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We have produced films of light emitting porous silicon (LEPSi) thinner than 1 μm, lifted them off the silicon wafer by an electropolishing step, and deposited them onto sapphire windows where they remain attached by van der Waals or electrostatic forces. Although free‐standing LEPSi films had been obtained before, our films are one order of magnitude thinner, luminesce strongly, and have excellent mechanical properties because of the sapphire substrate. The important steps in this procedure are discussed, and the structural, chemical, and optical properties of these films as measured using a variety of probes are reported. These films are semitransparent in the visible and thus make several new optical measurements possible. In particular, the results of photoinduced absorption measurements performed with 100 fs time resolution are presented.

We present the results of an investigation of various means to enhance or suppress the formation of porous silicon. The first method involves a lithographic process using silicon nitride to produce sub‐0.5 μm light emitting porous silicon (LEPSi) lines adjacent to fully protected silicon regions. The second method consists of amorphizing regions of the wafer prior to anodization with high energy/high dose ion implantation, followed by anodization and annealing. In this method, LEPSi is produced in the unimplanted regions only. Using focused ion‐beam implantation ∼100 nm patterns have been obtained. The third method utilizes low energy/low dose bombardment (ion milling/reactive ion etching) with argon ions prior to anodization. Under appropriate bombardment conditions, we have observed a strong enhancement of the formation rate of LEPSi, possibly due to the generation of a large number of defects on the wafer surface. Our results demonstrate that porous silicon light emitting diodes (LEDS) and silicon electronic devices can coexist in close proximity (within 1 μm or less) on the same chip.

Spontaneous, highly periodic, often permanent surface gratings or "ripples" can develop on the surface of almost any solid or liquid material illuminated by a single laser beam of sufficient intensity, under either pulsed or CW conditions. The grating periods are such that the incident laser beam is diffracted into a tangential wave which skims just along or under the illuminated surface. These spontaneously appearing surface ripples are generated by a runaway growth process analogous to stimulated Brillouin or Raman scattering or small-scale self focusing, but having many of the same properties as Wood's anomalies in diffraction gratings. Hence, it seems appropriate to refer to these spontaneous surface structures as "stimulated Wood's anomalies."

We report the observation and analysis of higher-order optical diffraction patterns arising from linear combinations of the primary laser-induced gratings or ripples on germanium surfaces. These higher-order surface structures presumably arise from nonlinear interactions between superimposed primary gratings with different grating wave vectors. For gratings produced by normally incident laser beams on Ge 〈111〉 surfaces, the diffraction patterns exhibit a strongly hexagonal symmetry.

The methods of femtosecond optical spectroscopy have been used to study the ultrafast electronic processes in undoped alloys of amorphous hydrogenated silicon prepared by rf glow discharge. The transient changes in reflectivity and transmission are studied as a function of wavelength, carrier density and alloy composition. The results are explained by taking into account hot carrier relaxation in the extended states, trapping into weakly localized bandtail states and thermalization in those bandtail states.

Spontaneous periodic surface structures, or ripples, are frequently observed after illumination of metals, semiconductors, and dielectrics by intense laser pulses. We develop a theory which predicts the observed spacing, polarization, and growth properties of these ripples. In this model, one or several Fourier components of a random surface disturbance scatter light from the incident beam very nearly along the surface. The interference of this diffracted optical wave with the incident beam then gives rise to optical interference fringes which can reinforce the initial disturbance. Sinusoidal corrugations on either metallic or molten surfaces seem to provide strong positive feedback for ripple growth, whereas sinusoidal gratings in temperature, electron-hole density, or dielectric constant seem much less well correlated with observations.

Recently it was reported that high quality, 10–100 μm thick, single-crystalline Si films were formed on oxidized single-crystalline Si wafers by the lateral epitaxial growth over oxide (LEGO) process. Although this recrystallization process is reliable and reproducible, periodic regions of dislocations in the otherwise relatively dislocation-free Si film were not well understood. In this paper, therefore, the film stress and defect properties are investigated in detail, and devices made in recrystallized wafers are compared with devices in conventional wafer structures. Stress levels were found to be too low to cause defects, with TEM data suggesting an impurity mechanism (SiO2 precipitation) for small dislocation loops and slight crystalline misorientation for long dislocation lines in the periodic, defective areas. Device results confirmed that LEGO is a viable alternative to the dielectric isolation (DI) technology.

A Raman microprobe has been used to detect the changes in crystallinity, stress and homogeneity of thin films of silicon-on-insulator and germanium-on-insulator (GIO) structures. Silicon films recrystallized with a graphite strip heater at a substrate temperature of 1200°C show little or no stress, but those recrystallized with a continuous wave laser at a substrate temperature of 500°C have a residual stress of 4 × 109 dyn cm−2. Some of the strip-heater-recrystallized samples consist of large crystals of different orientations. As-deposited GOI is found to be unstressed.

The optical absorption of a-Si:H,F a-Si 0.4 , Ge 0.6 : H,F superlattices have been measured over a range of well sublayer widths. The absorption data have been analyzed to yield a defect density per superlattice interface of ≈1010 cm−2. Electron and hole drift mobilities data suggest a transition from scattering dominated transport in states at the top of the barriers for short superlattice periods, to recombination dominated transport at the bottom of the wells for large periods. The data are used to estimate a characteristics scattering length of ≈3 nm in bulk a-Si0.4Ge0.6:H,F.

We have measured the complex dielectric function ϵ of picosecond laser-produced molten silicon from the near infrared to the near ultraviolet parts of the spectrum by time-resolved reflectivity measurements. We find that ϵ is well described by a Drude model with the plasma frequency ωp = 2.50 × 1016 rad/s and the relaxation time 1730 1692 τ = 212 × 10−18 s.

We have used the methods of picosecond time‐resolved reflectivity to measure the carrier lifetime in fine grain polycrystalline silicon films grown by low pressure chemical vapor deposition at 625 °C. After monatomic hydrogen diffusion or implantation with phosphorus ions followed by high temperature annealing (1150 °C), the trapping time τ increased from 40 to 150 ps, consistent with passivation of the grain boundaries or an increase in grain size, respectively. If implantation was not followed by annealing, τ decreased to less than 10 ps, while if it was followed by low temperature annealing (900 °C), which approximately restored the original grain size, τ recovered to 50 ps, very close to the trapping time measured in the as‐grown samples. In all cases, we found indications that trapping of carriers was much faster than their subsequent recombination.

The generation of solitons and other related periodic pulse evolutions in a passively mode-locked dye laser is controlled by adjustment of group velocity dispersion, self-phase modulation, and spectral filtering. Without spectral filtering, periodic pulse evolutions reminiscent of higher-order solitons are observed. The pulses differ from the classic solitons because of additional shaping mechanisms. With spectral filtering, pulses are generated that can be described analytically as true asymmetric N=2 solitons. The introduction of the filter appears to simplify the intracavity shaping mechanisms in such a way that the nonlinear Schrodinger equation becomes a good approximation for pulse generation in the laser. The remarkable stability achieved allows for accurate characterization and control. >

The Raman microprobe technique can provide quantitative information on structure, composition, homogeneity, and stress, with 1-μm resolution. It is also a nondestructive technique that requires no special sample preparation and can be used in situ. After an introduction to Raman scattering and a description of the technique itself, the author reviews specific applications in the fields of laser recrystallization and laser damage of layered structures.

A detailed study of the growth of amorphous hydrogenated fluorinated silicon (a-Si:H, F) from a DC glow discharge in SiF/sub 4/ and H/sub 2/ is discussed. The electrical properties of the films can be varied over a very wide range. The bulk properties of the best films that were measured included an Urbach energy E/sub u/=43 meV, a deep-level defect density N/sub s/=1.5*10/sup 15/ cm/sup -3/, and a hole drift mobility of 8*10/sup -3/ cm/sup 2/ V/sup -1/ s/sup -1/, which reflects a characteristic valence band energy of 36 meV. It was found that E/sub u/, N/sub s/, and the density of surface states N/sub ss/ are related to each other. Under the deposition condition of the films with the best bulk properties, N/sub ss/ reaches its highest value of 1*10/sup 14/ cm/sup -2/. It is suggested that in growth from SiF/sub 4//H/sub 2/, the density of dangling bonds at the growing surface is very sensitive to the deposition conditions. >

We show that the duration and coherence time of ultrashort laser pulses can be measured by a transient grating technique. In the method, a self-diffracted signal is recorded as a function of delay between two identical pulses that induce the grating in an absorbing medium. An integrated-intensity grating analysis that considers fourth-order partial-coherence effects describes the diffraction efficiency of the grating. This method, which is easily implemented, inexpensive, and can be used at any wavelength, is demonstrated with low-repetition-rate picosecond lasers.

Raman spectroscopy is widely used to study the vibrational and structural properties of single crystals. More recently, in the fields of chemistry, physics, and engineering, interest in the properties of small objects has arisen. Small objects include microcrystalline materials, artificially layered semiconductor structures, and colloidal suspensions. The reason for this recent interest is that these objects often display unique electronic properties which may find applications in devices. Since Raman spectroscopy is a convenient nondestructive tool, it is not surprising that it has been used to characterize such objects. In this review, we consider almost exclusively results obtained with semiconductors.

A method for enhancing the sensitivity of time-resolved reflectivity measurements in semiconductors is discussed. By appropriate choice of the angle of incidence near Brewster's angle, the photoinduced reflectivity changes are easily increased by one order of magnitude. Two examples, the lifetime of a dense electron-hole plasma and the picosecond laser-induced melting transition in silicon, illustrate the method. Extension to the case of thin films is possible, as demonstrated by experiments performed on thin polycrystalline silicon films. Possible complications due to the use of very tightly focused beams and ultrashort pulses near a strong resonance are considered and, together with other considerations of a more experimental nature, lead to general guidelines for signal-enhancement and easy implementation and data analysis. >

The electronic and thermal contributions to photoinduced changes in the optical properties of hydrogenated amorphous silicon (a‐Si:H) films can be distinguished in pump‐probe experiments by an appropriate choice of the probe wavelength. Intraband absorption decreases strongly with carrier localization.

We have studied both theoretically and experimentally the dynamics of laser‐induced electron‐hole plasmas created at the surface of silicon using the strong synergy observed during simultaneous illumination by two picosecond pulses of different wavelengths. In a range of intensities up to those capable of producing a phase transition, we find good agreement between the results of our experiments and the predictions of a conventional model for pulsed laser heating.

The ultrafast dynamics of photogenerated carriers is monitored at room temperature by femtosecond pump-and-probe optical spectroscopy in undoped a-Si:H and its alloy with C. We find that above 5×1019cm−3 the recombination is bimolecular and non-radiative, with a coefficient B=3.5×10−9cm3sec−1 and below 5×10−10cm−3, it is monomolecular, with a time constant τ = 7 psec.

We observe, for the first time, structural modifications that extend many microns beyond the area where laser‐induced damage is visible by high resolution optical microscopy. These modifications are recorded by a Raman microprobe, which is sensitive to stress and microcrystallinity.

We have used picosecond laser pulses to produce transiently molten silicon layers and to measure their dielectric function ϵ at 1064 nm by reflectivity measurements in a pump and probe configuration. Starting approximately 100 ps after excitation, ϵ is a constant equal to - 27.3 + j 48.7. Immediately following excitation but before ϵ becomes constant, the reflectivity changes, which is interpreted as evidence for superheating.

We report the first observation and full characterization of the weak photoluminescence signal at and above the Tauc gap in a-Si:H. Detailed spectral dependence on temperature (from 10 K to 500 K), Tauc gap, deep defect density and laser power density is reported. The origin of the signal is discussed in terms of the carrier dynamics.

The Drude parameters of liquid silicon at the melting temperature have been obtained from time‐resolved reflectivity measurements at 1064, 532, and 355 nm following melting of an optically thick layer by a picosecond visible laser pulse. The ratio of the electron density N to the electron mass m is found to be equal to 2.17×1059 m−3 kg−1 and the relaxation time τ to be equal to 212 as. These values are compared to previous results obtained by cw ellipsometry between 1 and 0.4 μm and by nanosecond time‐resolved ellipsometry of 632.8 nm.

Various gradual transitions on silicon, gallium arsenide (and copper) surfaces can be produced by repetitive subthreshold picosecond laser illumination. The dramatic decrease of the threshold energy is discussed in terms of the possible laser- induced transformations in the material.

We studied the growth of amorphous (a-Si:H,F) and of microcrystalline (μc-Si) silicon over trench patterns in crystalline silicon substrates. All deposited films show elbows at the trench mouth and are uniformly thick on the trench walls. Therefore, surface diffusion is not important. The results of a Monte-Carlo simulation suggest that film growth is controlled by a single growth species with a low sticking coefficient plus a highly reactive etching species.

We report experimental results on spontaneous periodic surface structures or ripples which occur on the surface of crystalline or ion‐implanted semiconductors, using high‐power picosecond laser pulses. We suggest that these surface ripples develop as the result of an exponentially growing interaction between the incident laser wave front and a scattered surface optical wave.

The etching of a-Si:H by H radicals in Hg-sensitized photo-CVD is studied as a function of deposition temperature, etch temperature and film hydrogen content. For films deposited at the same temperature, etch rates increased as etch temperature decreased from 230°C to 100°C and were temperature independent for temperatures between 100°C and 40°C. Etch rate increased with decreasing deposition temperature, but did not change when hydrogen content was varied by changing pressure and dilution. Under conditions producing high etch rates, microcrystalline films of Si:B:H and SiGe:H were prepared at temperatures as low as 100°C.

We report the deposition and comprehensive evaluation of a hydrogenated, fluorinated amorphous silicon‐germanium alloy with an optical gap of 1.28 eV. This low‐gap alloy of the a‐Si, Ge system possesses a small midgap defect density (6.5×1016 cm−3), and useful electron (σph/σd=23) and hole (LD=0.13 μm) transport properties. The alloy was grown by radio‐frequency plasma‐enhanced decomposition of SiF4, GeF4, and H2 in a reactor built to ultrahigh‐vacuum specifications.

Using a conventional rf glow discharge, we have grown microcrystalline p+ SiC:H films having conductivities 2–2×10−3 (Ω cm)−1 and activation energies 0.05–0.1 eV with carbon concentrations 0–6 at. %, respectively. Increasing the carbon content suppresses the microcrystallinity. The choice of substrate is crucial to initiating the immediate onset of microcrystalline growth in thin (∼200–400 A) films.

We study the interface between un-alloyed a-Si:H,F and an a-Si0.4,Ge0.6:H,F alloy using superlattice structures. From infrared spectroscopy we estimate a width of 8 A for the excess hydrogen layer, and X-ray diffraction data give us a width of 2Å contributing to the width of the diffraction peak. Vibrational Raman scattering data show that the ratios of the number of Si-Si, Ge-Ge and Si-Ge bonds is not altered by changing the number of interfaces. This fact allows us to establish an upper limit for the interface width of 3 atomic layers, i.e. one layer in each partner plus an intermediate layer.

The pulsewidth and coherence time of a picosecond Nd:YAG laser has been measured by the method of self‐diffraction, either in the two pulses or in the three pulses configuration. Experimental results have been obtained with thin dye cells or thin semiconductor films. A theoretical model has been developed that explains our results.

There is an increasing need for non-destructive characterization techniques that require no special sample preparation, can be used in-situ, and provide quantitative information with good spatial resolution. The Raman microprobe fulfills these requirements. We illustrate the capabilities of the Raman microprobe by presenting results obtained on thin films after laser-induced damage, after laser or rapid thermal annealing, and on heteroepitaxial semiconductor layers. The prospects for two and three-dimensional imaging of important properties such as composition, strain, and crystalline quality or orientation are discussed.

We demonstrate that Raman scattering with one micron spatial resolution is an ideal tool to characterize the effects of pulsed laser irradiation of solid surfaces. In particular, we observe changes in crystallinity, stress, and uniformity which extend many microns beyond the visible boundary between processed and unprocessed silicon thin films.

A Raman microprobe is used to detect the changes in crystallinity, stress and homogeneity on thin films of SOI irradiated by a single laser pulse. Maximum, isotropic, planar tensile stress increased from 5x109 dyn/cm2 to 12x109 dyn/cm2 in the processed area and stress of 8x109 dyn/cm2 was observed 20 μm beyond the damaged region. Crystallite sizes of 50 Å to greater than 300 Å were observed inside the processed area. Results obtained with different pulsed laser frequencies are compared and significant differences in the processing effects are observed.

The transport properties of intrinsic micro-crystalline and amorphous SiGe films are investigated. In the amorphous system it is found that the electron lifetime is relatively independent of band gag while the electron mobility of a-Ge:H is a factor of ten less than that of a-Si:H. Micro-crystalline silicon films have greater excess photo-conductivity as compared to a-Si:H predominantly due to longer effective lifetimes. Films containing micro-crystalline silicon in a matrix of a-SiGe:H have been prepared and are found to have comparatively poor transport.

We report the first femtosecond spectroscopic investigation of ultrafast electronic processes in $a$-Si:H. By tuning the probe wavelength in a wide range around the optical gap, we are able to follow the time evolution of carrier trapping in band-tail states. At moderate injected carrier density, the band-tail states are populated by multiple trapping. As more carriers are injected, the shallow band-tail states saturate and deeper band-tail states capture carriers by direct trapping, which is a faster process (1-2 ps) than multiple trapping (10 ps).

The physics of highly excited semiconductors has recently been the subject of many studies, especially in connection with the problem of pulsed laser annealing. Careful experiments [1–3] have now been performed with the temporal resolution required to demonstrate that one single picosecond or femtosecond pulse can produce melting at the surface of silicon. Since the carrier relaxation time is very short ( 1020 cm−3) and hot (Te > 2000°K) plasma close to but below the melting phase transition in Si and GaAs.

The complex dielectric function of molten silicon produced after picosecond illumination is found to be well described by a Drude model from 1.1 eV to 3.5 eV. Close to the melting temperature, we obtain ωp = 2.50 1016rad/s and τ = 212 10−18s. Transiently, the liquid temperature can exceed the melting temperature or even the boiling temperature Tb. We observe this transient heating and model it with a relatively simple computer code which includes superheating of the liquid above Tb. These measurements are possible thanks to a novel pump and probe technique.

We report novel experimental and theoretical results of our investigation of hot carrier dynamics in disordered materials. Femtosecond optical spectroscopy and cw photoluminescenceappear to be very promising techniques for the study of carrier relaxation in the extended statesand carrier trapping in the weakly localized states. In this paper, we focus on undoped a -Si:H. INTRODUCTION Hydrogenated amorphous silicon (a- Si:1 -I) is a techno- logically useful material for thin film devices including solar cells. Due to its high electronic quality, it is also an ideal material for the study of electronic relaxation in disorderedsystems. We have started a systematic study of carrier dynamics in a -Si:H and related alloys grown in S. Wagner's laboratory at Princeton University. Because relaxation is thought to occur with characteristic times between 10's offemtoseconds and 10's of picoseconds, we have almost exclusively used the methods of femtosecond optical spec-troscopy. In addition, cw hot photoluminescence measure-ments are in progress.The paper is organized as follows. In the first section

During the past few years, laser‐induced surface ripples have been studied by several groups. A stage has been reached where most properties are understood, although some significant observations remain unexplained. After a brief historical review followed by a summary of our present understanding, we discuss some results that we have obtained very recently using a Raman microprobe.

Microcrystalline Si was grown from SiF4 and H2 by plasma-enhanced chemical vapor deposition. The films are almost completely crystalline with a crystallite size (determined from Raman spectra) of about 60 Å. The optical absorption and the electrical conductivity of these films were studied. With increasing hydrogen content in the films, the dark conductivity decreases strongly and the activation of the conductivity increases. We explain the conductivity qualitatively in terms of a grain boundary model.