Mobile genetic elements, chiasmata, and the unique organization of beta-heterochromatin

Beta-heterochromatin in Drosophila and the Syrian hamster share a similar DNA organization, few unique sequences, and scrambled repeats of mobile elements without tandem repetition. DNA in alpha-heterochromatin is tandemly repetitious, and we now show that the repeat unit can either contain or lack a mobile element. The tandem repeat organization of alpha-heterochromatin is presumably due to a concertina-like mechanism of unequal exchange between repeat units. Although both heterochromatin types are late replicating and can incorporate mobile retroposons, the sequence distinction between the two heterochromatins appears to be due to a property conferred by chiasmata upon the process of homologous recombination in beta-heterochromatin but not in alpha-heterochromatin. Chiasmata seem to suppress the concertina mechanism of unequal exchange and impart to beta-heterochromatin its nontandem, scrambled repeat organization.     

Investigation of strained InxGa1−x As/InP quantum wells fabricated by metalorganic compound hydride epitaxy

An investigation was made of the possibility of using reduced-pressure MOC hydride epitaxy to fabricate highly strained (compressive stress) In_xGa_1−x As/In_0.53Ga_0.47As quantum wells on indium phosphide (100) substrates. The photoluminescence properties of these heterostructures were investigated. It was shown that these heterostructures are potentially useful for laser diodes emitting in the 1.5–2 μm range, which is important for environmental monitoring. Pis’ma Zh. Tekh. Fiz. 24, 46–51 (November 26, 1998)     

Studying the characteristics of pulse-pumped semiconductor 1060-nm lasers based on asymmetric heterostructures with ultrathick waveguides

High-power semiconductor lasers based on asymmetric quantum-dimensional separate confinement InGaAs/GaAs heterostructures with ultrathick waveguides were fabricated by means of metalorganic hydride vapor phase epitaxy technology. The laser characteristics were studied in a pulsed pumping regime, in which the emission was excited by current pulses of 100 ns duration at a repetition frequency of 10 kHz and an amplitude of up to 200 A. The passage to a pulsed lasing regime allowed the active region heating to be reduced and the output power to be increased to 145 W for a laser diode with a 100-μm exit aperture. The results obtained for the pulsed lasing regime show that saturation of the output power-current characteristic observed in the continuous-wave regime is fully determined by overheating of the active region of a semiconductor laser.     

Effect of the active region thickness on characteristics of semiconductor lasers based on asymmetric AlGaAs/GaAs/InGaAs heterostructures with broadened waveguide

The effect of the active region thickness on the basic characteristics of high-power semiconductor lasers based on AlGaAs/GaAs/InGaAs asymmetric separate-confinement heterostructures grown by MOCVD epitaxy has been studied. It is shown that the threshold current, temperature sensitivity of the threshold current density, internal quantum efficiency of stimulated emission, and differential quantum efficiency are improved as the active region thickness increases. It is demonstrated that the maximum attainable optical emission power of a semiconductor laser and the internal quantum efficiency of photoluminescence are the most sensitive to defect formation in the heterostructure and become lower as the critical thickness of the strained In_xGa_1–x As layer in the active region is exceeded.     

Self-organized nanosize InP and InAsP clusters obtained by metalorganic compound hydride epitaxy

Results are presented of investigations of the growth of self-organizing nanosize InP and InAsP clusters in an In_0.5Ga_0.5P matrix. The structure was characterized by low-temperature photoluminescence and transmission electron microscopy. The photoluminescence measurements revealed highly efficient radiative recombination from the quantum dots and indicated that the structures were of good optical quality. The average density and size of the InP clusters, determined from the results of the transmission electron microscope measurements, are 3×10⁹ cm⁻² and 80 nm, respectively. Pis’ma Zh. Tekh. Fiz. 24, 1–7 (August 26, 1998)     

High-power laser diodes based on asymmetric separate-confinement heterostructures

Asymmetric separate-confinement laser heterostructures with ultrawide waveguides based on AlGaAs/GaAs/InGaAs solid solutions, with an emission wavelength of ～1080 nm, are grown by MOCVD. The optical and electrical properties of mesa-stripe lasers with a stripe width of ～100 μm are studied. Lasers based on asymmetric heterostructures with ultrawide (>1 μm) waveguides demonstrate lasing in the fundamental transverse mode with an internal optical loss of as low as 0.34 cm^?1. In laser diodes with a cavity length of more than 3 mm, the thermal resistance is reduced to 2°C/W, and the characteristic temperature T ₀= 10°C is obtained in the range 0–100°C. A record-breaking wallplug efficiency of 74% and an output optical power of 16 W are reached in CW mode. Mean-time-between-failures testing for 1000 h at 65°C with an operation power of 3–4 W results in the power decreasing by 3–7%.     

Selection of power semiconductor devices for parallel connection

The method of selecting power semiconductor devices for group parallel connection on the basis of models developed on the basis of the measured electrical and thermal parameters and characteristics of certain devices are considered. Experimental data are obtained using the ADIP-6 test equipment. The electrical part of the model is constructed on the basis of the electrical parameters of the volt–ampere characteristic in a high-conductivity condition, and the thermal part is developed on the basis of the electrothermalanalogy method. The parameters of the thermal model are determined on the basis of the measured value of the thermal junction-to-case resistance in the steady-state thermal mode. The electrical and thermal parts of the model are connected through the temperature dependences of the volt–ampere characteristic parameters. A sample of ten power diodes is considered as an example, parallel connections consisting of two diodes are constructed of them, and the electrical and thermal processes are analyzed in this group using the simulation method for all possible combination of the diodes. The potentially reliable and unreliable diode combinations were specified on the basis of the simulation results. The temperature of the semiconductor structure is used as a criterion for determining reliability. A connection in which the temperature of the semiconductor structures of the devices included in the connection did not exceed the admissible limit value was considered as potentially reliable. Two connections, referred to as “potentially reliable” and “potentially unreliable” connections, are considered as an example.     

Optical and structural properties of ingaasp miscibility-gap solid solutions grown by MOVPE on GaAs(001) substrates

Optical and structural properties of InGaAsP solid solutions grown by MOVPE at 600°C on GaAs(001) substrates are studied. The photoluminescence spectra of InGaAsP solid solutions with a composition corresponding to the miscibility gap contain a main band and an additional auxiliary band. It is established that both bands are related to band-to-band radiative transitions; i. e., the studied layer includes two solid solutions with different compositions and different band gaps. It is shown that the observed high-energy shift of the additional band with an increasing level of excitation is governed by the nanometer size of domains in the corresponding solid solution. This conclusion is consistent with the results of TEM study, which revealed the presence of a periodic structure comprised of alternating domains with different compositions. This structure of alternating domains extends along the [100] and [010] directions with a characteristic period of 10 nm.     

Application of silicon carbide fibers for increasing the service parameters of abrasive cutting wheels

Abrasive cutting wheels reinforced with silicon carbide fibers are produced and tested. Their wear resistance for cutting steel pipes is much greater than in commercially produced wheels reinforced by glass network. The use of silicon carbide fibers makes it possible to produce wheels 1.0–1.5 mm in thickness while analogous commercial wheels are only 3.0 mm in thickness. Decreasing the thickness of the wheel helps save a scarce material, reduces the power rating of the equipment, and decreases the width of the cut.Translated from Poroshkovaya Metallurgiya, No. 10, pp. 91–93, October, 1992.