HIGHLY STABLE FIELD-EFFECT TRANSISTORS BASED ON ORGANIC/NANO MATERIALS

Abstract

Field-effect transistors (FETs) are the fundamental element of modern electronics. FETs are commonly used as switches, and one of the major applications is the backplane for displays. For decades, silicon has been chosen as a channel material of FETs to drive displays. As display technology has made a rapid progress, however, the demand for new FET technology has also arisen. High resolution, flexibility, transparency, low manufacturing cost, and broad applicability are the representative features of next-generation displays. Considering those expected features of the future displays, a variety of nanomaterials have been introduced for FET applications. Among various types of nanomaterials, organic semiconductors and low-dimensional nanomaterials, in particular transition metal dichalcogenides (TMDCs), have gained considerable attention as the great candidates for the channel materials in next-generation FETs. There has been a huge progress on a drawback of each material such as low field-effect mobility of organic semiconductors and scalability issue of TMDCs. The last piece for practical uses of organic semiconductors and TMDCs, hence, would be the stability issues. In this thesis, the stability issues of FETs based on organic semiconductors and nanomaterials, particularly TMDCs, were discussed in terms of environmental stability and operational stability. Firstly, we demonstrated the improved air stability of n-type OFETs with an electrically active interfacial layer. With the interfacial layer composed of poly(9-vinylcarbazole) (PVK), the air stability of n-type OFETs based on N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13) was remarkably improved. In addition, the high glass transition temperature of PVKs enabled thermal post annealing of the active layer, which resulted in a high electron mobility of 0.61 cm2/V·s. This high mobility was maintained at 90% and 59% after 4 days and 105 days stored in air, respectively. The PVK interfacial layer reduced the trapped charges in the OFETs for air exposure because of electron donating property of PVKs. Next, we demonstrated the novel method to overcome the tradeoff between mobility and bias stability of the OFETs regarding a self-assembled monolayer (SAM)-treatment. Four types of silazane-based SAMs with different alkyl chain lengths in the range of 1 to 8 were used. The mobility was increased from 0.29 cm2/V·s (without SAM-treatment) to 0.46 cm2/V·s, 0.61 cm2/V·s, 0.65 cm2/V·s, and 0.84 cm2/V·s after the SAM-treatment with an alkyl chain length of 1, 3, 4, and 8, respectively. On the other hand, inverse proportional relationship was observed between the bias stability and the SAM alkyl chain length. To overcome this tradeoff, a novel method for interface engineering, two-step SAM-treatment, was introduced. By treating long SAM and short SAM in sequence, both the high mobility and good bias stability were achieved. With the two-step SAM-treatment, the OFET showed the improved bias stability as the short SAM-treated OFETs, maintaining the high mobility as the long SAM-treated OFETs. Lastly, the stability issues of TMDC FETs are discussed. We demonstrated highly stable molybdenum disulfide (MoS2) FETs with a negligible hysteresis gap via multiple annealing scheme, followed by systematic investigation for long term air stability with time of MoS2 FETs with (or without) CYTOP passivation. The extracted life time of the device with CYTOP passivation in air was dramatically improved from 7 to 377 days, and even for the short-term bias stability, the experimental threshold voltage shift, outstandingly well matched with the stretched exponential function, indicates that the device without passivation has approximately 25% larger the barrier distribution than that of device with passivation. This work manifests that CYTOP encapsulation can be one of efficient methods to isolate external gas (O2 and H2O) effects on the electrical performance of FETs, especially with low dimensional active materials like MoS2. This thesis discusses the stability issues of FETs based on organic semiconductors and TMDCs. Electrically active interfacial layer was employed to improve the air stability of the OFETs, and two-step SAM-treatment was introduced to overcome the tradeoff between the bias stability and mobility of the OFETs regarding on the SAM-treatment. In addition, highly stable MoS2 FETs were fabricated via multiple annealing schemes followed by the systematic investigation of CYTOP passivation effect on the long term air stability and short term bias stability of the MoS2 FETs. The proposed methods to solve the stability issues of the FETs based on organic semiconductor and TMDCs can be also used in other types of nanoelectronic devices.