The presence of trace water vapor in process gases such as phosphine, used for compound semiconductor epitaxial growth, can negatively affect the optical and electrical properties of the final device. Therefore, sensitive H2O measurement techniques are required to monitor precursor purity and detect unacceptable contamination levels. A commercial cavity ring-down spectrometer that monitors an H2O absorption line at a wavelength of 1392.53 nm was investigated for service in high purity PH3. Spectral parameters such as the line shape of water vapor in the presence of PH3 as well as background features due to PH3 were measured at different pressures and incorporated into the data analysis software for accurate moisture readings. Test concentrations generated with a diffusion vialbased H2O source and dilution manifold were used to verify instrument accuracy, sensitivity, linearity, and response time. H2O readings at 13.2 kPa corresponded well to added concentrations (slope=0.990+/-0.01) and were linear in the tested range (0-52.7 nmol mol-1). The analyzer was sensitive to changes in H2O concentration of 1.3 nmol mol-1 based on 3sigma of the calibration curve intercept for a weighted linear fit. Local PH3 absorption features that could not be distinguished from the H2O line were present in the purified PH3 spectra and resulted in an additional systematic uncertainty of 9.0 nmol mol-1. Equilibration to changing H2O levels at a flow rate of 80 std cm3 min-1 PH3 occurred in 10-30 minutes. The results indicate that cavity ring-down spectroscopy (CRDS) at 1392.53 nm may be useful for applications such as on-line monitoring (and dry-down) of phosphine gas delivery lines or the quality control of cylinder sources.
The work function variations of NO2 and H2S molecules on Pd-adsorbed ZnGa2O4(111) were calculated using first-principle calculations. For the bonding of a nitrogen atom from a single NO2 molecule to a Pd atom, the maximum work function change was +1.37 eV, and for the bonding of two NO2 molecules to a Pd atom, the maximum work function change was +2.37 eV. For H2S adsorption, the maximum work function change was reduced from −0.90 eV to −1.82 eV for bonding sulfur atoms from a single and two H2S molecules to a Pd atom, respectively. Thus, for both NO2 and H2S, the work function change increased with an increase in gas concentration, showing that Pd-decorated ZnGa2O4(111) is a suitable material in NO2/H2S gas detectors.
Gallium
oxide (Ga2O3) has especially become
popular because of its established applications in semiconductors.
Of five polymorphs, monoclinic β-Ga2O3 is the most thermodynamically stable phase. However, orthorhombic
Ga2O3 (also known as ε-Ga2O3 or κ-Ga2O3) is gaining increasing
interest due to its high lattice symmetry and peculiar ferroelectricity.
Although the structural approach for estimating Ga2O3 has been studied both theoretically and experimentally, ε-Ga2O3 and κ-Ga2O3 are
still confused. In this study, ε-Ga2O3 epilayers are grown on c-plane sapphire by metal–organic
chemical vapor deposition with a multistep growth process. A thin
annealed ε-Ga2O3 buffer layer is grown
in the first step. The sequent growth steps with slow, fast, or combination
of slow then fast growth rate significantly influence the quality
of epilayers compared with that of directly grown Ga2O3. Through a detailed transmission electron microscopy (TEM)
characterization of these Ga2O3 epilayers, the
structural relationship between orthorhombic κ-Ga2O3 and hexagonal ε-Ga2O3 is
elucidated. A series of first-principles density functional theory
calculations are also carried out to confirm the argument.
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