Multiple phosphorus chemical sites in heavily phosphorus-doped diamond

Abstract

We have performed high-resolution core level photoemission spectroscopy on a heavily phosphorus ͑P͒-doped diamond film in order to elucidate the chemical sites of doped-phosphorus atoms in diamond. P 2p core level study shows two bulk components, providing spectroscopic evidence for multiple chemical sites of doped-phosphorus atoms. This indicates that only a part of doped-phosphorus atoms contribute to the formation of carriers. Diamond is a semiconductor with remarkable physical properties such as a wide band gap, very high electric breakdown, and high thermal conductivity. Therefore, the investigation of p-and n-type diamonds is very important for electrical applications. A p-type diamond is obtained by boron ͑B͒ doping, and the acceptor level in light doping is located at about 0.37 eV from the valence band maximum. 1 On the other hand, an n-type diamond is obtained with doping of nitrogen or phosphorus ͑P͒. Nitrogen gives a deep donor level with an activation energy of 1.7 eV. 2,3 This means that its application as a device is difficult because of the nearly zero excitation of electrons into the conduction band at room temperature. P gives a shallower donor level of 0.6 eV from the conduction band minimum. 4,5 This is the reason for the success of making controlled carrier concentration in n-type diamond films by light P-doping. 3 One of the possible research directions of P-doped diamond is to obtain metallic samples because of the possible occurrence of superconductivity as in heavily B-doped diamond. 6,7 Recently, heavily P-doped diamond with P concentration ͑n p ͒ as high as ϳ10 20 cm −3 has been obtained by microwave plasma assisted CVD ͑MPCVD͒. 8 While n p near 10 20 cm −3 is very close to the calculated critical concentration ͑1.2ϫ 10 20 cm −3 ͒ of a metal-insulator transition ͑MIT͒, 9 the resistivity shows semiconducting temperature dependence. This implies that carrier compensation plays a crucial role in P-doped diamond. 10 Theoretical studies predict that P-hydrogen ͑P-H͒ and P-vacancy ͑P-V͒ complexes compensate electrons. 11,12 However, multiple chemical sites of doped-phosphorus atoms have not been experimentally confirmed yet. Note that previous photoemission spectroscopy ͑PES͒ from a phosphorus ion irradiated diamond using monochromated Al K␣ x-ray radiation could not discuss P chemical sites because of a lower signal to noise ratio. 13 In this study, we have performed high-resolution soft x-ray core-level PES to understand P chemical sites in a heavily P-doped diamond. In order to measure very weak signals, we used a very high intensity soft x-ray from a thirdgeneration synchrotron radiation facility, SPring-8. From high-resolution core level PES, we succeeded to observe multiple P 2p core level signals. From the depth dependence of P 2p and spectral analysis, we confirmed the existence of at least two bulk chemical sites in this heavily P-doped diamond, indicating that only a part of doped-phosphorus atoms dopes carriers to the diamond. From the comparison between P 2p core level spectra an...

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