Tikalon Blog by Dev Gualtieri

Persistent Photoconductivity

November 27, 2013

Selenium would have been an uncommon material were it not for two electrical properties discovered in the very early days of electronics . The most useful of these is its ability to act as a type of rectifier called a metal rectifier . This rectifier action (changing alternating current to direct current ) was discovered by Charles Fritts , circa 1886, but such rectifiers were only made practical in the 1930s .

The selenium rectifier elements, looking like the fins of today's semiconductor heat sinks (see photograph), were aluminum or steel plates, coated with a thin layer of bismuth or nickel , and then coated with a thick (~0.0025 inch ) layer of selenium. Heat treatment ensured conversion of the selenium to its gray, hexagonal phase .

A selenium rectifier with inch square plates. Selenium rectifiers were common when I first started experimenting with electronics in the early 1960s.

(Photograph by Arnold Reinhold, viaWikimedia Commons.)

Each selenium element had a breakdown voltage of about twenty-five volts , but putting many in series gave a high voltage rectifier. Each selenium element had a voltage drop of about a volt, compared with about 0.7 volts for silicon rectifiers . Selenium rectifiers were useful for high current rectification, as for automotive battery chargers ; but the voltage drop of multiple series connections , combined with the high current, resulted in a hot rectifier (up to 130°C ) and a service life of about 50,000 hours.

The other useful property of selenium is its photoconductivity , which was discovered in 1873 by the English electrical engineer , Willoughby Smith . As often happens in science , this discovery was accidental . Smith was doing electrical measurements on submarine telegraph cables , and he found that the selenium resistance reference material he was using became more conductive upon exposure to light .[1]

Complementary to selenium's photoconductivity was the 1877 discovery of the photovoltaic effect in selenium by W.G. Adams and R.E. Day.[2] Charles Fritts used this effect to make a selenium-gold solar cell in 1883.[3] This solar cell had less than 1% efficiency , but it was an innovation, nonetheless. The best photovoltaic cells of today have better than 35% efficiency, but residential panels have far less than that .

Selenium's photoconductivity was crucial to the 1938 invention of Xerography by Chester Carlson . Carlson found that a plate coated with "cuprous oxide or the metallic variety of selenium," would lose charge in the areas exposed to light. Thus, an image of a printed page would still have charge at the black letters, and none from the white background sheet. A fine, black powder, sticking to the charged surface, would form a copy of the image.[4]

The photoconductivity of selenium was a surprise, and you would think that few such surprises would still exist for simple materials. Strontium titanate (SrTiO₃) was first synthesized by Leon Merker at the National Lead Company , and a process for its synthesis was patented by Merker in 1956.[5] Now, more than fifty years later, physicists at Washington State University (Pullman, Washington) have discovered persistent photoconductivity in strontium titanate.[6-8]

Persistent photoconductivity means that shining a light on the material increases the conductivity not just for the period of light exposure, but for a significant time thereafter. In the case of strontium titanate, this time period is many days, which means that it would be possible to make an optical memory device using this effect.[7]

Mounting a strontium titanate specimen (white square) for measurement.

(Still image from a YouTube video.[8])

Washington State University doctoral student , Marianne Tarun , discovered the effect. A research team comprised of Tarun, fellow student, Farida Selim , now at Bowling Green State University , and professor Matthew McCluskey , found through experiments that the effect was not from some dopant , but it was an effect of light on pure strontium titanate.[7] Said Tarun, "It came by accident... It's not something we expected. That makes it very exciting to share."[7]

Exposing strontium titanate at room temperature to light energy at its intrinsic bandgap of 2.9 eV or higher increases the free-electron concentration by more than two orders of magnitude .[6] The conductivity jumps by a factor of 400, albeit from a low initial level.[7]

The higher conductivity remains for days with negligible decay, and the authors speculate that the persistent photoconductivity is due to excitation of electrons from a titanium vacancy defect to the conduction band , with subsequent electron recapture by the vacancy having a very low probability .[6]

Measuring photoconductivity of a strontium titanate specimen.

(Still image from a YouTube video.[8])

Says paper co-author, Matthew McCluskey, alluding to the possibility of using strontium titanate as a holographic memory ,

“The discovery of this effect at room temperature opens up new possibilities for practical devices. In standard computer memory, information is stored on the surface of a computer chip or hard drive. A device using persistent photoconductivity, however, could store information throughout the entire volume of a crystal.”[7]

The Washington State University research was funded by the National Science Foundation .[7]

Persistent Photoconductivity

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