domingo, 25 de julio de 2010


Figure shows the low-temperature (T=10 K) photoluminescence spectra of a representative series of as-grown samples whose N concentration varies over about two orders of magnitude. At the very early stage of N incorporation in GaAs (N concentration less than 0.01%, bottommost spec-trum in Figure ), the low-temperature PL spectra are characterized by a number of sharp lines (line width ≈0.5 meV) between 1.40 and 1.48 eV, which are due to the recombination of excitons localized on N complexes. These lines are attributed to carrier recombination from electronic levels due to N pairs and/or clusters  and are superimposed on a broad band also related to N doping. The luminescence intensity associated with these transitions varies from line to line and increases with y (not shown here). An exact assignment of each line to a given N complex is made rather

difficult by the strong dependence of the material optical properties on the growth conditions, as extensively reported in the literature.

Free-electron to neutral-carbon acceptor (e,C) and free-exciton (E−) recombinations of GaAs are observed at 1.493 eV and 1.515 eV, respectively. As the nitrogen concentration is increased further (y=0.043 and 0.1%), the energy of the excitonic recombination from the material's band gap, E−, as well as the (e,C) recombination band start redshifting very rapidly, coexisting with and taking in the levels associated with the N complexes, whose energies do not change with N concentration. This highlights the strongly localized character of the N isoelectronic traps, contrary to that of shallow impurities, whose wavefinctions overlap at smaller concentrations (1016–1018 cm−3). Eventually, at higher N concentrations (alloy limit, y>0.1%), the GaAs1−yNy band-gap keeps redshifting along with the C-related states. The dramatic variation of the GaAs host band-gap with y is accompanied by other major effects on the electronic and optical properties of GaAs1−yNy. Indeed, with increasing N concentration, the electron effective mass increases , a sizable Stokes shift between emission and absorption is observed , the band-gap dependence on temperature and hydrostatic pressure decreases, and N resonant states move to higher energy. In the following discussion, we show that most of the above-mentioned changes can be fully and reversibly counteracted by irradiation with atomic hydrogen.

                                        ROSSANA HERNANDEZ
                              ELECTRONICA DEL ESTADO SOLIDO

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