Variation of the Photoelectric Threshold of Boron Doped Diamond Films
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Mat. Res. Soc. Symp. Proc. Vol. 509 ©1998 Materials Research Society
(4.87, 5.38 and 5.64 eV). The surface morphology was studied with a JEOL T-300 scanning electron microscope (SEM). Carbon phases were determined from Raman spectra obtained with 514.5 nm radiation from an Argon ion laser and measured backscattered light with a SPEX 1404 double monochromator. The total photocurrent was measured as a function of photon energy using the Xenon source and monochromator discussed above.
I
0 ppm
1.36 ppm 2.91 ppm 3.81 ppm 4.75 ppm
Figure 1. SEM images of diamond films grown with 0, 1.96, 2.91, 3.83 and 4.75 ppm of diborane added. Figure 1 shows SEM images of diamond films grown using diborane concentrations of 0, 1.96, 2.91, 3.83 and 4.75 ppm. In general, the average crystalline size decreases with increasing boron concentration, in agreement with previous results [5]. The crystal orientation is predominantly (111) with large (100) grains, depending on the boron concentration. While undoped film contains (100) diamond crystals, film doped with 1.96 ppm diborane is multi-faceted with large grained (100) crystals and at a diborane concentration of 2.91 ppm, microcrystalline features appear. At a hydrogen-diborane concentration of 3.83 ppm, the microcrystalline features persist, but large (111) crystals also are observed. This latter feature is similar to that observed with the 1.96 ppm diborane sample. At a diborane concentration of 4.75 ppm, the majority of crystallographic orientation is (111). In addition, small diamond crystallites with no clear crystallographic orientation are observed. The appearance of such structures is common for CVD diamond films exhibiting (111) orientation. This is partly because the (111) surfaces are structurally defective. Tsai et al., [6] have used transmission electron microscopy to observe that (111) surfaces contain much greater defect density than (100) surfaces. It is evident from the SEM results that a diborane concentration of 1.96 ppm, significantly improved the crystal quality of the diamond film, whereas higher boron concentrations deteriorated the films crystal quality. In addition, boron induces the growth of (1 11), probably due to propensity of the 144
4.75 ppm C: a,) C:
3.83 ppm 2.91 ppm 1.96 ppm 0 ppm
1050
1250
1450
1650
Raman Shift cm[-1] Figure 2. Raman spectra of polycrystalline diamond with different boron concentrations. (111) surfaces to incorporate impurities. Figure 2 shows the Raman spectra of polycrystalline diamond films grown with different boron concentrations. The Raman spectra have been peak fitted with Gaussian and Lorentzian line shapes and fitting parameters, integrated intensities, line widths of individual lines and the areas under each peak were determined. All spectra show the 1333 cm-1 line characteristic of single-crystal diamond and a broad peak extending from about 1540 to 1580 cml. This latter peak arises from graphite and amorphous carbon. The slight broadening and shift of the 1333 cm"1 peak in the boron doped films is probably due
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