Compound Semiconductor Nanostructures Research Articles

Colloidal Synthesis of γ-MnS Nanorods with Uniform Controlled Size and Pure ⟨002⟩ Growth Direction.

One dimensional (1D) compound semiconductor nanostructures have unique anisotropic optical, electrical, and physical properties. Synthesis of large scale 1D nanostructures with pure crystallographic growth direction by a colloidal route and finding an easy method to prove it were significant for further exploring their unique anisotropic properties. Additionally, MnS is one of the most important optoelectronic and magnetic semiconductors. Herein, the large scale γ-MnS nanorods with completely pure ⟨002⟩ growth direction were first synthesized and convinced by solid evidence using the X-ray diffraction method. Compared with the standard diffraction pattern of γ-MnS powder, the ⟨002⟩ oriented long γ-MnS nanorods showed only the (100),(110), (200), and (210) peaks while other diffraction peaks disappeared. This study opened a door for the synthesis of the 1D colloidal nanostructures with pure crystallographic growth direction at large scale, benefiting the manufacture of a novel apparatus based on their anisotropic properties.

Enhanced conduction in CdSe nanowires on 200 keV phosphorous negative ion implantation

For II–VI compound semiconductor nanostructures, ion implantation is considered to be an area of significant interest due to its potential use in the fabrication of the extensive variety of novel optoelectric devices. CdSe nanowires with diameter 80 nm have been grown via the template-assisted electro-deposition method and were subsequently implanted with 200 keV negative phosphorous ions at fluence ranging from 1011 ions/cm2 to 1013 ions/cm2. SRIM-TRIM simulation studies are done to understand the course and nature of events occurring during implantation. Pre- and post-implanted samples were characterized by Scanning electron microscope (SEM), X-Ray diffraction (XRD), UV–vis spectroscopy and Probe station with a Keithley source meter. Structural studies reveal no modification in crystal structure upon implantation. An increase of phosphorous content led to an apparent red shift in the optical band gap and an improvement in conductivity. These modifications are attributed to the ionization and the creation of damage on implantation.

High-Gain 200 ns Photodetectors from Self-Aligned CdS-CdSe Core-Shell Nanowalls.

1D core-shell heterojunction nanostructures have great potential for high-performance, compact optoelectronic devices owing to their high interface area to volume ratio, yet their bottom-up assembly toward scalable fabrication remains a challenge. Here the site-controlled growth of aligned CdS-CdSe core-shell nanowalls is reported by a combination of surface-guided vapor-liquid-solid horizontal growth and selective-area vapor-solid epitaxial growth, and their integration into photodetectors at wafer-scale without postgrowth transfer, alignment, or selective shell-etching steps. The photocurrent response of these nanowalls is reduced to 200 ns with a gain of up to 3.8 × 103 and a photoresponsivity of 1.2 × 103 A W-1 , the fastest response at such a high gain ever reported for photodetectors based on compound semiconductor nanostructures. The simultaneous achievement of sub-microsecond response and high-gain photocurrent is attributed to the virtues of both the epitaxial CdS-CdSe heterojunction and the enhanced charge-separation efficiency of the core-shell nanowall geometry. Surface-guided nanostructures are promising templates for wafer-scale fabrication of self-aligned core-shell nanostructures toward scalable fabrication of high-performance compact photodetectors from the bottom-up.

Graded morphology in bulk-heterojunction solar cells based on colloidal semiconductor nanostructures: Directed charge-separation and facile carrier transport

We introduce density gradient of p- and n-type compound semiconductor nanostructures in bulk-heterojunction (BHJ) solar cells. The graded BHJs (GBHJs) were formed with the p-type copper-zinc-tin-sulfide (Cu2ZnSnS4) nanoparticles and n-type Bi2S3 nanorods, both of which were based on nontoxic and earth-abundant elements and were grown at a moderate reaction temperature (180 °C). The concentration gradient of the nanostructures in the GBHJs provided a preferred directionality of nano-depletion regions for directed charge separation and also a controlled vertical segregation for ever-increasing carrier-transport pathways during the charge-extraction process. A comparison of solar cell characteristics having a bilayer, a BHJ, and a GBHJ structure is being presented. The performance of the thickness-optimized devices for highest efficiency shows that the GBHJ offered an improved short-circuit current as compared to the bilayer and the BHJ structures and a higher fill-factor as compared to the BHJ device. The overall energy conversion efficiency (η) of GBHJ exceeded that of the other two heterojunctions. The advantages of GBHJ structures in yielding an improved η have been explained through an increased exciton dissociation process along with a lower carrier recombination as compared to the bilayer and the BHJ structures, respectively. Series and shunt resistances, which were derived from current-voltage characteristics and impedance spectroscopy, supported such analyses.

III-V/Si on silicon-on-insulator platform for hybrid nanoelectronics

The unique properties of SOI wafers enable the integration of heterogeneous materials with distinct functionalities in different layers. In particular, III-V compound semiconductors are very attractive for low-noise and high-speed electronic and photonic components integrated on a single chip. We have developed a CMOS compatible and fully integrated solution for the integration of III-V compound semiconductors with silicon technology for optoelectronic applications. InAs compound semiconductor nanostructures are synthesized in SOI wafers using the combined ion beam implantation and millisecond liquid-phase epitaxial growth. Optoelectronic and microstructural investigations carried out on implanted, annealed, and selectively etched samples confirm the formation of high-quality III-V compound semiconductor nanostructures.

Unified model of droplet epitaxy for compound semiconductor nanostructures: Experiments and theory

We present a unified model of compound semiconductor growth based on kinetic Monte Carlo simulations in tandem with experimental results that can describe and predict the mechanisms for the formation of various types of nanostructures observed during droplet epitaxy. The crucial features of the model include the explicit and independent representation of atoms with different species and the ability to treat solid and liquid phases independently. Using this model, we examine nanostructural evolution in droplet epitaxy. The model faithfully captures several of the experimentally observed structures, including compact islands and nanorings. Moreover, simulations show the presence of Ga/GaAs core-shell structures that we validate experimentally. A fully analytical model of droplet epitaxy that explains the relationship between growth conditions and the resulting nanostructures is presented, yielding key insight into the mechanisms of droplet epitaxy.

Vertical “III–V” V-Shaped Nanomembranes Epitaxially Grown on a Patterned Si[001] Substrate and Their Enhanced Light Scattering

We report on a new form of III-V compound semiconductor nanostructures growing epitaxially as vertical V-shaped nanomembranes on Si(001) and study their light-scattering properties. Precise position control of the InAs nanostructures in regular arrays is demonstrated by bottom-up synthesis using molecular beam epitaxy in nanoscale apertures on a SiO(2) mask. The InAs V-shaped nanomembranes are found to originate from the two opposite facets of a rectangular pyramidal island nucleus and extend along two opposite <111> B directions, forming flat {110} walls. Dark-field scattering experiments, in combination with light-scattering theory, show the presence of distinctive shape-dependent optical resonances significantly enhancing the local intensity of incident electromagnetic fields over tunable spectral regions. These new nanostructures could have interesting potential in nanosensors, infrared light emitters, and nonlinear optical elements.

Transfer Printing of Compound Semiconductor Nanostructures on Heterogeneous Substrates

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Morphology control of gallium nitride grown on silicon nanoporous pillar array: From cone-strings to nanowires

Fascicle arrays of gallium nitride (GaN) nanostructures were grown on silicon nanoporous pillar array (Si-NPA) by a reactive chemical vapor deposition method. Through adjusting the distance between the gallium source and Si-NPA substrate, the morphology of GaN nanostructures was tuned from cone-strings, cone-strings plus nanowires to nanowires, accompanied with the average diameter changed from ∼800nm to ∼13nm. Both the cone-strings and the nanowires were found growing along [0001] direction. These results indicate that Ga concentration is a key factor in determining both the morphology and the average diameter of GaN nanostructures. The growing process of the GaN nanostructures was explained under the frame of vapor–liquid–solid deposition mechanism. Our method might be expanded to the growth of other compound semiconductor nanostructures on patterned silicon substrates for constructing functional nanodevices.

Photochemical growth of cadmium-rich CdS nanotubes at the air–water interface and their use in photocatalysis

Cadmium-rich CdS nanotubes were directly obtained at the air–water interface by a new photochemical route. Under ultraviolet light irradiation, branch-like lamellas were first formed on the surface of the precursor solution, and then they bent into nanotubes because of the composition difference between the two sides of lamellas during photochemical reactions and sulfur–air reactions. A typical nanotube has one spherical seal and one open end. Most of the cadmium was contained in the tubes and a little was doped in the tube-walls during their formation. Such nanotubes showed higher photocatalytic activity than the corresponding pure CdS nanotubes in the photodegradation of methylene blue because of the existence metal cadmium. This route is green, template/surfactant-free, reproducible and can be extended to prepare other binary compound semiconductor nanostructures containing elements that can react either with air or another gas.

III-V compound semiconductor nanostructures on silicon: epitaxial growth, properties, and applications in light emitting diodes and lasers

Significant developments have occurred in the area of III-V compound semiconductor nanostructures. The scope of developments includes quantum dots and nanowires epitaxially grown on Si substrates, as well as their applications in light emitting diodes and lasers. Such nanoscale heterostructures exhibit remarkable structural, electrical, and optical properties. The highly effective lateral stress relaxation, due to the presence of facet edges and free surfaces, enables the achievement of nearly defect-free III-V nanoscale heterostructures directly on Si, in spite of the large lattice mismatches and the surface incompatibility. With the incorporation of multiple quantum dot layers as highly effective three-dimensional dislocation filters, self-organized quantum dot lasers monolithically grown on Si exhibit, for the first time, relatively low threshold current (Jth = 900 A/cm2) and very high temperature stability (T0 = 244 K). III-V semiconductor nanowire light emitting diodes on Si, with emission wavelengths from UV to near-infrared, have also been demonstrated.

Direct integration of III–V compound semiconductor nanostructures on silicon by selectiveepitaxy

Direct integration of III–V compound semiconductor (GaAs) on silicon (Si) substrates hasbeen demonstrated using selective epitaxy on patterned substrates. GaAs was grown directlyon patterned (001), (011), and (111) Si substrates covered with 60 nm thick thermally grownSiO2. After growth, GaAs crystals with both dot and wire shapes show preferred{011} facets. GaAs grown on particular wire patterns which are parallel to{011} surfaces shows uniform nanowire structure. Transmission electron microscopy showsdislocation-free GaAs with low density of anti-phase domain boundaries along [111]directions. Metal–oxide–semiconductor structures with aluminum oxide as gate dielectricwere fabricated using GaAs nanowires on Si. Capacitance–voltage measurements show clearinversion and accumulation.

Soft photocurable nanoimprint lithography for compound semiconductor nanostructures

Soft photocurable nanoimprint lithography is used to transfer nanoscale features to GaAs substrates. Only ambient conditions without externally applied imprinting pressure are required during the imprinting step. The pattern which is transferred consists of a dense array of 100nm features, either one-dimensional lines or two-dimensional holes. Both wet etching and dry etching are used to etch the GaAs for pattern transfer. Examination by atomic force microscopy and scanning electron microscopy confirms faithful pattern transfer; defects are few enough and patterned areas are large enough for optoelectronic device applications.

Toward Atom-by-Atom Assembly of Compound Semiconductor Nanostructures: Mechanical Atomic Discrimination and Atomic Manipulation at Room Temperature

An atomic force microscope (AFM) under noncontact and nearcontact regions operated at room-temperature (RT) in ultrahigh vacuum, is used as a tool for topography-based atomic discrimination and lateral atomic manipulations of two intermixed atomic species on semiconductor surfaces. Noncontact AFM topography can give height difference between two intermixed atomic species. Therefore, noncontact AFM topography is a kind of atom selective image and hence can serve for atomic discrimination in simple cases. Besides, site-specific force curves provide the chemical covalent bonding forces between the tip apex and the atoms at the surface. Here, we introduced both examples related to topography-based atomic discrimination using selected Sn and Si adatoms in Sn/Si(111)-(√3×√3) surface. Recently, under nearcontact region, we found a lateral atom-interchange manipulation phenomenon at RT in Sn/Ge(111)-c(2×8) intermixed sample. This phenomenon can interchange an embedded Sn atom with a neighbor Ge atom at RT. Using the vector scan method under nearcontact region, we constructed “Atom Inlay”, that is, atom letters “Sn” consisted of 19 Sn atoms embedded in Ge(111)-c(2×8) substrate. Using these methods, now we can assemble compound semiconductor nanostructures atom-by-atom. Keywords: Atom-by-atom assembly, Atomic discrimination, Atomic force microscopy, Site-specific force spectroscopy, Lateral atom manipulation, Atom-interchange manipulation

複素ナノ構造体を組み立てる―原子間力顕微鏡で原子を識別・操作・組立―

An atomic force microscope (AFM) under noncontact and nearcontact regions operated at room-temperature (RT) in ultrahigh vacuum, is used as a tool for topography-based atomic discrimination and atomic-interchange manipulations of two intermixed atomic species on semiconductor surfaces. Noncontact AFM topography based site-specific force curves provide the chemical covalent bonding forces between the tip apex and the atoms at the surface. Here, we introduced an example related to topography-based atomic discrimination using selected Sn and Si adatoms in Sn/Si(111)-(√3 ×√3 ) surface. Recently, under nearcontact region, we found a lateral atom-interchange manipulation phenomenon at RT in Sn/Ge(111)-c(2×8) intermixed sample. This phenomenon can interchange an embedded Sn atom with a neighbor Ge atom at RT. Using the vector scan method under nearcontact region, we constructed “Atom Inlay”, that is, atom letters “Sn” consisted of 19 Sn atoms embedded in Ge(111)-c(2×8) substrate. Using these methods, now we can assemble compound semiconductor nanostructures atom-by-atom.

Controlled Synthesis of Zinc Selenide Nanostructures using Oil-Water-Amphiphilic Block Copolymer Liquid Crystals

ABSTRACTA technique for simultaneous size and shape control of compound semiconductor nanostructures using poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) block copolymer/water/p-xylene lyotropic liquid crystals as templates is reported. Luminescent quantum dots of ZnSe were grown in the spherical domains of reverse (water-in-oil) cubic liquid crystals, hollow nanospheres and hollow nanotubes were grown around the dispersed spherical and cylindrical oil domains of the normal (oil-in-water) cubic and hexagonal phases respectively, and free-standing quantum wells (nanoplates or nanolaminates) were grown in the lamellar liquid crystals. The ZnSe nuclei were formed at room temperature by a spontaneous and irreversible reaction between zinc acetate that was dissolved in the water phase, and hydrogen selenide that was allowed to diffuse into the liquid crystalline templates. The nanostructures were characterized by HR-TEM, XRD, and optical spectroscopy. The shape of the nanocrystals can be controlled by selecting the structure of the templating phase. The size of the nanocrystals can be controlled by the size of the nanodomains and the concentration of the zinc precursor in them.

Controlled Growth of III-V Compound Semiconductor Nano-Structures and Their Application in Quantum-Devices

This paper reviews our work on controlled growth of self-assembled semiconductor nanostructures, and their application in light-emission devices. High-power, long-life quantum dots (QD) lasers emitting at ~1 µm, red-emitting QD lasers, and long-wavelength QD lasers on GaAs substrates have successfully been achieved by optimizing the growth conditions of QDs.

Compound Semiconductor Nanostructures by Template Wetting

Compound Semiconductor Nanostructures by Template Wetting

Projective techniques for the growth of compound semiconductor nanostructures

We investigate the geometric deposition of compound semiconductors through shallow shadow masks as a means to obtain 3D nanostructures in situ. A high spatial resolution in the order of 10 nm and good material quality are achieved by epitaxy with directed molecular beams through stationary masks. Based on our study of growth dynamics new processes were tailored for the deposition of compound semiconductors. These expand the degrees of freedom of directional shadow growth. We would like to give an overview of this alternative technology, and present late results from experiments with an optimised molecular beam epitaxy (MBE) set-up. Projective techniques are demonstrated with sample structures. We discuss their respective potential and limitations. An outlook shows the perspective of further increase of the complexity of feasible structures by increasing the complexity of the masks (3D nanostructured).