Ian W. Boyd

A study of thin silicon dioxide films using infrared absorption techniques

Infrared transmission spectra of a series of silicon dioxide (SiO2) films grown on silicon wafers from a HCl and O2 gas mixture at 850 °C, have been studied for film thicknesses down to 28 A. The validity of Lambert‐Bouguer’s Law for such thin films has been confirmed, and the apparent absorption coefficient calculated for the absorption at 1065 cm−1 is in good agreement with previously published data for thicker, vapor‐deposited, and thermally grown films. A continuous shift of the absorption near 1065 cm−1 has been found, moving from an asymptotic limit maximum of ∼1070 cm−1 for thick films towards smaller wave numbers for thinner films. Various possibilities for the origin of this shift are discussed.

Simultaneous measurement of the two-photon coefficient and free-carrier cross section above the bandgap of crystalline silicon

We report what is to our knowledge the first simultaneous measurement of the two-photon absorption coefficient and the free-carrier cross section above the bandgap in a semiconductor. This is also the first observation of two-photon absorption of 1 μm radiation in single-crystal Si at room temperature in a regime where a two-photon stepwise process involving indirect absorption followed by free-carrier absorption is usually dominant. A critical pulsewidth (and fluence) is established below (and above) which two-photon absorption cannot be neglected. Pulses that range from 4 to 100 ps in duration are then used to isolate the irradiance-dependent two-photon absorption from the fluence-dependent free-carrier absorption. We obtain an indirect two-photon absorption coefficient of 1.5 cm/GW and extract a free-carrier cross section of 5 \times 10^{-18} cm2by using a simple technique that does not require a knowledge of the actual carrier density.

Optical limiting in GaAs

We have used two-photon absorption, self-defocusing, and optically-induced melting in GaAs to limit 1 μm picosecond pulsed radiation. The contribution to the limiting action from each of these mechanisms is discussed and demonstrated. Additionally, we measure a two-photon absorption coefficient of 26 cm/GW, which is in good agreement with the smallest values reported in the literature. A pulse-width study of the nonlinear absorption was conducted to isolate the effects of two-photon-generated free-carrier absorption. Results indicate that, even though the number of free-carriers is sufficient to severely defocus the incident beam, free-carrier absorption does not measurably contribute to the nonlinear absorption.

Nonlinear-optical energy regulation by nonlinear refraction and absorption in silicon.

We demonstrate a new silicon picosecond nonlinear-optical energy regulator for 1-microm radiation. The device has a high transmission for low input energies and a low transmission for high input energies and clamps the output at a constant value. We attribute this optical Zener action to nonlinear refraction and absorption induced in the silicon by the intense picosecond pulses.

Absorption of infrared radiation in silicon

The absorption of 9–11 μm radiation by thin wafers of lightly doped, n‐type Si has been measured at several lattice temperatures from 300 to 800 K. The temperature dependence of the absorption coefficient at λ=10.6 μm is extracted from the data and compared with previous measurements and also with recent theoretical models. A novel processing technique is described in which coupling of the CO2 laser radiation to the Si lattice is significantly enhanced by the simultaneous absorption of radiation from an argon laser.

Laser‐enhanced oxidation of Si

A scanning cw argon ion laser has been used to rapidly heat Si in a fixed oxygen atmosphere to produce thin oxide layers on the surface. Analysis of the growth rate of these films reveals an enhancement over the normal thermal equilibrium rate of oxidation for silicon dioxide layers grown by furnace in the temperature range 850–1050 °C, and is thought to be a result of the photoionizing effect of the band‐gap photon flux. A simple model incorporating this characteristic is shown to qualitatively agree with the experimental results, and clearly indicates the importance of the density of broken Si–Si bonds in the oxidation reaction.

Oxidation of silicon surfaces by CO2 lasers

We report for the first time, the use of a focussed CO2 laser beam and a controlled oxygen atmosphere to induce localized oxidation on the surface of a silicon wafer. These thin oxide films have been compared by infrared spectrometry with thin furnace‐grown layers. We conclude that the laser‐grown oxides are compositionally similar to conventional layers, and can be described by the formula SiO2. In contrast the half‐width of the Si‐O stretching vibration at 1070 cm−1 was found to be consistently less than for furnace‐grown oxides. By fabricating simple Al‐SiO2‐Si‐Al diodes, the dielectric properties of the films have been studied.

Various phase transitions and changes in surface morphology of crystalline silicon induced by 4–260‐ps pulses of 1‐μm radiation

We report the first pulse width study of the various morphological changes and bulk phase transitions of single crystal silicon irradiated by 1‐μm pulses of 4–260‐ps duration. In particular, we find that amorphous silicon is formed from the melt contrary to published expectations, but only for pulse widths less than 10 ps. We also find that the single shot melting threshold is pulse width dependent. Additionally, we observe the growth of multishot damage and of periodic ripple patterns with pulses as short as 4 ps.

Structural changes produced in silicon by intense 1‐μm ps pulses

The numerous bulk and surface structural changes observed in c‐Si following melting with 1‐μm pulses that range from 4 to 260 ps in duration and fluences from about 0.6 to 2.8 J cm−2 are examined by Nomarski and transmission electron microscope techniques. For melting pulse widths 30 ps or longer, recrystallization from the melt was observed. By contrast, for the shorter pulses (∼7 ps), the steep temperature gradients that accompany the onset of two‐photon absorption associated with pulses of this width produce an undercooled melt. Under these conditions, the resolidification velocities are evidently too high to allow epitaxial regrowth from the crystalline substrate and, for the first time, regions of amorphous and large‐ and fine‐grain polycrystalline silicon are observed to form directly on a crystalline underlayer. In addition, alternate stripes of amorphous and crystalline material are produced by these short pulses. These are associated with localized melting, demonstrating that uniform surface melt...

PULSEWIDTH-DEPENDENCE OF NONLINEAR ENERGY DEPOSITION AND REDISTRIBUTION IN Si, GaAs AND Ge DURING 1 μm PICOSECOND IRRADIATION

Abstract We have used 1 μm pulses ranging in duration from 4–260 ps to measure the pulsewidth dependence of the nonlinear absorption, melting threshold, and resolidification morphologies of Si, GaAs, and Ge. With these materials, we have been able to quantify a variety of nonlinear absorption processes with a single excitation wavelength. We find that the fluence required to melt Si and GaAs is roughly proportional to the square root of the pulsewidth while that required for Ge is nearly pulsewidth independent. A crystalline-to-amorphous transition is observed in Si for pulses less than 10 ps and in GaAs for all pulsewidths, but no such transition is observed in Ge. These observations are shown to be consistent with the various energy deposition and redistribution mechanisms present in each material. Finally, we have used the active nonlinearities in Si and GaAs to construct optical limiters designed to protect sensitive optical components from intense 1 μm radiation.