Mechanisms of polarity inversion during the MOVPE growth of III ‐nitrides on sapphire investigated by high resolution transmission electron microscopy

Abstract

The most important compound semiconductors for applications in optoelectronics crystallize in the sphalerite or wurtzite structure, which contain polar axes. Though the polarity is of crucial importance, existing concepts of polarity control in wurtzite III‐N films, grown on nonpolar substrates, are based on empiricism and a basic understanding of the elementary mechanisms behind is still missing. A common concept is deposition of a buffer layer between the polar layer and the nonpolar substrate. In metalorganic vapor phase epitaxy (MOVPE) growth of III‐nitrides such buffer layers are formed in three steps: first the surface of the sapphire is exposed to ammonia, commonly known as nitridation; then a thin layer of AlN or GaN is deposited onto the nitridated surface at relatively low growth temperatures, which is subsequently annealed at high temperatures. This classical process will result in high quality metal‐polar films. N‐polar growth is achieved by nitridation of sapphire surface followed by layer deposition at high temperature. As opposed to metal‐polar films, the resulting N‐polar films are characterized by their rough surface morphologies, which is attributed to the presence of metal‐polar inversion domains. The importance of the nitridation step in improving structural and optical properties has been pointed out in numerous reports. While the chemical processes of the nitridation step were studied in detail, very little and contradictory work was presented on structural aspects that mediate polarity. This concerns the crystalline structure of the nitridation layer and the interface between the nitridation layer and the sapphire substrate. The event of aberration corrected transmission electron microscopes (TEM) now open new possibilities to resolve even single oxygen atomic column with high spatial resolution and thus to study this . We performed a detailed study on the structure of nitridation layer and different buffer layers with respect to the polarity control by aberration corrected high resolution TEM as well as scanning TEM. We showed that sapphire nitridation results in a rhombohedral AlON‐layer that converts the initially N‐polar nucleated AlN to Al polarity. We performed contrast simulations for phase contrast imaging as well as for Z‐contrast STEM imaging. The result of these simulations and the corresponding experimental images are shown in Fig.2. The AlON layer, however, dissolves under high temperature growth conditions typical for III‐nitrides, the initially N‐polar AlN is reestablished and acts as a N‐polar template. Therefore, we suggest that the role of the low temperature buffer is to protect the unstable AlON layer upon further growth. The deeper understanding of the processes, governing the polarity inversion in III‐N films, will allow optimizing growth conditions and improving the quality of N‐polar thin films, therefore opening an access to the novel device concepts based on polarity engineering.