Broadband Ge/SiGe quantum dot photodetector on pseudosubstrate

We report the fabrication and characterization of a ten-period Ge quantum dot photodetector grown on SiGe pseudosubstrate. The detector exhibits tunable photoresponse in both 3- to 5- μm and 8- to 12- μm spectral regions with responsivity values up to about 1 mA/W at a bias of −3 V and operates under normal incidence radiation with background limited performance at 100 K. The relative response in the mid- and long-wave atmospheric windows could be controlled through the applied voltage.     

Direct Interband Light Absorption in Strongly Prolated Ellipsoidal Quantum Dots’ Ensemble

Within the framework of adiabatic approximation, the energy levels and direct interband light absorption in a strongly prolated ellipsoidal quantum dot are studied. Analytical expressions for the particle energy spectrum and absorption threshold frequencies in three regimes of quantization are obtained. Selection rules for quantum transitions are revealed. Absorption edge and absorption coefficient for three regimes of size quantization (SQ) are also considered. To facilitate the comparison of obtained results with the probable experimental data, size dispersion distribution of growing quantum dots by the small semiaxe in the regimes of strong and weak SQ by two experimentally realizing distribution functions have been taken into account. Distribution functions of Lifshits–Slezov and Gaussian have been considered.     

与InP晶格相匹配的Ga_xIn_(1-x)P_yAs_(1-y)半导体的光学常数计算

采用半经验的紧束缚方法计算了与InP晶格相匹配的Ga_xIn_(1-x)P_yAs_(1-y)半导体的电子联合状态密度及介电常数虚部,并根据Kramers-Kronig关系式求得了光透明区的折射率常数.     

Cuprate Superconducting Characteristics on the Doping Scale. A Multiband Approach

Some results on the doping dependence of cuprate superconductivity characteristics by an multiband approach are presented. A nonmonotonic dependence of the critical coherence length agrees with the experiment. Periodic spatial structures of T _c , etc. are expected at underdoping by Leggett noncritical mode fluctuations. The paired carrier effective mass diminishes moderately with doping. The inverse plane penetration depth shows a dependence with a maximum dominated by the supercarrier density. For the latter, the isotope effect is predicted and calculated. It is decisive for the isotope effect in the superfluid stiffness. This work was supported by the Estonian Science Foundation Grant No. 6540.     

A minimum variance distortionless response filter based on the bifrequency spectrum for single-channel noise reduction

This paper deals with the problem of single-channel noise reduction in the short-time Fourier transform (STFT) domain. Many algorithms have been developed to solve this important problem, most of which generally assume that the STFT coefficients in different frequency bands are uncorrelated, so the noise reduction is achieved by applying a gain function to the STFT of the noisy speech in each frequency band. However, this assumption is not accurate and the STFT coefficients of speech signals between neighboring frequency bands are correlated in practice due to the use of small lengths of the fast Fourier transform (FFT) and overlap add/save techniques in implementation. This paper formulates the noise reduction problem by taking into account the interband correlation using the so-called bifrequency spectrum. Based on this formulation, a single-channel minimum variance distortionless response (MVDR) filter is derived, which is shown to be able to significantly improve the signal-to-noise ratio (SNR) and meanwhile maintain the desired speech not much distorted. Simulations are presented to justify the claimed merits of the developed MVDR filter.     

Influence of delta-doping on the hole capture probability in Ge/Si quantum dot mid-infrared photodetectors

We study the effect of delta-doping on the hole capture probability in ten-period p-type Ge quantum dot photodetectors. The boron concentration in the delta-doping layers is varied by either passivation of a sample in a hydrogen plasma or by direct doping during the molecular beam epitaxy. The devices with a lower doping density is found to exhibit a lower capture probability and a higher photoconductive gain. The most pronounced change in the trapping characteristics upon doping is observed at a negative bias polarity when the photoexcited holes move toward the δ-doping plane. The latter result implies that the δ-doping layers are directly involved in the processes of hole capture by the quantum dots.     

Interband Model Scenario of Cuprate Superconductivity

A multiband approach for hole- and electron-doped cuprate superconductors is developed. The model electron spectrum includes nodal and antinodal defect (polaronic) subbands created by doping. Bare gaps between itinerant and defect subbands close with extended doping. The overlap conditions determine phase diagram special points. There are analogies for both types of doping despite the activation of different sublattices. Illustrative calculations have given self-consistent results reproducing qualitatively doping dependences of T _c , superconducting gaps and pseudogaps, supercarrier density and effective mass, coherence length and penetration depth, etc. Interband pairing scenario is suggested to be an essential aspect of cuprate superconductivity. This work was supported by the Estonian Science Foundation grant No. 7296.     

Cuprate Interband Superconducting Density for Doping Driven Spectral Overlaps

We calculate the superconducting density for a model cuprate with gapped itinerant and defect subsystems brought into spectral overlap by progressive doping. The interband nature of the pairing prevents the manifestation of the normal state gaps in the superconducting density. These “extrinsic” gaps drive the pseudogap behavior until the overlap concentrations are reached. Further doping changes the nature of the quasiparticle minimal excitation energy and a pseudogap is transformed into the corresponding superconducting gap. The Uemura-type plot contains a sublinear segment at underdoping.     

Well-defined electrical properties of high-strain resonant interband tunneling structure

The GaAs/InAs high-strain resonant interband tunneling diodes (HSRITDs) have been implemented by metal organic chemical vapor deposition (MOCVD). The current-voltage characteristics of variable quantum well and barrier thickness grown on (1 1 1) GaAs substrates are investigated. Experimental results reveal that the quantum barrier and well layer will influence current-voltage properties such as the peak current density, valley current density, and peak-to-valley current ratio (PVCR). Both peak current and valley current density decrease with increasing layers width. This result also exhibits the variation of PVCR with layers width.     

Relaxation of Superconducting Fluctuations in a Two-Component Scenario with Intra- and Interband Pairings

We study the relaxation of superconducting fluctuations in a two-component (two-band) model where the interband pair-transfer interaction and intraband effective attractions of electron-phonon nature are taken into account. The corresponding free energy and the system of Landau–Khalatnikov equations are derived. One can distinguish two different time scales in the damping of the fluctuations of superconducting order parameters in a two-band scenario. The formation of these relaxation channels is caused by the interband interaction leading to the redistribution of damping processes of the initially independent bands. The relevant relaxation times characterize the kinetics of critical and noncritical fluctuations which appear as the certain linear combinations of the deviations from the equilibrium band superconducting orders. The peculiarities of the temperature behavior of the relaxation times in dependence on intra- and interband couplings are established.