Signatures of N incorporation in Raman and optical properties of GaP/GaNP core/shell nanowires

• GaP/GaNP core shell NWs is a novel material system that has been most recently suggested for applications in solar cells. Adding nitrogen not only allow to tune the bandgap energy of GaNP alloy but also causes splitting of conduction band (CB) states, promising for intermediate band solar cells with improved efficiency. The purpose of this work is to investigate effects of N incorporation on band structure of such GaP/GaNxP1-x core/shell NWs using photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. Structural quality of the wires will be also evaluated from Raman measurements.The GaP/GaN0.009P0.991core/shell NWs studied in this work were grown on Si (111) substrates by gas-source molecular beam epitaxy (MBE). The GaP NW cores were grown under the vapor liquid-solid (VLS) mechanism, whereas the GaNP shell was formed via the step-mediated growth. The resulted core/shell NWs were found to have an axial length of about 2.5 μm, a total diameter of about 220 nm, and a typical diameter of the GaP core of ~110 nm. According to performed TEM measurements, the NWs predominantly have zincblende structure with some inclusions of the wurtzite crystal phase. Excellent structural quality of the wires was concluded based on the performed Raman measurements. The Raman scattering spectra were found to contain several first-order Raman modes including intense and sharp peaks at 366 and 403 cm-1 and weaker modes at 387, 397 and 499 cm-1. The first two modes are typical for zinc-blende GaP and are related to transverse-optic (TO) and longitudinal-optical (LO) phonons, respectively. The spectral positions of these modes were unaffected by the N incorporation indicating that the formed GaNP shell is unstrained. The Raman mode at 499 cm-1 peak is related to the Ga-N bond vibrations, confirming the formation of the GaNP alloy. The 397 cm-1 peak can be identify as a surface optical (SO) phonon mode due to its sensitivity to the dielectric constant of an external medium.It is also found that incorporation of N causes a dramatic increase of the PL intensity, which can be easily detected at room temperature even from a single wire. This is accompanied by a shortening of the PL decay time revealed from the performed transient PL measurements. We attribute these changes to the N-induced transformation of the band gap from the indirect one in GaP to a direct band gap in the GaNP alloy. Secondly, N incorporation causes a red shift of the fundament absorption edge revealed via the PLE measurements due to the bowing effect. The red shift of the conduction band (CB) edge is accompanied by a strong blue shift of the Γ CB state. This is ascribed to the splitting of the host CB states that are strongly perturbed by N. The revealed changes in the band structure are potentially beneficial for the applications of GaNP/GaP NWs in novel intermediate band solar cell structures with high efficiency.

Subject headings

NATURVETENSKAP  -- Fysik -- Den kondenserade materiens fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Condensed Matter Physics (hsv//eng)

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