Investigation of Thin Film Growth of B 12 As 2 by Chemical Vapor Deposition
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Investigation of Thin Film Growth of B12As2 by Chemical Vapor Deposition R.Nagarajan1 , J.H.Edgar1, J.Pomeroy2 , M.Kuball2 and T.Aselage3 1. Department of Chemical Engineering, Kansas State University, Manhattan, KS66506, USA. 2. H.H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, UK. 3. Department 2525/MS0613, Sandia National Laboratories, Albuquerque, NM 87185-06513, USA. ABSTRACT The chemical vapor deposition of icosahedral boron arsenide, B12As2, on 6H-SiC (0001) (on and off-axis) substrates was studied using hydrides as the reactants. The effects of temperature and reactant flow rates on the phases deposited and the crystal quality were determined. The growth rate increased with temperature from 1.5µm/h at 1100°C to 5 µm/h at 1400°C and decreased thereafter. X-ray diffraction revealed that the deposits were amorphous when the deposition temperature is below 1150° C. Above 1150°C, smooth B12As2 films were formed on 6H-SiC substrates with an orientation of (0001) B12As2 parallel to 6H-SiC (0001). Raman spectroscopy confirmed the strongly caxis oriented nature of B12As2 film on 6H-SiC. INTRODUCTION Rhombohedral boron pnictides B12As2 and B12P2 are wide band gap semiconductors [1, 2]. Their crystal structure is shown in Fig.1. [3]. With suitable control of carrier densities, B12As2 may find application in robust semiconductor devices. One such example is the fabrication of beta-cells which finds application in converting high-energy beta radiation directly into electricity.
Figure 1. Structure of B12As2
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There have been only a few reports on the chemical vapor deposition of B12As2 [4-6]. Hirayama et al [4] reported crystalline B12As2 films on and Si substrates. Correia at al [5] made B12As2 films on Si and other substrates, but the films were either amorphous or polycrystalline depending on the gas flow conditions. Most recently, Wang et al [6] deposited crystalline B12As2 films on 6H-SiC substrates employing diborane and arsine in hydrogen at 1150ºC. This choice of 6H-SiC as the substrate was an improvement over previous studies, as the basal plane lattice parameter of B12As2 is closely matched to twice that of SiC. SiC is much more thermally stable than silicon. In this paper we report the growth of B12As2 thin films on 6H-SiC substrates by CVD. EXPERIMENTAL B12As2 thin films were prepared by CVD using diborane and arsine mixtures in a hydrogen atmosphere. The substrate was kept inside a tantalum carbide coated graphite susceptor that was inductively heated. The reactants were 1% B2H6 in H2 and 2%AsH3 in H2.The hydrogen carrier gas and the B2H6 –H2 was controlled at a constant flow rate of 5 SLM and 55 sccm respectively, while the flow rate of the AsH3 – H2 gas was varied from 20 sccm to 85 sccm. B12As2 films were grown on 6H-
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