D. K. Hwang

Comparative studies on the stability of polymer versus SiO2 gate dielectrics for pentacene thin-film transistors

The authors report on the electrical reliabilities of poly-4-vinyl phenol (PVP) and SiO2 gate dielectrics for pentacene thin-film transistors (TFTs). SiO2 films were grown by dry oxidation and PVP films were prepared by spin coating and subsequent cross-linking at 175°C for 15min. The pentacene TFTs with the PVP cured for 15min exhibited a large hysteresis and an abnormal drain-current increase under a gate bias stress over time, while the other TFT with SiO2 displayed a small hysteresis but its drain current decreases with time. The hysteresis behaviors induced by PVP and SiO2 were opposite to each other in the gate bias swing direction, due to the difference in hysteresis mechanism between the two types of TFTs. Comparing their hysteresis behavior, the authors fabricated a far more reliable pentacene TFT with PVP by extending the PVP curing time to 1h. Our improved device with PVP exhibited no hysteresis and persistent toughness to the gate bias stress.

Hysteresis mechanisms of pentacene thin-film transistors with polymer/oxide bilayer gate dielectrics

We have studied the electrical stability of organic poly-4-vinyl phenol (PVP)/inorganic oxide bilayer gate dielectrics for low-voltage pentacene thin-film transistors (TFTs). Curing conditions of spin-cast PVP influence on the drain current-gate bias hysteresis behavior; long term curing reduces the magnitude of the hysteresis, which can also be reduced by decreasing the PVP thickness. The electron charge injection from gate electrode plays as another cause of the electrical hysteresis. These instabilities are categorized into the following three: channel/dielectric interface-induced, slow polarization-induced, and gate charge injection-induced hystereses. By examining the hysteresis behavior of pentacene TFTs with five different combinations of bilayer dielectric, we clarified the instability mechanisms responsible for the electrical hysteresis.

ZnO-based nonvolatile memory thin-film transistors with polymer dielectric/ferroelectric double gate insulators

The authors report on the fabrication of a top-gate ZnO thin-film transistor (TFT) with a polymer dielectric/ferroelectric double-layer gate insulator that was formed on patterned ZnO through a sequential spin-casting process of 450-nm-thick poly-4-vinylphenol (PVP) and 200-nm-thick poly(vinylidene fluoride/trifluoroethylene) [P(VDF/TrFE)]. Compared to the single P(VDF/TrFE) layer, double layer shows remarkably reduced leakage current with the aid of the PVP buffer. TFT with the PVP/P(VDF/TrFE) double layer exhibits a field effect mobility of 0.36cm2∕V and a large memory hysteresis in the transfer characteristics due to the ferroelectric P(VDF/TrFE). The retention of the device lasted over 2h.

Observation of optical properties related to room-temperature ferromagnetism in co-sputtered Zn1−xCoxO thin films

Abstract We report on the analysis of optical transmittance spectra and the resulting ferromagnetic characteristics of sputtered Zn1−xCoxO films. Zn1−xCoxO films were prepared on (0001)-oriented Al2O3 substrates by the radio-frequency (rf) magnetron co-sputtering method. The XRD results showed that the crystallinity of films was properly maintained up to x=0.30 and no second phase peaks were detected up to x=0.40. The transmittance spectra showed both the increase of the absorption band intensity and the red shift of the absorption peak as well as the band edge with increasing x. We have proved experimentally that these changes depend on Co concentration. These optical properties suggest that sp–d exchange interactions and typical d–d transitions become activated with increasing x, which leads to the enhancement of ferromagnetic properties in Zn1−xCoxO films as shown in the AGM results. Therefore, it is concluded that the ferromagnetism derives from the substitution of Co2+ for Zn2+ without changing the wurtzite structure.

Formation of p-type ZnO film on InP substrate by phosphor doping

Abstract ZnO thin film was initially deposited on InP substrate by radio frequency (rf) magnetron sputtering and the diffusion process was performed using the closed ampoule technique where Zn3P2 was used as the dopant source. To verify the junction formation of ZnO thin films, the electrical properties were measured, and the effects of Zn3P2 diffusion on ZnO thin films were investigated. It is observed that the electrical property of the film is changed from n-type to p-type by dopant diffusion effect. Based on the results, it is confirmed that ZnO thin films can be a potential candidate for ultraviolet (UV) optical devices.

Low-voltage high-mobility pentacene thin-film transistors with polymer/high-k oxide double gate dielectrics

We report on the fabrication of pentacene-based thin-film transistors (TFTs) with poly-4-vinylphenol (PVP)/yttrium oxide (YOx) double gate insulator films. The minimum PVP and YOx layer thicknesses were chosen to be 45 and 50nm, respectively. The PVP and YOx double dielectric layers with the minimum thicknesses exhibited a high dielectric capacitance of 70.8nF∕cm2 and quite a good dielectric strength of ∼2MV∕cm at a leakage current level of ∼10−6A∕cm2 while the leakage current from either PVP or YOx alone was too high. Our pentacene TFTs with the 45nm thin PVP∕50nm thin YOx films operated at −5V showing a high field effect mobility of 1.74cm2∕Vs and a decent on/off current ratio of 104. Our work demonstrates that the PVP∕YOx double layer is a promising gate dielectric to realize low-voltage high-mobility organic TFTs.

Low voltage complementary thin-film transistor inverters with pentacene-ZnO hybrid channels on AlOx dielectric

The authors report on the fabrication of complementary thin-film transistor (CTFT) inverters with pentacene (p type) and ZnO (n type) hybrid channels on rf-deposited AlOx dielectrics. All deposition processes were carried out on glass substrates at low temperatures (<100°C). Since our p-channel TFT showed somewhat equivalent field mobility of 0.11cm2∕Vs compared to that of the n-channel device (0.75cm2∕Vs), the hybrid CTFT inverters were designed with identical dimensions for both channels. The CTFT device demonstrated an excellent voltage gain of ∼21 with a low static power dissipation of ∼1.5nW at a low supply voltage of 7V.

Comparative study of the photoresponse from tetracene-based and pentacene-based thin-film transistors

We report on the photoresponse from tetracene-based and pentacene-based thin-film transistors (TFTs) with semitransparent NiOx source/drain electrodes and SiO2∕p+-Si substrate. Both organic TFTs have been fabricated with identical channel thickness and device geometry. Compared with pentacene-based TFTs, the tetracene-TFT exhibited superior potentials as a photodetector in the visible and ultraviolet range although it showed a field mobility (μ=0.003cm2∕Vs) which is two orders of magnitude lower than that of the pentacene-based TFT (μ=∼0.3cm2∕Vs). The tetracene-TFT displayed a high photo-to-dark current ratio (Iph∕Idark) of 3×103, while that of the pentacene-TFT was only ∼10.

Flexible semitransparent pentacene thin-film transistors with polymer dielectric layers and NiOx electrodes

We have fabricated the flexible semitransparent pentacene-based thin-film transistors (TFTs) with poly-4-vinylphenol (PVP) dielectric layers which were deposited by spin coating on a thermostable plastic substrate with a conductive film. For the source∕drain (S∕D) electrodes of our flexible pentacene TFTs both Au and semitransparent NiOx have been tested. It was found that NiOx was better matched to the pentacene channel for the S∕D contacts than Au. Our flexible pentacene TFTs with semitransparent NiOx contacts exhibited mobility of ∼0.24cm2∕Vs higher than that achieved with Au contacts (∼0.14cm2∕Vs) and also demonstrated a higher initial drain current.

Transparent thin-film transistors with pentacene channel, AlOx gate, and NiOx electrodes

We report on the fabrication of pentacene-based transparent thin-film transistors (TTFT) that consist of NiOx, AlOx, and indium-tin-oxide (ITO) for the source-drain (S/D) electrode, gate dielectric, and gate electrode, respectively. The NiOx S/D electrodes of which the work function is well matched to that of pentacene were deposited on a 50-nm-thick pentacene channel by thermal evaporation of NiO powder and showed a moderately low but still effective transmittance of ∼25% in the visible range along with a good sheet resistance of ∼60Ω∕◻. The maximum saturation current of our pentacene-based TTFT was about 15μA at a gate bias of −40V showing a high field effect mobility of 0.9cm2∕Vs in the dark, and the on/off current ratio of our TTFT was about 5×105. It is concluded that jointly adopting NiOx for the S/D electrode and AlOx for gate dielectric realizes a high-quality pentacene-based TTFT.