domingo, 25 de julio de 2010

Restoration of the host band-gap by hydrogenation: dilute, amalgamation, and alloy limits

We describe first the effects of hydrogen irradiation on the optical properties of GaAs1−yNy/GaAs epilayers in the very dilute nitrogen limit (y<0.01%). Figure shows the effect of hydrogen irradiation on the sample. Hydrogenation at various H doses, dH, leads to a progressive and finally complete quenching of the Nrelated lines as well as of the broad underlying band. The dH=5×1015 ions/cm2 spectrum closely reproduces that of pure GaAs, where only two bands are observed, namely, the longitudinal optical (LO) phonon replicas of the C-related free-to-bound transition at 1.4934 eV. This H-induced passivation has never been reported before for any isoelectronic impurity, except for a weak reduction in the luminescence intensity of a few N-related lines in GaP:N. Note that a 100% passivation of impurity luminescence bands is hardly attainable even in the common case of H passivation of shallow impurities in GaAs or Si.

We now move to the so-called amalgamation limit, corresponding to the existence of both localized and extended (or Bloch-like) states in the material electronic structure. Figure illustrates such a case for GaAs1−yNy with y=0.1%. The bottom curve shows the PL spectrum of the H-free sample. H irradiation leads first to a passivation of the N cluster states and then to an apparent reopening of the GaAs1−yNy band-gap toward that of the GaAs reference (top curve). As a matter of fact, both the (e,C) and the E− recombination bands converge to those of the GaAs reference. The energy separation between these two transitions decreases with increasing N concentration, most likely dueto the increase of the tensile strain with increasing x. Indeed, for increasing N concentration, the top of thevalence band acquires a more pronounced light-hole character and, in turn, the binding energy of the acceptor impurity decreases. Similar results have been observed in the full alloy limit as shown in the following section.

                                                  ROSSANA C HERNANDEZ C
                                      ELECTRONICA DEL ESTADO SOLIDO