is equal to the reciprocal of the probability A spontaneous transition per unit of time. However, if the luminescence quantum yield is less than unity, i.e., the luminescence is partially quenched, then the exponential decay law is preserved only in the simplest case, when the quenching probability Q is constant. In this case, τ=1/(A+Q), and the quantum yield η=A/(A+Q), where Q is the probability of a nonradiative transition. However, Q often depends on the time elapsed from the moment of excitation of a given molecule, and then the luminescence decay law becomes more complicated. The kinetics of stimulated luminescence in the case of one metastable level is determined by the sum of two exponentials.

Luminescence is the emission of light by certain materials in a relatively cold state. It differs from the radiation of incandescent bodies, for example, or coal, molten iron and wire heated by an electric current. Luminescence emission is observed:

• in neon and fluorescent lamps, televisions, radars and fluoroscope screens;
• in organic substances such as luminol or luciferin in fireflies;
• in some pigments used in outdoor advertising;
• with lightning and northern lights.

In all of these phenomena, light emission is not the result of heating the material above room temperature, which is why it is called cold light. The practical value of luminescent materials lies in their ability to transform invisible forms of energy into

Sources and process

The phenomenon of luminescence occurs as a result of the absorption of energy by a material, for example, from a source of ultraviolet or X-ray radiation, electron beams, chemical reactions, etc. This brings the atoms of the substance into an excited state. Since it is unstable, the material returns to its original state and the absorbed energy is released as light and/or heat. Only outer electrons are involved in the process. The efficiency of luminescence depends on the degree of conversion of excitation energy into light. The number of materials with sufficient efficiency for practical use is relatively small.

Luminescence and incandescence

The excitation of luminescence is not connected with the excitation of atoms. When hot materials begin to glow as a result of incandescence, their atoms are in an excited state. Although they vibrate already at room temperature, this is enough for the radiation to occur in the far infrared region of the spectrum. As the temperature rises, the frequency of electromagnetic radiation shifts to the visible region. On the other hand, at very high temperatures, such as those created in shock tubes, the collisions of atoms can be so violent that electrons separate from them and recombine, emitting light. In this case, luminescence and incandescence become indistinguishable.

Luminescent pigments and dyes

Ordinary pigments and dyes have color, since they reflect that part of the spectrum that is complementary to the absorbed one. A small part of the energy is converted into heat, but no noticeable radiation occurs. If, however, the luminescent pigment absorbs daylight in a certain part of the spectrum, it can emit photons that differ from those reflected. This occurs as a result of processes within the dye or pigment molecule by which ultraviolet light can be converted into visible light, such as blue light. Such luminescence techniques are used in outdoor advertising and in laundry detergents. In the latter case, the "clarifier" remains in the fabric not only to reflect white, but also to convert ultraviolet radiation to blue, which compensates for the yellowness and enhances the whiteness.

Early research

Although lightning, northern lights and the dim glow of fireflies and mushrooms have always been known to mankind, the first studies of luminescence began with synthetic material, when Vincenzo Cascariolo, an alchemist and shoemaker from Bologna (Italy), in 1603 heated a mixture of barium sulfate (in the form of barite, heavy spar) with coal. The powder obtained after cooling emitted a bluish glow at night, and Cascariolo noticed that this could be restored by exposing the powder to sunlight. The substance was called lapis solaris, or sunstone, because the alchemists hoped it could turn metals into gold, symbolized by the sun. The afterglow aroused the interest of many scientists of that period, who gave the material other names, including "phosphorus", which means "bearer of light."

Today, the name "phosphorus" is only used for the chemical element, while microcrystalline luminescent materials are called phosphor. Cascariolo's "phosphorus" appears to have been barium sulfide. The first commercially available phosphor (1870) was "Balmain's paint" - a solution of calcium sulfide. In 1866, the first stable zinc sulfide phosphor was described - one of the most important in modern technology.

One of the first scientific studies of luminescence, which is manifested in the decay of wood or flesh and in fireflies, was carried out in 1672 by the English scientist Robert Boyle, who, although not aware of the biochemical origin of this light, nevertheless established some of the main properties of bioluminescent systems:

• glow cold;
• it can be suppressed by such chemical agents as alcohol, hydrochloric acid and ammonia;
• radiation requires access to air.

In 1885-1887, it was noticed that crude extracts obtained from West Indian fireflies (fire nutcrackers) and from folad clams produce light when mixed.

The first effective chemiluminescent materials were non-biological synthetic compounds such as luminol, discovered in 1928.

Chemi- and bioluminescence

Most of the energy released in chemical reactions, especially oxidation reactions, is in the form of heat. In some reactions, however, some of it is used to excite electrons to higher levels, and in fluorescent molecules, to chemiluminescence (CL). Studies show that CL is a universal phenomenon, although the luminescence intensity is sometimes so low that the use of sensitive detectors is required. There are, however, some compounds that show bright CL. The best known of these is luminol, which, when oxidized with hydrogen peroxide, can produce a strong blue or blue-green light. Other strong CL substances are lucigenin and lofin. Despite the brightness of their CLs, not all of them are effective in converting chemical energy into light energy, since less than 1% of the molecules emit light. In the 1960s, oxalic acid esters oxidized in anhydrous solvents in the presence of highly fluorescent aromatic compounds were found to emit bright light with an efficiency of up to 23%.

Bioluminescence is a special type of CL catalyzed by enzymes. The luminescence yield of such reactions can reach 100%, which means that each molecule of the reacting luciferin passes into the radiating state. All bioluminescent reactions known today are catalyzed by oxidation reactions occurring in the presence of air.

Thermally stimulated luminescence

Thermoluminescence does not mean thermal radiation, but an increase in the light radiation of materials whose electrons are excited by heat. Thermally stimulated luminescence is observed in some minerals, and above all in crystal phosphors after they have been excited by light.

Photoluminescence

Photoluminescence, which occurs under the action of electromagnetic radiation incident on a substance, can be produced in the range from visible light through ultraviolet to x-rays and gamma rays. In photon-induced luminescence, the wavelength of the emitted light is usually equal to or greater than the wavelength of the exciting light (i.e., equal to or less than the energy). This difference in wavelength is due to the incoming energy being converted into vibrations of atoms or ions. Sometimes, when exposed to a laser beam intensely, the emitted light may have a shorter wavelength.

The fact that PL can be excited by ultraviolet radiation was discovered by the German physicist Johann Ritter in 1801. He noticed that phosphors glow brightly in the invisible region beyond the violet part of the spectrum, and thus discovered UV radiation. The conversion of UV to visible light is of great practical importance.

At high pressure, the frequency increases. The spectra no longer consist of a single 254 nm spectral line, but the emission energy is distributed over spectral lines corresponding to different electronic levels: 303, 313, 334, 366, 405, 436, 546 and 578 nm. High-pressure mercury lamps are used for illumination, since 405-546 nm correspond to visible bluish-green light, and when part of the radiation is transformed into red light using a phosphor, the result is white.

When gas molecules are excited, their luminescence spectra show broad bands; not only do the electrons rise to higher energy levels, but the vibrational and rotational motions of the atoms as a whole are simultaneously excited. This is because the vibrational and rotational energies of the molecules are 10 -2 and 10 -4 of the transition energies, which add up to form many slightly different wavelengths that make up one band. In larger molecules, there are several overlapping bands, one for each type of transition. The radiation of molecules in solution is predominantly ribbon-like, which is caused by the interaction of a relatively large number of excited molecules with solvent molecules. In molecules, as in atoms, the outer electrons of molecular orbitals participate in luminescence.

Fluorescence and phosphorescence

These terms can be distinguished not only on the basis of the duration of the glow, but also on the basis of the method of its production. When an electron is excited to a singlet state with a residence time of 10 -8 s, from which it can easily return to the ground state, the substance radiates its energy in the form of fluorescence. During the transition, the spin does not change. The ground and excited states have a similar multiplicity.

An electron, however, can be raised to a higher energy level (called an "excited triplet state") by reversing its spin. In quantum mechanics, transitions from triplet to singlet states are forbidden, and, consequently, their lifetime is much longer. Therefore, the luminescence in this case has a much longer period: phosphorescence is observed.

Electroluminescence is the emission of light under the influence of an electric field or a flowing current. When an electric field is applied to a semiconductor (called a phosphor), impact ionization of atoms by electrons occurs due to the electric field, as well as the emission of electrons from the capture center. As a result, the concentration of free carriers will exceed the equilibrium one and the semiconductor will be in an excited state, i.e. in a state in which its internal energy exceeds the equilibrium at a given temperature.

The device of an electroluminescent emitter (capacitor): a thin layer (up to 20 microns) of a semiconductor (zinc sulfide) is deposited on a metal base, a thin layer of metal transparent to visible light is applied on top of it. When a source (constant or variable) is connected to the metal layers, a greenish-blue glow appears, the brightness of which is proportional to the U value of the source. If zinc selenide is included in the composition of the phosphor, then a white, yellow or orange glow can be obtained.

Flaws:

Low performance;

Unstable parameter;

Low brightness of the glow;

Small resource.

Electroluminescence is also observed in semiconductor diodes, when current flows through the diode, with direct connection. In this case, electrons pass from the n-region to the p-region and recombine with holes there. Depending on the band gap, photons have frequencies in the human visible or invisible part of the light spectrum, made of silicon, emit invisible infrared light.

For LEDs, materials with a band gap from 1.6 eV to 3.1 eV (these are red and violet colors) are used, and therefore it is widely used to create digital indicators, optocouplers, and lasers.

Advantage:

manufacturability;

High performance;

Long service life;

Reliability;

Micro miniature;

High monochromaticity of radiation.

By design, LEDs are distinguished: injection, semiconductor lasers, superluminescent (occupying intermediate values ​​​​and used in fiber optic lines), with a controlled glow color.

ZSI- sign-synthesizing indicators - in which the image is obtained using a mosaic on independently controlled "electric signal-to-light" converters.

ZSI uses the glow that occurs in phosphors placed in a strong electric field. Structurally, they are a group of capacitors, in which one of the plates is made transparent, and the other is not transparent.

When the source is connected to the plates, the phosphor starts to glow.

If a transparent electrode is made of one shape or another, then the glow zone will repeat the shape. The color of the section depends on the composition of the phosphor. Used in displays.

Luminance brightness depends on U value and frequency: U=160-250V, f=300-4000Hz.

Power consumption hundredths-tenths of a watt, brightness 20-65cd/m 2 .

cathodoluminescence. When the gas is removed from the flask (at a pressure of ≈ 1.3 Pa), the glow of the gas weakens and the walls of the flask begin to glow. Why? Electrons knocked out of the cathode by positive ions rarely collide with gas molecules at such a discharge and therefore, accelerated by the field, hitting the glass causes its glow, the so-called cathodoluminescence, and the flow of electrons is called cathode rays.

Low voltage vacuum luminescence. According to the mechanism of action, it does not differ from high-voltage and is advisory in nature.

Essence - the phosphor is bombarded with electrons, which excite the phosphor and lead to violation of thermodynamic equilibrium. Electrons appear, the energy of which is greater than the energy for the conduction band, and holes, which have an energy lower than the ceiling of the valence band. Due to the instability of the nonequilibrium state, the process of recombination begins with the emission of photons by the cathodes, which is accompanied by emission.

If recombination is carried out through a trap, then after a while the carriers can return to their places, which increases the afterglow.

Low voltage luminescence is characterized by:

Type of phosphor;

The depth of penetration in the crystal of the bombarding electrons;

Low-voltage voltage is used (units-tens of volts);

Used in vacuum ZSI;

Heating voltage = 5V;

U a \u003d (20-70) B;

Anode current segment (1-3) mA.

Advantages of vacuum ZSI:

High brightness of the glow;

Multicolor;

Minimum energy consumption;

Great speed.

Disadvantages: it is necessary to have three power sources, the fragility of the design.

Security questions for topic 2:

1 The concept of polarization.

2 Types of polarization.

3 What determines the electrical conductivity of a dielectric?

4 Specify the types of electrical breakdown.

5 Indicate the features of ferroelectrics.

6 Piezo effect and its application.

7 Indicate the types of gas discharge and their features.

8 Features of electroluminescence and cathodoluminescence.