Strained GaAs/InGaAs Core-Shell Nanowires for Photovoltaic Applications
© Moratis et al. 2016
Received: 10 December 2015
Accepted: 21 March 2016
Published: 1 April 2016
We report on the successful growth of strained core-shell GaAs/InGaAs nanowires on Si (111) substrates by molecular beam epitaxy. The as-grown nanowires have a density in the order of 108 cm−2, length between 3 and 3.5 μm, and diameter between 60 and 160 nm, depending on the shell growth duration. By applying a range of characterization techniques, we conclude that the In incorporation in the nanowires is on average significantly smaller than what is nominally expected based on two-dimensional growth calibrations and exhibits a gradient along the nanowire axis. On the other hand, the observation of sharp dot-like emission features in the micro-photoluminescence spectra of single nanowires in the 900–1000-nm spectral range highlights the co-existence of In-rich enclosures with In content locally exceeding 30 %.
KeywordsCore-shell GaAs/InGaAs nanowires Nanostructure characterization Micro-Raman Photoluminescence Cathodoluminescence Micro-photoluminescence
Semiconductor nanowires (NWs) and NW-based heterostructures are currently under extensive research, due to the unique properties and prominent quantum phenomena emerging from them. For instance, there are numerous reports of NW structures containing quantum dots, which can act as efficient sources of single photons or entangled photon pairs [1–5]. Another application area where NW arrays attract wide interest is in next-generation cost-effective and high-efficiency photovoltaic (PV) devices, based on two main reasons: first, their relaxed lattice-matching requirements, due to easy strain accommodation at the NW free surface, providing flexibility in substrate selection and band-gap engineering. Second, the possibility for lesser material utilization, due to enhanced light absorption in NW arrays, based on their inherent anti-reflecting properties , increased light trapping within the array , and resonant wave-guiding properties . Recently, a single-junction solar cell based on InP NWs  with an efficiency η ≈ 13.8 % and a single GaAs NW PV device  with η ≈ 40 % have been reported, showing the real potential of NW solar cells to compete directly with other thin-film solar cell technologies. Another direction worth exploring for NW solar cells is the possibility to utilize piezoelectric (PZ) fields present in strained NW heterostructures, in order to obtain enhanced NW-based PV devices. As first estimated by Boxberg et al. , a significant axial PZ field of 60–80 kV/cm can develop in InAs/InP core-shell NWs, which can be used for efficient carrier sweeping towards the device electrodes. As we have recently shown in , axial PZ fields of the order of 7–8 kV/cm may even exist in core-shell GaAs/AlGaAs NWs, having a very small lattice mismatch of 0.05 %. Such fields are already significant for the operation of a PV device and should be taken into account in any related device work. In this work, we report on the preliminary results of the growth and characterization of strained core-shell GaAs/InGaAs NWs, a NW heterostructure which is expected to exhibit pronounced PZ effects.
GaAs/InGaAs core-shell NWs are grown in a VG Semicon V80H III-As solid-source molecular beam epitaxy (MBE) system via the vapor liquid solid (VLS) mechanism on n+ Si (111) substrates (manufactured by Siltronix). The native oxide-capped Si substrate is loaded into the MBE growth chamber without prior chemical treatment and is annealed in situ at 650 °C for 10 min in order to create pinholes in the native oxide layer that act as nucleation sites for GaAs NWs. The core-shell configuration is achieved by first growing GaAs NWs (“core”) and then encapsulating the GaAs core by a two-dimensional (2D) epitaxial growth of InGaAs shell. A substrate temperature of 600 °C is used for the growth of the GaAs core NWs. Ga droplets are deposited in situ at a flux of 0.4 ML/s for 20 s to act as catalysts that initiate the VLS growth of the GaAs core NWs. Note that the Ga and As fluxes are quoted in terms of the equivalent 2D growth rates estimated from RHEED oscillation analysis during Ga- and As-limited GaAs growths on GaAs (100) surfaces. Following Ga droplet deposition, the shutter of the Ga Knudsen cell (K-cell) is closed for 20 s to allow Ga to relax on the substrate surface and form droplets of uniform diameter. The growth of the GaAs NWs is performed for 30 min using a Ga flux of 0.4 ML/s and an As/Ga flux ratio of ~2. These growth parameters result in GaAs core NWs with a diameter of ~60 nm, height of ~3 μm, and density of 2 × 108 cm−2.
InGaAs shell growth duration and nominal In content of the samples grown for this study
Set of samples
InGaAs shell growth duration (min)
Nominal In content (%)
40 for all 3 samples
2, 5, 9.5
1, 2, 4
9.5 for all 3 samples
5, 10, 20
9.5 for all 3 samples
The surface morphology and the structural characteristics of the NWs are investigated by performing field emission-scanning electron microscopy (FE-SEM) on the as-grown samples. The optical properties of the samples are studied by performing macro-PL measurements using a 325-nm He-Cd Kimmon laser. The macro-PL experimental setup consists of a closed circuit liquid He-cooled cryostat in which the sample is mounted and excited by a laser beam focused using a 10-cm quartz lens. The excitation power used for PL is limited to 0.23 mW because of photo-bleaching effects due to the activation of surface-related non-radiative channels , which are especially pronounced in the GaAs core NW reference samples. The PL signal is analyzed by a 0.5-m spectrograph with a 600 gr/mm grating and is recorded by a liquid nitrogen (LN)-cooled charge-coupled device (CCD) camera.
In addition, micro-Raman, cathodoluminescence (CL) and micro-PL measurements have been performed on individual GaAs/InGaAs core-shell NWs dispersed on Si substrate with gold metal grid to assist with the identification and tracking of individual NWs of interest. A Nicolet Almega XR micro-Raman setup is used for the micro-Raman measurements, which consists of a 473-nm diode laser with the beam focused to a spot size of 0.5 μm using a ×100 microscope objective with 1.25 numerical aperture (NA). The Raman spectra are analyzed by a 2400-gr/mm grating and are recorded by a CCD camera. For the micro-PL measurements, the sample (with NWs dispersed on grid) is mounted on a LN flow cryostat and excited using the 750-nm cw line of a tunable Ti:Sapphire laser, with the beam focused to a spot size of ~1 μm using a ×40 microscope objective with 0.65 NA. The micro-PL signal is analyzed by a 0.75-m spectrograph utilizing a 600-gr/mm grating and is recorded by a back-thinned LN-cooled CCD camera with high quantum efficiency. CL measurements at 5 K are carried out in a FEI Inspect F50 FE-SEM system, which is equipped with a Gatan cryogenic stage and a custom in-house-made light collection system. The CL mappings are recorded over a ±3-nm spectral window using a Horiba IHR 550 spectrometer equipped with a photomultiplier.
Results and Discussion
Next, we attempt to characterize the actual incorporation and distribution of In in the core-shell NWs. Energy-dispersive X-ray (EDX) analysis on a range of relatively large areas (1–105 μm2) of the as-grown samples reveal In contents that are in good agreement with the nominal values. For instance, EDX gives an In content of 7.5 % for the sample of Fig. 2, compared with the 9.5 % nominal concentration. However, due to the relatively low NW density, most of the In signal in such EDX scans originates from the polycrystalline InGaAs 2D layer formed at the interface with the Si substrate in between the NWs. In other words, the EDX results may not necessarily represent the true level of In incorporation in the InGaAs NW shell. EDX measurements on single NWs were not conclusive because the In signal was below the detection limit of the system.
where, aside from the elastic constants and deformation potentials of the core material, f is the lattice mismatch between GaAs and InGaAs and (1 − η) is the cross-sectional area ratio between the InGaAs shell and the full core-shell structure, including the GaAs cap layer. The dependence on the shell thickness is contained in this area ratio. Using GaAs parameters , the average core and core-shell NW diameters and a 3-nm GaAs cap layer thickness, we can reproduce the data as represented by the solid line in Fig. 4d, assuming a lattice mismatch of ~0.1 %. This value corresponds to an average In concentration in the shell layer of merely ~1.5 %, in good agreement with the micro-Raman results.
Strained core-shell GaAs/In x Ga1−x As nanowires with nominal In content between 2 and 9.5 % have been successfully grown on Si (111) substrates, with the majority of the nanowires growing vertical to the substrate. The as-grown nanowires are 3–3.5 μm long, have a density of about 108 cm−2, a core diameter of ~60 nm, and an InGaAs shell thickness ranging from 1 to 50 nm, depending on the shell growth duration. By applying a range of optical and structural characterization techniques, we conclude that the In incorporation in the nanowires is on average significantly smaller than the nominal In content estimated from 2D InGaAs growth calibrations. The In content also exhibits a gradient along the nanowire axis, with higher In content at the tip compared with the lateral facets, which can possibly be attributed to different In incorporation rates in the various crystallographic planes of the nanowire surface. Moreover, the dot-like emission spectra of single nanowires show evidence of the formation of In-rich enclosures with an In content higher than 30 %.
This work is funded by the Chair of Excellence program of the “Laboratoire d’excellence LANEF in Grenoble (ANR-10-LABX-51-01)” and the European Social Fund and National resources through the THALES program “NANOPHOS,” the ARISTEIA II program “NILES,” and the PROENYL research project, Action KRIPIS, project MIS-448305.
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