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Research On SnO_x Thin-Film Transistors And CMOS Integrated Circuits Based On Oxide Semiconductors

Posted on:2020-10-16Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y P LiFull Text:PDF
GTID:1368330572490783Subject:Microelectronics and Solid State Electronics
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Oxide semiconductors are highly attractive especially for the new generation flexible,transparent,and wearable electronics due to their low cost,low deposition temperatures,high carrier mobilities(1?100 cm2/Vs),good transparency in the visible-light region,and ease of large-area manufacturing.Oxide-semiconductor thin-film transistors(TFTs)have already been commercialized to replace amorphous silicon(a-Si)for backplane drivers in flat-panel displays.There is also huge potential in other integrated circuits and high-frequency applications on glass or flexible substrates beyond flat-panel displays.Most oxide semiconductors are n-type,such as amorphous indium gallium zinc oxide(a-IGZO)and ZnO.Only a very limited number of oxide semiconductors exhibit p-type conduction.Recently,some oxides have been proved to be promising p-type oxide semiconductors.In general,however,p-type metal-oxide semiconductors are much less studied than their n-type counterparts.Developing CMOS technology is an urgent issue for oxide semiconductors.P-type TFTs are essential in order to fabricate oxide-based CMOS integrated circuits(ICs)with low static power consumption,high noise margin,high yield,and high reliability for actual applications.Realizing sophisticated all-oxide circuits operating in complementary mode has been challenging due to the difficulty in making high-performance p-type oxide TFTs matched with their n-type counterparts.Thus,extraordinary efforts have been made for developing p-type oxide materials and TFTs which are comparable to their n-type counterparts in performance.To date,ternary Cu-bearing oxides,binary copper oxides,tin monoxide(SnO),NiO,etc.have been known as p-type oxides.Among these p-type oxides,SnO is one of the most promising because of its native p-type conductivity and high hole mobility.CMOS inverters based on n-type oxides such as In2O3,SnO2,IGZO,and ZnO,and p-type SnO have been reported.However,systematic study on CMOS circuits based on oxide semiconductors is still limited to dateThis thesis focuses on the fabrication of high-performance SnOx TFT and CMOS circuits based on p-type SnO TFTs and n-type IGZO TFTs,and systematically studies their working principle and electrical performances.The main research contents are as follows.1)An extremely sensitive dependence of the electronic properties of SnOx film on sputtering deposition power is discovered experimentally.The carrier transport sharply switches from n-type to p-type when the sputtering power increases by less than 2%.The best n-type carrier transport behavior is observed in TFTs produced at a sputtering power just below a critical value(120 W).In contrast,at just above the critical sputtering power,the p-type behavior is found to be the best with the TFTs showing the highest on/off ratio of 1.79 × 104 and the best subthreshold swing among all the sputtering powers that we have tested.A further increase in the sputtering power by only a few percent results in a drastic drop in on/off ratio by more than one order of magnitude.The highest on/off ratio of our p-type SnOx TFT is among the best values in reported single-gated TFTs.Scanning electron micrographs,x-ray diffraction spectra,x-ray photoelectron spectroscopy,as well as TFT output and transfer characteristics are analyzed.These studies suggest that the sputtering power critically affects the stoichiometry of the SnOx film and the electric performance of the TFT.It is also suggested that the high on/off ratio could be related to the formation of Sn clusters and the resulting reduction of Sn interstitials and the related mid-gap states,which occurred most strongly at an annealing temperature of 250 0C.The revealed extremely sensitive dependence of SnO film properties on sputtering conditions may provide useful clues to future work in order to further reduce the off current of p-type SnO TFTs which remains a key bottleneck issue in achieving satisfactory oxide-semiconductor CMOS circuits for practical applications.2)SnO is the only oxide semiconductor to date that has exhibited ambipolar behavior in TFTs.Ambipolar behavior is observed in SnOx TFTs fabricated at a high sputtering power of 200 W and post-annealed at 150-250 0C in ambient air.X-ray-diffraction patterns show polycrystallisation of SnO and Sn in the annealed SnOx films.Scanning-electron-microscopy images reveal that microgrooves occur after the films are annealed.Clusters subsequently segregate along the microgrooves,and our experiments suggest that they are most likely Sn clusters.Atomic-force-microscopy images indicate an abrupt increase in film roughness due to the cluster segregations.An important implication of this work is that excess Sn in the film,which has been generally thought to be detrimental to the film quality,may promote the ambipolar conduction when it is segregated from the film to enhance the stoichiometric balance.The segregation of excess Sn on the sidewalls of the microgrooves leads to the reduction of density of subgap trap states by Sn interstitials in SnO,making it possible to shift the Fermi level effectively by the gate voltage.As the ambipolar oxide TFT is highly attractive for CMOS-like applications,our results may have useful implications in achieving and optimizing ambipolar behavior in SnOx films for thin-film-based circuits.In addition,the optimum processing temperature for the ambipolar SnO,TFT is below 200 0C so that the results are relevant to possible applications on flexible substrates such as polyimide.3)CMOS inverter based on the n-type IGZO TFTs and p-type SnO TFTs are presented.Both IGZO and SnO are deposited at room temperature using the radio-frequency magnetron sputtering method,which is highly industrial compatible for large-area film deposition.The highest process temperature is 225 0C,making it possible to fabricate the CMOS ICs on flexible substrates.First,the CMOS inverter using an initiatory layout and semiconductor process is fabricated.At a supply voltage of 10 V,the complementary inverter shows an extremely high gain of 112 with a geometric aspect ratio of 5.The dynamic response of the inverter is also examined.The delay time of the inverter measured from dynamic response is 27.75 ?s at a supply voltage of 10 V.Second,the CMOS inverters using the optimized layout and semiconductor process are fabricated for large-scale or large-area integration.The geometric aspect ratio of the inverter is 6.The inverters show rail-to-rail output voltage behavior,low average static power consumption of 8.84 nW,high gain up to 131.57,high noise margin level up to?40%supply voltage,high yield of 98%,and high uniformity with negligible standard deviation.4)CMOS Logic gates(NAND and transmission gates)and 3-stage ring oscillators using the initiatory layout and semiconductor process are presented.The NAND and transmission gates all exhibit ideal rail-to-rail output voltage behavior.At a supply voltage of 20 V,the 3-stage ring oscillators are able to operate at 32.87 kHz,and the stage delay is 5.07 ?s.Our study indicates that other complementary logic gates and sophisticated circuits for potential larger-area transparent/flexible electronics with large noise margin and low static power consumption can be realized based on n-type IGZO and p-type SnO TFTs.5)Various CMOS circuits,including NAND,NOR,XOR,d latches,full adders,and 7-,11-,21-,and 51-stage ring oscillators,are fabricated using the optimized layout and semiconductor process.The NAND,NOR,XOR,d latches,and full adders show desirably ideal input-output characteristics.The d latch and full adder are typical sequential logic circuit and combinational logic circuit,respectively.The desiably ideal input-output characteristics of the NAND,NOR,XOR,d latch and full adder indicate that our oxide semiconductors have the capability to construct complicated CMOS ICs for various applications in flexible and transparent electronics.7-,11-,21-,and 51-stage ring oscillators are presented to systematically evaluate the speed,uniformity,and yield of our CMOS circuits.The stage delay is?1?s with a supply voltage of 10 V,which is highly attractive for some newly emerging applications such as human-machine interfacing.The linear dependence of the oscillation period on stage number of ring oscillator indicates the extremely high uniformity over a large sample area of 1.5 cm × 1.5 cm.In addition,the 51-stage RO composed of 104 TFTs(52 IGZO TFTs and 52 SnO SnO TFTs)is the first medium-scale CMOS circuit fully based on oxide semiconductors,indicating the high yield of our oxide-based CMOS technology.This work may inspire constructions of low-power,large-area,large-scale,and high-performance transparent/flexible CMOS circuits fully based on oxide semiconductors for applications beyond flat-panel displays.
Keywords/Search Tags:oxide semiconductor, tin monoxide(SnO), thin-film transistor(TFT), complementary metal-oxide-semiconductor(CMOS), integrated circuit(IC)
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