H. Kurz

Phase transformation on and charged particle emission from a silicon crystal surface, induced by picosecond laser pulses

Ultrafast melting and resolidification on a (111) sufarce of a silicon crystal wafer, induced by 20‐ps pulses at 532‐nm wavelength, is accompanied by the emission of charged particles. This emission is studied as a function of pulse energy, in combination with time‐resolved reflectivity changes and post‐annealing morphology. The data provide evidence that thermalization time for a dense carrier plasma and the lattice is shorter than 10−11 s.

Picosecond time‐resolved plasma and temperature‐induced changes of reflectivity and transmission in silicon

A pump‐and‐probe technique is used to perform picosecond time‐resolved measurements of reflectivity and transmission changes in silicon. The results provide direct evidence that lattice heating, melting, or boiling can occur on a picosecond time scale. Detailed analysis of the data provides information about the dynamics of the electron‐hole plasma prior to melting and the kinetics of ultrafast phase transitions and crystal regrowth.

Time‐resolved temperature measurement of picosecond laser irradiated silicon

Time‐resolved reflectivity and transmission measurements of crystalline silicon films reveal lattice heating through the temperature dependence of the complex index of refraction. The temperature rise, which is much higher than derived by others from Raman scattering experiments, occurs in a surface layer of 100‐nm thickness.

Space-time resolved reflectivity measurements of picosecond laser-pulse induced phase transitions in (111) silicon surface layers

The changes in reflectivity of a silicon surface, irradiated by a green picosecond pulse, are probed during and following that pulse with a spatial resolution of 10 μm. The data indicate the development of a liquid phase, and a resolidification either into a single crystal or an amorphous phase. The latter has a characteristic ring-type pattern, and occurs only at locations where the incident picosecond laser fluence lies between 0.2 and 0.26 J/cm2. The reflectivity data appear to be in good quantitative agreement with a “simple heating” model, in which the electrons and phonons maintain a local thermodynamic equilibrium on a picosecond time scale.

Optical heating of electron‐hole plasma in silicon by picosecond pulses

Using a novel three‐pulse technique, essential information about the density, optical effective mass, and kinetics of laser‐generated plasmas in silicon has been obtained.

Nonlinear photoemission from picosecond irradiated silicon

Three distinctly different regimes of photoelectric emission are observed over a wide fluence range of uv-laser pulses irradiating single-crystal silicon samples. The role of the electron-hole plasma in the nonlinear photoemission is demonstrated by temporal correlation measurements. The diffusion properties of hot carriers are analyzed by investigating the influence of energy transport by hot carrier diffusion on the fluence threshold for melting with uv photons.

Phenomenology of picosecond heating and evaporation of silicon surfaces coated with SiO2 layers

Picosecond time-resolved reflectivity measurements on bare silicon surfaces and silicon surfaces with oxide layers reveal very fast heat diffusion and material evaporation on subnanosecond time scales. With a thick oxide layer resolidification of a molten silicon surface can take place in a few hundred picoseconds. At high laser fluences, vaporization processes take only a couple of 100 ps.

Time-resolved spectroscopy of plasma resonances in highly excited silicon and germanium

The dynamics of the electron-hole plasma in silicon and germanium samples irradiated by 20 ps. 532 nm laser pulses has been investigated in the near infrared by the time-resolved picosecond optical spectroscopy. The experimental reflectivities and transmission are compared with the predictions of the thermal model for degenerate carrier distributions through the Drude formalism. Above a certain fluence, a significant deviation between measured and calculated values indicates a strong increase of the recombination rate as soon as the plasma resonances become comparable with the band gaps. These new plasmon-aided recombination channels are particularly pronounced in germanium. 15 refs., 8 figs.

Picosecond Time-Resolved Detection of Plasma Formation and Phase Transitions in Silicon

The interaction of laser pulses with strongly absorbing media has received a great deal of attention during the past five years, especially in silicon [1–3]. An important issue has been the time scale of energy transfer between a dense electron-hole plasma and the lattice. If this energy transfer were insignificant during a picosecond pulse, it is clear that the plasma temperature would attain much higher values. We have presented ample evidence [4,5] that melting of a silicon surface layer can take place during a 20 ps laser pulse. There is a sharply defined fluence threshold (0.2 J/cm2 for a 20 ps pulse at λ = 532 nm on silicon) above which the reflectivity changes to a value characteristic of liquid phase, the evaporation of silicon atoms becomes appreciable, and an amorphous phase may be created by rapid resolidification, with cooling rates exceeding 1013 °C/sec, of a thin (10 nm) molten layer on top of a cool crystalline substrate.

Picosecond Time-Resolved Optical Studies of Plasma Formation and Lattice Heating in Silicon

Time-resolved studies of reflectivity and transmission at 0.532 μm, 1.064 vm and 2.8 μm of thin silicon films following irradiation with ps pulses at 0.532 μm have been performed. The formation of the electron-hole plasma and the evolution of lattice temperature is investigated as a function of pump fluence and time delay. Quantitative determination of the plasma densities and lattice temperature up to the melting temperature shows that the maximum plasma density is limited to ∼ 1 × 10 21 cm −3 by Auger recombination even on a time scale of picoseconds at fluences sufficient to cause the phase transition. The thermal nature of the phase transition is confirmed.