Role of strain and properties of N clusters at the onset of the alloy limit in GaAs 1-x N x A. Polimeni,* F. Masia, † G. Pettinari, R. Trotta, M. Felici, ‡ and M. Capizzi CNISM and Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 2, 00185 Roma, Italy A. Lindsay and E. P. O’Reilly Tyndall National Institute, Lee Maltings, Cork, Ireland T. Niebling and W. Stolz Department of Physics and Material Sciences Center, Philipps-University, Renthof 5, 35032 Marburg, Germany P. J. Klar Institute of Physics I, Justus-Liebig-University, Heinrich-Buff-Ring 16, 35392 Giessen, Germany Received 21 December 2007; revised manuscript received 13 February 2008; published 28 April 2008 In GaAs 1-x N x , the band gap energy decreases very rapidly with x and the electron effective mass shows a quite unusual compositional dependence characterized by a sudden doubling for x  0.1%. In this work, we investigate the origin of this behavior by photoluminescence measurements under hydrostatic pressure in as-grown and hydrogenated GaAs 0.9989 N 0.0011 samples. First, we show that two nitrogen pair states emitting at 1.488 and 1.508 eV contribute mainly, but to a different extent, in determining the steep increase in the electron mass observed for x  0.1%. Tight-binding supercell calculations assign the 1.488 eV levels to iso- lated N pairs and the 1.508 eV states to N pairs perturbed by a nearby N atom, in disagreement with previous attributions but consistent with the electron mass data. Second, photoluminescence at high hydrostatic pressure discloses that these N pair states show quite different rates of passivation by hydrogen. By combining these findings with the calculated lattice energies associated with each N complex, we conclude that strain relaxation is a key mechanism driving the interaction of hydrogen with N atoms in GaAs 1-x N x . DOI: 10.1103/PhysRevB.77.155213 PACS numbers: 78.55.Cr, 71.55.Eq, 71.20.Nr, 71.15.m I. INTRODUCTION GaAs 1-x N x / GaAs is an interesting semiconductor system since its optical and transport properties display several un- usual behaviors as the nitrogen concentration x is increased from 0 to 6%, where phase separation effects eventually occur. 1,2 In particular, the interaction of spatially localized levels of N with extended states of the conduction band CB of the GaAs host determines a large counterintuitive decrease with x of the GaAs 1-x N x band gap. 3 As a result of this inter- action, the entire CB undergoes a distortion, 4 which could be exploited for the fabrication of terahertz sources. 5 Initially, the above mentioned band gap reduction was accounted for in terms of an anticrossing interaction between the GaAs CB states and a single N level located at about 160 meV above the conduction band minimum. 6 The inadequacy of the band anticrossing model in describing the compositional depen- dence of the whole electronic properties of GaAs 1-x N x was later evidenced by a series of experiments. 7–10 For instance, accurate measurements of the electron effective mass 9 m e and gyromagnetic factor 10 g e as a function of x revealed strong deviations from the smooth dependence predicted on the basis of a two-level anticrossing. In particular, m e doubles as soon as x = 0.1% and displays a nonmonotonic dependence for greater x values. On the theoretical side, the crystal symmetry breaking induced by various N complexes NCs was taken into account in pseudopotential calculations by Kent and Zunger. 11 Those authors showed that N incor- poration leads to the formation of N-related deep states in the gap and to a strong perturbation of the states of the GaAs host. This perturbation results in a band gap decrease in GaAs 1-x N x , as well as in an enhanced localized character of the CB minimum. Later, Lindsay and O’Reilly highlighted the crucial role of NC states in determining the value of the electron mass. 12 In particular, it was found that the peculiar compositional dependence of m e was ascribable to a mixing of the CB states with k-space delocalized levels due to dif- ferent specific nitrogen complexes pairs, triplets, and possi- bly higher-order clusters. As a matter of fact, when the red- shifting CB edge crosses specific NC states, there is a loss of  character of the CB minimum and an ensuing increase in m e . 9–12 A particular attention has been dedicated to the composi- tional interval around x = 0.1%, where, apart from the men- tioned doubling of the electron effective mass, other impor- tant changes in the electronic properties of GaAs 1-x N x occur. For example, the scaling law ruling the band gap reduction E g varies from E g  x for x  0.15% to E g  x 2/3 for x  0.15% see Ref. 13. In this compositional interval, also the emission spectra at low temperature T  40 K exhibit marked qualitative changes. In fact, for x  0.2%, individual recombination lines related to N complexes are not anymore observable since they have been taken in by the redshifting CB edge 11 and the free-exciton recombination is not spec- trally distinguishable from a broad emission band skewed to lower energy. 14–17 This band arises from excitons localized on potential fluctuations, as usual for semiconductor alloys, 18,19 thus rendering the spectral characteristics of GaAs 1-x N x somewhat more similar to those of “conven- tional” alloys. Optical measurements under hydrostatic pressure P pro- vided several valuable information on the unusual electronic PHYSICAL REVIEW B 77, 155213 2008 1098-0121/2008/7715/1552138 ©2008 The American Physical Society 155213-1