Phosphorus Doping of Boron Carbon Alloys
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ABSTRACT Phosphorus doped boron carbon alloy films were made by chemical vapor deposition from a single source compound, dimeric chloro-phospha(Ill) carborane ((C2B10H10) 2(PCI) 2). Phosphorus doped B5C materials exhibit increases in the band gap from 0.9 eV to 2.6 eV. INTRODUCTION Phosphorus will readily alloy with boron forming both boron phosphide (BP) [1] and B 12P2, which is structurally similar to conventional icosahedral boron carbide [2,3]. The B 12P2 structure is obtained by replacing the three-atom chain of the B4C crystal with two phosphorus atoms; the icosahedra contain only boron in B12P2. Phosphorus is precluded from occupying an icosahedral site due to its relatively large size compared boron. Because of the structural similarity between B 12P2 and boron carbides, it is anticipated that phosphorus will substitute at a chain position, and result in a larger band gap. Since the molecular structure of dimeric chloro-phospha(III)carborane ([C2B loHl oPCl] 2) has similarities with orthocarborane (closo- 1,2-dicarbadodecaborane or C2B 10H 12), we anticipated that boron-carbon-phosphorus alloys could be fabricated by plasma enhanced chemical vapor deposition (PECVD) using this compound as a single source molecule in place of orthocarborane.
Because of the similarity in molecular structures (as seen in Figure 1) between dimeric chlorophospha(EI)carborane and orthocarborane, the materials grown by PECVD from these source molecules should have similar structures. Orthocarborane has been successfully used to make boron-carbon alloy heteroj unction diodes [4-7], homojunction diodes [5], tunnel diodes [5-6] and transistors [7] by chemical vapor deposition (CVD).
1
2
10
Carbon
0
O
Boron
Chlorine
S
Hydrogen
Phosphorous
Fig. 1. Schematic diagrams of the source molecules orthocarborane (C2B10H12) and dimmic chloro-phospha(L1)carborane ([C 2B 1oH 10PC112 ) at the left and right, respectively.
1031 Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
EXPERIMENTAL Boron-carbon-phosphorus or phosphorus doped boron-carbon alloys were prepared by plasma enhanced chemical vapor deposition (PECYD) on Si(l 11) and glass substrates using the single source compound dimeric chloro-phospha(Ill)carborane (shown in Figure 1). The plasma reactor used in fabricating these materials has been described in detail elsewhere [8] and is operated at 13.56 MHz at 30 Watts. The general procedure for growing the films has been described in detail elsewhere [4-8]. The optical gap of the deposited materials was measured by a double beam, double monochromator, Perkin Elmer Lambda-9 series spectrophotometer. Both the transmittance (T) and reflectance (R) of the phosphorus doped boron carbon alloy thin films deposited on quartz glass substrates were measured. The absorption coefficient (Cc)of the films was assumed to be proportional to the absorption (A) determined from transmittance and reflectance measurements. The source molecule chloro-phospha(III) carborane was synthesized using standard procedures described els
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