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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.

Electrical conductivity has been studied as a function of temperature in amorphous silicon containing varying amounts of Si crystallites (nc-Si:H), prepared by plasma enhanced chemical vapor deposition (PECVD) of silane mixed with hydrogen. HRTEM shows the presence of nanocrystals in the films, prepared under high hydrogen dilution. The relative fraction of crystallinity, χ c is estimated using Raman spectroscopy and has been changed by varying the hydrogen dilution and RF power. The room temperature conductivity at first changes very little as χ c increases but shows a sharp increase as χ c crosses a certain threshold value. Our findings are consistent with the percolation theory calculations. The conductivity is found to be thermally activated and also gives a good fit to the T − 1/4 law, for variable range hopping. Further, the prefactors and the slopes in both cases are found to be correlated, through a Meyer–Neldel type relationship, whose origin is not clear at present. This latter relationship (hopping MNR) is like the conventional MNR and many materials having diverse conduction mechanisms, obey it. We take a look at the derivation of the T − 1/4 law and try to see if the hopping MNR can be explained in all materials in general and in nc-Si:H in particular. No explanation is available in either case, at present.

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.

A Meyer–Neldel rule (MNR)-type relationship is found to exist between the slope (activation energy) and the intercept (prefactor), in the T − 1 / 4 plot of hopping conductivity. This new relation (hopping MNR) is obeyed by a large number of systems, including those obeying the conventional MNR. Further, all the materials lie on a single straight line on the hopping MNR plot. Hopping in the exponentially rising density of localized states can give such a relationship in amorphous semiconductors. This may be used to roughly estimate a parameter, e.g., the density of states at the Fermi level or its slope, if either is known from some other experiment. However, like conventional MNR, a general explanation is missing for hopping MNR also.

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.

The kinetics of tartaric acid oxidation by peroxodisulphate ion has been studied to elucidate the mechanism of oxidation. Uncommon to many peroxodisulphate reactions it shows a variation in order. It is found to progress bimolecularly in the beginning and unimolecularly till completion. H+ ion and O2 gas accelerate the reaction. Some observations with change of surface revealed that area/volume ratio Influences the rate of reaction. In order to explain the observed data the chain mechanism has been proposed and rate expressions have been derived to explain the change of order from two to one.

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.

Silicon thin films are deposited using plasma enhanced chemical vapor deposition (PECVD) of silane, argon, hydrogen mixture at various pressures in the range of 2–8 Torr. Raman scattering shows these to be amorphous in the pressure range 6–8 Torr, and nanocrystalline in the range 2–4 Torr. The volume fraction of nanocrystals is estimated by fitting the Raman data to three peaks and is found to be ∼75% for the films deposited at low pressure, density of states of these films was measured. It is observed that the electrical conduction in these films depends on the crystalline volume fraction (ρ), estimated from the laser Raman Spectroscopy. Temperature dependence electrical conductivity shows that at lower temperatures thermionic emission dominates for the films with lower ρ, whereas, hopping is the main conduction mechanism for the films having high ρ. The density of states is estimated from the space charge limited currents (SCLC) observed at high fields. Photoconductivity at room temperature is also measured. The amorphous films are found to be more photosensitive than the nanocrystalline one. In the context of these findings, changes in the properties of silicon from amorphous to nanocrystalline are described.

Exposure of porous silicon (PS) to moisture is found to increase its photoluminescence (PL) intensity, whereas it decreases drastically upon light soaking. On the other hand, coating by a thin layer of polystyrene protects PS against light induced degradation without changing PL significantly. Further, polystyrene coating protects the PS from humidity also. FTIR shows appearance of new bonding configurations at the PS/polymer interface, whcih may account for the improved stability. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of light soaking on the boron doped and undoped a-Si:H samples is studied after coating them with a thin film of polystyrene. We find that in both cases, the photo-degradation is reduced. This can be understood if, in addition to the bulk, light soaking affects the surface also.

Fresh porous silicon (PS) samples show photoluminescence (PL) in visible. After a short dip in water, the PL peak shifts towards blue. However, a short dipping of PS in toluene and pentane does not change PL. The intensity of PL decreases upon exposing PS briefly to acetone. The peak position however, remains unchanged. X-Ray diffraction (XRD) and atomic force microscopy (AFM) show the presence of nanocrystals in PS. After dipping in water, XRD and AFM show structural changes. However, dipping in pentane, toluene and acetone has no effect on the XRD and AFM. Using AFM data, John–Singh model of quantum confinement explain the PL results of PS treated with water, toluene and pentane. However, the changes in PL after acetone treatment cannot be explained within this model.

Light soaking in air rapidly decreases photoluminescence (PL) of porous silicon (PS) and increases electron spin resonance (ESR) signal. In vacuum, a short light exposure (

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.

We report the effect of fast thermal quenching (FQ) and light soaking (LS) on electronic transport in lithium-doped hydrogenated amorphous silicon (a-Si:H(Li)), and compare the two instabilities in the same specimen. We find that the metastable changes are qualitatively similar to those in phosphorus-doped hydrogenated amorphous silicon. The films are in thermal equilibrium above the temperature T E, which is lower for heavier doping. However, for a given sample, the TE values are about the same for FQ and LS. Isothermal relaxation of conductivity [sgrave] of the metastable states follows a stretched exponential, but the relaxation parameters for FQ and LS are different. We also find that the function Q = In [sgrave] - eS/k shows a large change after LS but hardly changes after FQ. The results are explained by assuming that the recombination of carriers at the local minima of the potential fluctuations present on a-Si: H(Li) gives rise to structural changes. The observed increase in the slope of ...

Films of a-Ge 20 Se 80 prepared by vacuum evaporation, are exposed to visible light and irradiated with 1 MeV Krypton ions. The changes in bonding configurations are studied by Raman spectroscopy. Three bands are observed in the virgin films and are identified as corresponding to the stretching mode of GeSe 4/2 corner-sharing tetrahedra (201 cm −1 ) and to the vibrations of Se atoms in four member rings composed of two edge sharing tetrahedra (215 cm −1 ) and Se–Se chains (263 cm −1 ). Upon ion irradiation the band positions remain unchanged and no new band appears. In contrast, light soaking produces an additional band at 257 cm −1 . This bond has been identified as arising from the stretching vibrations of Se atoms in helical chain-like and ring-like arrangements. A study of the relative areas under the peaks shows that ion-irradiation causes the Se–Se bridges linked with GeSe 4/2 units to convert into Se–Se chains and the number of GeSe 4/2 bonds remain unchanged. Upon light soaking, the density of corner-sharing GeSe 4/2 bonds increases, no significant variation is observed in the Se bonds. In addition, the width of the peak at 201 cm −1 decreases after ion irradiation but increases upon light soaking. These data show that ion irradiation induces bond changes in Ge 20 Se 80 , which are different from those, created by light soaking.

Effects of rapid cooling (FQ) from high temperature and light soaking (LS) at room temperature using heat filtered white light on transport properties of lithium doped a-Si:H(Li) have been studied for varying lithium (Li) concentrations while keeping the hydrogen content fixed. This study was possible as Li was introduced by thermal diffusion after a-Si:H films were prepared by glow discharge technique. We find that the equilibrium temperature T E is lower for a-Si:H(Li) with larger lithium concentration. The isothermal relaxation studies in the metastable FQ and LS states show that for a given state, relaxation is always slower for less doped a-Si:H(Li). These results can be understood in terms of diffusion of hydrogen, if we assume that the presence of lithium increases the diffusion coefficient of hydrogen in a-Si:H.

Sub-band gap absorption measured using constant photocurrent measurement and photothermal deflection spectroscopy have been used as complementary techniques to obtain the density of surface states in lithium doped hydrogenated amorphous silicon [a-Si:H(Li)]. This method has the advantage that it obviates the need to make identical samples of varying thicknesses. We find a surface state density of ≈5 × 1012 cm−2 in a-Si:H(Li), for the two Li concentrations studied.

Thermally induced metastability has been studied on P doped LPCVD a-Si (low pressure chemical vapour deposited amorphous silicon) films as a function of the concentration of hydrogen in these films. The hydrogen concentration has been varied over two orders of magnitude, from 0.06 at.% in as-deposited films to 15 at.% in heavily implanted films. We find that thermal metastability effects are present even at the lowest concentrations of hydrogen. These become larger as hydrogen concentration increases. Our studies indicate that hydrogen plays an important role in thermal equilibration of a-Si:H and supports the hydrogen glass model.

Metastabilities in lithium-doped a-Si:H films are studied after fast quenching from high temperatures (FQ) as well as after exposing to visible light (LS). We have studied the relaxation of a new FQ+LS state obtained by light soaking a-Si:H films in FQ state. We find that FQ+LS state is also metastable; however, in this case the isothermal relaxation does not follow a stretched exponential. Further, the relaxation of conductivity in FQ+LS state can be explained, if the return of the Fermi level from this state to the state obtained after slow cooling is taken to be the sum of the relaxations of Fermi levels in the FQ and LS states. This suggests that FQ and LS are independent.

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 electronic structure of hydrogenated amorphous silicon (a-Si:H) is in a state of metastable equilibrium and can change upon application of external stimuli. We study the effect of thermal quenching and light soaking in lithium-doped a-Si:H, on its conductivity and thermopower. We present evidence showing that the metastable state obtained after fast quenching is different than that obtained after light exposure. Experiments on chalcogenides show that they are not affected by thermal quenching although they change upon light soaking. This is in contrast with lithium doped a-Si:H in which both effects are observed. Our experiments suggest that hydrogen present in a-Si:H plays an important role by controlling heterogeneities and potential fluctuations in a-Si:H. Light soaking appears to enhance these potential flucutations, whereas fast cooling seems to have little effect on them.

Effect of thermal and light induced metastabilities on conductivity (σ) and thermopower (S) in phosphorus doped a-Si:H films has been studied. It has been found that although both conductivity and thermopower show a change upon fast quenching, the function Q=ln σ−eS/k does not change. This suggests that quenching results in a new frozen-in structure, in which the position of the Fermi level is changed, but there is no change in the potential fluctuations. No measurable effect of light soaking is observed in our films.

Thermal and light induced metastabilities have been studied on lithium doped a-Si:H prepared by the glow-discharge technique. The equilibration temperature, T E , is found to be the same (393 K) for the two metastabilities. The relaxation can be fitted to a stretched exponential in both cases. However, the relaxation parameters are found to be different. Thermopower results along with conductivity have been analyzed to show that light soaking changes the potential fluctuations, whereas fast quenching leaves them unchanged.

The hydrogen in hydrogenated amorphous silicon (a-Si: H) makes it behave like a hydrogen glass. Above a temperatureTE, which is analogous to the glass transition temperature, the hydrogen is able to move more freely than belowTE. This motion of hydrogen is believed to be responsible for the observed thermal and light induced metastabilities in a Si: H. However, the changes in the microstructure of the bonded hydrogen upon thermal quenching are found to influence the electronic properties of a-Si: H, in a manner, which is different from light soaking. Our studies suggest that the light soaking changes the potential fluctuations in lithium doped a-Si: H, whereas the thermal quenching does not.

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.

Lac Processing & Product Development Division, Indian Lac Research Institute, Namkum, Ranchi-834 010, Jharkhand, India E-mail : lppd@ilri.ernet.in Fax : 91-0651-2260202 Manuscript received 17 February 2006, revised 13 July 2006, accepted 25 August 2006 (E)-7-Hexadecen-1,16-olide (ambrettolide), having musk-like odour being used as a fixative in perfumery industry. It was synthesized from \(threo\)-aleuritic acid (9,10,16-trihydroxyhexadecanoic acid) by simple procedure.

The effect of ambient conditions on light soaking (LS) in porous silicon (PS) sample is studied. In vacuum LS on porous silicon increases dark current (DC) while in presence of water vapor it decreases DC. Interestingly, LS gives a higher DC in vacuum as well as in presence of water vapor for boron doped hydrogenated amorphous silicon a-Si:H(B) samples. A thin layer of polystyrene polymer almost eliminates the light induced degradation of porous silicon (PS) layers but can do so only to a smaller extent in a-Si:H(B). This shows that the effect of LS in PS is different than the effect of LS in a-Si:H(B) and that the surface plays a more important role in PS than in a-Si:H.

In undoped a-Si:H thin films, light soaking (LS) decreases dark current and photocurrent and increases the electron spin resonance signal monotonically with increasing exposure time, (Staebler-Wronski Effect, SWE). On the other hand, in porous silicon (PS) the light-soaking effect is non-monotonic. For short durations, LS increases dark current, photo current and photoluminescence and decreases the ESR signal. Long exposures of PS, however, have the opposite effect, which is similar to the SWE observed in a-Si:H. All metastabilities in PS as well as in a-Si:H can be removed by annealing, but not by exposure to infrared light. We find that the combined effect of NH3vapor exposure and LS on PS yields a final state that depends upon the order in which the two steps are performed. This is in contrast to the corresponding observation in a-Si:H. We could arrest the light-induced degradation of PS over long times by coating with a thin polystyrene, which resulted in constant PL and ESR intensities. However, in the case of a-Si:H the polymer coating decreases the SWE, but does not eliminate it. The results can be understood, if we were to propose that LS affects the surface in PS, but affects mainly the bulk in a-Si:H.

Conversion of benzaldehyde to L-phenylacetyl carbinol (L-PAC) was achieved with immobilized, growing cells of Saccharomyces cerevisiae in different reactors. Product formation increased (31%) with the subsequent initial reuses of the entrapped cells. Biomass production and PAC formation depleted (40 and 57%, respectively) after 4-5 continuous growth and biotransformation cycles. With the regeneration of the biocatalysts, catalytic activity of the cells was resumed. The highest yields were in a stirred tank reactor (29 g PAC) from 77 g benzeldehyde with 14 repeated uses of entrapped cells.

Exposure to ammonia (NH3) increases the dark current (DC) in porous silicon (PS), but evaporated selenium (Se) deposited on PS decreases DC. Photoluminescence (PL) measurement shows that there are two types of centers. PL in one region of PS (Peak ∼ 800nm) initially increases with the NH3 exposure and then decreases. But the PL from another region of PS has a peak at ∼ 780nm and it decreases continuously with the NH3 exposure. Dipping PS in water and drying in air shifts the PL peak at 800 nm to 744 nm. Atomic Force Microscopy (AFM) shows that the as prepared sample has wires of diameters 2.4nm, 3.4nm, 4.6nm and bigger. However, in the AFM images of the water treated sample the wires of diameters 3.4nm and 4.6nm are absent. The PL results are explained using the AFM data and the John-Singh model of quantum confinement.

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.

The effect of light soaking (LS) on nanocrystalline porous silicon (nc-Si) is found to depend on the ambient conditions. Light soaking in air decreases the photoluminescence (PL) intensity and increases the number of dangling bond density measured by electron spin resonance (ESR) monotonically as the exposure time is increased. In vacuum, however, short light exposures increase the PL intensity and decrease the ESR signal, but longer exposures have the opposite effect. Thus PL intensity has a maximum as a function of exposure time. The temperature dependence of LS in vacuum is measured at 300, 325, and 350K. We found that the maximum PL intensity occurs at smaller exposure times as the temperature is increased. These results are explained in terms of two kinds of photostructural changes having opposite LS effects on the PL. Coating nc-Si with a thin layer of polystyrene stabilizes PL against light soaking as well as water vapor, without affecting the PL intensity significantly. Modified bonding configurat...

Hydrogenated amorphous silicon (a-Si:H) has potential fluctuations, which arise from the heterogeneities, variations in the bond angles, bond lengths, and nonuniform distribution of hydrogen and dopants, etc. We have estimated the potential fluctuations present in undoped and doped a-Si:H from the optical as well as from the transport measurements. In the optical measurements, the subgap absorption has been used to estimate the width of the potential fluctuations. In the transport method, the conductivity and the thermopower measured as a function of temperature are used to provide the width of the potential fluctuations, as shown by Overhof and Beyer. We find that in all cases, the width of the potential fluctuations measured by the optical measurements is about 20 times larger than that measured by the transport measurements on the same sample. This is explained by noting that optical absorption is a local process, which is sensitive to the short-range potential fluctuations by the order of 3 A, whereas...

The deformation dipole model (DDM) originally formulated by Hardy (1960) has been re-analysed in the light of recent data. The existing tables of ionic radii and the values of overlap integrals have been used to predict the effective ionic charges in crystals of alkali halides.

We have grown amorphous silicon-germanium alloys by rf glow discharge decomposition of disilane, germane, silicon tetrafluoride and hydrogen. Films with an optical band gap in the range of 1.72 to 1.2e V have been made. The optimized films show the following properties: 1) incorporation of germanium is uniform across the length of the sample; 2) hydrogen is bonded to silicon and germanium uniformly without any preference; 3) the slope of the valence band tail remains constant as the optical gap is lowered down to 1.25eV and 4) use of silicon tetrafluoride in the gas mixture improves the quality of the material. Photothermal deflection spectroscopy and space charge limited conduction were used to obtain information about the deep states. Even for the lowest band gap material, the density of states at the Fermi level is about 10 17 cm −3 eV −1 which demonstrates the high quality of the material.

Using Schottky diodes, the density of localized states (DOS) has been estimated in thin films of hydrogenated amorphous silicon (a-Si:H), by measuring steady state capacitance as a function of bias, C ( V ), frequency, C ( ω ) and temperature, C ( T ). The DOS at the Fermi-level is found to lie between 10 16 −10 17 cm −3 eV −1 and is in agreement with the results obtained by space charge limited currents (SCLC) and isothermal transient capacitance (ICTS) measurements performed on the same diode. The assumptions used in deducing the DOS from each of the measurements have been spelled out and the limitations pointed out. This DOS has been compared with that obtained by other methods, e.g., deep level transient spectroscopy (DLTS), transient currents (TCUR) and the Field effect by others. It is suggested that the differences in DOS reported in the literature may be because different samples might not have identical DOS.

A dc glow discharge apparatus for preparing amorphous silicon films from silane gas is described. The films are characterized by electron microscopy, infrared spectroscopy, electrical conductivity and photoconductivity. The deposition parameters which give good photoconducting films are established. The Staebler-Wronski effect is studied and is found to be smaller in vacuum than in air. A photovoltage is observed in structures with gold as the Schottkybarrier metal. The conversion efficiency of the device is about 1%. The results are compared with those in the literature, and the improvements which might result in a better conversion efficiency are pointed out.

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.

The uncatalysed oxidation of glycollic acid by peroxodi- sulphate has been investigated. The kinetics are peculiar in as much that the rate of peroxodisulphate decomposition is observed to decrease with increase in glycollic acid concentration. The rate law in low peroxodisulphate concentration is — dcS 2 O 8 2-/dt = k(S 2 O 8 2- )/(CH 2 OH.COOH) 0.2 . On the other hand when (S 2 O 8 2 -) ≥ (CH 2 OH. COOH), the rate law has the form— dcS 2 O 8 2- /dt = k(S 2 O 8 2 -) 0,8 /(CH 2 OH.COOH) 0,2 .Product analysis reveals the C — C cleavage path to approximate only a very small fraction (≈ 7 %) of the overall oxidation. Mechanism for the uncleaved and the cleaved path ways is suggested.