Growth of doped transparent conducting oxides by oxygen plasma assisted atomic beam epitaxy
Keywords: delafossite
cuprous oxide
transparent conducting oxides
oxygen plasma assisted atomic beam epitaxy

Interest exists in the development of transparent conducting oxide materials, which have diverse applications in areas such as transparent coatings for display technologies, solar cells, and optoelectronics. Since many of the applications require the use of thin film forms, the need is to establish useful experimental approaches to the fabrication of such structures. One relatively new method in this area is oxygen-plasma assisted atomic beam epitaxy (OPABE) in which oxide layers are grown under normal molecular beam epitaxy (MBE) conditions with the addition of an oxygen atom beam to ensure full oxidation of the depositing metallic species. Work in this area has to date mainly focussed on the growth of relatively stable oxides such as ZnO, MgO and In2O3 which are the strongly thermodynamically favoured reaction products, across a broad range of reaction conditions. In contrast, the present work is concerned with the growth of Cu2O and a range of delafossite materials, namely CuInO2, CuCrO2 and CuGaO2, which are expected to require much more sensitive control to achieve the desired reaction product.
Studies of the OPABE growth of Cu2O on MgO (100) and MgO (110) substrates have been carried out, using a broad range of physical techniques to characterise the grown Cu2O deposits. It is demonstrated that CuO is the favoured reaction product at low growth temperatures, although Cu2O becomes increasingly favoured as the growth temperature increases. Alternatively, it is also shown that a novel bilayer growth method, whereby some pure Cu is deposited prior to oxide growth, can be used to form the desired Cu (I) phase. Varying crystal orientations are seen, depending on the exact growth conditions; core level and valence band X-ray photoelectron spectroscopy (XPS), optical band gap and atomic force microscopy (AFM) measurements are used to characterise the deposits.
Further growth investigations of the delafossite compounds CuInO2, CuCrO2 and CuGaO2 using OPABE are also recorded, and for the case of CuInO2, comparison is also made with the pulsed laser deposition approach. For all three materials systems, oriented crystal growth on basal planes sapphire substrates is seen, with either the (001) plane or the (015) plane orienting parallel to the substrate depending on the growth temperature, provided approximately correct metal fluxes are used as set by the Knudsen-cell temperatures. The typical valence band electronic structure of delafossite materials is observed in all three cases, and XPS peak shifts suggest that the layers can be electrically doped by adding appropriate metal fluxes during growth. AFM measurements show the grown films are relatively rough and it is suggested that the growth mode follows an island growth mechanism in which oriented three dimensional islands formed at the start of growth gradually enlarge and coalesce as the film thickens. Optical absorption measurements are consistent with the generally accepted optical band gaps of the materials concerned.