Excitonic molecules trapped by quantum dots and isoelectronic impurities in many-valley semiconductors

Excitonic molecules with two to six excitons trapped by quantum dots or isoelectronic impurity centers are studied theoretically and experimentally for the first time. Such excitonic molecules can exist only in many-valley semiconductors. A model of an excitonic molecule, valid for a large number of valleys in the conduction and valence bands, is examined. The formation kinetics of excitonic molecules is discussed. A material feature of excitonic molecules bound on quantum dots is the existence of tunneling transitions of excitons between quantum dots. Fiz. Tekh. Poluprovodn. 33, 1123–1125 (September 1999)     

Evidence of thermally activated transfer of excited carriers between CdSe/ZnSe quantum dots

Temperature dependent photoluminescence and cathodoluminescence of selfassembled CdSe/ZnSe quantum dots grown by metalorganic vapor phase deposition were investigated. We found an unusual large red shift and a narrowing of the photoluminescence peak with temperature increases. Cathodoluminescence studies of a small number of quantum dots showed that the broad peak observed in the photoluminescence spectra is, in fact, made up of a series of narrower peaks, coming from quantum dots of different sizes. While the intensity of luminescence from small dots drops monotonously with temperature rises, that from the large dots displays a peculiar behavior. It actually increases within the temperature range of 140–170 K, the same range in which the photoluminescence peak shows narrowing. The simultaneous increase of luminescence from some quantum dots and decrease from others are believed to be responsible for the red shift and narrowing of the observed photoluminescence peak. A simple analytically solvable rate equation model was used to understand the spectral data. We suggest that the unusual behaviors observed can be understood as resulting from a transfer of thermally activated carriers from small to large quantum dots.     

Luminescence properties of InAs/GaAs quantum dots prepared by submonolayer migration-stimulated epitaxy

The results of an investigation of the luminescence properties of an ensemble of InAs quantum dots, obtained by submonolayer migration-stimulated epitaxy on singular and vicinal GaAs(100) surfaces, are reported. The largest width at half-height of the photoluminescence line is observed in samples with a 3° disorientation, indicating that the size-variance of the quantum dots is largest in this case. Quasiequilibrium quantum dots are formed either with a long sample holding time in an arsenic flow or with a larger quantity of deposited indium. Fiz. Tekh. Poluprovodn. 31, 912–915 (August 1997)     

Lateral association of vertically coupled quantum dots

The modification produced in the structural and optical properties of vertically coupled In_0.5Ga_0.5As quantum dots in a GaAs matrix by increasing the number of deposited layers of quantum dots has been investigated. It was shown that the deposition of a sequence of In_0.5Ga_0.5As quantum-dot planes separated by narrow (of the order of the height of the quantum dots) GaAs layers gives rise to an interaction between neighboring vertically coupled quantum dots. This interaction shifts the photoluminescence line due to the recombination of nonequilibrium carriers via states of the quantum dots into the region of lower photon energies. Fiz. Tekh. Poluprovodn. 31, 851–854 (July 1997)     

Photoconductivity of copper-compensated gallium phosphide

Autocompensation effects in GaP after diffusive copper doping, amphotericity of electrical activity of the copper impurity, and the kinetics of interband photoconductivity and infrared quenching are explained in terms of a model of reconstruction of the bonds between the copper impurity and its immediate environment. In this picture, the acceptor state is associated with substitutional Cu atoms with tetrahedral coordination, while the donor state is associated with Cu atoms bonded to only two of the four phosphorus atoms. Fiz. Tekh. Poluprovodn. 32, 1165–1169     

Thermal stability of vertically coupled InAs-GaAs quantum dot arrays

The effect of high-temperature annealing on the optical properties of vertically coupled InAs quantum dots in a GaAs matrix and on the performance of a quantum-dot laser are studied. A strong blue shift of the photoluminescence peak and lasing line, as well as changes in the photoluminescence intensity and temperature dependence of the threshold current density are observed. The reason for this behavior is probably a reduction in the carrier localization energy due to a partial mixing of the In and Ga atoms as well as an improvement in the quality of the low-temperature grown Ga(Al)As layers achieved by high-temperature annealing. Fiz. Tekh. Poluprovodn. 31, 104–108 (January 1997)     

Photoluminescence of InAs quantum dots grown on disoriented GaAs substrates

The photoluminescence spectra in an external magnetic field of an ensemble of InAs quantum dots grown by molecular beam epitaxy on a (001) GaAs substrate with a disorientation in the [010] direction are studied. A redistribution of the photoexcited carriers among different groups of dots under the influence of the magnetic field is observed. The concentration of quantum dots is determined by analyzing the data. Fiz. Tekh. Poluprovodn. 33, 1084–1087 (September 1999)     

Electron properties of surface InGaAs/InAlAs quantum wells with inverted doping on InP substrates

The electron-transport and optical properties of heterostructures with a surface InGaAs/InAlAs quantum well in the cases of inverted δ doping with Si atoms (below the quantum well) and of standard δ doping (above the quantum well) are compared. It is shown that, in the case of inverted doping, the two-dimensional electron density in the quantum well is increased in comparison with the case of the standard arrangement of the doping layer at identical compositions and thicknesses of other heterostructure layers. The experimentally observed features of low-temperature electron transport (Shubnikov–de Haas oscillations, Hall effect) and the photoluminescence spectra of heterostructures are interpreted by simulating the band structure.     

Contributions of the interior and surface states of charge carriers to the emission spectra of CdS quantum dots in borosilicate glasses

The photoluminescence spectra of CdS quantum dots grown in a borosilicate glass by sol-gel technology are recorded and analyzed. It is shown that the photoluminescence spectra of the samples are related to annihilation of free (interior) excitons in the ground state and excited state. Emission associated with the surface states of the quantum dots is detected in the region around 2.7 eV for the first time. The emission is due to recombination of electrons localized at the surface with heavy holes in the free states of the quantum dots. Resonance excitation of the structures makes it possible to reveal the specific features of the localized surface states responsible for the photoluminescence band. The properties of the band are, to a large extent, similar to the properties of the emission bands of both three-dimensional media (amorphous semiconductors and substitution alloys) and two-dimensional systems (quantum wells and superlattices).     

Annihilation of nonradiative recombination centers in GaAs/AlGaAs multiquantum well structures as a result of exposure to plasma

The effect of exposure to a low-energy plasma (CF₄, Ar, Kr) on the photoluminescence properties of GaAs/AlGaAs multiquantum well structures is examined. It is shown that the photoluminescence of the quantum wells in the surface region is quenched after plasma exposure and the depth of this region increases with increase of exposure time. The photoluminescence intensity from the quantum wells located beyond this region increases. We associate these changes in the photoluminescence intensity with the effect of plasma-induced nonequilibrium point defects diffusing with anomalous rapidity into the depth of the structure. Fiz. Tekh. Poluprovodn. 31, 1436–1439 (December 1997)     

铜掺杂多孔硅的光致发光及其载流子的复合过程

采用浸泡镀敷的方法在多孔硅表面形成了一镀铜层,通过对掺铜前后多孔硅的光致发光(PL)谱和傅里叶变换红外(FTIR)吸收光谱的研究,讨论了铜在多孔硅表面的吸附对其光致发光的影响。实验表明,掺铜多孔硅的光致发光谱出现两个发光带,其中能量较低的发光带随主发光带变化,并使多孔硅的发光峰位蓝移。多孔硅发光峰位的蓝移,是由于在发生金属淀积的同时伴随着多孔硅表面Si的氧化过程(纳米Si氧化为SiO2)的缘故。     

Specific Features and Nature of the 890 nm Photoluminescence Band Detected in SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Films after Low-Temperature Annealing

A band with a peak at 890 nm is detected in the photoluminescence spectra of SiO _x (x ≈ 1.3) films deposited by thermal evaporation of SiO and annealed in air at 650–1150°C. The 890-nm band appears after low-temperature (∼650°C) annealing and exhibits a number of features: (i) as the annealing temperature is elevated to 1150°C, the position of the band peak remains unchanged, whereas the intensity increases by two orders of magnitude; (ii) the effects of the annealing atmosphere (air, vacuum) and the excitation wavelength and power density on the intensity of the 890-nm band differ from the corresponding effects on the well-known bands observable in the ranges 600–650 and 700–800 nm; and (iii) the photoluminescence decay is first fast and then much slower, with corresponding lifetimes of ∼9 and ∼70 μs. The observed features are inconsistent with the interpretation of photoluminescence observed in SiO _x so far. Specifically, the earlier observed photoluminescence was attributed to transitions between the band and defect states in the matrix and between the states of band tails, transitions inside Si nanoclusters, and intraion transitions in rare-earth impurity ions. Therefore, we consider here the possibility of attributing the 890-nm band to transitions in local centers formed by silicon ions twofold- and/or threefold-coordinated with oxygen; i.e., we attempt to interpret the 890-nm band in the same manner as was done for luminescence in SiO₂ glasses and films slightly deficient in oxygen.     

Long-wavelength emission in structures with quantum dots formed in the stimulated decomposition of a solid solution at strained islands

When an array of strained InAs nanoislands formed on a GaAs surface is overgrown by a thin (1–10 nm) layer of an indium-containing solid solution, stimulated decomposition of the solid solution is observed. This process causes the formation of zones of elevated indium concentration in the vicinity of the nanoislands. The volume of newly formed InAs quantum dots increases as a result of this phenomenon, producing a substantial long-wavelength shift of the photoluminescence line. This effect is enhanced by lowering the substrate temperature, and it depends weakly on the average width of the band gap of the solid solution. The indicated approach has been used successfully in achieving room-temperature emission at a wavelength of 1.3 μm. Fiz. Tekh. Poluprovodn. 33, 990–995 (August 1999)     

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.     

Injection heterolaser based on an array of vertically aligned InGaAs quantum dots in a AlGaAs matrix

Arrays of vertically aligned InGaAs quantum dots in a AlGaAs matrix have been investigated. It is shown that increasing the band gap of the matrix material makes it possible to increase the localization energy of quantum dots relative to the edge of the matrix band, as well as the states of the wetting layer. The use of an injection laser as the active region makes it possible to decrease the thermal filling of higher-lying states, and thereby decrease the threshold current density to 63 A/cm² at room temperature. A model explaining the negative characteristic temperature section observed at low temperatures is proposed. The model is based on the assumption that a transition occurs from nonequilibrium to equilibrium filling of the states of the quantum dots. Fiz. Tekh. Poluprovodn. 31, 483–487 (April 1997)     

Characterization of GaAs/ InxGa1−x As quantum-dot heterostructures by electrical and optical methods

Results of electrical and optical studies of GaAs/In_xGa_1−x As heterostructures are reported. The aim of these studies was to identify the quantum dots and develop a technology of their growth by spontaneous transformation of an In_xGa_1−x As layer. The surface charge at the depth of the quantum dots and their surface density as a function of the deposition time of this narrow-band material are estimated by C-V profiling. A photoluminescence study of the quantum dots revealed peculiarities of the filling of their electron states at various excitation levels. The influence of Coulomb interactions on the optical properties of the quantum dots is discussed. Fiz. Tekh. Poluprovodn. 32, 111–116 (January 1998)     

InGaAs/GaAs structures with quantum dots in vertical optical cavities for wavelengths near 1.3 µm

Semiconductor heterostructures with vertical optical cavities with active regions, based on arrays of InAs quantum dots inserted in an external InGaAs quantum well, have been obtained by molecular-beam epitaxy on GaAs substrates. The dependences of the reflection and photoluminescence spectra on the structural characteristics of the active region and optical cavities have been investigated. The proposed heterostructures are potentially suitable for optoelectronic devices at wavelengths near 1.3 μm. Fiz. Tekh. Poluprovodn. 33, 629–633 (May 1999)     

InGaAs/InP heterostructures with strained quantum wells and quantum dots (λ=1.5–1.9 µm)

Strongly strained In_xGa_1−x As/In_0.53Ga_0.47As/InP heterostructures with indium content x=0.69−1.0 in the active region were investigated experimentally and theoretically. Two types of structures were obtained by vapor-phase epitaxy from metalorganic compounds: 1) with isolated compression-strained quantum wells and 2) with self-organized nanosize InAs clusters (quantum dots). The temperature dependence of the quantum radiation efficiency of samples with quantum wells in the temperature range 77–265 K is characterized by T ₀=43 K. One reason for the low value of T ₀ is electron delocalization in the active region. The maximum radiation wavelength obtained in structures with quantum dots is 1.9 μm at 77 K. Fiz. Tekh. Poluprovodn. 33, 1105–1107 (September 1999)     

Spin injection in GaAs/GaSb quantum-well heterostructures

GaAs/GaSb type-II quantum-dot heterostructures were grown by molecular-beam epitaxy. The circularly polarized photoluminescence of these samples in a magnetic field up to 4.7 T in the Faraday configuration was investigated. It was found that the emission from quantum dots in a magnetic field is σ⁻-polarized, which corresponds to the electron-spin component along the magnetic-field vector of +1/2. The degree of polarization increases with increasing excitation intensity. The observed effect is explained in terms of spin injection from the GaSb matrix, where spin orientation appears owing to the Zeeman splitting of the conduction band. An increase in the degree of polarization occurs due to a reduction in the charge-carrier radiative lifetime in type-II quantum dots with increasing excitation level.     

杂质Si对InAs自组织量子点均匀性的影响

研究了较低掺杂浓度时InAs量子点中直接掺杂Si对其发光特性的影响.光致发光谱(PL)的测量表明,与未掺杂样品相比,掺杂样品发光峰稍微蓝移,同时伴随着发光峰谱线明显变窄.该结果表明,在生长InAs层时直接掺杂,有利于形成大小分布更均匀的小量子点.该研究对InAs自组织量子点在器件应用方面有一定的意义.