Fabrication And Characterization Of InGaZnO Thin-Film Transistors And Non-Volatility Memories

Recently,metal-oxide semiconductor materials represented by amorphous indium-gallium-zinc oxide(a-InGaZnO:IGZO)have been widely developed and used.For example,IGZO thin-film transistors(TFTs)have started replacing traditional amorphous-silicon transistors in large-area displays.IGZO is an n-type semiconductor with a large bandgap and excellent performance,including high electron mobility.optical transparency,low process temperature and excellent uniformity.As such,a wide variety of applications can greatly benefit from the easy deposition of IGZO on flexible substrates,such as flexible and transparent electronics.For the development of wearable devices,it is necessary to research on suitable memory devices.A standand memory chip requires two basic components:select-gate transistors and non-volatility memory devices,both of which are the focuses of this thesis.In this thesis,one of the main tasks is to explore low-power-consumption IGZO TFTs,which are suitable as select-gate transistors in memory circuits.In addition,charge-trapping memory(CTM)and floating-gate memory(FGM)devices are still the main non-volatile memory components in the current commercial memory applications.The former has been widely studied,because it is suitable for 3D NAND integration technology.However,the traditional memory devices are based on silicon,which are not suitable for transparent flexible electronics.Therefore,a main objective of this thesis is to develope CTM devices based on IGZO channel and improve the device performance by improving the gate stack layers.In order to further reduce the operation voltage of the memory device.we also proposed a new low-power-consumption non-volatile resistive-change random access memory(ReRAM)based on IGZO functional layer,and improved its characteristics by using simple thin-film technologies.The thesis includes the following contents:1.Low-voltage IGZO TFTsThe simplest method to reduce the operation voltage is reducing the thickness of the gate dielectric layer.However,the gate leakage current may become severe when the dielectric is too thin.Many researchers have tried to use high-dielectric-constant(high-K)gate dielectrics to reduce the TFTs operation voltage.In order to be better used in transparent and flexible devices,it is important to develop high-K dielectric materials grown at low temperatures.In this thesis,we first prepared Al2O3 and HfO2 high-K dielectrics by low-temperature ALD process,and then fabricated the HfAIO dielectric films by combining the advantages of the two materials.The chemical constituents and energy band of the various films were determined by X-ray photoelectron spectroscopy(XPS).The band gaps of Al2O3 and Hf02 dielectric films prepared at low temperature(150?)were 6.99 eV and 4.50 eV,respectively.And the band gap of HfAIO dielectric was between the two values.As the Al content in HfAlO increased,the band gap also increased with fewer defects in the films.The leakage current of dielectrics could be effectively suppressed with a wide enough band gap and few defects.Furthermore,we also fabricated metal-insulator-semiconductor(MOS)devices,and further analyzed the electrical characteristics of dielectric materials through current-voltage(I-V)and capacitance-voltage(C-V)measurements.The electron transport mechanism in Al2O3 was founded to be consistent with Fowler-Nordheim(F-N)tunneling mechanism by fitting the I-V curves.The electron transport through the Hfo2 and HfAlO films has also been analyzed,indicating the Poole-Frenkel(P-F)emission and F-N tunneling mechanisms in different voltage ranges.Dielectric characteristics of the films at different growth temperatures were obtained from the C-V results,such as permittivity,flat band voltage(Vfb),the density of border trapped oxide charge(Nbt)and fixed oxide charge(Nfc).The surface morphology of the high-K dielectric film was also studied by atomic force microscope(AFM).The surface roughness(RMS)of the 5 nm Al2O3 and HfO2 films was only 0.15 and 0.20 nm,respectively.The ultra-thin high-K films prepared at a low temperature have been proved with high uniformity and stability,and can hence be applied to IGZO TFTs as dielectric layers.Different types of dielectrics have been experimented to study their effects on IGZO TFTs operations.IGZO TFTs with ultrathin high-? materials have been demonstrated to be capable of operating not only at 1V but also at 0.5 V with desirable properties,which greatly reduced the power consumption of TFTs devices.Firstly,the IGZO TFTs based on 5 nm Al2O3 dielectric not only operated at a low voltage of 1V but also exhibited desirable properties including a low threshold voltage(Vth=0.3 V),a small subthreshold swing(SS=100 mV/decade).a high saturation mobility(?sat=6.3 cm2/Vs)and a high on/off current ratio(Ion/Ioff=1.2 ×107).Forthermore,an ultra-low IGZO TFT operation voltage of 0.5 V was achieved by a very high gate capacitance of 5 nm H fO2(COx=300 nF/cm2).As such,the IGZO TFTs exhibited desirable properties such as low power devices,including a small SS of 75 mV/decade,a low Vth of 0.1 V,and a high Ion//Ioff 1×106.When the HfAlO films were used as the dielectrics,the IGZO TFTs also exhibited desirable properties.The leakage currents were smaller than that of HfO2 based TFTs and the currents of on state were larger than that of Al2O3 based TFTs.Furthermore,the IGZO TFTs showed a steep subthreshold slope,negligible hysteresis and a high Ion/Ioff ratio.2.IGZO CTM based on high-K gate stacksA main objective of this thesis is to develope CTM devices based on IGZO channel and improve the device performance by optimising the gate stack layers.Al2O3 films showed different characteristics with different oxygen reaction sources at a low ALD growth temperature.The defect density of AI2O3 films could be modulated by controlling oxygen sources.Thus,Al2O3 films with different properties could be used as tunneling layer,blocking layer or charge-trapping layer in IGZO CTM devices,respectively.In order to achieve good non-volatile memory characteristics,the low defect Al2O3 films(H2O-Al2O3)grown by trimethylaluminum(TMA)and H2O were used as the blocking and tunneling layers of CTM devices.The Al2O3 film(O3-Al2O3)with some defects grown by O3 as the oxygen source was used as the charge-trapping layer of CTM devices.The chemical compositions of two types of Al2O3 films were analyzed by XPS.The traps in O3-Al2O3 may be caused by the residual carbon-related oxidation state.The trap energy level was found to be deep,which contributes to the retention characteristics.IGZO CTM devices showed a large memory window of 8.2 V obtained by programming voltage of+20 V and very good data retention characteristics.A 7.3 V memory window could be maintained for 10 years according to the trend of data.Furthermore,multi-level-storage states were achieved in our IGZO CTM with different programming voltages.There are many reports on the research of HfO2 as charge-trapping layer in the literatures.IGZO CTM devices based on Al2O3/HfO2/Al2O3 gate stack were fabricate with either O3-Al2O3 or HfO2 was used as charge-trapping layer.HfO2 was more suitable to matching the energy band of the tunneling layer due to a smaller bandgap than O3-Al2O3,which can improve the programming efficiency of the CTM device.However,a 6 V memory window was obtained by programming voltage of+20 V.Under the same conditions O3-Al2O3 enabled a higher charge storage capacity and a larger memory window.In addition,the charges were trapped in the shallow trap energy level in HfO2 film,which were easier to escape compared with the deep trap energy level in O3-Al2O3 layer,resulting in poor data retention characteristics of CTM devices.Comparing the two devices,O3-Al2O3 was found more suitable for CTM devices,owing to its large memory window and good data retention characteristics.To further investigate the possibility of using O3-Al2O3 in IGZO CTM devices,we tried to use O3-Al2O3 monolayer film as both tunneling layer and charge-trapping layer in the IGZO CTM device.The performance of IGZO CTM device was explored in such an extreme case.When the physical tunneling layer was removed,the programming voltage of the device dropped by 6 V,and the memory window still maintained a large enough value.A large memory window of 7.9 V was obtained by programming voltage of±14 V.Furthermore,our IGZO CTM device can also achieve a multi-level storage with different programming voltages.Although the IGZO channel was in direct contact with the O3-Al2O3 layer due to the removed of the tunneling layer,the memory device could still maintain good retention characteristics,and a 6 V memory window could be maintained for 10 years.By further simplifying the fabrication process of traditional CTM devices,more possibilities were provided in the non-volatile memory field.3.High-performance IGZO ReRAMIn order to further reduce the operating voltage of memory devices,we designed IGZO ReRAM with both Ag/IGZO/Al and AI/IGZO/Al structures.A suitable oxygen plasma treatment(OPT)reduced the surface roughness of the Al bottom electrode and generated an Al2O3 buffer layer,which improved the ReRAM performance significantly,such as a high on/off ratio and excellent switching uniformity.After the OPT,the endurance of ReRAMs was also enhanced.The on/off current ratios reached 104 and 105 for 100 endurance cycles in the A1 and Ag top electrode devices,respectively.Furthermore,the ReRAMs memory window remained nearly constant during a retention test of 105 s,and the information storage in our devices could be maintained for a much longer time(10 years)according to the trend of data.The conduction mechanisms of the two device structures were examined using Schottky emission and space-charge-limited-current mechanisms.Importantly,the switching voltage can be reduced to 3 V and-1 V for set and reset operations.which is quite suitable for the low-power applications.The various devices studied above were all prepared below 150?.which have promising potentials for the transparent and flexible thin film circuit and memory applications.Since IGZO TFTs have already been adopted in the display industry,the addition of IGZO-based low-temperature,low-voltage and high-performance TFTs and memories could enable more functional oxide-based circuits for a range of applications,such as wearable electrons and smart labels.