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The article presents a physical and mathematical model developed by the authors. The model allows analyzing the amplitude-frequency characteristic (AFC) of electroencephalograms (EEG) of a human brain. The proposed method of EEG processing, in contrast to the previously used one, is based on approximating the AFC by an equation that contains a set of coefficients. This method is convenient for comparing data obtained from different subjects and, as found, has diagnostic significance. Previously, only the frequency / amplitude ratio or its inverse was used for evaluations. The results achieved indicate the possibility of differentiating patients of various neuropsychic profiles according to the values of the parametric indicators obtained by approximating the EEG amplitude-frequency response.

The defect properties of InGaAsN dilute nitrides grown as sub-monolayer digital alloys (SDAs) by molecular beam epitaxy for photovoltaic application were studied by space charge capacitance spectroscopy. Alloys of i-InGaAsN (Eg = 1.03 eV) were lattice-matched grown on GaAs wafers as a superlattice of InAs/GaAsN with one monolayer of InAs (<0.5 nm) between wide GaAsN (7–12 nm) layers as active layers in single-junction solar cells. Low p-type background doping was demonstrated at room temperature in samples with InGaAsN layers 900 nm and 1200 nm thick (less 1 × 1015 cm−3). According to admittance spectroscopy and deep-level transient spectroscopy measurements, the SDA approach leads to defect-free growth up to a thickness of 900 nm. An increase in thickness to 1200 nm leads to the formation of non-radiative recombination centers with an activation energy of 0.5 eV (NT = 8.4 × 1014 cm−3) and a shallow defect level at 0.20 eV. The last one leads to the appearance of additional doping, but its concentration is low (NT = 5 × 1014 cm−3) so it does not affect the photoelectric properties. However, further increase in thickness to 1600 nm, leads to significant growth of its concentration to (3–5) × 1015 cm−3, while the concentration of deep levels becomes 1.3 × 1015 cm−3. Therefore, additional free charge carriers appearing due to ionization of the shallow level change the band diagram from p-i-n to p-n junction at room temperature. It leads to a drop of the external quantum efficiency due to the effect of pulling electric field decrease in the p-n junction and an increased number of non-radiative recombination centers that negatively impact lifetimes in InGaAsN. [ABSTRACT FROM AUTHOR]

Heterostructures with multiple strongly coupled quantum wells, such as super-multiperiod (SMP) superlattices (SLs), are promising semiconductor devices, which may contain hundreds or even thousands of layers with 100 or more periods synthesized by molecular beam epitaxy (MBE) to high structural perfection. The proposed characterization method employs matched application of high-resolution x-ray diffractometry (XRD), reflectometry (XRR), and, for the first time, the deep XRR (DXRR) allows the study of SMP structures, as well as high-accuracy determination of the thicknesses of layers, roughness/diffuseness of boundaries using the rigorous scattering theory, and composition of solid solutions. Combining these methods with scanning transmission electron microscopy (STEM) enables characterization of SMP SLs and independent determination of these same parameters. The differences between the expected and obtained layer thicknesses by XRD and XRR were 1%–3% for AlGaAs/GaAs structures. The samples were characterized by sharp interfaces with the root-mean-square width of the transition layers of the order of a few Ĺ, which is consistent with the XRR/DXRR and STEM analysis. Based on the data obtained for the thicknesses of layers, the composition of Al0.3Ga0.7As has been accurately determined by the x-ray methods. These results may be considered as the first step in the analysis of MBE-grown SMP structures with a number of periods up to 1000. [ABSTRACT FROM AUTHOR]

Gallium phosphide is an important material in terms of III-V semiconductors integration on Si. In this work we study photovoltaic properties of GaP:Be/GaNP (E g ~ 2.0eV)/GaP:Si p-i-n heterostructure grown on Si (100) with GaP buffer by plasma-assisted molecular beam epitaxy. Correlation between the structural and optoelectronic properties of the fabricated device was studied by scanning and transmission electron microscopy (TEM), electron beam induced current (EBIC) and deep-level transient spectroscopy (DLTS) techniques. The I–V characteristic of the fabricated mesa diode demonstrates short circuit current of 2.2 mA/cm2 and open circuit voltage of 0.8 V. TEM studies of the epilayer structural properties demonstrate high density of antiphase domains in the n-doped GaP buffer layer; however, they found to be annihilated in dilute nitride layer, where mainly threading dislocations were formed. EBIC investigation shows that crystalline imperfections of the heterostructure leads to poor carriers transport affecting the energy conversion efficiency. We performed numerical modeling concerning presence of the structural defects and discuss their influence on the diode performance. The carried out study is the initial step on the way to development of the growth technique of the GaP-based dilute nitride direct bandgap materials on Si for photonic and photovoltaic applications. • For the first time MBE-grown GaNP-based solar cell on Si is fabricated. • The fabricated device is thoroughly characterized with I–V, DLTS and EBIC measurements. • Theoretical model of the device is introduced to find out prospects for future development of the photovoltaic devices of the proposed design. [ABSTRACT FROM AUTHOR]