Hydrogen-induced states near the GaAs band edges A. Amore Bonapasta* Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Materiali, Via Salaria Km. 29, 5-CP 10, 00016 Monterotondo Scalo, Italy Mario Capizzi Istituto Nazionale di Fisica della Materia, Dipartimento di Fisica, Universita` di Roma (La Sapienza), Piazzale Aldo Moro 2, I-00185 Roma, Italy Paolo Giannozzi Istituto Nazionale di Fisica della Materia, Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy ~Received 7 May 1998! In the present study the link between the inclusion of H species in GaAs and the appearance in the band gap of near-band-edge energy levels has been investigated. The equilibrium geometries and the electronic struc- tures relative to different locations of H atoms, ions, and dimers in the GaAs lattice have been evaluated by first-principle local-density-functional methods. Atomic arrangements in GaAs identical to those found for different equilibrium geometries of the H species, but with the H species removed, have also been investigated in order to separate the effects of the lattice deformations due to the H inclusion from those produced by the formation of H-host-atom bonds. Discrete near-edge levels appear in the band gap, which are induced by the H interactions with the GaAs lattice as well as by charge effects in the case of H ions. Finally, radiative transitions from the conduction-band to the valence-band near-edge states are shown to account for the main features of the emission bands observed in the photoluminescence spectra of hydrogenated III-V compounds. @S0163-1829~99!01304-1# I. INTRODUCTION Hydrogen efficiently passivates various defects in cova- lent semiconductors, in particular deep and shallow donors and acceptors, and has therefore been the object of a large number of experimental and theoretical investigations. 1–3 The H atom is an interstitial impurity characterized by a small size, an amphoteric behavior, and a high reactivity, features that lead to the rich and complex phenomenology reported for H in semiconductors. Negative and positive hy- drogen ions easily diffuse in covalent and III-V semiconduc- tors and form complexes with shallow and deep impurities, which turn out to be passivated. The existence of the neutral H species in semiconductors has instead been questioned by several theoretical studies in Si and GaAs, which suggest a negative-U behavior of H. 1,4–7 Finally, dihydrogen com- plexes and molecules may also form. 6,8–10 The formation of complexes with the impurities as well as the interaction of H with the host crystal atoms is accompanied by a sizable re- laxation of the host lattice. 1,2 The theoretical estimates of this relaxation are supported by ion channeling 11 and x-ray measurements 12 in hydrogenated, highly doped Si, as well as by muon spin relaxation measurements in GaAs. 13 It is well known that the lattice deformations affect the optical properties of semiconductors. In particular, lattice de- formations may give rise to energy levels near the band edges, which are related to states localized in real space. Discrete broad bands, a few tenths of an eV below the band- gap emission, have been reported in the photoluminescence ~PL! spectra of boron-doped Si treated by reactive-ion etch- ing and plasma etching based on deuterium. 14 All those bands have been attributed to electron-hole recombination in heavily damaged regions, where electron and holes can be localized in potential wells caused by the strain from the hydrogen- or deuterium-induced microscopic defects, of un- known origin. 14 However, similar bands were observed in either n-type or p-type Si exposed to a H plasma where ar- rangements were made in order to eliminate the damage re- sulting from direct immersion in the plasma. 15 In that case, the H-induced PL bands have been related to H stabilized platelets. A new structured band has also been reported a few tens of meV below the band-gap energy in the PL spectra of hydrogenated, doped Si, 16 and of different hydrogenated III-V epilayers and heterostructures. 17–21 These bands are ei- ther unexplained, 16,17 associated to H-plasma damage 21 or tentatively attributed to transitions involving localized levels which are associated to small, local lattice deformations. 18–20 Finally, a continuum distribution of levels near the band edges gives rise to an exponential absorption edge, the Ur- bach’s tail, in amorphous materials. The aim of the present work is to explain the above dis- crete PL bands in terms of transitions between energy levels close to the valence- or conduction-band edges which are associated to the lattice deformations induced by the intro- duction of H species in the host lattice. No detailed theoret- ical study has been performed yet, at least to our knowledge, on this issue. Local-density-functional calculations of equi- librium geometries, electronic charge distributions, and elec- tronic eigenvalues have been performed for different loca- tions of neutral and charged H species in the GaAs lattice. Atomic arrangements in GaAs identical to those found for different equilibrium geometries of the H atom~s!, but that the H atom(s) is (are) removed, have also been considered in PHYSICAL REVIEW B 15 FEBRUARY 1999-I VOLUME 59, NUMBER 7 PRB 59 0163-1829/99/59~7!/4869~12!/$15.00 4869 ©1999 The American Physical Society