Showing papers by "Werner Wesch published in 2009"

Radiation damage in ZnO ion implanted at 15 K

Abstract: Commercial O-face (0 0 0 1) ZnO single crystals were implanted with 200 keV Ar ions. The ion fluences applied cover a wide range from 5 × 10 11 to 7 × 10 16 cm −2 . The implantation and the subsequent damage analysis by Rutherford backscattering spectrometry (RBS) in channelling geometry were performed in a special target chamber at 15 K without changing the target temperature of the sample. To analyse the measured channelling spectra the computer code DICADA was used to calculate the relative concentration of displaced lattice atoms. Four stages of the damage evolution can be identified. At low ion fluences up to about 2 × 10 13 cm −2 the defect concentration increases nearly linearly with rising fluence (stage I). There are strong indications that only point defects are produced, the absolute concentration of which is reasonably given by SRIM calculations using displacement energies of E d (Zn) = 65 eV and E d (O) = 50 eV. In a second stage the defect concentration remains almost constant at a value of about 0.02, which can be interpreted by a balance between production and recombination of point defects. For ion fluences around 5 × 10 15 cm −2 a second significant increase of the defect concentration is observed (stage III). Within stage IV at fluences above 10 16 cm −2 the defect concentration tends again to saturate at a level of about 0.5 which is well below amorphisation. Within stages III and IV the damage formation is strongly governed by the implanted ions and it is appropriate to conclude that the damage consists of a mixture of point defects and dislocation loops.

Ultrathin membranes in x-cut lithium niobate

Abstract: Ion-beam enhanced etching is used to pattern a bulk lithium niobate crystal with ultrathin membranes. By the implementation of an air gap beneath the membrane, high index contrast is achieved. A buried amorphous layer, created by irradiation with He ions, is removed by means of wet chemical etching in hydro-fluoric acid. Membranes having thicknesses down to 200 nm are fabricated. The etched air gaps and the membranes exhibit a uniform thickness over the entire etched area, and their widths can be purposefully adjusted over a wide range by choosing appropriate ion energies and fluences as well as annealing conditions.

In situ observation of surface oxide layers on medical grade Ni-Ti alloy during straining.

Abstract: Medical grade Ni-Ti alloys with shape memory or pseudo-elastic behavior exhibit good biocompatibility because of an electrochemically passive oxide layer on the surface. In this work, the mechanical stability of surface oxide layers is investigated during reversible pseudo-elastic deformation of commonly applied medical grade Ni-Ti wires. Surface oxide layers with varying thickness were generated by varying annealing times under air atmosphere. The thicknesses of the surface oxide layers were determined by means of Rutherford backscattering spectrometry. In situ scanning electron microscopy investigations reveal a damage mechanism, which is assumed to have a significant influence on the biocompatibility of the material. The conditions that lead to the appearance of cracks in the surface oxide layer or to the flaking of surface oxide layer particles are identified. The influence of the thickness of the surface oxide layer on the damage mode is characterized. The possible impact of the damaged surface oxide layer on the material's biocompatibility and the potentials to reduce or avoid the damage are discussed.

Structural modification of swift heavy ion irradiated amorphous Ge layers

Abstract: Swift heavy ion (SHI) irradiation of amorphous Si (a-Si) at non-perpendicular incidence leads to non-saturable plastic flow. The positive direction of flow suggests that a liquid phase of similar density to that of the amorphous solid must exist and accordingly a-Si behaves like a conventional glass under SHI irradiation. For room-temperature irradiation of a-Si, plastic flow is accompanied by swelling due to the formation of voids and a porous structure. For this paper, we have investigated the influence of SHI irradiation at room temperature on amorphous Ge (a-Ge), the latter produced by ion implantation of crystalline Ge substrates. Like a-Si, positive plastic flow is apparent, demonstrating that liquid polymorphism is common to these two semiconductors. Porosity is also observed, again confined to the amorphous phase and the result of electronic energy deposition. Enhanced plastic flow coupled with a volume expansion is clearly responsible for the structural modification of both a-Si and a-Ge irradiated at room temperature with swift heavy ions.

Rapid ion-implantation-induced amorphization of InxGa1−xAs relative to InAs and GaAs

Abstract: We report on the rapid implantation-induced amorphization of the ternary InxGa1−xAs alloys Unlike AlxGa1−xAs, InxGa1−xAs did not exhibit amorphization kinetics intermediate between the two binary-alloy extremes Instead, our investigation of the crystalline-to-amorphous phase transformation over the entire stoichiometry x range demonstrated that InxGa1−xAs alloys with x= 006–053 were rendered amorphous at fluences less than that required for both InAs and GaAs Implantation-induced disorder was quantified with Rutherford backscattering spectroscopy in the channeling configuration and fit to the Hecking model to yield the probabilities of direct-impact and stimulated amorphizations The phase transformation was dominated by stimulated amorphization, which was a maximum at the stoichiometry x= 034 most easily amorphized, while the probability of direct-impact amorphization was effectively stoichiometry independent From extended x-ray-absorption fine-structure spectroscopy measurements of unimplanted InxGa1−xAs alloys, separate stoichiometry-dependent In-As and Ga-As bond lengths were measured Distortion in both the bond-length and bond-angle distributions was apparent though structural perturbation was primarily accommodated in the latter as consistent with measured deviations from the tetrahedral bond angle We attribute the relative ease with which the InxGa1−xAs alloys were amorphized to the presence of localized regions of strain due to structural distortion Equivalently, atomistic configurations comprised of strained bond lengths and bond angles represent pre-existing and preferential sites for stimulated amorphization To demonstrate the general applicability of our model, we report on preliminary measurements with the InxGa1−xP alloys which also exhibit a bimodal bondlength distribution and distortion in the bond-angle distribution Comparable amorphization behavior to that of the InxGa1−xAs alloys has been observed

Annealing behavior of lithium niobate irradiated with He-ions at 100 K

Abstract: In order to investigate the formation and the annealing of defects, x - and z -cut lithium niobate ( LiNbO 3 ) crystals were irradiated with 200 keV He-ions at a temperature of 100 K. Subsequently, thermal treatment was performed at temperatures of 250 °C and 300 °C, respectively. The defect concentration was measured by means of Rutherford-backscattering spectroscopy (RBS) using 1.4 MeV He-ions immediately after the irradiation at 100 K, after warming to room temperature as well as after thermal treatment. After the irradiation the x -cut crystal is apparently more damaged compared to z -cut which is a consequence of the preferential arrangement of displaced Nb-atoms on vacant octahedral sites. Upon thermal treatment the samples show a completely different annealing behavior. At an ion fluence of 1 × 10 16 cm - 2 , the defect concentration decreases for x -cut material, but increases for z -cut samples with increasing annealing time. This is explained by the formation of aligned dislocation loops during the thermal treatment.

Strain-driven defect evolution in Sn+ implanted Si/SiGe multilayer structure

Abstract: We report on secondary defect evolution in a multilayered Si/SiGe structure after 1 MeV Sn + -ion implantation to a fluence of 2 × 10 14 cm −2 followed by thermal annealing in a dry nitrogen atmosphere. Formation of a buried amorphous layer is registered after ion implantation. Thermal treatment leads to formation of dislocation loops in an EOR-defect band, and a mixture of tangle dislocations and “clamshell” defects at the depth of 200–500 nm. In addition, self-assembling of voids in a near-surface SiGe layer structure is observed. The voids are of nanometer size and are preferably located in thin SiGe layers. The results are discussed in terms of the separation of the vacancy and interstitial depth profiles attributed to the preferential forward momentum of recoiling Si atoms. The compressively strained SiGe layers play the role of vacancy accumulator, prevent in-surface diffusion of vacancies and, in this way, result in self-assembling of voids inside compressively strained SiGe layers.

N – K edge NEXAFS study of the defects induced by indium implantation in GaN

Abstract: N – K edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy is applied in order to determine implantation-induced changes in the electronic structure of GaN. The samples were implanted with 700 keV In ions and fluencies in the range 5×1013 – 1 × 1016 ions/cm2. The NEXAFS results are discussed in combination with Rutherford backscattering (RBS) characterization which assesses the implantation induced damage. The main implantation effects on the NEXAFS spectra are: (a) a fluence-dependent broadening of the NEXAFS peaks, (b) emergence of a pre-edge shoulder (RL1) that is attributed to N split-interstitials and (c) appearance of a post-edge sharp peak (RL2) that is attributed to molecular N2 trapped in the GaN matrix. The RL2 is characterized by fine structure due to vibronic transitions that result from a change of the vibrational quantum number along with the electronic transition. The concentration of the interstitials and the N2 molecules as well as the width of the NEXAFS peaks, have a sigmoidal dependence on the logarithm of the ion fluence, following the behaviour of the defect concentration deduced from the RBS measurements.

Effect of implantation and annealing regimes on ion-beam synthesis of InAs nanocrystals

Abstract: We reported the formation of nanosized InAs crystallites in silicon wafers by means of As (245 keV, 4.1·10 cm) and In (350 keV, 3.7·10 cm) implantation. The implantation was carried out at 25 and 500 ◦C. In order to verify the effect of getter on precipitates growth an additional procedure was carried out for the samples implanted with As and In species at the room temperature. This procedure included the implantation of H+2 ions with the energy of 100 keV at 1.2·10 16 cm. Afterwards, the samples were annealed at 900 ◦C for 60 min in inert ambient. In order to characterize the implanted layers, Rutherford backscattering spectrometry in combination with the channelling (RBS / C) and transmission electron microscopy (TEM) techniques were used. TEM has revealed InAs nanocrystals in implanted samples after the annealing. It has been shown that average size and size distribution of InAs clusters depend on implantation temperature and annealing duration. Significant diffusional redistribution of implanted species has been revealed after “hot” implantation and post-implantation annealing. We have suggested that it is caused by non-equilibrium diffusion. The radiation-enhanced diffusivities at “hot” implantation have been determined for the abovementioned experimental conditions.

Ion beam synthesis of Mn/Sb clusters in silicon

Abstract: In order to investigate the formation of Mn/Sb clusters embedded in crystalline silicon, sequential ion implantation with fluences of 1 × 1016 at cm−2 and 2 × 1016 at cm−2, respectively, was used to incorporate Mn and Sb ions at high concentrations into Si(0 0 1). Based on investigations with Rutherford backscattering spectroscopy (RBS) and corresponding channelling measurements (RBS/c), we report on a temperature dependent redistribution of the implanted species during the rapid thermal annealing process governed by the radiation-induced defects. Additionally performed cross-sectional TEM analyses, including EDX measurements, clearly show the presence of hexagonal shaped elementary Sb precipitates as well as compound clusters consisting of Mn and Sb, which are aligned to the crystal structure of the host silicon. In electron magnetic resonance measurements many samples exhibit broad resonance bands persisting up to approximately 60 K. For out-of-plane rotations, the bands show a weak angular dependence of the resonance field but a strong angular dependence of the intensity. Zero-field-cooled and field-cooled magnetization curves were measured on selected samples with a SQUID magnetometer between 10 and 400 K at different applied fields. The curves show a weak magnetic signal generated by different magnetic phases while at least one can be ascribed to superparamagnetic nanoparticles of MnSb.

Er+ implantation in SnO2:SiO2 layers: Structure changes and light emission

Abstract: Structure changes and light emission behavior in Er + implanted SnO 2 :SiO 2 layers are studied, using transmission electron microscopy (TEM), Rutherford backscattering (RBS) and cathodoluminescence (CL). SnO 2 :SiO 2 layers of different composition deposited with RF magnetron sputtering on Si wafers were implanted with 200 keV Er + to a fluence of 3 × 10 15 cm −2 at room temperature. The implanted structures were then annealed at 600–1000 °C for 30 min, resulting in the formation of crystalline SnO 2 nanoclusters. Cross-section TEM revealed a strong reduction of the SnO 2 crystallite size down to several nanometers in the implanted area of the SnO 2 :SiO 2 layer as compared to the undoped layer. In addition, a very narrow layer of SnO 2 nanocrystals appears at the SiO 2 /Si interface. Several narrow CL emission peaks and wide bands were found which could be related to the decay of SnO 2 free excitons, to oxygen deficiency centers in SiO 2 and to transitions between the energy levels in the Er ions, apparently located at nanoclusters. The mechanisms of nanostructuring as well as the emission process are discussed.

Photonic crystals in lithium niobate by ion-beam enhanced etching

Abstract: A photonic crystal defect waveguide fabricated in a freestanding LiNbO 3 membrane is presented. The three dimensional patterning was done by means of ion-beam enhanced etching.

Damage formation in Ge during Ar+ and He+ implantation at 15 K

Abstract: Ar + and He + ions were implanted into Ge samples with (1 0 0), (1 1 0), (1 1 1) and (1 1 2) orientations at 15 K with fluences ranging from 1×10 11 to 1×10 14 cm −2 for the Ar + ions and fluences ranging from 1×10 12 to 6×10 15 cm −2 for the He + ions. The Rutherford backscattering (RBS) technique in the channelling orientation was used to study the damage built-up in situ. Implantation and RBS measurements were performed without changing the target temperature. The samples were mounted on a four axis goniometer cooled by a close cycle He cryostat. The implantations were performed with the surface being tilt 7° off the ion beam direction to prevent channelling effects. After each 300 keV Ar + and 40 keV He + implantation, RBS analysis was performed with 1.4 MeV He + ions. For both the implantation ions, there is about no difference between the values found for the damage efficiency per ion for the four different orientations. This together with the high value (around 5 times higher than that found in Si), gives rise to the assumption of amorphous pocket formation per incident ion, i.e. direct impact amorphization, already at low implantation fluences. At higher fluences, when collision cascades overlap, there is a growth of the already amorphized regions.

Ion Beam Synthesis of Transition Metal Nanoclusters in Silicon

Abstract: The synthesis of materials combining ferromagnetism and semiconducting properties is of great interest for the development of devices for future electronics. Possible implementations of adding the spin degree of freedom to conventional semiconductors are the formation of diluted magnetic semiconductors (DMS’s) and the synthesis of magnetic clusters embedded in semiconducting matrices. Despite the technological importance of Si, the former research has mostly been focused on II–VI, III–V and other compound semiconductors. Beside various layer deposition techniques, ion implantation in combination with subsequent thermal treatment is an excellent way to introduce the necessary high concentration of foreign atoms into the substrate. Especially the formation of magnetic clusters in semiconductors or DMS layers is of interest because they offer the possibility to achieve Curie temperatures above room temperature, which is a drawback of common DMS structures. In the present paper we have studied the formation of MnAs and MnSb nanoclusters in Si by co‐implantation of Mn, As and Sb in combination with subsequent thermal annealing. In certain windows of the implantation and annealing parameters both Mn and As or Mn and Sb rich crystalline nanoclusters are formed that are partly phase‐separated. The influence of the formation parameters on size, size distribution and composition of the nanocrystals as well as the role of atom diffusion are discussed. Results of magnetic analyses are presented as well.

Radiation Losses of Photonic Crystal Waveguides in LiNbO^3Membranes

Abstract: The impact of the air gap separating a photonic-crystal membrane from the underlying substrate on radiation losses is investigated. The proposed photonic-crystal geometry can be fabricated by means of ion-beam enhanced etching.