Junshuai Li

Formation mechanism and optical properties of InAs quantum dots on the surface of GaAs nanowires.

Formation mechanism and optical properties of InAs quantum dots (QDs) on the surface of GaAs nanowires (NWs) were investigated. This NW-QDs hybrid structure was fabricated by Au-catalyzed metal organic chemical vapor deposition. We found that the formation and distribution of QDs were strongly influenced by the deposition time of InAs as well as the diameter of GaAs NWs. A model based on the adatom diffusion mechanism was proposed to describe the evolution process of the QDs. Photoluminescence emission from the InAs QDs with a peak wavelength of 940 nm was observed at room temperature. The structure also exhibits a decoupling feature that QDs act as gain medium, while NW acts as Fabry-Perot cavity. This hybrid structure could serve as an important element in high-performance NW-based optoelectronic devices, such as near-infrared lasers, optical detectors, and solar cells.

Growth and photoluminescence of InxGa1−xAs quantum dots on the surface of GaAs nanowires by metal organic chemical vapor deposition

InxGa1−xAs (x = 0.6-1) quantum dots are grown on the {112} side facets of GaAs nanowires by metal organic chemical vapor deposition. The emission spectrum of quantum dots exhibits a multi-peak structure due to size and composition fluctuations. The emission wavelength of quantum dots ranges from 857 nm to 930 nm at 77 K, which is distinctly blueshifted relative to that on traditional GaAs (100) planar substrate. The emission linewidth of an ensemble of quantum dots increases from 29.5 meV to 40.5 meV with increasing the In content, indicating a broader composition distribution of quantum dots.

Axially connected nanowire core-shell p-n junctions: a composite structure for high-efficiency solar cells

A composite nanostructure for high-efficiency solar cells that axially connects nanowire core-shell p-n junctions is proposed. By axially connecting the p-n junctions in one nanowire, the solar spectrum is separated and absorbed in the top and bottom cells with respect to the wavelength. The unique structure of nanowire p-n junctions enables substantial light absorption along the nanowire and efficient radial carrier separation and collection. A coupled three-dimensional optoelectronic simulation is used to evaluate the performance of the structure. With an excellent current matching, a promising efficiency of 19.9% can be achieved at a low filling ratio of 0.283 (the density of the nanowire array), which is much higher than the tandem axial p-n junctions.

Self-catalyzed growth of pure zinc blende ⟨110⟩ InP nanowires

We demonstrate the self-catalyzed vapor-liquid-solid growth of ⟨110⟩ InP nanowires (NWs) by metal organic chemical vapor deposition. The ⟨110⟩ InP nanowire is formed via a spontaneous kinking from the original ⟨111⟩ growth direction, which is attributed to instabilities at the liquid/solid interface caused by a fast In incorporation into the droplet. The NW length before kinking has a nearly linear relationship with the diameter, offering a way to control the NW morphology for different applications. The ⟨110⟩ nanowire exhibits pure zinc blende crystal structure and a narrower emission linewidth in comparison with a typical ⟨111⟩ nanowire, demonstrating its potential applications in high-performance electronic and photonic devices.

Anomalous photoconductive behavior of a single InAs nanowire photodetector

We report on a bare InAs nanowire photodetector which exhibits an anomalous photoconductive behavior. Under low-power illumination, the current is smaller than the dark current, and monotonously decreases as the excitation power increases. When the excitation power is high enough, the current starts to increase normally. The phenomenon is attributed to different electron mobilities in the “core” and “shell” of a relatively thick nanowire originating from the surface effect, which result in a quickly dropped “core current” and slowly increased “shell current” under illumination.

Fabrication and optical properties of multishell InAs quantum dots on GaAs nanowires

Hybrid nanostructures combining nanowires with quantum dots promote the development of nanoelectronic and nanophotonic devices with integrated functionalities. In this work, we present a complex nanostructure with multishell quantum dots grown on nanowires. 1–4 shells of Stranski-Krastanov InAs quantum dots are grown on the sidewalls of GaAs nanowires by metal organic chemical vapor deposition. Different dot shells are separated by 8 nm GaAs spacer shells. With increasing the number of shells, the quantum dots become sparser and tend to align in one array, which is caused by the shrinkage of facets on which dots prefer to grow as well as the strain fields produced by the lower set of dots which influences the migration of In adatoms. The size of quantum dots increases with the increase of shell number due to enhanced strain fields coupling. The spectra of multishell dots exhibit multiwavelength emission, and each peak corresponds to a dot shell. This hybrid structure may serve as a promising element in nanowire intermediate band solar cells, infrared nanolasers, and photodetectors.

Controllable growth and optical properties of InP and InP/InAs nanostructures on the sidewalls of GaAs nanowires

The growth and optical properties of InP and InP/InAs nanostructures on GaAs nanowires are investigated. InP quantum well and quantum dots (QDs) are formed on the sidewalls of GaAs nanowires successively with increasing the deposition time of InP. The GaAs/InP nanowire heterostructure exhibits a type-II band alignment. The wavelength of the InP quantum well is in the range of 857–892 nm at 77 K, which means that the quantum well is nearly fully strained. The InP quantum dot, which has a bow-shaped cross section, exhibits dislocation-free pure zinc blende structure. Stranski-Krastanow InAs quantum dots are subsequently formed on the GaAs/InP nanowire core-shell structure. The InAs quantum dots are distributed over the middle part of the nanowire, indicating that the In atoms contributing to the quantum dots mainly come from the vapor rather than the substrate. The longest emission wavelength obtained from the InAs QDs is 1039 nm at 77 K. The linewidth is as narrow as 46.3 meV, which is much narrower than those on planar InP substrates and wurtzite InP nanowires, suggesting high-crystal-quality, phase-purity, and size-uniformity of quantum dots.

Analysis of critical dimensions for axial double heterostructure nanowires

Critical dimensions for an axial double heterostructure nanowire are studied by using finite-element method based on the energy equilibrium criteria. Results show that the critical dimensions are determined by stress fields generated at two interfaces. The dislocations incline to emerge at higher interface when the indium content is less than 10%, while at lower interface when exceeds 10%. Two critical radiuses are obtained: One is called dislocation-free critical radius, below which the structure is coherent regardless of the thickness. The other is named dislocation-unavoidable critical radius, above which dislocations are always energetically favored. The simulated results are in good agreement with the experimental data. We also find that the dislocation-free critical radius here is smaller than that of single heterostructure, and independent of the mediumlayer thickness. This work may serve as a guide to the fabrication of coherently strained double heterostructure nanowires

Realization of Stranski–Krastanow InAs quantum dots on nanowire-based InGaAs nanoshells

We investigate the formation and optical properties of InAs quantum dots on an InGaAs nanosubstrate. We find that Stranski–Krastanow InAs quantum dots are hardly formed on Au-catalyzed InGaAs nanowires due to the phase separation as well as stacking faults. High-quality Stranski–Krastanow InAs quantum dots are realized on a pure zinc blende InGaAs shell radially grown on a GaAs nanowire core. The quantum dots have a large size and regular shape, residing on a wetting layer of several nanometers. For optical characterization, we fabricate a “dot-in-well” structure by capping the quantum dots with InGaAs/GaAs double layers. Photoluminescence from the quantum dots is observed at 77 K, with a peak wavelength of 954 nm, which is distinctly redshifted compared with that of InAs quantum dots directly grown on GaAs nanowires. This work shows the potential of growing Stranski–Krastanow QDs on more types of NWs and obtaining longer wavelengths for wider applications.

Morphological and temperature-dependent optical properties of InAs quantum dots on GaAs nanowires with different InAs coverage

We report a study on the morphological and temperature-dependent optical properties of InAs quantum dots on GaAs nanowires with different InAs coverage. We find that the size, density, and distribution of quantum dots strongly depend on the InAs coverage. At higher coverage, the quantum dots exhibit a longer peak wavelength and broader linewidth at low temperature, suggesting a larger size and increased size fluctuations. Particularly, a great difference in the linewidth dependence on temperature for different InAs coverage is observed, corresponding to a different result of competition between electron-phonon scattering and thermal penetration of carriers between neighboring quantum dots.