Morphological TEM studies and magnetoresistance analysis of sputtered Al‐substituted ZnO films: The role of oxygen

Abstract

In this article, we report on the synthesis of thin, epitaxial films of the transparent conductive oxide Al:ZnO on (0001)‐oriented synthetic sapphire substrates by DC sputtering from targets with a nominal 1 at.% Al substitution. The deposition was carried out at an unusually low substrate temperature of only 250 °C in argon–oxygen mixtures as well as in pure argon. The impact of the process–gas composition on the morphology was analysed by transmission electron microscopy, revealing epitaxial growth in all the cases with a minor impact of the process parameters on the resulting grain sizes. The transport properties resistivity, Hall effect and magnetoresistance were studied in the range from 10 to 300 K in DC and pulsed magnetic fields up to 45 T. While the carrier density and mobility are widely temperature independent, we identified a low field–low temperature regime in which the magnetoresistance shows an anomalous, negative behaviour. At higher fields and temperatures, the magnetoresistance exhibits a more conventional, positive curvature with increasing field strength. As a possible explanation, we propose carrier scattering at localised magnetic trace impurities and magnetic correlations. Cross‐sectional HAADF‐STEM image of an Al:ZnO film grown without oxygen in the sputtering gas. The inset is a false‐colour [100] high resolution STEM image (inverted annular bright field: green, annular dark‐field: red) of the ZnO crystal structure.

Low-temperature positive magnetoresistance in ZnO-based heterostructures

“…The observed negative MR can be attributed to the weak localization (WL) effect [11]. The observed positive MR here demonstrates two typical characteristics to note: it is suppressed (1) at the higher T and (2) for the larger n H value, which is consistent with the reported observations [4,[6][7][8]. The channel width of the formed 2DEG confined in the ZnO layer near the hetero-interface is ∼3 nm [21,22].…”

“…Experimentally, the large linear positive MR usually appears up in narrow-bandgap or zero-bandgap materials such as InSb [30], HgCdTe [31], graphene [32,33], and Bi 2 Se 3 [34][35][36][37]. But the linear MR is rarely reported in In ZnO-based transport systems, the appearance of the positive MR at cryogenic temperatures is usually reported to be related to the magnetic scattering [2][3][4][5][6][7][8][9], which presumably causes the spin splitting. In this picture, the conduction band splits into two sub-bands respectively for spin-up and spindown electrons.…”

“…In ZnO without magnetic doping, the positive MR at low temperatures was also attributed to the same mechanism, in which some defects such as oxygen vacancies are postulated to play the same role as the magnetic 3 Author to whom any correspondence should be addressed. element to provide the local moment [6][7][8][9]. Recently, the large positive MR in Zn 1-x Mg x O/ZnO heterostructures was explained by the skew scattering of electrons by the localized paramagnetic moments [10].…”