Abstract
Two-dimensional electron gases (2DEGs) at the ZnO/ZnMgO interface are promising for applications in spintronics and quantum computing due to the combination of low spin-orbit coupling and high electron mobility. Growing high mobility 2DEGs requires high quality substrates with low impurity densities. In this work we demonstrate a ZnO substrate sample treatment combining high temperature rapid thermal annealing and chemical etching to improve the surface quality for the subsequent growth of 2DEGs. This process enables the growth of a 2DEG with low-temperature mobility of 4.8×104 cm2V−1s−1. An unannealed control sample shows a scattering rate at least three times greater than the annealed sample.
Highlights
Two-dimensional electron gases (2DEGs) form at the ZnO/ZnMgO due to a combination of polarisation and band-gap discontinuities at the interface [1]
In MBE-grown ZnO/ZnMgO 2DEGs, interface roughness scattering and impurity scattering have been shown to be the dominant factors for electron mobility [8]
In this work we investigate the combination of substrate annealing and hydrochloric acid etching of hydrothermally grown single crystal ZnO substrates as a preparatory step before 2DEG growth
Summary
Two-dimensional electron gases (2DEGs) form at the ZnO/ZnMgO due to a combination of polarisation and band-gap discontinuities at the interface [1] These 2DEGs combine exceptional electron mobilities and consequent long diffusion lengths [2] in a low spin-orbit-coupling material [3]. High mobility 2DEGs require a low density of scattering centres These can be formed by crystalline defects [8], ionized donors [9], alloy scattering [8], electron–electron scattering [10] or atomic impurities [11]. In MBE-grown ZnO/ZnMgO 2DEGs, interface roughness scattering and impurity scattering have been shown to be the dominant factors for electron mobility [8] It is known, for instance, that lithium acts as an acceptor in ZnO and can significantly reduce ZnO conductivity [15,16,17] and harm 2DEG formation [11]. Whilst exceptionally low impurity ZnO crystals have been grown using a platinum-lined autoclave [17,18], since the decommissioning of this equipment, only ZnO substrates grown in rhenium/iridium-lined autoclaves are available
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