Theory of hydrogen diffusion and reactions in crystalline silicon

Abstract

The behavior of hydrogen in crystalline silicon is examined with state-of-the-art theoretical techniques, based on the pseudopotential-density-functional method in a supercell geometry. Stable sites, migration paths, and barriers for different charge states are explored and displayed in totalenergy surfaces that provide immediate insight into these properties. The bond-center site is the global minimum for the neutral and positive charge states; in the negative charge state, the tetrahedral interstitial site is preferred. The positive charge state is energetically favorable in p-type material, providing a mechanism for passivation of shallow acceptors: electrons from the H atoms annihilate the free holes, and formation of H-acceptor pairs follows compensation. Also addressed are the issues of molecule formation and hydrogen-induced damage. A number of different mechanisms for defect formation are examined; hydrogen-assisted vacancy formation is found to be an exothermic process. ' ' have offered interpretations of the passivation data seeking to unravel the underlying mechanisms. Attempts to explain the observed phenomena led to a number of contradictory assumptions regarding the nature of the charge states of H along its diffusion path, and hence about the H-impurity reactions that can occur. Particular models were advanced for the structure of the hydrogen-impurity complexes that are a result of passivation. The electronic structure of these complexes is such that all impurity levels are removed from the band gap. A complete understanding of the passivation process can only be obtained, however, by considering 39 10 791

Universal alignment of hydrogen levels in semiconductors and insulators

“…This behavior of hydrogen was confirmed and described in great detail with a variety of experimental techniques [8][9][10][11]. In parallel, theory and computation [12][13][14][15] elucidated the physics of hydrogen's interactions with semiconductors, and established the correlation between its atomic and electronic structure. In a compound semiconductor, H + wants to reside in the vicinity of the anion, which can be explained based on Coulomb attraction between the proton and the electronic charge density around the more electronegative anion.…”

“…We obtain the formation energy of interstitial hydrogen in charge state q (where q= −1, 0, or +1) by placing the hydrogen impurity in a volume of host material, calculating the total energy E tot (H q ) of this structure, and subtracting the energy E tot (bulk) of a corresponding volume of pure host material [15,16]:…”

On the Mechanisms of Hydrogen Implantation Induced Silicon Surface Layer Cleavage

“…The calculation were based on the Si-Si bond length change caused by the incorporation of an H atom to the bond-centered site, predicted by theory to be 0.83 Angstrom. [31] The calculation were facilitated by TRIM [75] derived damage and hydrogen distributions, which were modified to provide depth scales compatible with the data presented in figure 5-4 The result of the strain calculations as a function of hydrogen occupation of the bondcentered sites is presented in figure 5-18. As can be seen, the depth of maximum out-ofplane tensile strain increases with increasing amount of H residing at the bond-centered site, as expected considering the increasing influence of H to the total out-of-plane tensile strain.…”

“…Theory predicts a change from the Si-Si bond length of 2.35 Angstrom to a Si-H BC -Si bond length of 3.18 Angstrom. [31] Pre-straining the lattice with radiation damage results in a smaller length change upon H incorporation, thereby reducing the strain energy barrier to this process. Indeed, previous research has shown that the presence of this strain reduces the barrier to place H and H-H between neighboring Si-Si atoms, which lie along the <111> direction.…”

“…(see figure 5-21) Previous research has shown that the blister depth for non-bonded samples is equal to the Ion-Cut depth for bonded samples. [31] The high anneal temperature and long anneal duration were chosen to ensure intense surface blistering. The base vacuum in the annealing treatment was 5 × 10 -8 Torr.…”