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Non-equilibrium response of MOS devices to a linear voltage ramp—1. Bulk discrete traps
360
World Abstracts on Microelectronics and Reliability
trum of Si doped with B and Ga are reported. These lines are interpreted as two-hole bound exciton transitions which leave the acceptor in an even parity excited state. The energies of some of these even-parity acceptor excited states are reported here for the first time. The energies of these newly identified levels will provide another test of the recently improved calculations of the acceptor energy levels. Dielectric anomaly in amorphous Sin~0 .~Au~ system. T. SANO, M. IWAMI, A. HIRAKI and K. MORIGAKI. Solid St. Comnlu~ts 23, 971 (1977). In amorphous Si~00 ~Au., system the dielectric constant is decreasing with increasing frequency or with decreasing temperature at x ~ 5.5'~,, This is explained by considering the dispersion of the variable range hopping contribution to the dielectric constant. The dielectric constant at sufficiently high frequency or at sufficiently low temperature is increasing rapidly with increasing x in the region x < 5.5~, which predicts the occurrence of the nonmetal-metal transition. Ferromagnetic interaction between spins in amorphous Si-Ge flints. S. HASEGAWA, S. YAZAKI and T. SHIMIZU. Solid St. Communs 23, j 901 (1977). Temperature dependence of the ESR signal intensity in amorphous Si-Ge films annealed at various temperatures is investigated. As a result, it is found that spins undergo a ferromagnetic interaction and the Curie-Weiss O in the relation X = C/(T - O) distributes between 40 and 135 K. The ferromagnetic behavior suggests that spins are inhomogeneously distributed and closely located together. Photo-excitation of electrons from ionized silver acceptors in silicon. P. MIGLIORATO, C. T. ELLIOT and A. W. WERE. Solid St. Communs 24, 117 (1977). We report the investigation of transient extrinsic photoconductivity, involving the excitation of electrons from ionized silver acceptors in silicon. The photoconductivity of our samples is characterized by very long (several seconds) rise and decay times. A theoretical model is proposed, whereby we obtain a good fitting of the experimental data and derive the values of the optical and thermal cross-section for the silver acceptor level. Electronic behaviours of the gap states in amorphous semiconductors. H. OKAMOTO and Y. HAMAKAWA. Solid St. Communs 24, 23 (1977). An extention of the Adler-~Yoffa's calculation on the localized gap states in amorphous semiconductors has been made with a more realistic model which takes into account an influence of the diffused gap states and impurity states. A considerable large difference in the electron occupation probability of the gap states with sign of the correlation energy U has been examined on the tetrahedrally bonded and chalcogenide glasses. Variations of the Fermi level as a function of doped impurity concentration and gap state density have been studied. Nucleation of electron-hole drops in Si. J. SHAH, A. H. DAYEM and M. COMBESCOT. Solid St. Communs 24, k71 (1977). Time resolved luminescence spectroscopy in Si, for pump intensities very close to threshold for electron-holedrops (EHD) formation, shows that optically excited carriers first form excitons which then begin to form E H D after a delay. The origin of this delay has been determined. The measured time of formation of E H D varies by approximately four orders of magnitude for a 30°~i change in pump intensity, providing a direct confirmation of the predictions of the nucleation theory. Effect of heat treatment on electron traps in n-type GaAs. S. SUBRAMANIAN,S. GUHA and B. M. ARORA. Solid-St. Electron. 20, 799 (1977). Annealing of bulk and vapor phase epitaxial (VPE) n-type GaAs at about 600°C for one hour in an apparatus of fused quartz (GE 204 or vitrosil) is seen to cause considerable reduction in the free electron concen-
tration (Ns) and the commonly observed 0.85 eV electron traps (Nr) in these materials. Heating at elevated temperatures in the same environment causes an n-type sample to convert to p-type. The reduction in Ns and Nr is found to be considerably smaller for comparable heat treatments in a vacuum system free from quartz, tt is suggested that the observed reduction is caused by in-diffusion of impurities like Cu and Li which are present in the types of quartz used in the experiments. Non-equilibrium response of MOS devices to a linear voltage ramp--1. Bulk discrete traps. K. BOARD and J. G. SIMMONS. Solid-St. Electron. 20, 859 (1977). A study is presented of MOS devices containing discrete bulk traps, subjected to a triangular voltage waveform, such that the rate of change of voltage is sufficiently high to take the device into the nonequilibrium mode of operation. Consequently, the dynamics of the system response are related to the parameters of the traps involved in the generation and recombination processes occurring within the device. The technique is in contrast with Kuhn's method in which the device, and hence the various generation and recombination processes; are always in quasi-equilibrium Analytical parametric equations relating the current, and hence the small-signal capacitance and the gate voltage to the width of the depletion region, are obtained. From these equations current and small-signal capacitance vs gate voltage plots are obtained as a function of sweep rate and temperature, for both forward and reverse voltage sweeps. These plots are rich in structure. and physical discussion relating to the salient features of these plots is presented. Suggestions aremade of how various device parameters such as generation rate and life-time, trap density, and capture cross-section, can be extracted from the theory. Thermionic-field emission through silicon Schottky barriers at room temperature. J. M. SHANNON. Solid-St. Electron. 20, 869 (1977). Current transport via thermionic-fie!d emission through Ni Si diodes having implanted surface layers has been studied for surface fields between l() ~ and l0 s V m ~ ~. The well defined potential profile in these structures enables a close comparison to be made between the calculated and measured thermionic-field emission currents and it is shown that the change of reverse current with field can be accurately predicted using the appropriate electron effective mass in the direction of current flow. For typical barrier heights, the field dependence of the electron current through a triangular barrier with image force correction is very nearly exponential with an effective tunnelling distance between 25 and 30 A at 300 K. Single crystal silicon photocells for terrestrial use: state of the art and future prospects. J. MtCHEL Acta Electronica 20, (2) 133 (1977). The main drawback of the single crystal silicon photocells with a P/N junction is their cost which means that the electric energy thus produced is expensive. The aim of this paper is to show that some solutions can be found to reduce this cost. Firstly an industrial fabrication process for the photocells is described with reference to its limitations and the problems encountered. Secondly the possibilities of reducing the panel cost are investigated: increase in the diameter of the silicon ingot and a decrease in the thickness of the silicon used for each wafer: modifications of the realization process with respect to the N ' layer, the metallization deposit and the antireflective coating; optimization of encapsulation by increasing photocell density and panel surface. Another means of reducing the cost of the energy produced is to increase the energy conversion efficiency by improving the short-circuit current (better absorption and greater spectral sensitivity) and the open-circuit voltage. In conclusion, some data are provided for an economic comparison between single crystal silicon photocells and photocells made from less expensive base material (especially polycristalline silicon).
World Abstracts on Microelectronics and Reliability
trum of Si doped with B and Ga are reported. These lines are interpreted as two-hole bound exciton transitions which leave the acceptor in an even parity excited state. The energies of some of these even-parity acceptor excited states are reported here for the first time. The energies of these newly identified levels will provide another test of the recently improved calculations of the acceptor energy levels. Dielectric anomaly in amorphous Sin~0 .~Au~ system. T. SANO, M. IWAMI, A. HIRAKI and K. MORIGAKI. Solid St. Comnlu~ts 23, 971 (1977). In amorphous Si~00 ~Au., system the dielectric constant is decreasing with increasing frequency or with decreasing temperature at x ~ 5.5'~,, This is explained by considering the dispersion of the variable range hopping contribution to the dielectric constant. The dielectric constant at sufficiently high frequency or at sufficiently low temperature is increasing rapidly with increasing x in the region x < 5.5~, which predicts the occurrence of the nonmetal-metal transition. Ferromagnetic interaction between spins in amorphous Si-Ge flints. S. HASEGAWA, S. YAZAKI and T. SHIMIZU. Solid St. Communs 23, j 901 (1977). Temperature dependence of the ESR signal intensity in amorphous Si-Ge films annealed at various temperatures is investigated. As a result, it is found that spins undergo a ferromagnetic interaction and the Curie-Weiss O in the relation X = C/(T - O) distributes between 40 and 135 K. The ferromagnetic behavior suggests that spins are inhomogeneously distributed and closely located together. Photo-excitation of electrons from ionized silver acceptors in silicon. P. MIGLIORATO, C. T. ELLIOT and A. W. WERE. Solid St. Communs 24, 117 (1977). We report the investigation of transient extrinsic photoconductivity, involving the excitation of electrons from ionized silver acceptors in silicon. The photoconductivity of our samples is characterized by very long (several seconds) rise and decay times. A theoretical model is proposed, whereby we obtain a good fitting of the experimental data and derive the values of the optical and thermal cross-section for the silver acceptor level. Electronic behaviours of the gap states in amorphous semiconductors. H. OKAMOTO and Y. HAMAKAWA. Solid St. Communs 24, 23 (1977). An extention of the Adler-~Yoffa's calculation on the localized gap states in amorphous semiconductors has been made with a more realistic model which takes into account an influence of the diffused gap states and impurity states. A considerable large difference in the electron occupation probability of the gap states with sign of the correlation energy U has been examined on the tetrahedrally bonded and chalcogenide glasses. Variations of the Fermi level as a function of doped impurity concentration and gap state density have been studied. Nucleation of electron-hole drops in Si. J. SHAH, A. H. DAYEM and M. COMBESCOT. Solid St. Communs 24, k71 (1977). Time resolved luminescence spectroscopy in Si, for pump intensities very close to threshold for electron-holedrops (EHD) formation, shows that optically excited carriers first form excitons which then begin to form E H D after a delay. The origin of this delay has been determined. The measured time of formation of E H D varies by approximately four orders of magnitude for a 30°~i change in pump intensity, providing a direct confirmation of the predictions of the nucleation theory. Effect of heat treatment on electron traps in n-type GaAs. S. SUBRAMANIAN,S. GUHA and B. M. ARORA. Solid-St. Electron. 20, 799 (1977). Annealing of bulk and vapor phase epitaxial (VPE) n-type GaAs at about 600°C for one hour in an apparatus of fused quartz (GE 204 or vitrosil) is seen to cause considerable reduction in the free electron concen-
tration (Ns) and the commonly observed 0.85 eV electron traps (Nr) in these materials. Heating at elevated temperatures in the same environment causes an n-type sample to convert to p-type. The reduction in Ns and Nr is found to be considerably smaller for comparable heat treatments in a vacuum system free from quartz, tt is suggested that the observed reduction is caused by in-diffusion of impurities like Cu and Li which are present in the types of quartz used in the experiments. Non-equilibrium response of MOS devices to a linear voltage ramp--1. Bulk discrete traps. K. BOARD and J. G. SIMMONS. Solid-St. Electron. 20, 859 (1977). A study is presented of MOS devices containing discrete bulk traps, subjected to a triangular voltage waveform, such that the rate of change of voltage is sufficiently high to take the device into the nonequilibrium mode of operation. Consequently, the dynamics of the system response are related to the parameters of the traps involved in the generation and recombination processes occurring within the device. The technique is in contrast with Kuhn's method in which the device, and hence the various generation and recombination processes; are always in quasi-equilibrium Analytical parametric equations relating the current, and hence the small-signal capacitance and the gate voltage to the width of the depletion region, are obtained. From these equations current and small-signal capacitance vs gate voltage plots are obtained as a function of sweep rate and temperature, for both forward and reverse voltage sweeps. These plots are rich in structure. and physical discussion relating to the salient features of these plots is presented. Suggestions aremade of how various device parameters such as generation rate and life-time, trap density, and capture cross-section, can be extracted from the theory. Thermionic-field emission through silicon Schottky barriers at room temperature. J. M. SHANNON. Solid-St. Electron. 20, 869 (1977). Current transport via thermionic-fie!d emission through Ni Si diodes having implanted surface layers has been studied for surface fields between l() ~ and l0 s V m ~ ~. The well defined potential profile in these structures enables a close comparison to be made between the calculated and measured thermionic-field emission currents and it is shown that the change of reverse current with field can be accurately predicted using the appropriate electron effective mass in the direction of current flow. For typical barrier heights, the field dependence of the electron current through a triangular barrier with image force correction is very nearly exponential with an effective tunnelling distance between 25 and 30 A at 300 K. Single crystal silicon photocells for terrestrial use: state of the art and future prospects. J. MtCHEL Acta Electronica 20, (2) 133 (1977). The main drawback of the single crystal silicon photocells with a P/N junction is their cost which means that the electric energy thus produced is expensive. The aim of this paper is to show that some solutions can be found to reduce this cost. Firstly an industrial fabrication process for the photocells is described with reference to its limitations and the problems encountered. Secondly the possibilities of reducing the panel cost are investigated: increase in the diameter of the silicon ingot and a decrease in the thickness of the silicon used for each wafer: modifications of the realization process with respect to the N ' layer, the metallization deposit and the antireflective coating; optimization of encapsulation by increasing photocell density and panel surface. Another means of reducing the cost of the energy produced is to increase the energy conversion efficiency by improving the short-circuit current (better absorption and greater spectral sensitivity) and the open-circuit voltage. In conclusion, some data are provided for an economic comparison between single crystal silicon photocells and photocells made from less expensive base material (especially polycristalline silicon).