Niels Benson

Complementary organic field effect transistors by ultraviolet dielectric interface modification

The realization of p- and n-type pentacene organic field effect transistors and an organic inverter stage is reported based on selective ultraviolet (UV) modification of the polymer dielectric in air. Apart from the UV radiation treatment, the device structures are identical. The achieved field effect carrier mobilities for both transistor types are ≈0.1cm2∕Vs. Similar performance data for both transistor types as well as an observed low current hysteresis qualify the UV treatment for organic complementary metal oxide semiconductor (O-CMOS) technology. The realized O-CMOS inverter exhibits stable operation below its supply voltage, as well as a gain of 17.

The role of Ca traces in the passivation of silicon dioxide dielectrics for electron transport in pentacene organic field effect transistors

Recently, n-type transport in organic field effect transistors (OFETs) incorporating pentacene on a silicon dioxide (SiO2) dielectric has been demonstrated by Ahles et al. [Appl. Phys. Lett. 85, 4499 (2004)]. The electron transport was made possible by modifying the dielectric/semiconductor interface using traces of Ca. While the facilitation of electron current in pentacene remained unclear at that point, an interface near filling of electron trap states in the transistor channel or on the SiO2 dielectric could be suggested as a possible explanation. In the following the influence of the Ca interlayer on the n-type transport in pentacene based OFETs will be correlated with an x-ray photoelectron spectroscopy analysis of the SiO2/Ca interface, in dependence of the Ca layer thickness. It is demonstrated that for low thicknesses an oxidized Ca insulator is formed on the SiO2 dielectric, allowing for the observed pentacene electron transport. The formation of the oxide is suggested to compensate available el...

A sintered nanoparticle p-n junction observed by a Seebeck microscan

A nanoparticular p-n junction was realized by a field-assisted sintering process, using p-type and n-type doped silicon nanoparticles. A spatially resolved Seebeck microscan showed a broad transition from the positively doped to the negatively doped range. Overshoots on both sides are characteristic for the transition. Despite the tip size being much larger than the mean particle size, information about the dopant distribution between the particles is deduced from modeling the measured data under different assumptions, including the limited spatial resolution of the tip. The best match between measured and modeled data is achieved by the idea of doping compensation, due to the sintering process. Due to a short time at high temperature during the field-assisted sintering process, solid state diffusion is too slow to be solely responsible for the observed compensation of donors and acceptors over a wide range. Therefore, these measurements support a densification mechanism based on (partial) melting and rec...

A new adaptive light beam focusing principle for scanning light stimulation systems.

In this article a novel principle to achieve optimal focusing conditions or rather the smallest possible beam diameter for scanning light stimulation systems is presented. It is based on the following methodology: First, a reference point on a camera sensor is introduced where optimal focusing conditions are adjusted and the distance between the light focusing optic and the reference point is determined using a laser displacement sensor. In a second step, this displacement sensor is used to map the topography of the sample under investigation. Finally, the actual measurement is conducted, using optimal focusing conditions in each measurement point at the sample surface, that are determined by the height difference between camera sensor and the sample topography. This principle is independent of the measurement values, the optical or electrical properties of the sample, the used light source, or the selected wavelength. Furthermore, the samples can be tilted, rough, bent, or of different surface materials. In the following the principle is implemented using an optical beam induced current system, but basically it can be applied to any other scanning light stimulation system. Measurements to demonstrate its operation are shown, using a polycrystalline silicon solar cell.

Organic CMOS technology by interface treatment

In the present paper a new concept towards O-CMOS technology is presented substantiating the importance of the semiconductor/dielectric interface for charge carrier transport in organic semiconductors. It will be demonstrated that by controlling the interface properties of either SiO2 or PMMA, unipolar p- and n-type OFETs can be realized using a single organic semiconductor and even a single metal for source and drain contacts. Two dielectric/semiconductor interface modifications are considered for the realization of complementary OFETs on the basis of pentacene, otherwise known for its exclusive hole transporting properties. Selective modification of the SiO2 dielectric interface with traces of vacuum deposited Ca, allows for electron transport in pentacene and the realization of complementary pentacene OFETs on a single substrate. By this technique electron traps are removed due to a reaction of atomic Ca with oxygen from available hydroxide groups, resulting in the formation of an oxidized Ca layer. In a second approach, it is demonstrated that by selective UV treatment of a PMMA dielectric surface, unipolar n-type pentacene OFETs can be converted to unipolar p-type by the introduction of electron traps in the form of -OH and -COOH groups at the PMMA interface. Both methods allow for the realization of CMOS organic inverter stages with decent electrical properties.

Soluble Metal Oxo Alkoxide Inks with Advanced Rheological Properties for Inkjet-Printed Thin-Film Transistors

Semiconductor inks containing an indium-based oxo alkoxide precursor material were optimized regarding rheology requirements for a commercial 10 pL inkjet printhead. The rheological stability is evaluated by measuring the dynamic viscosity of the formulations for 12 h with a constant shear rate stress under ambient conditions. It is believed that the observed superior stability of the inks is the result of effectively suppressing the hydrolysis and condensation reaction between the metal oxo alkoxide precursor complex and atmospheric water. This can be attributed to a strong precursor coordination and the resulting reduction in ligand exchange dynamics of the solvent tetrahydrofurfuryl alcohol which is used as the main solvent in the formulations. It is also shown that with a proper selection of cosolvents, having high polar Hansen solubility parameter values, the inks drop formation properties and wettability can be fine-tuned by maintaining the inks rheological stability. Good drop jetting performance without satellite formation and high drop velocities of 8.25 m/s were found with the support of dimensionless numbers and printability windows. By printing single 10 pL ink dots onto short channel indium-tin-oxide electrodes, In2O3 calcination at 350 °C and a solution-processed back-channel protection, high average saturation mobility of approximately 10 cm2/(V s) are demonstrated in a bottom-contact coplanar thin-film transistor device structure.

Dielectric interface modification by UV irradiation : a novel method to control OFET charge carrier transport properties

The charge carrier transport in organic field effect transistors (OFETs) is determined by the transport properties of the insulator / organic semiconductor interface. We demonstrate that an adequate treatment of this interface results in a polarity change of the OFET charge carrier transport properties, without further altering the device structure. Illuminating the utilized PMMA polymer dielectric, by using UV radiation, leads to the introduction of mainly electron traps at the dielectric interface. This results in the suppression of the electron transport for an otherwise n-type pentacene OFET. However, as a consequence of trapped electrons in the near surface layer of the PMMA dielectric, the hole transport of the device is enabled though a hole blocking source/drain metallization. This effect, as well as the impact of the UV irradiation on the PMMA dielectric will be discussed in detail. The UV treatment yields a PMMA interface rich on polar groups. The influence of these groups on the OFET characteristics is investigated by studying several polymer dielectrics with varying content of the emerging groups.

Note: Automated optical focusing on encapsulated devices for scanning light stimulation systems

Recently, a scanning light stimulation system with an automated, adaptive focus correction during the measurement was introduced. Here, its application on encapsulated devices is discussed. This includes the changes an encapsulating optical medium introduces to the focusing process as well as to the subsequent light stimulation measurement. Further, the focusing method is modified to compensate for the influence of refraction and to maintain a minimum beam diameter on the sample surface.

Scanning Light Stimulation System With Active Focus Correction at

Recently, we introduced a scanning light stimulation system with an automated focus correction. The method is however resolution limited by a CMOS sensor used to focus the light beam. Here, to achieve a higher resolution, we enhanced the light beam focusing process by combining it with an edge detection technique. By this modification, the system resolution is improved down to a 1/e2 -diameter of 4.3 μm, as demonstrated using an example measurement on a multicrystalline solar cell. Furthermore, it is shown that the obtained resolution is mainly limited by height variations of the positioning system, and methods to compensate these limitations are discussed using example measurements on a quantum well solar cell.

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— A low-temperature polysilicon active-matrix process without the need for ion implantation to dope drain and source areas of TFTs has been developed. A doped silicon layer is deposited by PECVD and structured prior to the deposition of the intrinsic silicon for the channel. The dopant is diffused and activated during the excimer-laser crystallization step. N-channel test TFTs with different geometries were realized. The TFT properties (mobility, on/off ratio, saturation, etc.) are suitable to realize AMLCDs and AMOLED displays and to integrate driver electronics on the displays. In addition to simple TFTs, a full-color 4-in. quarter-VGA AMLCD was realized. The complete display (including photolithographic masks, active-matrix backplane, and color-filter/black-matrix frontplane), and an addressing system were developed and manufactured at the Chair of Display Technology, University of Stuttgart, Germany. The substitution of ion doping by PECVD deposition overcomes a major limitation for panel sizes in poly-Si technology and avoids large investment costs for ion-implantation equipment.