S. Guha

Formation of metastable species in hydrogen thyratrons

Direct spectroscopic evidence is presented for the formation of metastable molecular and atomic species in hydrogen thyratrons. Because these states have long radiative lifetimes and energies greater than 11 and 10 eV, respectively, their behavior may effect thyratron operation.

The effects of focusing on parametric oscillation

The threshold pump intensity for singly and doubly resonant parametric oscillators and the efficiency of doubly resonant parametric oscillators is calculated for the general case of unequal confocal beam parameters.

Growth of InxGa1−xAs on patterned GaAs(100) substrates

The influence of finite substrate size on misfit dislocation densities in strained systems is examined through the growth of InxGa1−xAs on nonplanar patterned (100) GaAs substrates consisting of parallel mesas of widths between 6500 A and 1.3 μm and of macroscopic length. Cross‐sectional transmission electron microscopy (XTEM) studies on In0.11Ga0.89As films reveal that while the mean misfit dislocation spacing for the growth in the nonpatterned region was ∼1500 A, for the growth on the mesas no misfit dislocations running parallel to the mesa length were observed. This is likely due to strain relief at the mesa edges, possibly brought about by the ability to transfer strain energy from cluster coalescence boundaries to the mesa edges and/or reduced cluster coalescence boundaries when the mesa size becomes comparable to or less than the effective migration length.

Raman microprobe study of narrow InxGa1−xAs stripes on patterned GaAs(100) substrates

The strain and structural variations across InGaAs ridges (∼1 μm) and valleys (∼3 μm) grown on patterned GaAs(100) substrates are examined via Raman microprobe spectroscopy. High spatial resolution scans across these structures show significant variations in the Raman features (phonon frequency, linewidth, and intensity). Variation in the GaAs‐like longitudinal optical mode frequency suggests a large difference in strain found in the InGaAs on top of the 1 μm ridges and in the valleys. Concomitant measurement of phonon linewidth and Ar+ laser‐induced heating are also indicative of the same. These results, in conjunction with transmission electron microscope studies, demonstrate that Raman microprobe spectroscopy is a powerful tool for probing patterned semiconductor structures on a microscopic scale.

An explanation for the directionality of interfacet migration during molecular beam epitaxical growth on patterned substrates

An explanation of the interfacet migration behavior observed in molecular beam epitaxical growth of GaAs and AlGaAs on patterned GaAs(100) substrates is presented on the basis of the nature of the ledge–ledge interaction.

The nature and control of morphology and the formation of defects in InGaAs epilayers and InAs/GaAs superlattices grown via MBE on GaAs(100)

Initial stages of molecular beam epitaxial (MBE) growth of highly mismatched InxGa1-xAs/GaAs(100) have been studied by planar and cross-sectional transmission electron microscopy. For In0.5Ga0.5As growth, we find drastic differences in morphology obtained by reducing the growth temperature form 475 to 420°C. We also observe differences in morphology between alloy growth and short period superlattice ((InAs)n/(GaAs)m (m = 1 monolayer, n = 2 monolayers) growth of equivalent effective composition. In the case of growth by formation of large islands, we present direct evidence of strain relief at the island edges and discuss defect formation in these islands.

Some optical and electron microscope comparative studies of excimer laser‐assisted and nonassisted molecular‐beam epitaxically grown thin GaAs films on Si

We present results of Raman and Rayleigh scattering, near‐band‐edge photoluminescence, and electron microscope examination of GaAs thin films (<0.3 μm) grown on Si(100)+4°〈011〉 substrates via molecular‐beam epitaxy with part of the 2‐in. wafer under KrF excimer laser irradiation. Comparative examination of laser‐assisted and nonassisted growth areas on the same wafer reveals differences in the nature and degree of strain and structural defects. It is found that the Rayleigh scattering intensity is higher and the longitudinal‐optical mode Raman peak is broader in the non‐laser‐irradiated region indicating a higher degree of disorder. The luminescence is higher from the laser‐irradiated region. These differences between the non‐laser‐ and laser‐irradiated regions on the same wafer are partially caused by a difference in the film thickness of the two regions. Near‐edge photoluminescence peak position reflects the presence of compressive stress in these thin GaAs films (both in the non‐laser‐ and laser‐irradi...

A Study of Discharge Processes in Hydrogen Thyratrons

The purpose of this paper is to provide a discussion of the fundamental processes occurring in hydrogen thyratrons. Recent experimental data pertaining to electron densities, energies, and excitation processes occurring in devices during their normal operation are presented. Electron densities are observed to be lower than 2 × 1014 cm-3, corresponding to current densities of approximately 100 A/cm2. The presence of both molecular and atomic species affecting recovery and voltage reerection is reported, and mechanisms for this are discussed. Streak camera data showing a delay in breakdown of the gridanode relative to the grid-cathode region are also presented.

Thyratron operation using Helium for high-power and high-repetition rate applications

Results of a study of the helium thyratron for high-power applications are reported. The helium thyratron operates at high voltages and currents, comparable to hydrogen, and, for certain of the conditions studied, recovers faster than hydrogen. These results suggest that helium thyratrons should be seriously considered for high-power applications, such as excimer lasers.

Laser-Assisted Molecular Beam Epitaxical Growth Of GaAs On Si (100)

We have grown GaAs on Si (100) substrates misoriented 4° from the [110] direction using a KrF pulsed excimer laser-assisted Molecular Beam Epitaxy. In this work, we report the systematic study of 2000 A GaAs films grown on Si using a two step growth sequence. Raman scattering, Rayleigh scattering, near band edge photoluminescence, cross-sectional TEM microscopy, in situ RHEED, and optical surface roughness profiles are used to characterize the differences between laser irradiated and non-irradiated areas of the samples. We find a reduction of defects, an enhancement of photoluminescence intensity, and no evidence for laser-induced melting for power levels of up to 25 MW/cm2. Photoluminescence linewidths at 4.5K are comparable to widths observed for 1 micrometer thick GaAs films grown on Si.