GaAs/GaInP nanowire solar cell on Si with state-of-the-art Voc and quasi-Fermi level splitting.

With their unique structural, optical and electrical properties, III-V nanowires (NWs) are an extremely attractive option for the direct growth of III-Vs on Si for tandem solar cell applications. Here, we introduce a core-shell GaAs/GaInP NW solar cell grown by molecular beam epitaxy on a patterned Si substrate, and we present an in-depth investigation of its optoelectronic properties and limitations. We report a power conversion efficiency of almost 3.7%, and a state-of-the-art open-circuit voltage (VOC) for a NW array solar cell on Si of 0.65 V. We also present the first quantification of the quasi-Fermi level splitting in NW array solar cells using hyperspectral photoluminescence measurements. A value of 0.84 eV is obtained at 1 sun (1.01 eV at 81 suns), which is significantly higher than qVOC. It indicates NWs with a better intrinsic optoelectronic quality than what could be expected from TEM images or deduced from electrical measurements. Optical and electronic simulations provide insights into the main absorption and electrical losses, and guidelines to design and fabricate higher-efficiency devices. It suggests that improvements at the n-type contact (GaInP/ITO) are key to unlocking the potential of next generation NW solar cells.

Stable and high yield growth of GaP and In0.2Ga0.8As nanowire arrays using In as a catalyst.

We report the first investigation of indium (In) as the vapor-liquid-solid catalyst of GaP and InGaAs nanowires by molecular beam epitaxy. A strong asymmetry in the Ga distribution between the liquid and solid phases allows one to obtain pure GaP and In0.2Ga0.8As nanowires while the liquid catalyst remains nearly pure In. This uncommon In catalyst presents several advantages. First, the nanowire morphology can be tuned by changing the In flux alone, independently of the Ga and group V fluxes. Second, the nanowire crystal structure always remains cubic during steady state growth and catalyst crystallization, despite the low contact angle of the liquid droplet measured after growth (95°). Third, the vertical yield of In-catalyzed GaP and (InGa)As nanowire arrays on patterned silicon substrates increases dramatically. Combining straight sidewalls, controllable morphologies and a high vertical yield, In-catalysts provide an alternative to the standard Au or Ga alloys for the bottom-up growth of large scale homogeneous arrays of (InGa)As or GaP nanowires.

GaAs epilayers grown on patterned (001) silicon substrates via suspended Ge layers.

We demonstrate the growth of low density anti-phase boundaries, crack-free GaAs epilayers, by Molecular Beam Epitaxy on silicon (001) substrates. The method relies on the deposition of thick GaAs on a suspended Ge buffer realized on top of deeply patterned Si substrates by means of a three-temperature procedure for the growth. This approach allows to suppress, at the same time, both threading dislocations and thermal strain in the epilayer and to remove anti-phase boundaries even in absence of substrate tilt. Photoluminescence measurements show the good uniformity and the high optical quality of AlGaAs/GaAs quantum well structures realized on top of such GaAs layer.

Evidence and control of unintentional As-rich shells in GaAs1-x P x nanowires.

We report on the detailed composition of ternary GaAsP nanowires (NWs) grown using self-catalyzed vapor-liquid-solid (VLS) growth by molecular beam epitaxy. We evidence the formation of an unintentional shell, which enlarges by vapor-solid growth concurrently to the main VLS-grown core. The NW core and unintentional shell have typically different chemical compositions if no effort is made to adjust the growth conditions. The compositions can be made equal by changing the substrate temperature and the P/As flux ratio in the vapor phase. In all cases, we still observe the existence of a P-rich interface between the GaAsP NW core and the unintentional shell, even if favorable growth conditions are used.

Measuring and Modeling the Growth Dynamics of Self-Catalyzed GaP Nanowire Arrays.

The bottom-up fabrication of regular nanowire (NW) arrays on a masked substrate is technologically relevant, but the growth dynamic is rather complex due to the superposition of severe shadowing effects that vary with array pitch, NW diameter, NW height, and growth duration. By inserting GaAsP marker layers at a regular time interval during the growth of a self-catalyzed GaP NW array, we are able to retrieve precisely the time evolution of the diameter and height of a single NW. We then propose a simple numerical scheme which fully computes shadowing effects at play in infinite arrays of NWs. By confronting the simulated and experimental results, we infer that re-emission of Ga from the mask is necessary to sustain the NW growth while Ga migration on the mask must be negligible. When compared to random cosine or random uniform re-emission from the mask, the simple case of specular reflection on the mask gives the most accurate account of the Ga balance during the growth.

Determination of n-Type Doping Level in Single GaAs Nanowires by Cathodoluminescence.

We present an effective method of determining the doping level in n-type III-V semiconductors at the nanoscale. Low-temperature and room-temperature cathodoluminescence (CL) measurements are carried out on single Si-doped GaAs nanowires. The spectral shift to higher energy (Burstein-Moss shift) and the broadening of luminescence spectra are signatures of increased electron densities. They are compared to the CL spectra of calibrated Si-doped GaAs layers, whose doping levels are determined by Hall measurements. We apply the generalized Planck's law to fit the whole spectra, taking into account the electron occupation in the conduction band, the bandgap narrowing, and band tails. The electron Fermi levels are used to determine the free electron concentrations, and we infer nanowire doping of 6 × 1017 to 1 × 1018 cm-3. These results show that cathodoluminescence provides a robust way to probe carrier concentrations in semiconductors with the possibility of mapping spatial inhomogeneities at the nanoscale.

Engineered Coalescence by Annealing 3D Ge Microstructures into High-Quality Suspended Layers on Si.

The move from dimensional to functional scaling in microelectronics has led to renewed interest toward integration of Ge on Si. In this work, simulation-driven experiments leading to high-quality suspended Ge films on Si pillars are reported. Starting from an array of micrometric Ge crystals, the film is obtained by exploiting their temperature-driven coalescence across nanometric gaps. The merging process is simulated by means of a suitable surface-diffusion model within a phase-field approach. The successful comparison between experimental and simulated data demonstrates that the morphological evolution is driven purely by the lowering of surface-curvature gradients. This allows for fine control over the final morphology to be attained. At fixed annealing time and temperature, perfectly merged films are obtained from Ge crystals grown at low temperature (450 °C), whereas some void regions still persist for crystals grown at higher temperature (500 °C) due to their different initial morphology. The latter condition, however, looks very promising for possible applications. Indeed, scanning tunneling electron microscopy and high-resolution transmission electron microscopy analyses show that, at least during the first stages of merging, the developing film is free from threading dislocations. The present findings, thus, introduce a promising path to integrate Ge layers on Si with a low dislocation density.

Formation of stable Si-O-C submonolayers on hydrogen-terminated silicon(111) under low-temperature conditions.

In this letter, we report results of a hydrosilylation carried out on bifunctional molecules by using two different approaches, namely through thermal treatment and photochemical treatment through UV irradiation. Previously, our group also demonstrated that in a mixed alkyne/alcohol solution, surface coupling is biased towards the formation of Si-O-C linkages instead of Si-C linkages, thus indirectly supporting the kinetic model of hydrogen abstraction from the Si-H surface (Khung, Y. L. et al. Chem. - Eur. J. 2014, 20, 15151-15158). To further examine the probability of this kinetic model we compare the results from reactions with bifunctional alkynes carried out under thermal treatment (<130 °C) and under UV irradiation, respectively. X-ray photoelectron spectroscopy and contact angle measurements showed that under thermal conditions, the Si-H surface predominately reacts to form Si-O-C bonds from ethynylbenzyl alcohol solution while the UV photochemical route ensures that the alcohol-based alkyne may also form Si-C bonds, thus producing a monolayer of mixed linkages. The results suggested the importance of surface radicals as well as the type of terminal group as being essential towards directing the nature of surface linkage.