Reactions of silicon tetrachloride and germanium tetrachloride with oxygen and oxides of nitrogen
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. INTRODUCTION The oxidation reactions of SiCl4 and GeCl4 to form SiO2 and GeO2 are of considerable interest since they are the primary reactions occurring in the fabrication of optical waveguides by the Modified Chemical Vapor Deposition (MCVD) process. These reactions, together with hydrolysis, may also play an important role in the Vapor phase Axial Deposition (VAD) and Outside Vapor Deposition (OVD) techniques for optical waveguide production. The kinetics of the oxidation process play a decisive role in determining the temperature at which oxide particles form. This in turn may affect particle size and growth rates and may also influence the composition of particles formed by oxidation of SiCl4/GeCl4 mixtures. In addition, the ability to generate GeO 2 particles at temperatures lower than those required for the reaction of GeCl4 with oxygen is critical to the fabrication of pure GeO2 core optical waveguides by the MCVD technique, as will be described later. II. EXPERIMENTAL A continuous flow system, which simulates the actual MCVD process, was used in these studies. The rate and chemical composition of the oxidizing gas flow were controlled and measured by a set of Unit Instruments flow controllers, with typical total flow rates around 250 cc/min. Most of the flow passed directly to the reaction chamber, but an accurately measured amount was first passed through a GeCl4 or SiCl4 bubbler before entering the reactor. This flow serves as a carrier gas; it resides in the bubbler long enough to emerge saturated with GeCl4 or SiCl4 vapor. The flow rate of the carrier gas and temperature of
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Bell Laboratories Summer Research Program Participant, 1986. J. Mater. Res., Vol. 4, No. 3, May/Jun 1989
the bubbler were accurately measured so that the amount of GeCl 4 or SiCl4 delivered could be determined from available vapor pressure data.' The chemical reactor was a laminar flow U-tube constructed of 6.7 mm ID fused quartz tubing, positioned inside a cylindrical furnace capable of reaching 1250 CC. A 10 mil chromel-alumel thermocouple was placed between the arms of the U-tube to monitor the reactor temperature. The thermocouple was interfaced to an AT & T PC6300 data acquisition computer and to an Omega series CN-2010 temperature controller, which controlled the furnace temperature via a feedback loop. A Perkin-Elmer PE683 infrared spectrometer was used to measure the IR spectrum of the effluent gas stream over the wavelength range of interest. The AT & T PC6300 was used to control the spectrometer and acquire and store the spectra. The furnace temperature, bubbler temperature, time and spectrometer attributes (slit width, scan speed, etc.) were also recorded along with the spectra. Figure 1 gives a schematic summary of the complete system. The experimental procedure involved oxidizing a given flow composition at varying temperatures. Typical experi-
CONTROLLERS
TEMPERFITURE CONTROLLER D /U-TUBE FILTER
BUBBLER
CELL
FURNRCE
FIG. 1. Schematic of experimental apparatus. © 1989 Materials Research Society
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