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Hydrogen in diamond

Hydrogen in semiconductors has attracted much attention the last two decades, and appears to induce fundamental changes in the electronic properties of the host material. The role played by hydrogen in the formation of defects has been addressed, and its interation with the defects existing in the sample or with impurities has been investigated. It was found, for example, that dangling bonds existing on grain boundaries or point defects can interact with hydrogen and be neutralized. The presence of hydrogen could therefore, in that manner, reduce the density of state in amorphous diamond [73], for instance. It can also decrease the hole concentration in p-type semiconductor [74], or neutralize dopant in n-type materials [75].

In CVD grown diamond, the abundance of hydrogen is due to the growth conditions themselves, since its presence in the ambient plasma is required to promote diamond bonding over graphite bonding. Landstrass et. al. have experimentally shown [76] that the behavior of diamond subjected to the action of hydrogen from a hydrogen plasma is very similar to that of diamond film. In that case, hydrogen passivates electrically active defects, resulting in a substantiel reduction in the resistivity.

When doped with boron, the CVD grown diamond has a p-type character, and the presence of hydrogen makes difficult the control of this experiment. Indeed, it has been shown that hydrogen is attracted to boron in p-type diamond [58]. This passivation process is possible if one considers first the formation of H+ ions by the compensation of free holes by electrons. These free holes are created by the partial ionization of boron acceptors, which leads also to the formation of B- ions. The coulombic interaction between H+ and the negatively charged acceptor impurities B- would then lead to the formation of acceptor-hydrogen complexes according to [58]

\begin{displaymath}H^+ + B^- \rightarrow (HB)^o.
\end{displaymath} (5.1)

The compensation of free holes is possible only if the energy levels induced by the presence of hydrogen, are located above the fundamental level of boron. The migration of hydrogen in diamond should also be essential for the formation of (HB)$^{\rm o}$. A detailed comprehension, at an atomistic level, of H in diamond is thus of basic importance, and requires understanding the relative energetics of different possible sites for hydrogen in diamond, and its motion in the diamond lattice. The energies of several interstitial sites were recently calculated by different groups, using first principle and semiempirical methods [77,78,16]. In these calculations, first and second neighbor lattice relaxations [77,78] were applied to carbon clusters terminated by H atoms, and full relaxation to carbon supercells [16]. The Bond Center (BC) site was found to be lower in energy than the tetrahedral (T$_{\rm d}$) site, by amounts of 2.7 eV [77], 1.9 eV [78], and 1.6 eV [16]. More recently [79], ab initio calculations showed that the energy of a H at the BC site is lower by 0.95 eV than at a T$_{\rm d}$ site. Different pathways for the motion of H in diamond were also considered [16,80], and various barriers were predicted for the migration of H in diamond, ranging from 0.4 eV to 5.1 eV.

The location of H in semiconductors (C, Si) is best determined experimentally by electron paramagnetic resonance (EPR) [81] or by the measurements of the location of muonium (the light pseudoisotope of H) in the crystal [82]. The EPR signal in CVD diamond [81] reveals the presence of dangling bonds associated with hydrogen atoms. Muon spin resonance ($\mu$SR) measurements [82] indicate that two paramagnetic forms of muonium exist: the ``normal'' muonium (Mu), with an isotropic hyperfine interaction, and the ``anomalous'' muonium (Mu$^\star$), with an anisotropic hyperfine interaction. For Si, it is well accepted [83] that Mu resides on a T$_{\rm d}$ site, and Mu$^\star$ on a BC site. The experimental situation for muons in diamond is less clear, with the location of Mu$^\star$ not unambiguously established [84]. In contrast to the case of H in Si, for which the calculations fully support the experimental findings [83], for diamond, calculations failed to precisely verify the measured hyperfine interaction [84], or appear to be fortuitously good considering the small cluster investigated [85].

The electronic states induced by hydrogen depend on the interstitial site occupied. Simple molecular bonding arguments can explain the position of the energy levels obtained. In the case of hydrogen in a bond center site in diamond, the bonding states of the carbon atoms in diamond can couple to the 1s state of hydrogen to form one occupied state in the valence band, and one corresponding unoccupied state in the conduction band. The remaining antibonding states of the carbon atoms creates a defect level in the upper part of the energy gap [83]. Therefore, if the bond center is indeed the most favorable site for hydrogen in diamond, the energy level induced should be above the fundamental level of boron. Neutral hydrogen atom in boron-doped diamond will thus contribute its electron to the compensation of free holes from the ionization of boron acceptors, leading to the formation of (BH)$^{\rm o}$ complexes. This process results in the neutralization of the dopant atoms by hydrogen, which prevents the doping control of diamond.

next up previous contents
Next: The goal of the Up: Impurities and defects in Previous: The <100> split-interstitial
David Saada