K. Gärtner

Axial dechanneling: II. Point defects

Abstract The model of axial dechanneling presented in part I is applied to crystals with point defects. The additional change in transverse energy due to scattering by point defects is calculated with the same procedure as used for scattering by thermally displaced atoms. Point defects with fixed and randomly distributed positions in the channel are considered. For heavily damaged (Si) and weakly damaged (Si, Ge, Ta) crystals the magnitude and temperature dependence of the calculated dechanneling is in good agreement with the experimental data. The different temperature dependence of dechanneling in〈111〉 and 〈110〉 silicon measured by Howe et al. may be explained by 〈110〉-splits found by Gotz et al.

Three-step amorphisation process in ion-implanted GaN at 15 K

GaN layers were implanted at 15 K with 150 keV O, 300 keV Ar or 800 keV Xe ions. The subsequent damage analysis was performed by Rutherford backscattering of He ions in channelling configuration at the same temperature. At this low temperature thermal effects can be widely excluded. However, the dependence of the damage concentration on the ion fluence suggests that the damage evolution in GaN is dominated by a pronounced recombination of the primarily produced defects within the collision cascades. Furthermore, a strong influence of the ions themselves has to be assumed in order to understand the experimental results. Such effects occur already at rather low ion fluences. Our results indicate an amorphisation of GaN proceeding in three steps.

AXIAL DECHANNELING IN COMPOUND CRYSTALS WITH POINT DEFECTS AND DEFECT ANALYSIS BY RBS

Abstract The general channeling concept of Lindhard is extended to the description of channeling phenomena in compound crystals. Based on this modified concept the description of axial dechanneling in elementary crystals elaborated by Gartner et al. is also extended to compound crystals. It is applied to the case of compound crystals containing two different kinds of point defects described by two different configurations of uncorrelatedly displaced lattice atoms. Based on this description the analysis of point defects by Rutherford Backscattering Spectrometry (RBS) can be extended to compound crystals and to more complex point defect structures. The progress in defect analysis obtained is demon-strated for two examples of radiation damage in GaAs.

Axial dechanneling: III. Dislocations

Abstract The model of axial dechanneling given in part I is extended and applied to crystals with dislocations. The dechanneling matrix is determined by the change of transverse energy of ions moving in a distorted channel. The analysis of some RBS data provides integral dislocation densities in good agreement with the results of TEM measurements. Because of the mutual influence of the dechanneling contributions from the perfect crystal and from the dislocations the dechanneling parameter is not only a function of the integral density of dislocations (as assumed so far) but depends also on the shape of the distribution.

LATERALLY RESOLVED CRYSTALLINE DAMAGE IN SINGLE-POINT-DIAMOND-TURNED SILICON

Abstract By using an ultra-stiff lathe and a diamond tool, silicon can be turned like ductile materials. We have prepared diamond-turned silicon samples, and characterised them with ion microbeam Rutherford backscattering and channelling, together with cross-sectional transmission electron microscopy (TEM) and other techniques. The channelling spectra of the as-turned samples have been interpreted using a new algorithm, and are shown to be consistent with the amorphous surface layer and the dislocation rich subsurface layer seen with TEM. The lateral resolution of our microbeam enables us to probe and image regions with clearly different damage; these are analysed and interpreted.

Investigation of defects in weakly damaged ion implanted GaAs layers

Abstract From the analysis of the near edge optical properties it has previously been concluded that weakly damaged ion implanted GaAs layers contain high concentrations of point defect complexes consisting preferably of vacancies and anti-site defects. The investigation of the temperature dependence of the dechanneling of light ions provides further information about the structure of such layers. With this method it was shown that, in room temperature nitrogen implanted GaAs, slightly and heavily displaced atoms exist with mean values of the displacement distance r a = 0.018 nm and r a = 0.065 nm from the atomic strings, respectively. The concentrations of the two defect types varies with the ion fluence. The results indicate that defect annealing and recombination during room temperature implantation of GaAs produces a resulting damage structure consisting of point defect complexes which contain vacancies and anti-site defects. As the implant fluence is increased the formation of further defects (interstitials also) becomes probable.

Investigation of defects in GaAs by dechanneling

Abstract Weakly damaged GaAs layers are characterized by a slight increase of the yield of backscattered light ions. In such cases, displaced atoms with preferred positions in the lattice are more probable than a random distribution. Information about the position of the displaced atoms can be provided by the temperature dependence of the dechanneling. By this the position and the depth distribution of the defects can be obtained. The investigation of room-temperature nitrogen-implanted GaAs has shown that two characteristic positions for displaced atoms — ra,1 = 0.018 nm and ra,2 = 0.065 nm — fit the experimental data very well over a broad range of ion fluences (2 × 1013 ⩽ Ni ⩽ 1 × 1016cm−2). The observed behaviour may be understood assuming defect complexes containing heavily and slightly displaced atoms, the concentration of which varies with the ion fluence and with the dose rate.

Computer simulation studies of low energy B implantation into amorphous and crystalline silicon

Abstract Computer simulation results of the depth profiles of B ions implanted with energies in the range of 0.2 to 5 keV into amorphous and 〈100〉 Si are presented and discussed. For simulation different binary collision computer codes (MARLOWE, TRIM, BCCRYS) were used. The influence of the interatomic interaction potential is carefully checked. The best overall agreement with experimental data is obtained when using the “individual potential”. Low energy channeling is proved to be quite different from high energy channeling. Multiple interactions, which are important for channeling, are shown to be insufficiently included in the existing binary collision codes.

Defect characterization of low-energy recoil events in silicon using classical molecular dynamics simulation

Abstract We classify the defects generated by silicon recoils as a function of energy up to 200 eV, using classical molecular dynamics simulations and analysis of the geometry of each isolated defect. The majority of defects in this energy range are vacancies and interstitials, the latter mostly in split-〈110〉 configuration and less frequently in tetrahedral interstitial positions. Besides Frenkel pairs, bond defects and di-interstitials are found with significantly lower probability. The fraction of defects belonging to none of these types is less than 5% for recoil events below 200 eV, but rises sharply at higher energies and remains almost constant at a value of 40% between 300 and 500 eV. Moreover, we determine the projected range and the pair distance distribution of the defects. Throughout the paper we compare results obtained with the Tersoff and the Stillinger–Weber interatomic potential.

RBS and optical investigations of defects in weakly damaged GaAs

Abstract Weakly damaged GaAs layers were investigated by optical transmission and reflection measurements and the Rutherford backscattering (RBS) channeling technique. The samples show characteristic near edge optical properties which are caused by high concentrations of point defects. The analysis of the temperature dependence of the RBS minimum yield points at the existence of slightly displaced lattice atoms with concentrations ≳ 50 at.%. The results suggest, in agreement with those of the optical measurements, that vacancies, antisite defects and/or complexes are responsible for the high concentration of slightly displaced atoms.