Zinc oxide as a semiconductor

Chemists realize old dream

Hydrogen atoms release their electron into the conduction band of zinc oxide. In principle, such a doping is the basis for the use of ZnO as a semiconductor. The effect shown in the picture, however, is favorable only for the n-doping, it interferes with the equally important p-type doping. © RUB
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Zinc oxide is a multi-talent: its uses range from food additives to sunscreens. Even as a semiconductor, it is important, but the big breakthrough is still pending. Reason: The incorporation of special foreign atoms in the crystal lattice does not work perfectly. The reason for this is now Bochum chemists have come a decisive step closer. They proved in experiments that hydrogen atoms disturb the process.

The key to the routine use of ZnO as a semiconductor therefore lies in the control of the concentration of hydrogen in the production of high-purity zinc oxide, according to the researchers in the journal "Physical Review Letters".

Doping brings movement in the semiconductor

In the manufacture of semiconductor devices, it depends on the doping: the integration of foreign atoms in the crystal lattice of the solid. These either emit an electron (n-doping) or take up an electron and thus create a "hole" in the solid (p-doping). These mobile electrons or holes then provide the electrical conductivity of the otherwise insulating semiconductor - so the doping brings "movement" in the semiconductor.

What is standard with conventional semiconductor materials such as silicon or germanium, however, causes problems with zinc oxide. In particular, due to difficulties with the p-type doping, the production of semiconductor components such as transistors or light-emitting diodes has not hitherto been possible. This requires a pn junction, a boundary zone between p- and n-doped regions. Therefore, zinc oxide is currently used in the semiconductor field only for a few special applications.

Hydrogen is always in it

Significant progress has been made in recent years in the production of high purity zinc oxide. Recently, zinc oxide blue LEDs have been introduced. However, there are still significant problems with doping. Researchers at the Ruhr University Bochum headed by Professor Christof Wöll have now been able to identify a significant obstacle in the production of high-purity zinc oxide. display

In their experiments - motivated by the interest in the catalytic properties of ZnO - they were able to show that hydrogen atoms always lead to n-doping. They succeeded in reversibly doping zinc oxide substrates with hydrogen and completely removing the hydrogen by heating.

For the first time, they were able to comprehensively confirm theoretical predictions from the year 2000. The corresponding changes in the charge carrier concentration, essential for the function of electronic components manufactured from zinc oxide, could be detected by the researchers with a special technique by measurements at different temperatures.

Impurities with hydrogen - as a result of the manufacturing processes practically unavoidable - thus stand in the way of a targeted p-doping. The electrons emitted by the H atoms to the ZnO immediately fill up the holes generated by the p-doping. High purity, in particular the absence of hydrogen, therefore plays a crucial role in the production of high-purity zinc oxide.

Controversy ended

The researchers also ended a scientific controversy, because so far it was often postulated that the doping problems would be tempted by defects in the zinc oxide crystal lattice, by additional Zn atoms or oxygen vacancies. The results obtained in Bochum form the basis for the production of high-performance ZnO-based electronic components.

At present, the researchers are working hard to achieve a p-type doping by incorporating suitable foreign atoms in highly pure, ie hydrogen-free zinc oxide substrates.

The technique used, a special variant of electron spectroscopy, is normally used for a different purpose: the study of chemical processes on zinc oxide surfaces.

(idw - Ruhr-University Bochum, 29.12.2008 - DLO)