Microscopy of thin AlN layers grown by MBE on (111) silicon

Abstract

After the discovery of graphene many other materials were subject of research in a 2D form among them some of them showed bandgap at room temperature. There were a lot of theoretical papers which predicted the properties of the appropriate candidates. AlN layer is widely used as a buffer layer for the epitaxial growth of III‐Nitrides on SiC, Si and Al 2 O 3 substrates. In this work, the formation of a few monolayers thick graphene‐like AlN (g‐AlN) layer on a (111)‐oriented silicon substrate in ammonia molecular beam epitaxy (MBE) has been studied using various transmission electron microscopy (TEM) techniques. Flat, ultrathin layer of AlN was deposited on an ammonia treated (111) Si substrate (1). Two samples with thick (15‐25 monolayer) and thin (3‐4 monolayer) AlN were prepared. Cross sectional TEM specimens were prepared using the conventional mechanical polishing and low‐energy (<1 keV) low‐angle Ar ion milling. TEM images (not shown) used to measure the AlN layer thicknesses as X and Y nm in the two samples. The chemical composition of the interface structure between the AlN and Si substrate was studied by spectrum imaging; combining energy dispersive X‐ray spectroscopy (EDXS) and scanning TEM (STEM) in an aberration‐corrected FEI Titan 80‐200 ChemiSTEM microscope. Figure 1 shows the Si and N map of the sample containing 15‐25 AlN monolayers. The elements distribution across the interface extracted as linescans and shown in Fig. 1b suggests nitrogen enrichment between the Si and AlN. The interface structure was further studied by aberration‐corrected TEM using an FEI Titan 80‐300 kV TEM operated at 300 kV. Figure 2 shows the boundary structure very clearly, where ‐ based on the measured distances (3.3 Å and 1.9 Å) ‐ we suppose that two layers of Si 3 N 4 phase were grown onto the silicon probably due to the ammonia treatment prior the AlN deposition. The first three sheets of AlN was measured to be as 2.86 Å and 2.60 Å that are higher than the wurtzite‐type AlN (2.5 Å) suggesting that the AlN does not immediately follow the wurtzite‐type stacking. The results promise not only the formation of few layer AlN with different properties from the thick layers, but also their possible integration into the silicon device technology.