Ordering of scandium into monolayers of aluminium nitride and its implication for AlScN based nitride semiconductor devices

Abstract text (incl. figure legends and references)

Recently the incorporation of scandium (Sc) into aluminium nitride (AlN) attracted severe attention for III-V nitride semiconductors. The sufficiently larger ionic size of Sc increases significantly the spontaneous polarisation of AlN (1), which is e.g. responsible for the formation of a two dimensional electron gas at the heterojunction to another, compositional slightly different nitride compound (2). This allows to fabricate high-electron mobility transistors (HEMT) with very high switching frequencies and high efficiencies.

However, the growths of well-crystalline AlScN material is impeded by a huge immiscibility gap between AlN and ScN. The material films deposited by magnetron sputtering or recently epitaxy are considered to exist only metastable (3). Intense annealing at temperatures beyond 1400°C for three hours, lead in this study to the formation of a layered structure of the compound, observed for the first time. The structural properties were intensely studies by atomic resolution STEM-HAADF images, EDS and EELS spectroscopy and will be reported in this contribution.

The results from STEM-HAADF images show clearly that bright atomic monolayers are developed every 5-8 layers, after annealing at 1700°C. They run straight through the whole crystal and extend several µm (Fig. 1a) along the basal plane. Following EDS data, the bright appearance is due to an ordering of Sc into these layers (Fig. 1 b,c) coupled to a strong increase in oxygen content. The Sc coordination changes from four-fold in the wurtzite structure to six-fold as preferred by Sc. Whereas the sequence of cation positions in atomic STEM-HAADF images is strongly determined, the sequence of bright layers varies randomly between 5 and 8, leading to a partial disorder along c-direction.

Further structural observations after annealing of samples at 1400°C allow some insight into their formation. Rarely platy seed of two octahedral Sc-bearing layers with only one intermittent AlN layers were observed. They are assumed to be thermodynamically favourable, similar to GdN platelets formed in Gd-doped GaN (4), but in contrast, significantly lower strain allows their two dimensional growths in the Sc-bearing system. Hence, this model of metastable formation resembles thermodynamical exsolution lamellae of mono-atomic widths.

The presented observations are not only intriguing from a structural point of view, the obtained insight into the material behaviour has also implications on the growths of future ScAlN material for semiconductor devices as well as their long-time stability during device operation.

References:

(1) Akiyama et al., Adv. Mater. 21, 593–596 (2009). (2) Leone et al., Phys. Status Solidi RRL. 14, 1900535 (2020). (3) A. Moram and S. Zhang, J. Mater. Chem. A. 2, 6042–6050 (2014). (4) Wu et al., Acta Materialia. 76, 87–93 (2014).

Fig. 1: Left: atomic resolution STEM-HAADF image showing bright Sc-bearing layers at variable distance. Right: EDS elemental distribution maps and integrated counts perpendicular the layers.