Monolayer Fluctuations Research Articles

Entropy and fluctuations of monolayers, membranes, and microemulsions

Conformational entropy of the interfaces in microemulsions and surfactant solutions has been predicted, under some circumstances, to have significant effect upon their thermodynamic behavior. Continuum theories have led to a consistent phenomenological description of the effects of undulations, but conclusive experimental evidence for or against the predicted effects remains elusive in all but the lamellar phase. Work on the bicontinuous phases has led to an increasing focus on the role of topology and Gaussian rigidity, and on the role of anharmonic (in curvature) contributions to the bending energy.

Luminescence of Excitons Localized by Monolayer Interface Fluctuations in ZnSe/MgS Superlattices Grown by Metalorganic Vapor Phase Epitaxy

Luminescence properties of ZnSe/MgS superlattices (SLs) grown by metalorganic vapor phase epitaxy (MOVPE) were characterized using photoluminescence (PL) and reflection measurements. At low temperature, PL peaks originating from localized excitons which were confined by monolayer fluctuations were clearly observed. It was found that the luminescence efficiency of localized excitons was very high, probably nearly 100%. The thermalization of localized excitons to free excitons with the increase of temperature leads to a blue shift of the PL peak energy and an abrupt decrease of the luminescence intensity. This decrease of the luminescence intensity is attributed to the capture of free excitons by nonradiative centers. A SL which shows luminescence from continuously distributed tailed states exhibited a temperature dependence much different from that of the above SLs which show luminescence from the localized excitons defined by the interface fluctuations.

Optical studies of localized excitons in symmetric coupled quantum wells

We present evidence for coupling between spatially separated excitonic states in a GaAs symmetric coupled quantum well. A split excitonic peak is observed in the photoluminescence spectrum of our sample and is identified as due to islands of monolayer fluctuation in the well width. We use the technique of far-infrared modulated photoluminescence to show that a resonant coupling exists between the excitonic states when the incident far-infrared photon energy is approximately equal to the splitting as measured in the photoluminescence spectrum. The most likely mechanism for this coupling is the dipole–dipole interaction.

Electronic states and optical gain in strained CdS/ZnS quantum structures

Ultrathin, coherently strained CdS layers have been grown epitaxially on ZnS with nominal thicknesses below the critical value for strain relaxation. These CdS/ZnS quantum structures, which show efficient photoluminescence and optical gain in the deep blue to ultraviolet spectral range, have been analyzed with respect to the dimensionality of the electronic states. It has been found that in wide-gap II-VI quantum structures small monolayer fluctuations result in such strong localization of excitons that the localization depth reaches energies around 100 meV. Consequently, the luminescence and gain can well be explained by optical transitions from an ensemble of spatially distributed exciton states in which the deepest and decoupled states can be considered as individual, three-dimensionally confined excitons.

A mini-trough for the study of membrane insertion ability of proteins

A mini-trough for the study of membrane insertion ability of proteins was designed and constructed. This trough was made up of teflon, small and handy, having 1.5 ~ 2.0 ml subphase volumes and 6 cm 2 surface areas for the monolayer spreading. With the unique construction, the vortex flow of fluids driven by the magnetic stir bar in the trough is converted into the laminar flow, which has no influence on the monolayer stability and surface pressure measurement. So the fluctuations of monolayers spread at the surface of subphase are small and the protein samples injected into the subphase could rapidly mix with the bulk solution, so as to make the response time is much rapid. Using this mini-trough, membrane insertion ability of melittin and apocytochrome c was studied.

High quality CdTe/Cd1−xMgxTe quantum wells grown on GaAs (100) and (111) substrates by molecular-beam epitaxy

We report the successful molecular-beam epitaxy growth of high quality CdTe/Cd1−xMgxTe quantum wells (x=0.27–0.52) on GaAs (100) and (111)B substrates. The effectiveness of the two different buffer layers for providing nearly lattice-matched templates were verified by x-ray diffraction and photoluminescence. Optical properties of excitons in the above quantum wells were investigated. It is observed that, for the (100) sample, the exciton displays double peaks, the peak splitting originating from exciton localization on the monolayer fluctuation of the quantum well width. Our experiments indicate that the influence of such fluctuations can be reduced, and interfaces of exceptional perfection can be obtained by growth of the CdTe/CdMgTe quantum well structures on GaAs (111)B-oriented substrates or by a combined growth of conventional molecular-beam epitaxy and migration-enhanced epitaxy on GaAs (100)-oriented substrate. The photoluminescence spectra indicate that significant reduction of the influence of the monolayer fluctuation was indeed accomplished by these procedures.

Photoluminescence spectroscopy of growth-interrupted GaAs/AlAs single quantum wells subjected to hydrogenation

Photoluminescence excitation and emission spectroscopy (PLE and PL) have been used to investigate growth interrupted GaAs/AlAs single quantum wells (SQW). Evidence that growth interruption leads to smooth interfaces which vary in height by integer monolayer steps is provided by changes in the PLE and PL spectra from a broad single band for noninterrupted SQW to narrow multiple peak features for growth interrupted samples. However, growth interruption results in a dramatic reduction in the intensity of the photoluminescence signal above 30 K. Hydrogenation treatment has been found to recover the photoluminescence intensity in the growth-interrupted samples which we attribute to passivation of nonradiative centers incorporated during the growth interruption process. Similar, though less dramatic, effects have been observed for noninterrupted samples. Time-resolved photoluminescence spectroscopy and decay time measurements show that hydrogen passivates carbon acceptors also incorporated during interruption, and in addition, demonstrate the migration process of excitons from the narrower to the wider well widths in these SQW. This latter exciton kinetic information fully supports the view that growth interruption results in large lateral islands with monolayer fluctuations in well width.

Metalorganic vapor phase epitaxy of wide-gap II–VI semiconductors for optoelectronic applications: current status and future trends

The current status and future trends to overcome the major problems of wide-gap II–VI compounds grown by metalorganic vapor phase epitaxy (MOVPE) which are p-type doping and the understanding of interface properties ofZnSSeZnSe-based heterostructures will be discussed. Since a low growth temperature is required to reduce defects and impurities and to increase the sticking coefficient of dopant atoms, a matched precursor combination of zinc and selenium compounds or an additional growth assistance (e.g. plasma stimulation) must be employed. The optical and electrical properties of ZnSe doped with nitrogen will be discussed. Emphasis will be put on the fact that most of the MOVPE grown ZnSe:N layers remain highly resistive or that they show only low free hole concentrations. Occurring compensation mechanisms such as parasitic compensating donors associated with nitrogen or compensating nitrogen-hydrogen complexes may be the reason. The ability of MOVPE to handle high vapor pressure elements such as sulphur favours this technology for the growth of sophisticated quantum wells and superlattices to achieve electrical and optical confinement in laser structures and to push the emission wavelength further into the blue. Scanning transmission electron microscope, photoluminescence (PL) and X-ray measurements were used for the analysis of the interface properties. Growth optimization of ZnSSeZnSe interfaces results in monolayer fluctuations at the interfaces. High excitation PL experiments show that room temperature stimulated emission is possible with this kind of structures. To realize high bit rate data transmission in the blue spectral range at 2.7 eV the physical properties of optoelectronic modulators based on ZnSSeZnSe superlattices were examined.

X-ray photoelectron spectroscopy and atomic force microscopy surface study of GaAs(100) cleaning procedures

The chemical composition of GaAs(100) surfaces after HCl/H2O wet etching sequences has been examined with x-ray photoelectron spectroscopy (XPS). XPS measurements show that oxides and chlorides are removed from the GaAs surface by the HCl solution etching in 10–20 min and subsequent H2O rinse in several seconds. The surface of the GaAs substrate thus prepared has been examined with atomic force microscopy. The surface flatness is so improved by the HCl/H2O treatment that the surface undulation remains within a ±1 monolayer fluctuation over a 1×1 μm surface area.

Optical investigations on p-type MOCVD-grown GaAs/Al xGa 1− xAs MQW

Absorption and PL investigations of MOCVD-grown p-doped (10 17 cm −3) 5 nm wide MQW have revealed excitonic transitions which can be associated with the heavy hole transition (1E-1HH) and that bound to neutral impurities. The optical spectra, while exhibiting both intrinsic and extrinsic excitonic emission well as e-A° transitions, however, reject the interpretation of the results as being due to monolayer fluctuations. The dissociation energy for the neutral impurity-exciton complex ( E dis) is found to be 10 meV.

X-ray photoelectron spectroscopic and atomic force microscopic study of GaAs etching with a HCl solution

GaAs(100) substrates covered with native oxides have been etched with a HCl solution for 10–20 min and subsequently rinsed with water for several seconds. The surface of the GaAs substrate thus prepared has been examined with atomic force microscopy. The surface flatness is improved by this wet etching and the surface undulation remains within a ± 1 monolayer fluctuation over a 1 μm × 1 μm surface area. X-ray photoelectron spectroscopy has been used to examine the chemical composition of the GaAs(100) surfaces after the HCl/H 2O wet etching sequences. UV photoirradiation of the GaAs surface treated with the HCl solution has been performed to characterize the chlorides on the GaAs surface.

Tailoring InAs/Ga 1- xIn xSb superlattices for long-wavelength IR applications

A systematic study has been carried out on the design of long-wavelength infrared detector materials from InAs/GaSb and InAs/GaInSb supelattices with absorption energies below 250 meV, i.e. λ > 5 μm. The influence from layer thicknesses and alloy composition on cut-off wavelength and optical matrix element has been analysed. A three-band envelope-function model including strain effects was used to calculate conduction-band electron and valence-band light-hole states, while an effective-mass approximation was used to describe heavy-hole states. In order to achieve useful absorption coefficients, the period of such superlattices must be less than typically 20 monolayers. Calculations revealed that the absorption can be increased with almost unaffected cut-off wavelength by reducing the GaInSb thickness. The effects of including a small amount of In in the GaSb in order to reach longer wavelengths were studied. One conclusion is that although it makes longer wavelengths possible, it also makes the cut-off wavelength much more sensitive to monolayer and composition fluctuations.

Photoluminescence study of HgTe-Hg 0.9Cd 0.1Te superlattices

Infrared photoluminescence (PL) measurements have been performed on 17 [211]-oriented superlattices with energy gaps spanning the range 110–495 meV. The spectra for 7 different samples display double peaks, which are attributed to monolayer fluctuations in the average thickness of the quantum wells. The peak splittings vary systematically with PL energy gap ( E p), and are accurately reproduced by orientation-dependent band structure calculations if the magnitude of the fluctuations is taken to be 1.5 monolayers. These data confirm that the low-temperature [211] growth produces islands with monolayer average smoothness over lateral dimensions of at least 500–1000 Å. Results are compared with earlier determinations of the interface roughness correlation length from magneto-transport measurements. In agreement with theoretical predictions, the decrease of d E p/d T with increasing E p is as rapid as that in the Hg 1- x Cd x Te alloy.

Optical and X-ray analysis of ZnS xSe 1−x/ZnSe superlattices grown on GaAs by metalorganic vapour phase epitaxy

X-ray diffractometry, Raman backscattering and photoluminescence were applied to characterize ZnS x Se 1- x /ZnSe superlattices, grown on GaAs(100) by metalorganic vapour phase epitaxy (MOVPE). As sulphur precursor materials diethylsulphide (DES) and H 2S were compared. Furthermore, we investigated the influence of different kind of buffer layers, the effect of increasing the number of periods and the consequences of stabilization during growth interruptions. Superlattices with a high crystal quality and very regular periodicity were obtained for 120-period structures, grown with DES as precursor, without intentional buffer layer. They show very narrow folded acoustical phonons in the Raman spectrum and their X-ray diffraction pattern contains not only sharp satellite peaks, whose fine structure reveals monolayer fluctuations, but also fringes which are due to interference from the entire stack.

Photoreflectance and photoluminescence characterizations of GaAs/Al χGa 1-χAs single quantum well interface structures

Photoreflectance (PR) and photoluminescence (PL) techniques were utilized as complementary tools to characterize GaAs/Al χGa 1-χAs single quantum well structures in the temperature range of 4–300 K. The samples were grown by MBE on GaAs (001) substrates exactly oriented or slightly misoriented 4° toward (111) Ga. Sharp PR features concerning pseudosmooth interfaces were observed for the sample with 4° tilt. Temperature dependence of PL peak positions as well as profiles showed different behaviour for sample grown with and without 4° misorientation. Tripeak structures of PL spectra corresponding to monolayer fluctuation are analyzed.

Topological defects in lamellar phases: passages, and their fluctuations

In lamellar phases of ternary amphiphilic mixtures, passages are tube-like connections of neighboring oil- (or water-) layers. Passages form spontaneously due to the thermal fluctuations of the monolayers (or membranes), which separate oil and water. Analogous membrane conformations also exist in binary amphiphilic mixtures, where the membranes are amphiphilic bilayers. We study the shape and the fluctuations of passages. Our analysis is based on the curvature model of fluid membranes, both with and without spontaneous curvature. Passages have a very small bending energy, and show strong fluctuations. The mean radius, the fluctuations of the radius, and the asphericity of the passage are calculated. All these quantities are experimentally accessible

Enhancements in MBE-grown high-speed GaAs and In 0.35Ga 0.65As MQW laser structures using binary short-period superlattices

The luminescence properties of molecular-beam epitaxially grown GaAs/AlGaAs multiple quantum-well laser structures with core and cladding layers consisting of either binary short-period superlattices (SPSLs) or ternary alloy layers are compared. Photoluminescence enhancements in the SPSL laser structures are attributed to improved gettering of background impurities. Low-temperature cathodoluminescence topography reveals regular 1–2 monolayer fluctuations in the QW widths of the ternary alloy laser structures, which are greatly reduced in the SPSL laser structures. The SPSL lasers demonstrate lower threshold currents than the ternary alloy devices. Modulation bandwidths of 16 and 21 GHz have been achieved for 3×200 μm 2 SPSL lasers containing three GaAs or In 0.35Ga 0.65As QWs, respectively, in the active region.

Theory of two-dimensional magnetic polarons in an external magnetic field

Magnetic self-trapping of an exciton in a quantum well is investigated by a variational method in the adiabatic approach. It is shown that the exciton magnetic polaron (EMP) can be formed in a quantum well at temperatures up to the critical temperature (Tc), which is found to depend on the characteristics of the structure. Tc decreases with increasing external magnetic field. The EMP binding energy in the CdTe/Cd1-xMnxTe quantum well structure is calculated. Primary localization of the exciton at a monolayer fluctuation of the interface is taken into account. The conditions for EMP formation in one-, two- and three-dimensional (1D, 2D and 3D) structures are analysed on a comparative basis.

Magneto-Optical Study of Reduced Deimensionality and Quantum Confinement in CdTe/MnTe Multiple Quantum Wells

CdTe/MnTe multiple quantum well (MQW) structures were grown on sapphire (0001) substrates. Magnetic circular dichroism (MCD) spectra show that these samples have sharp heterointerfaces with monolayer fluctuations. From the temperature dependence of the MCD peak intensity, it is found that the magnetism at heterointerfaces differ distinctly from that of MnTe barrier layers.

High valence-band offset of GaSbAs-InAlAs quantum wells grown by molecular beam epitaxy

Photoluminescence measurements were made at 2–300 K on GaSbAs-InAlAs single quantum wells lattice matched to InP grown by molecular beam epitaxy. The experimental exciton transition energy from the first electron subband to the first heavy hole subband was analyzed with calculated values obtained from the envelope function method. The quantum well exciton transition energy for well widths of 10–300 Å is fitted to a large valence-band offset (ΔEv) of 0.93 ΔEg. The exciton linewidth increases with decreasing well width. The two dominant exciton line broadening mechanisms were found to be monolayer fluctuations of the well width and the band filling of electron.