R. B. Hammond

Melt dynamics of silicon‐on‐sapphire during pulsed laser annealing

Transient electrical conductance measurements have been made on 0.45‐μm silicon‐on‐sapphire during pulsed laser annealing with 25‐ns ruby irradiation. The photoconductive contribution to the transient current was sufficiently small that the entire melt and resolidification process could be directly observed. The technique yields quantitative measures of melt depths, melting velocities (5–13 m/s), and solidification velocities (2.8–3.3 m/s). Combined with the complementary techniques of time‐resolved reflectivity, energy transmission, and calorimetric energy absorption, transient conductance provides a powerful new diagnostic for investigating melt dynamics.

80‐MW photoconductor power switch

The application of photoconductors to fast rise time, high‐power switching is discussed. We report the successful switching of a 100‐kV system to generate a 1.8‐kA, <5‐ns rise time, 200‐ns duration electrical pulse in a 25‐Ω load using a single photoconductor switch excited by a Q‐switched Nd: glass laser. The photoconductor was a 2.5‐cm‐long bar of single‐crystal, high‐resistivity silicon with a 0.5×0.5 cm cross section. Only a depth of about 1 mm of one side was used for conduction.

Temperature dependence of the exciton lifetime in high‐purity silicon

We observe that the exciton lifetime in high‐purity silicon is strongly temperature dependent. To describe our data we propose a theory that includes capture, evaporation, and recombination of excitons at neutral impurities. We suggest applications of exciton‐decay measurements to the determination of impurity content, capture cross sections, and electron‐hole droplet nucleation kinetics.

Pulsed soft x-ray response of InP:Fe photoconductors

We report measurements of the energy‐dependent pulse response of InP:Fe photoconductors to soft x rays in the energy range 200 eV–3 keV. The experiments were performed with x‐ray pulses from the Stanford Positron Electron Asymmetric Ring storage ring at the Stanford Synchrotron Radiation Laboratory. The detector had an optical impulse response of 180 ps (full width at half‐maximum) and an active area of 1×1 mm. An array of filters (carbon, magnesium, and beryllium) was used to unfold the energy response of the detector to the soft x rays. Direct measurements of the time structure of the ring bunches were limited by the bandwidth of the real time recording system (400 MHz), but inidividual ring pulses were easily recorded with a detector pulse amplitude of 65 mV for the unfiltered beam. The detector response was constant over the energy range examined at 2.7×10−3 A/W.

Electron velocity overshoot observed in an impulse-excited GaAs photoconductor

Abstract We have studied the impulse photoresponse of a GaAs photoconductor excited by 0.17-ps, 610-nm laser pulses. At intermediate average electric fields in the photoconductor, 3.3 − 10 kV/cm, overshoots were observed in the current transient. At both high, 17 kV/cm, and low, 1.7 kV/cm, average electric field, the current transient from the photoconductor showed little or no overshoot. Overshoot was not observed at high electric field because of the limited, 6-ps, time resolution of the measurement; while at the low field, 1.7 kV/cm, no electron-velocity overshoot is expected in GaAs. The duration and field dependences of the observed overshoots are consistent with the transient- electron-velocity overshoot that has been predicted in GaAs by Monte Carlo transport calculations.

Measurements of the melt dynamics in laser annealed semiconductors

Abstract Combined measurements of the transient electrical conductance, optical reflectance and absorbed energy have been used to determine the dynamics of the annealing process during laser irradiation of bulk silicon and silicon on sapphire. The combined transient conductance and optical reflectance yield the regrowth velocity and melt duration in bulk silicon; in silicon on sapphire the measurements also yield the melt velocity and maximum melt depth. Measurement of the absorbed energy provides energy coupling data used in numerical calculations of the melt dynamics for direct comparison with experiment in bulk silicon and the result indicates annealing proceeds by a purely thermal process.

Scanning Microellipsometer For The Spatial Characterization Of Thin Films

A polarization-modulated ellipsometer was constructed to investigate the optical properties of surfaces and transparent thin films. In the latter case, the measurement gives a unique determination of the index of refraction n and film thickness t. Using a HeNe laser light source, the beam was focused to a spot size of 50 μm. By stepping the sample across the focal point of the laser beam in both x and y directions, the spatial uniformity could be measured. This apparatus was particularly useful for optical profiling laser-annealed oxide films grown on GaAs. A new technique for laser annealing native oxides on GaAs produced the need for observing spatial structure with spatial resolution of less than 100 μm. Here the laser pulse produced a crater in the oxide due to localized healing and subsequent densification of the film (Figs. 1 & 2). This technique allows profiling of film index of refraction and thickness across the laser irradiated area--about 2 to 3 mm in our case. A number of applications in microelectronics are suggested.

Measurement of the Soft X-Ray Temporal and Spectral Response of InP:Fe Photoconductors

X-ray pulses (300 ps FWHM) from the SPEAR storage ring at the Stanford Synchrotron Radiation Laboratory (SSRL) were used to study the energy-dependent pulse response of InP:Fe photoconductors in the soft x-ray region (0.8 to 3 keV). The detector sensitivity (2.7×10–3 A/W) has been measured to be independent of the photon energy in this range. The temporal response of the photoconductor to x-rays is similar to optical excitation (FWHM ≃ 180 ps).

Transient Conductance Measurements During Pulsed Laser Annealing

Measurements were made of the conductance of single crystal Au-doped Si and silicon-on-sapphire (SOS) during irradiation with 30 nsec ruby laser pulses. After the decay of the photoconductive response, the sample conductance is determined primarily by the thickness and conductivity of the molten layer. For the single crystal Au-doped Si, the solid-liquid interface velocity during recrystallization was determined from the current transient to be 2.5 m/sec for energy densities between 1.9 and 2.6 J/cm 2 , in close agreement with numerical simulations based on a thermal model of heat flow. SOS samples showed a strongly reduced photoconductive response, allowing the melt front to be observed also. For complete melting of a 0.4 μm Si layer, the regrowth velocity was 2.4 m/sec.

Laser annealing of native oxides on GaAs

Annealing of native oxides grown on GaAs has been performed using a pulsed KrF laser. This process allows the oxides to be heated to temperatures well above 500 °C without arsenic loss from the GaAs substrate. The physical, chemical, and electronic properties of the oxide are markedly changed by laser processing.