Electronic Band Structure Engineering and Doping of Transparent Oxide Semiconductors towards Improved Optoelectronic Properties and Bipolar Conductivity


Student thesis: Doctoral Thesis

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Award date7 Apr 2021


Transparent oxide semiconductor (TOS) materials have shown widespread significance for application in the optoelectronic and energy industry. These wide-bandgap semiconducting oxides combine high electrical conductivity and optical transparency, and this unique property makes them well suited to a variety of applications such as in portable electronics, flat panel displays, energy-efficient smart windows and solar cells. Among TOSs, those showing n-type character have been widely explored and several of them (such as highly doped In2O3:Sn, SnO2:F and ZnO:Al) show exceptional conductivity and visible range transparency, hence they can be classified as transparent conducting oxides. However, p-type TOS showing comparable performance to their n-type counterparts is still lacking. Due to this lack of feasible p-type TOS, the applications of TOSs are limited to unipolar (n-type TOSs) based devices. A few p-type metal oxides have shown some promising performance, for example, Nickel Oxide (NiO). Motivated by the diverse applications of semiconducting NiO and the possibility to further improve their transport, electrical and optical properties, this dissertation is devoted to exploring doping, defects, and electronic band structure engineering in NiO. Furthermore, with recent efforts targeted towards replacing In2O3:Sn with TM-doped In2O3 that show improved carrier mobility. We also explore in this dissertation, improvements in the electrical mobility of n-type In2O3 doped with transition metal (TM) and comprehensively describe the effects of free carriers on the optical properties of TM-doped In2O3.

Native defects in semiconductors play an important role in their optoelectronic properties. This dissertation begins by making a comprehensive study on intrinsic vacancy defects induced changes in the electronic and optical properties of NiO. The systematic study compared the optoelectronic properties of stoichiometric NiO, oxygen-rich NiO with Ni vacancies (NiO:VNi), and Ni-rich NiO with O vacancies (NiO:VO). The experimental results are directly compared to first principles DFT + U calculations. Computational results confirm that gap states are present in both NiO systems with vacancies. Gap states in NiO:VO are predominantly Ni 3d states, while those in NiO:VNi are composed of both Ni 3d and O 2p states. The increase in sub-gap absorptions in NiO:VNi compared to NiO and NiO:VO samples is attributed to gap states observed in the electronic density of states. The relation between native vacancy defects and the electronic and optical properties of NiO is demonstrated.

Doping provides a reliable method to improve the carrier density in semiconductors. Next in this dissertation, acceptor doping in p-type NiO to improve their optoelectronic properties and electrical properties stability is explored. This is achieved by a combined experiment and computational study of the effects of acceptor dopants, including Li, Ag, and Cu on the properties of NiO. Measured electrical properties of the undoped and doped Oxygen-rich NiO (NiO1+δ) show an increase in conductivity and hole concentration for the doped samples. In general, superior electrical properties compared to previously reported efforts for doping in NiO is achieved. Li showed the highest doping efficiency for achieving highly conducting p-type NiO with >40% transmittance in the visible range for a 100-nm-thick film. A remarkable increase in the temperature-dependent Hall mobility is also observed in the doped samples. Based on the small-polaron hoping model, the conduction mechanism in the doped samples is analyzed, which revealed a hopping dominated activation with energies in the range of 172–208 meV. A p-CuxNi1-xO1+δ and an n-type ZnO p-n heterojunction was fabricated, showing good rectification with a type-II band alignment.

Controlling the electronic structure of materials by alloying is a longstanding endeavour in semiconductor physics as this offers a possibility to tune several properties of the parent materials across a wide composition range. This dissertation further explores band structure engineering by alloying of NiO with n-type CdO and ZnO TOSs (NixCd1-xO, NixZn1-xO) to achieve wide-gap oxides exhibiting both n- and p-type bipolar conductivity in NixCd1-xO and p-type conductivity in RS- NixZn1-xO alloys. It is found that stoichiometric NixCd1−xO alloys are n-type in the composition range of 0 ≤ x ≤ 0.52 and insulating for higher x due to an increase in the valence band maximum with Ni alloying. On the other hand, O-rich alloys are p-type conducting for x ≥ 0.38. This demonstrates that in the alloy composition range of 0.38 < x < 0.52, n-type and p-type NixCd1−xO alloys can be synthesized by controlling the oxygen stoichiometry. It is further shown that by Li and Cu doping the conductivity in the p-type regime of these alloys is improved, which also leads to a wider composition window for bipolar doping in NixCd1−xO. Specifically, by Li doping, the p-type alloy composition can be extended to x ≥ 0.3 so that the bipolar doping window is expanded to 0.30 ≤ x ≤ 0.52. A p-Ni0.7Cd0.3O:Li/n-Ni0.45Cd0.55O quasi-homojunction was fabricated and a rectification ratio ∼102 with an ideality factor of ∼2.9 was obtained. The demonstrated quasi-homojunction structure also showed > 60% transmittance in the visible spectrum. In the NiO-ZnO alloy system study, it is found that the Zn1−xNixO alloy thin films undergo a phase transition from wurtzite (WZ) to rocksalt (RS) structure as the Ni content x increases to x ∼0.27. Nominally undoped alloy thin films sputtered in pure Ar are semi-insulating, while O-rich RS-Zn1−xNixO thin films with relatively high x (e.g., x ≥ 0.5) sputtered in Ar+1.4% O2 exhibit good p-type conductivity at room temperature due to the drastic increase in the valence band maximum of RS Zn1−xNixO which can enable the ease of formation of acceptor defects. However, Li doping in RS Zn1−xNixO led to a less remarkable improvement in their p-type properties compared to NixCd1-xO and this is attributed to a deeper Li acceptor level in this alloy. Lastly, optical properties studies of TM-doped In2O3 reveal remarkable carrier density-dependent changes in the optical properties. For instance, the free-carrier absorption at a wavelength of 1300 nm in the NIR region show an increase with carrier concentration from 103 to 104 cm-1, similarly, the plasma wavelength, λp for the TM-doped In2O3 thin films decreases with carrier concentration. Significant Burstein-Moss and band-gap renormalization effect also lead to a notable increase in the optical bandgap with high carrier concentration up to 4.2 eV for thin films with carrier density ~8.2 × 1020 cm-3. Also, the refractive index, n and the high-frequency dielectric constant, ε∞ of the TM doped thin films decreases with an increase in the electron concentration. The electron effective mass, m* show an increase with electron concentration following the non-parabolic conduction model up to a carrier concentration < 7 × 1020 cm-3, above this concentration, conduction band modification by anti-crossing interaction between the TM d levels and the In2O3 host conduction states is observed.