Metalorganic chemical vapor deposition (MOCVD) has emerged as a promising technique for scalable synthesis of transition metal dichalcogenides as it enables growth at high temperatures and chalcogen overpressures which are beneficial for epitaxial growth of monolayers and provides good control over precursor flux which is necessary for the synthesis of heterostructures. Our work has focused on MOCVD growth of epitaxial semiconducting TMDs (MoS2, WS2 and WSe2) on 2” diameter c-plane sapphire substrates. Steps on the sapphire surface can be used to control the orientation of TMD domains and reduce the number of inversion domain boundaries. In situ spectroscopic ellipsometry is demonstrated to be an effective real time monitor of TMD growth even at the sub-monolayer level which can be exploited to track surface coverage as a function of time under varying growth conditions. The ability to precisely control and modulate precursor flux during growth is used to synthesize in-plane heterostructures that enable localized exciton confinement and emission.
Wafer-scale epitaxial growth of semiconducting transition metal dichalcogenide (TMD) monolayers such as MoS2, WS2 and WSe2 is of significant interest for device applications to circumvent size limitations associated with the use of exfoliated flakes. Epitaxy is required to achieve single crystal films over large areas via coalescence of TMD domains. Our research has focused on epitaxial growth of 2D semiconducting TMDs on sapphire substrates using metalorganic chemical vapor deposition (MOCVD). Steps on the miscut sapphire surface serve as preferential sites for nucleation and can be used to induce a preferred crystallographic direction to the TMD domains which enables a reduction in inversion domain boundaries in coalesced films. The step-directed growth is dependent on the surface termination of the sapphire which can be altered through pre-growth annealing in H2 and chalcogen-rich environments. Uniform growth of TMD monolayers with significantly reduced inversion domains is
The growth of thin Bi2Se3 films on (0001) sapphire substrates by metalorganic chemical vapor deposition (MOCVD) was investigated. A two-heater configuration was employed to pre-crack the metalorganic sources upstream of the
substrate while maintaining a low substrate temperature (<250°C). Epitaxial Bi2Se3 films with (006) x-ray rocking curve full-width-at-half-maximum values on the order of 160 arcsecs were obtained at growth rates of ~6 nm/min or lower while higher growth rates resulted in polycrystalline films. The background electron concentration of the films was
found to depend strongly on the substrate temperature and Se/Bi inlet ratio. Bi2Se3 films with a room temperature electron concentration of 6.7x1019 cm-3 and mobility of 155 cm2/Vs were obtained at 200°C with a Se/Bi ratio of 80. Higher substrate temperature and lower Se/Bi ratios resulted in an increase in electron concentration and corresponding reduction in mobility. The results demonstrate the potential of MOCVD for the growth of Bi2Se3 and related materials for topological insulator studies.
The epitaxial growth of <110> silicon nanowires on (110) Si substrates by the vapor-liquid-solid growth process was investigated using SiCl4 as the source gas. A high percentage of <110> nanowires was obtained at high temperatures and reduced SiCl4 partial pressures. Transmission electron microscopy characterization of the <110> Si nanowires revealed symmetric V-shaped {111} facets at the tip and large {111} facets on the sidewalls of the nanowires. The symmetric {111} tip faceting was explained as arising from low catalyst supersaturation during growth which is expected to occur given the near-equilibrium nature of the SiCl4 process. The predominance of {111} facets obtained under these conditions promotes the growth of <110> SiNWs.
Radial p-n silicon nanowire (SiNW) solar cells are of interest as a potential pathway to increase the efficiency of
crystalline silicon photovoltaics by reducing the junction length and surface reflectivity. Our studies have focused on the
use of vapor-liquid-solid (VLS) growth in combination with chemical vapor deposition (CVD) processing for the
fabrication of radial p-n junction SiNW array solar cells. High aspect ratio p-type SiNW arrays were initially grown on
gold-coated (111) Si substrates by CVD using SiCl4 as the source gas and B2H6 as the p-type dopant source. The
epitaxial re-growth of n-type Si shell layers on the Si nanowires was then investigated using SiH4 as the source gas and
PH3 as the dopant. Highly conformal coatings were achieved on nanowires up to 25 μm in length. The microstructure of
the Si shell layer changed from polycrystalline to single crystal as the deposition temperature was raised from 650oC to
950oC. Electrical test structures were fabricated by aligning released SiNWs onto pre-patterned substrates via fieldassisted
assembly followed by selective removal of the n-type shell layer and contact deposition. Current-voltage
measurements of the radial p-n SiNWs diodes fabricated with re-grown Si shell layers at 950°C demonstrate rectifying
behavior with an ideality factor of 1.93. Under illumination from an AM1.5g spectrum and efficiency for this single
SiNW radial p-n junction was determined to be 1.8%, total wire diameter was 985 nm.
A lithography-free method for producing freestanding one-dimensional gold nanoparticle arrays encapsulated within
silicon dioxide nanowires is reported. Silicon nanowires grown by the vapor-liquid-solid technique with diameters
ranging from 20 nm to 50 nm were used as the synthesis template. The gold nanoparticle arrays were obtained by coating
the surface of the silicon nanowires with a 10 nm gold film, followed by thermal oxidation in an oxygen ambient. It was
found that the thermal oxidation rate of the silicon nanowires was significantly enhanced by the presence of the gold thin
film, which fully converted the silicon into silicon dioxide. The gold-enhanced oxidation process forced the gold into the
core of the wire, forming a solid gold nanowire core surrounded by a silicon dioxide shell. Subsequent thermal treatment
resulted in the fragmentation of the gold nanowire into a uniformly spaced array of gold nanoparticles encapsulated by a
silicon dioxide shell, which was observed by in situ annealing in transmission electron microscopy. Analysis of many
different silicon nanowire diameters shows that the diameter and spacing of the gold nanopaticles follows the Rayleigh
instability, which confirms this is the mechanism responsible for formation of the nanoparticle array.
The fabrication of high density arrays of semiconductor nanowires is of interest for nanoscale electronics, chemical and biological sensing and energy conversion applications. We have investigated the synthesis, intentional doping and electrical characterization of Si and Ge nanowires grown by the vapor-liquid-solid (VLS) method in nanoporous alumina membranes. Nanoporous membranes provide a convenient platform for nanowire growth and processing, enabling control of wire diameter via pore size and the integration of contact metals for electrical testing. For VLS growth in nanoporous materials, reduced pressures and temperatures are required in order to promote the diffusion of reactants into the pore without premature decomposition on the membrane surface or pore walls. The effect of growth conditions on the growth rate of Si and Ge nanowires from SiH4 and GeH4 sources, respectively, was investigated and compared. In both cases, the measured activation energies for nanowire growth were substantially lower than activation energies typically reported for Si and Ge thin film deposition under similar growth conditions, suggesting that gold plays a catalytic role in the VLS growth process. Intentionally doped SiNW arrays were also prepared using trimethylboron (TMB) and phosphine (PH3) as p-type and n-type dopant sources, respectively. Nanowire resistivities were calculated from plots of the array resistance as a function of nanowire length. A decrease in resistivity was observed for both n-type and p-type doped SiNW arrays compared to those grown without the addition of a dopant source.
KEYWORDS: Silicon, Germanium, Gold, Nanolithography, Group IV semiconductors, Doping, Transmission electron microscopy, Scanning electron microscopy, Boron, Nanowires
The fabrication of semiconductor nanowires, in which composition, size and conductivity can be controlled in both the radial and axial direction of the wire is of interest for fundamental studies of carrier confinement as well as nanoscale device development. In this study, group IV semiconductor nanowires, including Si, Ge and SixGe1-x alloy nanowires were fabricated by vapor-liquid-solid (VLS) growth using gaseous precursors. In the VLS process, gold is used to form a liquid alloy with Si and Ge which, upon supersaturation, precipitates a semiconductor nanowire. Nanoporous alumina membranes were used as templates for the VLS growth process, in order to control the diameter of the nanowires over the range from 45 nm to 200 nm. Intentional p-type and n-type doping was achieved through the addition of either trimethylboron, diborane or phosphine gas during nanowire growth. The electrical properties of undoped and intentionally doped silicon nanowires were characterized using field-assisted assembly to align and position the wires onto pre-patterned test bed structures. The depletion characteristics of back-gated nanowire structures were used to determine conductivity type and qualitatively compare dopant concentration. SiGe and SiGe/Si axial heterostructure nanowires were also prepared through the addition of germane gas during VLS growth. The Ge concentration in the wires was controllable over the range from 12 % to 25% by varying the inlet GeH4/SiH4 ratio.
Laser action is demonstrated in InGaN/GaN double heterostructures with cleaved facets. Hydride vapor phase epitaxy is used to grow a 10-micrometer-thick buffer layer of GaN on (0001) sapphire, and metal-organic vapor phase epitaxy is used to subsequently grow a GaN/In0.09Ga0.91N/GaN double heterostructure. One-mm-long cavities are produced by cleaving the structure along the (1010) plane of the sapphire substrate. A pulsed Nitrogen laser is used for optical excitation. At room temperature, the laser threshold occurs at an incident power density of 1.3 MW/cm2. Above threshold, the differential quantum efficiency increases by a factor of 34, the emission linewidth decreases to 13.5 meV, and the output becomes highly TE polarized.
The near-band gap and the 'yellow' optical transitions in n- type GaN grown by MOCVD have been studied by photoluminescence experiments. The excitation density ranged from 5 X 10-6 W/cm-2 to 50 W/cm-2. The UV PL intensity increases linearly in the entire range of excitation density. The yellow PL intensity exhibits a linear dependence oat low excitation densities and a square-root dependence at high excitation densities. A theoretical model is developed describing the intensity of the two radiative transition between continuum states and one defect level deep in the band gap as a function of the excitation density, free carrier and defect concentrations. The calculated dependences of the two luminescence channels follow power laws with exponents of 1/2 and 1 depending on excitation density. These dependences are in very good agreement with experimental results. The measured intensity of the yellow luminescence does not saturate at high excitation densities. This rules out the possibility that the yellow PL could arise from a sequential transition via two deep levels in the gap. It is shown that the intensity modulation that frequently appears in the PL spectra is caused by a micro-cavity which is formed by the semiconductor-substrate and semiconductor-air interfaces. Finally, the dependence of the yellow luminescence intensity on n-type doping concentration indicates that the deep center causing the yellow luminescence is an acceptor-like level.
InGaN double heterostructure light emitting diodes (DH-LEDs) were fabricated on hydride vapor phase epitaxy (HVPE) GaN- on-sapphire substrates. These substrates consisted of a thick HVPE GaN layer grown directly on sapphire and eliminated the need for the growth of a low-temperature buffer layer for GaN epitaxy on sapphire. Homojunction and DH-LEDs have been fabricated with various composition InGaN active regions resulting in strong electroluminescence in the blue, green, and yellow portion of the visible spectra. These devices had turn-on voltages as low as 3.6 volts.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.