Effects Of Doping On Physical Properties Of Metal Oxide Semiconductors And Its Application In Thin Film Transistors

Metal oxide semiconductor is a kind of unique material with both optical transparency and conductivity.Due to its high field-effect mobility,high optical transparency and good environmental stability,people pay more and more attention to transparent semiconductor materials.This kind of semiconductor materials can be used in the next generation of electronic products,including high-performance,flexible and transparent electronic applications,with increasing scientific research and market demand.Similar to silicon materials,metal oxide semiconductors can be divided into n-type semiconductors for electron transport and p-type semiconductors for hole transport.Due to the fact that these materials generally have dispersive conduction band minimum and localized valence band maximum,the electrical parameters such as conductivity and mobility of n-type materials are much better than those of p-type materials,which leads to the fact that the current practical application of metal oxide semiconductors is limited to monopole electronic circuits and hinders the development of transparent complementary metal oxide integrated circuits.On the other hand,although the conductivity of n-type metal oxides is relatively high,the high carrier concentration is difficult to control,resulting in its stability to be improved.Copper oxide CuO is one of the few p-type oxide semiconductors.Compared with other p-type oxides?such as cuprous oxide Cu₂O,nickel oxide NiO,stannous oxide SnO,delafossite materials,etc.?,the chemical state of CuO is more stable,the resources are more abundant,and its tunable optical properties also expand the research space.Indium oxide In₂O₃ is a typical n-type oxide semiconductor.Indium tin oxide?ITO?and indium gallium zinc oxide?IGZO?,which are widely used in the photoelectric display industry,are based on the preparation of In₂O₃.Their excellent electrical properties,high transparency and simple preparation technology make them always in the research upsurge.Among the numerous film processing methods,solution process is favored by people because of its low cost and simple operation.In addition,solution method also has unique advantages in large-scale manufacturing and control of stoichiometric ratio.Despite these advantages,it is not easy to optimize the performance of solution processed oxide transistors under the conditions of high mobility,low threshold voltage,steep sub threshold voltage swing,electrical uniformity and high switch ratio.In particular,in the case of process simplification,when transistors own high mobility,they usually exhibit low switching ratio and large sub threshold swing.In addition,these devices show a large negative threshold voltage,which limits their low-power performance and is not conducive to active matrix drive applications.In short,many problems must be overcome in order to obtain high-performance solution processed devices.Nevertheless,a large number of literatures show that doping is an effective way to improve oxide semiconductor films.By changing the doping elements and doping concentration,the crystal quality,surface morphology,electron and band structure of the main material can be regulated,and then its electrical properties such as conductivity,intrinsic carrier concentration,etc.can also be improved and optimized.In this paper,the optical and electrical properties of metal oxides are optimized by doping.Ni doped CuO and Hf doped In₂O₃ and their transistors are prepared by solution method.The main contents and innovations of this paper are as follows:?1?.The band structure of CuO and CuO doped with 50%Ni was analyzed by the first principles calculation.CuO thin films with Ni doping concentration of0%,10%,20%,30%,50%(Cu_1-xNi_xO,x=0,0.1,0.2,0.3,0.5)were prepared.The refractive index and extinction coefficient of Cu_1-xNi_xO films were extracted by ellipsometry,and the influence of different doping concentration on the absorption coefficient and optical conductivity of Cu_1-xNi_xO films was analyzed.The influence of Ni doping concentration on the electronic structure and optical properties of CuO was studied systematically and deeply by theoretical calculation and experimental data analysis.The calculation based on the density functional theory revealed a strong hybridization of O 2p and Cu 3d orbits near the conduction band minimum?CBM?and valence band maximum?VBM?of CuO films.The Ni addition is found to enhance the carrier mobility because the weaker localization of O 2p states at the VBM is observed in 50%doped CuO.To confirm the theoretical results,the ellipsometric spectra of solution-processed CuO films doped by Ni ions?from 0%to50%?have been fitted and the optical constants have been uniquely extracted.The optical conductivity has a linear increase with the Ni doping concentration,which results from the decreased electron traps.Four electronic transitions are observed at about 2.75 eV,3.27 eV,4.01 eV,and 4.90 eV,and the physical origins have been discussed.?2?.In addition,Ni doped CuO thin film transistors with Ni concentration of0.1%,0.5%,1%and 5%have been fabricated and the optical properties of the channel layer films are characterized through the analysis of ellipsometry.The effects of Ni doping concentration on the mobility,on-off ratio and subthreshold swing of CuO thin film transistor were studied.The results show that the refractive index,absorption coefficient and optical conductivity of Cu_1-xNi_xO?x=0.001,0.005,0.01,0.05?films can be increased by appropriate Ni light doping,and the probability of electron transition from the valence band maximum to the conduction band minmum is also increased,which maybe lead to a larger hole concentration.The integration of Cu_1-xNi_x O films into TFTs demonstrates the typical p-type semiconducting behavior.The optimized Cu₀.₉₉₅Ni_0.005O TFT shows the excellent performance with a carrier mobility of 0.01cm²V^-1s^-1,threshold voltage of 3.72 V,and subthreshold swing of 5.35 V/dec.The achievement represents a significant step toward the development of complementary metal–oxide semiconductor circuits.?3?.In₂O₃ thin film transistors with Hf doping concentration of 0%,1%,2%,3%and 5%(In_1-xHf_xO,x=0,0.01,0.02,0.03,0.05)were prepared by solution method.The crystal quality and chemical bond valence state of the channel layer film were characterized,and the effect of Hf doping concentration on the mobility and bias stability of In₂O₃ transistors was studied.In addition,the deterioration of the performance of In_1-xHf_xO transistors over time was further explored.In this paper,Hf-doped In₂O₃ transistors have been fabricated by solution process.The crystal quality of In₂O₃ thin films decreases with the increase of doping concentration,but the oxygen vacancy in the films decreases significantly under the effect of Hf-O bond.The oxygen vacancy concentration in the thin film decreases from 22.35%in the In₂O₃ film to 12.78%in the In_0.95Hf_0.05O film.The decrease of oxygen vacancy leads to the reducing of carriers in In₂O₃ transistor and the mobility,but at the same time improves the stability of the transistors under the gate voltage of20V.With the increase of doping concentration,the threshold voltage shift of In_1-xHf_xO transistor decreased from 4.53 V to 0.18 V after gate bias was applied for3600 s.In addition,the performance degradation of In_1-xHf_xO transistor over time is also studied.The mobility of In_1-xHf_xO transistors decreased greatly after 3 weeks,and the threshold voltage shift increased about 0.4³ times after the bias voltage was applied,which confirms the necessity of the passivation process.