Epitaxial stabilization of orthorhombic cuprous oxide films on MgO(110)
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Y. Sun, G.K. Wong,c) and J.B. Kettersonb) Department of Physics and Astronomy and Materials Research Center, Northwestern University, Evanston, Illinois 60208 (Received 5 October 2000; accepted 27 December 2000)
Continuous epitaxial films of cuprous oxide (Cu2O) have been formed by the thermal oxidation of 1.5-m-thick Cu metal films deposited on MgO(110) substrates. These films melted at 1118 °C in air, in agreement with equilibrium phase diagrams. Upon cooling from the liquid, a highly crystalline, epitaxial, 2.5-m-thick Cu2O film was formed. X-ray diffraction spectroscopy revealed that the Cu2O film crystal structure was orthorhombically distorted from the bulk cubic crystal structure. High-resolution transmission electron microscopy showed that the film is coherent, and energy dispersive x-ray spectroscopy showed that interdiffusion is limited to the interface. These results suggest that a new epitaxially stabilized phase of Cu2O has been formed.
I. INTRODUCTION
Cuprous oxide (Cu2O) is of interest as a material in which Bose–Einstein condensation (BEC) of excitons may be observed.1 Its structure, cuprite,2 is very rare, with the only isomorph being Ag2O.3 Cu2O is known to have a negative thermal expansion coefficient below room temperature4 and elastic moduli which soften under pressure.5 Several authors have suggested a structural instability in this material3–5 and predicted a phase transition under pressure.4,5 This paper presents experimental observations that are consistent with a bulk Cu2O structural instability. It is observed that an orthorhombically distorted structure of Cu2O could be epitaxially stabilized on a MgO(110) substrate upon cooling a Cu2O film from the melt. This process bypasses the coherency strain obstacle to growing planar Cu2O films6 and allows for optical studies of excitons in Cu2O films. II. EXPERIMENTAL PROCEDURES
MgO(110) single-crystal substrates (Coating & Crystal Technologies, Kittaning, PA) were cleaned with acetone and blow-dried with pulses of dry nitrogen before a)
e-mail: [email protected] Also with the Department of Electrical and Computer Engineering, Northwestern University. c) Also with the Department of Physics, Hong Kong University of Science and Technology, Hong Kong. b)
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J. Mater. Res., Vol. 16, No. 4, April 2001 Downloaded: 13 Apr 2015
being placed into an Edwards 306A thermal evaporator. Cu slugs (99.999%) (Materials Research Corp., Orangeburg, NY) were used as the source material, and molybdenum source boats were used to melt the Cu metal for evaporation. The chamber base pressure before deposition was 1 × 10−6 torr. Cu films of 1.5-m thickness were deposited at a rate of 0.3 nm/s as measured by a quartz crystal microbalance. Oxidation of the Cu films to form Cu2O films and various heat treatments were performed in a vacuumsealed quartz tube which is inserted into a box furnace as shown in Fig. 1. This apparatus allows one to maintain both the temperature and partial pressure of oxygen (pO2) within the Cu2O phase region
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