GR-Fri-5 - Self-organised ordering of scandium into monolayers of aluminum nitride and its implication for materials growth and AlScN based semiconductor devices

1. Growth
Ulrich Bläß¹ , Mingjian Wu², Boris Epelbaun¹, Elke Meissner³, Sven Besendörfer^1
1 Fraunhofer Institute for Integrated Systems and Device Technology (IISB), Erlangen, Germany.
² Friedrich-Alexander-Universität Erlangen-Nürnberg (IMN), Erlangen, Germany.
³ Fraunhofer Institute for Integrated Systems and Device Technology (IISB), Erlangen, Germany. Friedrich-Alexander-Universität Erlangen-Nürnberg (LEB), Erlangen, Germany.

Abstract text
Within the last decade, scandium (Sc) bearing aluminium nitride (AlN) increasingly appears as a promising material for next generation of semiconductor devices. This is due to a 4-5 times higher piezoelectric response, compared to pure AlN [1]. The enhanced piezoelectric response led e.g. to a high 2D electron gas concentration in high-electron mobility transistors enabling high efficiencies and switching frequencies, which are essential parameters e.g. for power electronic applications and others.

All these Al_1-xSc_xN thin films with x up to 0.4 are considered being thermodynamically metastable due to a huge miscibility gap between AlN and ScN, even substrate misfit strain may stabilise their existence [2].

In this study we demonstrate that excessive annealing between 1400° and 1700°C led to a new kind of Sc-ordering [3]. Scanning transmission electron microscope (STEM) investigations reveal at atomic resolution that a self-organised ordering process led to a complete ordering of Sc-atoms into basal monolayers, which are recurring statistically every 5 to 8 layers. Inside these layers, Sc-atoms change their coordination from former tetrahedral in disordered AlN wurtzite structure to octahedral in these layers. This local equilibration of Sc-atoms is assumed as driving force for the formation of the layered phase. Whether this is triggered by a high oxygen content present in these layers, is discussed. In between these layers, Sc-free AlN occurs in the wurtzite structure. The sequence of atomic positions along the hexagonal c-direction is strictly ordered, except the statistic recurrence of Sc-layers. Thermo­dynamically these Sc-rich layers are interpreted as exsolution lamellae of minimal atomic width, for which tiny nuclei has been first observed at 1400°C.

This study demonstrates that the metastable existence of disordered AlScN of wurtzite structure is limited in temperature relatively close to e.g. MOVPE process temperatures. To obtain excellent material for electronic devices, the growth temperature should be sufficiently high to gain good quality instantaneously, but below the onset of significant diffusion to prevent structural degradation.

[1] M. Akiyama et al., Adv. Mater. 21, 593–596 (2009).

[2] M. A. Moram and S. Zhang, J. Mater. Chem. A. 2, 6042–6050 (2014).

[3] U.W. Bläß et al., J. Mater. Chem. C, DOI: 10.1039/d4tc04545a (2025).