Contact resistance effects in organic n-channel thin-film transistors

Organic thin-film transistors (TFTs) have undergone tremendous progress in the past few years. Their great potential in terms of mechanical flexibility, light weight, low-cost and large-area fabrication makes them promising candidates for novel electronic products, such as sensor arrays, radio-frequency identification tags and flexible displays. For the realization of these applications and to improve their performance it is necessary to miniaturize organic TFTs to dimensions of a few micrometers or even less. However, with such aggressive scaling, the device physics becomes more and more important. Especially the contact resistance at and close to the interface between the organic semiconductor and the source/drain metal limits the performance of the organic TFTs at miniaturized dimensions. In this thesis, the contact properties of bottom-gate, top-contact n-channel organic transistors are investigated using the gated four-probe (GFP) technique. The TFTs employ the small-molecule semiconductor N,N'-bis-(1H,1H-heptafluorobutyl)-1,7-dicyano-perylene-3,4:9,10-tetracarboxylic diimide (PDI-FCN2) and are fabricated on flexible plastic substrates. The transistors are air-stable and can be operated at low voltages of about 3 V, owing to a thin hybrid gate dielectric composed of aluminum oxide and a self-assembled molecular monolayer. The GFP method enabled a detailed investigation of the influence of a multitude of factors, including the gate-source and drain-source voltages, the contact length, the temperature and the contact metal, on the contact resistance. The contact properties are found to be very sensitive to the thickness of the organic semiconductor layer. In transistors in which this layer is relatively thin, ohmic contact resistances as small as about 0.6 Ohm cm were measured. In transistors with a relatively thick organic semiconductor layer, device physics becomes more complex and the current-voltage characteristics of the contact become nonlinear. Space-charge limited currents are identified as the probable origin of this nonlinear behavior. The good performance of the PDI-FCN2 transistors allows the realization of simple integrated circuits. Flexible organic complementary ring oscillators are fabricated and signal propagation delays per stage as short as 6 µs are demonstrated at a supply voltage of 3.6 V

A detailed study of self-assembled (Al,Ga)InP quantum dots grown by molecular beam epitaxy

We report on the structural and optical properties of self-assembled (Al,Ga)InP quantum dots (QDs) with varying material composition embedded in a (Al0.30Ga0.70)0.51In0.49P matrix. The samples were grown by gas-source molecular beam epitaxy. Atomic force microscopy was used to study the structural properties of the quantum dots, revealing a strong dependence of the morphology on the material composition. Low-temperature ensemble photoluminescence was observed between 590 nm and 720 nm. Temperature and excitation power dependent, as well as time resolved measurements were performed, indicating a significantly reduced electron confinement and a reduced overlap of the electron/hole wavefunctions for Al- and/or Ga-rich compositions

A detailed study of self-assembled (Al,Ga)InP quantum dots grown by molecular beam epitaxy

We report on the structural and optical properties of self-assembled (Al,Ga)InP quantum dots (QDs) with varying material composition embedded in a (Al0.30Ga0.70)0.51In0.49P matrix. The samples were grown by gas-source molecular beam epitaxy. Atomic force microscopy was used to study the structural properties of the quantum dots, revealing a strong dependence of the morphology on the material composition. Low-temperature ensemble photoluminescence was observed between 590 nm and 720 nm. Temperature and excitation power dependent, as well as time resolved measurements were performed, indicating a significantly reduced electron confinement and a reduced overlap of the electron/hole wavefunctions for Al- and/or Ga-rich compositions

Kinetics of Local and Systemic Leucocyte and Cytokine Reaction of Calves to Intrabronchial Infection with Chlamydia psittaci.

Infection of cattle with chlamydiae is ubiquitous and, even in the absence of clinical sequeleae, has a quantifiable negative impact on livestock productivity. Despite recent progress, our knowledge about immune response mechanisms capable of counteracting the infection and preventing its detrimental effects is still limited. A well-established model of bovine acute respiratory Chlamydia (C.) psittaci infection was used here to characterize the kinetics of the local and systemic immune reactions in calves. In the course of two weeks following inoculation, leukocyte surface marker expression was monitored by flow cytometry in blood and bronchoalveolar lavage fluid (BALF). Immune-related protein and receptor transcription were determined by quantitative real-time reverse transcription PCR in blood, BALF and lung tissue. An early increase of IL2RA, IL10 and HSPA1A mRNA expressions was followed by a rise of lymphocytes, monocytes, and granulocytes exhibiting activated phenotypes in blood. Monocytes showed elevated expression rates of CD11b, CD14 and MHC class II. The rates of CD62L expression on CD8hi T cells in blood and on CD4+ T cells in BALF were also augmented and peaked between 2 and 4 dpi. Notably, CD25 antigen expression was significantly elevated, not only on CD8dim/CD62L+ and CD8-/CD62L+ cells in blood, but also on granulocytes in blood and BALF between 2-3 dpi. From 4 dpi onwards, changes declined and the calves recovered from the infection until 10 dpi. The findings highlight the effectiveness of rapid local and systemic immune reaction and indicate activated T cells, monocytes and granulocytes being essential for rapid eradication of the C. psittaci infection

Analysis of CD8α⁺ blood lymphocytes after <i>C</i>. <i>psittaci</i> infection of calves.

<p>Definition of subpopulations (a), representative CD25 expression (b and c), proportions of subpopulations (d), CD62L expression (e) and CD25 expression (f) on subpopulations are given. For labelling of x-axis, sample numbers and statistical analysis see legend to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135161#pone.0135161.g002" target="_blank">Fig 2</a>.</p

Analysis of CD4⁺ blood and BALF lymphocytes after <i>C</i>. <i>psittaci</i> inoculation of calves.

<p>For blood lymphocytes, definition of subpopulations (a), representative CD25 expression (b), proportions of subpopulations (c), CD62L expression on CD4⁺ cells (d), and CD25 expression on CD4⁺/CD62L⁺ and CD4⁺/CD62L^- cells (e) are given. All post-inoculation values were compared to ai-values with the Wilcoxon signed rank test, and then <i>P-</i>values were adjusted according to Holm (# 0.05 < <i>P</i> ≤ 0.1; * 0.01 < <i>P</i> ≤ 0.05; ** 0.001 < <i>P</i> ≤ 0.01; *** <i>P</i> ≤ 0.001). For BALF lymphocytes, definition of subpopulations (f), proportions of subpopulations (g), CD62L expression on CD4⁺ cells (h), and CD25 expression on CD4⁺/CD62L⁺ and CD4⁺/CD62L^- cells (i) are given. All values of infected animals were compared to values of healthy controls using the Mann-Whitney U test with Holm adjustment of <i>P</i>-values. Data are presented as mean and standard deviation obtained with samples from n = 30 animals (n = 20 at 10 dpi). ai: one hour before inoculation; C: healthy control animals (n = 7); numbers below x-axis refer to days post inoculation; MFI: mean fluorescence intensity.</p

Gating of leukocyte subpopulations by flow cytometry.

<p>Forward versus sideward scatter plots of blood leukocytes (a) and bronchoalveolar fluid (BALF) cells (b).</p

Quantitation of <i>CXCL8</i> and <i>TNFRSF9</i> specific mRNA in BALF cells and lung tissue after <i>Chlamydia psittaci</i> inoculation of calves.

<p>Levels of mRNA encoding for Interleukin 8 and TNFRSF9 in BALF cells, in macroscopically normal and in inflamed lung tissue sampled 14 days after inoculation (dpi) are given as relative abundance. C: healthy controls. All values of infected animals were compared to values of healthy controls using the Mann-Whitney U test with Holm adjustment of <i>P</i>-values (# 0.05 < <i>P</i> ≤ 0.1; ** 0.001 < <i>P</i> ≤ 0.01).</p

Analysis of blood monocytes after <i>Chlamydia psittaci</i> inoculation of calves.

<p>Expression intensity of CD11b (a) is given for all cells in the monocyte population, whereas expressions intensities of CD14 (b) and MHC-II (c) are given only for the CD14⁺ cells in the monocyte population. For labelling of x-axis, sample numbers and statistical analysis see legend to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135161#pone.0135161.g002" target="_blank">Fig 2</a>.</p