ZnSe Quantum Wells Research Articles

Efficient Single-Photon Sources Based on Chlorine-Doped ZnSe Nanopillars with Growth Controlled Emission Energy.

Isolated impurity states in epitaxially grown semiconductor systems possess important radiative features such as distinct wavelength emission with a very short radiative lifetime and low inhomogeneous broadening, which make them promising for the generation of indistinguishable single photons. In this study, we investigate chlorine-doped ZnSe/ZnMgSe quantum well (QW) nanopillar (NP) structures as a highly efficient solid-state single-photon source operating at cryogenic temperatures. We show that single photons are generated due to the radiative recombination of excitons bound to neutral Cl atoms in ZnSe QW and the energy of the emitted photon can be tuned from about 2.85 down to 2.82 eV with ZnSe well width increase from 2.7 to 4.7 nm. Following the developed advanced technology, we fabricate NPs with a diameter of about 250 nm using a combination of dry and wet-chemical etching of epitaxially grown ZnSe/ZnMgSe QW structures. The remaining resist mask serves as a spherical- or cylindrical-shaped solid immersion lens on top of NPs and leads to the emission intensity enhancement by up to an order of magnitude in comparison to the pillars without any lenses. NPs with spherical-shaped lenses show the highest emission intensity values. The clear photon-antibunching effect is confirmed by the measured value of the second-order correlation function at a zero time delay of 0.14. The developed single-photon sources are suitable for integration into scalable photonic circuits.

Tailoring the optical properties of wide-bandgap based microcavities via metal films

We report on the tuning of the optical properties of II-VI-material-based microcavity samples, which is achieved by depositing Ag films on top of the structures. The micro-reflectivity spectra show a spectral shift of the sample resonance dependent on the metal layer thickness. By comparison of the experimental findings with the theoretical calculations applying the transfer matrix method on a metal-dielectric mirror structure, the influence of the metal layer particularly with regard to its partial oxidation was explored. Tamm plasmon modes are created at the interface between an open cavity with three ZnSe quantum wells and a metal layer on top. When tuning the excitonic emission relative to the mode by changing the sample temperature, an anticrossing of the resonances was observed. This is a clear indication that the strong coupling regime has been achieved in that sample configuration yielding a Rabi splitting of 18.5 meV. These results are promising for the realization of polariton-based optical devices with a rather simple sample configuration.

Real-time contrast-enhanced holographic imaging using phase coherent photorefractive quantum wells.

We demonstrate wide-field real-time and depth-resolved contrast enhanced holographic imaging (CEHI) using the all-optical phase coherent photorefractive effect in ZnSe quantum wells. Moving objects are imaged at large depth-of-field by the local enhancement of a static reference hologram. The high refresh rate of the holographic films enables direct-to-video monitoring of floating glass beads and of living Paramecium and Euglena cells moving in water. Depth resolution is achieved by tilting the incident laser beam with respect to the normal of the cuvette. This creates double images of the objects, which are analyzed geometrically and with Fresnel diffraction theory. A two-color CEHI set-up further enables the visualization of a concealed 95 µm thick wire behind a thin layer of chicken skin.

Molecular-beam epitaxy of heterostructures of wide-gap II–VI compounds for low-threshold lasers with optical and electron pumping

The paper presents basic approaches in designing and growing by molecular beam epitaxy of (Zn,Mg)(S,Se)-based laser heterostructures with multiple CdSe quantum dot (QD) sheets or ZnCdSe quantum wells (QW). The method of calculation of compensating short-period ZnSSe/ZnSe superlattices (SLs) in both active and waveguide regions of laser heterostructures possessing the different waveguide thickness and different number of active regions is presented. The method allowing reduction of the density of nonequilibrium point defects in the active region of the II–VI laser structures has been proposed. It utilizes the migration enhanced epitaxy mode in growing the ZnSe QW confining the CdSe QD sheet. The threshold power density as low as P thr ∼ 0.8 kW/cm2 at T = 300 K has been demonstrated for laser heterostructure with single CdSe QD sheet and asymmetric graded-index waveguide with strain-compensating SLs.

Blue lasing and strong coupling in ZnSe monolithic microcavities

AbstractMicrocavity structures which consist of a cavity formed by two distributed Bragg reflectors can be used to study light‐matter interaction in the weak and strong coupling regime. In this contribution microcavities with one or three ZnSe quantum wells embedded into the cavity are presented. In contrast to the sample with the single quantum well, strong light‐matter coupling can be observed for that one with three quantum wells.The sample possesses an energy splitting of about 19meV between the upper and lower polariton branch observed by reflectivity measurements. In addition, the transition from strong to weak coupling and the starting of the photonic lasing is demonstrated by excitation density dependent measurements. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Strong coupling in monolithic microcavities with ZnSe quantum wells

Strong light-matter coupling is demonstrated in monolithic microcavities with only three ZnSe quantum wells embedded. Basis for this observation is the excellent structural quality of the sample as confirmed by scanning transmission electron microscopy measurements. A comparative large energy splitting between the upper and lower polariton (LP) of about 19 meV is observed by reflectivity measurements in real and k-space. Efficient polariton relaxation is shown by photoluminescence measurements at low temperatures in k-space. These beneficial properties of the sample result in a nonlinear increase of the lower polariton population in excitation density dependent measurements before the photonic cavity emission becomes dominant.

Blue monolithic II-VI-based vertical-cavity surface-emitting laser

We report on laser operation of optically pumped, fully epitaxial blue vertical-cavity surface-emitting lasers. The ZnSe-based structures posses a bottom and top distributed Bragg reflector consisting of ZnMgSSe as high-index material and a short-period superlattice of MgS/ZnCdSe as low-index material. The cavity has an optical thickness of λ and contains three ZnSe quantum wells surrounded by ZnMgSSe barriers. To illustrate the specific issues related to this demanding material combination, we compare two epitaxial structures of different crystalline quality. A minimum threshold of 5 pJ is observed for laser emission at 443 nm comparable to values reported for nitride based vertical-cavity surface-emitting lasers.

Influence of electron density and trion formation on the phase-coherent photorefractive effect in ZnSe quantum wells

We investigate the influence of electron density and trion formation on the phase-coherent photorefractive effect in ZnSe single quantum wells by laser energy and temperature dependent degenerate four-wave-mixing experiments using 90-fs pulses. Two different structures with the same quantum well width and confinement energy but different barriers adjacent to the GaAs substrate are studied in order to compare the formation of a photorefractive electron density grating at specific excitation conditions. At temperatures below 35 K and laser excitation energy close to the exciton energy the formation of trions significantly suppresses the generation of an electron density grating. At lower excitation energies increasing space-charge fields reduce the trion binding energy which leads to an enhanced thermal ionization of trions resulting in a strong phase-coherent photorefractive effect. Due to the thermal dissociation of trions at temperatures exceeding 45 K a significant photorefractive effect exists even at exciton resonant excitation. The experimentally observed signal traces obtained at different excitation conditions are in good agreement with model calculations that are based on the optical Bloch equations, including inhomogeneous broadening at strong space-charge fields.

Magnetic-field-induced exchange effects between Mn ions and free carriers in ZnSe quantum wells through the intermediate nonmagnetic barrier studied by photoluminescence

Photoluminescence (PL) of the 50 nm $Zn_{0.9}Be_{0.05}Mn_{0.05}Se$/ $d$ nm $Zn_{0.943}Be_{0.057}Se$/ 2.5 nm $ZnSe$/ 30 nm $Zn_{0.943}Be_{0.057}Se$ structures is investigated as a function of magnetic field ($B$) and thickness ($d$) of intermediate $Zn_{0.943}Be_{0.057}Se$ nonmagnetic barrier between the $Zn_{0.9}Be_{0.05}Mn_{0.05}Se$ semimagnetic barrier and $ZnSe$ quantum well at the temperature 1.2 K. The rate of the shift of different PL bands of the structures under study is estimated in low and high magnetic fields. The causes of the shift rate increase under pass from low to high magnetic fields are interpreted. The peculiarities of the effect of the intermediate barrier on the luminescence properties of the structures are presented. It is shown that deformation of adjacent layers by the barrier plays a crucial role in the formation of these properties, especially in forming the $Mn$ complexes in the $Zn_{0.9}Be_{0.05}Mn_{0.05}Se$ layer. The change of the band gap as well as of the donor and acceptor levels energies under the effect of biaxial compression of the $Zn_{0.9}Be_{0.05}Mn_{0.05}Se$ layer by the $Zn_{0.943}Be_{0.057}Se$ are estimated. It is concluded that the $Zn_{0.943}Be_{0.057}Se$ intermediate barrier also appreciably changes the effect of giant Zeeman splitting of the semimagnetic $Zn_{0.9}Be_{0.05}Mn_{0.05}Se$ barrier energy levels on the movement of the energy levels of $ZnSe$ quantum well in a magnetic field and on polarization of the quantum well exciton emission.

Real-time single-shot three-dimensional and contrast-enhanced optical coherence imaging using phase coherent photorefractive quantum wells

We demonstrate two real-time optical coherence imaging acquisition modes using all-optical phase coherent photorefractive ZnSe quantum wells as dynamic holographic films. These films use the coherence of excitons for time-gating which provides depth information of an object according to the brightness profile of its holographic image. This quality allows depth-resolved imaging of moving particles with a resolution of a few micrometers in a single-shot three-dimensional mode. In a complementary contrast-enhanced mode moving particles are imaged by the local enhancement of a static reference hologram, enabling optical coherence imaging at a large depth-of-field.

Violet–green laser converter based on MBE grown II–VI green lasers with multiple CdSe quantum dot sheets, pumped by InGaN laser diode

AbstractThe violet–green laser converter based on a molecular‐beam‐epitaxy (MBE) grown CdSe quantum dot (QD) laser heterostructure pumped by a commercial InGaN laser diode (LD) emission has been fabricated and studied in detail. The optimized II–VI laser heterostructure consists of asymmetrical ZnSe/ZnSSe superlattice (SL) waveguide and active region comprising five CdSe QD sheets (QDS) placed in the centre of 2‐nm‐thick ZnSe quantum wells. The new laser structure design provides both a high homogeneity of optical pumping of the CdSe QDS due to tunnelling of charge carriers between the QDS separated by 5‐nm‐thick ZnSe/ZnSSe/ZnSe barriers and high optical confinement factor. Optimization of both cavity length of the II–VI laser and parameters of optical focusing system to obtain a narrow stripe with the length slightly exceeding the cavity length has been performed. As a result, the maximum achieved quantum efficiency and pulse output power in green have been as high as 8% and 65 mW, respectively.

Towards Bose-Einstein Condensation of Semiconductor Excitons: The Biexciton Polarization Effect

We theoretically predict a strong influence of stimulated exciton-exciton scattering on semiconductor luminescence. The stimulated scattering causes circularly polarized instead of unpolarized emission at the biexciton emission line in a degenerate gas of partly spin polarized excitons. The biexciton polarization effect increases with increasing exciton densities and decreasing temperatures and approaches almost unity in the ultimate case of Bose-Einstein condensation. Time- and polarization-resolved luminescence measurements evidence the biexciton polarization effect both in ZnSe and GaAs quantum wells.

An efficient electron-beam-pumped semiconductor laser for the green spectral range based on II–VI multilayer nanostructures

Emission characteristics of an electron-beam-pumped Cd(Zn)Se/ZnMgSSe semiconductor laser are studied. The laser’s active region consists of a set of ten equidistant ZnSe quantum wells containing fractional-monolayer CdSe quantum-dot inserts and a waveguide formed by a short-period superlattice with the net thickness of ∼0.65 μm. Lasing occurs at room temperature at a wavelength of 542 nm. Pulsed power as high as 12 W per cavity face and an unprecedentedly high efficiency of ∼8.5% are attained for the electron-beam energy of 23 keV.

Modelling of strained ZnSSe on relaxed ZnSSe-based structures for blue light emission

It has been a long time since the start of successful production in laboratory of light-emitting devices based on wide bandgap II–VI semiconductors, but the applications have been limited by the luck of materials that can emit blue light efficiently. In this work, we propose and model a ZnS x Se 1− x -based structure emitting blue light and resolve many defect problems due to the lack of substrates of ZnSSe-epitaxy. The optoelectronic properties of (n-ZnS 0.8Se 0.2-relaxed/(ZnSe/ZnS 0.2Se 0.8/ZnSe)-strained/n-ZnS 0.8Se 0.2-relaxed) and (n-ZnS 0.8Se 0.2-relaxed/triple quantum well (QW) (ZnSe/ZnS 0.2Se 0.8/ZnSe)-strained/ n-ZnS 0.8Se 0.2-relaxed) are studied and compared. The numerical self-consistent resolution of Schrödinger–Poisson equations leads to the potential profiles of conduction and valence bands. Variation of ZnSe /ZnS 0.2Se 0.8/ZnSe well thickness permits to optimize the confinement of electrons and heavy holes, the wave function overlap, the oscillator strength and the carrier density for the two structures. The optimized results show that pure blue light emission is achieved more efficiently with the second proposed stack using a triple ZnSe/ZnS 0.2Se 0.8/ZnSe QW in the active layer.

MBE Growth and Characterization of MgS-Rich Zinc-Blende ZnxMg1-xS1-ySey Alloys

Samples containing ZnMgSSe alloy were grown by using molecular beam epitaxy at 240 C and were analyzed by using X-ray interference. The alloy composition was found to be Zn0.20Mg0.80S0.64Se0.36. The surfaces of these layers were found to be extremely flat, unlike MgS layers of similar thickness grown under identical conditions, which show pronounced ridges. Structures with Zn0.20Mg0.80S0.64Se0.36 barriers were grown with ZnSe quantum wells and showed good quantum confinement with a sharp PL peak. Calculations of the phase stability of ZnMgSSe alloys suggest that an alloy of this composition should phase separate. However, samples with this composition are demonstrably single phase, and the discrepancy with the calculation can be removed if the enthalpy of formation of zinc-blende MgS is reduced by less than 2 % to 231 kJ mol 1 .

A comparison of ZnMgSSe and MgS wide bandgap semiconductors used as barriers: Growth, structure and luminescence properties

The wide bandgap alloy Zn 0.2Mg 0.8S 0.64Se 0.36 has recently been grown by molecular beam epitaxy (MBE) and been shown to be oxidation and acid resistant. This makes it attractive either as a replacement or adjunct to MgS in II–VI multilayers. In this paper we compare the structural and optical properties of MBE grown multilayer structures containing Zn 0.2Mg 0.8S 0.64Se 0.36 to those grown with the quaternary alloy replaced by MgS. Cross-sectional high-resolution transmission electron microscopy (HRTEM) and X-ray interference spectra of ZnSe/Zn 0.2Mg 0.8S 0.64Se 0.36/ZnSe multilayers show the Zn 0.2Mg 0.8S 0.64Se 0.36 layers are of good crystal quality and do not phase segregate. Layer interfaces are seen to be flat and Zn 0.2Mg 0.8S 0.64Se 0.36 does not introduce defects into the overlying ZnSe. Atomic force microscopy shows the surface of a 30 nm Zn 0.2Mg 0.8S 0.64Se 0.36 layer is atomically flat, in contrast with similar MgS layers, which show pronounced 1D surface ridges, indicating that the Zn 0.2Mg 0.8S 0.64Se 0.36 layers have not started to relax. ZnSe quantum wells grown with Zn 0.2Mg 0.8S 0.64Se 0.36 barriers show 77 K photoluminescence comparable in wavelength and intensity to ZnSe wells of similar thickness grown with MgS barriers. This has allowed us to demonstrate the use of the quaternary alloy, which resists oxidation in place of MgS in multilayer structures.

Holographic imaging using the phase coherent photorefractive effect in ZnSe quantum wells

We demonstrate wide-field real-time, and depth-resolved holographic imaging of various reflecting objects using the all-optical phase coherent photorefractive effect in ZnSe quantum wells. In contrast to current photorefractive quantum well devices, these devices use the coherence of excitons as a basis for holography. This has the advantage that the entire depth-resolved information of the object can be obtained from the brightness profile of the holographic image with a resolution of a few micrometers. In addition, the high diffraction efficiency and short refresh time of the phase coherent photorefractive effect allow high-speed video acquisition of moving objects.

Lasing of donor-bound excitons in ZnSe microdisks

Excitons bound to fluorine atoms in ZnSe have the potential for several quantum optical applications. Examples include optically accessible quantum memories for quantum information processing and lasing without inversion. These applications require the bound-exciton transitions to be coupled to cavities with high cooperativity factors, which results in the experimental observation of low-threshold lasing. We report such lasing from fluorine-doped ZnSe quantum wells in 3 and $6\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ microdisk cavities. Photoluminescence and selective photoluminescence spectroscopy confirm that the lasing is due to bound-exciton transitions.

Optically injected electron gas and formation of charged excitons in type-II ZnSe/BeTe quantum wells

We have investigated the effect of accumulation of photo-created electrons in ZnSe/BeTe type-II quantum well (QW) structures by means of a pump-and-probe method in high magnetic field up to 7 T. A cw-diode laser was used as a pumping light source. With increasing pumping intensity neutral exciton (X) absorption are weakened and a distinct peak structure due to negatively charged exciton (X−) was observed at a 6 meV lower energy side of the X in ZnSe QW layers. The magnetic field dependence of circular polarization of X− contains information about the density of twodimensional electron gas (2DEG). We found that high-density 2DEG (∼1×1011cm−2) is optically injected by successfully using type-II quantum structures without n-doped layers.

Interband absorption of multiple quantum wells including diluted magnetic semiconductors in magnetic fields

We have investigated theoretically the interband excitonic absorption spectra and the coupling behaviors of coupled symmetric triple Zn1−xCdxSe∕ZnSe quantum wells with diluted Mn doping in the central well under external magnetic field. The results show that the magnetic field can effectively adjust the energy levels and the optical properties of the coupled multiple quantum wells (CMQWs). The coupling behaviors of the CMQWs are very sensitive to the central well depth. The effect of the magnetic-field-introduced parabolic-confined potential on the main features of the absorption spectra can be neglected at moderate magnetic-field densities.