CuI as a Hole-Selective Contact for GaAs Solar Cells.

Carrier-selective contacts have emerged as a promising architecture for solar cell fabrication. In this report, the first hole-selective III-V semiconductor solar cell is demonstrated using copper iodide (CuI) on i-GaAs. Surface passivation quality of GaAs is found to be essential for open-circuit voltage (VOC), with good correlation between photoluminescence properties of the GaAs layer and the VOC. Passivation with <10 nm thick In0.49Ga0.51P layers is shown to provide an over 300 mV improvement. Oxygen-rich CuI is formed by natural oxidation in the atmosphere, and the increased oxygen content of ∼10% is validated by energy-dispersive X-ray measurements. The oxygen incorporation is shown to improve hole selectivity and thus solar conversion efficiency. Ultraviolet photoelectron spectroscopy indicates a high work function of ∼6 eV for the oxygen-rich CuI. With optimized GaAs surface passivation and oxygen-rich CuI, a VOC of nearly 1 V and a solar conversion efficiency of 13.4% are achieved. The solar cell structure includes only undoped GaAs, a surface passivation layer, and non-epitaxial CuI contact and is therefore very promising to various low-cost crystal growth methods. The results have a significant impact on III-V solar cell fabrication and costs as it (i) enables fully carrier-selective architectures, (ii) reduces cell fabrication complexity, and (iii) is suitable for layers grown by low-cost crystal growth techniques.

Room-temperature optically detected magnetic resonance of single defects in hexagonal boron nitride.

Optically addressable solid-state spins are important platforms for quantum technologies, such as repeaters and sensors. Spins in two-dimensional materials offer an advantage, as the reduced dimensionality enables feasible on-chip integration into devices. Here, we report room-temperature optically detected magnetic resonance (ODMR) from single carbon-related defects in hexagonal boron nitride with up to 100 times stronger contrast than the ensemble average. We identify two distinct bunching timescales in the second-order intensity-correlation measurements for ODMR-active defects, but only one for those without an ODMR response. We also observe either positive or negative ODMR signal for each defect. Based on kinematic models, we relate this bipolarity to highly tuneable internal optical rates. Finally, we resolve an ODMR fine structure in the form of an angle-dependent doublet resonance, indicative of weak but finite zero-field splitting. Our results offer a promising route towards realising a room-temperature spin-photon quantum interface in hexagonal boron nitride.

Tailoring Second-Harmonic Emission from (111)-GaAs Nanoantennas.

Second-harmonic generation (SHG) in resonant dielectric Mie-scattering nanoparticles has been hailed as a powerful platform for nonlinear light sources. While bulk-SHG is suppressed in elemental semiconductors, for example, silicon and germanium due to their centrosymmetry, the group of zincblende III-V compound semiconductors, especially (100)-grown AlGaAs and GaAs, have recently been presented as promising alternatives. However, major obstacles to push the technology toward practical applications are the limited control over directionality of the SH emission and especially zero forward/backward radiation, resulting from the peculiar nature of the second-order nonlinear susceptibility of this otherwise highly promising group of semiconductors. Furthermore, the generated SH signal for (100)-GaAs nanoparticles depends strongly on the polarization of the pump. In this work, we provide both theoretically and experimentally a solution to these problems by presenting the first SHG nanoantennas made from (111)-GaAs embedded in a low index material. These nanoantennas show superior forward directionality compared to their (100)-counterparts. Most importantly, based on the special symmetry of the crystalline structure, it is possible to manipulate the SHG radiation pattern of the nanoantennas by changing the pump polarization without affecting the linear properties and the total nonlinear conversion efficiency, hence paving the way for efficient and flexible nonlinear beam-shaping devices.

Temperature effects in contacts between a metal and a semiconductor nanowire near the degenerate doping.

We have investigated the nonlinear conductance in diffusion-doped Si:GaAs nanowires contacted by patterned metal films in a wide range of temperatures <i>T</i>. The wire resistance R_W and the zero bias resistance R_C, dominated by the contacts, exhibit very different responses to temperature changes. While R_W shows almost no dependence on <i>T</i>, R_C varies by several orders of magnitude as the devices are cooled from room temperature to T= 5 K. We develop a model that employs a sharp donor level very low in the GaAs conduction band and show that our observations are consistent with the model predictions. We then demonstrate that such measurements can be used to estimate carrier properties in nanostructured semiconductors and obtain an estimate for N_D, the doping density in our samples. We also discuss the effects of surface states and dielectric confinement on carrier density in semiconductor nanowires.

Can misfit dislocations be located above the interface of InAs/GaAs (001) epitaxial quantum dots?

InAs/GaAs(001) quantum dots grown by droplet epitaxy were investigated using electron microscopy. Misfit dislocations in relaxed InAs/GaAs(001) islands were found to be located approximately 2 nm above the crystalline sample surface, which provides an impression that the misfit dislocations did not form at the island/substrate interface. However, detailed microscopy data analysis indicates that the observation is in fact an artefact caused by the surface oxidation of the material that resulted in substrate surface moving down about 2 nm. As such, caution is needed in explaining the observed interfacial structure.