Excitonic recombination near the mobility edge in CdSe/ZnSe nanostructures

The low-temperature photoluminescence and Raman scattering in CdSe/ZnSe nanostructures with individual CdSe inserts of 1.5 and 3.0 monolayers in nominal thickness were studied. The energy position of the photoluminescene band is governed by interdiffusion of Cd and Zn into the insert regions, whereas the shape of this band is controlled by strong interaction of localized excitons with optical phonons in the Zn_1?xCd_xSe solid-solution insert. Multiphonon processes of excitonic relaxation with involvement of acoustic phonons at the Brillouin zone edges are also important. The results obtained are interpreted in the context of a model for the effective excitonic mobility edge.     

CdSe/ZnSe quantum dot structures grown by molecular beam epitaxy with a CdTe submonolayer stressor

A procedure for formation of CdSe quantum dots (QDs) in a ZnSe matrix is suggested. The procedure is based on the introduction of a CdTe submonolayer stressor deposited on the matrix surface just before deposition of the material of the QDs. (For CdTe/ZnSe structure, the relative lattice mismatch is Δa/a ≈ 14%.) The stressor forms small strained islands at the ZnSe surface, thus producing local fields of high elastic stresses controlling the process of the self-assembling of the QDs. According to the data of transmission electron microscopy, this procedure allows a considerable increase in the surface density of QDs, with a certain decrease in their lateral dimensions (down to 4.5 ± 1.5 nm). In the photoluminescence spectra, a noticeable (～150 meV) shift of the peak to longer wavelengths from the position of the reference CdSe/ZnSe QD structure is observed. The shift is due to some transformation of the morphology of the QDs and an increase in the Cd content in the QDs. Comprehensive studies of the nanostructures by recording and analyzing the excitation spectra of photoluminescence, the time-resolved photoluminescence spectra, and the cathodoluminescence spectra show that the emission spectra involve two types of optical transitions, namely, the type-I transitions in the CdSeTe/ZnSe QDs and the type-II transitions caused mainly by the low cadmium content (Zn,Cd)(Se,Te)/ZnSe layer formed between the QDs.     

Luminescence properties of MgxZn1-xSe prepared by Mg diffusion

The photoluminescence of ZnSe doped with the isoelectronic substituent Mg with a simple diffusion procedure has been studied. It was found that Mg easily enters substitutionally on Zn-site to form Mg_xZn_1-xSe. The diffused samples therefore show a graded bandgap due to a composition gradient in the surface region. In contrast to ZnSe the photoluminescence spectrum of Mg_xZn_1-xSe is dominated by a near bandgap emission at all temperatures between 1.5 and 300 K. The bandgap shift compared with “pure” ZnSe is estimated from photoluminescence excitation spectra. The luminous efficiency of Mg_xZn_1-xSe makes the material promising for future applications as light emitting diodes with a well defined narrow emission band at a wavelength determined by the Mg content x.     

Bright CdSe quantum dot inserted in single ZnSe nanowires

We report the evidence of CdSe quantum dot (QD) insertion in single defect-free ZnSe nanowire. These nanowires have been grown by molecular beam epitaxy in vapour-liquid-solid growth mode catalysed with gold particles. We developed a two-step process allowing us to grow very thin (from 15 to 5 nm) defect-free ZnSe nanowire on top of a nanoneedle, where all defects are localised. The CdSe QDs are incorporated to the defect-free nanowires part. Owing to the extraction efficiency of the nanowires and the reduced number of stacking fault defects in the two-step-process nanowires, a very efficient photoluminescence is observed even on isolated single nanowire. Time-resolved photoluminescence and correlation photon give evidences that the bright photon emission is related to the CdSe QD.     

一种新结构中ZnSe薄膜电致发光特性研究

用电子束蒸发的方法制备了一种新的ITO／SiO2／ZnSe／SiO2／Al薄膜电致(TFEL)发光器件。在交流电压驱动下,其有2个发光峰,分别位于466nm和560nm。通过研究器件(PL)激发(PLE)谱、光致发光、EL发光以及EL发光强度随驱动电压和频率的变化发现,器件的发光来源于ZnSe的带边发射和自激活发光中心。器件的发光机理与一般的无机电致发光有所不同。这里,SiO2作为电子加速层,ZnSe作为发光层,电子在SiO2层中的高电场作用下被加速到很高的能量,然后直接碰撞激发ZnSe分子使其发光。这种发光现象被称为固态类阴极发光。     

Three-dimensionally confined excitons and biexcitons in submonolayer-CdSe/ZnSe superlattices

We report on structural and optical investigations of submonolayer-CdSe/ZnSe superlattices with varying thicknesses of the ZnSe spacer layers. High-resolution electron microscopy images demonstrate the formation of two-dimensional nanoscale CdSe islands for submonolayer-CdSe depositions. The vertical island arrangement is anti-correlated for spacer layer thicknesses exceeding 30Å, while predominantly vertically correlated growth occurs for thinner spacers. The different vertical ordering of the CdSe islands results in two clearly distinguishable lines in photoluminescence and optical reflectance spectra, which are attributed to excitons localized in quantum dots (QDs) formed by vertically coupled and uncoupled (Cd,Zn)Se islands, respectively. δ-like emission of single QDs is demonstrated and the different carrier localization in uncoupled and coupled QDs is reflected in the polarization of the edge emission. Stimulated emission and resonant waveguiding effects are observed for both states. At the highest excitation densities, we observe saturation of the stimulated emission in edge geometry attributed to saturation of localized states.     

Optical properties of CdSe quantum dots grown on ZnSe and ZnBeSe by molecular beam epitaxy

We report detailed studies of the optical properties of CdSe quantum dots (QDs) grown on ZnSe and ZnBeSe by molecular-beam epitaxy (MBE). We performed steady-state and time-resolved photoluminescence (PL) measurements and observe that nonradiative processes dominate at room temperature (RT) in the CdSe/ZnBeSe QDs structures, though these nonradiative processes do not dominate in the CdSe/ZnSe QDs structures up to RT. We performed secondary ion-mass spectrometry (SIMS) measurement and propose that the oxygen incorporation in the ZnBeSe layers (possibly caused by the reactivity of Be) may contribute to the dominant nonradiative processes at high temperatures in the QDs grown on ZnBeSe.     

静压下（CdSe）_m／（ZnSe）_n－ZnSe超短周期超晶格量子阱的共振喇曼散射研究

在静压和液氮温度下观察到（ＣｄＳｅ）ｍ／（ＺｎＳｅ）ｎ短周期超晶格中重空穴激子的复合发光和多达４阶的类ＺｎＳｅＬＯ多声子喇曼散射，并观察到厚ＺｎＳｅ势垒层的带边发光和限制在厚势垒层中的类ＺｎＳｅＬＯ声子散射．结果表明，加压后（ＣｄＳｅ）ｍ／（ＺｎＳｅ）ｎ短周期超晶格中的类ＺｎＳｅ的１ＬＯ和２ＬＯ声子模频率分别以３．７６和７．１１ｃｍ－１／ＧＰａ的速率向高频方向移动，超晶格阱层光致发光峰的压力系数为５９．８ｍｅＶ／ＧＰａ．与（ＣｄＳｅ）ｍ／（ＺｎＳｅ）ｎ短周期超晶格共振时的类ＺｎＳｅ１ＬＯ声子模频率比与ＺｎＳｅ势垒层共振时的类ＺｎＳｅ１ＬＯ声子模频率低２．０ｃｍ－１，反映了（ＣｄＳｅ）ｍ／（ＺｎＳｅ）ｎ短周期超晶格中ＬＯ声子的限制效应     

Sub‐kilogram‐Scale One‐Pot Synthesis of Highly Luminescent and Monodisperse Core/Shell Quantum Dots by the Successive Injection of Precursors

The one‐pot synthesis of core/shell quantum dots (QDs) represents an attractive alternative to conventional synthesis techniques, where the core CdSe QDs are first purified and then an epitaxial shell of the desired thickness is obtained by the slow addition of shell precursors to a solution of the purified QDs at high temperature. We have developed a one‐pot synthesis procedure involving the successive injection of deliberately selected core‐ and shell‐forming reagents at appropriate temperatures. Sub‐kilogram quantities of highly luminescent and monodisperse core/shell QDs with desirable optical properties (full width at half maximum of photoluminescence (PL) band is ca. 30 nm) have been produced by the sequential growth of the core and shell in a controlled manner. This one‐pot method has also been extended to form water‐soluble core/double‐shell CdSe/ZnSe/ZnS QDs exhibiting high PL efficiency and stability.     

［（Cdse）_m（ZnSe）_n］_p－Znse多量子阱中的多声子散射

对用原子层外延方法，在［００１］晶向ＧａＡｓ衬底上生长的［（Ｃｄｓｅ）ｍ（Ｚｎｓｅ）ｎ］ｐ－ＺｎＳｅ应变量子阱结构，在１０～３００Ｋ温度范围内测量了喇曼散射光谱，观察到两种类ＺｎＳｅＬＯ声子限制模．利用改变样品温度和入射光能量实现了共振喇曼散射，观察到高达７阶的类ＺｎＳｅＬＯ声子模．并讨论了多声子喇曼散射和热萤光过程的区别．     

Optical an photoelectric properties odf ZnSe:Ti crystals

The photoconductivity and photoluminescence spectra of ZnSe:Ti crystals in the visible and infrared spectral regions are studied. It is established that the high-temperature impurity-induced photoconductivity of ZnSe:Ti crystals is defined by the optical transitions of electrons from the ³ A ₂(F) ground state to highenergy excited states, with the subsequent thermal transitions of electrons to the conduction band. The efficient excitation of intracenter luminescence of ZnSe:Ti crystals is achieved by light from the region of the intrinsic absorption of Ti²⁺ ions.     

Effect of iron impurities on the photoluminescence and photoconductivity of ZnSe crystals in the visible spectral region

The photoconductivity and photoluminescence of ZnSe:Fe crystals in the visible spectra region are studied. The scheme of optical transitions within Fe²⁺ impurity centers is established. It is shown that the high-temperature photoconductivity of ZnSe:Fe crystals is controlled by optical transitions of electrons from the ⁵ E(F) ground state to the higher levels of excited states of Fe²⁺ ions, with subsequent thermal activation of the electrons to the conduction band. Efficient excitation of intracenter luminescence of ZnSe:Fe crystals is attained with light corresponding to the region of intrinsic absorption in Fe²⁺ ions.     

Förster Resonance Energy Transfer and Harvesting in II–VI Fractional Monolayer Structures

We report on Förster resonance energy transfer in the dense arrays of epitaxial quantum dots (QDs), formed by fractional monolayer CdSe insertions within a ZnSe matrix. In such arrays comprising the QDs of different sizes, the energy transfer can take place between the ground levels of small QDs and the excited levels of large radiating QDs, when these states are in resonance. This mechanism provides directional excitation of a limited number of the large QDs possessing the excited levels. It reveals itself by the shrinkage of photoluminescence (PL) bands and the appearance of the narrow single excitonic lines in micro-PL spectra. The strong shortening of characteristic PL decay times in the energy-donating QDs is observed when the distance between them and the energy-accepting QDs decreases. Photoluminescence excitation spectroscopy demonstrates the switching of the dominant energy transfer mechanism at the energy predicted by theoretical modeling of the excitonic levels in the QD arrays. Our results pave the way for engineering of the architecture of excitonic levels in the QD arrays to realize efficient nano-emitters.     

Formation and characterization of self-organized CdSe quantum dots

We have investigated the formation and characteristic of self-organized CdSe quantum dots (QDs) on ZnSe(001) surfaces with the use of photoluminescence (PL) and transmission electron microscopy (TEM). Coherent CdSe QDs are naturally formed on ZnSe surfaces, when the thickness of CdSe layers is around 2 ML. The plan-view TEM images exhibit that CdSe QDs have a relatively narrow distribution of QD size, and that the density of CdSe QDs is about 10¹⁰ cm⁻². The base structure of the CdSe dot is rhombic, which has the long axis of about 20 nm in length along direction. The temperature dependence of macro-PL spectra reveals that the behavior of self-organized CdSe QDs is quite different from that of ZnCdSe quantum well (QW), resulting from characteristic features of zero-dimensional structures of QDs. Moreover, the macro-PL results suggest the existence of QW-like continuous state lying over QD states. Micro-PL measurements show several numbers of high-resolved sharp lines from individual CdSe QDs. The linewidth broadening with temperature depends on peak energy position of the QDs. The linewidths of lower energy lines, corresponding to larger size QDs, are more temperature dependent.     

Photoluminescence of nitrogen-doped zinc selenide epilayers

Photoluminescence (PL) studies of nitrogen doped ZnSe epilayers grown by molecular beam epitaxy have been performed as a function of excitation wavelength, power density, and temperature. The broad emission from heavily doped ZnSe:N is composed of two distinct bands which we label as N_I and N_II. The dominant band N_I appears at 2.54 eV, while the N_II band position is sensitive to excitation power and occurs between 2.55 and 2.61 eV. The N_I emission energy is insensitive to incident power or temperature over the ranges studied. Further, a 69 meV localized phonon of the N_I band is observed. We propose that the N_I band is related to transitions within a (V_se+-Zn-N_se-)0 close-associate pair. The N_II band displays characteristics consistent with the conventional donor acceptor pair model. A third band N_III at 2.65 eV is observed under high-power pulsed excitation. Previous studies of heavily doped ZnSe:N had suggested that the broad emission band was described by a modified donor-acceptor pair model. Our PL study does not support this previous model. In addition, our data suggests that singly ionized selenium vacancy complexes form in heavily doped ZnSe:N and play a role in compensation.     

Effect of conjugation with biomolecules on photoluminescence and structural characteristics of CdSe/ZnS quantum dots

The photoluminescence and photoluminescence excitation spectra, the X-ray diffraction patterns, and the effect of conjugation with biomolecules upon these characteristics are studied for silanized CdSe/ZnS quantum dots. Along with the band of annihilating excitons in the quantum dots, the luminescence spectra exhibit emission associated with defects. It is established that the emission spectrum of defects involves at least two components. It is shown that the defects are located mainly at the small-sized quantum dots; the defects responsible for the long-wavelength component are located mainly at the quantum dots larger in size than the quantum dots, at which the defects responsible for the short-wavelength component are located. It is found that conjugation with biomolecules induces not only the blue shift of the excitonic band, but transformation of the emission spectra of defects and an increase in the contribution of defects to the luminescence spectrum as well. The changes observed in the emission spectrum of defects are attributed to the formation of certain emission centers. It is shown that, when conjugated with biomolecules, the quantum dots experience increasing compression strains. This effect is responsible for the blue shift of the luminescence band of the quantum dots.     

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.     

Self-ordered CdSe quantum dots in ZnSe and (Zn, Mn)Se Matrices Assessed by transmission electron microscopy and photoluminescence spectroscopy

Single and multilayer sheets of self-assembled CdSe [quantum dots (QDs)] were grown by means of molecular beam epitaxy in both ZnSe and (Zn_0.9Mn_0.1)Se matrices. Both types of structure were assessed by means of transmission electron microscopy in the scanning, high-resolution, and diffraction-contrast modes. Complementary results from wider sample areas were obtained by means of photoluminescence spectroscopy. In one of the samples analyzed, a fractional monolayer of MnSe was deposited immediately before the CdSe deposition. A second structure grown under identical conditions, but without the MnSe fractional monolayer, was also analyzed. This comparison provides direct evidence for an enhanced size and shape homogeneity of 3D QDs caused by the presence of a tiny amount of MnSe at the interface. In the multilayer structure, we observed the co-existence of highly strained quasi-2D QDs and CdSe rich aggregates with compositional modulations on certain crystallographic planes in close proximity.     

Deep level electronic structure of ZnSe/GaAs heterostructures

We performed 1—2 keVcathodoluminescence measurements and He-Ne and HeCd excited photoluminescence studies of ZnSe/GaAs( 100) heterostructures grown by molecular beam epitaxy. Our goal was to investigate the deep level electronic structure and its connection with the heterojunction band offsets. We observed novel deep level emission features at 0.8, 0.98, 1.14, and 1.3 eV which are characteristic of the ZnSe overlayer and independent in energy of overlayer thickness. The corresponding deep levels lie far below those of the near-bandedge features commonly used to characterize the ZnSe crystal quality. The relative intensity and spatial distribution of the deep level emission was found to be strongly affected by the Zn/Se atomic flux ratio employed during ZnSe growth. The same flux ratio has been shown to influence both the quality of the ZnSe overlayer and the band offset in ZnSe/GaAs heterojunctions. In heterostructures fabricated in Se-rich growth conditions, that minimize the valence band offset and the concentration of Se vacancies, the dominant deep level emission is at 1.3 eV. For heterostructures fabricated in Zn-rich growth conditions, emission by multiple levels at 0.88,0.98, and 1.14 eV dominates. The spectral energies and intensities of deep level transitions reported here provide a characteristic indicator of ZnSe epilayer stoichiometry and near-interface defect densities.