InGaN based Light Emitting Diodes (LED) literature review

This is in a series of literature reviews on InGaN solar cells, which supported the comprehensive review by D.V.P. McLaughlin & J.M. Pearce, "Progress in Indium Gallium Nitride Materials for Solar Photovoltaic Energy Conversion"Metallurgical and Materials Transactions A 44(4) pp. 1947-1954 (2013). open access
Others: InGaN solar cells| InGaN PV| InGaN materials| InGan LEDs| Nanocolumns and nanowires| Optical modeling of thin film microstructure| Misc.

This page describes selected literature available on InGaN based LED and LASER devices.

InGaN/GaN nanowire green light emitting diodes on (001) Si substrates

Abstract: Progress in solid state lighting at the present time primarily involves the research and development of visible nitridebased light emitting diodes (LEDs) and perhaps lasers in the future. However, this development has been impeded due to the lack of high-quality and low-cost GaN substrate. Successful growth of GaN and InGaN nanowires on silicon and other mismatched substrates has been demonstrated recently. The nanowires exhibit significantly reduced defect density due to their large surface-to-volume ratio. A reduced strain distribution in the nanostructures also leads to a weaker piezoelectric polarization field. Other advantages include large light extraction efficiency and the compatibility with lo w-cost, large area silicon substrates. In the present study, author(s) have conducted a detailed investigation of the molecular beam epitaxial (MBE) growth and optical properties of (In)GaN nanowires directly on (001) Si in the absence of a foreign metal catalyst. Green LEDs have been fabricated with an ensemble of nanowires and the characteristics of these devices are also presented.

(In)GaN nanowires were grown on (001) Si substrates by RF plasma assisted MBE system.
radiative lifetimes in InGaN/GaN NWs were significantly smaller than that in quantum wells. This is due to better confmement of electrons and holes in dot-in-a-wire structure and weaker piezoelectric field caused by reduced strain in wires. However, nonradiative lifetime is also small, which is attributed to surface states due to large surface-to-volume ratio.
peak energy of 490 nm, and decay time of 0.095 ns and the stretching parameter 0.73 was recorded (for 490 nm).

Growth of InGaN/GaN multiple-quantum-well blue light-emitting diodes on silicon by metalorganic vapor phase epitaxy.

Abstract: Author(s) report the growth of InGaN/GaN multiple-quantum-well blue light-emitting diode (LED) structures on Si(111) using metalorganic vapor phase epitaxy. By using growth conditions optimized for sapphire substrates, a full width at half maximum (FWHM) (102) x-ray rocking curve of less than 600 arcsec and a room-temperature photoluminescence peak at 465 nm with a FWHM of 35 nm was obtained. Simple LEDs emitting bright electroluminescence between 450 and 480 nm with turn-on voltages at 5 V were demonstrated.

major problems for heteroepitaxy of GaN on Si include (i) considerable lattice mismatch (17%) larger than that between GaN and sapphire (13%), (ii)large difference (2 ppm/K) in thermal expansion coefficient (about same as with sapphire but of opposite sign), and (iii) nonpolar/polar character of silicon versus GaN.
Cracks were observed due to tension between the film and Si substrate. Areas between cracks were smooth.
Higher defect density in LED grown on Si made p doping less efficient, so p doping in p-GaN is expected to be lower than that of LED grown on sapphire. Further optimization of buffer layer may reduce turn-on voltage and improve reverse leakage current to same levels typically observed for LED structures grown on sapphire.
Have demonstrated a blue LED on silicon using an InGaN/GaN MQW as an active layer grown entirely by MOVPE.
Photoluminescence and electroluminescence from these LEDs are comparable to that on sapphire while optical power, turn-on voltage, and reverse bias current are still inferior.

Light-emitting diode extraction efficiency.

Abstract (further reading required: A model of optical processes in LED's was created that takes into account device geometry, light absorption in contacts and cladding layers, photon recycling, light randomization due to surface scattering and the benefit from encapsulation of the device into epoxy. Based on the results of our modeling, an optimization of the LED was proposed. Also, photoluminescence measurements of internal quantum efficiency were performed on the epi-layers used for LED fabrication.

Thinning down the active layer reduces considerably re-absorption losses in the active layer, especially in material with low internal quantum efficiency. This can also shift the operating point of the device towards the high-level injection regime.
Quality of the active layer material determines whether the preferred device design should be thick or thin.
For a high internal quantum efficiency device (>90%), one should minimize bulk absorption by making the device as thin as possible.
On the other hand, if the active layer has a low (<90%) internal efficiency, it's better to make a thick substrate device which allows the photons to see the device edges where 4 additional escape cones are present. This increases the photon to escape probability from the semiconductor on the very first surface bounce.
From the point of view of light extraction efficiency, the smaller device area is preferable, since light randomization happens only on the edges of the device.
Even though contacts cover only about 15% of the area of the device under consideration, the dependence of light extraction efficiency on the contact reflectivity is strong.

References

Meng Zhang et al., 2010. InGaN/GaN nanowire green light emitting diodes on (001) Si substrates. In Device Research Conference (DRC), 2010. Device Research Conference (DRC), 2010. IEEE, pp. 229-230.