The incorporation of Nickel and Phosphorus dopants into Boron-Carbon alloy thin films

The structural and electronic properties of nickeland phosphorus-doped boron–carbon (B5C) alloy thin films grown by plasma-enhanced chemical vapor deposition have been examined. The Ni-doped boron–carbon alloys were grown using closo-1,2-dicarbadodecaborane (C2B10H12) as the boron–carbon source compound and nickelocene (Ni(C5H5)2) as the nickel source. The phosphorus-doped alloys were grown using the single-source compound: dimeric chloro-phospha(III)-carborane ([C2B10H10PCl]2). Nickel doping increased the conductivity, relative to undoped B5C, by six orders of magnitude from 10−9 to 10−3 (Ω cm)−1 and transformed the material from a p-type semiconductor to an n-type. Phosphorus doping decreased the conductivity, relative to undoped B5C, by two orders of magnitude and increased the band gap from 0.9eV for the undoped material to 2.6eV. Infrared absorption spectra of the nickel-and phosphorus-doped B5C alloys were relatively unchanged from those of undoped B5C. X-ray diffraction suggests that the phosphorus-doped material may be a different polytype from the Ni-doped and undoped B5C alloys. With the growing demand for semiconductor materials which can operate at temperatures in excess of 200 °C, alternatives to silicon are being sought. Although silicon carbide has excellent thermal properties, it is difficult to manufacture and package. Alternatives to silicon carbide are the boron-rich materials, in particular boron–carbon alloys. Like boron carbide, the boron–carbon alloys are refractory materials with melting temperatures in excess of 2400 °C. These alloys typically have band gaps in excess of 0.7eV [1, 2] and low numbers of free carriers [1]. Boron–carbon alloys have been successfully grown by the technique of plasma-enhanced chemical vapor deposition (PECVD) [1, 3–5]. The alloy forms of boron carbide have the chemical composition B5C. The alloy materials typically have conductivities of 10−10 to 10−12 (Ω cm)−1 [1], compared to single-crystal boron carbide which has a conductivity of 1 (Ωcm)−1 [6]. Similar conductivities (10−8 (Ω cm)−1) have been observed for sputter-deposited boron–carbon alloy thin films [7]. Like crystalline boron carbide, the boron– carbon alloys consist of an icosahedral network [1, 8], where the icosahedra are composed of boron and carbon atoms. In contrast, the icosahedra in crystalline boron carbide, are almost exclusively constructed from boron atoms with the carbon atoms predominantly participating in the three atom chains that connect the icosahedral network [9–16]. The average crystallite size of the PECVD-grown alloys is approximately 100 Å [1, 8]. The PECVD process for growing boron–carbon alloys is well suited for semiconductor device manufacturing. Diodes and simple junction field effect transistors have been constructed from boron–carbon alloy thin films [1, 3, 4, 7]. Typically, these devices have been constructed from boron–carbon thin films grown on either Si or a boron thin film. These studies demonstrated that simple devices could be successfully constructed from boron–carbon alloy materials, regardless of long-range crystallinity. The B5C alloy thin films grown by PECVD are slightly p-type, i.e. essentially intrinsic [1, 3, 4]. In order to develop true boron–carbon homojunction devices, it is necessary to develop n-type materials by means of doping. The doping of boron–carbon alloys has only recently been examined [17], whereas the doping of crystalline boron-rich materials has been more extensively studied. In particular, β-rhombohedral boron, a p-type material, has been doped with Mn, P, and Fe [18–20], where only the latter was found to transform the material to n-type. Even though Mn and Fe have the same number of electrons in their outer shell (4s2), they yield p-and n-type materials, which suggests that they may have markedly different structures. Attempts to dope crystalline boron carbide with Si (B4.3C) and P (B10C) failed to transform the material to n-type [21, 22]. These examples demonstrate the complexity of introducing dopants into boron-rich materials in order to obtain n-type character. In this paper we present the results of Ni and P doping of boron–carbon alloy (B5C) thin films grown by PECVD. Transport, device characterization, and spectroscopic studies Published in Applied Physics A 67 (1998), pp. 335-342. Copyright © Springer-Verlag 1998. Used by permission. The publisher’s version is online @ http://springerlink.metapress.com/content/1432-0630/ The incorporation of Nickel and Phosphorus dopants into Boron-Carbon alloy thin films D. N. McIlroy1, S.-D. Hwang2, K. Yang2, N. Remmes2, P. A. Dowben2, A. A. Ahmad3, N. J. Ianno3, J. Z. Li4, J. Y. Lin4 , H. X. Jiang4 1 Department of Physics, University of Idaho, Moscow, ID 83844-0903; E-mail: [email protected] 2 Center for Materials Research and Analysis and the Department of Physics and Astronomy, University of Nebraska–Lincoln, Lincoln, NE 68588-0111 3 Center for Microelectronics and Optical Materials and the Department of Electrical Engineering, University of Nebraska–Lincoln, Lincoln, NE 68588-0511 4 Department of Physics, Kansas State University, Manhattan, KS 66506-2601 Submitted April 1997; accepted November 1997