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.

H 2 S gas interaction mechanisms of sputtered SnO2 and SnO2–CuO bilayer sensors with a varying distribution of the Cu catalyst on SnO2 are studied using Pt interdigital electrodes within the sensing film. Sensitivity to H2S gas is investigated in the range 20–1200 ppm. Changes induced on the surface, the SnO2–CuO interface, and the internal bulk region of the sensing SnO2 film upon exposure to H2S have been analyzed to explain the increasing sensitivity of three different sensors SnO2, SnO2–CuO, and SnO2 with CuO islands. SnO2 film covered with 0.6 mm diameter ultrathin (∼10 nm) CuO dots is found to exhibit a high sensitivity of 7.3×103 at a low operating temperature of 150 °C. A response speed of 14 s for 20 ppm of H2S, and a fast recovery time of 118 s in flowing air have been measured. The presence of ultrathin CuO dotted islands allow effective removal of adsorbed oxygen from the uncovered SnO2 surface due to spillover of hydrogen dissociated from the H2S–CuO interaction, and the spillover mechanism is sensed through the observed fast response characteristics, and the high sensitivity of the SnO2–CuO-dot sensor.

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Completely glassy thin films of Zr-Al-Cu-Ni exhibiting a large supercooled liquid region (DeltaT(x) = 95 K), very smooth surface (R(a) = 0.2 nm) and high value of Vickers hardness (H(v) = 940) were deposited by sputtering. The micro/nano-patterning ability of these films is demonstrated by focused ion beam etching (smallest pattern approximately 12 nm), as well as by the imprint lithography technique (smallest feature approximately 34 nm). These glassy films having very good mechanical and chemical properties, combined with superb nano-patterning ability, integrateable with silicon integrated circuit technology, are promising for fabrication of a wide range of two- or three-dimensional components for future nano-electromechanical systems.

Fabrication of a highly-sensitive UV detector based on the surface-acoustic-wave (SAW) principle is reported. The output voltage of the SAW oscillator constructed with a ZnO/LiNbO3 hybrid SAW filter was found to decrease linearly with increasing UV light intensity, and a very low intensity UV light of 10 μW/cm2 was detected easily. Our results show its potential use in wireless UV measurement systems.