Causes and mechanisms
At the edge of a diode laser, where light is emitted, a mirror is traditionally formed by cleaving the semiconductor wafer to form a specularly reflecting plane. This approach is facilitated by the weakness of the [110] crystallographic plane in III-V semiconductor crystals (such as GaAs, InP, GaSb, etc.) compared to other planes. A scratch made at the edge of the wafer and a slight bending force causes a nearly atomically perfect mirror-like cleavage plane to form and propagate in a straight line across the wafer.
But it so happens that the atomic states at the cleavage plane are altered (compared to their bulk properties within the crystal) by the termination of the perfectly periodic lattice at that plane. Surface states at the cleaved plane have energy levels within the (otherwise forbidden) band gap of the semiconductor.
The absorbed light causes generation of electron-hole pairs. These can lead to breaking of chemical bonds on the crystal surface followed by oxidation, or to release of heat by nonradiative recombination. The oxidized surface then shows increased absorption of the laser light, which further accelerates its degradation. The oxidation is especially problematic for semiconductor layers containing aluminium.[2]
Essentially, as a result when light propagates through the cleavage plane and transits to free space from within the semiconductor crystal, a fraction of the light energy is absorbed by the surface states where it is converted to heat by phonon-electron interactions. This heats the cleaved mirror. In addition the mirror may heat simply because the edge of the diode laser—which is electrically pumped—is in less-than-perfect contact with the mount that provides a path for heat removal. The heating of the mirror causes the band gap of the semiconductor to shrink in the warmer areas. The band gap shrinkage brings more electronic band-to-band transitions into alignment with the photon energy causing yet more absorption. This is thermal runaway, a form of positive feedback, and the result can be melting of the facet, known as catastrophic optical damage, or COD.
Deterioration of the laser facets with aging and effects of the environment (erosion by water, oxygen, etc.) increases light absorption by the surface, and decreases the COD threshold. A sudden catastrophic failure of the laser due to COD then can occur after many thousands hours in service.[3]
Improvements
One of the methods of increasing the COD threshold in AlGaInP laser structures is the sulfur treatment, which replaces the oxides at the laser facet with chalcogenide glasses.[4] This decreases the recombination velocity of the surface states.[2]
Reduction of recombination velocity of surface states can be also achieved by cleaving the crystals in ultrahigh vacuum and immediate deposition of a suitable passivation layer.[2]
A thin layer of aluminium can be deposited over the surface, for gettering the oxygen.[2]
Another approach is doping of the surface, increasing the band gap and decreasing absorption of the lasing wavelength, shifting the absorption maximum several nanometers up.[2]
Current crowding near the mirror area can be avoided by prevention of injecting charge carriers near the mirror region. This is achieved by depositing the electrodes away from the mirror, at least several carrier diffusion distances.[2]
Energy density on the surface can be reduced by employing a waveguide broadening the optical cavity, so the same amount of energy exits through a larger area. Energy density of 15–20 MW/cm2 corresponding to 100 mW per micrometer of stripe width are now achievable. A wider laser stripe can be used for higher output power, for the cost of transverse mode oscillations and therefore worsening of spectral and spatial beam quality.[2]
In the 1970s, this problem, which is particularly nettlesome for GaAs-based lasers emitting between 1 µm and 0.630 µm wavelengths (less so for InP based lasers used for long-haul telecommunications which emit between 1.3 µm and 2 µm), was identified. Michael Ettenberg, a researcher and later Vice President at RCA Laboratories' David Sarnoff Research Center in Princeton, New Jersey, devised a solution. A thin layer of aluminum oxide was deposited on the facet. If the aluminum oxide thickness is chosen correctly, it functions as an anti-reflective coating, reducing reflection at the surface. This alleviated the heating and COD at the facet.
Since then, various other refinements have been employed. One approach is to create a so-called non-absorbing mirror (NAM) such that the final 10 µm or so before the light emits from the cleaved facet are rendered non-absorbing at the wavelength of interest. Such lasers are called window lasers.
In the very early 1990s, SDL, Inc. began supplying high power diode lasers with good reliability characteristics. CEO Donald Scifres and CTO David Welch presented new reliability performance data at, e.g., SPIE Photonics West conferences of the era. The methods used by SDL to defeat COD were considered to be highly proprietary and have still not been disclosed publicly as of June, 2006.
In the mid-1990s IBM Research (Ruschlikon, Switzerland) announced that it had devised its so-called "E2 process" which conferred extraordinary resistance to COD in GaAs-based lasers. This process, too, has never been disclosed as of June, 2006.
Further reading
Graduate thesis about COD in high power diode lasers from 2013
References
- Kamiyama, Satoshi; Mori, Yoshihiro; Takahashi, Yasuhito; Ohnaka, Kiyoshi (1991). "Improvement of catastrophic optical damage level of AlGaInP visible laser diodes". Applied Physics Letters. 58 (23): 2595. Bibcode:1991ApPhL..58.2595K. doi:10.1063/1.104833.
https://en.wikipedia.org/wiki/Catastrophic_optical_damage
A thin layer of aluminium can be deposited over the surface, for gettering the oxygen.[2]
Another approach is doping of the surface, increasing the band gap and decreasing absorption of the lasing wavelength, shifting the absorption maximum several nanometers up.[2]
Current crowding near the mirror area can be avoided by prevention of injecting charge carriers near the mirror region. This is achieved by depositing the electrodes away from the mirror, at least several carrier diffusion distances.[2]
Energy density on the surface can be reduced by employing a waveguide broadening the optical cavity, so the same amount of energy exits through a larger area. Energy density of 15–20 MW/cm2 corresponding to 100 mW per micrometer of stripe width are now achievable. A wider laser stripe can be used for higher output power, for the cost of transverse mode oscillations and therefore worsening of spectral and spatial beam quality.[2]
https://en.wikipedia.org/wiki/Catastrophic_optical_damage
https://en.wikipedia.org/wiki/Cleavage_(crystal)
https://en.wikipedia.org/wiki/Miller_index#Crystallographic_planes_and_directions
https://en.wikipedia.org/wiki/Specular_reflection
https://en.wikipedia.org/wiki/Surface_states
https://en.wikipedia.org/wiki/Phonon
https://en.wikipedia.org/wiki/Laser_pumping
https://en.wikipedia.org/wiki/Thermal_runaway
https://en.wikipedia.org/wiki/Positive_feedback
https://en.wikipedia.org/wiki/Chalcogenide_glass
https://en.wikipedia.org/wiki/Getter
https://en.wikipedia.org/wiki/Anti-reflective_coating
https://en.wikipedia.org/wiki/Current_crowding
https://en.wikipedia.org/wiki/P%E2%80%93n_junction
https://en.wikipedia.org/wiki/Electrical_contact
https://en.wikipedia.org/wiki/Electron_mobility
https://en.wikipedia.org/wiki/Current_crowding
https://en.wikipedia.org/wiki/Electromigration
https://en.wikipedia.org/wiki/Aluminium_gallium_indium_phosphide
https://en.wikipedia.org/wiki/Bipolar_junction_transistor
Aluminium gallium indium phosphide (AlGaInP, also AlInGaP, InGaAlP, GaInP, etc.) is a semiconductor material that provides a platform for the development of novel multi-junction photovoltaics and optoelectronic devices, as it spans a direct bandgap from deep ultraviolet to infrared.[1]
AlGaInP is used in manufacture of light-emitting diodes of high-brightness red, orange, green, and yellow color, to form the heterostructure emitting light. It is also used to make diode lasers.
Formation
AlGaInP layer is often grown by heteroepitaxy on gallium arsenide or gallium phosphide in order to form a quantum well structure. Heteroepitaxy is a kind of epitaxy performed with materials that are different from each other. In heteroepitaxy, a crystalline film grows on a crystalline substrate or film of a different material.[citation needed]
This technology is often used to grow crystalline films of materials for which single crystals cannot 1D view.[citation needed]
Another example of heteroepitaxy is gallium nitride (GaN) on sapphire.[2]
Properties
AlGaInP is a semiconductor, which means that its valence band is completely full. The eV of the band gap between the valence band and the conduction band is small enough that it is able to emit visible light (1.7 eV - 3.1 eV). The band gap of AlGaInP is between 1.81 eV and 2 eV. This corresponds to red, orange, or yellow light, and that is why the LEDs made from AlGaInP are those colors.[1]
| Optical properties | |
|---|---|
| Refractive index | 3.49 |
| Chromatic dispersion | -1.68 ÎĽm−1 |
| Absorption coefficient | 50536 cm−1 |
Zinc blende structure
AlGaInP's structure is categorized within a specific unit cell called the zinc blende structure.[3] Zinc blende/sphalerite is based on a face-centered cubic lattice of anions. It has 4 asymmetric units in its unit cell. It is best thought of as a face-centered cubic array of anions and cations occupying one half of the tetrahedral holes. Each ion is 4-coordinate and has local tetrahedral geometry. Zinc blende is its own antitype—you can switch the anion and cation positions in the cell and it has no effect (as in NaCl). In fact, replacement of both the zinc and sulfur with carbon gives the diamond structure.[4]
Applications
AlGaInP can be applied to:
- Light emitting diodes of high brightness
- Diode lasers
- Quantum well structures
- Solar cells (potential). The use of aluminium gallium indium phosphide with high aluminium content, in a five junction structure, can lead to solar cells with maximum theoretical efficiencies (solar cell efficiency) above 40%[1]
AlGaInP laser
A diode laser consists of a semiconductor material in which a p-n junction forms the active medium and optical feedback is typically provided by reflections at the device facets. AlGaInP diode lasers emit visible and near-infrared light with wavelengths of 0.63-0.76 ÎĽm.[5] The primary applications of AlGaInP diode lasers are in optical disc readers, laser pointers, and gas sensors, as well as for optical pumping, and machining.[1]
LED
AlGaInP can be used as an LED. An LED is composed of a p-n junction which contain a p-type and an n-type. The material used in the semiconducting element of an LED determines its color.[6]
AlGaInP is one of type of LEDs used for lighting systems. Another is indium gallium nitride (InGaN). Slight changes in the composition of these alloys changes the color of the emitted light. AlGaInP alloys are used to make red, orange and yellow LEDs. InGaN alloys are used to make green, blue and white LEDs.[citation needed]
Safety and toxicity aspects
The toxicology of AlGaInP has not been fully investigated. The dust is an irritant to skin, eyes and lungs. The environment, health and safety aspects of aluminium indium gallium phosphide sources (such as trimethylgallium, trimethylindium and phosphine) and industrial hygiene monitoring studies of standard MOVPE sources have been reported in a review.[7] Illumination by an AlGaInP laser was associated in one study with slower healing of skin wounds in laboratory rats.[8][medical citation needed]
See also
- Indium phosphide
- Indium gallium phosphide
- Aluminium gallium phosphide
- Indium gallium arsenide phosphide
References
- Rodrigo, SM; Cunha, A; Pozza, DH; Blaya, DS; Moraes, JF; Weber, JB; de Oliveira, MG (2009). "Analysis of the systemic effect of red and infrared laser therapy on wound repair". Photomed Laser Surg. 27 (6): 929–35. doi:10.1089/pho.2008.2306. hdl:10216/25679. PMID 19708798.
- Notes
- Griffin, I J (2000). "Band structure parameters of quaternary phosphide semiconductor alloys investigated by magneto-optical spectroscopy". Semiconductor Science and Technology. 15 (11): 1030–1034. doi:10.1088/0268-1242/15/11/303.
- High Brightness Light Emitting Diodes:G. B. Stringfellow and M. George Craford, Semiconductors and Semimetals, vol. 48, pp. 97–226.
https://en.wikipedia.org/wiki/Aluminium_gallium_indium_phosphide
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