Preparation And Properties Of Organic Thin Film Transistor Gas Sensors

Organic Thin Film Transistors （OTFT） gas sensor is a novel and fast developinggas sensor device at the field of international chemical sesors in recent years. OTFT gassensor has many distinctive advantages compared with other traditional gas sensors,such as small size, low cost, simple fabricated technology, multi-parameter detection,easy integration. This dissertation adopted OTFT as sensing component, and carried outa series of research at the electrical characteristics of OTFT and sensing properties ofOTFT gas sensor on the basis of the fabrication technology and parameter optimizationof the device. The main content includes following points:1. Thin-Film Transistors were made in a bottom contact configuration on aSb-doped n+-type Czochralski （CZ）-grown silicon wafer. The gate dielectric layer wasthermal SiO₂. The source and drain interdigital electrodes were formed by depositingtitanium （Ti） film （20nm） and gold （Au） film （50nm） on the top of the gate dielectric.During the fabrication process of TFT, different thickness of the insulator layer （125nm,195nm and300nm） and different ratios of channel width to length （40,160and640）were designed. Copper Pthalocyanine （CuPc） and α-Sexithiophene （α-6T） weredeposited onto the electrode layer and used as the organic active layer by vacummevaporation. CuPc TFTs were used to investigate the dependence of electricalcharacteristics and sening properties to H2S gas on the thickness of the insulator layer atroom temperature. α-6T TFTs were used to investigate the dependence of electricalcharacteristics and sening properties to NO₂gas on the channel parameters at roomtemperature. The results showed that the TFTs with195nm thick-insulator layer and160ratio of channel width to length exhibite the optimal electrical characteristics andsening properties. Carrier transport theory of OTFT was used to understand themechanism of this process and results.2. The gas-sensing properties and sensing mechanism of TFT based metalpthalocyanines were investigated systematically. The TFT based CuPc （p type）semiconductor film was fabricated to study its sensing properties to H2S. The resultsshowed that CuPc TFT exhibited good response/recovery, repeatability and selectivity at room temperature. It was found that oxygen in air played important roles in theproperties of OTFT gas sensor combing with surface morphology of CuPc film. Thepolarities and abilities of donating or trapping electrons of gas molecur influenced thesensing response of OTFT gas sensor directly. At the same time, CuPc, CobaltPhthalocyanine （CoPc, p type） and Copper Hexadecafluorophthalocyanine (F₁₆CuPc, ntype) were fabricated on TFTs to study their electrical characteristics and seningproperties to nerve agent simulants-dimethyl methylphosphonate （DMMP）. The resultsshowed that the CoPc and F₁₆CuPc TFTs exbihited good electrical characteristics, butshowed weaker response to DMMP with same concentration compared with CuPc TFT.The Cu ion in CuPc may be helpful to the catalysis and degradation of DMMP andCuPc TFT exhibited better respone.3. Poly （3-hexylthiophene）（P3HT） composite materials were fabricated on TFTs asactive layer by spray-deposited method. The sening properties to formaldehyde （HCHO）of P3HT/ZnO composite TFTs were investigated. The results showed that P3HT/ZnOcomposite TFT exhibited better response and recovery to HCHO compared with P3HTTFT. The surface morphology revealed that the P3HT/ZnO composite film exhibitedthree-dimensional network structure and existed many holes, which provided adsorptionsites and helped to the adsorption and desorption of HCHO gas molecules. The interfaceof n type ZnO and p type P3HT formed p-n heterojunction. The carriers accumulated atthe interfaces could make up the defects of semiconductor film effectively and benefitedthe transport process of electrons or holes, which improved the electrical and sensingcharacteristies to some extent. It was found that the different airbrush mass （thethickness of film） and the ratio of ZnO in P3HT/ZnO composite had dependence onmorphology of the film and the properties of OTFT gas sensor. The optimizedP3HT/ZnO composite film TFT exhibited good response/recovery, repeatability andselectivity at room temperature, but the stability needed further improvements.Moreover, P3HT/iron oxide （Fe₂O₃） and P3HT/indium oxide/tin oxide （simpled asInSnO） TFTs were fabricated with the same technology and compared with P3HT/ZnOTFTs. The results showed that P3HT/ZnO TFTs exhibited optimum electrical andHCHO-sensing properties. The size and energy band of nano-materials and theelectrical properties were adapted to analyze the experiment results.4. The gas sensing properties of P3HT-ZnO heretolunction TFTs and P3HT/ZnO composite film TFTs were compared and analyzed. The results showed these twodevices had good electrical characteristics. But P3HT-ZnO heretolunction TFTsexhibited weaker response to NO₂than P3HT/ZnO composite TFTs at room temperature.The morphology of film and carrier transport theory were considered to analyze thereason. F₁₆CuPc-CuPc heretolunction TFTs showed some response and no recovery toNO₂at room temperature. The properties of F₁₆CuPc-CuPc heretolunction TFTs neededfurther researches.