Loading related content. Jump to main content. Jump to site search. You do not have JavaScript enabled. Please enable JavaScript to access the full features of the site or access our non-JavaScript page. Issue 4, From the journal: Materials Chemistry Frontiers.
Recent developments in III—V semiconducting nanowires for high-performance photodetectors. Lifan Shen , ab Edwin Y. This article is part of the themed collection: Materials Chemistry Frontiers Review-type Articles. This enables nanowire detectors to achieve very high sensitivity, provided that light can be efficiently coupled into the nanowires. Several methods have been proposed to achieve light coupling efficiency, such as placing the nanowires in an optical resonant cavity.
The engineers also show that molecular oxygen absorbed at the surface of zinc oxide ZnO nanowires capture free electrons present in n-type ZnO nanowires and make them especially good at keeping holes and electrons apart. The oxygen mechanism the authors outline explains much of the enhanced sensitivity reported in ZnO nanowire photodetectors. The engineers fabricated and characterized UV photodetectors made from ZnO nanowires with diameters of to nanometers and lengths ranging from 10 to 15 micrometers.
The researchers studied the photoconductivity of zinc oxide nanowires over a broad time range and under both air and vacuum. According to Wang, this work also highlights how moving to the nanoscale can sometimes throw intuitions out the window. Soci, A. Zhang, B. Each year, NSF receives more than 40, competitive proposals and makes about 11, new awards. Those awards include support for cooperative research with industry, Arctic and Antarctic research and operations, and U. Get News Updates by Email.
Connect with us online NSF website: nsf. Data is shown for a dielectric shell with index 1. The imaginary component for the index of the dielectric shell is assumed to be 0.
The real part of refractive index for these dielectrics varies between 1. Large evanescent fields enhance in-coupling of incident light to the guided modes supported in the nanowire due to better overlap of the field components. Figure 4 a,b show that with a dielectric shell, real n eff of the mode increases and hence smaller diameter nanowire is required for better in-coupling of incident light.
The imaginary part of n eff is also larger for the nanowire with the dielectric shell and describes the propagation loss for the mode. Higher propagation loss for the resonant mode in the nanowire with the dielectric shell may result from stronger absorption in the nanowire due to better confinement. The x-axis is the diameter of the nanowire, shell thickness is not included.
The white circles indicate the nanowire and shell positions. The complex effective indices are also indicated. There have been reports in the literature of dielectric shells improving the absorption characteristics of nanowires. All these reports have looked at the absorption characteristics in a nanowire in horizontal illumination configuration. To gauge the effect of dielectric shells on absorption characteristics of a vertical nanowire and its effect on the performance of wavelength selective nanowire-based devices it is necessary to compare the global maximum in the absorption of a bare nanowire to the global maximum in the absorption of a nanowire with a shell, at a given wavelength.
However, this requires scanning a huge parameter space of nanowire diameter and dielectric shell thickness, which makes the task impractical. We now discuss a relatively simple approach to identify the optimal nanowire-shell parameters for maximizing absorption in a nanowire, compare the absorption characteristics of a bare nanowire with that of a nanowire with a dielectric shell and discuss the implications for photodetector applications.
As discussed earlier the maximum in absorption in a nanowire corresponds to the maximum of the product of overlap integral and confinement factor. We can evaluate this product for the guided modes in a bare nanowire and a nanowire with a dielectric shell to identify the optimal nanowire dimensions for maximum absorption.
This is computationally less intensive and can be done either analytically or numerically using 2D simulations. Figure 4 c shows this data for a shell of index 1. As is expected from the guided mode behavior shown in Fig. As the dielectric shell thickness increases the overlap between the guided mode and the incident field increases, but the mode confinement inside the nanowire reduces as a result of lower index contrast between the nanowire and the surrounding medium. Thus there exists an optimal shell thickness for maximizing absorption that can be identified using this method.
Based on the above arguments direct comparison of absorption in a nanowire of fixed diameter with and without shell, which has been done in all reports to date, is not appropriate since the optimal nanowire dimensions are different for maximizing absorption without or with a dielectric shell. The optimal nanowire diameter and the shell thickness for maximizing absorption are also listed. The absorption efficiency is determined by normalizing the absorption cross section to the nanowire core projected area.
The area of dielectric shell is not included as it does not contribute to the generation of photocarriers. The maximum absorption efficiency for a nanowire with a shell of index of 1. To highlight the importance of optimizing the nanowire-shell dimensions for device applications we also evaluate the absorption efficiency for non-optimal nanowire-shell dimensions.
Figure 4 c shows that there is only a narrow range of optimal dimensions to maximize the absorption efficiency. This has implications for nanowire photodetectors and shows that the entire device structure needs to be properly designed for optimal optical performance. With proper optimization, a dielectric shell can enhance the photocurrent generated in a nanowire photodetector but non-optimal shell thickness may considerably degrade the nanowire device performance.
The diameter-dependent resonant waveguide modes supported in III-V semiconductor nanowires enable design of photodetectors with wavelength tunable peak response, without having the need to change the material of the nanowire. Nanowire photodetectors have the potential to reduce the active semiconductor volume by a factor equal to the nanowire absorption efficiency compared to planar photodetectors fabricated from the same semiconductor material.
Nanowires with varying diameters and wavelength scale separation between them can be used to realize multi-color focal plane array photodetectors with very small footprint. For practical devices it is necessary to identify the optimal nanowire diameter and thickness of the dielectric shell used for isolating the electrical contacts to maintain these advantages.
We have illustrated a simple approach to optimize these parameters without having the need to scan a huge parameter space with computationally intensive approaches. Three dimensional FDTD simulations were used for absorption calculations.
The nanowire was illuminated with a broadband plane wave source and a three dimensional box of power monitors around the nanowire was used to determine the power inflow into the box. Wavelength dependent complex refractive index was used for the nanowire. Perfectly matched layer PML boundaries are used to minimize reflections from boundaries for calculating absorption in the nanowires.
Two dimensional FDTD simulations were used for obtaining the mode profiles. The required mode was launched inside an infinitely long nanowire and the electric field intensity was recorded by placing a field monitor across the nanowire cross-section.
The dispersion relationship, confinement factor and the overlap integral were calculated using MODE solutions package. The overlap integral between the resonant mode supported in the nanowire and the incident plane wave was calculated using , where the subscript 1 denotes the resonant mode and the subscript 2 denotes the incident plane wave.
How to cite this article : Mokkapati, S. Optical design of nanowire absorbers for wavelength selective photodetectors. Ning, C. Semiconductor nanolasers. Physica status solidi b , — CAS Google Scholar. Grzela, G.
0コメント