Problem of optimal control in the system of semiconductor quantum points

For the operation of spin information swapping in the system of two vertically superposed layers of tunnel-coupled semiconductor quantum points, a problem of optimization of the time function of control voltage pulse was formulated. The corresponding problem of optimal control with terminal constraints was stated and solved.     

Photovoltaic Ge/Si quantum dot detectors operating in the mid-wave atmospheric window (3-5 micrometers)

ABSTRACT: Ge/Si quantum dots fabricated by molecular-beam epitaxy at 500C are overgrown with Si at different temperatures Tcap, and effect of boron delta-doping of Si barriers on the mid-infrared photoresponse was investigated. The photocurrent maximum shifts from 2.3 to 3.9 micrometer with increasing Tcap from 300 to 750C. Within the sample set, we examined devices with different positions of the delta-doping layer with respect to the dot plane, different distances between the delta-doping layer and the dot plane d, and different doping densities pB. All detectors show pronounced photovoltaic behavior implying the presence of an internal inversion asymmetry due to the placing dopants in the barriers. The best performance was achieved for the device with Tcap=600C, pB=12x10^{11} cm^{-2}, and d=5 nm in a photovoltaic regime. At a sample temperature of 90K and no applied bias, a responsivity of 0.83 mA/W and detectivity of 8x10^{10} cmHz^{1/2}/W at lambda=3.4 micrometer were measured under normal incidence infrared radiation.     

Quantum Gates with Spin States in Continuous Microwave Field

The physical basis of the method of implementing quantum logical operations in a continuous microwave field in a system of two electrons with different g-factors and a constant exchange interaction is developed. A small variable additive ΔB(t) to the magnetic field is proposed as a control action. It is obtained that a simple functional dependence ΔB(t), based on harmonic functions, allows us to perform elementary quantum logical operations and their sequences. This method is adapted to the experimental conditions.     

Dependence of the Luminescence Properties of Ordered Groups of Ge(Si) Nanoislands on the Parameters of the Pit-Patterned Surface of a Silicon-on-Insulator Substrate

Semiconductors - This paper presents the results of studies of the luminescence properties of structures with Ge(Si) quantum dots (QDs), in which the pit-patterned surface of a silicon-on-insulator...     

Formation of ordered groups of quantum dots during Ge/Si heteroepitaxy

A new approach to the creation of circularly ordered Ge nanoislands by epitaxy on the surface of a heterophase structure consisting of a Si(100) substrate with premade seeds in the form of SiGe nanodisks or SiGe nanorings is developed. It is shown that the spatial configuration of islands in the group is due to the nucleation of the islands in the area of local minima of the elastic energy density on the surface of a circular seed. On the basis of this approach, a number of multilayer structures with vertically aligned ring groups of quantum dots were grown. The elemental composition and luminescent properties of the ordered structures are studied.     

Two-dimensional strain distribution in elastically anisotropic heterostructures

We consider the two-dimensional distribution of elastic strain in semiconductor heterostructures — quantum wires characterized by anisotropy of the elastic properties. The deformation is caused by the mismatch in lattice parameters between the material of the quantum wire and its environment (matrix).Such deformations affect the position of the energy bands, so that they should be taken into account in the calculation of the electronic states. It is shown that the strain distribution in an anisotropic medium is a linear combination of two distributions relating to transversely stretched modifications of the original quantum wire.     

Effect of the growth rate on the morphology and structural properties of hut-shaped Ge islands in Si(001)

The effect of Ge deposition rate on the morphology and structural properties of self-assembled Ge/Si(001) islands was studied. Ge/Si(001) layers were grown by solid-source molecular-beam epitaxy at 500?°C. We adjusted the Ge coverage, 6?monolayers (ML), and varied the Ge growth rate by a factor of 100, R = 0.02-2?ML?s(-1), to produce films consisting of hut-shaped Ge islands. The samples were characterized by scanning tunnelling microscopy, Raman spectroscopy, and Rutherford backscattering measurements. The mean lateral size of Ge nanoclusters decreases from 14.1?nm at R = 0.02?ML?s(-1) to 9.8?nm at R = 2?ML?s(-1). The normalized width of the size distribution shows non-monotonic behaviour as a function of R and has a minimum value of 19% at R = 2?ML?s(-1). Ge nanoclusters fabricated at the highest deposition rate demonstrate the best structural quality and the highest Ge content (～0.9).     

Interlevel optical transitions and many-body effects in a dense array of Ge/Si quantum dots

We have investigated experimentally the mid-infrared normal-incidence response of holes confined in array of Ge/Si self-assembled quantum dots. The dots have a lateral size of approximately 15 nm and a density of 3×10¹¹ cm⁻². An in-plane polarized absorption in the 70–90 meV energy range is observed and attributed to the transition between the first two states in the dots. As the hole concentration in the dot ground state is increased, the absorption peak shifts to higher energies, its linewidth is reduced, and the lineshape is changed from an asymmetric to symmetric one. We attribute all features to a depolarization-type effect caused by collective interlevel excitations.     

Silicon-germanium nanostructures with quantum dots: Formation mechanisms and electrical properties

The generally accepted notions about the formation mechanisms for germanium islands with nanometer-scale sizes in a Ge-on-Si system are reviewed on the basis of analysis of recent publications. The presence of elastic strains in the epilayers and in the three-dimensional Ge islands on Si is a key factor that not only initiates a morphological transition from a planar film to an island-containing film (the Stranski-Krastanov mechanism) but also influences the subsequent stages of the islands’ evolution, including their shape, size, and spatial distribution. In many cases, this factor modifies appreciably the classical mechanisms of phase-formation and their sequence up to the quasi-equilibrium coexistence of three-dimensional Ge nanoislands at the surface of the Si substrate. The methods for improving the degree of the ordering of nanoislands to attain the smallest possible sizes and large density of areal distribution of these islands are discussed. The published data on optical absorption in the multilayered Ge-Si systems with quantum dots are considered; these data are indicative of an anomalously large cross section of intraband absorption, which makes this class of nanostructures promising for the development of photodetectors of the infrared region of the spectrum. The results of original studies of electrical and optical properties of heterostructures that involve Ge quantum dots and are synthesized by molecular-beam epitaxy on the Si substrates are reported.