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Text | Epitaxial growth and properties of cobalt-doped ZnO on -Al2O3 single-crystal substrates | 001
PHYSICAL REVIEW B 70, 054424 (2004)
Epitaxial growth and properties of cobalt-doped ZnO on -Al2O3 single-crystal substrates
A. C. Tuan,1 J. D. Bryan,2 A. B. Pakhomov,3 V. Shutthanandan,4 S. Thevuthasan,4 D. E. McCready,4 D. Gaspar,4 M. H. Engelhard,4 J. W. Rogers, Jr.,4 K. Krishnan,3 D. R. Gamelin,2 and S. A. Chambers5,*
1Department of Chemical Engineering, University of Washington, Seattle, Washington, USA
2Department of Chemistry, University of Washington, Seattle, Washington, USA
3Department of Materials Science and Engineering, University of Washington, Seattle, Washington, USA 4Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, USA 5Fundamental Science Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, USA
(Received 19 August 2003; revised manuscript received 21 May 2004; published 30 August 2004)
Co-doped ZnO CoxZn1−xO is of potential interest for spintronics due to the prediction of room-temperature ferromagnetism. We have grown epitaxial CoxZn1−xO films on Al2O3 012 substrates by metalorganic chemi- cal vapor deposition using a liquid precursor delivery system. High concentrations of Co x 0.35 can be uniformly incorporated into the film without phase segregation. Co is found to be in the +2 oxidation state, independent of x, by both surface-sensitive core-level x-ray photoemission and bulk-sensitive optical absorp- tion spectroscopies. This material can be grown n-type by the deliberate incorporation of oxygen vacancies, but not by inclusion of 1 at. % Al. Semiconducting films remain ferromagnetic up to 350 K. In contrast films without oxygen vacancies are insulating and nonmagnetic, suggesting that exchange interaction is mediated by itinerant carriers. The saturation and remanent magnetization on a per Co basis was very small 0.1 B / Co , even in the best films. The dependence of saturation magnetization, as measured by optical magnetic circular dichroism, on magnetic field and temperature, agrees with the theoretical Brillouin function, demonstrating that the majority of the Co II ions behave as magnetically isolated S=3/2 ions.
With silicon-based electronics approaching fundamental fabrication and performance limits, the use of quantum me- chanical spin states in semiconductor devices represents an exciting new concept. For example, the additional degree of freedom provided by spin would enable the development of spin-transistors, high performance nonvolatile memories, and polarized light emitting devices. In the emerging field of quantum computing and communication, the electron spin is a natural candidate for the qubit–the fundamental unit of quantum information.1 Loss and Vincenzo show that these spin qubits, when located in quantum-confined structures, satisfy all the requirements for a scalable quantum computer.2 These quantum-confined structures include semi- conductor quantum wells and quantum dots with a wide range of dimensions. The common goal of both analog and digital spintronics is to gain control over and effectively utilize the dynamics of spins in semiconductor device structures.
Ferromagnetism provides an ideal way to achieve the spin polarization necessary for realization of spintronic technolo- gies, provided the spin injection material remains ferromag- netic at or above room temperature and is capable of high efficiency spin injection into semiconductor heterostructures. Given these requirements, ferromagnetic metals are very at- tractive because they have Curie temperatures that are well above room temperature. However, ferromagnetic metals are typically inefficient spin injectors, exhibiting spin injection efficiencies on the order of 1%.3 This low efficiency is due to the sizeable conductivity mismatch that exists at the metal-semiconductor interface. A simple resistor model has
PACS number(s): 73.61.Le
been proposed to explain this phenomenon and reveals that the spin polarization is proportional to the ratio of the resis- tances in the metal and the semiconductor. One way to cir- cumvent the conductivity mismatch issue has been shown experimentally by Hanbicki et al.4 By using an insulating AlGaAs Schottky barrier between Fe and a GaAs-AlGaAs quantum well structure, these workers demonstrated 30% spin injection efficiency at room temperature.
A superior approach, in principle, is to use a diluted mag- netic semiconductor (DMS) whose conductivity has been tuned (by altering the dopant level) to match that of the channel material. For example, BeyMnxZn1−x−ySe and Mn,Zn Se have been utilized as efficient spin injectors, with spin polarizations between 40% and 50%, into Al , Ga As-GaAs- Al , Ga As quantum well structures.5,6 However, these materials are Brillouin paramagnets, and the spin polarization effect can be maintained only with an ex- ternal magnetic field. Thus, DMS materials that are strongly ferromagnetic at and above room temperature are highly de- sirable. Indeed, a few unconventional semiconductors have exhibited room temperature ferromagnetism. These include MnxGa1−xN,7 CoxTi1−xO2,8 and CoxZn1−xO.9
Interest in ZnO-based DMSs was initially generated by the theoretical work of Dietl and coworkers. Their calcula- tions showed that both Mn-doped ZnO and Mn-doped GaN would exhibit above-room-temperature ferromagnetism if the materials were grown with substitutional Mn+2 ions and sufficiently high levels of some p-type dopant 3 1020 atoms / cm3 .10 Recently, Mn-doped GaN films with Curie temperatures greater than 300K have been synthesized,7 while Mn-doped ZnO has been shown to be ferromagnetic with a Curie temperature of 250 K.11
1098-0121/2004/70(5)/054424(9)/$22.50 70 054424-1 ©2004 The American Physical Society
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