ZnS Barrier Research Articles

Electron ground state energy level determination of ZnSe self-organized quantum dots embedded in ZnS

Optical and electrical characterization of the ZnS self-organized quantum dots (QDs) embedded in ZnS by molecular beam epitaxy have been investigated using photoluminescence (PL), capacitance–voltage (C−V), and deep level transient Fourier spectroscopy (DLTFS) techniques. The temperature dependence of the free exciton emission was employed to clarify the mechanism of the PL thermal quenching processes in the ZnSe QDs. The PL experimental data are well explained by a two-step quenching process. The C−V and DLTFS techniques were used to obtain the quantitative information on the electron thermal emission from the ZnSe QDs. The correlation between the measured electron emission from the ZnSe QDs in the DLTFS and the observed electron accumulation in the C−V measurements was clearly demonstrated. The emission energy for the ground state of the ZnSe QDs was determined to be at about 120 meV below the conduction band edge of the ZnS barrier, which is in good agreement with the thermal activation energy, 130 meV, obtained by fitting the thermal quenching process of the free exciton PL peak.

Magnetic tunnel junctions for magnetic random access memory applications

Magnetic Tunnel Junctions (MTJ) are in the center of a large number of studies in spin polarized transport, because of their potential use in Magnetic Random Access Memory (MRAM) applications. The most commonly used tunnel junction is composed of an aluminium-oxide as a tunnel barrier and presents very interesting tunnel magnetoresistance (TMR) of about 40%. Nevertheless, this barrier needs an oxidation step and has a large resistance, which is not suitable for electronic devices. Thus a MTJ with an alternative ZnS barrier grown by sputtering on Si(111) substrate at room temperature is presented here with the following structure: Fe 6 nmCu 30 nm(CoFe) 1.8 nmRu 0.8 nm(CoFe) 3 nmZnS x (CoFe) 1 nmFe 4 nmCu 10 nmRu 3 nm. The hard magnetic bottom electrode consists of the artificial antiferromagnetic structure in which the rigidity is ensured by the antiferromagnetic exchange coupling between two FeCo layers through a Ru spacer layer. This alternative barrier is most suitable for electronic devices and we will discuss its possible application in MRAM.

Tunnel magnetoresistance in magnetic tunnel junctions with ZnS barrier

A first experimental evidence of a significant tunneling magnetoresistance signal of about 5% at 300 K for a magnetic tunnel junction consisting of hard and soft magnetic layers separated by a 2 nm ZnS semiconducting barrier is reported. The samples have been grown by sputtering on Si(111) substrate at room temperature and have the following structure: Fe6 nmCu30 nmCoFe1.8 nmRu0.8 nmCoFe3 nmZnSxCoFe1 nmFe4 nmCu10 nmRu3 nm. The hard magnetic bottom electrode consists of the artificial antiferromagnetic structure in which the rigidity is ensured by the antiferromagnetic exchange coupling between two FeCo layers through a Ru spacer layer. Barrier impedance scanning microscope (BISM) measurements reveal a good homogeneity of the barrier thickness. Electric transport measurements over square tunnel elements with lateral sizes between 3 and 100 μm, exhibit a typical tunnel current–voltage variations and tunnel resistance of 2–3 kΩ μm2 with small variations which never exceed a factor of 2, which is in good agreement with the BISM results. This good reproducibility of the junctions is very promising for MRAMs and transistors applications.

P‐56: Blue‐Emitting dc CaS:Pb Electroluminescent Device

AbstractA new dc TFEL device having a blue‐emitting CaS:Pb active layer and a ZnS barrier layer was fabricated using atomic layer deposition technique. This device shows the threshold voltage of 17 V, the maximum luminescence of 85 cd/m2 at 46 V, and the luminous efficiency ranging from 0.36 to 0.12 lm/W.

Tunnel magnetoresistance in magnetic tunnel junctions with a ZnS barrier

We report on the junction magnetoresistance in magnetic tunnel junctions of the hard–soft type with magnetic layers separated by a ZnS barrier. The hard magnetic bottom electrode consists of an artificial antiferromagnetic structure in which the rigidity is ensured by the antiferromagnetic exchange coupling between two FeCo layers through a Ru spacer layer. The samples were grown by sputtering on Si (111) wafers at room temperature and have the following structure: Fe6 nmCu30 nm(CoFe)1.8 nmRu0.8 nm(CoFe)3 nmZnSx(CoFe)1 nmFe4 nmCu10 nmRu3 nm. The square tunnel elements, with lateral sizes of 10, 20, 50, and 100μm, exhibit typical tunnel resistance of 2–3 kΩ μm2 and nonlinear zero field current–voltage (J–V) variation. The most interesting result is the observation of junction magnetoresistance of about 5% at room temperature with a 2 nm thick ZnS barrier.

The Subbands and Resonant Tunneling of a Two-Dimensional Electron Gas in a HgCdTe Metal-Insulator-Semiconductor Structure

Electron tunneling spectroscopy was performed at 77 and 4.2 K for the measurement of the tunneling current as a function of the bias voltage, which provided the information on the subbands and resonant tunneling of a two-dimensional electron gas confined in an n-type HgCdTe accumulation layer in the Hg1-xCdxTe–ZnS–In junction structure. Our analysis of the tunneling current versus applied bias measured at 77 K indicates that the subband energy level in a Hg0.79Cd0.21Te accumulation layer of a HgCdTe metal-insulator-semiconductor (MIS) structure is located at -59 meV for the ground state and -13 meV for the first excited state relative to the Fermi level. In addition, negative differential resistance was observed for the Hg0.79Cd0.21Te at 4.2 K when the applied bias was larger than the difference between the work function of Indium and the electron affinity of ZnS. Our calculation based on transfer matrix method suggests that this negative conductance be attributed to Fowler-Nordheim tunneling induced by adjusting the transmission width of a ZnS barrier.

Localized excitonic states in ZnS–ZnSe single quantum wells

We present low temperature photoluminescence spectra taken from an 11Å ZnSe quantum well in ZnS barriers. The samples are grown by the technique of photo-assisted vapour phase epitaxy (PAVPE) and the spectra show evidence for interface disorder. The observed dependences of the excitonic luminescence on excitation power and temperature are interpreted by a model involving excitonic localization below an exciton mobility edge. This mobility edge is measured for these samples to be 6 meV below the free exciton energy in the ideal quantum well.

Raman scattering study of the longitudinal optical phonons in ZnSe–ZnS strained-layer superlattices

Raman scattering studies were performed on ZnSe–ZnS strained-layer superlattices with different strains. In the optical phonon regime, the Raman spectra of the ZnSe–ZnS superlattices with a superlattice axis along [001] show optical phonons confined in the ZnSe well and ZnS barrier layers, and display a two-mode behavior corresponding to ZnSe-like and ZnS-like modes. We report the experimental results, by means of Raman scattering of confined longitudinal optical phonons in ZnSe–ZnS strained-layer superlattices. The Raman frequency shift dependence on the layer thickness, superlattice period, and interface strain is presented and discussed.

ZnS/Si/ZnS Quantum Well Structures for Visible Light Emission

AbstractSilicon multiple quantum wells confined by ZnS barriers have been grown by MOCVD. Calculations indicate that well widths must be less than 15Å for visible light emission, basically independent of the band offsets. No near-infrared photoluminescence (except bulk Si band-edge emission) was observed from samples with 70Å or greater Si layers, using stimulation at 325 nm with a He-Cd laser; hole trapping in ZnS may play a role. However, we found evidence of yellow light emission from samples which possibly contain Si quantum dots embedded in ZnS.

Carrier Confinement Effects in Epitaxial Silicon Quantum Wells Prepared by MOCVD

ABSTRACTSilicon multiquantum wells ranging in width from 3 to 15 nm were deposited on closely lattice-matched ZnS barriers. MOCVD was used to deposit the ZnS films using diethyl zinc and hydrogen sulfide as the precursors; disilane was used to deposit silicon layers at low temperatures. Single and multiple silicon nano-layers were observed by transmission electron microscopy and secondary ion mass spectrometry. Photoluminesence studies revealed emissions peaks which were blue-shifted with respect to the edge emission from bulk silicon substrates. The observation of emission from silicon nanostructures shifted to wavelengths as short as the 800-850 nm range is consistent with the effects of quantum confinement in silicon nanostructures.

ZnS/ZnSe strained-layer superlattices under pressure.

First-principles density-functional calculations of the electronic properties of ZnS/ZnSe (001) strained-layer superlattices are used to investigate the influence of hydrostatic pressure on the valence-band offset (VBO). Three different strain modes corresponding to various values of the relative thicknesses of the two types of layers are considered. The pressure coefficients of the VBO's are found to be very sensitive to the strain mode. A I\ensuremath{\rightarrow}II type conversion associated with the conduction-band crossover between the ZnSe well layers and ZnS barrier layers is found, in agreement with recent experimental data. The pressure behavior of the key quantities (VBO's, bulk moduli, energy gaps) is discussed for various strain modes.

ZnS/ZnSe Superlattices under Pressure

Self-consistent linear muffin-tin orbital method is used to calculate the band structure of ZnS/ZnSe (001) strained-layer superlattice and investigate the influence of hydrostatic pressure on the valence band offset (VBO). Three different strain modes corresponding to various values of the relative thicknesses of both materials are considered. A I —> II type conversion associated with the conduction-band crossover between the ZnSe well and ZnS barrier layers is found in agreement with recent experimental data. PACS numbers: 71.25.Tn, 73.20.Dx

Interface properties and the effect of strain of ZnSe/ZnS strained-layer superlattices

The ZnSe/ZnS alternately layered system is one of the fundamental examples of the wide-gap II–VI compound strained-layer superlaticee (SLS) used to understand the basic physics associated with optical, electronic and structural properties. Interface properties of ZnSe/ZnS SLSs have been extensively investigated by means MeV transmission electron microscopy (TEM), ion Rutherford backscattering (RBS)/channeling, X-ray diffraction (XRD), X-ray photoelectron emission spectra (XPS) and cross-sectional TEM measurements. It is shown that the ZnSe/ZnS SLS can be grown coherently up to a layer thickness of about 100 Å and to exhibit good crystalline quality. The experimental X-ray diffraction patterns are analyzed in terms of the kinematic approximation from which the strains being induced in each layer can be quantatively evaluated; the ZnSe well is certainly under compressive strain, while the ZnS barrier is under tensile strain. Band offsets in the conduction and valence bands in the strained layers are experimentally determined to be about 0.25 and 0.73 eV by XPS, respectively, and are theoretically considered as a function of the well width thickness using the ‘model solid theory’. The <110> channeling in the SLS shows much higher dechanneling yields than along the <100> growth direction. This may be derived from the small angle change in the inclined crystal directions at each interface which result in significant <100> dechanneling, and can be discussed in terms of the bond relaxation model or beam steering effects. The periodic oscillatory structure of the superlattices in RBS/channeling spectra has been observed in this SLS system. The interface properties can be understood in terms of the excitonic linewidth broadening which is influenced by either the interface roughness or by well width fluctuation. Photoluminescence and absorption spectroscopies of the ZnSe/ZnS and ZnSe/ZnSSe SLS are used to characterise the interface, and to clarify the energy separation of the degenerate hole bands due to a compressive stran in the ZnSe quantum well. The effects of ion irradiation and thermal stability during annealing in SLS are briefly introduced with a view to providing the fundamental physical properties of the creation of lattice defects and impurity diffusion. A Fibonacci SLS sequence with a quasi-periodic superlattice system demonstrates the possibility of new functional devices.

Hydrostatic-pressure-induced type-I → type-II conversion in ZnSe-ZnS strained-layer superlattices

A conversion from type-I to type-II under hydrostatic pressure in band lineups of a wide-bandgap ZnSe-ZnS strained-layer superlattice is observed for the first time. This type conversion results from a Г-point crossover of ZnSe well and ZnS barrier conduction bands and occurs at approximately 31 kbar. From the crossover pressure, the conduction-band offset is estimated to be 68.8 meV. This value is consistent with the result of a Kronig-Penney analysis of our photoluminescence excitation measurements and can be supported by semi-quantitative calculations that modify the model-solid theory.

Hydrostatic pressure dependence of two-dimensional exciton luminescence in ZnSe/ZnS strained-layer superlattices

Photoluminescence spectra of a ZnSe/ZnS strained-layer superlattice (SLS) are studied as a function of hydrostatic pressure at 77 K. The Γ-Γ conduction-band crossover between ZnSe well and ZnS barrier layers, leading to the formation of type-II band lineups, occurs at approximately 31 kbar. The luminescence quenching induced by the type conversion is discussed.

Type conversion under hydrostatic pressure in ZnSe-ZnS strained-layer superlattices.

A conversion from type-I to type-II in a ZnSe-ZnS strained-layer superlattice under hydrostatic pressure is observed. The hydrostatic-pressure dependence of the integrated intensity and the linewidth of the n=1 heavy-hole exciton emission spectra is represented by a couple of straight lines with a clear kink at about 31 kbar. Moreover, the emission peaks appear below the band-gap energy of the ZnSe well layers above 31 kbar. All of these features are well explained by the type conversion associated with the \ensuremath{\Gamma}-\ensuremath{\Gamma} conduction-band crossover between the ZnSe well and ZnS barrier layers.

II–VI widegap superlattices

We review our recent results of the excitonic properties in ZnSeZnS and Cd xZn 1−xSZnS strained-layer superlattices (SLSs). The most important physical insights in the II–VI widegap superlattices are to understand the relationship between the optical properties of quasi-two-dimensional exciton and strain because the well layer frequently receives biaxial compression or tension. The strain thus causes the significant shifts of the bandgap and splitting of the valence band. Semi-quantative calculations lead to an expectation that ZnSeZnS SLS always exhibits a type I band lineup within 100 Å thicknesses of the ZnSe well at a constant ZnS barrier width of several tens angstrom. This is in good agreement with the experimental results of exciton absorption and its luminescence excitation spectra. The Cd 0.3Zn 0.7SZnS SLSs with a range of well widths can produce intense excitonic emissions around 3.4 eV at room temperature due to the quantum confinement of excitons in the ternary CdZnS well. In order to elucidate localisation and relaxation processes of excitons, we have for the first time reported a multiple-LO-phonon emission process in the excitation spectra. The electric-field studies suggest that the concomitant decrease in intensity and the energy downshift of the exciton line may originate from the quantum confined Stark effect.

Defect creation and the stability of hetero-interfaces in ZnSe-ZnS strained-layer superlattices.

Photoluminescence (PL) and X-ray diffraction measurements have been performed to investigate the effects et thermal annealing and ion irradiation on the interface properties of low-pressure MOCVD-grown ZnSe-ZhS strained-layer superlattices (SLSs). It is shown from the evaluation of PL linewidth that a fluctuation of the heterointerface between the ZnSe well and the ZnS barrier layers can be completely controlled, within one monolayer. In the present SLS, the interface characteristics are retained against annealing in H2 atmosphere up to 580°C and no additional strain was observed. On the other hand, N+-ion irradiation and the subsequent annealing studies reveal that irradiation-induced defects quench the photoluminescence bands and consequently the fluctuation at the interface takes place. The results of Ar+-ion irradiation also suggest that such defects generation causes an additional fluctuation of the interface from about one monolayer to two monolayer.

Correlation Between Defect Production and Excitonic Emission in the Irradiated ZnSe-ZnS Strained-Layer Superlattices

ABSTRACTThe effects of thermal heat-treatment, N+ -ion, Ar+ -ion, γ-ray and electron-beam irradiations have been studied on MOCVD-grown ZnSe-ZnS strained-layer superlattices (SLSs) by means of low-temperature photoluminescence and X-ray diffraction measurements. As-grown SLS structure seems to be stable against the heat-treatment compared to ZnSe thin film, but after the defects are introduced in the SLS by irradiation, additional fluctuation at the interface between the ZnSe well and the ZnS barrier layers takes place. However, no significant intermixing and the changes of strain can be detected in the irradiated SLS.

Characterization of the interface of ZnSe-ZnS strained-layer superlattices by MeV transmission electron microscopy and ion-channeling

Interface properties of ZnSeZnS strained-layer superlattices (SLS) have been investigated by means of MeV transmission electron microscopy and ion-channeling measurements, and by X-ray diffraction analysis using the kinematic approximation. It is shown that the ZnSeZnS SLS can be grown coherently up to a layer thickness of about 200 Å and exhibits good crystalline quality. The ZnSe well is under compressive strain, while the ZnS barrier is under tensile strain. The 〈110〉 channeling in the SLS shows much higher dechanneling yields than along the 〈100〉 growth direction. This may be derived from the small angle change in the inclined crystal directions at each interface which result in significant 1̌10〉 dechanneling.