氧化硅缓冲层对于退火形成锗量子点的作用研究

在化学氧化得到的二氧化硅薄层覆盖的硅衬底上,室温淀积锗膜并进行后期退火处理.实验表明,不同于传统退火过程形成大岛,通过一定工艺的控制可以获得高密度(~10¹¹ cm^-2)的均匀锗量子点.研究了后期退火温度对量子点的结构影响的局部反常规律并进行了原因分析.利用拉曼和荧光光谱研究了其应力和发光特性,发现在可见(500 nm)和近红外(1350 nm)的两个光致荧光峰出现. 关键词： 锗量子点 二氧化硅 退火     

Sensitizing effects of ZnO quantum dots on red-emitting Pr-doped SiO2 phosphor

In this study, red cathodoluminescence (CL) (λ_emission=614 nm) was observed from Pr³⁺ ions in a glassy (amorphous) SiO₂ host. This emission was enhanced considerably when ZnO quantum dots (QDs) were incorporated in the SiO₂:Pr³⁺ suggesting that the ZnO QDs transferred excitation energy to Pr³⁺ ions. That is, ZnO QDs acted to sensitize the Pr³⁺ emission. The sol–gel method was used to prepare ZnO–SiO₂:Pr³⁺ phosphors with different molar ratios of Zn to Si. The effects of the ZnO QDs concentration and the possible mechanisms of energy transfer from ZnO to Pr³⁺ are discussed. In addition, the electronic states and the chemical composition of the ZnO–SiO₂:Pr³⁺ phosphors were analyzed using X-ray photoelectron spectroscopy (XPS).     

Densely packed Ge quantum dots grown on SiO2/Si substrate

We studied the growing process of Ge dots on silicon substrates covered with an ultrathin silicon dioxide buffer layer which was formed with simple chemical procedure. Uniform and densely packed (10¹¹ cm⁻²) quantum dots (QDs) were obtained by optimizing the growth parameter with the MBE method. The influence of temperature, coverage, as well as the post-annealing process, on the epitaxial and non-epitaxial nanodots formation was evaluated. Nano-sized high density quantum dots were also realized with different growing conditions, whose structural and growing mechanism were discussed under the help of SEM and RHEED results.     

Energy of excitons and acceptor-exciton complexes to explain the origin of ultraviolet photoluminescence in ZnO quantum dots embedded in a SiO2 matrix

Assuming finite depth and within the effective mass approximation, the energies of exciton states and of the acceptor-exciton complexes confined in spherical ZnO quantum dots (QDs) embedded in a SiO₂ matrix are calculated using a matrix procedure, including a three-dimensional confinement of carrier in the QDs. This theoretical model has been designed to illustrate the two emission bands in the UV region observed in our experimental Photoluminescence spectrum (PL), with the first emission band observed at 3.04 eV and attributed to the bound ionized acceptor-exciton complexes, and the second one located at 3.5 and assigned to the free exciton. Our calculations have revealed a good agreement between the matrix element calculation method and the experimental results.     

Electroluminescence with micro-watt output from ultra-small sized Si quantum dots/amorphous SiO2 multilayers prepared by laser crystallization method

We report the fabrication of Si quantum dots (QDs)/SiO₂ multilayers by using KrF excimer laser (248 nm) crystallization of amorphous Si/SiO₂ multilayered structures on ITO coated glass substrates. Raman spectra and transmission electron microscopy demonstrate the formation of Si QDs and the size can be controlled as small as 1.8 nm. After laser crystallization, Al electrode is evaporated to obtain light emitting devices and the room temperature electroluminescence (EL) can be detected with applying the DC voltage above 8 V on the top gate electrode. The luminescent intensity increases with increasing the applied voltage and the micro-watt light output is achieved. The EL behaviors for samples with different Si dot sizes are studied and it is found that the corresponding external quantum efficiency is significantly enhanced in sample with ultra-small sized Si QDs.     

Optical properties of acceptor-exciton complexes in ZnO/SiO2 quantum dots

The binding energy E_b of the acceptor-exciton complex (A⁻,X) as a function of the radius (or of the impurity position of the acceptor) and the normalized oscillator strength of (A⁻,X) in spherical ZnO quantum dots (QDs) embedded in a SiO₂ matrix are calculated using the effective-mass approximation under the diagonalzation matrix technique, including a three-dimensional confinement of the carrier in the QD and assuming a finite depth. Numerical results show that the binding energy of the acceptor-exciton complexes is particularly robust when the impurity position of the acceptor is in the center of the ZnO QDs. It has been clearly shown from our calculations that these physical parameters are very sensitive to the quantum dot size and to the impurity position. These results could be particularly helpful, since they are closely related to experiments performed on such nanoparticles. This may allow us to improve the stability and efficiency of the semiconductor quantum dot luminescence which is considered critical.     

The studies of Ge quantum dots on strained Si0.7Ge0.3 layer by photoluminescence and deep level transient spectroscopy

Systematic studies of Ge quantum dots (QDs) grown on strained Si_0.3Ge_0.7 layer have been carried out by photoluminescence (PL) and deep level transient spectroscopy (DLTS). In PL measurements, two peaks around 0.7 eV are distinguished, which are assigned to two types of QDs observed by atomic force microscopy (AFM). Large blueshifts of the PL peaks from small QDs with the increase of excitation power are observed and attributed to the band bending effects typical for type-II band alignment. From DLTS measurements, the energy levels of holes in both types of QDs are derived, which shift with the change of the number of holes in QDs due to their charge energy. By comparing results from PL and DLTS measurements, further understanding of band alignment with the increase of the number of excitons in QDs is deduced.     

Synthesis and nonlinear optical characterization of SnO2 quantum dots

The present paper demonstrates the preparation and characterization of SnO₂ semiconductor quantum dots. Extremely small ∼1.1 and ∼1.4 nm SnO₂ samples were prepared by microwave assisted technique with a frequency of 2450 MHz. Based on XRD analysis, the phase, crystal structure and purity of the SnO₂ samples are determined. UV-vis measurements showed that, for the both size of SnO₂ samples, excitonic peaks are obtained at ∼238 and ∼245 nm corresponding to ∼1.1 nm (sample 1) and ∼1.4 nm (sample 2) sizes, respectively. STM analysis showed that, the quantum dots are spherical shaped and highly monodispersed. At first, the linear absorption coefficients for two different sizes of SnO₂ quantum dots were measured by employing a CW He-Ne laser at 632.8 nm and were obtained about 1.385 and 4.175 cm⁻¹, respectively. Furthermore, the nonlinear refractive index, n₂, and nonlinear absorption coefficient, β, were measured using close and open aperture Z-scan respectively using the same laser. As quantum dots have strong absorption coefficient to obtain purely effective n₂, we divided the closed aperture transmittance by the corresponding open aperture in the same incident beam intensity. The nonlinear refraction indices of these quantum dots were measured in order of 10⁻⁷ (cm²/W) with negative sign and the nonlinear absorption coefficients were obtained for both in order of 10⁻³ (cm/W) with positive sign.     

Temperature dependence of the electron distribution in a GaAs matrix with embedded InAs quantum dots

We report on the effect of the Debye averaging process on the CV characteristics of a sample containing four coupled planes of InAs self-assembled quantum dots. The observed electron distribution presented a dynamical dependence of the temperature during the C–V measurements which was explained in terms of the screening length dependence on the temperature. In addition, using the C–V data, we calculated the electron density at the planes containing the InAs dots and we have observed a high-temperature stability: the electron density at the quantum dots remained constant over a large range of temperature.     

PbSe量子点近红外光源的CH_4气体检测

研制了一种新型的PbSe量子点近红外光源,其光致发光谱较窄,能有效匹配气体的红外吸收峰。采用配位溶剂法合成出5.1 nm的PbSe量子点,并将其沉积到Ga N芯片上(沉积厚度为165.5μm),经过紫外光照处理和固化后制成了光致发光的近红外光源。该光源第一激子吸收峰位于1 592 nm,光致发光峰位于1 665 nm,其发射光谱包含了1 625~1 840 nm之间的CH_4气体的全部的吸收谱。利用其进行CH_4气体浓度检测实验,结果表明,系统的检测下限可以达到100×10~(-6),检测误差为2%。这种由PbSe量子点近红外光源构成的新型检测系统具有低功耗、低成本和高效能等优点,将其应用在气体检测中时可以略去滤光片,其在红外气体检测领域中有着较广阔的应用前景。     

Room-temperature observation of current bistability and fine structures in germanium quantum dots/SiO2 resonant tunneling diodes

The steady-state and time-dependent current–voltage (I–V) characteristics are experimentally investigated in Ge quantum dot (QD)/SiO₂ resonant tunneling diodes (RTDs). Ge QDs embedded in a SiO₂ matrix are naturally formed by thermal oxidation of Si_0.9Ge_0.1 nanowires (30 nm×50 nm) on silicon-on-insulator substrates. The average dot size and spacing between dots are 9±1 and 25 nm, respectively, from TEM observations, which indicate that one or two QDs are embedded between SiO₂ tunneling barriers within the nanowires. Room-temperature resonant oscillation, negative differential conductance, bistability, and fine structures are observed in the steady-state tunneling current of Ge-QD/SiO₂ RTDs under light illumination. Time-dependent tunneling current characteristics display periodic seesaw features as the Ge-QDs RTD is biased within the voltage regime of the first resonance peak while they exhibit harmonic swing behaviors as the RTD is biased at the current valleys or higher-order current peaks. This possibly originates from the interplay of the random telegraph signals from traps at the QD/SiO₂ interface as well as the electron wave interference within a small QD due to substantial quantum mechanics effects.     

微腔中CdSe量子点荧光增强效应

文章主要研究了CdSe量子点微腔结构,微腔结构包括上下分布式布拉格反射镜(DBR),中间的有源层为溶解在聚甲基丙烯酸甲酯(PMMA)中的CdSe胶体量子点.采用传递矩阵法模拟微腔的反射光谱,对实验测试曲线进行较好的拟合.通过测试微腔结构的光致荧光(PL)光谱,其半峰宽(FWHM)由未加入微腔的CdSe量子点样品的27.9 nm,减小到微腔结构的7.5 nm,在微腔中的量子点,由于腔模式的出现,其发光谱的品质因数增加了3.6倍,达到了荧光增强的效果. 关键词： CdSe量子点 微腔效应 荧光增强     

Tunneling current of the Coulomb-coupled quantum dots embedded in n-n junction

Taking account of the electron--electron (hole) and electron--hole interactions, the tunneling processes of the main quantum dot (QD) Coulomb-coupled with a second quantum dot embedded in n--n junction have been investigated. The eighteen resonance mechanisms involved in the tunneling processes of the system have been identified. It is found that the tunneling current depends sensitively on the electron occupation number in the second quantum dot. When the electron occupation number in the second dot is tiny, both the tunneling current peaks and the occupation number plateaus in the main QD are determined by the intra-resonance mechanism. The increase of the electron occupation number in the second dot makes the inter-resonance mechanism participate in the transport processes. The competition between the inter and intra resonance mechanisms persists until the electron occupation number in the second dot reaches around unity, leading to the consequence that the inter-resonance mechanisms completely dominate the tunneling processes.     

Luminescence of CsPbBr3-like quantum dots in CsBr single crystals

Luminescence and decay kinetics of the Pb²⁺ aggregates in CsBr host crystals were measured in the 4–300 K temperature interval and in 10⁻¹⁰–10⁻³ time scale. Their emission properties are similar to those of CsPbBr₃ bulk crystal showing a subnanosecond free exciton emission in the 520–540 nm spectral region and slower trapped exciton emission in the 530–580 nm spectral region. An efficient energy exchange between the free and trapped exciton states is shown by the temperature dependencies of emission spectra. The quantum size effect is demonstrated in the high energy shift and broadening of the absorption and emission spectra and an estimate of the size of the CsPbBr₃-like aggregates is provided. Independent evidence of the presence of the CsPbBr₃ and Cs₄PbBr₆ aggregated phases in the CsBr host was obtained by X-ray structural studies.     

Optical properties of Si-rich SiO2 films in relation with embedded Si mesoscopic particles

Optical properties of Si-rich SiO₂ films prepared by an RF cosputtering method are discussed. From the infrared and Raman spectroscopy together with the electron microscopy, it is shown that Si mesoscopic particles embedded in solid matrices with the sizes ranging from ˜ 10 nm (nanocrystals) to less than ˜1 nm (clusters) can be obtained by the cosputtering and post-annealing. The absorption and photoluminescence spectra are presented. For our samples, a red luminescence peak analogous to that of porous Si is observed for films containing Si clusters rather than nanocrystals. Raman spectra which evidence the success in the heavy doping of B atoms into Si nanocrystals are also discussed.     

Time-resolved studies of single quantum dots in magnetic fields

We present time-resolved and time-integrated spectroscopy of single InAs quantum dots grown in a GaAs matrix. We observe a number of interesting features in the spectra, including the zero field splitting of exciton and biexciton lines due to quantum dot asymmetry. By the application of an in-plane magnetic field, the normally optically active and inactive exciton states become mixed, enabling us to optically probe the normally inaccessible ‘dark’ states. Time resolved measurements on the mixed states show decay times several times longer than the exciton lifetime at zero field, which we show to be consistent with a dark exciton lifetime orders of magnitude longer than that for bright exciton.     

超小晶粒锡掺杂CsPbBr3蓝光量子点的合成及其光学性能研究

近年来,铅卤钙钛矿CsPbX₃ (X=Cl,Br或I)因其具有荧光波段可调、荧光量子产率高(Photoluminescence quantum yield,PLQY)以及荧光半峰宽窄等优点而被广泛应用于光电器件领域.然而,与PLQY接近于100%的绿光和红光相比,蓝光卤素钙钛矿的PLQY仍比较低.在此,采用过饱和结晶的方法在室温下合成了粒径低于4 nm的超小晶粒锡(Sn)掺杂CsPbBr₃量子点,并对其结构特性和光学特性进行了研究.结果表明:随着SnBr₂添加量的增大,量子点晶粒的粒径略微减小,荧光发射峰发生蓝移,粒径由3.33 nm (SnBr₂为0.03 mmol)减小到2.23 nm(SnBr₂为0.06 mmol时),对应的荧光发射峰由490 nm蓝移至472 nm.当SnBr₂添加量为0.05 mmol时合成的超小晶粒锡掺杂CsPbBr₃量子点显示出最优的光学性能,其粒径约为2.91 nm,对应的XRD各晶面衍射峰强度最强,...     

The preferential formation site of dislocations in InAs/GaAs quantum dots

In this paper, taking elastic anisotropy into consideration, we use a dislocation position dependent model to calculate the preferential formation site of the pure edge and 60° mixed dislocation segment in different shaped InAs/GaAs quantum dots (QDs). From the result, it is clear that for the pure edge dislocations the most energy favorable position is always the base center of the quantum dots. While as to the 60° mixed dislocations, the positions near to the edge of the quantum dot base are the energy favorable area and the exact position is changed with different aspect ratio of the quantum dot.     

Magnetic properties of Mn-oxide nanoparticles dispersed in an amorphous SiO2 matrix

Samples of Mn-oxide nanoparticles dispersed in an amorphous SiO₂ matrix with manganese concentration 0.7 and 3 at% have been synthesized by a sol-gel method. Transmission electron microscopy analysis has shown that the samples contain agglomerates of amorphous silica particles 10-20 nm in size. In silica matrix two types of Mn-rich particles are dispersed, smaller nanoparticles with dimensions between 3 and 10 nm, and larger crystalline areas consisting of aggregates of the smaller nanoparticles. High-temperature magnetic susceptibility study reveals that dominant magnetic phase at higher temperatures is λ-MnO₂. At temperatures below T_C=43 K strong ferrimagnetism originating from the minor Mn₃O₄ phase masks the relatively weak magnetism of λ-MnO₂ with antiferromagnetic interactions. Magnetic field dependence of the maximum in the zero-field-cooled magnetization for both the samples in the vicinity of 40 K, and a frequency shift of the real component of the ac magnetic susceptibility in the sample with 3 at% Mn suggest that the magnetic moments of the smaller Mn₃O₄ nanoparticles with dimensions below 10 nm are exposed to thermally activated blocking process just below the Curie temperature T_C. Appearance of a maximum in the zero-field-cooled magnetization for both the samples below 10 K indicates possible spin glass freezing of the magnetic moments at low temperatures which might occur in the geometrically frustrated Mn sublattice of the λ-MnO₂ crystal structure.     

Quantum cascade multi-electron injection into Si-quantum-dot floating gates embedded in SiO2 matrices

The use of silicon-quantum-dots (Si-QDs) as floating gates in metal-oxide-semiconductor-field-effect-transistors (MOSFETs) has been attracting great attention. It has been reported that large decreases in drain current are observed within a very short time in Si-QDs memories, indicating that the collective motion of electrons occurs during electron injection into Si-QDs. In this study, we present a theoretical report which indicates that the interaction length between QDs is about 5-10 nm. From these results, we propose a mechanism for “quantum cascade multi-electron injection”.