Atomic Layer Epitaxy Technique Research Articles

Fabrication of Plasmonic Nano-Slit Pore Double Layer for Single Molecule Detection

We fabricated the plasmonic nano-slit pore platforms for single molecule detection and other various applications. The nano-apertures on the Au/insulator/Au/SiN flat membrane, in addition to the 200 nm thick Au film on ~ 20 nm thick SiN membrane were fabricated using a focused ion beam drilling technique. The insulating materials of SiO2 or Al2O3 were deposited using an atomic layer epitaxy technique. The nanopores with a few nanometer size were fabricated under the proper control of drilling depth and pore opening. Optical characteristics were carried out dependent upon the pore opening width and sample thickness. The broad emission spectra from the (7x 7) slit pore array are obtained via spp-mediated emission on the ~200 nm thick Au/~ 20 nm thick SiN flat samples. A sharp strong infrared emission peak is also obtained due to Au nanoparticle. The fabricated Au double layer platform will be investigated for a single molecule sensor device and a possible radiative thermal energy transfer device.

Diminishing the Induced Strain and Oxygen Incorporation on Aluminium Nitride Films Deposited Using Pulsed Atomic-Layer Epitaxy Techniques at Standard Pressure MOCVD

A pulsed atomic-layer epitaxy growth technique has been introduced to substantially diminish the induced strain and oxygen incorporation on aluminium nitride films grown at standard pressure by metal organic chemical vapour deposition. The qualities of the as-deposited aluminium nitride films were studied by varying the aluminium nitride nucleation layer growth temperature at 700°C, 800°C, 900°C, 1000°C and 1100°C, respectively. The compressive strain inside the as-deposited aluminium nitride films, induced by the hetero-epitaxial growth on sapphire, was investigated through Raman spectroscopy by focusing on the evolution of E2 (high) peak frequency, where almost stress-free aluminium nitride films were attained at nucleation layer growth temperature of 1100°C. Then, the correlation between luminescence defect and level of foreign impurities respective to the varied nucleation layer growth temperatures were also systematically analysed through photoluminescence spectroscopy and x-ray photoelectron spectroscopy, respectively.

Effects of gas-flow sequences on the self-limiting mechanisms of GaAsN films grown by atomic layer epitaxy

We adapted the atomic layer epitaxy (ALE) technique to grow GaAsN thin films. The effects of the gas flow sequences used on the self-limiting mechanism (SLM), the N incorporation ratios, and the concentrations of residual impurities were investigated. A pulse of the N precursor (monomethylhydrazine) was added after that of the Ga precursor (trimethylgallium) or the As (trisdimethylaminoarsenic) precursor, which were part of the conventional sequence used for the growth of GaAs films via ALE. If it was noticed that the SLM was in effect, the N precursor was supplied on the Ga- or As-terminated surface (the corresponding sequences and their resulting films are labeled as On-Ga and On-As, respectively). The On-As film exhibited rough surfaces, nonuniform N concentration, and disordered lattices owing to defects. On the other hand, in the case of the On-Ga sequence, GaAsN films with high-quality crystals were grown with the SLM in effect and the concentration of N being a few percent. The concentrations of the residual impurities (C and H) in the On-Ga film were low. This demonstrated that the On-Ga sequence was effective for growing GaAsN thin films on precisely controlled surfaces using the ALE technique.

Water Adsorption and Dissociation on Cu Nanoparticles

The reaction of H2O dissociation on Cu nanoparticles prepared by an atomic layer epitaxy (ALE) technique is discussed in this article. The activation energy of H2O dissociation, desorption energy of H2O, active sites for H2O adsorption, and structural changes of the Cu surface were studied using temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), in situ IR spectroscopy, and X-ray absorption spectroscopy (XAS). The reduced Cu nanoparticles of the ALE-Cu/SiO2 catalyst possess a slightly positive charge (Cuδ+) due to the effect of the nanosized particles. The very low activation energy of H2O dissociation (23 kJ/mol) and the exothermic dissociation heat were obtained from a series of H2-TPR experiments on Cu nanoparticles. It is suggested that the Cu nanoparticles might be partially oxidized to Cu2O, while some oxygen atoms are proposed to be located on the surface of the Cu particles over the course of water adsorption.

Characteristics of high Al content AlGaN grown by pulsed atomic layer epitaxy

Al 0.91Ga 0.09N epilayers have been obtained by pulsed atomic layer epitaxy (PALE) technique on sapphire (0 0 0 1) substrates. Deep ultraviolet (DUV) photoluminescence (PL) spectroscopy and Raman scattering spectrum have been employed to study the optical transitions in Al 0.91Ga 0.09N epilayers. We found the exciton-phonon interaction by fitting the asymmetric PL peak, in which the transverse optical phonon (TO) and the longitudinal optical (LO) phonon are the main contributor. The abnormal S-shaped temperature dependence of the PL band peak is less pronounced or has disappeared. Further analysis shows that there possibly exists a high density of deeper localized state (∼90 meV) in Al 0.91Ga 0.09N. The formation of these localized states provides a favorable condition for efficient light emission.

Carbon Dioxide Hydrogenation on Cu Nanoparticles

The reaction mechanism and active sites for CO2 adsorption during the reverse water-gas shift (RWGS) reaction on silica-supported Cu nanoparticles were investigated. The Cu nanoparticles prepared by an atomic layer epitaxy technique were shown to strongly bind CO2 molecules, as evidenced by the two main peaks with maxima near 353 (α peak) and 525 K (β peak) observed during temperature-programmed desorption experiments. The high-temperature peak (525 K), which corresponds to CO2 in the β state, indicates that β-state CO2 was the dominant species for the RWGS reaction. The values for the activation energy of CO2 desorption were 33.4 and 74.4 kJ/mol for α-CO2 and β-CO2, respectively. The sites for CO2 adsorption were different from those for CO adsorption (defect sites and sites with highly dispersed Cu particles). It is likely that CO2 hydrogenation occurs on a low-index plane. We propose that the mechanism for the RWGS reaction mainly involves formation of formate species.

ChemInform Abstract: Alloying in Cu/Pd Nanoparticle Catalysts.

In this paper we show a new application of the atomic layer epitaxy technique to prepare binary alloy nanoparticle catalysts on various supports. The technique is used to prepare Cu/Pd bimetallic catalysts on SiO2 and on γ-Al2O3. The samples were characterized by means of transmission electron microscopy (TEM) and extended X-ray absorption fine structure (EXAFS) measurements. The EXAFS results show alloying in the Cu/Pd samples with a Cu-rich surface for the alumina-supported sample and random alloying for the silica-supported sample. From TEM and EXAFS measurements the particle sizes are determined to be in the range 20−60 A.

Nickel sulfur thin films deposited by ECALE: Electrochemical, XPS and AFM characterization

This paper reports on a study of the electrodeposition of nickel sulfide thin films on Ag(1 1 1) by the Electrochemical Atomic Layer Epitaxy (ECALE) technique. The multilayer growth of nickel sulfide has been studied by cyclic and stripping voltammetry. The composition of the films was studied by XPS analysis and the morphology was studied by Atomic Force Microscopy (AFM). The phenomenological observation of the hydrogen evolution reaction (HER) suggests the presence of electrocatalytic properties of the thin films obtained.

Confined Electrodeposition of CdS in the Holes Left by the Selective Desorption of 3-Mercapto-1-propionic Acid from a Binary Self-Assembled Monolayer Formed with 1-Octanethiol

Confined electrodeposition can be achieved through the use of suitable templates, by which the electrodeposition occurs in natural or artificial holes of an insulating layer on a conducting substrate. Here, we present the electrodeposition of CdS on the holes left by the selective desorption of 3-mercaptopropionic acid (MPA) from a binary self-assembled monolayer (SAM) formed on Ag(111) with 1-octanethiol (OT). The electrodeposition of a compound is quite demanding, since it requires the right stoichiometry. In addition, the surface underpotential deposition phenomena exploited by electrochemical atomic layer epitaxy (ECALE) technique ensures that the surface available for electrodeposition after the selective desorption is still Ag(111). Parallel electrochemical experiments show that the amount of compound electrodeposited is consistent with this free Ag(111) surface, and the morphological analysis performed both by atomic force microscopy (AFM) and by lateral force microscopy (LFM) confirm the electrochemical data.

Strain-induced nitrogen incorporation in atomic layer epitaxy growth of InAsN/GaAs quantum wells using metal organic chemical vapor deposition

We report on the growth of high-quality high-indium-content (Ga)InAsN/GaAs quantum wells grown using low-pressure metal organic chemical vapor deposition. The growth was performed employing a strain-controlled atomic layer epitaxy technique. We verified experimentally that the strain enables the incorporation of nitrogen atoms during the atomic layer epitaxy growth of InAsN monolayers on GaAs. Photoluminescence and secondary ion mass spectroscopy measurements indicate that about 2.5% of the nitrogen was incorporated in the grown layers. Utilizing this strain-controlled atomic layer epitaxy technique, we designed and demonstrated highly strained InAsN/GaAs short-period superlattice structure suitable for applications in optical communication.

Active sites on Cu/SiO 2 prepared using the atomic layer epitaxy technique for a low-temperature water–gas shift reaction

The atomic layer epitaxy (ALE) technique has been used to prepare uniform copper nanoparticles dispersed on a silica support (ALE-Cu/SiO 2 with 2.85 ± 0.32 nm ), which are highly active in the water–gas shift reaction. Infrared spectra of CO adsorption are employed to study the active sites on ALE-Cu/SiO 2 surface, suggesting that two major active sites are found on the copper surface, namely defect sites and highly dispersed Cu particles and/or isolated Cu atoms sites. We report here that the defect sites on these small Cu particles or isolated Cu atoms provide high activity for the water gas shift reaction. The high efficiency of the water gas shift reaction on the ALE-Cu/SiO 2 catalyst may be ascribed to its strong activity in promoting H 2O dissociation. Nanoscale Cu particles may be involved in strong interactions with the SiO 2 support, leading to a partially electropositive state as a result of interactions with oxygen atoms at the surface of the support, even if the copper is reduced.

Atomic Layer Epitaxial Growth of Gaas on Porous Silicon Substrate

GaAs thin film has been grown on porous silicon by metal organic chemical vapour deposition (MOCVD) for different growth temperatures using atomic layer epitaxy (ALE) technique. The morphology of GaAs layer was investigated by atomic force microscopy (AFM). The effect of growth temperature is studied using photoluminescence measurements (PL).The photoluminescence spectra revealed a dissymmetry form toward high energies attributed to strain effect resulting from the lattice mismatch between GaAs and porous Si substrate.

Ternary Cd xZn 1− xSe deposited on Ag (1 1 1) by ECALE: Electrochemical and EXAFS characterization

This paper reports the characterization of ternary II–VI semiconductor nanocrystals, deposited by the electrochemical atomic layer epitaxy (ECALE) technique. In particular, morphological and structural properties of the ternary compounds of formula Cd x Zn 1− x Se deposited on Ag (1 1 1) have been characterized as a function of composition. The number of the attainable x values is limited by the necessity of using well-defined ZnSe/CdSe deposition sequences. However, the quantitative analysis carried out on the basis of both electrochemical and extended X-ray absorption fine structure (EXAFS) experiments indicates that the ECALE method is a successful way of controlling the composition of Cd x Zn 1− x Se. In addition, the electrochemical measurements show that the amount of deposition is minimum in correspondence to the compound with x = 0.5, thus corroborating the hypothesis of a higher degree of disorder suggested both by morphological and structural investigation. The morphology was studied by atomic force microscopy (AFM). The structure of the films is estimated by EXAFS which is a powerful technique for the analysis of the local structure around chosen atoms.

Low-temperature water gas shift reaction on Cu/SiO2 prepared by an atomic layer epitaxy technique

An atomic layer epitaxy technique was used to produce nanoscale 2.9-3.4 nm copper particles supported on silica, and the nanoscale Cu/SiO2 catalysts can show surprisingly high activity for the water gas shift reaction, in comparison with the 5.6 wt% Pt/SiO2 and 10.3 wt% Cu/SiO2 prepared by the impregnation method.

Patterned growth of CdS by combined electrochemical atomic layer epitaxy and microcontact printing techniques

Ultrathin films of well-ordered CdS on Ag(1 1 1) can be attained by electrochemical atomic layer epitaxy (ECALE) technique, which is based on self-limiting reactions, such as underpotential deposition. The layer-by-layer electrodeposition on single crystal faces allows a precise control of film thickness and yields semiconducting nanostructures of high crystallinity. In this paper, the ECALE method has been used to deposit CdS on Ag(1 1 1) covered by patterns of hexadecanethiol, C 16SH. The patterned Ag(1 1 1) was obtained by using the microcontact printing technique of a polydimethylsiloxane (PDMS) stamp previously immersed in C 16SH. The stamp used in this work produced thiol strips 320 nm large, with an 850 nm pitch. The deposition of CdS takes only place on the uncovered silver substrate whereas it is prevented on the thiols monolayers, yielding CdS patterned according to the used stamp. The deposit has been characterized by electrochemical techniques and AFM measurements.

Properties of Cu(thd)2 as a Precursor to Prepare Cu/SiO2 Catalyst Using the Atomic Layer Epitaxy Technique

The new Cu/SiO2 catalyst is developed by the atomic layer epitaxy (ALE) method. The ALE-Cu/SiO2 catalyst with high dispersion and nanoscale Cu particles appears to have very different catalytic properties from those of the typical Cu-based catalysts, which have satisfactory thermal stability to resist the sintering of Cu particles at 773 K. Due to the formation of small Cu particles, the ALE-Cu/SiO2 can strongly bind CO and give high catalytic activity for CO2 converted to CO in the reverse water-gas-shift reaction. The catalytic activity decreases in the order of 2.4% ALE-Cu/SiO2 =... 2% Pt/SiO2 > 2% Pd/SiO2 > 10.3% IM-Cu/SiO2.

Growth of p-type ZnO thin films by using an atomic layer epitaxy technique and NH 3 as a doping source

Nitrogen-doped, p-type ZnO thin films have been grown successfully on sapphire (0001) substrates by atomic layer epitaxy (ALE) using Zn(C 2H 5) 2 [Diethylzinc, DEZn], H 2O and NH 3 as a zinc precursor, an oxidant and a doping source gas, respectively. The lowest electrical resistivity of the p-type ZnO films grown by ALE was 210 Ω cm with a hole concentration of 3.41 × 10 16 cm − 3 . Low temperature-photoluminescence analysis results support that the nitrogen ZnO after annealing is a p-type semiconductor. Also a model for change from n-type ZnO to p-type ZnO by annealing is proposed.

Carrier confinement and interband transition properties of InAs/GaAs quantum dots grown by using atomic layer epitaxy

The electrical and the optical properties of InAs/GaAs quantum dots (QDs) grown by using atomic layer epitaxy (ALE) technique were investigated by using capacitance–voltage (C–V) and photoluminescence (PL) measurements. C–V curves showed that the plateaus related to the zero-dimensional carrier confinement effect existed and that the number of electrons occupying the InAs QD was approximately 7. The full width at half maxima of the interband transitions from the ground electronic subband to the ground heavy-hole subband and from the first excited electronic state to the first excited state heavy-hole subband were not significantly affected by the temperature variation, indicative of strong confinement of the carriers occupying the InAs QDs. These results can help improve understanding for applications of InAs/GaAs QDs grown by using ALE in high-efficiency electronic and optoelectronic devices.

Photoluminescence Studies of ZnO thin films grown by atomic layer epitaxy

ZnO thin films have been grown on sapphire substrates by the atomic layer epitaxy technique using diethyzinc and H 2 O as reactant gases at substrate temperatures of 170°C and 400°C and annealed at temperatures of 800–1000°C. In order to investigate the effect of annealing treatment on the optical properties of ZnO films, the films have been annealed at various annealing temperatures after deposition. After the annealing treatment the optical properties of ZnO thin films were characterized by photoluminescence (PL). A strong free exciton emission with a weak defect-band emission in the visible region was observed at room temperature. Based on the full-width at half-maximum measurement of the free exciton emission of 67.54 meV have been made at room temperature. Both the PL intensity peaks in the UV light and visible light regions are increased by annealing the ZnO film in an oxygen atmosphere. In contrast, only the PL intensity peak in the UV light region is selectively increased by annealing it in a nitrogen atmosphere. Also, the ZnO film deposited at 170°C shows better PL characteristics than that deposited at 1000°C.

Comparison of ECR plasma pretreatment techniques for ZnO atomic layer epitaxy on the sapphire substrate

ZnO films were grown on sapphire substrates treated with ECR plasma by the atomic layer epitaxy (ALE) technique. ZnO nucleation enhancing effects of oxygen, hydrogen, and argon plasma treatments were compared. The incubation period for ZnO nucleation was measured by using scanning electron microscopy (SEM) and Auger electron emission spectrometric (AES) analysis. The incubation period for ZnO nucleation on the Al2O3 substrate not treated with plasma was more than 40 ALE cycles. The incubation period was shortened down to less than 35 cycles by argon or hydrogen ECR plasma pretreatment and to less than 30 cycles by oxygen ECR plasma pretreatment. It was found that a microwave power of 300W and a plasma exposure times of 10 min were appropriate for oxygen ECR plasma treatment of sapphire substrate surfaces to enhance ZnO nucleation. Higher power and longer exposure time would not be effective or would rather aggravate than enhance ZnO nucleation. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)