Jürgen Reif

Ripples revisited: non-classical morphology at the bottom of femtosecond laser ablation craters in transparent dielectrics

Abstract Studying femtosecond laser ablation of wide band gap insulators, with BaF2 as a representative, we observe a complex structure of fine ripples at the bottom of the ablated crater, which are oriented perpendicular to the beam polarization. A second, wider periodic structure oriented parallel to the beam polarization appears superimposed on the first one. To check the idea of an interference pattern translating into ripples, a controlled interference was created in the target in a non-collinear two-beam experiment. However, no signature of it was observed in the ablated spot. This calls the classical interpretation for ripples formation into question. More likely, we assume that the ripples structures are due to self-organization structure formation during the relaxation of the highly non-equilibrium surface after explosive positive ion emission.

High-resolution investigations of ripple structures formed by femtosecond laser irradiation of silicon

We report on the structural investigation of self-organized periodic microstructures (ripples) generated in Si(100) targets after multishot irradiation by approximately 100-fs to 800-nm laser pulses at intensities near the single shot ablation threshold. Inspection by surface sensitive microscopy, e.g., atomic force microscopy (AFM) or scanning electron microscopy (SEM), and conventional and high-resolution transmission electron microscopy reveal complex structural modifications upon interaction with the laser: even well outside the ablated area, the target surface exhibits fine ripple-like undulations, consisting of alternating crystalline and amorphous silicon. Inside the heavily modified area, amorphous silicon is found only in the valleys but not on the crests which, instead, consist of highly distorted crystalline phases, rich in defects.

Self-Assembled Surface Patterning and Structural Modification upon Femtosecond Laser Processing of Crystalline Silicon

Upon femtosecond laser ablation (Ti:Sapphire; 800 nm, 100 fs, under ultra-high vacuum) from crystalline silicon (001), the surface morphology and structural changes w er examined ex-situ by optical, scanning electron microscopy, and Raman spectroscopy. Af ter repetitive illumination with several thousand laser pulses at an intensity below the single hot damage threshold (10 12 W/cm), self-assembled periodic nanostructures with periods of 200 nm resp. 60 0-700 nm develop at the crater bottom. Raman spectroscopy reveals a phase transfor mation inside the crater from Si-I to the polymorphs Si-III, Si-XII, hexagonal Si-wurtzite (Si-IV) , and amorphous silicon. The ablation dynamics was monitored by time-of-flight mass spect ros opy, showing the emission of superthermal positive ions with a kinetic energy of about 7 eV. The r esults suggest that the ablation, leaves behind a severely perturbed crystal surface. The resul ting instability relaxes by a selforganization, independent of the initial, and surrounding, crystal structure.

Temperature behaviour of extended defects in solar grade silicon investigated by photoluminescence and EBIC

Abstract Spatially resolved photoluminescence and EBIC were used to characterise a sample of multicrystalline silicon in the temperature range 80–300 K. The dislocation related lines in the spectrum-D1, D2, D3 and D4 correlate with the total recombination activity measured by EBIC. The temperature dependent EBIC behaviour was utilised to access the contamination level at the dislocations in low quality regions of the sample. The temperature dependence of D1 line shows a maximum at about 150 K. The decrement of D1 peak area upon temperature decreasing below 150 K could be related to the appearance of D3 and D4 lines in the photoluminescence spectra. The peak widths of D1 and D2 show opposite temperature dependence. D1 width decreases and D2 becomes broader upon decreasing temperature. Two additional lines with energies below the energy of band-to-band luminescence were observed together with the D bands at 80 K. They could be related to phonon replication of the band edge luminescence peak and can be seen on FZ-Si too.

Luminescence of Silicon Implanted with Phosphorus

We applied photoluminescence and transmission electron microscopy (TE M) to characterise phosphorous implanted samples after annealing. The implantation was carr ied out at 750 keV with doses ranging between 1 x 10 13 cm and 2 x 10 cm. We show that room temperature luminescence can be used for non-destructive monitoring of implanted sampl e . The implantation defects were studied by TEM. The photoluminescence spectra taken at 80 K indicate a considerable D-band radiation related to these defects. The room temperature spec tra are found to depend strongly on the annealing treatment performed (RTA vs. furnace annea ling). The band-to-band luminescence does not show quenching, but instead increases upon increase of t mperature for the highest implantation dose. Introduction Photoluminescence based techniques like SiPHER (Si liconPHotoEnhancedRecombination) [1] are nowadays available for non-destructive/non-contact defect monitoring of s ilic n in semiconductor industry. The technique is based on the idea that the intensity of the radiative band-to-band recombination decreases at crystal defects, due to the dominance of on-radiative recombination channels at the defects. Differences in the radiative recombina tion could be mapped with a resolution of a few μm. Despite the successful commercial appli c tion of SiPHER in semiconductor industry, according to our knowledge, a detailed background regarding i ts capabilities is still missing. So, it is still an open question, whether room-temperature lum inescence can be applied to monitor implantation-related defects in Si because the strong Aug er recombination in the implanted layers may dominate and cause quenching of the radiative recombination. Application of Si as an optoelectronic material is limited due t o its indirect band gap. Nevertheless, considerable effort is being made for the development of fficient Si-based lightemitting devices to be used for optical data transmission in future IC. Recently, light-emitting diodes prepared by boron implantation in Cz Si were demonstrated to show considerable roomtemperature electroluminescence at about 1.13 μm [2]. An alternative emission at 1.5 μm is provided by the D1 line of the well-known defect-related D-lines i n Si. It was observed to appear even at room temperature and might therefore be a notable candida te for Si based light emission, too. In this paper we deal with both aspects of Si based luminesce nce, i.e. application for defect monitoring and for light emission. As to the latter aspect, we ana lyse the impact of phosphorous implantation (instead of implantation of boron) on the band-to-band lumines ce ce. We also discuss the features of the D-band emission in the implanted structures. Solid State Phenomena Online: 2003-09-30 ISSN: 1662-9779, Vols. 95-96, pp 289-296 doi:10.4028/www.scientific.net/SSP.95-96.289

Residual Stress Distribution and Silicon Phase Transformation Induced by Rockwell Indentation at Different Temperatures, Studied by Means of Micro-Raman Spectroscopy

Rockwell microindentations obtained with a load of 1.5 N were made on (110) Cz-silicon substrates at temperatures of 70 and 600°C. The residual stress fiel ds and the silicon polymorphs induced by loading and unloading were studied by micro-Raman spe ctro copy. Mapping of the indented zones showed that the residual stress varies along the imprint radius. The indentations made at different temperatures show different str ss distributions. At 600°C the region of tensile stress is broad and the entire magnitude of t h residual stress variation is small. High compressive stress was measured at the imprint center. The stress becomes tensile in the pileup region and alters to compressive again away from the i ndentation. At 70°C the region with tensile stress is completely missing and there i s no residual stress outside the imprint. The size of the dislocation rosette around the imprint was correlated to the size of the stressed region outside the indentation. Polymorphs of silicon (Si-III, Si-XII, Si-IV), as well as amorphous material were observed within the indenter contact area. The variety of crystal phases s hows a tendency to diminish with increasing temperature. Scanning Electron Microscopy was applied to investigate the impri nt morphology as a function of the indentation temperature. Introduction The stress distribution in large diameter silicon wafers induce d by their weight during the thermal treatment has been the subject of some recent studies [1,2]. However, there is a lack of studies regarding the processes occurring in microscopic scale at th area of contact with the supporting pins. The indentation of silicon and its related microstructural changes have been a focus of research over the last decade [3-9]. Imprints in silicon prepar ed t room temperature have been studied extensively by a variety of techniques [3-5, 10]. Silicon has been indented by Solid State Phenomena Online: 2003-09-30 ISSN: 1662-9779, Vols. 95-96, pp 513-518 doi:10.4028/www.scientific.net/SSP.95-96.513

Pulsed Laser Deposition of Hafnium Oxide on Silicon

Hafnium oxide films were prepared by Pulsed Laser Deposition (PLD). The influence of laser wavelength (fundamental, second and third harmonic of a Nd:YAG laser), used for evaporation, and substrate temperature on the film morphology, chemical structure and interfacial quality were investigated yielding the following results: While the laser wavelength exhibits minor influence on layer structure, the substrate temperature plays a critical role regarding morphological and chemical structure of the produced hafnium oxide / silicon stacks. Atomic Force Microscopy (AFM) images show a clear transition from smooth layers consisting of small area crystallites to very rough surfaces characterized by large craters and regular, plane features when the growth temperature was increased. These facts suggest a chemical instability which is confirmed by X-ray Photoelectron Spectroscopy (XPS). Investigations of the hafnium and silicon core level spectra indicate the occurrence of silicon dioxide and hafnium silicide in the case the samples were produced at elevated temperatures.

On ultra-short laser pulse induced instabilities at the surface of non-metallic solids

The impact of intense femtosecond laser pulses on dielectric targets results in a non-equilibrium state of the surface. We consider the influence of this instability on ablation and surface relaxation phenomena. Important consequences of the laser-material coupling and energy dissipation are addressed such as transient and permanent modification of the surface. From experiments on ablation products kinetics, Coulomb explosion upon multiphoton surface ionization has been established as the initial mechanism for desorption of fast positive ions from dielectric surfaces. We refer to the role of surface defects responsible for ion yield enhancement and the nature of defects by detecting laser induced fluorescence. Additionally, observations point to a set-in of a thermal emission process at higher laser intensity. Investigating the dynamics of particle emission, we find ultra-short timescales for the coherence of electronic excitation and energy relaxation via transient phases, the latter related to the coupling strength of the various solids. The surface morphology after ablation is modified, with regular nano- and micro-structures of features originated from self-organization of surface instabilities.