R. N. Zitter

SATURATED OPTICAL ABSORPTION THROUGH BAND FILLING IN SEMICONDUCTORS

If a monochromatic light beam absorbed by a semiconductor generates enough carriers to fill band states up to and including those of the optical transition, the optical absorption will be saturated and a condition of transparency should result. Analysis shows that in this regime the absorption coefficient varies inversely with light intensity.

Infrared and Raman spectra of polyethylene oxide in the low frequency region

The far infrared and Raman spectra of polyethylene oxide (PEO) between 5 and 150 cm−1 has been measured for three crystalline samples with different degrees of polymerization. Lines are observed at 37, 52, 81, and 107 cm−1 in the infrared and at 37, 56, and 78 cm−1 in the Raman spectrum. Theoretical calculations show that the 81 and 107 cm−1 lines are assignable to the torsional motion of the PEO helical chain about the C–O bond. The absorptions at 37 and 52 cm−1 have not been previously reported or predicted by any normal mode calculations of the isolated 7/2 helix although they seem to arise from intramolecular rather than lattice vibrations.

Deformation of an electrorheological chain under flow

The simplest of all structures in an electrorheological fluid is a single chain of dielectric particles. The shape of a chain deformed by flow in a square tube is observed and is found to be reasonably consistent with a model of chain force based on point‐dipole nearest‐neighbor interactions.

Photoeffects in CdTe with acid−grown Te layers

Immersion of CdTe in dilute HNO3 produces a layer of tellurium whose thickness grows with immersion time. These Te−CdTe junctions rectify and are photosensitive. Spectral response measurements show (a) a flat band extending to photon energies of 1.5 eV, characteristic of interband transitions in CdTe, (b) a peak at 1.4 eV in CdTe without a Te layer which disappears as the layer is added, and (c) a peak at 0.59 eV observed only with a Te layer present. The 1.4−eV peak is consistent with reports from electroluminescence experiments, but there are no previous reports of an effect in CdTe with energy near 0.59 eV. The latter peak may result from states created at the Te−CdTe interface.

Current-generated spatially periodic space charge in semiconductors

A steady‐state current can generate significant regions of space charge in semiconductors. A first integral is given here for the nonlinear differential equations describing the process, which should be helpful in reducing computing time for calculations. A rigorous proof is presented to show that spatially oscillating charge distributions are produced when ωpτ ≳1/2 where ωp is the plasma frequency and τ is the average scattering time. For collisionless transport, a complete analytic solution is obtained showing perfect periodicity with properties determined by the injecting contact.

Current generated periodic space charge in semiconductor diodes with non‐Ohmic injection

Calculations are carried out which show minima or quasiminima in the voltage‐current characteristics of semiconductor diodes for values of the plasma frequency/scattering time product of the order of unity. In this regime the minima are evident only for non‐Ohmic injection conditions. The results are consistent with reported experiments.

Evidence of current‐generated spatially periodic space charge in a semiconductor

The voltage–current characteristics of pure p‐type germanium samples at 4.2 K show the features expected from theoretical models which predict that current generates a spatially periodic distribution of space charge, carrier velocity, and potential. The effects are observed at temperatures up to 68 K.

Magnetotransport in spatially periodic space‐charged semiconductors

A ballistic transport model which predicts the generation of spatially periodic space charge in a semiconductor by a steady‐state current is extended to include the effect of a transverse magnetic field. Analytic solutions are obtained showing how the magnetic field affects the periodicities of both longitudinal and transverse potentials.

Magnetic‐field effects on current‐generated spatially periodic potentials in p‐Ge at 4.2 K

Voltage‐current curves of pure p‐type germanium diodes at 4.2 K show sequences of minima which have been attributed to current‐generated spatially periodic space‐charge distributions along the current path. The effects of applied magnetic fields on the minima, as reported here, are found to be in good agreement with the periodic space‐charge model.

FALLING BALL EXPERIMENTS IN A DILUTE ELECTRORHEOLOGICAL FLUID

The velocity v of a glass ball falling through a dilute electrorheological fluid of silica gel particles in oil is measured as a function of applied electric field E for various particle sizes and concentrations. The fields used range up to the point where the ball ceases to fall. The results are consistent with a model using two types of field‐dependent retarding force, one proportional to E2 and the other proportional to vE2. The latter is proportional to both particle size and particle volume fraction, while the former is independent of particle size and is significant only above a volume fraction c0 where it is proportional to c−c0. The force in E2 is related to the yield stress and is attributed to chain breaking by the ball. The force in vE2 is attributed to transient chain deformations when the ball impacts chains.