Impurity band structure of boron-doped homoepitaxial diamond

Abstract

Impurity band structure of boron-doped diamond is investigated using p + / p − / p + mesa structures with thin p − layers, where p + is heavily doped diamond with variable range hopping conductivity and p − is less-doped diamond with valence-band conductivity. From the frequency and temperature dependence of the conductivity of the mesa structures, it is found that holes injected into the p − layer stay at the 2p states of boron for an extremely long time and contribute to the conductivity. From the equivalent circuit analysis it is concluded that, when the boron concentration exceeds ϳ4 ϫ 10 18 cm −3 , the 2p wave functions of boron begin to overlap, and an impurity band forms at 2p states, which is located ϳ0.07 eV above the valence-band maximum. Simultaneously, the Fermi level rises to the impurity band by a strong electron-optical phonon interaction and a variable range hopping is realized in this band.

Evolution of the Impurity Band to Diamond-Like Valence Bands in Boron Doped Diamond

“…Simultaneously, the Fermi level rises to this impurity band and the relaxation time of the excited holes decreases drastically. 12,13) Takeuchi et al obtained similar results of the impurity band structure with the carrier concentration close to and below N c using a total photoelectron emission yield spectroscopy. 14) This is a remarkable feature of diamond and it is important to understand how the impurity band evolves into solid bands in boron doped diamond.…”

“…In previous papers we showed how boron impurity evolved from isolated level to an impurity band. 12,13) Now, the first principles calculation shows that the impurity band continues to evolve into the solid valence band through Mott transition. Here, we present an idea of the evolution, schematically shown in Fig.…”

“…At N c1 the average distance between borons is µ4 nm, and the 2p wavefunction starts to overlap that of the neighboring boron atoms, and E F rises to the E 2p position. 13) Hence, the supercell condition is partially satisfied and the 2p wavefunctions have k dependence. The appearance of k dependence is indicated by the dashed line.…”

“…This is because the Fermi level rises to the 2p excited states and the relaxation time of the excited carriers decreases drastically. 13) As the temperature decreases, the absorption coefficient at the peak increases by 20% and a shoulder at ³0.3 eV becomes obvious, which is caused by the increase of the absorption component at 0.305 eV. Such weak temperature dependence of the absorption spectra suggests that despite some boron remaining as isolated impurity, most boron forms the impurity band.…”

Boron‐doped Homoepitaxial Diamond CVD from Microwave Plasma‐Activated Ethanol/Trimethyl Borate/Hydrogen Mixtures

“…The peak at 2460 cm −1 is identified as the first excited‐state transition of the boron dopant, the broad peak observed at ∼2800 cm −1 involves the unresolved second and third excited transitions of the boron acceptor, while the peak at 4125 cm −1 is the one‐phonon‐assisted electronic transitions of excited states displaced by one diamond optical phonon. All boron‐related bands in the samples under study are broader than the corresponding peaks in lightly B‐doped single‐crystal diamonds due to electron‐phonon interaction at the boron sites 22. Quantifying the total neutral boron acceptors, we used an amplitude of 1280 cm −1 one‐phonon band and the integral of the 2800 cm −1 absorption peak.…”