Xuanjun Yan

Organic heterojunction and its application for double channel field-effect transistors

Heterojunction organic field-effect transistor (OFET) based on p-type copper phthalocyanine (CuPc) and n-type hexadecafluorophthalocyaninatocopper (F16CuPc) was demonstrated. The heterojunction OFETs can be operated in normally-on (depletion-accumulation) mode, which attributes to the existence of a new conductive channel at the interface of heterojunction. The new channel is originated from accumulation of electrons and holes induced by the interface dipole. Compared with the device with CuPc single layer, the double channel transistor displays improved field-effect mobility from 0.017to0.042cm2∕Vs, and threshold voltage shifts from −17to+19V. In addition, ambipolar electric characteristics have been observed from the heterojunction OFETs.

Ambipolar organic field-effect transistors with air stability, high mobility, and balanced transport

Ambipolar organic field-effect transistors (OFETs) based on the organic heterojunction of copper-hexadecafluoro-phthalocyanine (F16CuPc) and 2,5-bis(4-biphenylyl) bithiophene (BP2T) were fabricated. The ambipolar OFETs eliminated the injection barrier for the electrons and holes though symmetrical Au source and drain electrodes were used, and exhibited air stability and balanced ambipolar transport behavior. High field-effect mobilities of 0.04cm2∕Vs for the holes and 0.036cm2∕Vs for the electrons were obtained. The capacitance-voltage characteristic of metal-oxide-semiconductor (MOS) diode confirmed that electrons and holes are transported at F16CuPc and BP2T layers, respectively. On this ground, complementary MOS-like inverters comprising two identical ambipolar OFETs were constructed.

Bottom-contact organic field-effect transistors having low-dielectric layer under source and drain electrodes

An organic thin-film transistor (OTFT) having a low-dielectric polymer layer between gate insulator and source/drain electrodes is investigated. Copper phthalocyanine (CuPc), a well-known organic semiconductor, is used as an active layer to test performance of the device. Compared with bottom-contact devices, leakage current is reduced by roughly one order of magnitude, and on-state current is enhanced by almost one order of magnitude. The performance of the device is almost the same as that of a top-contact device. The low-dielectric polymer may play two roles to improve OTFT performance. One is that this structure influences electric-field distribution between source/drain electrodes and semiconductor and enhances charge injection. The other is that the polymer influences growth behavior of CuPc thin films and enhances physical connection between source/drain electrodes and semiconductor channel. Advantages of the OTFT having bottom-contact structure make it useful for integrated plastic electronic devices.

Improved n-type organic transistors by introducing organic heterojunction buffer layer under source/drain electrodes

N-type organic thin-film transistors (OTFTs) employing hexadecafluorophthalocyaninatocopper (F16CuPc) as active layer and p-type copper phthalocyanine (CuPc) as buffer layer are demonstrated. The highest field-effect mobility is 7.6×10−2cm2∕Vs. The improved performance was attributed to the decrease of contact resistance due to the introduction of highly conductive F16CuPc∕CuPc organic heterojunction. Therefore, current method provides an effective path to improve the performance of OTFTs.

Characterization of zinc oxide crystal nanowires grown by thermal evaporation of ZnS powders

Abstract Semiconductor single-crystal ZnO nanowires have been successfully synthesized in bulk quantities by a simple and low cost process based on thermal evaporation of ZnS powders onto silicon substrates with the presence of Au catalyst. XRD of the product proves that the nanowires are the wurtzite structure of ZnO. Scanning electron microscopy and transmission electron microscopy show that the ZnO nanowires have diameters about 20–60 nm and lengths up to several tens of micrometers. The growth of ZnO nanowires is controlled by the conventional vapor–liquid–solid mechanism. The dependence of photoluminescence on temperatures was examined by He–Cd laser.

Organic thin-film transistors having inorganic/organic double gate insulators

Bottom-contact organic thin-film transistors (BC OTFTs) based on inorganic/organic double gate insulators were demonstrated. The double gate insulators consisted of tantalum pentoxide (Ta2O5) with high dielectric constant (κ) as the first gate insulator and octadecyltrichlorosilane (OTS) with low κ as the second gate insulator. The devices have carrier mobilities larger than 10−2cm2∕Vs, on/off current ratio greater than 105, and the threshold voltage of −14V, which is threefold larger field-effect mobility and an order of magnitude larger on/off current ratio than the OTFTs with a Ta2O5 gate insulator. The leakage current was decreased from 2.4×10−6 to 7.4×10−8 A due to the introduction of the OTS second dielectric layer. The results demonstrated that using inorganic/organic double insulator as the gate dielectric layer is an effective method to fabricate OTFTs with improved electric characteristics.

Organic thin-film transistors with improved characteristics using lutetium bisphthalocyanine as a buffer layer

Organic thin-film transistors (OTFTs) with a buffer layer sandwiched between source/drain electrodes and organic semiconductor are demonstrated. An intrinsic molecular semiconductor, Lutetium bisphthalocyanine (LuPc2), is used as the buffer layer due to its high carrier density (1016cm3). Compared with conventional OTFTs, the introduction of the buffer layer leads to on-state current increases from 700nA to 2.5μA, field-effect mobility increases from 0.7×10−2 to 1.58×10−2cm2∕Vs, and threshold voltage downshifts from −21 to −11V for the linear region. The on/off current ratio is improved to a level of 104. Mechanisms of performance improvement are attributed to include the difference of the Fermi level and interface dipolar between LuPc2 and Au. Our results demonstrate that it is an effective method to improve linear region characteristics by using a molecular semiconductor as the buffer layer.

A simple large-scale synthesis of coaxial nanocables: silicon carbide sheathed with silicon oxide

Abstract We reported a simple method to synthesize coaxial nanocables with silicon carbide core and amorphous silicon oxide sheath just by exposure of Au-coated silicon substrates to carbon monoxide at 1100 °C. The as-grown product was characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and micro-Raman spectroscopy. The obtained nanocables were in large scale, several tens of micrometers long, with the core a few nanometers to ten or more nanometers in diameter. The vapor–liquid–solid mechanism was proposed to elucidate the growth process.