Led

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Optoelectronic devices are generally divided into two main categories.

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Those that convert electrical current to electromagnetic radiation (light) and those that convert light into electrical current

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Luminescence

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Luminescence is defined as the optical radiation due to electronic excitation. When the excited system goes back to the ground state energy is emitted in the form of EM radiation.

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There are different types of luminescence, depending on how the electronic excitation is originally created.

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Photoluminescence -electronic excitation is created by incident light.

2. Cathodoluminescence -electronic excitation is created by an electron beam

3. Radioluminescence -electronic excitation is created by ionizing radiation (β-rays)

4. Electroluminescence -electronic excitation is created by an electrical f ield.

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LEDs usually work by

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electroluminescence. Electric current, i.e. electron and holes, are passed to the device by an applied bias. These electrons and holes recombine to emit light.

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LEDs depend on recombination of electrons and holes. There are 3 main mechanisms of this recombination

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1. Interband transitions 2. Defect transitions 3. Intraband transitions.

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In LEDs the radiative transition must be maximized relative to the nonradiative transition.

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This can be accomplished by choosing the right materials and the right external bias.

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EL. Recombination

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LED is a pn junction. Assume the n region is heavily doped so that the depletion width lies mostly in the p side. At equilibrium the Fermi levels line up and there is a built in potential. When a forward bias is applied, electrons and holes are injected into the depletion region. These recombine, and radiation is emitted whose wavelength depends on the band gap of the p type material. This is called injection EL. Recombination is a statistical spontaneous process and emission takes place in all directions.

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Heterostructures

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If we can confine the electrons and holes to a small region (like a potential well) then it is possible to increase radiative recombination eff iciency. This can be achieved by using a heterojunction. The difference in band alignment between a GaAs based homojunction and GaAs-AlGaAs heterojunction LED is shown in figure 4.

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Homo and heterostructure band alignment.The heterojunction has a higher quantum efficiency since the carriers are localized in GaAs. Thus, recombination occurs only in the i-GaAs region. For both LEDs, the emitted wavelength is the same.

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Consider a LED formed by using AlGaAs and GaAs.

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AlAs is an indirect semiconductor with a band gap of 2.16 eV and GaAs a direct band semiconductor with a gap of 1.42 eV. AlxGa1−xAs, formed by substitutional doping of Ga with Al, is a direct band gap for x < 0.4 and its band gap depends on x, given by 1.43 + 1.247x. For x > 0.4, this becomes an indirect band gap semiconductor. For the heterostructure junction shown in figure 5 the band gap of AlGaAs is 2.0 eV, which corresponds to x = 0.45.

Two heterostructure junctions are formed in this device. At equilibrium the Fermi levels line up and the depletion region lies mostly in the GaAs region, which is lightly doped. When a forward bias is applied, electrons and holes are injected into the depletion region (GaAs) where they recombine radiatively, with energy equal to the band gap of GaAs. The wide band gap AlGaAs acts as confining layers for the carriers.

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The double heterostrucutre device is usually fabricated in such a way that emission takes place from one surface. This is shown in f igure 6. This is usually accomplished by suitable deposition of the electrical contacts.Another example of a double heterostructure system is GaAs1−xPx with GaAs.

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Light emission in the LED is from .....

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band to band transitions i.e. from conduction to valence band.

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What is responsible for the finite width of the LED emission?

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4 LEDline width Light emission in the LED is from band to band transitions i.e. from conduction to valence band. These band transitions also involve thermal fluctuations which cause a slight deviation in the energy of the carriers and are responsible for the finite width of the LED emission.

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Consider light emission in a LED. The energy of the radiation is given by

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The spontaneous emission rate is given by the product of the density of available states, with the occupation probability. This can be written as

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The line width (full width at half maximum) is given by ∆λ

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Theoretical line width in a LED. The line width is determined by two opposing functions, the density of available states, which increases as the square root of the energy and the occupation probability that decreases exponentially with energy. As temperature increases line width increases, since the occupation probability increases while density of states is unchanged.

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(a) PL spectrum from a GaAs LED, at two different temperatures. The spectrum shows a narrowing at lower temperature. (b) Plot of the photon energy vs. temperature showing the peak shift to higher wavelengths (lower energy) with increase in temperature. Due to lattice expansion there is a reduction in band gap with temperature.

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The commonly used LED materials

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GaAs

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LEDs are made of direct band gap semiconductors and since the visible region lies above energy of 1.8 eV , materials with band gap above this value are chosen. Most the commonly used LEDs are based on the GaAS system. GaAs is a direct band gap semiconductor, but its band gap lies in the IR region (1.42 eV ). So higher band gap materials like AlGaAs, GaAsP, GaP (indirect band gap semiconductor) are used with suitable substitutional doping to ‘tune’ the band gap to the required value

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AlGaAs

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AlGaAs -While AlAs is an indirect band gap semiconductor, AlGaAs for Al < 0.45 is a direct band gap semiconductor. This is typically used in the infra red and red regions of the spectrum. AlGaAs can be directly grown on GaAs substrates.

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InAlGaP

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InAlGaP -this covers a wider region in the visible spectrum, from red to green. This is due to the higher band gap of the base GaP system i.e. 2.3 eV . But GaP is an indirect band gap semiconductor so there is only a limited composition region. This also limits the maximum energy of this system. GaP based devices can also be grown on GaAs substrates, due to the low lattice mismatch.

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InGaN

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InGaN -this is a higher band gap LED that covers the green, blue, and violet region of the spectrum. Due to lattice mismatch this cannot be grown on GaAs but usually sapphire, SiC, or GaN is used as a substrate. The substrate cost increases the overall cost of the device.

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GaAsP

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GaAsP -this covers the middle of the visible region to the IR region of the spectrum. It is similar to the AlGaAs system and can be grown on GaAs substrates.

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LED material Growth challenges

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Most of the LED materials are grown by a vapor deposition process as thin films on suitable substrates. Typically, chemical vapor deposition (CVD) is used for the film growth. Variations of CVD, like plasma enhanced CVD (PECVD) or low pressure CVD (LCVD) are also employed. Achieving the right growth conditions to get the exact stoichiometry and microstructure is challenging. This is one of the reason that LED materials are chosen so that growth can be performed on GaAs, since it is one of the few compound semiconductors with extensive background literature along with lower substrate cost.

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ALD technique

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For growing very thin layers, especially on lattice mismatched substrates, atomic layer deposition (ALD) is used. ALD is a variation of the CVD process where the precursors are introduced one at a time to form an atomically thin layer on the substrate. ALD can be used for precise control of the process but it is very slow since growth happens layer-by-layer.

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Physical vapor deposition techniques for growing LED

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Physical vapor deposition techniques like sputtering and e-beam evaporation are also used for growing LEDs.The disadvantage of physical vapor deposition is that it produces polycrystalline films and there are usually a lot of defects in the film. These defects can reduce device efficiency by causing non-radiative recombination and some post deposition annealing is usually needed to eliminate them.

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Pulsed laser deposition (PLD)

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Pulsed laser deposition (PLD) is used for systems with complex stoichiometry and where preserving this is essential to get the right emission wavelength.

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LED efficiency metrics

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Internal quantum efficiency

External quantum efficiency

Power efficiency

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Internal quantum efficiency

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External quantum efficiency & Power efficiency

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Organic KED

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Organic LEDs or OLEDs, as they are more commonly known, are a new class of materials where the emissive layer is an organic compound sandwiched between two electrodes.

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OLED

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Organic or polymer materials are used in OLEDs and they work on a similar principle to solid state LEDs. Carriers are injected into the active emissive layer, where they recombine and emit light. For organic molecules, instead of valence and conduction bands, there are discrete electron energy states called HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) and recombination occurs across these levels. The color of the radiation depends on the energy gap between these two levels. Usually, the anode is a transparent material like Indium Tin oxide (ITO) so that the light emitted can be extracted out of the device. The cathode is usually a reflective material, like a metal film, and is deposited on the substrate.

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OLED construction

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Commonly used material for OLE

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Commonly used organic materials in OLEDs are organometallic chelates and f luorescent dyes. An example is Alq3 which stands for Tris (8-hydroxyquinolinato) aluminum and has the chemical formula Al(C9H6NO)3.

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Advantages and disadvantages of OLEDs

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The organic material is thermally vapor deposited on substrates of choice. The advantages of OLEDs are that they can be used to form flexible displays by depositing on suitable substrates. The devices are light weight, have wider viewing angles, and a faster response time. However, OLEDsare costly, and have a short lifespan due to degradation of the organic layer. The color balance, especially in the blue region, is not good and they are susceptible to water damage and consume more power than solid state.

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There are two different wavelength parameters: “Peak Wavelength” and “Dominant Wavelength” A common question is “What is the difference between them?”.

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Peak Wavelength - Peak wavelength is defined as the single wavelength where the radiometric emission spectrum of the light source reaches its maximum. More simply, it does not represent any perceived emission of the light source by the human eye, but rather by photo-detectors.Dominant Wavelength - Dominant wavelength is defined as the single wavelength that is perceived by the human eye. Generally one light source consists of multiple wavelength spectrums from the light source rather than one single wavelength. Our brains turn those multiple spectrums into a single color of light consistent with a single specific wavelength which is what we see when we look at the light. That’s the light source’s Dominant wavelength.In general, these two parameters are not drastically different, but it can pay to consider our application when using these two parameters.For example, if the LED is used in optical instruments and machines are being used to identify the wavelength, you should use Peak Wavelength for your LED selection.If the LED is used to backlight a display or otherwise illuminate or indicate something for human operators, you should use Dominant Wavelength for your LED selection.

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