For the first time, the international research team revealed the core mechanism for limiting the indium (In) content of indium gallium nitride ((In,Ga)N) films - a key material for blue light-emitting diodes (LEDs). Increasing the In content in the InGaN quantum well is a common method of moving the emission of Group III nitride based LEDs to the green, especially red, portions necessary for modern RGB devices. The new findings answer long-standing research questions: Why do this classic methods fail when we try to get efficient InGaN-based green and red LEDs?
Field projected state density (pDOS) of a 2 x 2 N atom (0001) GaN surface with 25% InN on the topmost surface. - For details, please refer to the content...
Despite advances in green LEDs and lasers, researchers are unable to overcome the 30% limit for indium in movies. The reason for this is still unclear: is it the fundamental reason for finding the right growth conditions or failing to overcome? Now, international teams from Germany, Poland and China have put forward new ideas on this issue and have revealed the mechanisms that have caused such restrictions.
The top graph shows the field projected state density (pDOS) of a 2 x 2 N atom (0001) GaN surface with 25% InN on the topmost surface. The (a) and (b) In atoms are located at the quadruple/triple coordination position, respectively. The blue curve represents the N atom and pDOS on the first surface layer atom. The gray shaded area indicates pDOS from the fourth surface layer, and the yellow shaded area indicates the highest occupied surface state. The arrow indicates the beginning of the lowest unoccupied surface state, while the horizontal solid line indicates the position of the highest occupied state. (Illustration) The yellow, red, and green density maps represent the partial charge densities of the states within the energy range shaded by yellow, red, and green, respectively, in pDOS. (Insets bottom) A schematic diagram of a tetrahedron formed by a triple-coordinated metal atom and three N atoms bonded thereto. The red/green contour surface shows the corresponding dangling key status. Dark/light spheres represent N atoms/atoms, respectively. The red/green spheres are In / Ga atoms, respectively. The dark green in the left illustration represents a ternary coordinated Ga atom.
In their work, scientists have tried to push the indium content to the limit by growing a single atomic layer of InN on GaN. However, independent of growth conditions, the indium concentration never exceeds 25% - 30% - this is an obvious basic limiting mechanism. Researchers used advanced characterization methods such as atom-resolved transmission electron microscopy (TEM) and in-situ reflection high-energy electron diffraction (RHEED), and found that once the indium content reaches about 25%, the (In,Ga)N monolayer is in a regular pattern. Arrangement - The single atomic column In alternates with the two atomic columns of the Ga atom. Comprehensive theoretical calculations show that atomic ordering is caused by specific surface reconstruction: indium atoms are combined with four adjacent atoms instead of the expected three. This creates a stronger bond between the indium and nitrogen atoms, which on the one hand can use higher temperatures during the growth process and provide better quality for the material. On the other hand, this sorting sets the limit of In content to 25%, which cannot be overcome under realistic growth conditions.
“Obviously, technical bottlenecks have hampered all attempts to shift from green to yellow and red regions, so new and original ways are needed to overcome these basic limitations,†said Dr. Tobias Schulz, Ph.D. In Leibniz-Institut fuer Kristallzuechtung; "For example, the growth of InGaN films on high quality InGaN dummy substrates will reduce the strain of the growth layer."
However, the discovery of sequencing may help overcome the well-known limitations of InGaN material systems: localization of charge carriers due to fluctuations in the chemical composition of the alloy. Therefore, the growth of a stable ordered (In,Ga)N alloy having a fixed composition at a high temperature can improve the optical properties of the device.
Acknowledgement:
The work is Leibniz-Institut fuer Kristallzuechtung, Berlin, Germany, Max-Planck-Institut fuer Eisenforschung, Düsseldorf, Germany, Paul-Drude Institut fuer Festkoerperelektronik, Berlin, Germany, Physics of High Voltage Institute (Warsaw, Poland) and artificial microstructures State Key Laboratory of Physics (Beijing, China).
The original text can be found at http://dx.doi.org/10.1103/PhysRevMaterials.2.011601 or https://
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