Characterisation of GaAsN layers grown by MOVPE

Mixed narrow band gap III–V alloys such as GaAsN and InGaAsN are attracting considerable interest for both their fundamental properties and the possibility to use them in novel solar cells for space applications.It has been shown that concentration of nitrogen as low as 0.01 lowers the band gap of the Ga1−xInxAs1−yNy and Ga1−xInxP1−yNy alloy by a significant fraction of eV, showing a “giant” bowing effect. The alloy Ga1−xInxAs1−yNy with y=0.35x is lattice-matched to GaAs (or Ge), and this material was used both as active layer for 1 eV laser and as fourth junction Ge lattice-matched in high-efficiency InGaP/GaAs/InGaAsN/Ge multijunction solar cells. The growth of GaAsN and the understanding of its physical properties is a necessary step to control the growth of the quaternary InGaAsN alloy.Undoped GaAs1−xNx layers were grown by means of a home-made MOVPE horizontal reactor on both GaAs and Ge substrates using Arsine and tri-methyl-gallium (TMG) precursors and hydrogen as carrier gas. Di-methyl-hydrazine (DMHy) was used as nitrogen source. The samples were grown at different temperatures (475–500 °C) and pressures (45–750 Torr), varying the molar ratio between DMHy and the other precursors.Samples were characterised by AFM, micro-Raman and spectral imaging Cathodoluminescence (CL) techniques; high temperature annealing (700–750 °C) were performed on samples in order to optically activate the N. Phase separation between GaAs and GaN was observed after annealing at high temperature.