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The bond constraint theory (BCT) dealing with the rigidity caused by bond constraints and the long-range potential fluctuations (LRPF) arising from the defects and heterogeneities in the disordered semiconductors are important for understanding the atomic and electronic properties of amorphous semiconductors. Here, they are applied to the already commercialized Ge X Sb Y Te1 − X − Y (GST) chalcogenide glasses used in the rewritable phase change memory (PCM) devices. The main concern at present is to improve their ability to withstand a large number of phase change cycles, by choosing the right composition. The two considerations (BCT and LRPF) are briefly described and tested on the most commonly used Ge2Sb2Te5 and the nearby compositions. While these considerations provide significant insight into their atomic and electronic structures, the ansatz linking them with the durability of the PCM devices need to be justified by more work.

The long range potential fluctuations (LRPFs) arising from the defects and heterogeneities in disordered semiconductors are important for understanding their atomic and electronic properties. Here, they are measured in GeXSbYTe1−X−Y (GST) chalcogenide glasses used in rewritable phase change memory (PCM) devices. It is found that the most commonly used composition Ge2Sb2Te5 has the smallest LRPF amongst its nearby compositions. This finding may be useful in the search for better PCM materials.

Long-range potential fluctuations have been quantified in amorphous and crystallised thin films of a family of Ge–Sb–Te (GST) chalcogenide glasses. Among the compositions studied, the width of the potential fluctuations is the smallest for amorphous Ge2Sb2Te5. This is also the most robust material in terms of the number of write–erase cycles for GST films when used as phase change memory materials. A plausible explanation for this observation is given and a criterion for selecting suitable compositions in optical memory devices is proposed.

An improper orientation of the sample electrodes with respect to the interference fringes in the steady-state photocarrier grating (SSPG) setup can lead to large errors in the measured diffusion length (L). A simple analysis shows that the measured L is always larger than the actual L if the sample is misaligned. The analysis compares well with the experimental data taken on a hydrogenated amorphous silicon sample.

From the recent analysis of the potential fluctuations in disordered semiconductors on the basis of optical and transport measurements [1] it was concluded that these two different kinds of phenomena evidence extremely different energy scales of the random potential in the same sample. We resolve this puzzle using for the analysis of experimental data the well-known theories of transport and optical absorption in a disordered system with long-range potential fluctuations, caused by charged impurities [2, 3]. The key point in our consideration is the essential difference between the density of states caused by the long-range fluctuations and the shape of the absorption coefficient. The latter is known to depend essentially not only on the fluctuation probability but also on the tunnelling efficiency of the optically excited electrons in the potential relief provided by the fluctuations [2]. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The inhomogeneities present in hydrogenated amorphous silicon (a-Si : H) give rise to potential fluctuations. They influence the electronic properties of a-Si : H in a profound manner. Nevertheless, very few theoretical and experimental studies acknowledge their presence, because of the inherent difficulties in dealing with them. We find that the width of the potential fluctuations obtained from transport measurements is much smaller than that obtained by optical methods. This may be because the former (transport) depends on long-range potential fluctuations, whereas the latter is sensitive to short-range ones. External perturbations, e.g. light soaking and thermal quenching, are found to have different effects on the long-range potential fluctuations. Light soaking (the Staebler–Wronski effect) seems to increase them, whereas thermal quenching leaves them unchanged.

The lateral photovoltage (LPV) has been measured between coplanar electrodes by illuminating hydrogenated amorphous silicon samples at various positions with a red laser spot. We find that the LPV ...

Large lateral photovoltages (LPVs) have been measured in hydrogenated amorphous silicon and silicon germanium thin films. We find that LPV increases upon light soaking (LS) the samples. The diffusion lengths of photocarriers have been measured by the steady state photocarrier grating (SSPG) technique in annealed and light soaked states. Our study shows that the knowledge of carrier diffusion lengths and photoconductivity is not sufficient in understanding LPV and that it can be explained in terms of potential fluctuations present in the sample.

The variation in the potential difference, observed between two coplanar electrodes on hydrogenated amorphous silicon (a-Si:H) films upon illumination with a laser beam, is studied as a function of the position of illumination, and the temperature of a-Si:H. The results are analysed in the light of diffusion of photogenerated carriers and yield the ratio of the diffusion lengths of electrons and holes.

Constant-photocurrent measurement and photothermal deflection spectroscopy have been used as complementary techniques to measure the density of surface states in hydrogenated amorphous silicon (a-S...

The effect of light soaking and thermal quenching on the electronic structure of hydrogenated amorphous silicon (a-Si:H) and chalcogenide glasses was studied. It was found that lithium dopeda-Si:H shows both light and thermal induced changes in electronic transport properties. In contrast, chalcogenides do not show any effect of thermal quenching, although they exhibit changes upon light soaking. By analysing the conductivity and thermopower data we have concluded that the light soaking increases the potential fluctuations present in lithium dopeda-Si:H, whereas quenching does not change them. A model qualitatively explaining these effects is presented.

Synthesis and some interesting properties of amorphous silicon-based superlattices are reported. Both quantum-well type and doping modulated structures are studied. Quantum confinement, phonon folding and persistent photoconductivity are some of the fascinating effects which are described and their current interpretations discussed.

Illumination with bandgap light induces changes in physical properties of many chalcogenide semiconductors. Fundamental aspects of strike reversible photodarkening (the light-induced red-shift in the optical absorption edge) in arsenic-chalcogen glasses are critically reviewed. For understanding photodarkening at the microscopic level, details of the changes in the atomic structure that accompany the shift in the absorption edge are of particular importance. Study of the structural changes by a variety of techniques has revealed a phenomenon rich in basic physics but has not led a coherent picture of the underlying microscopic mechanism. Application of advanced experimental probes providing more detailed structural information has clarified some of the fundamental changes in the atomic structure and their relation to changes in the electronic and mechanical properties. Modifications in short-range and intermediate-range order accompany photodarkening. The changes in short-range order in the form of increased AsAs bonding are very small and probably do not play a predominant role in the changes in the electronic structure. Evidence suggests that the primary effect of the light-induced changes is the modification of intermediate-range correlations.

Studies of amorphous (a-) semiconductors have been driven by technological advances as well as fundamental theories. Observation of electrical switching, for example, fueled early interest in a-chalcogenides. More recently a-chalcogenide switching has been applied quite successfully to DVD technology where the quest for the discovery of better-suited materials continues. Thus, switching provides researchers today with an active arena of technological as well as fundamental study. On the theoretical front, bond constraint theory and rigidity theory provide a powerful framework for understanding the structure and properties of a-materials. Applications of these theories to switching in a-chalcogenides holds the promise of finding the best composition suited for switching applications. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy is an ideally suited technique to investigate the switching properties of these materials. Results of previous EXAFS experiments will be presented and viewed through the lens of bond constraint theory and rigidity theory.

A simple and compact arrangement for flash evaporation is described. It uses a cell phone vibrator for powder dispensing that can be incorporated into a vacuum deposition chamber without any major alterations. The performance of the flash evaporation system is checked by making thin films of the optical memory chalcogenide glass Ge2Sb2Te5 (GST). Energy dispersive x-ray analysis shows that the flash evaporation preserves the stoichiometry in thin films.

Exposure to ammonia (NH3) increases the dark current (DC) in nanocrystalline silicon. Light soaking (LS) for short periods also enhances the dark current, which remains at a high value for a long time. Pumping alone is unable to restore the initial annealed state, but annealing brings it back. The final state obtained by LS and NH3 exposure depends on the order in which they are performed. Evaporated selenium (Se) deposited on nanocrystalline silicon decreases the DC. These effects cannot be explained entirely by the presence of a-Si : H alone, in our sample. DC and photoluminescence (PL) measurements indicate the presence of two types of center in our sample, which behave differently when exposed to NH3.

Persistent photoconductivity (PPC) has been studied in doping-modulated (npnp. . .) hydrogenated amorphous silicon (a-Si:H) multilayers as well as compensated a-Si:H films. The builtin field in the multilayers is varied by changing the dopant concentrations. It is concluded that charge separation in this builtin field alone cannot explain PPC in these npnp. . . structures. Experimental evidence is presented to argue that PPC in layered as well as compensated films arises from special centers made of phosphorous-boron complexes, which are in poor communication with the rest of the material.

Surface photovoltage (SPV) and dark conductance, G, in amorphous silicon (a-Si : H) have been measured using pulsed light, after exposing the heat-dried samples to cycles of light exposure (Staebler-Wronski effect) and moist nitrogen. The changes in SPV and G upon exposure to moisture can be understood only if surface states are assumed to dominate SPV. Thus, the sign of SPV does not give the direction of band bending at the surface. Furthermore, the final value of the SPV after a cycle is found to depend strongly on the sequence of light exposure and moisture. In contrast, the final G is almost the same for the two sequences. This suggests that light-soaking changes surface states as well as the bulk. Results on lithium-doped a-Si : H are also presented and are found to be qualitatively similar.

A persistent photoconductivity effect, which lasts for many minutes at room temperature after a brief exposure to light, is observed in multiple thin layers of alternating $a\ensuremath{-}\mathrm{Si}:\mathrm{H}$ and $a\ensuremath{-}\mathrm{Si}{\mathrm{N}}_{x}:\mathrm{H}$. In addition, the samples in the dark are found to get polarized upon application of low electric fields, parallel to the layers. The depolarization takes several minutes at 300 K. The results are explained in terms of deep traps at the interfaces.

Structures consisting of alternating thin layers of n and p type a-Si:H are prepared in (i) a single chamber system and (ii) a separate chamber system. Samples from either show similar persistent photoconductivity (PPC). nini … and ipipi … films show no PPC. These results are compared with the PPC on compensated a-Si:H samples. It is concluded that centers made of P-B complexes are responsible for PPC.

Two hundred and sixty-four samples of sera obtained from normal healthy persons belonging to different age groups were examined for haemagglutination-inhibiting antibodies against reovirus types 1–3 to assess the prevalence of reovirus infection in the northern part of India.Reovirus infection appeared as early as 6 months to 1 year of age. With reovirus types 1 and 2, the maximum incidence of 67 and 48% respectively occurred by the age of 10–20 years, whereas with type 3 infection the maximum incidence (73%) was reached by the age of 25 years. The incidence of reovirus type 2 infection in all the age groups was remarkably low in this series. A drop in the incidence of reovirus type 2 infection was also noticed after the age of 20 years.The difference in geometric mean titre of antibody in different ages against reovirus type 2 was highly significant, suggesting probably that most of the infection occurred by the age of twenty years. The difference in the geometric mean value of the titres of antibody against types 1 and 3 was not significant in different age groups. The levels of antibody against types 1 and 3 in all the age groups were almost the same, suggesting that infection, specially with reovirus type 3, was occurring in all the age groups even beyond 20 years of age.The authors wish to express their sincerest thanks to Dr Leon Rosen, Pacific Research Section, National Institute of Allergy and Infectious Diseases, Honolulu, Hawaii, for the supply of prototype strains of reoviruses, to Drs B. K. Aikat, Director Professor of Pathology, R. N. Chakravarti, Department of Experimental Medicine, O. N. Bhakoo, and Sucheta Thukral, Department of Paediatrics, Post- Graduate Institute of Medical Education and Research, Chadigarh, for their invaluable help and co-operation, to Prof. P. V. K. Iyer, Department of Mathematics, Panjab University, and to Mr H. D. Gupta, Biostatistician, Post-Graduate Institute of Medical Education and Research, Chadigarh for their statistical analysis of the results.The authors gratefully acknowledge the helpful criticism and suggestion of Dr C G. Pandit, Ex-Director, Indian Council of Medical Research, during the preparation of the paper.The authors acknowledge also the technical assistance of Mr Jagmohan Lai Pipat, Mr Gurmeet Singh Khatra, and Mr Amar Singh Saini.This work was partly supported by the Research Grant of the Indian Council of Medical Research sanctioned to one of the authors (S. R. P.).

An irrigation advance function relating dimensionless parameters for estimating advance distance during irrigation application is proposed. A dimensional analysis technique is utilized for the derivation considering dominant variables such as the distance of advance, elapsed time, inflow stream per unit top width of flow, slope of soil bed, acceleration due to gravity, exponent in the Kostiakov type accumulated infiltration function, hydraulic conductivity of soil bed, and absolute roughness of the bed. Field data obtained for sandy loam soil have been utilized to determine the constants involved in the derived relationship, and computations are made with a digital computer using the least square technique of curve fitting.

We have studied the effect of stress on light‐induced changes in photoconductivity and solar cell performance of amorphous silicon alloys. We find no correlation between degradation and stress in the material.

A thermistor may have more than one operating point under steady state conditions. The choice of the operating point depends upon the initial conditions. Various possible operations have been discussed from the point of view of stability. Methods of determining the transient behaviour when one or more parameters of a thermistor, operating under steady conditions, are suddenly changed have been described.

We show that persistent photoconductivity (PPC) observed in npn … multilayers of hydrogenated amorphous silicon (a-Si:H) and compensated thin a-Si:H films have a common origin. It is caused by special centers lying in the upper half of the mobility gap. The centers are not related to the presence of argon during deposition. Similarities of PPC between the layered and compensated films are shown. We also find that PPC in compensated samples, is not entirely a surface effect. Experiments are suggested to decide whether these centers are P-B complexes or can arise from the presence of boron alone.

The extramedullary plasma-cell tumors are very rare. About 150 cases have been described so far. The first case was reported by Shridde (1905). Since then, individual case reports appeared till Hellwig (1943) reviewed the entire literature on the subject. Later, Lewis and associates (1951) reported a case of their own and collected a total of 153 cases available on the subject. The distribution of these cases according to their location was upper respiratory tract 81, conjunctiva 50, lymph nodes 4; the rest were seen in pleura, mediastinum, spermatic cord, thyroid gland, ovary, stomach, skin, etc. In 1952 Sirsat added three more cases, involving the oral cavity, nasal cavity, and lymph node, respectively. The following case is considered worthy of record because of the extreme rarity of the condition in the skin: The tissue was obtained as a biopsy specimen from a ``cauliflower-like growth'' limited entirely to the skin of