MS-8-P-3000 Dependence of microsrtucture in AlN thin films on the annealing temperature for sapphire substrate

Aluminum nitride (AlN) crystallizes in wurtzite structure (P6₃mc) with the lattice parameters of a=0.311 nm and c=0.498nm. AlN is a very promising material for application not only to surface acoustic wave (SAW) devices and optoelectronic devices for very short wavelength regions but also substrate materials. In the course of research on metal-organic vapor phase epitaxy (MOVPE) of AlN, it was found that the crystallinity of AlN depends upon the annealing temperature of sapphire substrate. In this work, detail characterization was conducted to clarify the origin of its annealing temperature dependence.

Prior to the deposition of AlN, a sapphire (0001) substrate (0.2 degree off toward m-direction) was annealed at T_an=1150 - 1350^oC for surface-cleaning. Thereafter, AlN was deposited by MOVPE first at 1200^oC to be 100nm in thickness and then overgrown at 1500^oC. Thin-foil samples were made with a focused ion beam (FIB) mill (Hitachi. FB-2000K), followed by finishing with an argon-ion mill. TEM observation was performed with a JEM-2000EX microscope (JEOL). SEM observation and EBSD analysis were also carried out (Zeiss, Ultra55) for the AlN specimens without any prior treatment such as coating or etching.

It was found from the X-ray rocking curve (XRC) measurement that the twisting of AlN along c-axis is around 3 degree for T_an > 1250^oC, while very little twisting occurs for T_an < 1250^oC. Fig. 1 shows the results of TEM observation for the AlN specimens of T_an=1350^oC and 1225^oC. One can see that in both specimens, threading dislocations (TDs) have been formed and run upward. The densities of dislocations with screw and edge components are roughly estimated to be approximately 10⁹ cm^-2 and 10¹⁰ cm^-2, respectively. In the specimen of T_an=1350^oC, TDs are straight in shape and form a columnar structure of AlN. Fig.2 shows SEM and EBSD results of the specimens. In the SE (out-lens) image of the specimen of T_an=1350^oC shown in (a), one can see pits and steps. In the SE (In-lens) image (b), dark winding line-contrast is recognized. The locations of the line contrast coincide with those of steps in (a). The grain reference orientation deviation (GROD) map (the maximum deviation~3^o) shown in (c) indicates clearly that the dark line contrast corresponds to the boundaries of twisting domains. For the specimen of T_an=1225^oC, on the other hand, pits and step are seen in the SE-image (Out-lens) shown in (d), but dark line contrast is not recognized in (e), and the GROD map in (f) does not indicate the existence of twisting domains. These results suggest that low-angle grain boundaries can be observed in SE image (In lens). The origin of the contrast is not clarified.