Rich Ambient Research Articles

Ultrathin Zn and ZnO films on Cu(111) as model catalysts

To prepare well-defined models of ZnO-based catalysts, in particular of Cu/ZnO used for methanol synthesis, we studied the structure of Zn and ZnO thin films grown on a Cu(111) single crystal surface using metal vapor deposition. Structural characterization was performed by scanning tunneling microscopy, Auger electron spectroscopy, and low-energy electron diffraction. In agreement with previous studies, Zn wets the Cu surface forming mixed surface layer depending on Zn coverage. Surface oxidation of the Zn film into ZnO, as monitored by STM, showed that the reaction starts at step edges and propagates inside the terrace at increasing temperature. However, the process is affected by Zn migration into the Cu bulk and hence the film formation critically depends on the heating rate. In another approach using Zn deposition in oxygen ambient and subsequent annealing in vacuum, the resulted films were well-ordered and showed a long-range coincidence structure, assigned to the formation of a single ZnO(0001) layer on top of Cu(111). Independent of preparations conditions, the ZnO overlayer did not cover the entire surface, leaving considerable areas exposing Cu(111) or Cu2O/Cu(111) surface. Reactivity measurements for CO oxidation and reverse water gas shift reactions at nearly atmospheric pressures showed no promotional effects of the ZnO overlayer under conditions studied. Moreover, Zn irreversibly migrates into the Cu crystal bulk in an O2 rich ambient, and the surface chemistry is governed, in essence, by a poorly defined Cu-oxide film. However, the ZnO/Cu model catalysts are fairly stable in a mixture of CO2 and H2.

Characterisation of 4H-SiC MOS Capacitor with a Protective Coating for Harsh Environments Applications

Oxygen free Ohmic contacts are essential for the realisation of high performance devices. Ohmic contacts in SiC often require annealing under vacuum at over 1000 °C, whilst high-κ dielectrics are usually annealed in O2 rich ambient at temperatures of 800 °C or less, affecting the electrical and surface characteristics. Therefore, protection of the Ohmic contacts during the annealing of a high-κ dielectric layer is a key enabling step in the realisation of high performance MOS structures. In order to prevent damage during the high-k formation the use of silicon nitride as a passivation layer, capable of protecting the contacts during annealing, has been investigated. In this work we have investigated and compared silicon nitride protected high-κ dielectric SiC based MOS capacitors with the unprotected SiC MOS devices in terms of electrical and optical characteristics.

Residual stress and defect content in magnetron sputtered ZnO films grown on unheated glass substrates

Structural, vibrational, photoluminescence and photo electron spectroscopic properties of rf magnetron sputtered ZnO films grown on unheated glass substrates are discussed. Variations in the c-axis orientation, average crystallite size, residual stress, and presence of disoriented grains, with varying oxygen content (40%–100%) during film growth are found to correlate with variation in the E2(high) and A1(LO) modes of the Raman spectra. Room temperature photoluminescence (RTPL) spectra exhibit a characteristic splitting in the near-band-edge emission (NBE) with three emission peaks (around 355, 386 and 395 nm) under an excitation wavelength of 290 nm. The blue emission (443–455 nm) is significantly controlled by oxygen vacancies and seen in films grown under 80% oxygen content. Quenching of NBE emission, for films prepared under oxygen rich ambient, is caused by the increased non-radiative recombination centers owing to reduced size of ZnO nano-crystallites. The shifts in the optical band gap of films grown under different oxygen content supplement the observed changes in the nano-crystallite size and luminescence properties. X-ray photoelectron spectroscopy confirms the fractional changes in the oxygen vacancy content in films deposited under different oxygen content in the sputtering gas.

Homo-Epitaxial Growth on Low-Angle Off Cut 4H-SiC Substrate

The growth of 4H-SiC epilayers on 1.28o off-cut substrates is reported in this study and comparison when using standard 4o and 8o off-cut substrates is added. Growth at high temperature is needed for the polytype stability, whereas low C/Si is requested to decrease both triangular defects density and roughness of the grown surface. An in-situ etching with Si rich ambient allows the growth of epilayers with specular surface. The formation of Si droplets can be observed on the grown surfaces when lowering the growth temperature and appears first for the high off-cut angle.

Temperature-Dependent PL Intensity in FLA-Synthesized and Hydrogen-Modified Silica Layers with Silicon Nanocrystals

The temperature dependence of the photoluminescence intensity I in flash-lamp-anneal-synthesized silica layers with small silicon nanocrystals (nc's) and in similar layers heat-treated in hydrogen- (H-) rich ambient was used to examine how H treatments affect the "dark" fracture of nc's and the migration paths of optically induced excitations in such systems. It can be inferred from the data obtained that the fracture of "dark" nc's in the systems of interest is generally not large, hydrogen weakly passivates defects in small-size nc's, and the influence of H treatments on the parameters governing the temperature dependence of I is insignificant.

B profile alteration by annealing in reactive ambients

Suppression or stimulation of B diffusion and activation in Si (coimplanted with F) has been investigated by anneals in a N2 ambient enriched with H2 or O2. The H2 rich ambient leads to B desorption and therefore stimulated diffusion toward the surface, thus effectively reducing indiffusion. Annealing in an O2 rich ambient promotes deeper B diffusion by injection of Si interstitials. The impact of these ambients can further be modulated by combining the B implant with a shallow or deep F coimplant acting as Si interstitial trap. Deep F coimplant and H2 rich ambient offer attractive B diffusion and activation.

Effects of GaN template annealing on the optical and morphological quality of the homoepitaxially overgrown GaN layer

Growth of GaN based devices such as light emitting diodes and laser diodes often occurs on already prepared GaN templates grown on native (e.g. bulk GaN) and non-native (e.g. sapphire) substrates. High temperature annealing methods that are generally used for growth on other substrates cannot be completed on GaN templates due to the decomposition at temperatures above 800 °C in hydrogen rich ambient. The decomposition rate is higher than the Ga desorption rate from the surface under most conditions and causes Ga droplets to form on the surface which can hinder further growth. The effect of the annealing conditions on the GaN homoepitaxy on GaN templates is presented. In-situ annealing conditions were developed for annealing the GaN templates to remove any contamination while preventing significant Ga droplets from accumulating on the surface in both H2 and N2 environments to obtain optimized annealing conditions. As measured by AFM the RMS roughness of the GaN surfaces increases upon annealing, but becomes similar to the starting template after 1 μm, indicating complete recovery. The optical quality of the homoepitaxially overgrown layers as measured by PL shows improved band edge luminescence and band edge to yellow band ratio for annealing in H2 while it degrades for annealing in N2 only. Hall measurements show superior characteristics for annealing in H2 as well. These results will be especially important because bulk substrates are subjected to polishing damage that is known to negatively affect the surface condition and initial growth nucleation.

Effect of Surface Electronic States of p-Type GaN on the Blue-Light-Emitting Diodes

The surface electronic states of p-type GaN and its effect on the light-emitting diodes (LEDs) containing such p-GaN layers are studied. The surface band bending of p-GaN layers prepared from different growth conditions are determined by ultraviolet photoemission spectroscopy and X-ray photoemission spectroscopy. By either way, it is found that the surface band bending depends on the growth conditions and is reduced by increasing Mg concentration, growth temperature, and -rich ambient. A further reduction in band bending is found with a postannealing at in , resulting in a similar reduction of the operation forward voltage of the LED containing such a p-GaN layer. The mechanism involved is discussed.

Characteristics of n+–p junction leakage induced by tantalum pentoxide gate insulator and gate reoxidation

This letter will present the n+–p junction characteristics in tantalum pentoxide gate dielectric (Ta2O5) and gate reoxidation ambient. The n+–p junctions in n-type metal–oxide–silicon field effect transistor fabricated with different gate dielectrics and post-thermal conditions were characterized by current–voltage measurements. The current–voltage measurements of junction leakage of Ta2O5 gate dielectric without gate reoxidation (MT1) show the hump characteristics due to the precipitates of oxide near the junction depletion regime. And their leakage mechanism is phonon-assisted tunneling, which facilitates the hopping of electrons from the valence band into the shallow attractive Coulomb center via the interface states in the precipitate, leaving holes in the valence band. However, the junction leakage currents of Ta2O5 dielectric with gate reoxidation in hydrogen rich ambient (MT2) are higher than those of thermal oxide samples and their leakage mechanism is Schottky barrier lowering due to the enhanced diffusion of oxygen into the junction depletion region.

Annealing Effect on GaN Buffer Layer Surface

We investigate the effect of annealing parameters (carrier gas, duration) on GaN buffer layers grown by metalorganic vapor phase epitaxy. The samples were characterized in situ by a laser reflectometry (LR) set-up with a He–Ne laser beam and ex situ by atomic force microscopy (AFM). The reflectivity decreases when the temperature is about 1050 °C. This fact depends on the hydrogen flow rate in the H2 + N2 gas mixture and the thickness of the buffer layers. The correlation between this observation and the AFM images indicates that H2 generates a mass transport on the substrate surface and a three-dimensional (3D) mode surface morphology for thick buffer layers. For thin buffer layers this phenomenon is limited and the sample surfaces have a two-dimensional aspect (2D) similar to that obtained under N2 rich annealing ambient. Under this ambient, no surface transformation is observed for longer annealing duration, which means that all the surface transformations occur during the temperature ramp between 600 and 1050 °C.

Hydrogen enhancement of thermally induced interface degradation in thermal (111) Si/SiO 2 traced by electron spin resonance

The process of interface degradation induced in (111)Si/SiO 2 by postoxidation annealing (POA) in vacuum, previously identified by electron spin resonance (ESR) as creation of persistent P b (Si/Si 3) interface defects, is found to be strongly enhanced (≈6 times) when performed in H 2 ambient, and additionally, as sensed by ESR, the treshold temperature for creation is lowered from ≈640°C for vacuum to ≈550°C. It thus appears that POA in H 2 rich ambient, such as routinely applied to passivate interface states (preexisting P bs) naturally introduced during oxidation, effectively creates extra defect entities, which then have to be passivated additionally. The results point to the key role of the POA ambient in interface degradation. The data reveal the atomic nature of one of the mechanisms of the electrically long known H-induced POA generation of detrimental interface states. PACS numbers: 85.30.De; 68.35.Dv; 73.40.Ty; 76.30.Mi

Photoluminescence effects associated with thermally induced crystalline structure changes in CdS films

The characteristics of room temperature photoluminescence of cadmium sulfide films have been studied as the crystalline structure is changed by postdeposition thermal treatments in Ar or sulfur rich Ar ambient from cubic to hexagonal phase. The films, deposited by the chemical bath technique on a glass substrate, grow with a cubic polycrystalline structure. The behavior of the luminescence spectra is correlated with X-ray diffraction and optical absorption results. The luminescence emission from these samples is the well known green emission of CdS at ∼ 550nm. A shift in the location of this peak is observed to be associated with changes in the energy band gap and with crystalline structure transition from cubic to hexagonal. It has also been found that the structural transition occurs at lower temperature and more abruptly when the thermal treatment is given in an ambient without sulfur than when sulfur is added to the annealing ambient.

Continuous-Film Thickness Determination by AES of Cr Overlayers on Cu Surfaces

AbstractThe characteristics of Cr coverage of Cu surfaces, including determination of tc, the minimum average film thickness required for formation of a continuous film, have been studied in-situ by Auger Electron Spectroscopy (AES). Auger signal intensities of substrate and deposit were monitored during Cr film growth by vapor deposition in UHV. It was shown that substrate surface morphology (roughness) has a dominant effect on coverage rate and tc. Slower coverage rates and larger tc′s were effected by the presence of native oxides, substrate heating (to 330°C) and H2O-vapor rich (5×10−5 Torr) ambient during Cr deposition. Surface oxides seemed to affect more the coverage of a smooth than a rough surface. Conversely, substrate heating affected more the coverage of a rough surface. The combined effect of substrate heating and water vapor rich atmosphere was pronounced for both smooth and rough surface coverages. Some of the main factors controlling these effects are discussed.

Defect dominated conductivity in Zn 3P 2

The electrical resistivity and Hall coefficient of Zn 3P 2 have been measured for single crystal and thin polycrystalline film samples which were annealed over a range of equilibrium vapor compositions and temperatures. The room temperature electrical resistivity of single crystal samples annealed at 573 K varied from approximately 10 5Ω-cm for single crystals heated in equilibrium with zinc to 10 Ω-cm for those annealed in a phosphorus rich ambient. Hall measurements indicate that a variation in carrier concentration is responsible for these changes. The experimentally observed dependence of carrier concentration [ h° ], (cm −3) on phosphorus pressure is given by [ h°] = 1.32 · 10 16 [ p(P 4)] 0.13 for samples annealed at 573 K. The experimentally determined pressure dependence is in good agreement with a model based on phosphorus interstitials acting as acceptors. The pressure and temperature dependence of the carrier concentration yield the equilibrium constant K I for the formation of interstitial phosphorus defects according to the reaction 1 4 P 4 → P′ i + h° where K I = 10 42.4 ± 2 cm −6 torr 0.25[p(P 4)] −0.25 exp(−1.18 ev kT ) . The accommodation of phosphorus interstitials is discussed in light of the crystal structure of Zn 3P 2.