Lateral photoconductivity of multilayer Ge/Si structures with Ge quantum dots

Spectra of lateral photoconductivity of multilayer Ge/Si structures with Ge quantum dots, fabricated by molecular-beam epitaxy are studied. The photoresponse caused by optical transitions between hole levels of quantum dots and Si electronic states was observed in the energy range of 1.1–0.3 eV at T = 78 K. It was shown that the electronic states localized in the region of Si band bending near the Ge/Si interface mainly contribute to lateral photoconductivity. The use of the quantum box model for describing hole levels of quantum dots made it possible to understand the origin of peaks observed in the photoconductivity spectra. A detailed energy-level diagram of hole levels of quantum dots and optical transitions in Ge/Si structures with strained Ge quantum dots was constructed.     

Photoresistance of Si/Ge/Si structures with germanium quantum dots

An exponential decrease in the resistance of a Si/Ge/Si structure containing germanium quantum dots with an increase in the band-to-band optical excitation intensity is observed at 4.2 K. Two different exponential regions in the dependence of structure resistance on the optical excitation intensity are observed in elastically strained structures, but only one such region is observed in unstrained structures. The experimental results obtained are explained within the model of the hopping conduction of nonequilibrium electrons, which are localized at and between quantum dots in the strained structures, but are localized only between quantum dots in the unstrained structures.     

Photoconductivity of Si/Ge multilayer structures with Ge quantum dots pseudomorphic to the Si matrix

Longitudinal photoconductivity spectra of Si/Ge multilayer structures with Ge quantum dots grown pseudomorphically to the Si matrix are studied. Lines of optical transitions between hole levels of quantum dots and Si electronic states are observed. This allowed us to construct a detailed energy-level diagram of electron-hole levels of the structure. It is shown that hole levels of pseudomorphic Ge quantum dots are well described by the simplest “quantum box” model using actual sizes of Ge islands. The possibility of controlling the position of the long-wavelength photosensitivity edge by varying the growth parameters of Si/Ge structures with Ge quantum dots is determined.     

Type-II Ge/Si quantum dots

The electronic structure of spatially indirect excitons, multiparticle excitonic complexes, and negative photoconductivity in arrays of Ge/Si type-II quantum dots (QDs) are considered. A comparison is made with the well-known results for type-II III-V and II-VI QD heterostructures. The following fundamental physical phenomena are observed in the structures under study: an increase in the exciton binding energy in QDs as compared with that for free excitons in homogeneous bulk materials, a blue shift in the excitonic transitions during the generation of multiparticle complexes (charged excitons, biexcitons), and the capture of equilibrium carriers to localized states induced by the electric ield of charged QDs.     

Lateral photoconductivity of AlGaAs/InGaAs structures with quantum wells and self-organized quantum dots under interband illumination

The results of studying the special features of the dependence of lateral photoconductivity in AlGaAs/InGaAs structures with quantum dots and quantum wells on the intensity of interband light at low temperatures are reported. It is found that there is a threshold for the increase in photoconductivity. Oscillations of photoconductivity are observed at relatively high pulling fields. The effects of the pulling field and temperature on the photoconductivity are studied. The results are analyzed in terms of the theory of percolation of nonequilibrium charge carriers over localized states, taking into account the relaxation of stresses around quantum dots.     

Structures with vertically stacked Ge/Si quantum dots for logical operations

Ge/Si structures with vertically stacked quantum dots are simulated to implement the basic elements of a quantum computer for operation with electron spin states. Elastic-strain fields are simulated using the conjugate gradient method and an atomistic model based on the Keating potential. Calculations are performed in the cluster approximation using clusters containing about three million atoms belonging to 150 coordination spheres. The spatial distributions of the strain energy density and electron potential energy are calculated for different valleys forming the bottom of the silicon conduction band. It is shown that the development of multilayer structures with vertically stacked quantum dots makes it possible to fabricate deep potential wells for electrons with vertical tunnel coupling.     

Photoinduced and equilibrium optical absorption in Ge/Si quantum dots

The results of studies of the optical absorption spectra in Ge/Si quantum dot structures in the mid-infrared region are reported. Two types of structures different in terms of the method used for quantum dot formation and in terms of barrier layer thickness are explored. The photoinduced absorption associated with the nonequilibrium population of hole states and optical absorption in structures doped to different levels are investigated. Specific features that are associated with occupation of the ground and excited states of quantum dots and exhibit a polarization dependence are observed. From the experimental data, the energy spectrum of holes is determined for structures of both types.     

Photoluminescence and photoreflectance study of Si/Si0.91Ge0.09 andSi9/Ge6 quantum dots

Nanometer-scale quantum dots based on a series of Si/Si_0.91Ge_0.09 strained layer superlattices and a Si₉/Ge₆ strain-symmetrized superlattice were fabricated using electron beam lithography and reactive ion etching. They were investigated by photoluminescence and photoreflectance. It was found for the first time that the quantum efficiency of optical emission from the quantum well layers increased by over two orders of magnitude when the quantum dot sizes were reduced to ≤100 nm.     

Ge/Si waveguide photodiodes with built-in layers of Ge quantum dots for fiber-optic communication lines

The results of research aimed at the development of high-efficiency Ge/Si-based photodetectors for fiber-optic communication applications are reported. The photodetectors are designed as vertical p-i-n diodes on silicon-on-insulator substrates in combination with waveguide lateral geometry and contain Ge quantum-dot layers. The layer density of quantum dots is 1×10¹² cm^?2; the dot size in the plane of growth is ～8 nm. Unprecedentedly high quantum efficiency suitable for the range of telecommunication wavelengths is attained; specifically, in the waveguides illuminated from the end side, the efficiency was as high as 21 and 16% at 1.3 and 1.55 µm, respectively.     

Self-assembling Ge(Si)/Si(100) quantum dots

The morphological evolution of self-assembled epitaxial quantum dots on Si(100) is reviewed. This intensely investigated material system continues to provide fundamental insight guiding the growth of nanostructured electronic materials. Self-assembled quantum dots are faceted, three-dimensional islands which grow atop a planar wetting layer. Pure Ge growth at higher substrate temperatures results in narrower island size distributions but activates additional strain-relief mechanisms which will alter the optical and electronic properties of the dots. Optical and electrical characterization has shown that electrons and holes are confined to different regions of the dot. This results in a spatially indirect, type II recombination mechanism. Emerging device applications which exploit properties of these nanoscale Ge islands are discussed.     

Modification of quantum dots in Ge/Si nanostructures by pulsed laser irradiation

The goal of this study was the development of a method for the modification of a quantum dot (QD) structure in Ge/Si nanostructures by pulsed laser irradiation. The Ge_xSi_1?x QD structures were analyzed using data furnished by Raman spectroscopy. Frequency-dependent admittance measurements were used to study the energy spectrum of holes in the Ge/Si heterostructures with Ge_xSi_1?x QDs before and after the laser treatment. The obtained experimental data show that laser treatment makes it possible to reduce the sheet density of QDs, modify their composition, and increase the average size. The most important result is that the QD parameters become more uniform after the treatment with nanosecond laser pulses. In a sample with ODs of 8-nm average lateral size (six monolayers of Ge), the scatter of energy levels in the QD array is reduced by half after the treatment with 10 laser pulses.     

The band structure and photoluminescence in a Ge0.8Si0.2/Ge0.1Si0.9 superlattice with vertically correlated quantum dots

The energy band diagram of the multilayered Ge_0.8Si_0.2/Ge_0.1Si_0.9 heterostructures with vertically correlated quantum dots is analyzed theoretically. With regard to fluctuations of the thickness layer in the columns of quantum dots and to the exciton-phonon coupling, it is shown that the electron states constitute a miniband. The hole wave functions remain localized in the quantum dots. The spectrum of optical transitions calculated for a 20-layered structure at room temperature is in good agreement with the experimental photoluminescence spectrum that involves an intense band at about 1.6 μm. From theoretical considerations and experimental measurements, specific evidence for the miniband in the superlattice is deduced; it is found that the overlap integrals of the wave functions of electrons and holes and the integrated intensity of the photoluminescence band of the Ge quantum dots are described by quadratic functions of the number of the structure periods.     

Ge quantum dots light-emitting devices

Si photonics becomes one of the research focuses in the field of photonics.Si-based light-emitting devices are one of the most important devices in this field.In this paper,we review the Si-based light...     

Self-assembled Ge/Si dots for faster field-effect transistors

Self-assembled and coherently strained germanium nanostructured dots are grown on prepatterned Si substrates along ordered lines. These precisely aligned nanocrystals are proposed to make up the central unit of a dot-based field-effect transistor (DotFET). The strain-induced band edge splitting and the inherently smaller effective masses of charge carriers in Ge/Si dots promise faster transistors than are possible for conventional pure Si devices. Thick relaxed buffer layers-mandatory for any existing high-speed SiGe field-effect devices-are no longer required. The DotFET is straight-forward, defect-free, and fully compatible with current complementary metal-oxide-semiconductor (CMOS) technology     

Intraband photoconductivity induced by interband illumination in InAs/GaAs heterostructures with quantum dots

The effect of lateral intraband photoconductivity in undoped InAs/GaAs heterostructures with quantum dots (QDs) has been studied, with QD levels populated with carriers by means of interband optical excitation of varied power at different wavelengths. In the absence of interband illumination, no photoconductivity is observed in the mid-IR spectral range. At the same time, additional exposure of the structures to visible or near-IR light gives rise to a strong photoconductivity signal in the mid-IR spectral range (3?C5 ??m), associated with intraband transitions in QDs. The signal is observed up to a temperature of ??200 K. Use of interband optical pumping makes the intraband photoconductivity signal stronger, compared with similar structures in which doping serves to populate QD levels.     

Wannier-stark effect in Ge/Si quantum dot superlattices

Deep level transient spectroscopy (DLTS) measurements were performed to study electron emission from quantum states in a 20-layer Ge quantum-dot superlattice (QDSL) in a Ge/Si p-n heterostructure. It was established that the changes in the DLTS spectra depend heavily on the magnitude of the applied reverse bias U _r . Three regions of the reverse bias U _r were identified, corresponding to the manifestation of the three modes of the Wannier-Stark effect: Wannier-Stark ladder mode, Wannier-Stark localization, and nonresonant Zener tunneling mode. Furthermore, it was found that the appearance of DLTS peaks for all three modes is associated with electron emission from deep-level defects via Wannier-Stark localized states arising as a result of the splitting of the electron miniband of the Ge/Si QDSL.     

Linear chains of Ge/Si quantum dots grown on a prepatterned surface formed by ion irradiation

Semiconductors - The growth of Ge nanoclusters on a prepatterned Si (100) surface formed by imprint lithography in combination with subsequent irradiation with Ge+ ions is studied. The prepatterned...     

UHV/CVD自对准生长Ge量子点(英文)

在超高真空化学汽相淀积设备(UHV/CVD)上生长了小尺寸、大密度、垂直自对准的Ge量子点。采用原子力显微镜分析量子点的尺寸,可优化其生长温度和时间,在550℃,15s的条件下生长出尺寸最小的量子点,直径30nm,高约2nm。最上层多层结构Ge量子点中岛状量子点的比例为75%,其直径66nm,高11nm,密度4.5×109cm-2。利用透射电子显微镜分析了垂直自对准的Ge量子点的截面形貌,结果表明多层结构Ge量子点是垂直自对准的。Ge量子点及其浸润层的喇曼谱峰位分别为299cm-1和417cm-1,说明Ge量子点是应变的并且界面存在互混现象。采用光荧光谱分析了Ge量子点的光学特性。