Doping of Self-Catalyzed Nanowires under the Influence of Droplets

Non-〈111〉-oriented semiconductor nanowires: growth, properties, and applications

In recent years, non-〈111〉-oriented semiconductor nanowires have attracted increasing interest in terms of fundamental research and promising applications due to their outstanding crystal quality and distinctive physical properties. Here, a comprehensive overview of recent advances in the study of non-〈111〉-oriented semiconductor nanowires is presented. We start by introducing various growth techniques for obtaining nanowires with certain orientations, for which the growth energetics and kinetics are discussed. Attention is then given to the physical properties of non-〈111〉 nanowires, as predicted by theoretical calculations or demonstrated experimentally. After that, we review the advantages and challenges of non-〈111〉 nanowires as building blocks for electronic and optoelectronic devices. Finally, we discuss the possible challenges and opportunities in the research field of non-〈111〉 semiconductor nanowires.

Self-Powered InP Nanowire Photodetector for Single-Photon Level Detection at Room Temperature

Highly sensitive photodetectors with single-photon level detection are one of the key components to a range of emerging technologies, in particular the ever-growing field of optical communication, remote sensing, and quantum computing. Currently, most of the single-photon detection technologies require external biasing at high voltages and/or cooling to low temperatures, posing great limitations for wider applications. Here, InP nanowire array photodetectors that can achieve single-photon level light detection at room temperature without an external bias are demonstrated. Top-down etched, heavily doped p-type InP nanowires and n-type aluminium-doped zinc oxide (AZO)/zinc oxide (ZnO) carrier-selective contact are used to form a radial p-n junction with a built-in electric field exceeding 3 × 10⁵  V cm^-1  at 0 V. The device exhibits broadband light sensitivity and can distinguish a single photon per pulse from the dark noise at 0 V, enabled by its design to realize near-ideal broadband absorption, extremely low dark current, and highly efficient charge carrier separation. Meanwhile, the bandwidth of the device reaches above 600 MHz with a timing jitter of 538 ps. The proposed device design provides a new pathway toward low-cost, high-sensitivity, self-powered photodetectors for numerous future applications.

Red GaPAs/GaP Nanowire-Based Flexible Light-Emitting Diodes

We demonstrate flexible red light-emitting diodes based on axial GaPAs/GaP heterostructured nanowires embedded in polydimethylsiloxane membranes with transparent electrodes involving single-walled carbon nanotubes. The GaPAs/GaP axial nanowire arrays were grown by molecular beam epitaxy, encapsulated into a polydimethylsiloxane film, and then released from the growth substrate. The fabricated free-standing membrane of light-emitting diodes with contacts of single-walled carbon nanotube films has the main electroluminescence line at 670 nm. Membrane-based light-emitting diodes (LEDs) were compared with GaPAs/GaP NW array LED devices processed directly on Si growth substrate revealing similar electroluminescence properties. Demonstrated membrane-based red LEDs are opening an avenue for flexible full color inorganic devices.

Optimization of Ohmic Contacts to p-GaAs Nanowires

The performance of Ohmic contacts applied to semiconductor nanowires (NWs) is an important aspect for enabling their use in electronic or optoelectronic devices. Due to the small dimensions and specific surface orientation of NWs, the standard processing technology widely developed for planar heterostructures cannot be directly applied. Here, we report on the fabrication and optimization of Pt/Ti/Pt/Au Ohmic contacts for p-type GaAs nanowires grown by molecular beam epitaxy. The devices were characterized by current-voltage (IV) measurements. The linearity of the IV characteristics curves of individual nanowires was optimized by adjusting the layout of the contact metal layers, the surface treatment prior to metal evaporation, and post-processing thermal annealing. Our results reveal that the contact resistance is remarkably decreased when a Pt layer is deposited on the GaAs nanowire prior to the traditional Ti/Pt/Au multilayer layout used for p-type planar GaAs. These findings are explained by an improved quality of the metal-GaAs interface, which was evidenced by grazing incidence X-ray diffraction measurements in similar metallic thin films deposited on GaAs (110) substrates. In particular, we show that Ti exhibits low degree of crystallinity when deposited on GaAs (110) surface which directly affects the contact resistance of the NW devices. The deposition of a thin Pt layer on the NWs prior to Ti/Pt/Au results in a 95% decrease in the total electrical resistance of Be-doped GaAs NWs which is associated to the higher degree of crystallinity of Pt than Ti when deposited directly on GaAs (110).

Gradients of Be-dopant concentration in self-catalyzed GaAs nanowires

Effective and controllable doping is instrumental for enabling the use of III-V semiconductor nanowires (NWs) in practical electronics and optoelectronics applications. To this end, dopants are incorporated during self-catalyzed growth via vapor-liquid-solid mechanism through the catalyst droplet or by vapor-solid mechanism of the sidewall growth. The interplay of these mechanisms together with the competition between axial elongation and radial growth of NWs can result in dopant concentration gradients along the NW axis. Here, we report an investigation of Be-doped p-type GaAs NWs grown by the self-catalyzed method on lithography-free Si/SiO _x templates. The influence of dopant incorporation on the structural properties of the NWs is analyzed by scanning and transmission electron microscopy. By combining spatially resolved Raman spectroscopy and transport characterization, we are able to estimate the carrier concentration, mobility and resistivity on single-NW level. We show that Be dopants are incorporated predominantly by vapor-solid mechanism for low Be flux, while the relative contribution of vapor-liquid-solid incorporation is increased for higher Be flux, resulting in axial dopant gradients that depend on the nominal doping level.

Toward electrically driven semiconductor nanowire lasers

Semiconductor nanowire (NW) lasers are highly promising for making new-generation coherent light sources with the advantages of ultra-small size, high efficiency, easy integration and low cost. Over the past 15 years, this area of research has been developing rapidly, with extensive reports of optically pumped lasing in various inorganic and organic semiconductor NWs. Motivated by these developments, substantial efforts are being made to make NW lasers electrically pumped, which is necessary for their practical implementation. In this review, we first categorize NW lasers according to their lasing wavelength and wavelength tunability. Then, we summarize the methods used for achieving single-mode lasing in NWs. After that, we review reports on lasing threshold reduction and the realization of electrically pumped NW lasers. Finally, we offer our perspective on future improvements and trends.

Growth optimization and characterization of regular arrays of GaAs/AlGaAs core/shell nanowires for tandem solar cells on silicon

With a band gap value of 1.7 eV, Al_0.2Ga_0.8As is one of the ideal III-V alloys for the development of nanowire-based Tandem Solar Cells on silicon. Nevertheless, growing self-catalysed AlGaAs nanowires on silicon by solid-source molecular beam epitaxy is a very difficult task due to the oxidation of Al adatoms by the SiO₂ layer present on the surface. Here we propose a nanowire structure including a p.i.n radial junction inside an Al_0.2Ga_0.8As shell grown on a p-GaAs core. The crystalline structure of such self-catalysed nanowires grown on an epi-ready Si(111) substrate (with a thin native SiO₂ layer) was investigated by transmission electronic microscopy and photoluminescence. I(V) measurements performed on single nanowires have shown a diode-like behaviour corresponding to the radial p.i.n junction inside the Al_0.2Ga_0.8As shell. Moreover, a current generation under the electron beam was evidenced over the entire radial junction along the nanowires by means of electron beam induced current (EBIC) microscopy. The same structure was reproduced on patterned substrates with a SiO₂ mask, producing an ordered hexagonal array. High and uniform yields from 83% to 87% of vertical nanowires were obtained on 0.9 × 0.9 cm² patterned areas. EBIC mapping performed on these nanowires confirmed the good electrical properties of the radial junction within the nanowires.

Nanowires for High-Efficiency, Low-Cost Solar Photovoltaics

Solar energy is abundant, clean, and renewable, making it an ideal energy source. Solar cells are a good option to harvest this energy. However, it is difficult to balance the cost and efficiency of traditional thin-film solar cells, whereas nanowires (NW) are far superior in making high-efficiency low-cost solar cells. Therefore, the NW solar cell has attracted great attention in recent years and is developing rapidly. Here, we review the great advantages, recent breakthroughs, novel designs, and remaining challenges of NW solar cells. Special attention is given to (but not limited to) the popular semiconductor NWs for solar cells, in particular, Si, GaAs(P), and InP.

Deterministic Switching of the Growth Direction of Self-Catalyzed GaAs Nanowires

The typical vapor-liquid-solid growth of nanowires is restricted to a vertical one-dimensional geometry, while there is a broad interest for more complex structures in the context of electronics and photonics applications. Controllable switching of the nanowire growth direction opens up new horizons in the bottom-up engineering of self-assembled nanostructures, for example, to fabricate interconnected nanowires used for quantum transport measurements. In this work, we demonstrate a robust and highly controllable method for deterministic switching of the growth direction of self-catalyzed GaAs nanowires. The method is based on the modification of the droplet-nanowire interface in the annealing stage without any fluxes and subsequent growth in the horizontal direction by a twin-mediated mechanism with indications of a novel type of interface oscillations. A 100% yield of switching the nanowire growth direction from vertical to horizontal is achieved by systematically optimizing the growth parameters. A kinetic model describing the competition of different interface structures is introduced to explain the switching mechanism and the related nanowire geometries. The model also predicts that the growth of similar structures is possible for all vapor-liquid-solid nanowires with commonly observed truncated facets at the growth interface.

Growth and Fabrication of High-Quality Single Nanowire Devices with Radial p-i-n Junctions

Nanowires (NWs) with radial p-i-n junction have advantages, such as large junction area and small influence from the surface states, which can lead to highly efficient material use and good device quantum efficiency. However, it is difficult to make high-quality core-shell NW devices, especially single NW devices. Here, the key factors during the growth and fabrication process that influence the quality of single core-shell p-i-n NW devices are studied using GaAs(P) NW photovoltaics as an example. By p-doping and annealing, good ohmic contact is achieved on NWs with a diameter as small as 50-60 nm. Single NW photovoltaics are subsequently developed and a record fill factor of 80.5% is shown. These results bring valuable information for making single NW devices, which can further benefit the development of high-density integration circuits.

Nanoscale investigation of a radial p-n junction in self-catalyzed GaAs nanowires grown on Si (111)

One obstacle for the development of nanowire (NW) solar cells is the challenge to assess and control their nanoscale electrical properties. In this work a top-cell made of p-n GaAs core/shell NWs grown on a Si(111) substrate by Molecular Beam Epitaxy (MBE) is investigated by high resolution charge collection microscopy. Electron Beam Induced Current (EBIC) analyses of single NWs have validated the formation of a homogeneous radial p-n junction over the entire length of the NWs. The radial geometry leads to an increase of the junction area by 38 times with respect to the NW footprint. The interface between the NWs and the Si(111) substrate does not show any electrical loss, which would have led to a decrease of the EBIC signal. Single NW I-V characteristics present a diodic behavior. A model of the radial junction single NW is proposed and the electrical parameters are estimated by numerical fitting of the I-Vs and of the EBIC map. Solar cells based on NW arrays were fabricated and analyzed by EBIC microscopy, which evidenced the presence of a Schottky barrier at the NW/ITO top contact. Improvement of the top contact quality is achieved by thermal annealing at 400 °C, which strongly reduces the parasitic Schottky barrier.