Fractional Monolayer Research Articles

Properties of excitons in a wide CdTe-based quantum well with irregular insertions of a fractional monolayer of ZnTe

The irregular short period CdTe/ZnTe superlattice structure is investigated by both stationary and time-resolved optical spectroscopy with and without an external magnetic field as a perturbation. This study is aimed to emphasize the properties of radiative excitonic recombination in a superlattice of this type in comparison with the excitons confined in a single QW structure. The decay time of the excitons is about 400 ps which is deduced from the time-resolved measurements. Theg-factors of electrons and holes are obtained by the spin quantum beat measurements combined with Zeeman measurements. The experimental results show that theg-factors of holes in the irregular short period CdTe/ZnTe superlattice become dramatically different in comparison with the single CdTe/CdMgTe quantum wells.

Growth and excitonic properties of single fractional monolayer CdSe/ZnSe structures

Single fractional monolayer (FM) CdSe/ZnSe structures have been grown by molecular beam epitaxy (MBE), employing both conventional MBE and migration-enhanced epitaxy (MEE). A precise calibration of the FM mean thickness in the range of 0.15–3.0 ML has been performed for both techniques, revealing more than a 3.5 times lower Cd incorporation ability for the MEE mode at the same Cd and Se incident fluxes. Steady-state and time-resolved photoluminescence spectroscopy is used to characterize the intrinsic morphology of the CdSe FMs, with a special emphasis on the submonolayer thickness range. Both MBE and MEE grown samples exhibit inhomogeneity of the excitonic system, which can be explained by coexistence of a homogeneous alloylike layer and relatively large CdSe 2D clusters. The MEE samples display smaller fluctuations of the layer thickness and island sizes.

The influence of strained ZnTe islands on excitonic recombination and the self-organizing effects of ZnTe islands in a wide CdTe-based quantum well

Structures consisting of ZnTe fractional monolayers embedded coherently in a CdTe quantum well are investigated by means of optical measurements. This study is designed to study the influence of the strained ZnTe islands on excitonic recombination and to explore the dependence of the self-organizing effects of the ZnTe island on growth temperatures. The experimental results show that the strained ZnTe islands can strongly modify the excitonic transitions related to the quantization of the excitonic centre-of-mass motion (CM quantization). We have shown that the striking increase of the effective g-factor of the CM quantized excitons is due to the change of the hole g-factor involved since it is found experimentally that the electron g-factor is unchanged. The experimental results also show that a coherent distribution of the ZnTe islands is formed at higher growth temperatures , while a random distribution of the ZnTe islands is formed at lower growth temperatures . A coherent alignment of the islands along the growth direction is concluded.

Disorder-induced exciton localization in a fractional monolayer ZnSe/CdSe superlattice

Low-temperature spectra of excitonic emission and recombination dynamics have been studied in ZnSe/CdSe fractional monolayer (ML) superlattices (SLs). Two intrinsic emission bands are observed in SLs with the CdSe layer fraction ranging between 0.5 and 0.7 ML, reflecting inhomogeneity of the SL structure. The emission energies, line widths and decay times are interpreted in terms of random coupling of 1 ML thick extended (with respect to the exciton Bohr radius) CdSe islands in different layers of a disordered SL.

Disorder-induced exciton localization in a fractional monolayer ZnSe/CdSe superlattice

Disorder-induced exciton localization in a fractional monolayer ZnSe/CdSe superlattice

Structural and optical properties of CdTe/MnTe tilted superlattices grown on vicinal surfaces

We have grown CdTe/MnTe tilted superlattices by atomic layer epitaxy. These heterostructures are formed by depositing, in the step-flow growth mode, fractional monolayer superlattices (CdTe) m (MnTe) n , with p = m + n ≈ 1, onto 2° A and 2° B Cd 0.95Zn 0.05Te vicinal substrates. Transmission electron microscopy images reveal that we obtain a uniform short-range superlattice period and an in-plane modulation which is approximately 0.1 of that expected for perfect lateral atomic ordering. This reduced lateral modulation is evidenced by polarization anisotropy in optical experiments.

Ultrathin metal films on a metal oxide surface: Growth of Au onTiO2(110)

We have studied the growth of ultrathin Au films on the ${\mathrm{TiO}}_{2}$ (110) surface using low-energy ion scattering (LEIS), x-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). For substrate temperatures of 160 and 300 K, for fractional monolayer Au coverages, Au evaporated under UHV conditions appears to grow initially on the stoichiometric ${\mathrm{TiO}}_{2}$ (110) surface in a quasi-two-dimensional (2D) form. As the average Au coverage increases to one monolayer (1 ML) and higher, Au forms three-dimensional (3D) islands. The coverage at which 3D islands become apparent in LEIS data decreases as the surface temperature increases. No evidence of significant chemical interactions has been found between the Au and ${\mathrm{TiO}}_{2}$ substrate. By annealing ${\mathrm{A}\mathrm{u}/\mathrm{T}\mathrm{i}\mathrm{O}}_{2}$ to temperatures up to 775 K, the Au islands continue to grow; encapsulation of the Au islands by Ti suboxides is not observed. There is little or no CO adsorption on small Au clusters at 300 K (upper limit of \ensuremath{\sim}0.05 ML), based on sensitive LEIS measurements.

Optical spin resonance and transverse spin relaxation in magnetic semiconductor quantum wells

Ultrafast optical pulses are used to initiate and measure free-induction decays of coherent conduction electron spins and of embedded magnetic ${\mathrm{Mn}}^{2+}$ ions in a series of magnetic-semiconductor quantum wells. These time-resolved Faraday rotation experiments in transverse applied magnetic fields complement previous studies of spin dynamics in longitudinal fields by unambiguously distinguishing between the spin relaxation of electrons and holes, and by identifying a mechanism by which angular momentum is transferred from spin-polarized carriers to the sublattice of local moments. In transverse fields (Voigt geometry), the precession of the photoexcited spins about the field axis can be measured as an oscillatory induced Faraday rotation signal. We observe the THz free-induction decay of spin-polarized electrons in modest (4 T) magnetic fields and separately identify the more rapid spin relaxation of the holes as functions of field and temperature. The $g$ factors of the electrons and holes are accurately measured as a function of well width. The role of quantum confinement on the stability of the hole spin is discussed, with particular attention given to the observed ability of the transient hole-exchange field to coherently rotate a macroscopic ensemble of local ${\mathrm{Mn}}^{2+}$ moments. This ``tipping pulse'' initiates a free-induction decay in the sublattice of ${\mathrm{Mn}}^{2+}$ spins and enables electron paramagnetic resonance (EPR) studies of the fractional monolayer magnetic planes. These time-domain EPR measurements reveal a significant magnetic field dependence of the Mn transverse spin relaxation time.

P.2.043 The action of sertindole on negative symptoms in schizophrenia

Structural and optical properties of (AlAs)m/(GaAs)n type-I ultra short-period superlattices (USSLs) whose thickness ratio of AlAs and GaAs layers is 3:7 were studied. Especially, we paid attention to the superlattices (SLs) with the period length (Λ) of fractional monolayer (ML) grown by molecular beam epitaxy (MBE). From the photoluminescence (PL) measurements of the USSL, we found that the PL energies increase as the Λs decrease. The observed PL energies dependence on the Λs well agrees with the calculation results from a simple model based on Kronig–Penny model even in the case of the Λs are quite small.

P.2.042 The long term cardiovascular safety of sertindole

Structural and optical properties of (AlAs)m/(GaAs)n type-I ultra short-period superlattices (USSLs) whose thickness ratio of AlAs and GaAs layers is 3:7 were studied. Especially, we paid attention to the superlattices (SLs) with the period length (Λ) of fractional monolayer (ML) grown by molecular beam epitaxy (MBE). From the photoluminescence (PL) measurements of the USSL, we found that the PL energies increase as the Λs decrease. The observed PL energies dependence on the Λs well agrees with the calculation results from a simple model based on Kronig–Penny model even in the case of the Λs are quite small.

P.2.045 Involvement of adenosine A 2A receptors in the induction of c-fos expression by clozapine

Structural and optical properties of (AlAs)m/(GaAs)n type-I ultra short-period superlattices (USSLs) whose thickness ratio of AlAs and GaAs layers is 3:7 were studied. Especially, we paid attention to the superlattices (SLs) with the period length (Λ) of fractional monolayer (ML) grown by molecular beam epitaxy (MBE). From the photoluminescence (PL) measurements of the USSL, we found that the PL energies increase as the Λs decrease. The observed PL energies dependence on the Λs well agrees with the calculation results from a simple model based on Kronig–Penny model even in the case of the Λs are quite small.

Comparison of FEG-STEM and AES Measurements of Equilibrium Segregation of Phosphorus in 9% Cr Ferritic Steels

The equilibrium grain boundary segregation in a series of 9% Cr ferritic steel alloys, doped with different concentrations of P impurity, has been measured by analytical scanning transmission electron microscopy (STEM) and Auger electron spectroscopy (AES). Both P and Cr segregation to boundaries was detected using both methods. Different levels of sophistication have been used to process the spectral data in order to derive the equivalent fractional monolayer concentrations of the segregants. The distributions of measurements from individual grain boundaries are displayed as histograms, and the different analytical processes are contrasted by comparing the means and standard deviations of these distributions. The STEM and AES results for P are in agreement irrespective of the technique and analytical procedure used. However, a systematic divergence occurs in the Cr results, which is considered to arise from contributions from Cr-rich boundary precipitates to the AES spectra that cannot be accurately separated. © 1997 by John Wiley & Sons, Ltd.

Optical investigation of CdTe monomolecular islands in wide ZnTe/(Zn,Mg)Te quantum wells: Evidence of a vertical self-ordering

Continuous, piezomodulated, and time-resolved optical spectroscopies have been carried out on several samples basically consisting of a 38-nm-wide ZnTe quantum well incorporating monomolecular CdTe islands grown by molecular-beam epitaxy on nominal (001) ZnTe surfaces. During the growth of some samples, five regularly spaced monomolecular layers (single or half-monolayers) of CdTe were inserted in the wide ZnTe quantum wells. Our results indicate that, under certain conditions of deposition, (i) the fractional layers constitute CdTe islands separated by ZnTe “voids,” with in-plane size and spacing quite larger than the exciton Bohr diameter; and (ii) when several fractional monolayers are grown, these islands tend to stack on top of each other. The effects of this vertical self-ordering disappear when the conditions of deposition are changed. Moreover, it is shown that the CdTe inserts act as giant isoelectronic recombination centers, yielding intense and sharp photoluminescence lines with very short decay times (a few tens of picoseconds) in the range of λ≈517 nm.

X-ray-diffraction study of the lattice distortions induced by a fractional monolayer:ZnTe embedded in vicinal CdTe(001)

The distortion gradient of the interplanar distances induced by a ZnTe fractional monolayer embedded in vicinal B-type CdTe(001) is analyzed within a kinematical model from the diffuse scattering near the (004) Bragg peak of the substrate (scans along [00l]). It is explained how these x-ray measurements allow us to extract the relevant structural parameters of the sample. With this method, only the planes with distortions along the growth direction and without local bending give a significant contribution to the diffuse scattering along [00l]. The measured diffuse intensity only comes from a fraction of the sample volume. It is shown that the elasticity theory is very well verified in the monolayer limit by comparing the strains predicted by this theory to ab initio pseudopotential calculations. By assuming that this approach is still valid in the submonolayer range, the maximum Zn concentration per plane and the integrated Zn quantity are deduced from the distortion curves. The asymmetric profile of the Zn concentration can be explained by a nucleation of ZnTe islands. This analysis of the diffuse scattering has been proven to be very sensitive to the elastic deformation of the layers due to the large lattice mismatch between CdTe and ZnTe (about 6%), but not to the chemical composition of the cationic planes (Cd or Zn). In a more general way, it can easily be applied to systems having a large chemical contrast.

Probing the Schottky barrier with conduction electron spin resonance

The Schottky barrier of a heavily doped Si layer covered with a metal film is investigated using conduction electron spin resonance, which is utilized in situ in UHV to continuously monitor the metal overlayer development from a fractional monolayer through multiple layers. Measured increases in linewidth are compared with calculations based on a kinetic theory. For Al and Ag, the data can be explained both by flat-surface and island models, even though for Al only a flat-surface model with smooth surfaces is compatible with the experiments. In contrast, for Cu and Al-Cu bilayers, the data unambiguously favor the rough-surface island model.

Growth of CdTe/MnTe tilted and serpentine lattices on vicinal surfaces

We have grown by atomic layer epitaxy CdTe/MnTe tilted and serpentine superlattices. These heterostructures are formed by depositing in the step-flow growth mode fractional monolayer superlattices (CdTe)m(MnTe)n, with p=m+n∼1, onto 2 °A and 2 °B Cd0.95Zn0.05Te vicinal substrates. Transmission electron microscopy images reveal a good in-plane CdTe/MnTe separation and a uniform short-range superlattice period. The very existence of those superlattices imply that Te-based vicinal surfaces present a regular array of monomolecular steps, with no important step meandering and no step bunching.

Nitrogen adsorption and native contamination on Fe(211) surfaces

Nitrogen adsorption has been studied on clean Fe(211) surfaces. Negligible adsorption occurred at low or room temperature. However, a surface with particular contamination showed appreciable adsorption. Heating a clean surface in nitrogen resulted in a small surface N concentration with a (2 × 5) structure, but since the same occurred after heating a nitrogen-rich surface region in a high vacuum, it indicates that internal N diffuses to the surface at heating temperatures as low as 580 K. Since there is no clear evidence that any appreciable adsorption occurs from 80 to 580 K, speculation about a special site existing for initial adsorption, such as the C7 site, is not supported. Internal chlorine was detected on the surface after heavy exposures of nitrogen, hydrogen or oxygen, indicating that complex interactions occur with gases in the sub-surface region. Because used catalysts in ammonia catalysis show the presence of C and O, the structures on Fe(211) formed with internal contaminants S, C and external O were studied. Various ordered structures were found to form at fractional monolayer concentrations, and cannot be excluded from being involved in the catalysis process.

Cesium-Enhanced R. F. Magnetron Deposition of Carbon Nitride and Diamond-Like Carbon Films

ABSTRACTWe report the design, manufacturing, and testing of a new cesium enhanced negative carbon ion source that can be useful to synthesize hard and/or electron emitting carbon nitride and diamond-like carbon (DLC) thin films. The design of the source includes a conventional magnetron-sputtering gun, low voltage ion extraction lenses, and a cesium oven to provide cesium vapor for formation of a fractional mono layer of Cs on the carbon target. Cs reduces the surface work function of the carbon target and enhances the emission of negative carbon ions. Argon and argon-nitrogen gas mixtures were used to ignite and sustain the plasma in the chamber. We compare the properties of carbon nitride and DLC films deposited with and without cesium. Nitrogen composition in the Ar-N2 gas mixture was observed to be an important process parameter affecting mechanical properties of the film. The effect of the Cs oven temperature on deposition rate and current absorbed at the substrate was also investigated for RF powers from 0 to 150 W.

Enzymatic Hydrolysis of Cellulose (II): X-Ray Photoelectron Spectroscopy Studies on Cellulase Adsorption. Effect of the Surfactant Tween 85

The adsorption of proteins on particulate and fibrous celluloses was studied by means of X-ray photoelectron spectroscopy (XPS). The presence of protein (bovine serum albumin and Celluclast, a commercial cellulase from Trichoderma reesei) adsorbed on the fibres was detected by the N1s signal in the wide-field spectrum. The proteins adsorbed onto several types of cellulose could be compared, ires`pective of the fibres’ specific surface area. The fractional monolayer coverage of cellulose fibres could also be calculated.The influence of the surfactant Tween 85 on cellulase adsorption and enzymatic activity was also investigated. At low enzyme concentrations, the surfactant reduces the amount of adsorbed enzyme, simultaneously improving the reaction rate. The effect of the surfactant depends on the structural properties of the substrate. The higher the crystallinity of the substrate, the less effective is the enhancement of the reaction and the reduction of the amount of adsorbed enzyme by the surfactant.

Step ordering during OMCVD growth on non-planar substrates

We used atomic force microscopy (AFM) in air to investigate the structure of atomically smooth terraces separated by monoatomic steps which are formed during organometallic chemical vapor deposition (OMCVD) on non-planar substrates. We found that grooves along the [011̄]-direction can orient the steps on the (100) ridge between them in the [011]-direction. This orientation effect is sustained for a distance of about 500 nm from each groove. The distance between the oriented steps is determined by the initial misorientation angles of the substrate, and their width is nearly constant. We explain these step orientation phenomena as coming from the interplay between {311}A and (100) growth on the non-planar substrate. The step orientation control could be useful for preparation of improved nanostructures, such as quantum wells, quantum wires and fractional monolayer superlattices.