Band Structure Engineering, Defects and Doping in Rock-Salt and Wurtzite CdxZn1-xO Thin Films


Student thesis: Doctoral Thesis

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Award date3 Sep 2021


Transition Metal (TM) oxide semiconductors have unique properties that make them suitable for various optoelectronic applications, especially in solar energy conversion and as transparent conductors (TCs) for optoelectronic devices. By alloying different TM oxides, novel materials with properties tailored for new applications can be realized. The development of new TM oxide alloys requires the structural, electrical and optical properties to be well understood and tuned in a controllable way. Traditionally, to a first approximation, the electronic properties of an alloy system could be estimated by an extrapolation of the end point materials (virtual crystal approximation). However, in the case where the end point compounds are very different, especially when they have different crystal structures, a simple extrapolation is no longer valid and a more detailed study is required to understand the materials properties of these alloys.

This dissertation aims in developing a new TM oxide alloy system, the CdO-ZnO alloys which, depending on the alloy composition, can exhibit a small bandgap of ∼1.8 eV or a highly conducting material with high electron mobility and a large absorption edge (>3 eV). These unique properties of the CdO-ZnO alloy system arise from the structural mismatch of the end compounds, the wurtzite (WZ) ZnO and the rocksalt (RS) CdO. Wurtzite (WZ) structured CdxZn1−xO (with x<0.65) exhibits a direct band gap ranging from 1.8 to 3.3 eV, which make them attractive candidates for top cell materials for Si-based tandem solar cells as well as other optoelectronic devices that require a band gap in the visible light range. On the other hand, CdxZn1−xO alloys in the rocksalt (RS) structure (x>0.7) show high electrical conductivity and a larger intrinsic band gap compared to CdO, suggesting that they can be an ideal TC with a wide transparency window.

This work first explores the properties of structurally mismatched CdO-ZnO semiconducting alloy thin films synthesized by radio frequency magnetron sputtering over the entire composition range. Effects of growth conditions and post-deposition treatments on the structural, electrical and optical properties are investigated. The crystal structure of CdxZn1−xO thin films is found to depend on the alloy composition and growth conditions: WZ for x<0.4, RS for x>0.7, and a mixture of the two phases for 0.4<x<0.7. Transport properties are examined by variable temperature Hall effect measurements. The proportions of different scattering mechanism changes from ZnO (x = 0) to CdO (x = 1), with phonon scattering being more pronounced and defects scattering being less important as x increases. A donor activation energy of ∼130 meV, which is independent of x up to x∼0.6, is also extracted. Optical studies by spectroscopic ellipsometry show that both the optical constants and dielectric functions of CdxZn1−xO depend heavily on their crystal structure, with main features resembling that of ZnO for WZ and CdO for RS alloys.

Under normal sputtering conditions with pure Ar gas at a growth temperature of 230°C, composition regions for the pure WZ and RS CdxZn1−xO alloys are limited with a wide mixed phase region of 0.4<x<0.7. We attempt to extend the composition windows for both pure phase WZ and RS alloys by manipulating the density of native defects in the alloys via controlling the growth environment and temperature. It is found that the pure WZ phase CdxZn1−xO boundary can be extended from x∼0.4 to x∼0.7 when grown in an oxygen rich environment, resulting in an alloy oxide with a low band gap of ∼1.8 eV. Density functional theory calculations reveal that in an O-rich growth environment, O-interstitials have a low formation energy and WZ CdxZn1−xO alloys are stabilized by these native point defects. However, O-interstitials are acceptors in these alloys and the presence of high concentration of O-interstitials results in WZ-CdxZn1−xO with poor electrical properties.

The kinetics of the O-interstitial defects in O-rich WZ CdxZn1−xO alloys and thermal stability of the WZ phase are further studied by rapid thermal annealing (RTA) in different ambient environments. For O-rich WZ-CdxZn1−xO alloys with high Cd contents (x>0.5), the electrical conductivity increases by over 2 orders of magnitude after annealing in an inert environment at temperatures slightly above the deposition temperature (∼300°C). This suggests that native O-interstitial acceptor defects are highly mobile and can be removed at low annealing temperatures. Furthermore, the O-rich WZ-CdxZn1−xO alloys are found to be thermally stable with no phase separation up to ∼500°C. These results demonstrate that small gap (∼1.8 eV) WZ-CdxZn1−xO thin films with good electrical properties can be achieved by sputtering in an O-rich environment followed by low temperature RTA. These widely tunable band gap WZ-CdxZn1−xO (from 3.3 to 1.8 eV) with desirable electrical conductivity open up the new opportunities for all oxide optoelectronic applications from UV to the visible range.

RS-CdxZn1−xO alloys (x≥0.7) have increasing intrinsic gap from 2.2 eV (CdO) to 2.5 eV (x∼0.7) with increasing Zn content with high mobility of >60 cm2V−1s−1 . In order to be able to fully utilize their potentials for transparent conductor applications, increasing the conductivity and the optical gap of RS-CdxZn1−xO alloys is necessary and could be achieved through extrinsic doping. The effects of In doping on the electrical and optical properties are studied. We found that In-doping increases the electron concentration of RS-CdxZn1−xO alloys up to a saturation value of ∼7×1020 cm−3 accompanied by a drop in the electron mobility to <20 cm2V−1s−1 when the In doping concentration is higher than 3%. This doping saturation can be explained by compensation due to the formation of native acceptors in the alloys. Density functional theory calculations on the formation energies of native and dopant defects indicate a lower formation energy of compensating metal vacancy acceptors in RS-CdZnO compared to CdO, resulting in a lower saturation electron concentration as well as a lower mobility in CdZnO alloys and hence limits the conductivity and transparency. As a result, although RS-CdZnO alloys have a higher intrinsic band gap, due to the easy formation of native acceptors, a In doped RS alloy with x=0.8 can only achieve a conductivity ∼3000 S cm−1 with an optical band gap of ∼3.2 eV.

Finally, due to the modified electron band structure in WZ-CdxZn1−xO we expect that p-type doping of this material with acceptors, such as Ag and Sb, is a feasible route to overcome the challenging task of achieving stable and highly p-type conducting ZnO. In WZ-CdxZn1−xO the reduction in band gap energy with x has been shown to be mainly attributed to the up-shifting of their valance band maximum, while the position of the conduction band minimum is relatively stable. Hence most deep acceptors in ZnO would be shallower in CdxZn1−xO and effective p-type ZnO-based materials with a high hole concentration can be obtained. Our preliminary experimental and theoretical results suggest that although alloying ZnO with CdO can slightly lower the ionization energy of Ag Zn acceptors, it simultaneously lowers the formation energy of compensating Ag interstitial donors. As a result, p-type conductivity in Ag doped WZ-CdZnO alloys is still not observed. The possibility of p-type doping WZ-CdxZn1−xO using other acceptor species is discussed.