Morphology of heavily B-doped diamond films
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B-doped diamond films were synthesized by microwave plasma chemical vapor deposition using a mixture of methane (0.5% or 1.2%) and diborane (B2He) below 50 ppm on either Si substrates or undoped diamond films that had been synthesized using 0.5% or 1.2% methane. The surface morphologies of the synthesized films were observed by Secondary Electron Microscopy, and the infrared absorption and Raman spectra were measured. It was found that when diborane concentration was low, B-doped films preferred (111) facets. On the other hand, high diborane concentrations resulted in a deposition of needle-like material that was identified as graphite by x-ray diffraction.
I. INTRODUCTION Diamond is known to be a £>-type semiconductor when the crystal is doped with boron (B). 1 Structural,2"7 chemical, electrical,8"21 and optical22"25 properties of B-doped diamond films are currently under active investigation. Furthermore, electronic devices such as thermistors26"30 and field effect transistors31"41 have been fabricated using B-doped diamond films, demonstrating the feasibility of using diamond films for high power electric devices operational at high temperatures and in other harsh environments.42"44 B-doped diamond films have been shown to exhibit unexpected behavior; for instance, the Raman line of diamond at 1333 cm" 1 becomes sharper and more intense when about 1 ppm diborane (B 2 H 6 ) is added to the reaction gas.7'14'45 In addition, the intensity of the photoluminescence band at 5900 cm" 1 due to neutral vacancies decreases, also indicating that the crystal quality of diamond is significantly improved by B-doping.24'46 In the present article, B-doped diamond films were deposited on either Si substrates or diamond films by microwave plasma chemical vapor deposition (CVD) using methane, hydrogen, and diborane. The methane concentrations (cM) used were either 0.5% or 1.2%, and the B 2 H 6 concentrations (c B ) were =s50 ppm. Marked
changes in the film morphologies were observed as cB was increased. II. EXPERIMENTAL Undoped and B-doped diamond films were grown with an NIRIM-type microwave plasma CVD reactor described in Ref. 47. The substrates used were n-Si(lll) with a resistivity higher than 1000 Cl-cm. The gas pressure, the gas flow rate, and the substrate temperature were 32 Torr, 100 standard cubic centimeters per minute (seem), and 800 °C, respectively. Samples were prepared in four different ways as listed in Table I. Samples A and B were prepared for 7 h using cM = 0.5% and 1.2%, respectively, with cB =£ 50 ppm. On the other hand, for samples C and D, undoped diamond films were first deposited on Si substrates using cM — 0.5 and 1.2%, respectively, followed by the growth of B-doped layers using cM = 0.5% and different B 2 H 6 concentrations of cB ^ 50 ppm. III. RESULTS The major focus of the present work was the surface morphologies of B-doped films as observed by Scanning Electron Microscopy (SEM). Film analyses were
TABLE I. Samples prepared for this study. Growth conditions
Samples A
Samples B
Samples C
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