Peter D. Brewer

Measurement of the relative populations of I(2P01/2) and I(2P03/2) by laser induced vacuum ultraviolet fluorescence

The ground (2P03/2) and first excited (2P01/2) states of iodine atoms can absorb two photons at 304.7 and 306.7 nm, respectively, to reach 2D05/2 and 2D03/2 states. The excited atoms fluoresce twice, emitting an IR and then a VUV quantum. This is the basis of a new method for measuring the relative quantum yields of the two fine structure states at very short times after the atoms are formed. Quantum yields for I* production are reported for a number of alkyl halides and HI upon photodissociation.

Midwave infrared stimulated emission from a GaInSb/InAs superlattice

Use of a cracked Sb source and a postgrowth anneal procedure has been found to yield significant improvements in optical efficiencies of GaInSb/InAs superlattices grown by molecular beam epitaxy. Appreciable 5 μm band‐to‐band luminescence has been observed at room temperature, and stimulated emission at 3.2 μm has been demonstrated in an optically pumped structure. Intrinsic properties of this class of superlattices favor them for application as efficient infrared lasers operating at comparatively high temperatures.

Dry, laser‐assisted rapid HBr etching of GaAs

Dry, rapid etching of GaAs has been accomplished using an excimer laser (ArF, 193 nm) with HBr etching gas by photochemical initiation. Spatially uniform etch rates of up to 8 μm/min have been achieved on large‐area, masked substrates. Selective crystallographic etching is observed and controlled in the process.

Photon‐assisted dry etching of GaAs

UV radiation from an ArF laser has been used to perform large‐area etching of single‐crystal GaAs. The process is based on photochemical production of methyl or trifluoromethyl, and bromine radicals. The etching is anisotropic and features <1 μm have been made.

Two-photon induced fluorescence and resonance-enhanced ionization of sulfur atoms

Abstract Two-photon induced fluorescence and resonance-enhanced photoionization have been observed in atomic sulfur originating from both the 3P2,1,0 and the 1D2 states. Sulfur atoms are generated by the sequential multiphoton dissociation of CS2 at probing wavelengths. The two-photon absorption process involves the 3 3P2,1,0 → 4 3P2,0,1 or the 3 1D2 → 4 1F3 transitions with resolution of the individual J″ → J′ transitions in most cases. Intensities of the 33PJ″ → 4 3PJ′ transitions have been compared with Hartree-Fock calculated transition probabilities from the analogous transitions in atomic oxygen. Photoionization is observed in a three-photon (two to resonance) ionization originating from the 3P2,1,0 and the 1D2 states. Induced fluorescence is observed at 167 and 180 nm which is dipole-allowed radiation from the intermediate 3S01 and 1D02 states, respectively.

The adiabatic and diabatic reactions of S(1D) atoms with OCS: Internal state distribution of the S2 products

OCS at low pressure was photodissociated at 248 nm. It was shown by two photon laser induced fluorescence (LIF) that the concentration of the S(1D) atoms produced decays with time due to collisions with undissociated OCS molecules. In contrast, the S(3P) atom concentration rises from zero or nearly zero to a maximum and then subsequently decays. The S(1D) atoms react with OCS molecules to give S2 (X 3Σg), S2(a 1Δg), and S(3P). The rotational and vibrational distribution of the diatomic products was studied by LIF. The diabatic product S2(X 3Σg) was entirely in the v=0 state and was rotationally cold. The adiabatic product S2(a 1Δ) was probed in the v=1 state and had a fairly narrow non‐Boltzmann type rotational distribution peaking at J=65. We speculate that the reaction takes place through a C2v COS2 intermediate. The adiabatic and diabatic reactions are characterized by sequential and concerted C–S bond breaking, respectively.

Excimer laser projection etching of GaAs

We report the projection dry etching of GaAs using an excimer laser and HBr etching gas. Present experiments use photochemically generated reactants, which are spatially confined by gas phase collisions. Pattern transfer is accomplished by 1:1 imaging of the excimer laser light directly onto a GaAs substrate in a reaction cell filled with HBr gas. Resolution down to the laser beam image size is achieved through the addition of buffer gases.

Excimer laser‐assisted metalorganic vapor phase epitaxy of CdTe on GaAs

We have successfully grown CdTe (111) on GaAs (100) at 165 °C using a 248 nm excimer laser to photodissociate dimethylcadmium and diethyltellurium in the gas phase. Good crystalline quality of the layers is confirmed by x‐ray diffractometry. Growth rates up to 2 μm/h have been recorded in real time using time‐resolved reflectivity. Auger analysis reveals that the films are stoichiometric throughout the thickness of the layer, and that carbon and oxygen contaminants are below the level of detectability. We have used laser‐induced fluorescence spectroscopy to examine the photodissociation mechanism of diethyltellurium and have observed a linear dependence of Te atom production on excimer laser power.

Reversible modification of CdTe surface composition by excimer laser irradiation

KrF excimer laser irradiation of CdTe at fluences below the melt threshold (≤75 mJ/cm2) removes surface layers and produces reversible changes in the surface composition that depend upon the laser fluence and number of laser pulses delivered to the surface. At fluences above ∼40 mJ/cm2 a Te‐rich layer is obtained. A stoichiometric composition can be restored by irradiation at reduced laser fluence. The primary desorption products are Cd and Te2, and the velocities of these species are well described by a Maxwellian distribution. The fluence‐dependent changes in CdTe surface composition are consistent with a photothermal mechanism based on the competition between formation and desorption of Te2 and desorption of Cd atoms from the laser‐irradiated surface.

Photodissociation dynamics of ethylene sulfide at 193.3 nm

We have investigated the photodissociation dynamics of ethylene sulfide (thiirane) at 193.3 nm by studying Doppler broadened laser induced fluorescence spectra of atomic sulfur fragments. The only dissociative channel found produces S(1D) atoms and ethylene molecules. The measured average translational energy of the fragments is 20.5±4.5 kcal/mol. We have found an anisotropy in the velocity distribution of the atomic sulfur fragments (β=1.03±0.19), which confirms that the 193 nm transition is a parallel transition. We conclude that the dissociation is direct and symmetric, both C–S bonds breaking at once. The dissociation time is estimated to be about 300 fs.