P. Sakthivel

Evidence that blue luminescence of oxidized porous silicon originates from SiO2

We have analyzed red and blue luminescence from porous silicon as a function of oxidation parameters and feature dimension determined with an atomic force microscope. We have found correlation between blue luminescence intensity and the increase in feature size caused by oxidation. We have further shown that blue luminescence, is identical, with respect to spectrum and fast decay, to that of high microelectronic quality SiO2 grown on crystalline silicon using dry oxygen plus an organic chlorine compound. Thus, we conclude that blue luminescence originates from SiO2 film rather than from the silicon nanocrystals in the porous material. Intensity enhancement, as compared to SiO2 on crystalline wafers, comes from the gigantic surface area of porous silicon.

Dual‐laser ablation for particulate‐free film growth

A novel dual laser ablation process that leads to particulate‐free film growth is presented. A pulsed CO2 laser and an excimer (KrF) laser have been spatially overlapped on a Y2O3 target with a temporal delay between the pulses. The particulate density of the films grown by this method are at least three orders of magnitude smaller than the particulate density of a single excimer laser ablated film of similar thickness. In addition, a time‐of‐flight ion probe study indicates a sixfold enhancement of the plume species kinetic energies under dual‐laser ablation. The degree of the plume excitation is observed to depend strongly on the delay between the laser pulses.

Evidence for the physical basis and universality of the elimination of particulates using dual-laser ablation. I. Dynamic time-resolved target melt studies, and film growth of Y2O3 and ZnO

The application of a dual-laser ablation process, incorporating the addition of a synchronized CO2 laser to the traditional excimer (KrF) laser used for the ablation of targets in thin film deposition, has been previously demonstrated to be effective in the elimination of particulates in films of Y2O3 [J. Vac. Sci. Technol. A 13, 1171 (1995)]. It has been hypothesized that the efficacy of particulate removal is related to phase transformation from the solid to liquid phase prior to excimer laser ablation of the target material. In this series of two articles we present direct physical evidence of the dynamics of the phase transformation occurring on the target surface and its effect on the morphology of film growth. Pump–probe experiments have been conducted using the CO2 laser to probe the dynamic reflectivity of the target surface on the nanosecond timescale. These experiments were conducted for a range of materials spanning a wide range of thermal conductivity including a low thermal conductivity insul...

Study of ion activation in the in situ low-temperature laser deposition of superconducting YBa2Cu3O7-δ films

The presence of a pulsed discharge during the reactive deposition of high Tc thin films allows the lowering of substrate temperatures to ∼475 °C for in situ superconducting film deposition. We present the first ion probe analysis to study the role of this pulsed discharge on the excimer laser ablated YBa2Cu3O7−δ plume. The ionic enhancement in the laser‐ablated plume, during reactive deposition in the presence of a positively biased ring electrode, is studied both in terms of its extent and temporal characteristics. A significant increase in the forward‐directed incidence of oxygen ions on the depositing substrate, following in the wake of the highly directional plume, is demonstrated. The dependence of this ionic enhancement on ambient oxygen pressure and the bias voltage on the ring electrode is discussed.

Ultrasensitive spectral trace detection of individual molecular components in an atmospheric binary mixture

We present what is, to our knowledge, the first demonstration of the application of the laser homodyne interferometric technique to the quantitative identification of individual trace molecular constituents of a binary mixture in an ambient atmospheric background. Operation of the laser interferometric detection system to within a factor of 26 of the theoretical quantum noise limit, without extensive vibrationisolation, is observed. We realize the spectral identification of SF(6) and CF(2)Cl(2) mixed in various trace concentrations, without significant cross interference, using molecular spectral features overlapping the 10P CO(2) laser transitions.

Optical Detection of Slow Excited Neutrals in Plasma-Assisted Excimer Laser Ablation

The observation of a slow optically excited component in the excimer laser-ablated YBCO plume due to the presence of a biased ring electrode is reported. The temporal dynamics of the plume were investigated by using time-of-flight (TOF) optical emission spectroscopy (OES). Time-resolved emission signals reveal excitation and resultant fluorescence from slow-moving plume species in the presence of the discharge.

Dynamics of ionic enhancement in the plasma‐assisted laser deposition of high Tc superconductors

The presence of an oxygen discharge during the laser ablation‐deposition of superconducting films has been shown to facilitate low temperature growth. It has been speculated that the increased surface activation via ionic collisions and enhanced oxidation of the film in an oxygen plasma enabled the growth of the superconducting phase at a lower substrate temperature. To understand the effect of the oxygen discharge on the film growth, we have investigated the ionic content of the laser plume as it propagates in the oxygen discharge by utilizing an ion probe technique. The time resolved and integrated ion signals clearly reveal the ionic enhancement of the plume in presence of the oxygen discharge. The role of the background gas and level of excitation in the pulsed discharge on the ionic content of the plume is systematically analyzed.

Effect of Rapid Thermal Oxidation on Blue and Red Luminescence Bands of Porous Silicon

Photoluminescence (PL) of porous silicon after rapid thermal oxidation was studied using excitation energies of 5.0 and 6.4 eV. The emission spectra in both cases are dominated by a broad blue band centered at 430-450 nm and a much weaker red band positioned at 680-720 nm. Using atomic force microscopy (AFM), we have found a correlation between blue PL intensity and increase in feature size caused by progressive oxidation. The blue luminescence, found in porous Si only after oxidation, is identical, with respect to spectrum and fast decay, to that of thermally grown oxide on crystalline Si. Based on these findings, we conclude that the blue luminescence originates from SiO 2 rather than from Si nanocrystals embedded in the oxide matrix. The red luminescence, on the contrary, has a different origin. The intensity of red band is a function of oxidation temperature and at RTP above 1050 °C, the red band is completely eliminated. The red luminescence characteristics are discussed in relation to the possible mechanism.

Temporal synchronization of independently frequency tunable dual CO2 laser pulses

We describe a new method for the simultaneous emission of two CO2 laser pulses from a single transversely excited atmospheric discharge. The transient gains of the pulsed low‐pressure sections of two spatially separate hybrid cavities aligned on the discharge were temporally manipulated to allow low‐jitter pulse synchronization. Independently frequency tunable dual laser pulses over arbitrarily selected rotational transitions spanning the available P and R branches of the 9 and 10 μm CO2 laser transitions were obtained, overcoming limitations on possible dual frequency combinations that restricted previous techniques. The result of numerical calculations for the hybrid laser, using a four‐level rate equation model, supports the observed delay characteristics and provides a physical basis for understanding the dynamics of the synchronization.

Continuously variable distortion‐free attenuation of high‐power transversely excited atmospheric CO2 laser pulses

A new technique for the continuously variable linear attenuation of high‐power transversely excited atmospheric (TEA) CO2 laser pulses is reported. The use of a binary gas mixture comprising SF6 as the absorber and helium or argon as the buffer gas causes a significant enhancement in the saturation intensity, thereby allowing saturation‐free attenuation of TEA CO2 laser pulses up to 1 MW/cm2. A linearly variable dynamic attenuation range of 0–30 dB, and continuously variable attenuation up to 60 dB in a 10‐cm cell length, without spatial or temporal distortion in the attenuated beam is demonstrated.