Indium gallium nitride
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Indium gallium nitride (InGaN, InxGa1-xN) is a semiconductor material made of a mix of gallium nitride (GaN) and indium nitride (InN). It is a ternary group III/group V direct bandgap semiconductor. Its band gap can be tuned by varying the amount of indium in the alloy. The ratio of In/Ga is usually between 0.02/0.98 and 0.3/0.7.
Indium gallium nitride is the light-emitting layer in modern blue and green LEDs and often grown on a GaN buffer on a transparent substrate as, e.g. sapphire or silicon carbide. It has a high heat capacity and its sensitivity to ionizing radiation is low (like other group III nitrides), making it also a potentially suitable material for solar cell arrays for satellites.
It is theoretically predicted that in a composition regime between ~ 15% -85% Indium nitride spinodal decomposition should occur leading to In-rich and Ga-rich InGaN regions or clusters. However, local structure studies of InGaN did not show any evidence for strong phase segregation, however a signs of a weak phase segregation are observed [1].
GaN is a defect rich material with typical dislocation densities exceeding 108 cm-2. Light emission from InGaN layers grown on such GaN buffers used in blue and green LEDs is expected to be low because of non-radiative recombination at such defects. Nevertheless InGaN quantum wells, are efficient light emitters in green, blue, white and ultraviolet light-emitting diodes and diode lasers. In the indium-rich regions, with a lower bandgap than the surrounding material, most electron-hole pairs recombine and by the lower potential energy of these clusters carriers are hindered to diffuse and recombine non-radiatively at crystal defects.
The wavelength emitted, dependent on the material's band gap, can be controlled by the GaN/InN ratio, from near ultraviolet for 0.02In/0.98Ga through 390 nm for 0.1In/0.9Ga, violet-blue 420 nm for 0.2In/0.8Ga, to blue 440 nm for 0.3In/0.7Ga, to red for higher ratios and also by the thickness of the InGaN layers which are typically in the range of 2-3 nm.
This defect tolerance, together with a good spectral match to sunlight, also makes the material suitable for solar cells. It is possible to grow multiple layers with different bandgaps, as the material is relatively insensitive to defects introduced by a lattice mismatch between the layers. A two-layer multijunction cell with bandgaps of 1.1 eV and 1.7 eV can attain a theoretical 50% maximum efficiency, and by depositing multiple layers tuned to a wide range of bandgaps an efficiency up to 70% is theoretically expected.[2]
Quantum heterostructures are often built from GaN with InGaN active layers.
InGaN is often used together with other materials, eg. GaN, AlGaN, on SiC, sapphire and even silicon etc.
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[edit] Safety and toxicity aspects
The toxicology of InGaN has not been fully investigated. The dust is an irritant to skin, eyes and lungs. The environment, health and safety aspects of indium gallium nitride sources (such as trimethylindium, trimethylgallium and ammonia) and industrial hygiene monitoring studies of standard MOVPE sources have been reported recently in a review [3].
[edit] See also
[edit] References
- ^ V. Kachkanov, K.P. O’Donnell, S. Pereira, R.W. Martin, Phil. Mag. 87(13), 1999–2017 (2007)
- ^ A nearly perfect solar cell, part 2
- ^ Environment, health and safety issues for sources used in MOVPE growth of compound semiconductors; D V Shenai-Khatkhate, R Goyette, R L DiCarlo and G Dripps, Journal of Crystal Growth, vol. 1-4, pp. 816-821 (2004); doi:doi:10.1016/j.jcrysgro.2004.09.007
