Excitons in quantum—dot quantum—well nanoparticles

A variational calculation is presented for the ground-state properties of excitons confined in spherical core-shell quantum-dot quantum-well (QDQW) nanoparticles. The relationship between the exciton states and structure parameters of QDQW nanoparticles is investigated, in which both the heavy-hole and the light-hole exciton states are considered. The results show that the confinement energies of the electron and hole states and the exciton binding energies depend sensitively on the well width and core radius of the QDQW structure. A detailed comparison between the heavy-hole and light-hole exciton states is given. Excellent agreement is found between experimental results and our calculated 1s_e－1s_h transition energies.     

Ground state of excitons in quantum-dot quantum-well nanoparticles： stochastic variational method

Within the framework of effective mass approximation, the ground state of excitons confined in spherical core-shell quantum-dot quantum-well (QDQW) nanoparticles is solved by using the stochastic variational method, in which the finite band offset and the heavy (light) hole exciton states are considered. The calculated 1s_e－1s_h transition energies for the chosen CdS/HgS/CdS QDQW samples are in good agreement with the experimental measurements. Moreover, some previous theoretical results are improved.     

Optoelectronic characteristics of close-packed HgTe nanoparticles in the infrared range

Absorption, photoluminescence (PL), photoresponse, and I-V measurements were made for a close-packed HgTe nanoparticle film without organic capping materials to investigate its optoelectronic characteristics in the infrared (IR) range. In the absorption and PL spectra taken for the close-packed nanoparticle film, the wavelength of exciton peak was red-shifted, compared with 1-thioglycerol capped HgTe nanoparticles dispersed in solution. For the HgTe nanoparticle film, dark current was below several pA level, current was increased by about three orders of magnitude at a biased voltage of 3 V under the illumination, and photoresponse was very rapid compared with 1-thioglycerol capped HgTe nanoparticles. These optoelectronic characteristics illustrate that HgTe nanoparticles are one of promising materials for the photodetector in the IR range. Finally, the origin for the increase of photocurrent with increasing temperature observed in this study will be discussed.     

CdTe/CdZnTe多量子阱激子复合动力学性质的研究

报道了分子束外延制备的高质量CdTe/Cd_0.64Zn_0.36Te多量子阱结构的光学性质,由变温光致发光光谱讨论了随温度升高辐射线展宽和辐射复合效率降低的机理.在变密度激发的皮秒时间分辨光谱中,发现不同激发密度下发光衰减时间不同,并研究了它的机理.在高激发密度下观测到n=2的重空穴激子发光. 关键词：     

Temperature properties of exciton luminescence from CdTe quantum wells with different thicknesses in the CdTe/CdMnTe structure

The optical spectra of the CdTe/Cd_0.7Mn_0.3Te structure containing three CdTe quantum wells with nominal thicknesses of 16, 8, and 4 monolayers have been investigated. The temperature dependences of parameters of the exciton luminescence spectra (integrated intensity, full-width at half-maximum, position of the maximum, Stokes shift) for quantum wells with different thicknesses differ substantially. These differences are explained by a strong thickness dependence of the energy of Coulomb coupling in the exciton, the energy of localization of the exciton on bulges of the quantum well, and the degree of penetration of the exciton wave function into the barrier. At high excitation power densities, the emission contours of the quantum wells with thicknesses of 8 and 16 monolayers contain short-wavelength tails that correspond to optical transitions between excited quantum-well levels.     

水溶性CdTe/CdS纳米晶表面SiO2壳层的反相胶束法包覆

"提出了一种水相中制备CdTe/CdS核壳结构纳米粒子的方法.用Te粉作为碲源,用Na2S作为硫源,在50 ℃下制备了CdTe/CdS核壳结构纳米粒子. 用紫外可见吸收光谱和荧光光谱分析了CdS壳层对CdTe核的影响. 随CdS壳层厚度的增加,紫外可见吸收光谱和荧光光谱均发生了红移. CdS壳层厚度较薄时,CdTe/CdS纳米晶的荧光强度较CdTe纳米粒子有显著提高;而CdS壳层厚度较厚时,CdTe/CdS纳米晶的荧光强度会逐渐降低. 用反相胶束法在CdTe/CdS核壳结构纳米粒子的表面包被一层SiO2,     

Self consistent energy band structures for HgTe and CdTe

Self consistent energy band calculations have been performed for HgTe and CdTe with local density functional potentials, using the LMTO method in the atomic sphere approximation. Equilibrium volumes and bulk moduli are obtained in good agreement with experiment. In the case of HgTe the effect of spin-orbit interation is found to be important in obtaining the equilibrium volume. For both materials the energy bands (although in qualitative agreement with experiment) show a semiconducting gap (inverted in the case of HgTe) which is almost 1 eV lower than experiment. This seems to be a feature common to all calculations for semiconductors using existing local exchange and correlation potentials.     

Characterization of solution-synthesized CdTe and HgTe

We report the characterization of solution-synthesized CdTe and HgTe nanocrystals by X-ray diffraction, transmission electron microscopy, and photoluminescence. Methanol solutions of sodium telluride and cadmium iodide or mercury iodide, respectively, are reacted to precipitate the nanocrystalline metal tellurides, while the sodium iodide byproduct remains in solution. The existence of crystalline CdTe, HgTe, and ternary HgCdTe compounds has been demonstrated by powder X-ray diffraction after a post-synthesis sintering process. Precipitated crystallites from this synthesis were analyzed by transmission electron microscopy, which revealed that crystal diameters can vary from approximately 1 nm to 100 nm and that crystals are stoichiometric within the detection limit of the electron microprobe technique. Narrow size ranges can be selected and investigated due to an in-situ separation process in the electron microscope. Photoluminescence is found at energies above the bulk exciton energy for CdTe and is attributed to near-band-gap recombination which is blue-shifted due to quantum confinement. Both low defect luminescence and dark field imaging suggest a high crystalline quality. A comparative characterization by photoluminescence, transmission electron microscopy, and X-ray diffraction evaluates the effects of heat treatments during and after synthesis.     

Size-dependent carrier dynamics in self-assembled CdTe/ZnTe quantum dots

We investigate size-dependent carrier dynamics in self-assembled CdTe/ZnTe quantum dots (QDs) grown using molecular beam epitaxy and atomic layer epitaxy. Photoluminescence (PL) spectra show that the excitonic peak corresponding to transitions from the ground electronic subband to ground heavy-hole band in CdTe/ZnTe QDs shifts to a lower energy with increasing ZnTe buffer thicknesses. This shift of the PL peak can be attributed to size variation of the CdTe QDs. In particular, carrier dynamics in CdTe QDs grown on various ZnTe buffer layer thicknesses is studied using time-resolved PL measurements. As a result, the decay time of CdTe QDs is shown to increase with increasing ZnTe buffer layer thicknesses due to the reduction of the exciton oscillator strength in the larger QDs.     

Pressure-induced threshold optical pump intensity in a CdTe/ZnTe quantum dot

Pressure-induced binding energies of an exciton and a biexciton are studied taking into account the geometrical confinement effect in a CdTe/ZnTe quantum dot. Coulomb interaction energy is obtained using Hartree potential. The energy eigenvalue and wave functions of exciton and the biexciton are obtained using the self-consistent technique. The effective mass approximation and BenDaniel-Duke boundary conditions are used in the self-consistent calculations. The pressure-induced nonlinear optical absorption coefficients for the heavy hole exciton and the biexciton as a function of incident photon energy for CdTe/ZnTe quantum dot are investigated. The optical gain coefficient with the injection current density, in the presence of various hydrostatic pressure values, is studied in a CdTe/ZnTe spherical quantum dot. The pressure-induced threshold optical pump intensity with the dot radius is investigated. The results show that the pressure-induced electronic and optical properties strongly depend on the spatial confinement effect.     

Optical study of ZnP2 nanoparticles in zeolite Na-X

Nanoparticles of the II-V semiconductor (ZnP₂) were prepared and investigated. ZnP₂ nanoparticles were incorporated into zeolite Na-X matrix. Absorption, diffuse reflection (DR) and photoluminescence (PL) spectra of ZnP₂ nanoparticles were measured at the temperature of 77 K. Five bands B₁-B₅ are observed in both the DR and PL spectra demonstrating the blue shift from the line of free exciton in bulk crystal. We attribute the B₁-B₅ bands to five stable nanoparticles with size less than the size of zeolite Na-X supercage. We observed Stokes shift of the PL bands with respect to the absorption bands. This dependence of this Stokes shift on the particle size is nonmonotonic.     

Time-dependent photocurrent of a CdTe nanoparticle film under the above-gap illumination

Time-dependent photocurrent of a solid film of 1-thioglycerol-capped CdTe nanoparticles under the illumination of the 325-nm wavelength light is characterized to investigate the transport mechanism of photo-generated charge carriers in this nanoparticle film. Under the illumination of the above-gap light, photocurrent rises rapidly, and subsequently it decays or rises slowly, depending on the magnitude of bias voltage. A careful investigation into the variation in the magnitude of the current measured as a function of time while the light was switched on and off periodically reveals that rapidly and slowly respondent photocurrents overlap in the time-dependent photocurrent. Charge carriers contributing to the rapidly and slowly respondent photocurrents are electrons and holes separated from a fraction of excitons excited by the above-gap light. The transport behaviors of these charge carriers may explain the monotonously decay and slow rising of the photocurrent after its rapidly rising under the illumination for the solid film at unbiased and biased voltage, respectively.     

Thermal dissociation of excitons in a type-I GaAs/AlAs superlattice studied by time-resolved photoluminescence measurements

In order to investigate thermal properties of excitons, we have performed time-resolved photoluminescence (PL) measurements for a type-I (GaAs)₁₅/(AlAs)₁₅ superlattice. At low temperatures, PL signals show ordinal exponential time decay, whereas at high temperatures, the PL shows power decay. Such change of PL signals is understood by considering thermal dissociation of exciton into account. At low temperatures, recombination of bound excitons generates PL which shows exponential decay. At temperatures higher than the exciton binding energy, correlation between electrons and holes disappears. Recombination of free excitons causes PL which decays by a power function. By means of the least-squares fitting, we evaluate the portion of bound excitons, recombination time of bound and free excitons as functions of temperatures.     

Stress effects on the photoluminescence energies and binding energies of excitons in ultra-thin ZnTe/CdTe/ZnTe quantum wells

Using the variational method and the effective mass and parabolic band approximations, electron and heavy-hole ground-state energies and exciton and photoluminescence energies are calculated in ultra-thin quantum wells of CdTe/ZnTe heterostructures. The results indicate dependencies on the well width, the barrier height, and stress-related effects and occur because the wave functions of both free carriers and those bound in exciton form determine the system energy and are shaped by the geometry of the well. Critical system thicknesses were estimated for the point at which stress effects become negligible: a value of five monolayers was obtained based on the exciton binding energy, and a value of seven monolayers was obtained based on the free-carrier ground-state energy.     

半导体量子阱中自由激子稳态荧光特征

在不同晶格温度和不同激发光强度下，测量了四元系GaInAsSb/GaAlAsSb单量子阱中自由激子的荧光光谱，导出了稳态光谱测量条件下自由激子荧光强度与激发光强度和晶格温度的一般性公式.计算结果表明，激子相对占有数引起的温度和密度效应会影响激子发光的强度关系.根据本文的简单模型，线性比例系数I/I₀实际上综合地反映了量子阱中自由激子的荧光效率，而从激子荧光强度的Arrhenius图的最佳拟合中不仅可以得到激子的束缚能和激活能，而且还能估计出量子阱材料的本底浓度和散射时间常数. 关键词：     

Quantum wire lasers with high uniformity formed by cleaved-edge overgrowth with growth-interrupt anneal

High-quality T-shaped quantum wire lasers are fabricated by cleaved-edge overgrowth with the molecular beam epitaxy on the interface improved by a growth-interrupt high-temperature anneal. Micro-photoluminescence (PL) and PL excitation spectroscopy reveals unprecedented high quality of the wires, and structures of one-dimensional (1D) free excitons and 1D continuum states. At high pumping levels, PL evolves from a sharp free exciton peak via a biexciton peak to a red-shifted broad band. Lasing has been achieved with low lasing threshold. The lasing energy is on the red-shifted broad band and is about 5 meV below the free exciton. The observed shift excludes free excitons in lasing, and suggests contribution of highly Coulomb-correlated electron-hole plasma.     

Excitonic photoluminescence characteristics of amorphous silicon nanoparticles embedded in silicon nitride film

We have investigated the photoluminescence (PL) properties of amorphous silicon nanoparticles (a-Si NPs) embedded in silicon nitride film (Si-in-SiN_x) grown by helicon wave plasma-enhanced chemical vapor deposition (HWP-CVD) technique. The PL spectrum of the film exhibits a broad band constituted of two Gaussian components. From photoluminescence excitation (PLE) measurements, it is elucidated that the two PL bands are associated with the a-Si NPs and the silicon nitride matrix surrounding a-Si NPs, respectively. The existence of Stokes shift between PL and absorption edge indicates that radiative recombination of carriers occurs in the states at the surface of the Si NPs, whereas their generation takes place in the a-Si NPs cores and the silicon nitride matrix, respectively. The visible PL of the film originates from the radiative recombination of excitons trapped in the surface states. At decreasing excitation energy (E_ex), the PL peak energy was found to be redshifted, accompanied by a narrowing of the bandwidth. These results are explained by surface exciton recombination model taking into account there existing a size distribution of a-Si NPs in the silicon nitride matrix.     

Design principles and coupling mechanisms in the 2D quantum well topological insulator HgTe/CdTe

We present atomistic band structure calculations revealing a different mechanism than recently surmised via k · p calculations about the evolution of the topological state (TS) in HgTe/CdTe. We show that 2D interface (not 1D edge) TSs are possible. We find that the transitions from a topological insulator at critical HgTe thickness of n = 23 ML (6.453 [corrected] ?) to a normal insulator at smaller n is due to the crossing between two interface-localized states: one derived from the S-like Γ?(c) and one derived from the P-like Γ?(v) light hole, not because of the crossing of an interface state and an extended quantum well state. These atomistic calculations suggest that a 2D TS can exist in a 2D system, even without truncating its symmetry to 1D, thus explaining the otherwise surprising similarity between the 2D dispersion curves of the TS in HgTe/CdTe with those of the TS in 3D bulk materials such as Bi?Se?.     

Dependence of optical gain and interband transitions on the CdTe well width and temperature for CdTe/ZnTe single quantum wells

Optical gains and interband transition energies for CdTe/ZnTe single quantum wells (SQWs) with different CdTe well widths were investigated. Photoluminescence (PL) spectra for CdTe/ZnTe SQWs at various temperatures were experimentally obtained, and the corresponding optical gains were calculated by using an interacting pair Green’s function and by using an energy space integrated function. The peak energies in the gain spectra that take the Coulomb interaction between the electron and the hole into account were in qualitatively reasonable agreement with those determined from the PL spectra.     

Electron tunneling through double-electric barriers on HgTe/CdTe heterostructure interface

We investigate theoretically the carrier transport in a two-dimensional topological insulator of (001) HgTe/CdTe quantum-well heterostructure with inverted band, and find distinct switchable features of the transmission spectra in the topological edge states by designing the double-electric modulation potentials. The transmission spectra exhibit the significant Fabry-Pérot resonances for the double-electric transport system. Furthermore, the transmission properties show rich behaviors when the Fermi energy lies in the different locations in the energy spectrum and the double-electric barrier regions. The opacity and transparency of the double-modulated barrier regions can be controlled by tuning the modulated potentials, Fermi energy and the length of modulated regions. This electrical switching behavior can be realized by tuning the voltages applied on the metal gates. The Fabry-Pérot resonances leads to oscillations in the transmission which can be observed in experimentally. This electric modulated-mechanism provides us a realistic way to switch the transmission in edge states which can be constructed in low-power information processing devices.