Dielectrics Effect On CuPc Micro/Nano Single Crystal Field-effect Transistors Performance

In recent years,single-crystal organic field-effect transistors have been widely developed.In some areas such as sensors and RFID cards and other fields have begun to be applied,thus the stability of the device was more concerned.During the preparation of the field-effect transistor,it is essential to select dielectric,because it can affect device electrical performance and then affect the stability of device.In this paper,we select five kinds of dielectric widely used in field-effect transistor,respectively are SiO₂?PMMA?PS?PDMS and air dielectric.Copper phthalocyanine single crystal nanobelts is prepared by physical-vapor-transport method.Constructions of devices mainly use spin-coating,evaporation,electron beam lithography and mechanical probe transfer method.Basic electrical performances of devices are tested and we study threshold voltage stability of devices.Among four kinds of solid dielectric devices,PS dielectric devices have the highest electrical performance,the average mobility can be reached to 0.36±0.04 cm²/Vs and Ion/Ioff can reach to 105.The highest mobility of air dielectric devices is 1.16cm²/Vs and is the maximum reported value in copper phthalocyanine single crystal field-effect transistor.Ion/Ioff of the device can reach to 107.For solid dielectric,they could be distinguished by two kinds.SiO₂ and PMMA dielectric are polar dielectrics and PS and PDMS dielectric are nonpolar dielectrics.Transfer cures of solid dielectric devices are been multiply measured.The threshold voltage shift of polar dielectric devices is above 10 V and the threshold voltage shift of nonpolar dielectric devices is around 1V.This phenomenon can be explained by polar interaction between dielectric and semiconductor make semiconductor add a number of deep defects,causing the carrier trapped and eventually leads to a large shift of threshold voltage.In order to further validate the obtained results above,we prepared air dielectric devices,such devices can eliminate polar factors of polar dielectric.We get minimum shift of threshold voltage of five kinds of devices,and the value is 0.13±0.04 V which prove our conclusions above.We do some further study in threshold voltage stability,we rebuild the five kinds of dielectric device based on long time kept nanobelts and study its threshold voltage stability conditions.First we use air dielectric device based on long time kept nanobelts to study what changes of nonobelts take place during a long kept time.Compared to devices based on new growth nonobelts,the mobility of devices based on long time kept nanobelts are smaller and the absolute value of threshold voltage are larger which shows the shallow and deep defects of semiconductors increase during a long kept time.Then we test solid dielectric devices based on long time kept nanobelts and found threshold voltage shift of SiO₂ and PMMA such polar dielectric devices are respectively 2.18±1.59 V and 1.20±0.07 V,while threshold voltage shift of PS and PDMS such nonpolar dielectric devices are respectively 2.15±1.63 V and 1.41±0.43 V.We found polar dielectrics SiO₂ and PMMA devices based on long time kept nanobelts,its threshold voltage stability has been significantly improved,while the nonpolar dielectrics PS and PDMS devices,its threshold voltage stability declines slightly but remained relatively stable.We make a reasonable speculation for this phenomenon.The single crystal nanobelts during a long kept time,adsorbed substances constitute a natural surrounding passivation layer weakening the interaction of polar dielectric and semiconductor,resulting in a smaller threshold voltage shift.Long time kept nanobelts has been brought enough deep defects to it,resulting in polar effect of dielectric weakened.