Trace Water Detection in Phosphine By Cavity Ring-down Spectroscopy
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Trace Water Detection in Phosphine By Cavity Ring-down Spectroscopy Susan Y. Lehman, Kris A. Bertness, and Joseph T. Hodges1 National Institute of Standards and Technology, 325 Broadway, Boulder, CO 80305 1 National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899 ABSTRACT We are using cavity ring-down spectroscopy (CRDS) to measure concentrations of water in nitrogen and, for the first time to our knowledge, in phosphine. Water vapor concentrations have been measured in purified and unpurified phosphine, indicating a water mole fraction of (22.0 ± 1.0) x 10-6 in unpurified phosphine. After purification with an in-line, chemically-reactive purifier, the mole fraction of water in phosphine was less than 0.1 x 10-6. Mole fractions as high as (730 ± 60) x 10-6 have been measured in unpurified phosphine, suggesting that the H2O vapor concentration increases substantially with time as the gas is stored in a cylinder. The materials properties of AlInP grown by molecular beam epitaxy (MBE) with CRDS-characterized PH3 were found to be relatively insensitive to water contamination at the 22 µmol/mol level. INTRODUCTION Impurities in semiconductor source gases have a negative impact on the material and device performance. Water and molecular oxygen are particularly detrimental, especially for Alcontaining compounds, forming oxygen complexes with energies deep in the band gap of the resultant semiconductor and thereby increasing non-radiative recombination. Most techniques for detecting H2O in process gases have poor accuracy for concentrations below 1 part in 106 and require either long equilibration times or frequent calibration and control of ambient humidity. Cavity ring-down spectroscopy (CRDS) [1,2] is based on optical absorption but depends on a measurement of a time constant rather than detection of small changes in transmitted intensity. CRDS allows measurement of the H2O concentration with low uncertainty even for mole fractions in the vicinity of 1 part in 109. In this work we quantify the relationship between the content of H2O in a semiconductor source gas and the materials quality of resultant semiconductor films. We have used CRDS to measure the amount of water contaminating the source gas (phosphine). We then measured the correlation between this H2O and the amount of O incorporated in MBE-grown AlInP structures. EXPERIMENT A simplified schematic of the cavity ring-down apparatus is shown in Fig. 1. The cavity is composed of two high-efficiency mirrors, separated by approximately 75 cm, with reflectivity nominally 0.999985. These mirrors form an optical cavity with a finesse of greater than 2x105 and an effective path length of more than 50 km. The mirrors are mounted within a sealed volume constructed of standard ultra-high-vacuum stainless steel components; one mirror is mounted on a piezoelectric transducer to enable control of the cavity length. The length is stabilized to nm-levels by locking to a frequency-stabilized HeNe laser emitting at 633 nm, which is not shown
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