Aiming toward process control of industrial high yield / high volume CVD reactors, the potential of optical sensors as a monitoring tool has been explored. The sensors selected are based on both Fourier transform infrared spectroscopy (FTIR) and tunable diode laser spectroscopy (NIR-DLS). The former has the advantage of wide spectral capability, and well established data bases. NIR-DLS spectroscopy has potentially high sensitivity, laser spatial resolution, and the benefits of comparatively easier integration capabilities -including optical fibre compatibility. The proposed technical approach for process control is characterised by a "chemistry based" feedback system with in-situ optical data as input information. The selected optical sensors continuously analyze the gas phase near the surface of the growing layer. The spectroscopic data has been correlated with process performance and layer properties which, in turn establish data basis for process control. The new process control approach is currently being verified on different industrialised CVD coaters. One of the selected applications deals with the deposition of SnO 2 layers on glass based on the oxidation of (CH 3 ) 2 SnCl 2 , which is used in high volume production for low-E glazing.
W t-« a: ~ ~ « N Z 8 z o a: tu W ...J W 1.8 2.0 2.2 2.4 lIE (10-6 cm' V-I) FIG. 2. Electric field dependences of electron ionization rates at temperatures T = 77, 153, 223, and 293 K. Solid and broken lines are the values in (III) oriented and (100) oriented InP, respectively.surface of n--lnP was flat over the area of about 300 /lm diameter. The p+ region was formed by Cd diffusion with the depth of 1.6/lm so that this region be included in the area having a flat surface. Then the n+ -loP substrate was selectively etched in order to realize the hole injection into the high field region. The dark current was as low as 1 nA at 0.9 VB' and the maximum multiplication factor of 30 was obtained. The sufficient guard ring effect was ascertained from photosensitivity measurements of the two-dimensional light spot scanning over the diode surface. Figure 1 shows typical experimental Mn and Mp data at temperatures T = 293, 223, 153, and 77 K. Solid and broken curves are Mn and Mp , respectively. This diode has a reverse bias voltage dependence of quantum efficiency on the holeinjection condition, and the Mp 's in this figure were derived by fitting the theoretical collection efficiency of holes JO to the measured! unmultiplied photocurrent.From these photomultiplication data the electric field dependences of a were obtained at the four temperatures, and are depicted in Fig. 2. In this figure the experimental a's in (100) oriented InP 9 are shown for comparison. It is clear that there is no marked difference in a between the two crystallographic orientations at I.ow temperatures. This result indicates that the probability for electrons to ballistically reach the threshold states is negligible in InP at the measured electric field and temperature ranges. It is in agreement with the experimental results 7,8 and Monte Carlo simulations of Brennan and Hess. 11 tH. Shichijo and K. Hess, Phys. Rev. B 23, 4197 (1981).
An industrialized computer controlled fiber-optic laser diode gas analysis system is described. A unique signal processing scheme completely eliminates both non-gas related transmission variations and long term drift. Sensitivities better than 1 ppm.m and 200 ppm•m is routinely achieved for field installed systems for ammonia and oxygen, respectively. Due to the superior selectivity of laser diodes interference effects from coexisting gases, such as water vapor in ammonia measurements, is easily avoided.
A Gbit/sec optical fiber communication experiment was performed. The maximum speed was 3 Gbit/sec with a BH laser and 5 Gbits/sec with a TJS laser. A 500-m single-mode fiber was used, and the power was coupled by mounting the laser butt-end to the fiber, giving a minimum loss of 10 dB. The receiver employed a Si-avalanche photodiode with an impulse-response width of 210 psec (FWHM) and a 10-Gbit/sec and gate built from GaAs MESFETs. Bit-error-rate measurement values of <10(-9) prove the feasibility of optical fiber transmission at 5 Gbit/sec.
The spectral characteristics of a series of different VCSELs emitting at 850 nm and 762 nm have been examined, to assess the potential of this new kind of semiconductor laser for use in laser spectroscopy and fundamental metrology, particularly for environmental and energy monitoring of combustion processes. Adequate polarisation control, which is necessary for laser spectral purity, was provided through manufacture of a dumbbell or "figure-of-eight" shaped active region ofthe lasers at 850 nm. Spectral imeshape and linearity of the frequency tuning have been examined with a variety of methods such as optical interferometry and heterodyning. Optical heterodyrnng was used to measure a minimum linewidth of 90 MHz FWHM for a 762 nm VCSEL during single mode operation. Deviations from linearity ofup to some GHz of frequency tuning with current were observed with an original application of a Twyman-Green mterferometer, with basic current tuning rates of 58 GHz/mA and 130 GHz/mA for an 850 nm and a 762 nm VCSEL, respectively. The frequency stability for a 762 nm device was measured to be approximately MHz. A possible problem for the use of VCSELs in wavelength modulation spectroscopy (WMS) is that due to their extremely high tuning rate with current: ultra-low noise current drivers must be used in order to achieve high sensitivity detection. The WMS absorption spectrum with VCSEL of a number oflines ofthe oxygen molecule at 760 nm have been registered and studied.
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