It is even significant as semiconductor, although the great breakthrough in this field is yet to come. Perfect doping is not yet possible. A team of chemical scientists at the Ruhr-University in Bochum, working under the auspices of Prof. Christof Wöll, is a step closer to unveiling the reason. They were experimentally able to provide evidence that hydrogen atoms disturb the process.
Controlled concentration of hydrogen atoms during the production of intrinsic zinc oxide is thus the key to the routine use of ZnO as semiconductor. The scientists have documented their results in the journal Physical Review Letters.
Doping activates the semiconductor
Doping, i.e. the insertion of specific foreign atoms into the crystal lattice of a solid, is the most important factor during the production of semiconductor devices. These foreign atoms either release an electron (n-doping), or absorb an electron, thus creating a "hole" in the solid (p-doping). These mobile electrons or holes then bring about the electric conductivity of the otherwise isolated semiconductor, i.e. doping initiates "action" in the semiconductor. This standard process in the manufacturing of conventional semiconductors e.g. silicon or germanium has however been problematic for zinc oxide to date. In particular, it has been difficult to achieve p-doping, which made it impossible to construct semiconductor devices such as transistors or light emitting diodes (LEDs). Such devices require a pn-transition, a junction between the p-doped and the n-doped zones. Thus, in the field of semiconductors, zinc oxide is at present only used for a few special applications.
Hydrogen is always present
There has been a significant improvement in the production of intrinsic zinc oxide during the past few years. Blue LEDs made of zinc oxide have only been presented recently. There are, however, still numerous problems concerning doping. Research scientists at the Ruhr-University in Bochum have been able to identify a significant obstacle in the production of intrinsic zinc oxide. During experiments, which had actually been motivated by an interest in the catalytic properties of ZnO, they were able to show that hydrogen atoms always result in n-doping. They could reversibly dope zinc oxide substrates using hydrogen and then eliminate the hydrogen by heating. The scientists were thus able to verify theoretical predictions made in 2000. They used a special technique for measurements at diverse temperatures to verify the corresponding charge carrier concentrations. A sufficiently high density of these charge carriers is essential for the proper functioning of electronic devices. Hydrogen impurities are almost impossible to avoid during the production process, thus preventing the targeted p-doping. The electrons released from the H atoms to the ZnO immediately fill the holes caused by the p-doping. High purity, in particular a hydrogen-free environment, is thus a decisive factor for the production of intrinsic zinc oxide.
Controversy is history
The research team was thus also able to resolve a scientific controversy: to date it has often been postulated that the doping problems are caused by imperfections in the zinc oxide crystal lattice, by additional Zn atoms or oxygen defects. The results obtained in Bochum are a basis for the production of higher performance ZnO-based electronic circuits. Currently the scientists are doing intensive research to attain p-doping with an intrinsic, i.e. hydrogen-free, zinc oxide substrate by incorporating appropriate foreign atoms.
Technology actually intended for another purpose
The technology used, a special version of electron spectroscopy, is normally used for another purpose, namely for the investigation of chemical processes on the surfaces of zinc oxides. Such phenomena have been investigated at the Ruhr-University in Bochum for many years within the frameworks of SFB 558 (Focus Research Centre - Metal-Substrate Interactions in Heterogeneous Catalysis) due to the significance of zinc oxide for heterogeneous catalysis, particularly for the synthesis of methanol.
H. Qiu, B. Meyer, Y. Wang, Ch. Wöll: Ionization energies of shallow donor states in ZnO created by reversible formation and depletion of H interstitials. In: Physical Review Letters 101, 236401 (2008), DOI: 10.1103/PhysRevLett.101.236401
Prof. Christof Wöll, Department of Physical Chemistry I at the Ruhr-University Bochum, D-44780 Bochum, Germany, Tel: +49 (0) 234/32-25529, Fax: +49 (0) 234/32-14182, E-Mail: email@example.com
Dr. Josef König | idw
Silicon solar cell of ISFH yields 25% efficiency with passivating POLO contacts
08.12.2016 | Institut für Solarenergieforschung GmbH
Robot on demand: Mobile machining of aircraft components with high precision
06.12.2016 | Fraunhofer IFAM
In recent years, lasers with ultrashort pulses (USP) down to the femtosecond range have become established on an industrial scale. They could advance some applications with the much-lauded “cold ablation” – if that meant they would then achieve more throughput. A new generation of process engineering that will address this issue in particular will be discussed at the “4th UKP Workshop – Ultrafast Laser Technology” in April 2017.
Even back in the 1990s, scientists were comparing materials processing with nanosecond, picosecond and femtosesecond pulses. The result was surprising:...
Have you ever wondered how you see the world? Vision is about photons of light, which are packets of energy, interacting with the atoms or molecules in what...
A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.
Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent...
In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: The atoms form a new type of quantum liquid or quantum droplet state. These so called quantum droplets may preserve their form in absence of external confinement because of quantum effects. The joint team of experimental physicists from Innsbruck and theoretical physicists from Hannover report on their findings in the journal Physical Review X.
“Our Quantum droplets are in the gas phase but they still drop like a rock,” explains experimental physicist Francesca Ferlaino when talking about the...
The Max Planck Institute for Physics (MPP) is opening up a new research field. A workshop from November 21 - 22, 2016 will mark the start of activities for an innovative axion experiment. Axions are still only purely hypothetical particles. Their detection could solve two fundamental problems in particle physics: What dark matter consists of and why it has not yet been possible to directly observe a CP violation for the strong interaction.
The “MADMAX” project is the MPP’s commitment to axion research. Axions are so far only a theoretical prediction and are difficult to detect: on the one hand,...
16.11.2016 | Event News
01.11.2016 | Event News
14.10.2016 | Event News
07.12.2016 | Health and Medicine
07.12.2016 | Life Sciences
07.12.2016 | Health and Medicine