Au Catalyst Film Research Articles

Effect of additives on preparation of vertical holes in Si substrate using metal-assisted chemical etching

The influence of a polyethylene glycol (PEG) additive on the morphology of 10 μm diameter holes in a Si(001) substrate formed by metal-assisted chemical etching (MacEtch) was investigated. The addition of PEG to the etching solution suppressed deformation of the Au catalyst film, so that the verticality of the etched holes was significantly improved. There was a strong correlation between the deformation of the thin (10 nm) Au catalyst film and the bending of etched holes during the MacEtch process.

Influence of Additives on Formation of through-Si Via in Si Substrate Using Metal Assisted Chemical Etching

Recently, metal assisted chemical etching (MacEtch) attracts attention to fabricate a nano/microscale structures in Si substrate, such as nanowire and nanohole arrays. In this method, using a noble metal as a catalyst, only Si under the catalyst is selectively etched in the etching solution. Several papers have been already reported on the formation of microscale patterning using MacEtch. However, it is difficult to obtain high aspect vertical holes using MacEtch process because of its instability of etching direction for prolonged process as shown in figure (a). In this study, we present a new approach using surfactant agents as an additive in MacEtch solution to obtain vertical Si holes in Si substrates. Microscale holes were prepared in Si (100) substrates using the MacEtch process with a disc-shape patterned Au catalyst. The patterned Au discs ( 10 μm in diameter), which were deposited on a Ti / Si substrate using a photolithography lift-off process. For the MacEtch process, a mixture of hydrofluoric acid (HF), hydrogen peroxide (H2O2) and deionized water was used as the base etching solution. The PEG was added to the etching solution as an additive. The MacEtch of the Si substrate with patterned Au were performed for 120 min at 40 oC in the etching solution. Figure (a) shows cross-sectional SEM images of typical morphology of etched Si using MacEtch process without PEG in the etching solution. The etching direction of Si were changed during MacEtch as indicated with dashed arrow in Fig. (a), and these phenomena were observed in more than 60 % Si holes etched with MacEtch without PEG. Adding PEG in the etching solution, drastic improvement in verticality of etched Si holes were observed. Almost all holes were vertically etched to the Si substrate in PEG samples, and typical cross-sectional image of the etched holes is shown in Fig. (b). Fig. (c) and (d) shows enlarged image of Au catalyst films after MacEtch process. The catalyst in the bended hole prepared without PEG was deformed and partially detached from Si substrate, shown in Fig. (c). In contrast, in the case of straight holes etched with PEG shown in Fig. (d), the catalyst tightly contacted to the substrate, and showed relatively less defamation. Thus, there is a strong correlation between defamation of catalyst and formation of bended hole. The detailed mechanism of PEG additive in etching solution during Macetch process and the MacEtch results using other surfactants as additives will be presented. Figure 1

Tailoring the Green, Yellow and Red defect emission bands in ZnO nanowires via the growth parameters

Abstract The optical properties of ZnO nanowires, grown by the vapor-transport method using Au as catalyst over quartz, sapphire and SiO2/Si substrates, have been characterized as a function of the growth parameters such as temperature and time, and the thickness of the Au-catalyst film. The diameter and the length of the nanowires range from 40 to 200 nm and 0.2 to 2 µm, respectively. Room-temperature absorption and emission spectra show a well-defined exciton peak in the ultraviolet. Moreover, photoluminescence measurements show a very broad emission band in the visible range from 420 to 800 nm, which exhibits three distinct peak-like contributions in the Green, Yellow and Red at around 520, 590 and 720 nm (2.38, 2.10 and 1.72 eV), respectively. We observed that the intensity of the broad emission band in the visible increases relative to that of the exciton emission as the wire diameter decreases due to a surface effect. The origin of these visible contributions change as a function of the growth parameters and correspond to different defect-related recombination processes, which will be useful in the future applications of ZnO for optoelectronics.

Effect of a metal interlayer under Au catalyst for the preparation of microscale holes in Si substrate by metal-assisted chemical etching

Vertical microscale holes in Si(100) substrate were formed using a wet chemical method, metal-assisted chemical etching (MacEtch), with patterned Au catalyst films. Three types of samples were prepared for the MacEtch process: Au/Si, Au/SiO2/Si and Au/Ti/Si. To obtain well-defined holes in the Si substrate, the usefulness of an interlayer between the metal catalyst and the Si substrate was demonstrated. Here, the effect of the interlayer is discussed. The suppression of interdiffusion between the catalyst Au and the Si substrate by the interlayer was important to maintain the shape of the patterned catalyst layers, which influenced the morphology of the etched Si.

Low temperature rapid formation of Au-induced crystalline Ge films using sputtering deposition

We report low temperature (100–170°C) and rapid (10min) formation of crystalline Ge films between Au catalyst film and quartz glass substrate using a radio frequency magnetron sputtering deposition. The formation rate of crystalline Ge films between Au catalyst film and quartz glass substrate is proportional to the deposition rate of Ge film, namely the flux of Ge atoms. To obtain insights on the formation mechanism of crystalline Ge films, we studied dependence of grain size of Au films on annealing temperature and Au film thickness. Crystalline Ge films formed below Au films have random crystalline orientation with in-plane grain size from below 1μm. Small crystalline grain size of Au films is needed to form rapidly Au induced crystalline Ge films.

Modulating the morphology and electrical properties of GaAs nanowires via catalyst stabilization by oxygen.

Nowadays, III-V compound semiconductor nanowires (NWs) have attracted extensive research interest because of their high carrier mobility favorable for next-generation electronics. However, it is still a great challenge for the large-scale synthesis of III-V NWs with well-controlled and uniform morphology as well as reliable electrical properties, especially on the low-cost noncrystalline substrates for practical utilization. In this study, high-density GaAs NWs with lengths >10 μm and uniform diameter distribution (relative standard deviation σ ∼ 20%) have been successfully prepared by annealing the Au catalyst films (4-12 nm) in air right before GaAs NW growth, which is in distinct contrast to the ones of 2-3 μm length and widely distributed of σ ∼ 20-60% of the conventional NWs grown by the H2-annealed film. This air-annealing process is found to stabilize the Au nanoparticle seeds and to minimize Ostwald ripening during NW growth. Importantly, the obtained GaAs NWs exhibit uniform p-type conductivity when fabricated into NW-arrayed thin-film field-effect transistors (FETs). Moreover, they can be integrated with an n-type InP NW FET into effective complementary metal oxide semiconductor inverters, capable of working at low voltages of 0.5-1.5 V. All of these results explicitly demonstrate the promise of these NW morphology and electrical property controls through the catalyst engineering for next-generation electronics.

Low temperature VLS growth of ITO nanowires by electron beam evaporation method

Indium tin oxide (ITO) nanowires were grown at a lower substrate temperature of 400 °C via Au-catalyzed vapor–liquid–solid (VLS) growth mechanism by electron beam evaporation method. The grown nanowires had length and diameter of 0.8–1.2 μm and 20–50 nm, respectively for growth duration of 20 min. Transmission electron microscope studies confirm the single crystalline nature of the nanowires, and energy dispersive spectroscopy studies on the individual nanowires also confirm that nanowire growth proceeds via Au-catalyzed VLS growth mechanism. Transition in the growth mechanism from Au-catalyzed VLS growth to self-catalyzed VLS growth was observed as the growth temperature changed from 400 to 200 °C. Self-catalytic VLS growth as well as Au catalyzed VLS growth was observed in a growth temperature window of 350–250 °C. This transition in the growth mechanism is mainly due to differences in the growth kinetics of Au-VLS and self-catalyzed VLS mechanism. These results indicate a good understanding of ITO nanowires growth by e-beam evaporation method. Diameters of the nanowires were tuned in a broad range of 20–90 nm by changing the Au catalyst layer thickness. This catalyst-assisted and low temperature growth method can be implemented for precise diameter controlled synthesis of ITO nanowires with mono dispersed gold catalyst particles instead of Au catalyst film to tune the optical properties of the nanowires.

Patternable Large‐Scale Molybdenium Disulfide Atomic Layers Grown by Gold‐Assisted Chemical Vapor Deposition

A novel way to grow MoS2 on a large scale with uniformity and in desired patterns is developed. We use Au film as a catalyst on which [Mo(CO)6 ] vapor decomposes to form a Mo-Au surface alloy that is an ideal Mo reservoir for the growth of atomic layers of MoS2 . Upon exposure to H2 S, this surface alloy transforms into a few layers of MoS2 , which can be isolated and transferred on an arbitrary substrate. By simply patterning Au catalyst film by conventional lithographic techniques, MoS2 atomic layers in desired patterns can be fabricated.

Patternable Large‐Scale Molybdenium Disulfide Atomic Layers Grown by Gold‐Assisted Chemical Vapor Deposition

AbstractA novel way to grow MoS2 on a large scale with uniformity and in desired patterns is developed. We use Au film as a catalyst on which [Mo(CO)6] vapor decomposes to form a Mo‐Au surface alloy that is an ideal Mo reservoir for the growth of atomic layers of MoS2. Upon exposure to H2S, this surface alloy transforms into a few layers of MoS2, which can be isolated and transferred on an arbitrary substrate. By simply patterning Au catalyst film by conventional lithographic techniques, MoS2 atomic layers in desired patterns can be fabricated.

Formation of High Aspect Ratio GaAs Nanostructures with Metal-Assisted Chemical Etching

Periodic high aspect ratio GaAs nanopillars with widths in the range of 500-1000 nm are produced by metal-assisted chemical etching (MacEtch) using n-type (100) GaAs substrates and Au catalyst films patterned with soft lithography. Depending on the etchant concentration and etching temperature, GaAs nanowires with either vertical or undulating sidewalls are formed with an etch rate of 1-2 μm/min. The realization of high aspect ratio III-V nanostructure arrays by wet etching can potentially transform the fabrication of a variety of optoelectronic device structures including distributed Bragg reflector (DBR) and distributed feedback (DFB) semiconductor lasers, where the surface grating is currently fabricated by dry etching.

Synthesis and optical properties of silicon nanowires grown by different methods

We review our recent results on the growth and characterization of silicon nanowires (SiNWs). Vapour-phase deposition techniques are considered, including chemical vapour deposition (CVD), plasma-enhanced chemical vapour deposition (PECVD), high-temperature annealing, and thermal evaporation. We present complementary approaches to SiNW production. We investigate the low-temperature (down to 300 °C) selective nucleation of SiNWs by Au-catalysed CVD and PECVD. Bulk production of SiNWs is obtained by thermal-vapour deposition from Si/SiO powders in a high-temperature furnace. In this case, SiNWs grow either by condensing on Au catalyst films, or by self-condensation of the vapour in a lower-temperature region of the furnace. Finally, we also achieve controlled growth by thermolysis of nanopatterned, multi-layered Si/Au thin-film precursors. The as-produced wires are compared in terms of yield, structural quality, and optical properties. Raman and photoluminescence spectra of SiNWs are discussed.

Selective-area growth and field emission properties of Zinc oxide nanowire micropattern arrays

The patterned growth of ZnO nanowire arrays with different thickness and diameters has been achieved by using a simple method on the silicon substrates. The RF magnetron sputtering and photolithographic patterning processes have been employed to fabricate the patterned Au catalyst film, and the patterned ZnO nanowires with different thickness and diameters were synthesized via thermal evaporation vapor phase transport. The diameters of the as-synthesized ZnO nanowires were scattered in a range of 50–400 nm and a length up to ∼8 μm. The turn-on field of the patterned quasi-perpendicular and random ZnO nanowire at the current density of 0.1 μA/cm 2 is 2.4 and 2.8 V/μm, respectively. This approach must have a wide variety of applications such as FE-based flat panel displays, sensor arrays, and optoelectronic devices.

Synthesis and field emission of patterned SnO 2 nanoflowers

This article reports the synthesis and field emission of patterned SnO 2 nanoflowers obtained by a simple method. A patterned Au catalyst film was prepared on the silicon wafer by radio frequency (RF) magnetron sputtering and photolithographic patterning processes. The patterned SnO 2 nanoflowers arrays, with a unit diameter of ∼ 50 μm, were synthesized via vapor phase transport method. Field emission scanning electron microscopy (SEM) and X-ray diffraction (XRD) are used to identify the surface morphology and composition of the as-synthesized SnO 2 nanostructures. The mechanism of formation of SnO 2 nanostructures was also discussed. The measurement of field emission (FE) showed that the as-synthesized SnO 2 nanostructure arrays have a lower turn-on field of 2.6 V/μm at the current density of 0.1 μA/cm 2. This approach must have a wide variety of applications such as fabrications of micro-optical components and micropatterned oxide thin films used in FE-based flat panel displays and sensor arrays.

Field emission from patterned SnO2 nanostructures

A simple and reliable method has been developed for synthesizing finely patterned tin dioxide (SnO2) nanostructure arrays on silicon substrates. A patterned Au catalyst film was prepared on the silicon wafer by radio frequency (RF) magnetron sputtering and photolithographic patterning processes. The patterned SnO2 nanostructures arrays, a unit area is of ∼500μm×200μm, were synthesized via vapor phase transport method. The surface morphology and composition of the as-synthesized SnO2 nanostructures were characterized by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The mechanism of formation of SnO2 nanostructures was also discussed. The measurement of field emission (FE) revealed that the as-synthesized SnO2 nanorods, nanowires and nanoparticles arrays have a lower turn-on field of 2.6, 3.2 and 3.9V/μm, respectively, at the current density of 0.1μA/cm2. This approach must have a wide variety of applications such as fabrications of micro-optical components and micropatterned oxide thin films used in FE-based flat panel displays, sensor arrays and so on.

Patterned growth and field emission of ZnO nanowires

A simple and reliable method has been developed for the controlled patterned growth of ZnO nanowire arrays with different sizes on silicon substrates. A patterned Au catalyst film was prepared on the silicon wafer by a combination of r.f. magnetron sputtering and photolithographic patterning processes, and the patterned ZnO nanowires with different diameters were synthesized via vapor phase transport. Scanning electron microscopy (SEM) showed that the diameters of the nanowires were scattered in a range of 100–400 nm and the length up to 8 μm. The measurement of field emission (FE) revealed that the as-synthesized patterned ZnO nanowire arrays have a low turn-on field of 2.4 V/μm at the current density of 0.1 μA/cm2. This approach opens the possibility of creating micropatterns, one-dimensional nanostructures for application as FE-based flat panel displays, sensor arrays, and optoelectronic devices.

Selective growth of ZnSe and ZnCdSe nanowires by molecular beam epitaxy

Controlled growth of ZnSe and ZnCdSe nanowires is demonstrated by molecularbeam epitaxy using Au or Ag catalyst films in the temperature range400–550 °C.The highest density of small-diameter (10 nm), highly-crystalline ZnSe nanowires is achieved by usingAu at 400 °C. Direct growth onto transmission electron microscope grids clearly indicates a tip-growthregime. Pre-patterning of the catalyst film allows highly selective ZnSe deposition asprobed by photoluminescence and Raman spectroscopy. In similar conditions, the additionof Cd vapour in the MBE reactor allows the synthesis of ZnCdSe ternary nanowires.

Two-step evaporation process for formation of aligned zinc oxide nanowires

Two-step gas flow controlled evaporation process was introduced as a simple technique to fabricate aligned ZnO nanowires via the VLS growth mode. Using this method, well-aligned single crystalline ZnO nanowires was successfully synthesized on Au coated Si substrates. The diameter of ZnO nanowires was controlled from 40 to 150 nm and the ratio of length to diameter was observed to be larger than 30. Tetrapod-shaped ZnO nanostructures were formed using a mixture of Ar and O 2 gases, and it indicated that the partial pressure of oxygen during the synthesis is crucial for the growth of ZnO nanowire structures. Green emission induced from singly ionized oxygen vacancies was generated from ZnO nanowires with the excitonic emission in UV range. Higher intensity of the green emission was observed in thinner nanowires, which is attributed to their higher surface-to-volume ratio. Patterned structure of ZnO nanowires with a high fidelity was achieved by pattering the Au catalyst films.

Catalytic Growth of Semiconducting ZnO Nanowires by Reactive Evaporation Process

AbstractReactive evaporation process was introduced as a simple technique for the fabrication of aligned ZnO nanowires on a Si (100) surface. Single crystalline ZnO nanowire arrays were successfully synthesized on Au coated Si substrates via a VLS growth mechanism. The diameter of ZnO nanowires were observed to vary from 40 nm to 150 nm and the ratio of length to diameter was observed to be larger than 30. The diameter of ZnO nanowires was controlled by changing the processing temperature and by the thickness of Au catalyst film. Green emission induced from singly ionized oxygen vacancies was generated from ZnO nanowires with the excitonic emission in UV range. Higher intensity of the green emission was observed in thinner nanowires, which is attributed to their higher surface-to-volume ratio. Aligned structure formation and size control of ZnO nanowires could provide the potential for various nano-scale device applications.

Thermal desorption study of Oxygen adsorption on Pt, Ag & Au films deposited on YSZ

The Thermal Desorption or Temperature Programmed Desorption (TPD) technique has been used for the study of oxygen adsorption on Pt, Ag and Au catalyst films deposited on YSZ. The catalyst film was deposited on the one side of the YSZ specimen while on the other side gold counter and reference electrodes were deposited, constructing a three-electrode electrochemical cell similar to those used in Electrochemical Promotion studies. Oxygen adsorption has been carried out either by exposing the samples to gaseous oxygen (gas phase adsorption) or by the application of a constant current between the catalyst/working electrode and the counter electrode (electrochemical adsorption) or by mixed gas phase and electrochemical adsorption. Oxygen adsorption was carried out at temperatures between 200 and 480 °C. After exposure to gaseous oxygen, normal adsorbed atomic oxygen species have been observed on Pt and Ag surfaces while there was no detectable amount of adsorbed oxygen on Au. Electrochemical O2− pumping to Pt, Ag and Au catalyst films creates strongly bonded “backspillover” anionic oxygen, along with the more weakly bonded atomic oxygen. Electrochemical O2− pumping to Pt, Ag and Au catalyst films in presence of preadsorbed oxygen creates strongly bonded “backspillover” anionic oxygen, with a concomitant pronounced lowering of the Tp of the more weakly bound preadsorbed atomic oxygen. The two oxygen species co-exist on the surface. The activation energy for oxygen desorption or, equivalently, the binding strength of adsorbed oxygen was found to decrease linearly with increasing catalyst potential, for all three metal electrodes.

Chemistry at catalytic surfaces: The SO 2 oxidation on noble metals

The high temperature solid electrolyte cell air, M¦ ZrO 2 (8% Y 2O 3 )¦M , air-SO 2SO 3, where M is Pt, Ag, or Au, was used to monitor the oxygen activity on Pt, Ag, or Au catalyst films exposed to mixtures of O 2, SO 2, and SO 3 at temperatures above 400 °C and 1 atm total pressure. One electrode functioned simultaneously as both electrode and catalyst. The opencircuit EMF of the cell reflected the oxygen activity on the working catalyst and simultaneously the steady-state chemical kinetics were observed. For Pt it was found that the surface oxygen activity generally does not equal the gas phase P O 2 , but is determined by P SO 2 or the P SO 3 P SO 2 ratio, depending on the temperature and gas phase composition. These observations suggest that the rate-limiting step in the SO 2 oxidation on Pt is not generally the reaction between chemisorbed oxygen and gaseous SO 2, except at high temperatures and very low P so 2 , but rather the desorption of a chemisorbed phase of SO 3 or the adsorption of oxygen, depending upon the temperature, P SO 2 , P SO 3 , and P O 2 . The results with Au and Ag films show that chemisorbed SO 3 is also formed on these metals exposed to SO 2, O 2 mixtures. The values of surface oxygen activity compared to those obtained for Pt, provide an explanation for the weakly catalytic and noncatalytic properties of Au and Ag, respectively, for the SO 2 oxidation.