Wide bandgap semiconductors like the group III nitrides have recently emerged as important base materials for applications in optoelectronics and in high power high temperature electronics. However, for integrating these materials into circuits an adequate structuring technique is ecessary. Ion implantation is such a technique which is commonly used for standard semiconductors like Si but which still needs some development to apply it to the nitrides. The strong bonding of these materials ensures a high resistance to lattice damage and amorphisation but at the same time it considerably hampers the epitaxial regrowth of the lattice during post implant annealing. The PAC (perturbed angular correlation) technique was applied to study the annealing procedures of GaN, InN and AlN implanted with the PAC probe 181Hf . The probe atoms were implanted at the Bonn isotope separator with an energy of 160 keV and doses of 1013 cm−2. Subsequently an isochronal annealing programme was carried out and PAC spectra were taken after each annealing step. In all three lattices substantial fractions of the probe atoms occupied substitutional lattice sites after annealing. Directly after the implantation a damped oscillation was observed in the PAC spectra for GaN and InN. This indicates that at the end of the collision cascade a small fraction fs of the implanted probe atoms stops on well defined lattice sites. Least square fits to the data yield values for fs of 36% for GaN and 20% in the case of InN. These high values for fs as compared to AlN can be ascribed to the higher replacement collision probability due to the larger mass of the group III atom. The following annealing process for the three compounds showed quite different features.
In GaN immediately after the implantation most (64%) of the implanted 181Hf probes are situated in a lattice environment stronly disturbed by the implantation induced damage. The rest of the probes (fs = 36%) occupies less disturbed lattice sites. For this latter fraction annealing up to 673 K causes a strong drop of the damping parameter. During further annealing at higher temperatures the damping parameter keeps falling but slower whereas the fraction fs increases to 80% after the annealing step at 1373 K. In InN the initial fraction of probes on well defined lattice sites is fs = 19%. During annealing this fraction increases linearly with temperature until at 1173 K fs = 44% is reached. The corresponding damping factor remains essentially constant up to 600 K and then drops rapidly to its final value of 3.5%.
After the implantation all the 181Hf probes in AlN come to rest in a highly damaged lattice environment. Annealing leaves the spectrum unchanged until at 773 K a second, much narrower, frequency distribution appears in the spectrum. It’s fraction fs increases only slightly to 13% at 1173 K and it’s damping parameter does not decrease significantly up to 1773 K.
The electric field gradients (EFG) caused by the hexagonal lattice structure could be determined to Vzz(GaN) = 9.5(2)·1015 V/cm2, Vzz(InN) = 18.9(3)·1015 V/cm2 and Vzz(AlN) = 9.2(2)·1015 V/cm2 for the Hf probe. The orientation of the EFG was determined by measurements in different orientations of the samples. It was shown that the lattice EFG in GaN and InN is orientated parallel to the c-axis of the crystal whereas in AlN no significant differences were seen between in the spectra of different orientations due to the low single crystalline quality of the AlN material.