Study of n–i–p Axial/Core–Shell Hybrid GaAsSb Dense Nanowire-Based Near-Infrared Photodetectors on Graphene

Abstract

The van der Waals epitaxy integration of III–V compound nanowires (NWs) with graphene substrates is vital for the development of flexible, high-performance, and cost-effective optoelectronic devices. This article details the growth of high-density n–i–p core–shell (C–S) GaAs_1–xSb_x NWs on surface-functionalized monolayer graphene substrates using Ga-assisted molecular beam epitaxy. The impact of Te surfactant on the catalyst droplet, alongside oxygen plasma duration and key growth parameters, namely the lower substrate temperature pausing duration and V/III ratio, is studied, yielding a vertical core GaAs_1–xSb_x NW density of ∼60 μm^–2. Utilizing the optimal parameters, traditional (TCS) and hybrid (HCS) n–i–p C–S architectures are designed, comprising unique axial n-core multiheterostructures with an Sb gradient for bandgap engineering and a high Sb composition near the graphene surface, which is difficult to achieve on Si substrates. The hybrid structure includes an additional intrinsic GaAs_1–xSb_x axial segment over the top of the n-core to enhance absorption and minimize interface effects. High-resolution transmission electron microscopy images and corresponding selective area electron diffraction patterns of these NWs confirm their zinc blend structure. The absence of twins and stacking faults in HCS-configured NWs further attests to their high structural quality. The electrical performance of the ensemble NW devices with the HCS design outperforms TCS, exhibiting a higher responsivity (∼2100 A/W) and detectivity (2.7 × 10¹⁴ Jones), as well as a spectral response extending up to 1.5 μm on graphene. Temperature-dependent C–V and low-frequency noise measurements reveal the HCS photodetector’s good thermal stability, with consistent low capacitance, a low cutoff frequency of ∼6 Hz, and minimal shunt resistance variation with temperature. These results showcase that bandgap engineering of GaAsSb in a 1D configuration, coupled with the versatility of architectures offered by 1D geometry and inherent van der Waals forces in graphene, can be successfully exploited to fabricate high-performance photodetectors, advancing their use in the next era of flexible electronic devices.