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 necessary. 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 probes 181Hf and 111In. The probe atoms were implanted at the Bonn isotope separator with an energy of 160 keV and doses of 1013cm−2. Subsequently an isochronal annealing programme was carried out and PAC spectra were taken after each annealing step.
After implantation into GaN the major part of the Hf probes is built in on substitutional Ga sites and the level of lattice damage decreases with the annealing temperature. In the case of In the results also show a good recovery of the crystal lattice but reveal an unexpected development of the PAC-frequency, from which the local symmetry of the wurtzite GaN lattice at the probe site can be derived. The value of the frequency decreases by 30% with the annealing temperature implying a considerable change in the local lattice geometry. Another puzzling outcome is the unexpected large difference of the values for the PAC frequency for measurements with Hf and In. As it is known from RBS/Channeling measurements both probes occupy the Ga site. The ratio between the two frequencies should be determined only by the atomic and nuclear properties of the probes and usually has values of 6:1 for Hf and In. Indeed in many materials a factor of six to ten is observed. In GaN, in contrast, this ratio has values of 50:1. This gives again evidence of the unusual behaviour of In in GaN. Another difference is the maximum fraction of probe atoms incorporated on undisturbed substitutional lattice sites. Whereas for Hf this fraction is 80% for In only 60% is reached. RBS/Channeling measurements however show, that already directly after implantation 90% of the probe atoms are incorporated in substitutional sites. The discrepancy can be explained by the better sensitivity of the PAC technique to point defects in the neighbourhood of the probe atoms in comparison to the RBS. However, the difference between the PAC results with the two probes give hint to an additional defect coming up in the samples implanted with In.
In AlN substantial fractions of both probe atoms occupy substitutional, undisturbed lattice sites after annealing. A detailed investigation of the changes observed during the isochronous annealing programme indicates differences in the recovery process. After In implantation AlN shows considerable annealing of lattice damage already at unexpectedly low temperatures below 500°C. For Hf implantation AlN experiences a "reverse annealing" effect, i.e. in a temperature range of 400—500°C the substitutional fraction decreases before it increases again in another annealing regime at higher temperatures.
After implantation of In in InN a polycrystalline frequency equal to the one measured for In in In metal is observed probably due to In trapped in In clusters which are usually formed during the growth of InN or InGaN. With annealing the fraction showing this frequency decreases and is strongly damped which can be explained by diffusion of In out of the clusters. Only at high annealing temperatures (800°C) the frequency is again visible in the spectrum possibly due to the out diffusion of N and the segregation of In to indium surface clusters. The rest of the probe nuclei are incorporated on sites showing no single crystalline behaviour indicating the low crystalline quality of the film. Measurements with Hf also show only little annealing effect.
The electric field gradients (EFG) caused by the hexagonal lattice structure for GaN and AlN could be determined to Vzz(GaN) = 9.8(2)·1015V/cm2 and Vzz(AlN) = 16.2(2)·1015V/cm2 for the Hf probe, and Vzz(GaN) = 1.02(10)·1015V/cm2 and Vzz(AlN) = 5.5(2)·1015V/cm2 for the In 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 AlN is oriented parallel to the ˆc-axis of the crystal. To further study the different behaviour of Hf and In in GaN measurements were performed with different doses of implanted In and measurements at different temperatures were done. The dose dependent measurements suggest, that the variation of the frequency with annealing steps measured with In is due to the introduction of mechanical stress during the implantation and the relaxation of this stress during subsequent annealing. This theory was confirmed by XRD (x-ray diffraction) measurements which show a satellite peak in the spectra due to a lattice expansion in the implanted region. With annealing this peak shifts to smaller lattice parameters and for low dose implantation (5·1013cm−2) it disappears after annealing at 1000°C. For high dose implantation (5·1014cm−2) not all stress could be removed.
The temperature dependence of the EFG was studied in samples implanted with 181Hf and 111In and annealed at 1000°C. PAC measurements were performed at different temperatures between 16 and 1024 K. It was found, that after annealing the implantation induced damage, 65% of the implanted atoms were situated in regular undisturbed Ga lattice sites. The remaining fraction showed a puzzling behaviour insofar as its lattice surroundings changed reversibly from undisturbed at temperatures above 600 K to strongly disturbed at low temperatures. This can be explained by the trapping of a point defect near the probe at low temperatures which is detrapped at higher temperatures. Still unknown is the nature of this defect.