CdSe/ZnSe量子点的合成与荧光特性

采用低温成核生长与一步法相结合的方式合成了CdSe/ZnSe核壳结构量子点,并通过吸收光谱、荧光光谱、X射线衍射等分析手段证明了ZnSe壳层包覆成功.对加入空穴传输材料后CdSe/ZnSe量子点的荧光变化情况进行了深入的研究.稳态光谱结果表明.空穴传输材料对量子点发光有较强的淬灭作用;时间分辨光谱结果显示,随着空穴传输材料分子浓度的增加,量子点的荧光寿命明显缩短,其荧光淬灭过程可以解释为静态淬灭和动态淬灭过程.静态淬灭来源于量子点表面与空穴传输材料间的相互作用;而动态淬灭则来源于量子点到空穴传输材料的空穴转移过程.因此,量子点的壳层结构及空穴传输材料的种类对量子点的荧光淬灭起关键作用.     

Growth and photoluminescence study of ZnSe quantum dots

We report detailed photoluminescence (PL) studies of ZnSe quantum dots grown by controlling the flow duration of the precursors in a metal-organic chemical vapor deposition system. The growth time of the quantum dots determines the amount of blue shift observed in the PL measurements. Blue shift as large as 320 meV was observed, and the emission was found to persist up to room temperature. It is found that changing the flow rate and the total number of quantum dot layers also affect the peak PL energy. The temperature dependence of the peak PL energy follows the Varshni relation. From analyzing the temperature-dependent integrated intensity of the photoluminescence spectra, it is found that the activation energy for the quenching of photoluminescence increases with decreasing quantum dot size, and is identified as the binding energy of the exciton in ZnSe quantum dot.     

CdSe量子点的制备与荧光特性研究

主要讨论了CdSe量子点的制备及荧光特性。CdSe量子点由化学方法制备,通过选择不同的反应时间得到不同尺度的量子点样品。用荧光方法研究了量子点样品在石英衬底和有机溶剂中的荧光特性。实验表明,这些量子点都有良好的荧光特性。还用无限深球方势阱模型分析了量子点样品的电子态,并根据荧光参数估算了量子点的尺度．各样品荧光峰具有一致的半峰宽,表明CdSe量子点的成核过程在反应开始时同时完成。     

Polarization of the photoluminescence of quantum dots incorporated into quantum wires

The photoluminescence spectra of individual quantum dots incorporated into a quantum wire are studied. From the behavior of the spectra in a magnetic field, it is possible to estimate the exciton binding energy in a quantum dot incorporated into a quantum wire. It is found that the exciton photoluminescence signal emitted from a quantum dot along the direction of the nanowire axis is linearly polarized. At the same time, the photoluminescence signal propagating in the direction orthogonal to the nanowire axis is practically unpolarized. The experimentally observed effect is attributed to the nonaxial arrangement of the dot in the wire under conditions of a huge increase in the exciton binding energy due to the effect of the image potential on the exciton.     

Interface-related electrical properties of cadmium selenide films

An experimental method for determining the depletion depth in thin films of cadmium selenide is used to measure simultaneous variation of conductivity and the space-charge region. The temperature dependence of the conductance reveals donor levels distributed about 0.10 eV and 0.50 eV. Total depletion of films a few thousand angstroms thick produced by oxygen adsorption occurs by exhaustion of the shallow donors. Following exposure to the atmosphere at room temperature, the film conductance decreases by two orders of magnitude when heated in vacuum above 70°C. The subsequent time dependence of the current is shown to follow an Elovich-type relation. This effect is interpreted as conversion of physically adsorbed oxygen to chemisorbed oxygen. The role of evaporated silicon oxide layers in the prevention of the conversion process is discussed. The instability in thin film transistor threshold voltage above 200°C is described in terms of an interface desorption model.     

Specific features of electroluminescence in heterostructures with InSb quantum dots in an InAs matrix

The electrical and electroluminescence properties of a single narrow-gap heterostructure based on a p-n junction in indium arsenide, containing a single layer of InSb quantum dots in the InAs matrix, are studied. The presence of quantum dots has a significant effect on the shape of the reverse branch of the current-voltage characteristic of the heterostructure. Under reverse bias, the room-temperature electroluminescence spectra of the heterostructure with quantum dots, in addition to a negative-luminescence band with a maximum at the wavelength λ = 3.5 μm, contained a positive-luminescence emission band at 3.8 μm, caused by radiative transitions involving localized states of quantum dots at the type-II InSb/InAs heterointerface.     

Specific features of the epitaxial growth of narrow-gap InSb quantum dots on an InAs substrate

Semiconductors - Arrays of coherent InSb quantum dots (QDs) have been fabricated by liquid-phase epitaxy on InAs substrates in the temperature range T = 420–450°C. The QDs with a density...     

Baric properties of InAs quantum dots

In the context of the deformation potential model, baric dependences of the energy structure of InAs quantum dots in a GaAs matrix are calculated. Under the assumption of the absence of interaction between the spherical quantum dots of identical sizes, the energy dependence of the baric coefficient of energy of the radiative transition in the quantum dot is determined. A similar dependence is also found experimentally in the photoluminescence spectra under uniform compression of the InAs/GaAs structures. Qualitative agreement between the theory and experiment as well as possible causes for their quantitative difference are discussed. It is concluded that such factors as the size dispersion, Coulomb interaction of charge carriers, and tunnel interaction of quantum dots contribute to this difference.     

Electronic properties of interacting quantum dots

The electronic properties of two interacting dots connected to leads are studied in the Kondo regime. The configuration is such that one dot is inserted into the lead while the other is side-connected to it. The situation, where both dots are in the Kondo regime is investigated. We find that the development of a Kondo state related to the connected dot is mediated by the inserted dot. In this case, an anti-resonance appears in the density of states of the inserted dot, at the Fermi level. The equation of motion method is used to calculate the Green's functions of the system.     

Electrical properties of semiconductor quantum dots

A method, which makes it possible to obtain semiconductor particles V ≈ 10^?20 cm³ in volume (quantum dots) with a concentration of up to 10¹¹ cm^?2 and electrical contacts to each of them, is suggested. High variability in the electrical properties of such particles from a metal oxide (CuO or NiO) after the chemisorption of gas molecules is found.     

Electrical and structural properties of cadmium selenide thin film transistors

Stable thin film transistors based on cadmium senenide and silicon dioxide have been prepared. The degree of stability implies a decay of only 10% in drain current in 100 years of continuous d.c. operation. The decay is solely due to tunnelling of electrons into insulator traps and has a logarithmic time dependence. The devices have field effect mobilities up to 140 cm². Volt^?1 sec^?1, switching ratios in the range 10⁵–10⁶, and good reproducibility. The CdSe films contain the hexagonal structure and grain growth occurs during anneal. Grain size and distribution are reproducible from run to run.     

Influence of cadmium concentration on the optical and structural properties of cadmium selenide thin films

Thin films of cadmium selenide (CdSe) and Cd_xSe_100−x (x=54, 34) have been deposited by vacuum evaporation onto ultraclean glass substrates at room temperature from as-prepared powders. Fabricated samples were characterized using X-ray diffraction (XRD), UV–vis and Fourier transform infrared (FTIR) spectroscopy. XRD indicated the formation of polycrystalline Cd_xSe_100−x thin films polycrystalline in nature with the preferred orientation along the (002) plane. The crystallite sizes of thin films calculated by the Scherer formula were found to be in the range of 29–82 nm. The crystallinity of thin films degraded on increasing the Cd concentration in the Cd–Se system. The band gaps of thin films were obtained from their optical absorption spectra, which were found in the range of 1.69–2.20 eV. The band gap of the Cd₃₄Se₆₆ thin films was found to be very high because of the decrease of their crystallites sizes in comparison to the CdSe and Cd₅₄Se₄₆ thin films. From the FTIR spectra it was revealed that the Cd–Se peaks shifted to lower wavelengths with increase in selenium concentration. The SEM measurements for CdSe, Cd₅₄Se₄₆ and Cd₃₄Se₆₆ thin films reveal that the particle size and the crystalline nature decreased when Se content increased in the system.     

Long-time photoluminescence kinetics of InAs/AlAs quantum dots in a magnetic field

The influence of a magnetic field on the low-temperature photoluminescence (PL) kinetics of InAs/AlAs quantum dots is studied. It is found that the PL decay becomes faster upon application of the magnetic field. The results obtained are explained in the context of a model that considers the fine structure of exciton levels and their Zeeman splitting in the magnetic field.     

Electrical and optical properties of self-assembled quantum dots

One of the main directions of contemporary semiconductor physics is the production and study of structures with a dimension less than two, i.e. quantum wires (QWi) and quantum dots (QDs), in order to realise novel devices that make use of low-dimensional confinement effects.One of the promising fabrication methods is to use self-organised three-dimensional (3D) structures, such as 3D coherent islands, which are often formed during the initial stage of heteroepitaxial growth in lattice-mismatched systems. Quantum dots, for example, are believed to provide a promising way for a new generation of optical light sources such as injection lasers. While quantum well structures are already widely used in optoelectronic devices, QWi and QDs appear to be much more difficult to fabricate for this purpose. Some of the electrical and optical properties of self-assembled QDs will be reported in this paper.     

Infrared spectroscopy of chromium-doped cadmium selenide

The maximum optical-absorption cross section of Cr²⁺ ions was evaluated from near-infrared (NIR) absorption spectroscopy and direct measurements of the chromium concentration in Cr²⁺:CdSe crystals. The emission lifetime of the excited state, ⁵E, of Cr²⁺ was measured as a function of Cr²⁺ concentration in the 2×10¹⁷ −2×10¹⁸ ions/cm³ range and as a function of temperature from 77–300 K. Lifetime values were as high as ∼6 μs in the 77–250 K range and decreased to ∼4 μs at 300 K because of nonradiative decays. Assuming that most of the Cr dopant is in the Cr²⁺ state, an optical-absorption cross section σ_a of (1.94±0.56) × 10⁻¹⁸ cm² was calculated. Implications for laser performance are discussed.     

Resonant inelastic light scattering and photoluminescence in isolated nc-Si/SiO2 quantum dots

Observation at the room temperature the spectra of the resonant inelastic light scattering by the spatially confined optical phonons as well as the excitonic luminescence caused by confinement effects in the ensemble of isolated quantum dots (QDs) nc-Si/SiO₂ is reported. It is shown that the samples investigated are high purity and high crystalline perfection quality nc-Si/SiO₂ QDs without amorphous phase α-Si and contaminants. Comparison between the experimental data obtained and phenomenological model of the strong space confinement of optical phonons revealed the need of the more accurate form of the weighted function for the confinement of optical phonons. It is shown that simultaneous detection of the inelastic light scattering by the confinement of phonons and the excitonic luminescence spectra by the confined electron-hole pairs in the nc-Si/SiO₂ QDs allows selfconsistently to determine more accurate values of the diameter of the nc-Si/SiO₂ QDs.     

Specific features of ir photoluminescence of bismuth-doped silicon dioxide synthesized by plasmachemical method

Photoluminescence spectra and kinetics of the bismuth-doped silicon dioxide are studied in the wavelength interval 0.7?C1.6 ??m in the absence and in the presence of aluminum co-doping. It is demonstrated that a broad spectral band that is centered at 1420 nm and exhibits a decay time of 600 ??s dominates in the long-wavelength part of the spectral interval regardless of the presence of aluminum in the directly deposited unfused bismuth-doped silicon dioxide. The melting of the plasma-deposited aluminosilicate material leads to the shift of the center of the band to a wavelength of 1100 nm, whereas the melting of the aluminum-free material weakly affects the spectrum. It is demonstrated that an increase in the temperature to 700 K causes an increase in the intensity of the band centered at a wavelength of 1420 nm, which is most clearly seen under the excitation at a wavelength of 808 nm. An increase in the intensity is accompanied by an increase in the lifetime and a sharp decrease in the intensity of the band peaked at a wavelength of 825 nm. The spectral features and kinetics of photoluminescence are interpreted in the framework of the three-level scheme of electronic transitions for two types of bismuth defects that simultaneously exist in the aluminosilicate materials and exhibit close energies of the stationary electronic states.     

Geometric dependence of the dielectric properties of quantum dots arrays

The experimental techniques advances in the formation of artificial crystals based on quantum dots (QD) allow us to think about the possibility of generating novel materials. However, there is a lack of theoretical calculations which permits pre-designing the properties of the new materials. Taking advantage on a theoretical derivation of the electronic dielectric response of semiconductor nano-crystals using a tight binding framework [F. Trani, D. Ninno, G. Iadonisi, Phys. Rev. B 76 (2007) 085326], we calculate the dielectric function of QD arrays of finite size introducing the screening of surface polarization by means of a tuning static electric field. In previous calculations we report drastic geometric effects in the electronic structure of QD super-crystals, as the shape changes in 3D semiconductors, while in 2D the dielectric function peaks correspond to the predicted dipole transitions energies [J.F. Nossa, A.S. Camacho, J.L. Carrillo, Rev. Mex. Fis. 53 (7) (2007) 123]. With the aim of explaining the role of the dimensionality in nanosystems we apply the above method to III–V and II–VI QD 3D supercrystals of several geometries. In this report we study the dielectric behavior of finite 3D supercrystals built up of spherical, cylindrical and conical QD and particularly we discuss the shape dependence of the QD constituents on the response function and therefore on the surface polarization fields of finite arrays. Finally, we compare the results in 3D systems with the 2D systems.     

Photoelectric properties of GaAs/InAs heterostructures with quantum dots

The capacitive photovoltage and photoconductivity spectra of GaAs/InAs heterostructures with quantum dots is discussed. For these structures, which were fabricated by metallorganic gas-phase epitaxy, the photosensitivity spectrum has a sawtoothed shape in the wavelength range where absorption by the quantum dots takes place, which is characteristic of a δ-function-like density of states function. The spectra also exhibit photosensitivity bands associated with the formation of single-layer InAs quantum wells in the structure. An expression is obtained for the absorption coefficient of an ensemble of quantum dots with a prespecified size distribution. It is shown that the energy distribution of the joint density of states, the surface density of quantum dots, and the effective cross section for trapping a photon can all be determined by analyzing the photosensitivity spectrum based on this assumption. Fiz. Tekh. Poluprovodn. 31, 1100–1105 (September 1997)     

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.