Lattice site location of electrical dopant impurities in group-III nitrides

Dopants are impurities introduced in semiconductors in small quantities to tailor the material characteristics, the effects of which depend on the exact site the dopant occupies in the crystal lattice. The lattice location of impurities is, thus, crucial for the overall understanding of the semiconductor characteristics. In general, several techniques can be used to investigate the lattice site of an impurity, the most accurate and dedicated being emission channeling. However, a characteristic of this technique is that it requires the implantation of radioactive probes, usually created and accelerated in a radioactive ion beam facility. In some cases, emission channeling might however be the only technique capable to investigate the lattice sites occupied by the impurity atoms, provided an appropriate isotope for this technique can be used. For instance, the use of other methods such as Rutherford backscattering spectrometry, perturbed angular correlations, Mössbauer spectroscopy and extended X-ray absorption can be hindered by the lack of suitable probe atoms for these techniques, or by overlapping signals associated with the dopant and the constituent atoms. The work presented in this thesis aims at investigating the lattice site of electrical dopants in the group-III nitrides, GaN, AlN and InN. The nitrides have applications in a wide variety of optoelectronics (e.g. light emitting diodes), high temperature and high frequency electronics. The positive electrical doping is of utmost relevance, because the nitrides have an n-type (negative) characteristic background, and most of these applications require both a p-type (positive) and an n-type doping in order to obtain a p-n junction to produce diodes. Although group-II elements are expected to behave as acceptors, promoting p-type doping, the only one with established efficiency is Mg. Hence, particular attention was given to the Mg lattice site in nitrides in work done for this thesis. Recent developments in the setup allow the use of 27Mg (t1/2=9.45 min), produced at the ISOLDE-CERN facility, to perform emission channeling experiments and investigate this subject, while the mentioned alternative techniques are not applicable in this case. At ISOLDE, radioactive probes are created, accelerated and bombarded at relatively high energy (20−60 keV) into a single-crystal. These ions decay, emitting beta− particles in all directions. Their trajectories are influenced by the Coulomb interactions with the atoms of the material. This way, the beta− are channelled along the directions where the atoms are aligned due to symmetries of the crystal. Using this technique, we determined that Mg preferably occupies the cation site (Ga, Al or In) of the group-III nitrides. Moreover, in the case of GaN and AlN, at room temperature, a significant fraction of about 20% of Mg is also found in interstitial octahedral sites. At higher temperatures the interstitial Mg gradually migrates to the cation sites, and above 600°C it is found entirely in the substitutional sites of Ga and Al in GaN and AlN, respectively. The temperature required to migrate Mg to the cation site, where it is expected to behave as a p-type dopant, is well below the growth temperature of GaN and AlN. Moreover, the annealing temperature necessary to remove H impurities from the material and to activate the Mg-doped nitrides p-type is also above the Mg migration temperature. Our study of Mg lattice location in GaN and AlN contributes to settle a recent discussion prompted by theoretical predictions over the local structure distortion around different ionisation states of Mg in the nitrides, i.e. the precise location of Mg helps clarify the local structure surrounding it. The published data on nitrides implanted with Mg were used to consider the influence of the interstitial Mg in the electrical activity of the dopant, however new dedicated systematic measurements should be performed to draw more conclusive correlations. The lattice sites of several dopants, with properties similar to those of Mg, were also investigated by means of emission channeling, to understand which common factors contribute to the lattice location, and thus efficiency, of acceptors in the nitrides. From group-I of the periodic table, Na is a possible double-acceptor with high ionisation energy and, most importantly, of similar size to Mg. At room temperature, this element is found in the octahedral site and in the cation site of the GaN and AlN lattices. A significant conversion from interstitial to substitutional Na occurs after annealing at 900°C, revealing a higher stability of Na in the cation site, than Mg has shown. The comparison of this migration of the interstitial Na with that of Mg and Li in GaN and AlN reveals a correlation between the migration energy and the ionic radius of the group-I or group-II elements. The lattice sites of Ca and Sr -group-II elements heavier than Mgwere investigated in InN to complete the previous studies performed in GaN and AlN crystals, where these elements were found in the cation site. Similar to Mg, the largest fractions of the alkaline earths Ca and Sr were found in In sites, while a small fraction of Ca occupies the substitutional site of N. The Mn element, a magnetic dopant from group-VII, was suggested to also behave as an acceptor in InN. Mn has been assumed to occupy the cation site since its ionic radius is much closer to that of In3+ than to N. Nevertheless, in our experiment, the majority of Mn was found in the substitutional site of In. Simultaneously, a considerable fraction of Mn was distinguished in the N sites, which remains stable after a 400°C annealing. The fraction of Mn that is stable in the N site can contribute to the inefficiency of Mn as a magnetic and/or electrical dopant in InN. These findings are consistent with previous Mn emission channeling studies performed in GaN. For completion, we studied the lattice location of As in nitrides. Arsenic is from the same group as N and has a covalent radius similar to that of Al and Ga. Previous studies concluded that it is amphoteric in GaN, occupying both the substitutional site of N and Ga. We ascertained that As exhibits an amphoteric behaviour in InN and AlN, which altogether suggests that the size of As competes with its tendency to attract electrons, i.e. electronegativity, to define its preferential lattice site. In combination with results found in the literature, we conclude that the dopants (including but not exclusively the electrical dopants) in the nitrides exhibit a preference for the occupation of the cation site. After room temperature implantation, dopants with a small radius, in comparison with the cation, were also found in the octahedral (O) or the anion (N) sites of the nitrides lattice. Furthermore, these secondary locations depend on the dopants electronegativity. In other words, the size mismatch with the close-packed crystal structure clearly competes with the electronegativity to define a dopant’s lattice site in the nitrides. The influence of the dopant’s radius on its location is likely due to the large size difference between the cation and the N atoms of the material, hence it is more prominent in the case of InN.