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Notes: We have measured the microwave-induced melting and damage to the near-surface region of arsenic-implanted silicon for 1–2 μs pulses at a frequency of 2.856 GHz and an incident pulse power of up to 9 MW. Rectangular samples were irradiated by single-pass TE10 traveling wave pulses inside a WR-284 waveguide, and time-resolved in situ and post-irradiation studies were performed to characterize the material modifications induced by the microwave pulses. The test chamber where the specimens were irradiated was either evacuated to a pressure of 10−7–10−6 Torr or filled with a 30-psig pressure of Freon-12. Incident, transmitted, and reflected powers were monitored with directional couplers and fast diodes. The results of the time-resolved optical measurements for samples irradiated in vacuum show that melting of the near-surface region occurs for pulse powers exceeding 3 MW, and that the surface melting is accompanied by a large increase in the reflected microwave power. The onset of the enhanced reflectivity is measured at an earlier time as the microwave power is increased, and once the abrupt increase in the reflectivity is observed, it persists throughout the remainder of the pulse. Simultaneous with the onset of surface damage, we observe a large enhancement in the emission of light from the sample. Results are presented for the temporal behavior and spectral components of the fluorescence as a function of the incident microwave power. The gas pressure in the test cell was also monitored, and a large increase in the gas pressure was detected at the same pulse power as the threshold for the sudden increase in the microwave reflectivity. The large increments in the reflected microwave power, light emission, and gas pressure are attributed to the formation of a plasma due to gas breakdown at (or near) the sample surface.Examination of the irradiated specimens shows that the melting and damage are not homogeneous over the surface, and the degree of energy deposition from the microwave pulses depends on the ambient gas in the test cell. Using secondary ion mass spectrometry, we find that microwave pulses at a power of 8 MW cause melting and vaporization of the near-surface region up to depths that exceed 1 μm.

Notes: We show that an extremely shallow ((approximately-less-than)800 A(ring)) melt depth can be easily obtained by irradiating a thin (∼200 A(ring)) heavily doped silicon layer with a CO2 laser pulse. Since the absorption of the CO2 laser pulse is dominated by free-carrier transitions, the beam heating occurs primarily in the thin degenerately doped film at the sample surface, and there is little energy deposited in the underlying lightly doped substrate. For CO2 pulse-energy densities exceeding a threshold value of about 5 J/cm2, surface melting occurs and the reflectivity of the incident laser pulse increases abruptly to about 90%. This large increase in the reflectivity acts like a switch to reflect almost all of the energy in the remainder of the CO2 laser pulse, thereby greatly reducing the amount of energy available to drive the melt front to deeper depths in the material. This is in contrast to the energy deposition of a laser pulse that has a photon energy exceeding the band gap, in which case the penetration depth of the incident radiation is only weakly affected by the free-carrier density. Transmission electron microscopy shows no extended defects in the near-surface region after CO2 laser irradiation, and van der Pauw electrical measurements verify that 100% of the implanted arsenic dopant is electrically active. Calculated values for the melt depth versus incident pulse-energy density (EL) indicate that there exists a window where the maximum melt-front penetration increases slowly with increasing EL and has a value of less than a few hundred angstroms. For silicon specimens having a thin degenerately doped film at the surface and a lightly doped substrate, the two primary reasons for using a CO2 laser pulse to achieve very shallow melt depths are (1) the pulse energy is deposited only in the thin surface layer and (2) the melting of this layer causes the reflectivity to jump abruptly to a value of almost unity.

Notes: The absorption of CO2 laser radiation in p-type GaAs is dominated by direct free-hole transitions between states in the heavy- and light-hole bands. For laser intensities on the order of 10 MW/cm2, we report that the absorption associated with these transitions in moderately Zn-doped GaAs (∼1017 cm−3) begins to saturate in a manner predicted by an inhomogeneously broadened two-level model. At higher laser intensities surface melting occurs initially at localized sites in moderately doped material and more uniformly in heavily Zn-doped samples ((approximately-greater-than)1018 cm−3). As the energy density of the CO2 laser radiation is progressively increased further, the surface topography of the samples shows signs of ripple patterns, high local stress, vaporization of material, and exfoliation of solid GaAs fragments. Electron-induced x-ray emission data taken on the laser-melted samples show that there is a loss of As, compared to Ga, from the surface during the high-temperature cycling. By irradiating the samples in air, argon, and vacuum, we find that the vaporization rates are directly influenced by the ambient environment, particularly by the interaction of oxygen with the molten GaAs. Secondary ion mass spectrometry measurements are used to study the diffusion of oxygen from the native oxide and the incorporation of oxygen in the near-surface region of the GaAs samples that have been melted by a CO2 laser pulse. We find that oxygen incorporation does occur, and that the amount and depth of the oxygen incorporation depends on the laser energy density, number of laser shots, and ambient environment. For samples that are irradiated in argon or vacuum, we find that removal of the native oxide can be accomplished with CO2 laser pulses. Similar measurements are performed on Si-implanted GaAs, and results are reported for the redistribution of the implanted silicon atoms, the deviations from stoichiometry, and the incorporation of oxygen in the resolidified layer.

Notes: Characteristics of rapid thermal and pulsed laser annealing have been investigated in boron fluoride- (BF+2 and BF+3) -implanted silicon using cross-section and plan-view electron microscopy. The amorphous layers recrystallize by the solid-phase-epitaxial growth process, while the dislocation loops below the amorphous layers coarsen and evolve into a network of dislocations. The dislocations in this band getter fluorine and fluorine bubbles associated with dislocations are frequently observed. The secondary-ion mass spectrometry techniques were used to study concomitant boron and fluorine redistributions. The as-implanted Gaussian boron profile broadens as a function of time and temperature of annealing. However, the fluorine concentration peak is observed to be associated with dislocation band, and the peak grows with increasing time and temperature of annealing. The electrical properties were investigated using van der Pauw measurements. The electrical activation of better than 90% and good Hall mobility were observed in specimens with less than 500-A(ring) dopant-profile broadening. In pulsed laser-annealed specimens, the boron profile broadens both toward the surface and into the deeper regions of the crystal. However, the fluorine concentration profile exhibits a decrease in peak concentration with only a limited broadening.

Notes: The high-temperature diffusion of phosphorus into crystalline silicon causes the formation of electrically inactive phosphorus-rich precipitates near the surface. These precipitates decrease the carrier lifetime and mobility in the diffused layer, and thus lead to less than optimal diode characteristics of electrical junctions formed by diffusion of phosphorus into a p-type substrate. We show that the free-carrier absorption of a CO2 laser pulse can be used to completely dissolve the precipitates and remove dislocations in the diffused layer. Furthermore, we find that there are distinct advantages in depositing the pulse energy by way of free-carrier transitions, since the energy can be preferentially deposited in either confined doped layers or diffusion wells that are surrounded by lightly doped material. Our transmission electron microscopy results show that the annealing of the extended lattice defects is caused by melting of the near-surface region and subsequent liquid-phase epitaxial regrowth. Van der Pauw measurements are used to study the carrier concentration, mobility, and sheet resistivity of the samples before and after laser irradiation. The results of the electrical measurements show that there is a large increase in the carrier concentrations and a corresponding drop in the sheet resistivities of the laser irradiated samples. Using a Fourier transforminfrared spectrometer, we find that significant changes occur in the transmittance and reflectance spectra after CO2 laser annealing. Secondary ion mass spectrometry measurements are used to determine the redistribution of the phosphorus as a function of the pulse energy density. A time resolved pump-and-probe technique is utilized to measure the threshold for the onset of surface melting and the melt duration. We find that for energy densities greater than about 3 J/cm2, the reflectivity of the probe laser (at 633-nm wavelength) jumps rapidly to 70%, which is consistent with the reflectivity of liquid silicon. The interpretation of the laser induced changes in the electrical, optical, and structural properties is based on a thermal model, in which surface melting occurs for incident pulse energy densities exceeding a threshold value. Comparative calculations are reported for the melt depths and duration of surface melting, and good agreement is found. Other calculated results for the transient heating and cooling of the near-surface region are also reported.