Correlated optical and structural analyses of individual GaAsP/GaP core–shell nanowires

Abstract

We report on the structural and optical properties of GaAs 0.7 P 0.3 /GaP core-shell nanowires (NWs) for future photovoltaic applications. The NWs are grown by self-catalyzed molecular beam epitaxy. Scanning transmission electron microscopy (STEM) analyses demonstrate that the GaAsP NW core develops an inverse-tapered shape with a formation of an unintentional GaAsP shell having a lower P content. Without surface passivation, this unintentional shell produces no luminescence because of a strong surface recombination. However, passivation of the surface with a GaP shell leads to appearance of a secondary peak in the luminescence spectrum arising from this unintentional shell. The attribution of the luminescence peaks is confirmed by correlated cathodoluminescence and STEM analyses of the same NW.

Influence of surface passivation on the electrical properties of p–i–n GaAsP nanowires

“…17,18 One serious candidate for the top cell material in III-V/Si tandem devices is GaAsP. Several investigations of GaAsP NWs have been reported; 10,[19][20][21][22][23][24] however, the efficiency of GaAsP NW solar cells remains low. This is due to a more complex control over the optical and electrical properties of a ternary alloy and to a lack of knowledge of several fundamental parameters.…”

“…The surface recombination can be strongly reduced by either ex situ or in situ NW passivation. In the literature, analyses of the surface passivation mainly focus on the NW optical properties 19,22,23 with only a few assessments of the electrical properties. 14,21,27,28 In particular, there is only one report on the impact of passivation on the electrical properties of GaAsP NW solar cells; however, it is done for a core/shell architecture, while the passivation of axial junction GaAsP NWs remains unexplored.…”

“…22 In the present study, this passivation procedure is applied to active p-i-n junctions. To grow the shell, the Ga droplets were crystallized under element V flux in the presence of Be doping and then the GaAsP NW cores were in situ surrounded with an undoped 5-nmthick GaP shell at a substrate temperature of $500 C. 22,23 The absence of the catalyst and the low substrate temperature favor the NW lateral expansion by the vapor-solid growth. 30,31 The GaP shell growth was performed by opening the Ga and P shutters simultaneously, at the same Ga flux and V-to-III ratio as used for the NW core, for 2 min.…”

“…Indeed, structural and optical characterizations on undoped GaAsP NWs grown under similar conditions showed the formation of an unintentional GaAsP shell due to a parasitic lateral growth by the vapor-solid mechanism, which accompanies the axial NW growth. 23,24 This shell was shown to be optically inactive in bare NWs, but it produced luminescence in NWs passivated with a GaP shell. 23 In the present case of the p-i-n junction NWs, the unintentional shell formed around the base part during the deposition of the top pdoped segment is expected to be p-doped.…”

“…23,24 This shell was shown to be optically inactive in bare NWs, but it produced luminescence in NWs passivated with a GaP shell. 23 In the present case of the p-i-n junction NWs, the unintentional shell formed around the base part during the deposition of the top pdoped segment is expected to be p-doped. In addition to the vapor-solid unintentional growth, a p-doped shell can also be formed by the in-diffusion of the Be atoms impinging the NW sidewalls during the p-doped segment growth.…”