Growth and Properties of Microcrystalline Germanium-Carbide Alloys
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respectively. These radicals are believed to be the dominant species produced due to the heavy H2 dilution, which tends to suppress the formation of GeH2 and CH2. ECR processing is appropriate for growing this type of material for several reasons. First, reactive ion etching of the film during deposition by the H beam promotes crystalline growth at low substrate temperatures, as non-crystalline material is etched away. Low substrate temperatures are necessary to grow this material so that the Ge and C atoms do not phase separate. Secondly, the ECR plasma is relatively dense at low pressures and low pressures enhance the above etching process and promote faster growth rates. Finally, H passivation of the surface, which is a result of the H2 dilution, homogenizes the growing surface. This helps to suppress the formation of islands[7] leading to the desired 2-D growth. RESULTS AND DISCUSSION For each experiment, a set of parameters similar to those shown in Table 1 was used as a control set. Investigations were then made into how the methane and germane flow rate, substrate temperature, microwave power, chamber pressure, and the type of substrate deposited on affected the material properties of the resulting films. All experiments of deposition parameters were done on amorphous substrates, as a strain free surface was desired. The first parameter investigated was the flow rate of methane. The bandgap of the films was determined from the absorption data. The dependence of bandgap on methane flow rate is shown in Figure 2. As can be seen, the bandgap initially increases rapidly as the CH4 flow is increased, but then the values increase only nominally for high CH4 flow. The crystalline quality of the films was characterized by Raman spectroscopy. All of the films displayed the Ge crystalline peak at 300 cm- 1 . The full width at half maximum Heater/Sample Holder Restricting Orifice Magnets
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Table 1. Deposition base parameters. Value Parameter 3500 C Substrate Temperature 200 Watts Microwave Power 7 mTorr Chamber Pressure 0.5 sccm GeH4 Flow Rate 25 sccm H2 Flow Rate 0 - 22 sccm CH4 Flow Rate (FWHM) value of the Ge peak for the methane flow series is shown in Figure 3. The crystal order of the films was degraded as methane flow increased, but there is not a direct correlation between methane flow and the crystal quality of the films. Activation energy measurements were made on the films, and the values followed a similar trend as the Raman data, with energies ranging from 0.22 to 0.15 eV. The photosensitivities of the films were all close to unity, with dark conductivity values ranging from 102 s/cm for pure Ge to 100 s/cm range for the Gel-xCx samples. The decrease in conductivity was attributed to the incorporation of C. The cubic lattice parameter a was measured by XRD, and the results are shown in Figure 4. As expected, the unit cell became smaller as the flow of metha
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