D. L. Staebler

Reversible conductivity changes in discharge‐produced amorphous Si

A new reversible photoelectronic effect is reported for amorphous Si produced by glow discharge of SiH4. Long exposure to light decreases both the photoconductivity and the dark conductivity, the latter by nearly four orders of magnitude. Annealing above 150 °C reverses the process. A model involving optically induced changes in gap states is proposed. The results have strong implications for both the physical nature of the material and for its applications in thin‐film solar cells, as well as the reproducibility of measurements on discharge‐produced Si.

Optically induced conductivity changes in discharge‐produced hydrogenated amorphous silicon

Long exposure to light decreases the photoconductivity and dark conductivity of some samples of hydrogenated amorphous silicon (a‐Si : H). Annealing above ∼150 °C reverses the process. The effect occurs in the bulk of the films, and is associated with changes in density or occupation of deep gap states. High concentrations of P, B, or As quench the effect. Possible models involving hydrogen bond reorientation at a localized defect or electron‐charge transfer between defects are discussed. An example is shown where these conductivity changes do not affect the efficiency of an a‐Si : H solar cell.

Coupled‐Wave Analysis of Holographic Storage in LiNbO3

This paper considers two effects for directly studying thick‐phase holograms: (a) coupling between the two laser beams used to record a hologram and (b) interference between a readout beam and the diffracted beam within the hologram. Both effects were observed in experiments using single crystals of undoped LiNbO3. The results demonstrate that the holograms arise from electric field patterns caused by either diffusion or drift of photogenerated free electrons.

HOLOGRAPHIC PATTERN FIXING IN ELECTRO‐OPTIC CRYSTALS

This paper describes the results of an investigation into techniques for obtaining erasure resistant holograms in electro‐optic crystals. The most successful approach made use of thermally activated ionic drift during or after recording. The samples are heated for about 30 min at 100°C to obtain optically nonerasable holograms with as much as 50% diffraction efficiency in LiNbO3 or in doped Ba2NaNb5O15.

Multiple storage and erasure of fixed holograms in Fe−doped LiNbO3

Holograms were recorded and fixed simultaneously in heated (∼160°C) crystals of Fe−doped LiNbO3. With this procedure the crystals have the erase/write asymmetry required for multiple storage of high−diffraction−efficiency holograms. Five hundred fixed holograms, each with more than 2.5% diffraction efficiency, were recorded.

Stability of n‐i‐p amorphous silicon solar cells

Unencapsulated, amorphous silicon indium tin oxide/n‐i‐p/stainless‐steel solar cells were tested for stability. All cells have excellent shelf life. Changes occur during exposure to light, but can be controlled by the deposition conditions of the amorphous silicon. The changes are due to trapping and recombination of optically generated carriers in the i layer, and are reversibly annealed out above 175 °C. Preliminary life tests on two relatively stable cells showed a small initial drop to 5%, followed by a weak logarithmic decay that predicts only ∼20% further decrease in efficiency after 20 years in sunlight. Work is continuing on improving the efficiency and stability of these cells.

Improved electrooptic materials and fixing techniques for holographic recording.

This paper describes recent improvements in materials and techniques for storage of high efficiency phase holograms in electrooptic crystals. The storage performance of lithium niobate and barium sodium niobate was greatly enhanced by introducing transition metal impurities or by subjecting the undoped crystals to irradiation treatments. The latest materials combine good sensitivity with diffraction efficiencies that reach well over 50% for sample thickness of a few millimeters. In addition, the fixing techniques that were developed offer a simple means of achieving nondestructive readout for holographic information stored in these crystals.

Fe-Doped LiNbO 3 for Read–Write Applications

High erase sensitivity is observed in heavily reduced crystals of Fe-doped LiNbO(3). Only 12 mJ/cm(2) of incident 4880-A radiation erases a hologram, nearly 3 orders of magnitude less energy than previously required. However, the maximum diffraction efficiency that can be reached in these crystals is substantially reduced. These results are shown to be consistent with an extremely low density of Fe(3+) ions. The crystals are resistant to optical scattering effects usually observed in Fe-doped LiNbO(3).

Hologram storage in photochromic LiNbO3

The incorporation of Mn in Fe‐doped LiNbO3 leads to photochromic behavior involving photoreversible conversion of Fe3+ to Fe2+. This process can be used to advantage to control the holographic storage sensitivity of the material.

HOLOGRAPHIC STORAGE IN DOPED BARIUM SODIUM NIOBATE (Ba2NaNb5O15)

This paper describes the results obtained using doped crystals of Ba2NaNb5O15 for phase holographic storage. Very short‐lived holograms with less than 1% diffraction efficiencies were obtained in nominally pure crystals. Doping with Fe and Mo yields holograms with much longer decay times and diffraction efficiencies of 67% in a 0.32‐cm‐thick crystal. The energy density required to reach 40% diffraction efficiency was 6 J/cm2.