Junce Zhang

Phenomenological Model of the Growth of Ultrasmooth Silver Thin Films Deposited with a Germanium Nucleation Layer

The structural properties of optically thin (15 nm) silver (Ag) films deposited on SiO2/Si(100) substrates with a germanium (Ge) nucleation layer were studied. The morphological and crystallographical characteristics of Ag thin films with different Ge nucleation layer thicknesses were assessed by cross-sectional transmission electron microscopy (XTEM), reflection high-energy electron diffraction (RHEED), X-ray diffractometry (XRD), grazing incidence X-ray diffractometry (GIXRD), X-ray reflection (XRR), and Fourier transform infrared spectroscopy (FTIR). The surface roughness of Ag thin films was found to decrease significantly by inserting a Ge nucleation layer with a thickness in the range of 1 to 2 nm (i.e., smoothing mode). However, as the Ge nucleation layer thickness increased beyond 2 nm, the surface roughness increased concomitantly (i.e., roughing mode). For the smoothing mode, the role of the Ge nucleation layer in the Ag film deposition is discussed by invoking the surface energy of Ge, the bond dissociation energy of Ag-Ge, and the deposition mechanisms of Ag thin films on a given characteristic Ge nucleation layer. Additionally, Ge island formation, the precipitation of Ge from Ag-Ge alloys, and the penetration of Ge into SiO2 are suggested for the roughing mode. This demonstration of ultrasmooth Ag thin films would offer an advantageous material platform with scalability for applications such as optics, plasmonics, and photonics.

Indium phosphide nanowire network: growth and characterization for thermoelectric conversion

Indium phosphide (InP) nanowires were grown by metal organic chemical vapor deposition (MOCVD). InP nanowires grew in the structure of three-dimensional networks in which electrical charges and heat can travel over distances much longer than the mean length of the constituent nanowires. We studied the dependence of thermoelectric properties on geometrical factors within the InP nanowire networks. The InP nanowire networks show Seebeck coefficients comparable with that of bulk InP. Rather than studying single nanowires, we chose networks of nanowires formed densely across large areas required for large scale production. We also studied the role played by intersections where multiple nanowires were fused to form the nanowire networks. Modeling based on finite-element analysis, structural analysis, and transport measurements were carried out to obtain insights of physical properties at the intersections. Understanding these physical properties of three-dimensional nanowire networks will advance the development of thermoelectric devices.

Two-step growth and fabrication of thermoelectric devices employing indium phosphide nanowire networks

The ability to make a good electrical/thermal contact to a large area filled with semiconductor nanowires has been a major engineering challenge in developing this type of thermoelectric devices. A practical fabrication process of a top electrical/thermal contact onto a network of randomly oriented intersecting semiconductor nanowires was designed by implementing a sequence of two separated metal organic chemical vapor deposition processes for indium phosphide. In the first step, a nanowire network was grown on a substrate with indium phosphide nanowires grown axially. Subsequently, growth temperature and pressure were altered to change the axial growth to lateral growth that promoted the formation of indium phosphide extending over multiple nanowires. Possible growth mechanisms during the lateral growth and structural properties of the laterally grown segment will be discussed.

Single-crystal indium phosphide nanowires grown on polycrystalline copper foils with an aluminum-doped zinc oxide template

Abstract The growth of indium phosphide (InP) nanowires on transparent conductive aluminum-doped zinc oxide (AZO) thin films on polycrystalline copper (Cu) foils was proposed and demonstrated. AZO thin films and zinc oxide (ZnO) thin films, as comparison, were deposited on Cu foils by radio frequency magnetron sputtering. Subsequently, InP was grown by metal organic chemical vapor deposition with gold catalysts. InP nanowire networks formed on the AZO thin films, while no InP nanowires grew on the ZnO thin films. Morphological, crystalline, and optical properties of the InP nanowires on AZO thin films were compared with those of InP nanowires grown on silicon (Si) substrates. Zinc diffusion from AZO thin films into InP nanowire networks was suggested as the cause of substantial modifications on the optical properties of the InP nanowires on AZO thin films; redshift in photoluminescence spectra and a larger relative TO/LO intensity ratio in Raman spectra were observed, in comparison to those of the InP nanowires grown on Si substrates. In this paper, we proposed and demonstrated a new route to grow semiconductor nanowires on metals that potentially provide low-cost and mechanically flexible substrates and establish a reliable electrical contact by utilizing conductive oxide thin films as a template, which could offer a new material platform for such applications as sensors and thermoelectric devices.

Single-polycrystalline core-shell silicon nanowires grown on copper

The growth of silicon core-shell nanowires with a crystalline-core and a polycrystalline-shell on copper substrates pretreated with carbon via Plasma Enhanced Chemical Vapor Deposition (PECVD) was demonstrated. The nanowire diameters range from 120 to 250nm with 10-20nm crystalline cores. The overall large diameter enables easier methods of forming an electrical/thermal contact while the small core maintains the benefits of nanowires. By altering the copper surface with carbon, highly dense silicon nanowire networks can be directly grown on copper substrates, which could allow for efficient and economical incorporation of silicon nanowires into such applications as thermoelectric devices.

Study of Raman signal from indium phosphide nanowire networks coated with gold

Indium phosphide (InP) nanowire networks coated with gold were characterized by Raman spectroscopy. First, InP nanowire networks were grown via metal organic chemical vapor deposition (MOCVD) on silicon substrates with gold catalyst. Subsequently, gold was deposited by thermal evaporation on the grown InP nanowire networks. Different nominal thicknesses of gold were deposited, and then the goal coated InP nanowire networks were annealed in vacuum. Raman spectroscopy was used to study the dependence of InP phonon modes on the thickness of the gold coating. The study shows the gold coating decreases the longitudinal optical phonon mode signal of InP as the thickness increases. Publisher’s Note: This paper, originally published on 19 September 2013, was replaced with a corrected/revised version on 18 October 2013. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.

Deposition and characterizations of ultrasmooth silver thin films assisted with a germanium wetting layer

In this paper, silver thin films deposited on SiO2 substrates with a germanium wetting layer fabricated by electron-beam evaporation were studied. The characterization methods of XTEM, FTIR, XRD and XRR were used to study the structural properties of silver thin films with various thicknesses of germanium layers. Silver films deposited with very thin (1-5nm) germanium wetting layers show about one half of improvement in the crystallite sizes comparing silver films without germanium layer. The surface roughness of silver thin films significantly decrease with a thin germanium wetting layer, reaching a roughness minimum around 1-5nm of germanium, but as the germanium layer thickness increases, the silver thin film surface roughness increases. The relatively higher surface energy of germanium and bond dissociation energy of silver-germanium were introduced to explain the effects the germanium layer made to the silver film deposition. However, due to the Stranski-Krastanov growth mode of germanium layer, germanium island formation started with increased thickness (5-15nm), which leads to a rougher surface of silver films. The demonstrated silver thin films are very promising for large-scale applications as molecular anchors, optical metamaterials, plasmonic devices, and several areas of nanophotonics.

Sun to fibers (S2F): massively scalable collection and transmission of concentrated solar light for efficient energy conversion and storage

Concentrated solar energy has proven to be an efficient approach for both solar thermal energy applications and photovoltaics. Here, we propose a passive optical device, the Adiabatic Optical Coupler (AOC), that efficiently couples concentrated solar light from a primary solar concentrator into an optical fiber, enabling light collection and energy conversion/storage to be geographically separated, thus maximizing the overall system efficiency. The AOC offers secondary concentration of concentrated solar light through an adiabatic optical mode conversion process. Solar light, highly focused by this two stage concentrator, is delivered by optical fiber to either be subsequently converted to electricity or thermally stored. The ability to transport high energy light flux eliminates the need for high temperature working fluids in solar-thermal systems. In order to design the AOC and related peripherals, we used various modeling tools to cover different optical regimes at macroscopic and microscopic scales. We demonstrated a set of optical thin films with spatially varied refractive index up to 3 and negligible optical absorption by using proprietary sputtering technique to fabricate the AOC. We further studied the films using experimental measurements and theoretical analysis to optimize their optical properties. Preliminary cost analysis suggests that solar thermal power generation systems that employ our S2F concept could offer the cost and efficiency required to achieve the 2020 SunShot initiative levelized cost of electricity (LCOE) target. Success of this endeavor could change the energy conversion paradigm, and allow massively scalable concentrated solar energy utilization.

Aluminum oxide coating for post-growth photo emission wavelength tuning of indium phosphide nanowire networks

Semiconductor-oxide nanostructure devices can be a very intriguing material platform if optoelectronic properties of the original semiconductor nanostructures can be tuned by explicitly controlling properties of the oxide coating. This paper describes our finding that optical properties of semiconductor nanowires can be tuned by depositing a thin layer of metal oxide. In this experiment, indium phosphide nanowires were grown by metal organic chemical vapor deposition on silicon substrates with gold catalyst. The nanowires formed three-dimensional nanowire networks from which collective optical properties were obtained. The nanowire network was coated with an aluminum oxide thin film deposited by plasma-enhanced atomic layer deposition. We studied the dependence of the peak wavelength of photoluminescence spectra on the thickness of the oxide coatings. We observed continuous blue shift in photoluminescence spectra when the thickness of the oxide coating was increased. The observed blue shift is attributed to the Burstein-Moss effect due to increased carrier concentration in the nanowire cores caused by repulsion from an intrinsic negative fixed charge from the oxide surface. Samples were further characterized by scanning electron microscopy, transmission electron microscopy, and selective area diffractometry in an attempt to explain the physical mechanisms for the blue shift.

TEM study of nanofinger structures for surface enhanced Raman scattering

Chemical sensing applications utilizing surface enhanced Raman spectroscopy (SERS) have drawn significant attention recently. However, developing a reliable, high performance SERS platform remains a challenge. A novel SERS substrate based on nanofingers was successfully demonstrated to provide large enhancement reliably and showed great promise for practical applications. Capillary forces bring the gold caps on the nanofingers into close proximity upon exposure to a solution containing molecules of interest, trapping molecules within the gaps and producing greatly enhanced Raman signals. Transmission electron microscopy (TEM) was used to characterize the structure of the nanofingers, in particular the gaps between finger tips to improve the fundamental understanding of the structural-performance relationship.