Determination of Boron Concentration in Doped Diamond Films
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Determination of Boron Concentration in Doped Diamond Films S.N. Demlow1, T.A. Grotjohn1,2,, T. Hogan1, M. Becker2, J. Asmussen1,2 1 2
Michigan State University, Electrical and Computer Eng., East Lansing, MI 48824, U.S.A. Fraunhofer USA Center for Coatings and Laser Applications, East Lansing, MI 48824, U.S.A.
ABSTRACT The electrical characteristics of high quality single crystal boron-doped diamond are studied. Samples are synthesized in a high power-density microwave plasma-assisted chemical vapor deposition (CVD) reactor at a pressure of 160 Torr. The boron-doped diamond films are grown using diborane in the feedgas at concentrations of 0-0.25 ppm, and are compared to those grown previously with 1-10 ppm. The boron acceptor concentration is investigated using infrared absorption, and compared to the boron concentration obtained by SIMS. A four point probe is used to study the conductivity. The temperature dependent conductivity is analyzed to determine the boron dopant activation energy. INTRODUCTION Diamond’s exceptional properties, such as a wide bandgap, high breakdown voltage, and high electron and hole mobilities, make it a potentially useful semiconductor for hightemperature and high-power devices. Boron-doped p-type diamond has applications for the fabrication of electrodes for electrochemistry and electronic devices such as Schottky-barrier diodes and metal-semiconductor field effect transistors. The realization of useful devices requires the deposition of high quality, controlled conductivity films. This work expands on our previous work [1-5] on the deposition and characterization of high-quality boron-doped single crystal diamond (SCD). The repeatability of boron concentration in grown films for lower boron doping levels, the memory effect of residual boron in the reactor chamber, and the effect of small percentages of CO2 in the feedgas are investigated. The concentration of boron in the grown films is determined by FTIR methods and SIMS, and the two measurements are compared with each other and with the activation energy, as determined by temperature dependant conductivity measurements. EXPERIMENT Boron doped diamond was deposited on 3.5 mm x 3.5 mm HPHT diamond seeds using a microwave plasma-assisted CVD reactor [6]. The reactor operates at 2.45 GHz with a molybdenum substrate holder that is water cooled. In this study, samples SND01-SND05 and 174 were grown, and compared to other samples grown in the same reactor in previous studies [1-5]. The deposition conditions are given in Table 1. In some depositions, a small percentage of CO2 was added to the feedgas mixture. It has been shown that adding 0.25% oxygen to the feedgas mixture during CVD growth lowers the boron concentration in the grown diamond film. [20, 21]. The effect of using this same small percentage of CO2 as O2 however, was not apparent in the boron concentrations of the grown films. Before and after CVD growth, samples are cleaned in a boiling mixture of 1:1 sulfuric:nitric acid for 20 minutes, followed by 20 minutes of boiling in
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