The search for ferromagnetism above room temperature in dilute magnetic semiconductors has been intense in recent years. We report the first observations of ferromagnetism above room temperature for dilute (<4 at.%) Mn-doped ZnO. The Mn is found to carry an average magnetic moment of 0.16 mu(B) per ion. Our ab initio calculations find a valance state of Mn(2+) and that the magnetic moments are ordered ferromagnetically, consistent with the experimental findings. We have obtained room-temperature ferromagnetic ordering in bulk pellets, in transparent films 2-3 microm thick, and in the powder form of the same material. The unique feature of our sample preparation was the low-temperature processing. When standard high-temperature (T > 700 degrees C) methods were used, samples were found to exhibit clustering and were not ferromagnetic at room temperature. This capability to fabricate ferromagnetic Mn-doped ZnO semiconductors promises new spintronic devices as well as magneto-optic components.

The introduction of ferromagnetic order in ZnO results in a transparent piezoelectric ferromagnet and further expands its already wide range of applications into the emerging field of spintronics. Through an analysis of density functional calculations we determine the nature of magnetic interactions for transition metals doped ZnO and develop a physical picture based on hybridization, superexchange, and double exchange that captures chemical trends. We identify a crucial role of defects in the observed weak and preparation sensitive ferromagnetism in ZnO:Mn and ZnO:Co. We predict and explain co-doping of Li and Zn interstitials to both yield ferromagnetism in ZnO:Co, in contrast with earlier insights, and verify it experimentally.

Photoresponse characteristics of polycrystalline ZnO films prepared by the unbalanced magnetron sputtering technique have been analyzed for ultraviolet photodetector applications. Changes in the crystallographic orientation and the microstructure of the films due to in situ bombardment effects during film growth have been studied. Variations in photoresponse are correlated with the observed changes in the optical properties and the defect concentration in the films. ZnO films with (100) and (101) orientation possessing a small grain size exhibited a slow response with a rise time=1.99 s, whereas porous ZnO films with a mixed orientation (100), (002), and (101) and a larger grain size exhibited a fast response speed with a rise time=792 ms. The influence of trap levels on the slow and fast rising components of the photoresponse characteristics and the origin for a fast and a stable response have been identified. A slow rise in the photocurrent directly relates to the adsorption and desorption of oxygen on the film surface, and the fast rise is due to a bulk-related phenomena involving embedded oxygen. The magnitude of the photocurrent and the rise time are found to decrease considerably with increasing number of trap levels.

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