Showing papers by "Werner Wesch published in 2004"

Amorphous silicon exhibits a glass transition.

Abstract: Amorphous silicon is a semiconductor with a lower density than the metallic silicon liquid. It is widely believed that the amorphous-liquid transition is a first-order melting transition. In contrast to this, recent computer simulations and the experimental observation of pressure-induced amorphization of nanoporous silicon have revived the idea of an underlying liquid-liquid phase transition implying the existence of a low-density liquid and its glass transition to the amorphous solid. Here we demonstrate that during irradiation with high-energy heavy ions amorphous silicon deforms plastically in the same way as conventional glasses. This behaviour provides experimental evidence for the existence of the low-density liquid. The glass transition temperature for a timescale of 10 picoseconds is estimated to be about 1,000 K. Our results support the idea of liquid polymorphism as a general phenomenon in tetrahedral networks.

Effect of high electronic energy deposition in semiconductors

Abstract: Track formation due to high electronic energy deposition during swift heavy ion irradiation is well known in insulating materials as well as in some intermetallic compounds and metals. In semiconductors the physical situation was much less clear, and only during the last few years several experimental results on the effect of high electronic energy deposition were published, which are summarised and discussed in the present paper. Like in insulators, swift heavy ion irradiation may cause amorphous tracks in some semiconductors, such as InP, InSb, InAs, GaSb and Ge, if a certain value of the electronic energy deposition ee per ion and unit length characteristic for the material and the ion is exceeded. The critical values of ee are much higher than in the other materials, and the corresponding ion energies are close to the maximum ion energies available with the existing high energy accelerators. On the other hand, with cluster ions, as e.g. C60, tracks are easily formed in Si, Ge and GaAs with ion energies of several tens of MeV. Beside damage and track formation, annealing of damage was observed in the semiconductors, and it can be concluded that the effect of high electronic energy deposition represents itself as a competition between damage formation and annealing. Which of the two processes dominates is mainly determined by the electronic energy deposition. Qualitatively the observed behaviour can be explained in the framework of the thermal spike model. However, a quantitative description of all data available is not successful in the framework of this concept. This is obviously due to the fact that the effects are influenced not only by the electronic energy deposition, but also by a variety of other parameters, such as the ion velocity, the ion mass, the charge state of the impinging ions and the radial distribution of the electronic energy around the ions' path. All these parameters have to be taken into account within a theoretical description. However, the present situation is characterised by a lack of sufficient experimental data, and further systematic work is required to make progress in this interesting field.

Comparative study of radiation damage in GaN and InGaN by 400 keV Au implantation

Abstract: In x Ga 1− x N layers on sapphire with x =0 and x =0.1 were implanted with 380/400 keV Au ions. Implantation and subsequent damage analysis by Rutherford backscattering spectrometry with 1.4 MeV He ions backscattered under 170° were performed at 15 K without temperature change of the samples. At this temperature thermal effects can be neglected, which allows us to study the primary defect production. In In 0.1 Ga 0.9 N no difference is found between the Ga and In defect profiles. The defect concentration in the maximum of the distribution increases linearly with the ion fluence which can be explained by direct impact amorphisation and the growth of amorphous zones once they have formed. In GaN even at the low temperature of 15 K, recombination of defects within the collision cascades dominates. This causes two intermediate plateaus in the defect concentration versus the ion fluence before amorphous zones nucleate and final amorphisation of the layer is reached by a rapid growth of these amorphous zones.

Damage formation and annealing in InP due to swift heavy ions

Abstract: Virgin and pre-damaged InP samples were irradiated at room temperature (RT) and at liquid nitrogen temperature (LNT) with different fluences of 140 MeV Kr, 390 MeV Xe and 600 MeV Au ions. The pre-damaging was performed with 600 keV Ge ions at LNT to obtain different damage levels. The samples were analysed by means of Rutherford backscattering spectrometry (RBS) in random and channelling geometry. A relatively weak damage accumulation in virgin InP and a very significant defect annealing in pre-damaged InP occurs due to 140 MeV Kr irradiation. The damaging of virgin InP with 390 MeV Xe and 600 MeV Au is much more efficient in comparison with that of 140 MeV Kr. Further, annealing of the pre-damaged InP due to 390 MeV Xe irradiation is hardly visible. At LNT InP appears to be much more radiation-resistant to swift heavy ion (SHI) irradiation than at RT. Our results show that during SHI irradiation of InP both damage formation and damage annealing occur simultaneously. Whether the first or the second one plays a more important role depends on the SHI mass and energy.

Properties of buried waveguides produced by He-irradiation in KTP and Rb:KTP

Abstract: Buried waveguides were produced by He + -irradiation with energies between 1 and 3 MeV and ion fluences, which are sufficient to generate multiple buried amorphous layers in KTP and Rb:KTP, respectively. Within these layers the refractive index is decreased and they act as barriers for light guiding. The irradiation causes the formation of defects within the waveguide, reducing the optical transmittance. By post-irradiation thermal annealing light scattering and absorption inside the waveguides can be significantly reduced. In the case of Rb:KTP rapid thermal annealing up to temperatures of 500 °C was found to be suitable to abolish point defects and to prevent the Rb-diffusion at the same time. Due to adjacent regions of different refractive indexes non-symmetrical electric field distributions are obtained within the waveguide. For the completely irradiated waveguides the homogeneous refractive indexes around the waveguide cause fibre-like symmetrical electric field distributions in a single-mode waveguide undisturbed of surface effects. The samples were analysed by means of various complementary methods (Rutherford backscattering spectrometry, cross-sectional transmission electron microscopy, m-line spectroscopy). The near-field pattern and the attenuation of the waveguides were measured.

Laser irradiation of ion beam synthesized Ge nanocrystals in SiC

Abstract: Synthesis of group IV nanocrystals (NCs) by ion implantation and subsequent thermal annealing has opened ways to use indirect-gap semiconductors as new materials for optoelectronic applications. However, ion beam synthesis of NCs results in a broad size distribution with a relatively large mean size. In this work, the evolution of the size distribution of Ge rich NCs in SiC under multiple pulse excimer laser irradiation has been studied. The NCs were synthesized by 100 keV Ge+ implantation at 700 °C using ion fluences of 1 × 1016 cm−2 and subsequent thermal annealing at 1600 °C for 120 s. Post-irradiation was performed by means of a KrF excimer laser using 1000–10 000 pulses, laser fluences of 200–500 mJ cm−2 and pulse durations of 30 ns. Since the NCs are embedded in a near surface region up to a depth of 60 nm with a remaining lattice damage of only χmin≈10% solid state processes are to be expected. Rutherford backscattering spectrometry, energy dispersive X-ray analysis and time resolved reflectivity measurements during the laser irradiation confirmed the absence of surface melting. Cross-sectional transmission electron microscopy imaging revealed both a reduction of the amount of large NCs and an increase of the amount of small NCs with increasing laser pulses resulting in a reduced mean size and indicating the occurrence of inverse NC ripening processes under laser irradiation.