Selective-area Metalorganic Chemical Vapor Deposition Research Articles

Defect-Tolerant TiO2-Coated and Discretized Photoanodes for >600 h of Stable Photoelectrochemical Water Oxidation

Arrays of GaAs nanowires have been grown by selective-area metal–organic chemical-vapor deposition (MOCVD) onto photoactive planar Si substrates. This tandem, vertical-wire-array-on-planar absorber...

Defect-free InAsSb nanowire arrays on Si substrates grown by selective-area metal–organic chemical vapor deposition

We report the realization of defect-free InAsSb nanowire (NW) arrays on Si substrates by selective-area metal–organic chemical vapor deposition. We studied the effects of growth temperature and the morphology of patterned Si/SiO2 substrates on the formation of InAsSb NW arrays, and found that both the morphology and the yield of the NW arrays were sensitive to the growth conditions. By optimizing the growth conditions, we achieved high-quality InAsSb NW arrays, which exhibited a pure zinc-blende crystal structure without any defects. In addition, based on the as-grown NWs, we fabricated back-gated field effect transistors devices that showed an electron mobility of 2000 cm2 V−1 s−1 at room temperature.

Axial InAs(Sb) inserts in selective-area InAsP nanowires on InP for optoelectronics beyond 25 µm

In this work, we report on the growth of high yield small bandgap InAs and InAsSb inserts embedded in InAsP nanowires grown on an InP substrate by catalyst-free selective-area metal-organic chemical vapor deposition. It is observed that the growth of the inserts with high aspect ratios can be achieved by properly tuning the V/III ratio. Nanowire arrays with InAs(Sb) inserts exhibit strong photoluminescence at 77 K from interband transitions, spanning a wavelength range of 2.30–3.70 µm. In addition, the InAsP/InAs heterointerfaces are characterized by a scanning transmission electron microscope and an energy-dispersive X-ray spectroscopy. We believe that these results pave the way to engineering interband transitions and enabling hybrid integration for nanoscale optical devices at the mid-wavelength infrared.

Catalyst-free selective-area epitaxy of GaAs nanowires by metal-organic chemical vapor deposition using triethylgallium

We demonstrate catalyst-free growth of GaAs nanowires by selective-area metal-organic chemical vapor deposition (MOCVD) on GaAs and silicon substrates using a triethylgallium (TEGa) precursor. Two-temperature growth of GaAs nanowires—nucleation at low temperature followed by nanowire elongation at high temperature—almost completely suppresses the radial overgrowth of nanowires on GaAs substrates while exhibiting a vertical growth yield of almost 100%. A 100% growth yield is also achieved on silicon substrates by terminating Si(111) surfaces by arsenic prior to the nanowire growth and optimizing the growth temperature. Compared with trimethylgallium (TMGa) which has been exclusively employed in the vapor–solid phase growth of GaAs nanowires by MOCVD, the proposed growth technique using TEGa is advantageous because of lower growth temperature and fully suppressed radial overgrowth. It is also known that GaAs grown by TEGa induce less impurity incorporation compared with TMGa, and therefore the proposed method could be a building block for GaAs nanowire-based high-performance optoelectronic and nanoelectronic devices on both III–V and silicon platforms.

Modeling of semiconductor nanowire selective-area MOCVD growth

A numerical approach to the determination of gas kinetics in the case of non-planar nanostructure growth via the selective-area metal-organic chemical vapor deposition is developed. The direct simulation Monte-Carlo method is utilized to model the rarefied gas flow of precursor particles nearby the substrate. The computation is performed for the GaAs nanowire growth via the selective-area metal-organic chemical vapor deposition. The model allows the quantitative description of the decrease of nanowire length with the increase of distance between nanowires (the so-called synergetic effect). The optimal pitch of the mask that corresponds to the maximal nanowire length is found for typical growth conditions. In particular, our calculation shows that the optimal pitch increases with the increase of the nanowire diameter.

Selective-area InAsSb Nanowires on InP for 3 – 5 μm Mid-wavelength Infrared Optoelectronics

ABSTRACT We demonstrate high vertical yield InAs1-xSbx (0 < x ≤ 0.18) nanowire arrays grown on InP (111)B substrates by calalyst-free selective-area metal-organic chemical vapor deposition. High antimony composition is achieved by pulsing the arsenic flow to reduce the effective arsenic partial pressure while keeping the antimony partial pressure fixed. This increases the antimony vapor phase composition while allowing the antimony partial pressure to be kept low enough to avoid antimony condensation on the growth mask. InAsSb nanowire arrays show strong emission by photoluminescence at 77 K, covering a wavelength range of 3.77 – 5.08 μm. These results pave the way to engineering optical properties and enabling hybrid integration for nanoscale mid-wavelength infrared optical devices.

Selective-area growth of periodic nanopyramid light-emitting diode arrays on GaN/sapphire templates patterned by multiple-exposure colloidal lithography

Gallium nitride-based nanopyramid light-emitting diodes are a promising technology to achieve highly efficient solid-state lighting and beyond. Here, periodic nanopyramid light-emitting diode arrays on gallium nitride/sapphire templates were fabricated by selective-area metalorganic chemical vapor deposition and multiple-exposure colloidal lithography. The electric field intensity distribution of incident light going through polystyrene microspheres and photoresist are simulated using finite-different time-domain method. Nitrogen as the carrier gas and a low V/III ratio (ratio of molar flow rate of group-V to group-III sources) are found to be important in order to form gallium nitride nanopyramid. In addition, a broad yellow emission in photoluminescence and cathodoluminescence spectra were observed. This phenomena showed the potential of nanopyramid light-emitting diodes to realize long wavelength visible emissions.

Seeding layer assisted selective-area growth of As-rich InAsP nanowires on InP substrates.

We present the first demonstration of arsenic-rich InAs1-xPx (0 ≤ x ≤ 0.33) nanowire arrays grown on InP (111)B substrates by catalyst-free selective-area metal-organic chemical vapor deposition. It is shown that by introducing a thin InAs seeding layer prior to the growth of the nanowire, an extremely high vertical yield is achieved by eliminating rotational twins between (111)A and (111)B crystal orientations. InAsP nanowire arrays show strong emission of photoluminescence (PL) at room temperature, suggesting a significant reduction of surface state density compared with InAs nanowires. The phosphorus composition deduced from the PL peak energy is verified by energy-dispersive X-ray spectroscopy. The growth temperature shows a strong impact on the aspect ratio of InAs1-xPx nanowires with different phosphorus compositions. In addition, no PL emission is observed from nanowires grown with arsenic overpressure, likely due to an exchange of phosphorus with arsenic atoms at the surface which results in an increase in the surface state density. These results provide a path for the growth of heterojunctions based on As-rich InAs1-xPx for nanoscale short-wavelength infrared and mid-wavelength infrared optical devices.

Semipolar InGaN/GaN nanostructure light-emitting diodes on c-plane sapphire

The fabrication of electrically injected triangular-nanostripe core–shell semipolar III–nitride LEDs (TLEDs) is demonstrated using interferometric lithography and catalyst-free bottom-up selective-area metal–organic chemical vapor deposition (MOCVD). This alternative approach enables semipolar orientations on inexpensive, c-plane sapphire substrates, in comparison with planar growth on free-standing GaN substrates. Transmission electron microscopy and energy dispersive X-ray spectroscopy reveal nonuniform quantum well thickness and composition, respectively, as a function of location on the triangular stripes. The broad electroluminescence spectra, wavelength shift with increasing current density, and nonlinear light vs current characteristics are well correlated with the observed quantum-well nonuniformities.

High-Quality InAsSb Nanowires Grown by Catalyst-Free Selective-Area Metal-Organic Chemical Vapor Deposition.

We report on the first demonstration of InAs1-xSbx nanowires grown by catalyst-free selective-area metal-organic chemical vapor deposition (SA-MOCVD). Antimony composition as high as 15 % is achieved, with strong photoluminescence at all compositions. The quality of the material is assessed by comparing the photoluminescence (PL) peak full-width at half-max (fwhm) of the nanowires to that of epitaxially grown InAsSb thin films on InAs. We find that the fwhm of the nanowires is only a few meV broader than epitaxial films, and a similar trend of relatively constant fwhm for increasing antimony composition is observed. Furthermore, the PL peak energy shows a strong dependence on temperature, suggesting wave-vector conserving transitions are responsible for the observed PL in spite of lattice mismatched growth on InAs substrate. This study shows that high-quality InAsSb nanowires can be grown by SA-MOCVD on lattice mismatched substrate, resulting in material suitable for infrared detectors and high-performance nanoelectronic devices.

Analysis and optimization of the edge effect for III–V nanowire synthesis via selective area metal-organic chemical vapor deposition

Selective Area Metal-Organic Chemical Vapor Deposition (SA-MOCVD) is a promising technique for the scale-up of nanowire fabrication. Our previous study investigated the growth mechanism of SA-MOCVD processes by quantifying contributions from various diffusion sources. However, the edge effect on nanostructure uniformity captured by skirt area diffusion was not quantitatively analyzed. This work further improves our understanding of the process by considering the edge effect as a superposition of skirt area diffusion and “blocking effect” and optimizing the edge effect for uniformity control of nanowire growth. We directly model the blocking effect of nanowires in the process of precursor diffusion from the skirt area to the center of a substrate. The improved model closely captures the distribution of the nanowire length across the substrate. Physical interpretation of the edge effect is provided. With the established model, we provide a method to optimize the width of the skirt area to improve the predicted structural uniformity of SA-MOCVD growth.

Growth, structural and optical properties of ternary InGaN nanorods prepared by selective-area metalorganic chemical vapor deposition

Ternary InGaN nanorods were prepared on dielectric-masked nano-holes with selective area metalorganic chemical vapor deposition. To overcome the tendency for random nucleation of GaN at low temperatures, a pulsed growth procedure was introduced to enhance the diffusion length of Ga adatoms on SiO2, resulting in good selectivity at typical temperature ranges for InGaN. Photoluminescence from the InGaN nanorods can be tuned from near ultraviolet (400 nm) to blue-green (∼500 nm). Microstructural properties were characterized by transmission electron microscopy; threading dislocations from the underlying GaN template were terminated at the nanorod/template interface, resulting in dislocation-free nanorods. The height of dislocation-free InGaN nanorods is about 150 nm, which is much larger than the critical thickness for the onset of misfit dislocations in planar InGaN growth with typical thickness of less than 10 nm for an indium composition between 10 and 20%. The composition profile of In along the growth direction was examined by energy dispersive x-ray spectroscopic mapping and line scan. Oscillations of In composition along the growth direction were observed and are likely due to the kinetic competition between In and Ga adatoms. These InGaN nanorods are expected to be useful as templates for growing higher In composition nano-light-emitting diodes.

Optical, electrical, and solar energy-conversion properties of gallium arsenide nanowire-array photoanodes

Periodic arrays of n-GaAs nanowires have been grown by selective-area metal–organic chemical-vapor deposition on Si and GaAs substrates. The optical absorption characteristics of the nanowire-arrays were investigated experimentally and theoretically, and the photoelectrochemical energy-conversion properties of GaAs nanowire arrays were evaluated in contact with one-electron, reversible, redox species in non-aqueous solvents. The radial semiconductor/liquid junction in the nanowires produced near-unity external carrier-collection efficiencies for nanowire-array photoanodes in contact with non-aqueous electrolytes. These anodes exhibited overall inherent photoelectrode energy-conversion efficiencies of ∼8.1% under 100 mW cm−2 simulated Air Mass 1.5 illumination, with open-circuit photovoltages of 590 ± 15 mV and short-circuit current densities of 24.6 ± 2.0 mA cm−2. The high optical absorption, and minimal reflection, at both normal and off-normal incidence of the GaAs nanowire arrays that occupy <5% of the fractional area of the electrode can be attributed to efficient incoupling into radial nanowire guided and leaky waveguide modes.

Selective-area growth of thin GaN nanowires by MOCVD

We report the selective-area growth (SAG) of Ga-polar thin GaN nanowires on patterned GaN/sapphire (0001) substrates using metalorganic chemical vapor deposition (MOCVD) with a continuous gas supply. It has been found that the anisotropy in the growth rates of the (0001) and the {1–100} facets of GaN reaches a maximum at low precursor flow rates for both Ga source and NH3. It has also been revealed that the SAG efficiency which is dependent on pattern fill-factor should be properly taken into account in order to grow thin GaN nanowires. Based on these findings, we demonstrate the growth of GaN nanowires with a diameter of 50nm, which is the smallest reported so far by selective-area MOCVD. Optical properties of a single GaN nanowire have been investigated by low temperature micro-photoluminescence. It has been shown that an appropriate shell layer is effective to considerably improve the properties, suggesting the importance of controlling surface states.

Bottom-up Photonic Crystal Lasers

The directed growth of III-V nanopillars is used to demonstrate bottom-up photonic crystal lasers. Simultaneous formation of both the photonic band gap and active gain region is achieved via catalyst-free selective-area metal-organic chemical vapor deposition on masked GaAs substrates. The nanopillars implement a GaAs/InGaAs/GaAs axial double heterostructure for accurate, arbitrary placement of gain within the cavity and lateral InGaP shells to reduce surface recombination. The lasers operate single-mode at room temperature with low threshold peak power density of ∼625 W/cm2. Cavity resonance and lasing wavelength is lithographically defined by controlling pillar pitch and diameter to vary from 960 to 989 nm. We envision this bottom-up approach to pillar-based devices as a new platform for photonic systems integration.

Patterned Radial GaAs Nanopillar Solar Cells

Photovoltaic devices using GaAs nanopillar radial p-n junctions are demonstrated by means of catalyst-free selective-area metal-organic chemical vapor deposition. Dense, large-area, lithographically defined vertical arrays of nanowires with uniform spacing and dimensions allow for power conversion efficiencies for this material system of 2.54% (AM 1.5 G) and high rectification ratio of 213 (at ±1 V). The absence of metal catalyst contamination results in leakage currents of ∼236 nA at -1 V. High-resolution scanning photocurrent microscopy measurements reveal the independent functioning of each nanowire in the array with an individual peak photocurrent of ∼1 nA at 544 nm. External quantum efficiency shows that the photocarrier extraction highly depends on the degenerately doped transparent contact oxide. Two different top electrode schemes are adopted and characterized in terms of Hall, sheet resistance, and optical transmittance measurements.

Bottom-up Photonic Crystal Cavities Formed by Patterned III–V Nanopillars

We report on the formation and optical properties of bottom-up photonic crystal (PC) cavities formed by III-V nanopillars (NPs) via catalyst-free selective-area metal-organic chemical vapor deposition on masked GaAs substrates. This method of NP synthesis allows for precise lithographic control of NP position and diameter enabling simultaneous formation of both the photonic band gap (PBG) region and active gain region. The PBG and cavity resonance are determined by independently tuning the NP radius r, pitch a, and height h in the respective masked areas. Near-infrared emission at 970 nm is achieved from axial GaAs/InGaAs heterostructures with in situ passivation by laterally grown InGaP shells. To achieve out-of-plane optical confinement, the PC cavities are embedded in polydimethylsiloxane (PDMS) and removed from the growth substrate. Spatially and spectrally resolved 77 K photoluminescence demonstrates a strong influence of the PBG resonance on device emission. Resonant peaks are observed in the emission spectra of PC cavities embedded in PDMS.

InGaAs heterostructure formation in catalyst-free GaAs nanopillars by selective-area metal-organic vapor phase epitaxy

We investigate axial GaAs/InGaAs/GaAs heterostructures embedded in GaAs nanopillars via catalyst-free selective-area metal-organic chemical vapor deposition. Structural characterization by transmission electron microscopy with energy dispersive x-ray spectroscopy (EDS) indicates formation of axial InxGa1−xAs (x∼0.20) inserts with thicknesses from 36 to 220 nm with ±10% variation and graded Ga:In transitions controlled by In segregation. Using the heterointerfaces as markers, the vertical growth rate is determined to increase linearly during growth. Photoluminescence from 77 to 290 K and EDS suggest the presence of strain in the shortest inserts. This capability to control the formation of axial nanopillar heterostructures is crucial for optimized device integration.

Spectral and threshold performance of patterned quantum dot lasers

Semiconductor quantum dots have been widely researched as a means of improving the performance of optoelectronic devices. Self-assembly has been the dominant method of fabricating quantum dots because of its relative ease compared to more explicit techniques. We have developed a method for fabricating quantum dots in a more explicit manner using electron beam lithography and selective-area metal-organic chemical vapor deposition crystal growth. By eliminating the dependence on strain-driven self-assembly, we can avoid the size distribution and resulting inhomogeneously broadened emission spectrum associated with self-assembled quantum dot ensembles. We report on the threshold and spectral properties of patterned quantum dot lasers. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Simulation and design of the emission wavelength of multiple quantum well structures fabricated by selective area metalorganic chemical vapor deposition

Selective area metalorganic chemical vapor deposition (SA-MOCVD) is effective for the monolithic integration of semiconductor optical devices. Using appropriate patterns of SiO 2 masks on a substrate, we can fabricate multiple quantum wells (MQWs) of In 1− x Ga x As y P 1− y alloys with various emission wavelengths. Therefore, we can fabricate both passive elements and active components for different wavelengths on a substrate by a single growth. To make the best use of this SA-MOCVD process, we need a simulation tool that predicts the performance of the grown layer for a given mask pattern. We constructed a simulation that predicts the emission wavelength of MQW structures grown by SA-MOCVD. The simulation took into account the gas-phase diffusion of the precursors of In and Ga and their incorporation to the growth area. The rate parameters of these processes were extracted from the growth-rate profile in the SA-MOCVD of InP and GaAs. Based on these data, we simulated the photoluminescence (PL) peak wavelength of (1) In 1− x Ga x As y P 1− y bulk films and (2) MQWs consisting of these quaternary alloys. The simulated results agreed with experimental results, indicating the feasibility of computer-assisted design (CAD) of the mask patterns for SA-MOCVD.