Optoelectronic devices play an important role, so we should have some understanding of optoelectronic devices. In order to improve everyone’s understanding of optoelectronic devices, this article will introduce semiconductor optoelectronic devices.
Various functional devices made by utilizing semiconductor photo-Electronic (or electro-photon) conversion effect. It is different from semiconductor optical devices (such as optical waveguide switches, optical modulators, optical deflectors, etc.). The design principle of the optical device is based on the change of the propagation mode of the guided light by the external field, which is also different from the optoelectronic devices used by the early people. The latter only focuses on the reception and conversion of light energy (such as photoresistors, photocells, etc.). Early optoelectronic devices were limited to passive applications. The advent of semiconductor lasers as coherent optical carrier sources in the 1960s brought it into an active application stage. etc.) are extending functions that are difficult for electronics to perform. A new sub-discipline, optoelectronics, is developing rapidly, and optoelectronic devices constitute the core part of optoelectronics.
As early as the end of the 19th century, the photoelectric phenomenon in the semiconductor selenium had been studied, and later the selenium photovoltaic cell was applied, which was almost 80 years earlier than the invention of the transistor, but at that time people still lacked understanding of semiconductors and progress was slow. The research on the basic physical properties of semiconductors (such as energy band structure, electronic transition process, etc.) started in the 1930s, especially the research on the optical properties of semiconductors, has laid a physical foundation for the development of semiconductor optoelectronic devices. In 1962, RN Hall and MI Nathan successfully developed an injection-type semiconductor laser, which solved a high-efficiency optical information carrier source and expanded the application scope of optoelectronics, and optoelectronic devices developed rapidly.
1. Basic physical processes
From the point of view of energy band theory, the distribution of electronic states in semiconductors is shown in Figure 1. At room temperature, the states in the low-energy band (valence band) are basically filled with valence electrons, and the states in the high-energy band (conduction band) are basically filled with valence electrons. is left empty, and the two are separated by a forbidden band of width Eg. In this case, the conductive properties of semiconductors are very poor, and only electrons in the conduction band or empty states (holes) in the valence band can participate in conduction under the driving of an external field.
2. Internal photoelectric effect
When electrons in the valence band absorb photons with energy greater than the forbidden band width, they can transition to the conduction band, and at the same time leave holes in the valence band, which are collectively referred to as photogenerated carriers, and the resulting additional conduction phenomenon is called for the photoconductor. The current contributed by photogenerated carriers driven by an external field is called photocurrent. This photoelectron effect occurs in the semiconductor, so it is called the internal photoelectric effect. The internal photoelectric effect is the basis of all optoelectronic receiving and energy conversion devices.
3. External photoelectric effect
Electrons in semiconductors absorb high-energy photons and are excited to become hot electrons, which may overcome the constraints of the lattice field and escape from the body to become free electrons, which is also called the photoelectron emission effect. Figure 2 is an energy band diagram of a semiconductor with an ideal surface. EC and EV represent the bottom of the conduction band and the top of the valence band, respectively, E0 is the vacuum energy level in vitro, and x is the electron affinity (indicating that the electrons at the bottom of the conduction band escape from the body) The crystal binding energy to be overcome), EF is the Fermi level position, φ is the work function, and ET=x+EV is the photoelectron emission threshold energy.
Semiconductor surfaces are sensitive to ambient atmosphere and contact materials. The adsorption of foreign charges (positive or negative) on the surface layer causes the bending of the surface energy bands (up or down), which drastically affects the properties of photoelectron emission in semiconductors. E in Fig. 3 represents the potential energy of the downward bending of the surface energy band, the actual effective electron affinity xeff=xE. If E>x, then xeff becomes negative. The quantum yield of photoelectron emission from negative electron affinity (NEA) materials such as GaAs, InGaAsP in contact with Cs2O is considerable and is an important basis for the development of semiconductor photocathodes.
Electron-hole recombination luminescence effect In 1952, the phenomenon of injecting luminescence into silicon and germanium semiconductor materials was discovered. The non-equilibrium electron-hole pair injected into the semiconductor releases excess energy in some way and returns to the initial equilibrium state. Radiating photons is a way to release energy, but since germanium and silicon are both indirect band materials (the bottom of the conduction band and the top of the valence band are not in the same position in the momentum space), in order to satisfy the momentum conservation principle of the transition process (Figure 4), this It requires a large number of phonons to participate in the transition process at the same time, which is a many-body process. Therefore, the efficiency of interband recombination emission is very low (less than 0.01%). Many compound materials, such as GaAs and InGaAsP, are direct band materials (the bottom of the conduction band and the top of the valence band are in the same position in the momentum space), and the phonon transition process hardly requires the participation of phonons (Fig. 5). Therefore, the luminous efficiency is very high, and the internal quantum efficiency is almost 100% under large injection. The high-efficiency electron-hole pair recombination luminescence effect is the physical basis of all semiconductor light-emitting devices.
4. Classification of optoelectronic devices
Optoelectronic devices can be divided into bulk optoelectronic devices, forward and reverse junction optoelectronic devices, heterojunction and multi-junction optoelectronic devices.
Bulk optoelectronic devices are the simplest class of optoelectronic devices in structure. Semiconductor materials absorb incident photons with energy greater than the band gap and excite non-equilibrium electron-hole pairs (called intrinsic excitation). They participate in conduction under the external field, resulting in photoconductivity. In the case of non-uniform surface excitation, the diffusion of photo-generated carriers under the concentration gradient will lead to the establishment of an internal field, that is, the photovoltaic effect. The diffusion current is deflected by the action of the magnetic field, resulting in the opto-magnetoelectric effect. Based on these physical effects, photodetectors in various wavelength bands (especially in the infrared band) have been fabricated, such as InSb and HgCdTe photodetectors, which have been widely used in military affairs.
Bulk photodetectors can also be fabricated by doping deep-level impurities. Such as Au, Hg-doped Ge detector, is a very sensitive infrared detector. Photogenerated carriers are excited by deep-level impurity centers, called extrinsic excitation. Most of these detectors work at very low temperatures (such as liquid helium temperature 4.2K).
5. Forward junction optoelectronic devices
Under the large forward bias, a large number of unbalanced carriers will be injected near the junction region of the semiconductor PN junction, and various color light-emitting diodes can be made by using the composite luminescence effect. The red and green semiconductor indicators and digital tubes commonly used in electronic instruments are made of GaAsP, GaP, AlGaAs and other materials. Solid-state light-emitting tubes have low power consumption, small size and long life, and have gradually replaced vacuum tubes. The light-emitting tube made of GaAs has high luminous efficiency, and the emission wavelength is about 9000 angstroms, which belongs to the near-infrared band where the human eye is not sensitive. It is widely used as a light source for photoelectric control and early optical communication. The first semiconductor lasers were made of highly doped GaAs PN junctions. Although modern semiconductor lasers have been replaced by heterojunction devices, they are still basically forward junction structures.
6. Reverse Junction Optoelectronic Devices
In the PN junction, a strong internal field (up to 104 V/cm or more) is established in the junction region due to the transfer of charges on both sides, resulting in energy band bending and forming a PN junction barrier. Once the photo-generated carriers diffuse into the junction region, they are swept to both sides by the internal field to form the photo-generated current. Silicon photovoltaic cells and photodiodes are devices that use reverse junction characteristics. Silicon photovoltaic cells have been applied to artificial satellites as a solar power source, and China’s “Dongfanghong” No. 2 artificial satellite has used silicon photovoltaic cells. At present, the energy conversion efficiency of silicon photovoltaic cells is close to the theoretical value of 15%. Photodiodes are widely used light detection devices. In order to improve the quantum efficiency and response speed, the depletion region (ie the electric field region) must be enlarged as much as possible. Therefore, practical semiconductor photodiodes are all reverse biased, and the quantum efficiency can reach more than 80%. , the response time can be less than nanoseconds, Si-PIN detector used in optical fiber communication system is a typical one.
If a sufficiently large reverse bias is applied, the photogenerated carriers are accelerated under a strong electric field in a region near the junction, and their energies can reach the threshold for causing lattice impact ionization. This ionization process acts as an avalanche chain reaction, which results in an internal gain. Using this process, a fast and sensitive photodetector, called a semiconductor avalanche photodiode (APD), can be fabricated. It is used in long-distance, large-capacity optical fiber communication systems.
7. Heterojunction optoelectronic devices
Since the 1960s, semiconductor epitaxial growth technology has developed rapidly. Using epitaxial growth technology, different semiconductor single crystal films can be controlled to grow together to form heterojunctions or heterostructures. Appropriate selection of heterostructures can obtain some new electrical properties, such as unidirectional injection properties, carrier localization confinement effect, negative electron affinity, etc., optical window effect, optical waveguide properties, etc. The new properties of heterojunctions not only greatly improve the performance of the original optoelectronic devices, but also develop into many new functional devices (such as quantum well lasers, bistable optical devices, etc.). The invention of the double heterojunction laser was a major achievement in heterojunction research. After adopting the heterostructure, the active region of the laser can be precisely controlled in the order of 0.1 microns. Confining the injected carriers and light in this thin layer reduces the threshold current density of the laser by 2 to 3 orders of magnitude to below 103 A/cm2, thereby achieving low power consumption (milliwatts) and long lifetime (external). Push millions of hours), continuous wave work at room temperature, etc. Another achievement of heterojunctions in optoelectronics is the semiconductor photocathode that appeared in the 1970s. The previously used photocathode materials are positron affinity materials (such as Cs3Sb-CsO, etc.), the quantum yield is very low, and is basically determined by the hot electron relaxation time (10-12 seconds). Using the negative electron affinity of semiconductor heterojunctions (such as GaAs, InGaAsP-CsO, etc.), the quantum yield is increased by more than 3 orders of magnitude, and the quantum yield is determined by the non-equilibrium carrier lifetime (order of 10-8 seconds); Proper selection of materials can extend the response wavelength to the infrared band. This type of negative electron affinity photocathode is particularly suitable for military night vision.
The energy conversion efficiency of solar cells is improved by utilizing the heterojunction window effect. Compared with the theoretical limit of silicon photovoltaic cells, the energy conversion efficiency is multiplied. Among the more than 20 kinds of heterojunction photovoltaic cells developed, AlGaAs/GaAs has the highest conversion efficiency, reaching 23%. Heterojunction solar cells, although more expensive, are suitable for special purposes (such as space applications).
8. Multi-junction optoelectronic devices
According to the needs of device functional design, more than two multilayer heterojunctions can be grown continuously. This multi-junction optoelectronic device can be two-terminal, three-terminal or multi-terminal. The AlGaAs/GaAsPNPN negative resistance laser is a multi-junction two-terminal device, which is a composite functional device that combines an ordinary PNPN thyristor and a double heterolaser into one. In order to take into account both the electrical full conduction and the low threshold of the laser, the NpPpnP structure is usually made. Where capital letters indicate wide bandgap materials and lowercase letters indicate narrow bandgap materials. This negative resistance laser is suitable for photoelectric automatic control.
A phototransistor is a multilayer double junction three-terminal device, which is also a photodetector with internal current gain. It is not limited by impact ionization noise, making it comparable to semiconductor avalanche photodiodes for long-wavelength, low-noise detector applications.
The most typical multijunction device is the quantum well laser. The active region of the quantum well laser is composed of multi-layer superlattice materials. In the superlattice structure, the narrow bandgap material forms an extremely thin two-dimensional electron (or hole, or both) potential well, and the quasi-electron (or hole, or both) potential well in the conduction band is formed. The continuous electron states become quantized, and the recombination of electron holes occurs between these quantized discrete states, so the weakness of the energy band operation of semiconductor lasers can be overcome to a considerable extent. The spectral lines are narrowed, the temperature coefficient becomes smaller, and the emission wavelength can be tuned by changing the injection current density. It will expand the application field of semiconductor lasers.
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