New isoelectronic trap luminescence in gallium phosphide

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Abstract
The authors report a new bound-exciton (BE) series observed in absorption and luminescence of GaP vapour-grown needles containing significant NP impurities. Local-mode phonon replicas of the lowest-energy BE line indicate that the centre contains an isoelectronic (BGa-NP) associate. Remarkably strong ordering of the (111) axis of this associate exists relative to the (111) growth axis and (111) faces respectively in triangular needles and blade-morphology crystals. Strong polarity effects also occur; these centres are significant only in the P-terminated (111B) crystal face! Aside from these strong inhomogeneities, very unusual for a cubic (zincblende) crystal lattice, the Zeeman properties of these no-phonon lines are remarkably similar to several previously reported 'molecular' isoelectronic traps of C3v symmetry in GaP. The Gamma 9 (mJ=+or-3/2) hole states lies lowest, with a strong tensional local crystal-field splitting compared with electron-hole exchange. It is suggested that the BE series is due to interacting pairs, either (BGa-NP)-NP or (BGa-NP)-(BGa-NP), and that isolated (BGa-NP) does not produce a bound state. Three additional BE series are also reported without detailed study.
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