discovered_01_2015

FOCUS// THE HZDR RESEARCH MAGAZINE WWW.HZDR.DE 04 05 Cured crystals In order to insert foreign atoms into the semiconductors, Shengqiang Zhou and his team generate electrically charged atoms, accelerate these ions with an electric voltage and shoot them into the semiconductor. All over the world, semiconductors are ‘doped’ this way, as electronics experts call the process. Except that normally, the semiconductor gallium-arsenide is doped with carbon or tellurium, while HZDR researchers are using manganese. Like iron, manganese belongs to the group of ‘transitional metals’. First, the physicist directs a beam of manganese ions, accelerated to one hundred kilo-electron volts, onto a gallium-arsenide crystal in a high vacuum. The surface of the semiconductor is loaded with manganese atoms at a depth of a mere ten thousandth of a millimeter – one hundred nanometers. However, in addition to this intentional doping, the rather brutal bombardment of the crystal surface also damages the structure – up to the point where the semiconductor stops working. So HZDR researchers then proceed to shoot the material with a laser, using short light pulses that flash for a mere 20 to 30 billionths of a second (20 to 30 nanoseconds). This is sufficient to melt the top one hundred nanometers while the crystal underneath remains intact. Also, the surface does not remain liquid for very long. After about one hundred nanoseconds, the melted material re-crystallizes, supported by the intact crystal underneath. The surface that had been damaged by the ion beam heals, while some of the gallium atoms are replaced by manganese. Electronic and magnetic switches Just like any atom, manganese consists of an atomic nucleus that is embedded in shells of electrons. Electrons not only have an electric charge, but also a ‘spin’. This quantum- mechanical property acts like an intrinsic momentum that either rotates towards the right or the left. Manganese, like iron, has an incomplete inner shell, which is why each atom carries a net spin. These net spins of nearby atoms communicate with one another, making the material magnetic. A similar phenomenon occurs in the manganese-doped gallium-arsenide semiconductor that Shengqiang Zhou produced with the help of ion beams. Each manganese atom generates a spot of positive electronic charge, which in semiconductor technology is called a ‘hole’. These holes move, which makes the manganese atoms interact with one another. This way, the material can become ferromagnetic. ‘You can control the magnetic properties with an electric field, just like the logic circuits in a computer processor,’ Shengqiang Zhou is happy to report. This kind of magnetic semiconductor could revolutionize the electronics industry. The journey will be long. So far, the dual material only works at low temperatures of -100 degrees Celsius. ‘Next, we want to try to produce the same material in such a way that it can work in a computer at room temperature,’ the HZDR physicist explains. ION MACHINE: Low-energy ions knock atoms out of their normal positions in the crystal lattice. Photo: Claus Preußel

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