THE annoying flicker of fluorescent lights could be replaced by the cool white glow of zinc oxide LEDs, now that a US company has found a cheap way to crystallise the oxide from a solution.
White light-emitting diodes are usually made from crystals of the semiconductor gallium nitride grown on a sapphire base. But these crystals often develop flaws as they grow, and many end up unusable. The manufacturing process is so inefficient that the LEDs that do work are far too expensive for lighting homes or offices.
However, it is possible to use pure zinc oxide semiconductor crystals instead of the gallium and sapphire. The only problem is that you need to heat zinc oxide to thousands of degrees Celsius in order to crystallise it from its molten state, and that鈥檚 even more expensive.
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Now Larry McLandish, head of Ceramare Corporation in New Jersey, says his company can grow high-quality zinc oxide crystals from a solution at just 350 掳C, slashing the cost. McClandish says the trick is a patent-pending additive that helps to dissolve the zinc oxide in an alkaline solution, so that it can then be crystallised. He described the research this week at the 5th International Solvothermal Conference in East Brunswick, New Jersey.
Growing decent crystals is only the first step, however. LEDs need a junction of two types of semiconductor 鈥� positive 鈥減-types鈥� which lack electrons, and negative 鈥渘-types鈥� which have a surplus. Applying a voltage across the junction persuades electrons to cross and give up some of their energy in the form of light.
Zinc oxide crystals are normally only n-type, but Gang Xiong and a team at Wake Forest University in Winston-Salem, North Carolina, reported making p-type zinc oxide for the first time last year. Xiong says his p-type crystals aren鈥檛 yet good enough to produce light at a junction, but Ceramare鈥檚 new way of making zinc oxide crystals should help, as long as the two forms are compatible.