Definition: materials with a small band gap energy
Categories: optical materials, optoelectronics, physical foundations
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Author: Dr. Rüdiger Paschotta
Semiconductors are materials where the electronic band structure exhibits a relatively small band gap between the valence and conduction bands. They are of central importance for the electronics industry as base materials for diodes, transistors and other important electronic components, and semiconductor wafers are the starting point for electronic integrated circuits. However, semiconductors have also acquired a substantial role in the area of optics and photonics. The article focuses on that field.
Different kinds of semiconductors are relevant for optics and photonics. Single-element (group IV) semiconductors like silicon and germanium are subject to limitations due to their indirect bandgaps (see below); on the other hand, silicon is attractive for optoelectronics because of its wide use in electronics and the highly developed fabrication and processing technology. Compound semiconductors particularly of III–V type (e.g. gallium arsenide and indium phosphide), which often exhibit direct band gaps, have become quite important. Some II–VI semiconductors with small band gap (e.g. Hg1−xCdxTe, MCT) are used for infrared detectors, others like zinc selenides for transmissive optics like lenses.
The above-mentioned materials are all of inorganic type. There are also some organic semiconductors which may be used in organic photonics, but these are much less developed.
Optical Properties of Semiconductors
The band gap energy of semiconductors is below the photon energy of visible light; therefore, semiconductors exhibit strong absorption in that spectral region. However, for somewhat longer wavelengths, in the near to mid infrared, they can exhibit rather low propagation losses. Here, a helpful factor is the high purity and crystal quality of the usually used semiconductor materials, made with epitaxial methods. As the usually used semiconductors are made of relatively heavy elements such as gallium, indium and arsenic, they exhibit relatively low phonon energies, and that results in good infrared transmission up to relatively long wavelength. So one obtains a similarly wide or even wider transmission window as for dielectrics, only in a region of longer wavelengths.
The refractive index of semiconductors is relatively high – substantially larger than for typical dielectrics, often of the order of 3. The phase velocity and group velocity are correspondingly low, and chromatic dispersion is strong.
Absorption and Emission
For the absorption and emission of light in a semiconductor, it matters a lot whether the band gap is direct or indirect:
- Both processes work well for direct band gap semiconductors.
- With an indirect band gap, absorption for photon energies nearly band gap is much reduced, because it needs to be assisted by phonons. Only for higher photon energies, where phonons are not needed, stronger absorption can occur.
- Similarly, the rate of photon emission is much reduced in an indirect band gap material, and that can also much reduce the quantum efficiency of luminescence. This is frequently a problem for silicon-based devices, for example; the indirect band gap of silicon makes it hard (although not impossible) to realize light-emitting devices and amplifiers.
See the article on band gaps for more details.
Once carriers with a substantial density are excited by electrical or optical pumping, the absorption in certain wavelength regions is increased, and even optical amplification is possible. This is exploited in semiconductor lasers and semiconductor optical amplifiers. Here, one often uses p- and n-doped semiconductors in order to obtain increased electrical conductivity and p–n junctions where carriers can recombine.
Quantum Wells and Quantum Dots
Particularly in optoelectronics (see below), essential functions are often implemented not with bulk semiconductor material, but rather with quantum wells (very thin semiconductor layers) or with quantum dots. In both cases, the microscopic dimensions lead to a substantial modification of electronic properties by quantum effects.
Optical nonlinearities are quite strong for semiconductors, compared with those of dielectrics. For example, the nonlinear index of gallium arsenide (GaAs) around 2 μm wavelength is of the order of 10−17 m2/W . Also, the two-photon absorption coefficients are rather high, as long as the photon energy is more than half of the band gap energy. Besides, the χ(2) nonlinearity is also very strong compared to typical nonlinear crystals of dielectric type. Therefore, there are plenty of possibilities for nonlinear optics based on semiconductors.
Application of Semiconductors in Optics and Photonics
As in microelectronics, semiconductors for optics and photonics are usually used in the monocrystalline form. A fabrication process usually starts with a high quality semiconductor wafer and often involves some types of epitaxy and lithography.
The wide transparency range of semiconductors in the infrared spectral region is a good basis for applications in infrared optics. For example, silicon is widely used for optical windows and lenses with operation wavelengths around 2–8 μm; germanium is suitable for longer wavelengths around 2–14 μm. Zinc selenide is used e.g. for CO2 laser lenses, operated with high power levels. The strong absorption for visible or near-IR light in such materials can even be an advantage.
Crystalline mirrors are multilayer mirrors which are usually made of a monocrystalline semiconductor material. They are needed for special applications, where it is essential, for example, to reduce mechanical noise of thermal origin in interferometers.
Photonic crystals  contain microscopic structures with dimensions of the order of the wavelength. Semiconductors are attractive materials for photonic crystals, since they allow for a high refractive index contrast (in conjunction with air), and because they are well established fabrication techniques for microstructures in such materials.
See the article on photonic crystals for more details.
Photonic metasurfaces obtain very special optical properties due to nanoscale (sub-wavelength) structures. Some of them are made with semiconductor materials, particularly for operation in the infrared. The achievable high refractive index contrast is a beneficial aspect in that context, and even more so the prospects of establishing a new technological field of flat optics, based on ultra-thin semiconductor structures instead of much thicker optical components as used in traditional optics.
See the article on photonic metasurfaces for more details.
Nonlinear Frequency Conversion
Particularly III–V semiconductors as e.g. gallium arsenide (GaAs) are very suitable for nonlinear frequency conversion, e.g. for optical parametric oscillators and amplifiers operating in the mid-infrared. For example, the d14 coefficient of GaAs is as high as 94 pm/V for frequency doubling at 4.1 μm ; that is about an order of magnitude more than for relatively highly nonlinear dielectric crystals like LiNbO3, and conversion efficiencies scale with the square of that coefficient.
Furthermore, techniques of quasi phase matching (QPM) with periodic microscopic domains in gallium arsenide (→ orientation-patterned semiconductors) have been developed, which allow one to utilize the strong nonlinearity for many nonlinear interactions with basically arbitrary involved wavelengths within the wide transparency region [6, 9].
Applications of frequency conversion with semiconductors are found for example in spectroscopy and in the military domain, e.g. infrared countermeasures.
Semiconductor materials play various central roles in optoelectronics:
- They are used for light sources, mainly in the form of semiconductor lasers of various types, most of which are laser diodes.
- Light amplification is possible with semiconductor optical amplifiers. There are also superluminescent sources based on those.
- For modulating light, e.g. in telecom transmitters, electroabsorption modulators can be used.
- There are photoconductive switches, used e.g. for photoconductive sampling and for the generation and detection of terahertz radiation.
- Besides, semiconductors are applied in photodetectors such as photodiodes and phototransistors, metal–semiconductor–metal photodetectors (MSM detectors) and other photoconductive detectors, e.g. based on Hg1−xCdxTe (MCT). Semiconductors are also used for infrared photocathodes and dynodes for photomultipliers.
For photodetectors in the visible and near-infrared, one often uses silicon, which can easily be integrated with electronic components, e.g. based on the standard CMOS platform. That integration is generally more challenging for semiconductor lasers, which largely require III–V semiconductors, while most electronics are based on silicon. There are various technologies addressing that problem, either based on realizing semiconductor light sources with silicon or by combining (e.g. by wafer bonding) a silicon basis with III–V chips for optoelectronic functions. (See also the article on silicon photonics.) Besides, there are also electronics based on III–V materials, for example including very fast transistors.
Photonic integrated circuits are often developed on semiconductor platforms like indium phosphide (InP) or silicon (→ silicon photonics), or on hybrid platforms like silica-on-silicon, where the dielectric material silica is used on the semiconductor silicon. It is possible to realize waveguides for transporting light, typically with relatively large refractive index contrast, which allows for tight bending without excessive bend losses.
Thin semiconductor layers are also needed for improving the electron multiplication factor in microchannel plates.
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See also: dielectric materials, silicon photonics, semiconductor lasers
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