Lattice-Matched InGaAs–InAlAs Core–Shell Nanowires with Improved Luminescence and Photoresponse Properties

Core–shell nanowires (NW) have become very prominent systems for band engineered NW heterostructures that effectively suppress detrimental surface states and improve performance of related devices. This concept is particularly attractive for material systems with high intrinsic surface state densities, such as the low-bandgap In-containing group-III arsenides, however selection of inappropriate, lattice-mismatched shell materials have frequently caused undesired strain accumulation, defect formation, and modifications of the electronic band structure. Here, we demonstrate the realization of closely lattice-matched radial InGaAs–InAlAs core–shell NWs tunable over large compositional ranges [x(Ga)∼y(Al) = 0.2–0.65] via completely catalyst-free selective-area molecular beam epitaxy. On the basis of high-resolution X-ray reciprocal space maps the strain in the NW core is found to be insignificant (ε < 0.1%), which is further reflected by the absence of strain-induced spectral shifts in luminescence spectra and nearly unmodified band structure. Remarkably, the lattice-matched InAlAs shell strongly enhances the optical efficiency by up to 2 orders of magnitude, where the efficiency enhancement scales directly with increasing band offset as both Ga- and Al-contents increase. Ultimately, we fabricated vertical InGaAs−InAlAs NW/Si photovoltaic cells and show that the enhanced internal quantum efficiency is directly translated to an energy conversion efficiency that is ∼3–4 times larger as compared to an unpassivated cell. These results highlight the promising performance of lattice-matched III–V core–shell NW heterostructures with significant impact on future development of related nanophotonic and electronic devices.


x(Ga)/y(Al) NW
. Summarized dimensions (length, diameter) of the InGaAs NWs (core-only) and InGaAs-InAlAs core-shell NWs for the various different x(Ga)/y(Al). Also, for the core-shell NW structures the shell thickness and the length of a top segment that extends in the axial growth direction beyond the InGaAs NW core are given as extracted from the SEM data. Mean values and standard deviation are derived from approximately 20 NWs measured for each sample.

Finite element modelling (FEM) Simulations of XRD patterns
In order to analyze potential contributions from the InAlAs shell to the overall very small strain components, we correlated the Bragg peak positions of the XRD patterns with simulations. This allows us to estimate the approximate composition of the InAlAs shell. In particular, we exemplify this procedure for the NW sample with highest x(Ga)/y(Al) ~ 0.64.
For the lattice parameters of the core-only NWs, we evaluate the strain with respect to pure WZ material from the Bragg peak positions, assuming a biaxial strain state due to the mutual strain of ZB and WZ segments. For the core-shell NWs, the diffraction peaks are dominated by the signal from the core, as the shell thickness is very small. Hence, we see the influence of the shell only via shifts of the core Bragg peak. To quantify the shell composition, we performed finite element modelling (FEM) of the NWs using the geometrical parameters obtained from SEM and TEM. The chemical composition of the core was fixed to that of the respective core-only NWs, and the shell composition was varied between 0.4 < y(Al) < 0.8.
From the resulting strain distribution, we calculated the x-ray diffraction patterns and determined the Bragg peak shifts as a function of the shell composition, as shown in Figure   S2. We find that all samples exhibit only very small peak shifts, and the maximum deviation of shell composition y(Al) from the core composition x(Ga) is 7%. This corresponds to inplane strain values of the core below 0.1% and below 0.4% for the shells.

Photoresponse and EQE measurements
We measured the spectral photo-response and external quantum efficiency (EQE) of the surface passivated InGaAs-InAlAs core-shell NW device. This was realized by dispersing the light emitted from a tungsten-halogen lamp via a monochromator (Spex 340E, 1200 1/mm) onto the sample. The resulting wavelength-dependent photocurrent was detected by a pyroelectric detector and lock-in technique without bias. Calibration was further performed against a standardized Si solar cell, allowing to derive absolute values for EQE. As displayed in Figure S3 a photo-response is observed to illumination in the range of ~ 700 -1100 nm, with a maximum EQE of ~15 % at ~900-1000 nm which is typical for such NW/Si PV cell [1,2]. Approaching the Si absorption edge (> 1100 nm) the photoresponse is drastically decreased and EQE vanishes. This suggests a negligible contribution of the NWs to the photocurrent while most carriers are generated in the Si substrate. In addition, the photoresponse towards the ultraviolet region is also strongly reduced. . Further improvements in EQE will need to consider specific strategies, including optimization of the interwire distance, size and Ga-content of the NW arrays. All these will impact the absorption cross-section and additional spectral matching with the solar spectrum. Ultimately, decoupling the effective absorption and minority carrier collection will require implementation of the surface passivated InGaAs-InAlAs NWs into radially arranged p-n heterojunctions.