Defect Modulation Doping for Transparent Conducting Oxide Materials

Abstract

The doping of semiconductor materials is a fundamental part of modern technology. \nTransparent conducting oxides (TCOs) are a group of semiconductors, which holds the \nfeatures of being transparent and electrically conductive. The high electrical conductivity \nis usually obtained by typical doping with heterovalent substitutional impurities like in \nSn-doped In2O3 (ITO), fluorine-doped SnO2 (FTO) and Al-doped ZnO (AZO). However, \nthese classical approaches have in many cases reached their limits both in regard to \nachievable charge carrier density, as well as mobility. Modulation doping, a mechanism \nthat exploits the energy band alignment at an interface between two materials to induce \nfree charge carriers in one of them, has been shown to avoid the mobility limitation. \nHowever, the carrier density limit cannot be lifted by this approach, as the alignment of \ndoping limits by intrinsic defects. The goal of this work was to implement the novel \ndoping strategy for TCO materials. The strategy relies on using of defective wide band \ngap materials to dope the surface of the TCO layers, which results Fermi level pinning \nat the dopant phase and Fermi level positions outside the doping limit in the TCOs. \nThe approach is tested by using undoped In2O3, Sn-doped In2O3 and SnO2 as TCO \nhost phase and Al2O3 and SiO2−x as wide band gap dopant phase. \n \nThe study was divided into two parts by the approaches followed experimentally. The \nfirst part deals with a physical approach, in which sputtered TCOs are used as a host \nmaterials and covered with dopant layers. To test the versatility of the approach the \nsecond part deals with a chemical approach, in which SnO2 based nanocomposite films \nproduced in spray pyrolysis deposition. \n \nIn the physical approach, ITO/ALD-Al2O3, In2O3/ALD-Al2O3, and In2O3/sputtered \nSiO2−x thin film systems were exploited. The study was conducted mostly by photoelectron \nspectroscopy and Hall effect measurements. ITO films prepared in different \nconditions showed an increase of conductivity after ALD-Al2O3 deposition at 200 °C. \nThis was mostly due to an increase in carrier concentrations. However, Al2O3 deposition \nalso resulted in a chemical reduction of ITO. The diffusivity of compensating oxygen interstitial (Oi) defects at 200 °C is sufficiently screen the high Fermi level induced by Al2O3, which disable the use of defect modulation doping at this temperature. The \nresults indicate that achieving higher carrier concentration in ITO thin films requires \na control of the oxygen pressure in combination with low-temperature ALD process. \nUndoped In2O3 films also showed an increase of conductivity upon deposition of upto \n10-cycles of ALD-Al2O3. These increases indicate the occurrence of defect modulation \ndoping. However, in order to improve the interface properties and firmly prove the \nmodulation doping effect, more detailed studies required on the doped interfaces. The \napproach was further examined by depositing reactively sputtered SiO2−x dopant phase \nfrom Si target on the top of In2O3 films. The resulting conductivity of In2O3/sputtered \nSiO2−x do not show enhancement of electrical properties. This is due to the implantation \nof oxygen species during SiO2 deposition on the surface of In2O3, which counteract the \ndefect modulation doping by reducing concentration of oxygen vacancies (VO) in In2O3. \nTherefore, further studies on the deposition conditions of the dopant phase is still vital \nto see enhanced electrical properties. \n \nIn the chemical approach two different routes were followed: embedding nanoparticles \nin TCO host matrix and formation of demixed composite films. In the first route, \nAl2O3 and TiO2 nanoparticles (NPs) were chosen as dopant phases and were deposited \ntogether with SnO2 TCO precursors. Different characterization of the produced films \ndo not confirm the presence nanoparticles into tin oxide films. Therefore to realise \nmodulation effect further optimization deposition conditions and sample preparation \ntechniques are needed. For the second route, mixture of SnCl4 ·5(H2O) and Al(acac)3 \nprecursor solutions in different composition are used to produce SnO2/Al2O3 demixed \ncomposite films. Different physicochemical studies shows that under the deposition \nconditions followed during this study Al3+ preferably substitute Sn4+ than forming \nanother Al2O3 separated phase. Al was acting as an acceptor doping on SnO2 films. \nTherefore, enhanced conductivity was not observed on the probed samples. For this \nroute further optimization of deposition condition is clearly required. \n \nThe results of this dissertation are relevant for the usage of TCOs in the emerging field \nof oxide thin film electronics in particular in field where the surface to bulk ratio is much \nhigher than in conventional films, as the approach is near surface phenomena. However, \nfurther utilization of both the processing conditions and material selection are vital.