ALBERT

Notes: Short-period (Sim/Gen)N superlattices (SSLs) are grown step by step on a Si(001) substrates by solid source molecular beam epitaxy. Using the step-graded SSLs as buffer layers, 2000 Å uniform Si0.75Ge0.25 alloy layers are grown on the same substrates. The growth temperature of the SSLs and uniform layers is 500 °C. In the SSLs layers, m and n are the number of monolayers of Si and Ge, respectively. N is the period of (Sim/Gen) bilayers. The samples grown are characterized by x-ray diffraction, atomic force microscopy (AFM), and transmission electron microscopy (TEM) as a function of the step number of SSL layers. The SSLs show very smooth surfaces [the root-mean-square (rms) surface roughness is between 7 and 12 Å]. A dramatic decrease in roughness is observed in the uniform Si0.75Ge0.25 alloy layers, when even a one-step SSL is used as a buffer layer. A noticeable increase in rms roughness is seen in both SSL and alloy layers when the number of Ge monolayers is changed from one to two. AFM observation shows that the rms surface roughness behavior of the SSLs is reflected to their corresponding top alloy layers. The residual strains in alloy layers are considerably lower, with a maximum relaxation rate of about 80% for the sample with a seven-step SSL buffer. Cross-sectional TEM images show that strained SSL buffer layers effectively deflect threading dislocations in the substrate or confine the dislocations in the SSL buffer layers. © 2001 American Institute of Physics.

Notes: Y-Ba-Cu-O films were deposited on Al-coated Si substrates by the plasma-spray method. The Al buffer layer appears to be effective in yielding crack-free adhesive Y-Ba-Cu-O films. Resistance measurements indicate that the films exhibit a superconducting phase below 90 K. Results of x-ray microanalysis and x-ray photoelectron spectroscopy confirm that the Al buffer forms an Al2O3 layer and prevents precipitation of Cu at the film/substrate interface.

Notes: The purpose of this investigation is to study the electrical properties of the YBCO/YSZ/Si metal-insulator-semiconductor structure and the yttria-stabilized zirconia (YSZ)/Si interface. The YBCO and YSZ layers were epitaxially grown in situ on Si by pulsed laser deposition. Current-voltage measurements of devices fabricated on p-type Si(100) showed a small leakage current density at 292 K, which decreased further at 80 K. Comparison of capacitance-voltage measurements at 292 K for frequencies between 10 and 400 kHz showed a large variation of capacitance in the accumulation region demonstrating the presence of mobile ions in the YSZ layer. This variation is less pronounced at 80 K. A negative shift of about 5 V in threshold voltage from 292 to 80 K has been attributed to redistribution of charges in the YSZ buffer layer.

Notes: Nitrogen-doped hydrogenated amorphous carbon films have been deposited on silicon substrates by radio-frequency plasma-enhanced chemical vapor deposition using different N2/CH4 gas ratios from 0 to 3. The real and imaginary parts, n and k, of the complex index of refraction of these films have been determined for wavelengths between 300 and 830 nm by spectroscopic ellipsometry. Excellent agreement has been found between measured and modeled spectra, in which an empirical dielectric function based on classical Lorentz oscillator and Tauc joint density of states, and a linear void distribution along the thickness of the films have been assumed. Decrease in the optical energy gap and increase in the extinction coefficient, k, with increase in nitrogen concentration have been observed. Refractive index, n, increases rapidly with increase in nitrogen concentration up to 6.8 at. % (∼7.0 at. %) and then increases slowly with further increase in nitrogen concentration. For all the samples, n is found to be highest at the film-substrate interface which gradually decreases towards the film surface. © 2001 American Institute of Physics.

Notes: Hydrogenated amorphous silicon carbide (a-SiC:H) films were deposited with a radio-frequency plasma-enhanced chemical vapor deposition system which utilizes a dc electric field applied independently of the inductively coupled rf field. The source gases were SiH4 and CH4. It was found that application of an electric field directed out of the substrate surface enhances the growth rate and yields some improvements in photoconductivity. The compositions of the films were evaluated by x-ray photoelectron spectroscopy for a range of source gas mixtures. In order to assess the applicability of a-SiC:H thin films, heterojunction a-SiC:H/crystalline Si (c-Si) diodes were fabricated and their electrical characteristics evaluated. The diode capacitance-voltage results confirmed a step junction, which was consistent with the abruptness of the interface demonstrated by high-resolution transmission electron microscopy. The heterojunction diodes also showed good rectifying properties, suggesting promise for a-SiC@B:H in device applications.

Notes: A polyculture experiment with the large carp rohu, Labeo rohita (Hamilton), catla, Catla catla (Hamilton) and either mrigal, Cirrhinus mrigala (Hamilton) or common carp, Cyprinus carpio (L.) (as cash crop fish), and the small indigenous fish punti, Puntius sophore (Hamilton) (as food for the small-scale farmer family) was carried out at the Field Laboratory of the Faculty of Fisheries, Bangladesh Agricultural University, Mymensingh. The main objective was to compare polycultures of large carp in which the bottom feeder is either the native mrigal or the exotic common carp. Secondary objectives were to assess the effects of adding the small indigenous species punti to polycultures of large carp, and to compare the effects of mrigal and common carp on punti production and reproduction. It was found that (i) common carp damaged embankments, had no effect on catla, improved rohu performance by 50% and total fish production by 20%; (ii) punti addition did not affect rohu, catla and total yield, improved mrigal performance by 50%, and decreased common carp performance by 20%; and (iii) punti was not affected either by common carp or by mrigal. However, its performance was not satisfactory, probably owing to frequent netting, which might have hindered growth and breeding. In spite of the embankment damage caused by common carp, this bottom feeder seems to be more promising than mrigal, because it leads to higher fish production. The addition of punti to the large carp polyculture is a viable proposition, as it does not reduce cash crop production, and might be a good food source for a small-scale farmer's family.

Notes: A polyculture experiment with the large carp rohu, catla and either mrigal or common carp (as cash crop fish), and the small indigenous fish punti (as food for the farmer's family) was carried out at Bangladesh Agricultural University, Mymensingh. The main objectives were to compare polycultures of large carp in which the bottom feeder is either the native mrigal or the exotic common carp, and to assess the effects of adding the small indigenous species punti to those polycultures. The results of fish–fish interactions and overall fish production have already been reported. The present paper presents the effects on the water quality, and discusses fish–environment interactions. The main conclusions are: time changes in the pond environment were stronger than fish composition effects. The main practice affecting water quality was liming, that incresed alkalinity, pH and water transparency and decreased ammonia. Rain affected photosynthesis and the match-mismatch of the two steps of nitrification. The more that bottom feeding fish species disrupt the mud bottom, the stronger their effects on pond environment. Common carp produce the strongest disruption of the mud bottom, followed by punti and then by mrigal. Mud disruption produced by common carp leads to a stronger liming effect, nutrient release into the water, and provides more particles that rain-floods wash out, facilitating the mismatch of the two steps of nitrification, and increased phosphorus adsorption into the mud bottom. Mud disruption by punti is only enough to improve the liming effect. Mud disruption by mrigal is the least, hence less particles are resuspended, nitrification is not affected during floods and relatively more phosphate remains in the water available for photosynthesis. The bottom feeder common carp can be seen not only as a target-cultured fish but also as a management tool. Farmers can get double benefit in introducing common carp in the ponds as it enhances the effectiveness of lime application and increases the availability of nutrients to phytoplankton. Through the manipulation of species in the polyculture alone, farmers can maintain the environment better and also reduce input costs.