Microwave plasma-assisted deposition of boron doped single crystal diamond

Abstract

The homoepitaxial growth of diamond on single crystal substrates is done using microwave plasma-assisted chemical vapor deposition with a hydrogen-methane plasma. The doping of the deposited diamond with boron makes it a p-type semiconductor and with phosphorus makes it an n-type semiconductor. The doping of diamond is done to make diamond electronic devices. Diamond is of interest for electronics as it has a wide bandgap (5.4 eV), the highest thermal conductivity of all solid materials at room temperature, a high electric field breakdown strength (10 MV/cm), and high hole and electron carrier mobilities. This presentation will describe the boron doping of diamond by adding diborane into the feedgas of the plasma discharge. For electronic devices controlled doping levels of the boron in the diamond from 1015 cm-3 up to above 1020 cm-3 are desired. The primary control of the doping level is the amount of diborane added to the feedgas. Levels of diborane concentrations in the feedgas as measured by the [B]/[C] ratio in the feedgas range from 0.3 ppm to over 1000 ppm in our study. While the diborane concentration in the feedgas is of primary importance many other factors are also important to the boron concentration in the diamond including residual boron in the deposition system from previous runs, temperature of the substrate, preparation of the substrate, and conditions of the plasma discharge including its power density and pressure. An additional consideration in the doping of diamond is keeping unwanted impurities out of the deposition process. This is especially true of nitrogen impurities which are easily incorporated into the diamond during deposition and which compensates the boron doping, especially at low boron doping concentrations. This presentation will discuss measures taken in the design and operation of a microwave plasma diamond deposition system to control the boron doping and reduce the unwanted impurities.