Fundamental Limit of Blue LEDs

A scanning transmission electron microscopy image of the atomic ordering in (In, Ga)N monolayer: single atomic column, containing only indium atoms – shown by higher intensity on the image –, followed by two, containing only gallium atoms. (Source: IKZ)

Despite the progress in the field of green LEDs and lasers, the researchers could not overcome the limit of 30% of indium content in the films. The reason for that was unclear up to now: is it a problem of finding the right growth condi­tions or rather a funda­mental effect that cannot be overcome? Now, an inter­national team from Germany, Poland and China has shed new light on this question and revealed the mechanism respon­sible for that limi­tation. In their work the scien­tists tried to push the indium content to the limit by growing single atomic layers of InN on GaN.

However, inde­pendent on growth conditions, indium concen­trations have never exceeded 25% – 30% – a clear sign of a funda­mentally limiting mechanism. The researchers used advanced charac­terization methods, such as atomic reso­lution trans­mission electron micro­scope and in-situ reflec­tion high-energy electron diffrac­tion (RHEED), and disco­vered that, as soon as the indium content reaches around 25 %, the atoms within the (In, Ga)N mono­layer arrange in a regular pattern – single atomic column of In alter­nates with two atomic columns of Ga atoms.

Compre­hensive theo­retical calcu­lations revealed that the atomic ordering is induced by a particular surface recon­struction: indium atoms are bonded with four neigh­boring atoms, instead of expected three. This creates stronger bonds between indium and nitrogen atoms, which, on one hand, allows to use higher tempera­tures during the growth and provides material with better quality. On the other hand, the ordering sets the limit of the In content of 25%, which cannot be overcome under realistic growth condi­tions. “Apparently, a techno­logical bottle­neck hampers all the attempts to shift the emission from the green towards the yellow and the red regions of the spectra. Therefore, new original pathways are urgently required to overcome these funda­mental limi­tations”, states Tobias Schulz, scientist at the Leibniz-Institut für Kristall­züchtung; “for example, growth of InGaN films on high quality InGaN pseudo-sub­strates that would reduce the strain in the growing layer.”

However, the disco­very of ordering may help to overcome well known limi­tations of the InGaN material system: localization of charge carriers due to fluc­tuations in the chemical compo­sition of the alloy. Growing stable ordered (In, Ga)N alloys with the fixed compo­sition at high tempera­tures could thus improve the optical pro­perties of devices. The work is a result of a colla­boration between Leibniz-Institut für Kristall­züchtung (Berlin, Germany), Max-Planck-Institut für Eisen­forschung (Düsseldorf, Germany), Paul-Drude Institut für Festkörper­elektronik (Berlin, Germany), Institute of High-Pressure Physics (Warsaw, Poland), and State Key Labora­tory of Artificial Micro­structure and Meso­scopic Physics (Beijing, China). (Source: FVB)

Reference: L. Lymperakis et al.: Elastically frustrated rehybridization: Origin of chemical order and compositional limits in InGaN quantum wells, Phys. Rev. Mat. 2, 011601 (2018); DOI: 10.1103/PhysRevMaterials.2.011601

Link: Leibniz-Institute for Crystal Growth, Berlin, Germany

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