Inverse Pyramids Research Articles

Wave‐Interference Photonic Crystals and Space Charge Engineering Enable Efficient Broadband Faint Light Detection in Organic/Inorganic Hybrid Photodetectors

AbstractNoise current, detectivity, quantum efficiency, and response speed are critical metrics in evaluating organic/inorganic photodiodes. Herein, these metrics are simultaneously advanced in poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/native SiOx/n‐Si hybrid photodetectors, achieving excellent performance in detecting broadband faint light. Wave‐interference photonic crystals, comprising periodic microstructured inverse pyramids with nanometer‐scale mesa widths, are integrated into the Si absorber to effectively couple incident light for increased absorption, thereby balancing optics and conformal contact coverage. The developed photodetector comprises a deep depletion region and interfacial SiOx layer, enabling diffusion‐mitigated broadband photocarrier transport and effective charge collection, and suppressing carrier tunneling processes for low‐noise. An ultralow reverse dark current density of ≈2.46 × 10−8 A cm−2 at −0.4 V is realized for nanowatt‐level light detection. The photodetector showcases superlative properties among reported PEDOT:PSS–based inorganic heterojunctions, including a broadband external quantum efficiency of >≈80% from 340 to 960 nm (internal quantum efficiency of >≈90% from 380 to 840 nm), detectivity of >≈1012 Jones from 300 to 1140 nm, and microsecond response speed. This study provides practical insight for combining high‐absorption microstructures with space charge engineering for the development of high‐performance organic/inorganic hybrid photodetectors.

Plasmonic enhancement in gold coated inverse pyramid substrates with entrapped gold nanoparticles

Significant increase of plasmonic surface enhancement has been observed in gold coated micron sized inverse pyramids entrapping a gold nanoparticle. The amplification of both surface enhanced Raman and fluorescence signals was found and it was dependent on the size of the trapped gold nanoparticle. The enhancement was found to be 50 times higher for the 250nm diameter nanosphere, compared to the 50nm one. Finite differential time domain simulations were performed to determine the near-field distribution in the structure and showed that when the nanoparticle protrudes into the hotspot zone of the void, coupling of the electromagnetic field occurs and the plasmon-related near-field enhancement is concentrated into the close vicinity of the nanoparticle, mainly into the gaps around the contact points of the curved sphere and the flat pyramid surfaces.

Preparation and Characterization of Perforated SERS Active Array for Particle Trapping and Sensitive Molecular Analysis.

A gold-coated array of flow-through inverse pyramids applicable as substrate for entrapment and immobilization of micro-objects and for surface enhanced Raman spectroscopic measurements was fabricated using bulk micromachining techniques from silicon. Surface morphology, optical reflectance, immobilization properties, and surface enhanced Raman amplification of the array were modelled and characterized. It was found that the special perforated periodic 3D structure can be used for parallel particle and cell trapping and highly sensitive molecular analysis of the immobilized objects.

Conformal Electroless Nickel Plating on Silicon Wafers, Convex and Concave Pyramids, and Ultralong Nanowires.

Nickel (Ni) plating has garnered great commercial interest, as it provides excellent hardness, corrosion resistance, and electrical conductivity. Though Ni plating on conducting substrates is commonly employed via electrodeposition, plating on semiconductors and insulators often necessitates electroless approaches. Corresponding plating theory for deposition on planar substrates was developed as early as 1946, but for substrates with micro- and nanoscale features, very little is known of the relationships between plating conditions, Ni deposition quality, and substrate morphology. Herein, we describe the general theory and mechanisms of electroless Ni deposition on semiconducting silicon (Si) substrates, detailing plating bath failures and establishing relationships between critical plating bath parameters and the deposited Ni film quality. Through this theory, we develop two different plating recipes: galvanic displacement (GD) and autocatalytic deposition (ACD). Neither recipe requires pretreatment of the Si substrate, and both methods are capable of depositing uniform Ni films on planar Si substrates and convex Si pyramids. In comparison, ACD has better tunability than GD, and it provides a more conformal Ni coating on complex and high-aspect-ratio Si structures, such as inverse fractal Si pyramids and ultralong Si nanowires. Our methodology and theoretical analyses can be leveraged to develop electroless plating processes for other metals and metal alloys and to generally provide direction for the adaptation of electroless deposition to modern applications.

The biomass distribution on Earth

A census of the biomass on Earth is key for understanding the structure and dynamics of the biosphere. However, a global, quantitative view of how the biomass of different taxa compare with one another is still lacking. Here, we assemble the overall biomass composition of the biosphere, establishing a census of the ≈550 gigatons of carbon (Gt C) of biomass distributed among all of the kingdoms of life. We find that the kingdoms of life concentrate at different locations on the planet; plants (≈450 Gt C, the dominant kingdom) are primarily terrestrial, whereas animals (≈2 Gt C) are mainly marine, and bacteria (≈70 Gt C) and archaea (≈7 Gt C) are predominantly located in deep subsurface environments. We show that terrestrial biomass is about two orders of magnitude higher than marine biomass and estimate a total of ≈6 Gt C of marine biota, doubling the previous estimated quantity. Our analysis reveals that the global marine biomass pyramid contains more consumers than producers, thus increasing the scope of previous observations on inverse food pyramids. Finally, we highlight that the mass of humans is an order of magnitude higher than that of all wild mammals combined and report the historical impact of humanity on the global biomass of prominent taxa, including mammals, fish, and plants.

Gap Plasmons Multiple Mirroring from Spheres in Pyramids for Surface-Enhanced Raman Scattering

A reliable fabrication technique for obtaining a high density of regular nanogaps is critical for ultrasensitive surface-enhanced Raman scattering (SERS). However, nanogaps produced between nanostructures have suffered from the lack of controllability. Taking the sphere–film structure as a starting point for its straightforward fabrication technique and well-controlled gaps, we propose a novel nanostructure by combining Au nanospheres and inverse pyramidal holes. The proposed nanostructure is obtained by trapping Au nanospheres in inverse pyramidal hole arrays to create multiple, uniform, and reproducible geometrical gaps near the contact points between the nanospheres and the inverse pyramids. The combined nanostructure, referred to as SIP (spheres in pyramids), supports augmented plasmon hybridization due to mirroring of gap plasmons and induces much stronger electromagnetic enhancement at normal incidence when compared to the contacting sphere–film structure. In contrast to the sphere–film structure ex...

Comparative Analysis of SERS Substrates of Different Morphology

In this work the surface enhanced Raman scattering (SERS) performance of gold coated patterned silicon surfaces of different morphology and period was investigated. Arrays of inverse pyramids, spheres and rounded pyramids of different sizes were fabricated by photolithography and selective etching. Thin layer of gold was sputtered onto the surface of the samples. The SERS performance of the substrates was tested using a highly dissolved organic solution.

Comparison of Image Decompositions Through Inverse Difference and Laplacian Pyramids

In this work, the Inverse Difference Pyramid (IDP) and its modification – the Reduced IDP (RIDP), are compared and evaluated with the famous Laplacian Pyramid (LP) for multi-level decomposition of digital images. The comparison comprises: the structures of LP and IDP, the image representation through LP and IDP in the pixel space and in the spectrum space of the Fourier transform, the efficiency of both transforms for the aims of the progressive image transfer, the LP and the IDP computation graphs and the evaluation of the computational complexity of the algorithms for the 3-level reduced LP and the 3-level reduced IDP. The influence of the quantization noise on both decompositions is also analyzed. On the basis of the comparison are outlined the basic advantages of the RIDP for pipeline image processing. The results obtained could be used for the design of coders for image compression, aimed at real-time applications, etc.

Reverse Epitaxy of Ge: Ordered and Faceted Surface Patterns

Normal incidence ion irradiation at elevated temperatures, when amorphization is prevented, induces novel nanoscale patterns of crystalline structures on elemental semiconductors by a reverse epitaxial growth mechanism: on Ge surfaces irradiation at temperatures above the recrystallization temperature of 250 °C leads to self-organized patterns of inverse pyramids. Checkerboard patterns with fourfold symmetry evolve on the Ge (100) surface, whereas on the Ge (111) surface, isotropic patterns with a sixfold symmetry emerge. After high-fluence irradiations, these patterns exhibit well-developed facets. A deterministic nonlinear continuum equation accounting for the effective surface currents due to an Ehrlich-Schwoebel barrier for diffusing vacancies reproduces remarkably well our experimental observations.

Spectrally and time‐resolved cathodoluminescence microscopy of semipolar InGaN SQW on (11$\overline {2} $2) and (10$\overline {1} $1) pyramid facets

AbstractSemipolar InGaN/GaN single quantum wells (SQWs) grown on {11$\overline {2} $2} planes of an inverted pyramid surface and {10$\overline {1} $1} facets of single self assembled pyramids have been studied by spatially, spectrally, and time‐resolved cathodoluminescence (CL) microscopy. Mappings of local spectra and local transients provide the distribution of spectral and time‐resolved luminescence properties by peak wavelength images, time delayed CL images (TDCLIs), and initial lifetime maps. The SQW on inverse pyramids exhibit strong local differences in recombination kinetics – two orders of magnitude change in initial lifetime – correlated with a giant shift in emission energy of ∼1 eV along a facet. For single pyramids a migration process of indium adatoms from the upper facet to the edges leads to an emission of longer wavelengths at the edges and shorter wavelengths at the upper facet with respect to the base.

Three‐dimensional GaN for semipolar light emitters

AbstractSelective‐area epitaxy is used to form three‐dimensional (3D) GaN structures providing semipolar crystal facets. On full 2‐in. sapphire wafers we demonstrate the realization of excellent semipolar material quality by introducing inverse GaN pyramids. When depositing InGaN quantum wells on such a surface, the specific geometry influences thickness and composition of the films and can be nicely modeled by gas phase diffusion processes. Various investigation methods are used to confirm the drastically reduced piezoelectric polarization on the semipolar planes. Complete electrically driven light‐emitting diode test structures emitting in the blue and blue/green spectral regions show reasonable output powers in the milliwatt regime. Finally, first results of the integration of the 3D structures into a conventional laser design are presented.

Semipolar GaInN/GaN light‐emitting diodes grown on honeycomb patterned substrates

AbstractExcellent semipolar GaN material quality can be obtained by growing inverse GaN pyramids on full 2 inch c‐plane sapphire when combining the advantages of defect reduction via FACELO and selective area growth for faceted surfaces. The nearly defect free material is obtained by structuring the mask into a honeycomb pattern. When realizing full GaInN/GaN LED structures on those templates a relatively broad emission is observed during electroluminescence measurements.Furthermore, the dominant wavelength is found to be dependent on the applied current density. The pronounced shift is in good agreement with the wavelength shift along one single facet. Gas phase diffusion during the active area growth is believed to be the main reason for the varying indium content. The homogeneity could be influenced by changing the reactor pressure during the active area growth (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Properties of Blue and Green InGaN/GaN Quantum Well Emission on Structured Semipolar Surfaces

We investigate the properties of semipolar InGaN/GaN quantum wells (QWs) emitting in the blue and green spectral region. A single QW is grown on inverse V-shaped GaN pyramids which are formed by selective area epitaxy on a template masked with different hexagonally ordered patterns. Using photo- and locally resolved cathodoluminescence measurements the high material quality of the semipolar planes could be confirmed (full width at half maximum (FWHM) of D0X: 2.6 meV). Furthermore, it was found that the InGaN QW emission wavelength within one pyramid depends on the facet type as well as on the position along the facet.

Large-area Co-silicide nanodot arrays produced by colloidal nanosphere lithography and thermal annealing

We report here the successful fabrication of large-area size-tunable periodic arrays of cobalt and Co-silicide nanodots on silicon substrates by employing the colloidal nanosphere lithography (NSL) technique and heat treatments. The growth of low-resistivity epitaxial CoSi 2 was found to be more favorable for the samples with smaller Co nanodot sizes. The sizes of the epitaxial CoSi 2 nanodots can be tuned from 50 to 100 nm by varying the diameter of the colloidal spheres and annealing temperatures. The epitaxial CoSi 2 nanodots were found to grow with an epitaxial orientation with respect to the (0 0 1)Si substrates: [0 0 1]CoSi 2//[0 0 1]Si and (2 0 0)CoSi 2//(4 0 0)Si. From the results of planview HRTEM, XTEM, and SAED analysis, the epitaxial CoSi 2 nanodots were identified to be inverse pyramids in shape, and the average sizes of the faceted silicide nanodots were measured to decrease with annealing temperature. The observed results present the exciting prospect that with appropriate controls, the colloidal NSL technique promises to facilitate the growth of a variety of well-ordered silicide nanodots with selected shape, size, and periodicity.

Fabricated nano-disk InGaN/GaN multi-quantum well of the inverse hexagonal pyramids

Self-assembled inverted hexagonal pyramids with GaN:Mg and InGaN/GaN multi-quantum-well (MQW) structures were formed through photoelectrochemical wet etching. The formation mechanism of the inverted hexagonal pyramid consisted with a lateral etching process of the InGaN/GaN active layer, bottom-up etching process from N-face GaN direction, and anisotropic etching process. The photoluminescence (PL) intensity of GaN:Mg peak was enhanced in this inverted hexagonal pyramid caused by the quantum confinement effect of this nano-structure. These inverted hexagonal pyramids, consisting of the p-type GaN:Mg, nano-disk InGaN/GaN active layer, and n-type GaN:Si layer, are suitable for nano-scale optoelectronic devices.

Epitaxial growth of CoSi2 on Si(001) by reactive deposition epitaxy: Island growth and coalescence

Epitaxial CoSi2 layers, which are phase pure but contain {111} twins, are grown on Si(001) at 700 °C by reactive deposition epitaxy. Transmission electron microscopy analyses show that the initial formation of CoSi2(001) follows the Volmer–Weber mode characterized by the independent nucleation and growth of three-dimensional islands whose evolution we follow as a function of deposited Co thickness tCo in order to understand the origin of the observed twin density. We find that there are two families of island shapes: inverse pyramids and platelets. The rectangular-based pyramidal islands extend along orthogonal 〈110〉 directions, bounded by four {111} CoSi2/Si interfaces, and grow with a cube-on-cube orientation with respect to the substrate: (001)CoSi2||(001)Si and [100]CoSi2||[100]Si. Platelet-shaped CoSi2 islands are bounded across their long 〈110〉 directions by {111} twin planes (i.e. {111}(001)CoSi2||{111}Si) and their narrow 〈110〉 directions by {511}CoSi2||{111}Si interfaces. The top and bottom surfaces are {22¯1}, with {22¯1}CoSi2||(001)Si, and {1¯1¯1}, with {1¯1¯1}CoSi2||{11¯1}Si, respectively. The early stages of film growth (tCo≤13 Å) are dominated by the twinned platelets due to a combination of higher nucleation rates resulting from a larger number of favorable adsorption sites in the Si(001)2×1 surface unit cell and rapid elongation of the platelets along preferred 〈110〉 directions. However, at tCo≥13 Å island coalescence becomes significant as orthogonal platelets intersect and block elongation along fast growth directions. In this regime, where both twinned and untwinned island number densities have saturated, further island growth becomes dominated by the untwinned islands. A continuous epitaxial CoSi2(001) layer, with a twin density of 2.8×1010 cm−2, is obtained at tCo=50 Å.

Growth of size-tunable periodic Ni silicide nanodot arrays on silicon substrates

The fabrications of size-tunable periodic arrays of nickel metal and silicide nanodots on (001)Si substrates using polystyrene (PS) nanosphere lithography (NSL) and heat treatments have been investigated. The growth of epitaxial NiSi2 was found to be more favorable for the Ni metal nanodot arrays. The effect becomes more pronounced with a decrease in the size of the Ni nanodots. The sizes of the epitaxial NiSi2 nanodots were tuned from 38 to 110nm by varying the diameter of the PS spheres and heat treatment conditions. These epitaxial NiSi2 nanodots formed on (001)Si were found to be heavily faceted and the faceted structures were more prone to form at higher temperatures. Based on TEM, HRTEM and SAED analysis, the faceted NiSi2 nanodots were identified to be inverse pyramids in shape. Compared with the NiSi2 nanodot arrays formed using single-layer PS sphere masks, the epitaxial NiSi2 nanodot arrays formed from the double-layer PS sphere templates exhibit larger interparticle spacings and smaller particle sizes. Since the nanoparticle sizes, shapes and interparticle spacings can be adjusted by tuning the diameter of the PS spheres, stacking conditions, and heat treatment conditions, the PS NSL technique promises to be an effective patterning method for growth of other nanostructures.

P‐type doping in GaN through Be implantation

P-type doping through Be implantation in GaN is achieved by a two-step annealing process. Combined photoluminescence-Raman measurements showed Be-related band, indicating a low acceptor ionization energy of 140 meV, and resonance Raman features. X-ray diffraction revealed the lattice expansion due to Be-implantation. Scanning electron microscopy exhibited the surface defects morphology with hexagonal plane like inverse open pyramids. The effect of new annealing process on Be activation is discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of shape of compost blocks on the propagation, transplant establishment and yield of four vegetable species

SummarySeedlings of lettuce, leek, cauliflower and celery were raised in 12 cm3 of compressed blocking compost shaped as cubes, cylinders, inverse pyramids and pyramids contained in expanded polystyrene cells and as bare cubes and pyramids. Satisfactory establishment was obtained with all shapes of block when water was applied immediately after transplanting and, although differences in growth did occur, no single shape appeared to be best for all four species. In unirrigated soil, transplants in enclosed pyramids had the greatest outgrowth of new roots but the general benefits achieved by using any particular block shape were outweighed by irrigating immediately after transplanting. It is concluded that the choice of block shape may be dictated by the mechanical demands of the raising, handling and transplanting systems with little effect on transplant establishment or subsequent yield.

The Morphology of Organizations

Organization structures are generally assumed to take on the shape of a pyramid. To see whether this assumption is valid, an index of organization shape was devised and applied to 25 agencies of the federal government and four county health departments. Line personnel and the chiefs of line units did indeed form the familiar pyramidal pattern, but staff personnel and the chiefs of staff units formed inverse pyramids, while total personnel fell into a wide variety of patterns, none of which could reasonably be called pyramids. Shape did not correlate with organization age, size, mission, expenditures, expenditures per employee, or any combination of these factors. A possible conclusion is that line structures are governed by commonly assumed imperatives of span of control and convenience of communication, but staff people are distributed to organizational levels on the basis of organizational politics. If this is so, the distribution of personnel among administrative levels may indicate the locus of power in organizations.