Genesis of “Solitary Cations” Induced by Atomic Hydrogen

Abstract

Substitution of constituent atoms and/or changes of crystal structure are routinely used to tailor the fundamental properties of a semiconductor. Here, it is shown that such a tailoring can also be realized thanks to a novel hydrogen effect. Four hydrogen atoms can screen the effect the crystal potential has on a constituent cation, thus generating a solitary cation : an effectively isolated impurity, so chemically different from the unscreened constituent cations that it strongly perturbs the electronic properties of the material by increasing its fundamental band-gap energy. Such a hydrogen-induced screening effect is removed by thermal treatments, thus permitting reversible modifi cations of both the "crystal chemistry" and material's properties. This phenomenon, observed in InN and other topical nitrides, should permit the development of a new class of materials as well as the fabrication of photonic devices and optical integrated circuits with distinct, tailor-made regions emitting or absorbing light, all integrated onto a monolithic semiconductor structure.

Exceptional Hydrogen Uptake in Crystalline InxGa1–xN Semiconductors

“…In this case, hydrogenation does not induce any modification of the spectrum compared to the as-grown one, despite a very high H dose being used (d H = 3 × 10 18 ions cm –2 ). This observation also parallels what was observed via PL spectroscopy: the smaller x is, the smaller the blue-shift of the emission peak after hydrogenation, with the effect totally disappearing around x ∼ 0.49. − Figure c and d show the XANES spectra taken on two sample series with intermediate In concentrations ( x = 0.74 and x = 0.62, respectively) and hydrogenated at four doses (increasing from down to up). For these intermediate concentrations, the effects of hydrogenation on the XANES (transformation of the prepeak into a shoulder and smoothing of the other features at higher energies) are clearly observed only for the highest hydrogenation dose.…”

“…As noted in previous works, , the formation of In* complex induces strain on the structure, similar to what found with other H-related complexes in highly mismatched alloys . To assess the effect of this strain on the system’s energetics and formation energy at varying defect concentrations, the volume of the supercells was optimized for each simulated defect concentration.…”

“…InN is a degenerate concentrated nitride semiconductor characterized by high concentration of acceptors and donors . When solitary cation form in hydrogenated InN and In-rich In x Ga 1– x N, H concentration has been predicted to be very high based on the band gap opening produced but still not at the level observed in this present work …”

“…Despite inserting only one In* defect in a supercell of 100 atoms, where the other In atoms are not surrounded by H, the spectrum is dominated by the local configuration of the In* atom at the center of the cell. On the other hand, the relative insensitivity to x in the In* simulations supports the consideration that a solitary cation is somehow electronically isolated from the surrounding environment due to bonding with the H atoms …”

“…As such, Ω f tells us that the formation of the In* complex is not only possible but also favored, especially when the hydrogen flux is high enough. We have already shown , experimental evidence of the presence of In* complexes in InN and InGaN alloys with a slightly positive formation energy. In the present paper, the energy price to be paid is calculated at about 0.5 eV per H atom, that is of the order of magnitude of an hydrogen-bond (see for instance ref …”