In situpassivation of GaAsP nanowires

Abstract

We report on the structural and optical properties of GaAsP nanowires (NWs) grown by molecular-beam epitaxy. By adjusting the alloy composition in the NWs, the transition energy was tuned to the optimal value required for tandem III-V/silicon solar cells. We discovered that an unintentional shell was also formed during the GaAsP NW growth. The NW surface was passivated by an in situ deposition of a radial Ga(As)P shell. Different shell compositions and thicknesses were investigated. We demonstrate that the optimal passivation conditions for GaAsP NWs (with a gap of 1.78 eV) are obtained with a 5 nm thick GaP shell. This passivation enhances the luminescence intensity of the NWs by 2 orders of magnitude and yields a longer luminescence decay. The luminescence dynamics changes from single exponential decay with a 4 ps characteristic time in non-passivated NWs to a bi-exponential decay with characteristic times of 85 and 540 ps in NWs with GaP shell passivation.

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

“…Subsequently, we discuss the structural and optical properties of GaAsP/GaP core-shell NWs, and the origin of the double luminescence peak in these structures (section 3.2). where the droplet crystallization took place, shows a characteristic sequence of different crystal phases, which was reported in the literature 28 : starting with a pure ZB at the NW stem, the crystal first turns into a mixture of cubic and hexagonal phases, then to a pure wurtzite (WZ) phase and finally returns to a ZB phase at the topmost position. Detailed discussion about the crystal quality and crystal phase at different NW-heights can be found elsewhere.…”

“…In our previous study on GaP passivated GaAsP NWs, we observed reproducibly an unintentional GaAsP shell with a lower P content just under the final GaP shell. 28 To determine at which growth stage this unintentional shell is formed and how it affects the light emission from the NWs, we first characterize the structure and the luminescence of GaAsP NWs without the GaP passivating shell (section 3.1). Subsequently, we discuss the structural and optical properties of GaAsP/GaP core-shell NWs, and the origin of the double luminescence peak in these structures (section 3.2).…”

“…Detailed discussion about the crystal quality and crystal phase at different NW-heights can be found elsewhere. 28 In figure 1(b), a cross-sectional high-angle annular dark-field (HAADF)-STEM micrograph of the GaAsP NW in ‹111› zone axis is displayed. The HAADF brightness reveals four cross-sectional regions as indicated in the figure.…”

“…24 On the contrary, there are fewer reports on the shell passivation using III-P materials [25][26][27][28] and it is even more scarce to find a study of core-shell NWs when the core is a ternary alloy. Despite this lack of investigations, ternary alloy NWs and in particular GaAsP NWs [26][27][28] are of great interest for photovoltaic (PV) applications. Indeed, the optimal band gap for a III-V on Si tandem device lies around 1.7 eV 29 and therefore these devices require a ternary alloy for the top cell.…”

“…This binary GaP shell was shown to yield a large enhancement of the luminescence intensity and carrier lifetime. 28 In addition to the luminescence enhancement, passivation with this specific GaP shell was shown to result in a double emission peak in the GaAsP spectrum, which was at that time not fully understood. Such a spectral change is important for device applications and therefore it is crucial to understand its origin.…”