F. Harbsmeier

Ion beam mixing of ZnO/SiO2 and Sb/Ni/Si interfaces under swift heavy ion irradiation

We have investigated the irradiation induced interface mixing in ZnO/SiO2 (α-quartz) and Sb/Ni/Si thin layer systems under swift heavy ion irradiation in the electronic stopping power regime. The irradiations were carried out at 77 K using 100 MeV Ar, 260 MeV Kr, and 200 MeV Xe ions. For the ZnO/SiO2 system experiments were also carried out at lower ion energies (300, 600, and 900 keV, respectively) where nuclear stopping dominates. The alterations of the interface concentration profiles were determined by means of Rutherford backscattering spectrometry performed subsequently at the irradiated and the nonirradiated parts of the samples. While for the semimetal/metal Sb/Ni interface almost no mixing could be found after high-energy irradiation (mixing efficiency for Xe ions: k/Se<0.02 nm5/keV) the ceramic system ZnO/SiO2 strongly reacts upon high energy ion irradiation (Xe: k/Se=2.1 nm5/keV). The Ni/Si interface shows an intermediate effect (Xe: k/Se=0.2 nm5/keV). The mixing behavior found at high ion ener...

Interface mixing of CuOx/SiO2 bilayers by swift heavy ions

Abstract Thin films of CuOx (x=0, 0.5, 1) on SiO2-substrates were irradiated with heavy ions in the electronic stopping regime (some MeV/amu) and in the nuclear stopping regime (some keV/amu). The irradiation of the oxide layers with Ar, Kr and Xe ions of 90–260 MeV lead to strong interface mixing. The mixing efficiency seems to correlate with the bandgap of the toplayer and the estimation of the effective diffusion constant indicates interdiffusion in molten ion tracks. No mixing was observed after high energy irradiation of Cu/SiO2, which can be explained by the fact that the critical electronic stopping power for track formation has not been exceeded even with 230 MeV Xe ions. Irradiation of CuOx/SiO2 (x=0, 0.5, 1) with 900 keV Xe in all cases leads to only weak intermixing, which can be explained by the ballistic model.

Epitaxial regrowth of C- and N-implanted silicon and α-quartz

Abstract We have investigated the epitaxial regrowth and diffusion during thermal annealing of C- and N-implanted Si and α-quartz, utilising Rutherford Backscattering Spectroscopy in Channeling geometry (RBS-C) and nuclear reaction analysis (NRA). In both materials solid phase epitaxial regrowth was observed to occur at temperatures around 550°C (Si) and 1000°C (SiO 2 ). Epitaxial regrowth in Si was found to be strongly retarded compared to self-ion-implantation and the epitaxial recrystallisation front stopped at about the depth of maximum implant concentration even after annealing at 900°C. In fact, the implanted N-concentration profile in Si was found to be unchanged up to this temperature. This points at a chemical reaction between Si and the implanted C- and N-atoms, which obviously suppresses epitaxial recrystallisation. In SiO 2 almost no nitrogen could be detected in the N-implanted sample at T ≥900°C, i.e. no significant amount of impurity atoms was present during recrystallisation, which may hinder or suppress the regrowth process. A similar behavior is expected for the implanted C-atoms. In fact, epitaxial recrystallisation in the C-implanted α-quartz sample was found to be almost complete, except for a small amount of defects in the recrystallised matrix.

Diffusion of hydrogen implanted in α-quartz during air annealing

Abstract α-quartz samples were implanted with 20 keV H2+-ions to fluences of 5×10 16 H/cm2 at a target temperature of 77 K. Rutherford Backscattering in channeling geometry (RBS-C) revealed that an amorphous surface layer of 1.54×10 18 atoms/cm2 forms under these conditions. Resonant nuclear reaction analysis (RNRA) utilizing the 1H(15N, αγ)-resonance at 6.385 MeV beam energy was used to measure the implanted hydrogen profile. The samples were then annealed in air for 1 h at temperatures between 300°C and 950°C. After annealing, RBS-C and RNRA were again employed to study the alterations of the hydrogen profile and the amorphous layer induced by the heat treatment. In contrast to the observation with alkali ions no epitaxial regrowth could be detected even after the 950°C annealing. Below about 450°C also no changes of the hydrogen profile were observed, while at about 600°C almost all hydrogen has left the sample. This behavior fits nicely to the results obtained for other alkali implantations. Hydrogen as the lightest (and smallest) group-I atom becomes mobile at the lowest temperature and also the observed trend that the quality of the regrown layer decreases with decreasing atomic number of the implanted species has been confirmed, since no epitaxial recrystallization has taken place.

Solid state reaction in Si–C multilayers induced by ion bombardment

Abstract In order to investigate ion beam mixing and crystallization in the silicon–carbon system, Si/C multilayers were deposited on Si substrates and irradiated with Kr and Xe ions at temperatures to 873 K. The composition of the layer system as a function of depth before and after irradiation was analyzed by means of Rutherford backscattering spectroscopy (RBS). Changes in short-range order were monitored by X-ray absorption spectroscopy (XAFS). After irradiation at temperatures below 873 K, a disordered network of Si and C atoms forms, where the chemical short-range order is determined primarily by Si–Si bonds. At 873 K, the short-range order resembles that of crystalline SiC and XAFS results clearly indicate that crystallization and build-up of long-range order has initiated. Furthermore, it is demonstrated here that energy deposition by the ion beam is essential for initiating this solid state reaction, which cannot be achieved by thermal treatment only, even at higher temperatures.

Generation and relief of mechanical stresses in ion irradiated SiC and SiO2

Abstract We have investigated the generation and relief of mechanical stresses in 6HSiC and in α-quartz during bombardment with 50 keV Ne- and Na-ions at 77 K as a function of the applied ion fluence, by measuring the curvature change of the sample utilising mechanical surface profiling (MSP). In addition, the irradiation induced damage was depth profiled by means of Rutherford Backscattering Spectrometry in Channeling geometry (RBS-C). Density alterations could be deduced by combining the results on surface swelling obtained by MSP with the thickness of the ion beam modified layer observed with RBS-C. Bombardment with ion fluences below a critical fluence φc (nucleation regime) results in the accumulation of defect agglomerates and/or small amorphous zones in a still crystalline surrounding. At φc a coherent amorphous layer has formed, buried at the depth of maximum energy deposition. Further irradiation at higher fluences results in the growth of this layer perpendicular to the surface (growth regime). In the nucleation regime, strong compressive stresses of the order of 1.5–3 GPa are generated by the excess volume of the defect agglomerates. Only small volume swelling could be detected in this regime. After the amorphous layer extends to the surface, slow release of the compressive stresses could be observed, which was completed after application of φ = 3φc (SiC) and φ = 10φc (SiO2), respectively. The stress relaxation is accompanied by strong volume swelling perpendicular to the surface and a continuous decrease of the atomic density by up to about 20%. At still higher fluences, further growth of the amorphous layer occurs at reduced, but constant density.

Ion beam induced solid state reaction in Si/C layer systems

Abstract Ion irradiation (350 keV Xe ions) of Si/C layer systems leads to atomic mixing and a solid state reaction between the two constituents. The ballistic model underestimates the ion beam mixing rate and, in contradiction to the model, a quadratic dependence between the applied fluence and the thickness of the intermixed layer was found. This is a hint that chemical driving forces have to be taken into account. For the evolution of the chemical short range order in the intermixed material two temperature regions can be distinguished. If the irradiation is carried out below 873 K an amorphous network forms which exhibits no uniform short range order. At higher temperatures (∼873 K) a well-defined short range order forms which compares to that of crystalline SiC. Annealing experiments indicate that the phase formation is not a simple temperature effect, but the energy deposition by the ion beam is necessary for the formation of the SiC short range order.

Ion beam doping and epitaxial regrowth of α-quartz

Abstract Thermally induced epitaxial crystallization of thin a-SiO 2 films represents a promising procedure for removing the ion-beam damage induced during the fabrication of integrated optical devices. In this paper we report on the crystallization of a-SiO 2 films deposited on single crystalline α-quartz substrates, investigated by means of Rutherford backscattering spectrometry in channeling geometry (RBS-C). The epitaxial crystallization was achieved by means of a novel three-step procedure which uses high-fluence Cs + -ion doping of the films and subsequent annealing in air at 800–900°C. Similarly, amorphous SiO 2 layers, created by the ion irradiation of α-quartz samples, were epitaxially regrown after alkali post-implantation and annealing, thus demonstrating that the regrowth is independent of the production history of the amorphous film. Optical spectroscopy in the range 300–1100 nm showed the good optical properties of the regrown layer.

Amorphisation kinetics in ion-irradiated ceramics

We have investigated in detail the damage accumulation and amorphisation processes in covalently bound Si-based compounds like SiC, Si3N4 and SiO2 during low temperature ion bombardment. By combining ion beam analysis techniques with X-ray absorption, Raman spectroscopy and mechanical surface profiling we have gained new insights into the fundamental mechanisms underlying the amorphisation process and into the evolution of the short range order at the crystalline-to-amorphous transition and after further ion bombardment at higher fluences. In the present paper, the main results of this study shall be summarised.