Structure of the 2H-AlN/6H-SiC Interface
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551 Mat. Res. Soc. Symp. Proc. Vol. 423 01996 Materials Research Society
TEM cross section samples were thinned down to 100 pim by mechanical grinding and dimpled down to 10 pm. Electron transparency was achieved by ion milling with a LN2 cold stage at 5 kV. A final step at 3 kV was used to remove possible ion damage. HREM observations were made along the < 1120> direction on a Topcon 002B electron microscope operating at 200 kV with a point to point resolution of 0.18 nm (Cs=0.4 mm). Image simulations were carried out using the multislice package of the EMS software [9]. RESULTS The deposited AIN film and the SiC substrates present hexagonal symmetry structures with different stacking sequences. AIN exhibits the wurtzite structure, or 2H polytype in the Ramsdel notation, with a ...AaBbAaBb... stacking sequence, whereas SiC has the 6H polytype with the ...AaBbCcAaCcBbAa... sequence. Although the two materials have tetrahedrally coordinated structures with a good lattice match in the basal plane, their stacking is very different along the c direction. Due to the misoriention of the substrates, numerous steps are present at the interface. The displacement vector relating two terraces of the substrate separated by an interface step does not always correspond to a translation operation of the wurtzite structure. Therefore, the two crystals which grow on the two different terraces can present two different stacking sequences e.g.: ..AaBb.. and ..AaCc.. A simple procedure [10] has been devised in order to characterize all the possible interface steps, and at the AMN/SiC interface, only three types of steps were shown to exist. Firstly, there are steps for which the displacement vector corresponds to a translation operation of the wurtzite structure. An example is shown fig. 1. In this case, the step height is of 6 atomic bilayers ( atoms are black on the micrograph).
Fig. 1 HREM image of a 6 atomic bilayers step height. The Burgers circuit drawn on the micrograph shows that such a step does not present any dislocation character.
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The displacement vector introduced by the step is then 1 c translation of the SiC and corresponds to 3 c translations of the AIN structure, regardless the small lattice mismatch ( -1%). The Burgers circuit drawn on the micrograph does not point out any dislocation character for the step. A close examination of the step shows that it can be divided into two parts. The leftern part corresponds to the core (arrow fig. 1); the step face is vertical and extends along 4 atomic bilayers. Then, along a distance of 4.5 nm (dashed lines fig. 1), the step is lifted up another 2 bilayers, along the stacking misfit appears to be compensated. At this place, the coherency of the interface seems to be loss. The second kind of interface step is illustrated on figure 2. The step is 2 atomic bilayers high. The displacement vector d = CAIN + 1/3 introduced by the step was illustrated by the closure failure of the Burgers circuit drawn on the micrograph. The 1/3 component corresponds to the fault vector of t
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