ICMCTF2011 Session C2/F4-3: Thin Films for Photovoltaics and Active Devices: Synthesis and Characterization

Transparent conductive oxides (TCO´s) are well established materials for transparent electrodes in various applications. By using TCO´s a good combination of transparence and conductivity can be obtained. However, the conductivity of TCO´s is lower by a factor 100…1000 in comparison to metals. On the other hand side, the deposition of thin, transparent metal films is state of the art, for example on architectural glass. These films are transparent but non-conductive.

The aim of the recent study is to create transparent silver films with high electrical conductivity.

The reason of the non conductivity of classical thin silver films is the island like structure of these films. The atoms tend to form separated islands during the growth of the first atomic layers. Within the thickness range of approximately 10 nm (percolation threshold) the film converts to a closed layer with high electrical conductivity and low transparence.

The way to obtain a high conductivity below 10 nm film thickness is to avoid the island-like film growth at the beginning. This can be realized by using highly energetic silver ions for the film deposition. The silver ions are implanted into the topmost atomic layers of the substrate surface. Thus the nucleation into clusters or islands is prevented, a uniform coverage of the surface is provided and an electrical conductivity at very low film thicknesses is obtained.

The filtered high current pulsed arc technology (F-HCA) can provide multiple charged metal ions. Thin silver films produced by F-HCA were deposited on glass substrates. It could be shown, that films with a few nm in thickness can provide optical and electrical properties comparable to TCO´s in the sub-micron range. It can be estimated that thin transparent conductive metallic films have a great potential for a lot of applications, such as for displays, solar cells and others.

Transparent conducting oxides are attractive materials from a viewpoint of transparent electronics. One of the most promising applications is a transparent thin film transistor for next generation displays. When we apply these materials to an actual device, it is important to know energy positions of the valence band maxima (VBM) and the conduction band minima (CBM) with respect to the vacuum level. In this talk, the VBMs and the CBMs of transparent conducting oxides and related materials are determined by using ultraviolet photoelectron spectroscopy (UPS) and summarized in an energy band lineup. In addition, interfacial electronic structures between some of these materials and typical organic semiconductors measured by UPS will also be given as an application of the band lineup. High efficient carrier injection is an important issue to be solved to realize high performance heterojunction devices including OLEDs and the band lineup provide useful information to find out candidates of high efficient electrodes. We have examined RT-stable electride, C12A7:e^–, [1] and Cu⁺ based p-type degenerated semiconductors, LaCuOSe [2] and Cu_xSe, [3] as candidates of high performance cathode and anode, respectively.

[1] S, Matsuishi et al., Science, 301, (2003) 626.

[2] H. Hiramatsu et al., Appl. Phys. Lett. 91, (2007) 012104.

[3] H. Hiramatsu et al., J. Appl. Phys. 104, (2008) 113723.

Most organic light-emitting diodes (OLEDs) have a multilayered structure composed of functional organic layers sandwiched between two electrodes. Thin films of small molecules are generally deposited by thermal evaporation onto glass or other rigid or flexible substrates. The interface state between two organic layers in OLED device depends on the surface morphology of the layers and affects deeply the OLED performance. The variation in the morphology of an organic thin film depends on the substrate, the contamination of the substrate, the deposition rate and the substrate temperature. For organic films substrate temperature cannot be increased too much due to their thermal stability. However, studies in inorganic thin film indicate that it is possible modify the morphology a film by using substrate ultrasonic vibration in the kHz range without increasing the substrate temperature.

Mechanism of the instability for indium-gallium-zinc oxide thin film transistors caused by gate-bias stress performed in the dark and light illumination was investigated in this paper. The parallel V_t shift with no degradation of subthreshold swing (S.S) and the fine fitting to the stretched-exponential equation indicate that charge trapping model dominates the degradation behavior under positive gate-bias stress. In addition, the significant gate-bias dependence of V_t shift demonstrates that electron trapping effect easily occurs under large gate-bias since the average effective energy barrier of electron injection decreases with increasing gate bias. Moreover, the noticeable decrease of threshold voltage (V_t) shift under illuminated positive gate-bias stress and the accelerated recovery rate in the light indicate the charge detrapping mechanism occur under light illumination. Finally, the apparent negative V_t shift under illuminated negative gate-bias stress was investigated in this paper. The average effectively energy barrier of electron and hole injection were extracted to clarify that the serious V_t degradation behavior comparing with positive gate-bias stress was attributed to the lower energy barrier for hole injection.

This paper investigates the impact of N₂O plasma treatment on the light-induced instability of InGaZnO thin film transistors with SiO₂ passivation layer deposited by plasma-enhanced-chemical-vapor-deposition (PECVD). For untreated device, because the SiO₂ passivation layer deposited by PECVD would cause extra trap states, thus, the anomalous subthreshold leakage current is attributed to the source side barrier lowering effect induced by trap-assisted photogenerated hole. Contrarily, the device treated by N₂O plasma both on gate insulator and active layer could effectively suppress the instability under illumination. In order to clarify the influence of N₂O plasma treatment, device with N₂O plasma treatment only on gate insulator was investigated. The slight improvement of the light-induced subthreshold leakage current for N₂O plasma treatment on gate insulator demonstrates that post N₂O plasma treatment on active layer was critical to prevent the damage from the SiO₂ passivation process. On the other hand, the instability of threshold voltage (V_t) under illuminated negative bias stress (NBIS) was significantly improved by N₂O plasma treatment in contrast to the untreated device. Furthermore, the different dark recovery rate follows NBIS for untreated and N₂O plasma-treated device indicates different hole trapping levels in the energy band.

Hazardous effects of chlorine (Cl₂) gas on environment and biological system makes it one of the important pollution issues. Here, we report the fabrication and characterization of chemiresistive chlorine gas sensor based on copper(II) 1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine (CuPc(OBu)₈) films, deposited by low cost spin coating technique. CuPc(OBu)₈ films of thickness 50 nm were chosen for the fabrication of chemiresistive gas sensor. This gas sensor shows a response of the order of 90 % to few parts per million level of chlorine with response time of 5 minutes at room temperature 25 °C. The interactions between sensor and analytes followed first order kinetics with rate constant 0.5 ≤ k ≤ 1. The chemiresistive sensor showed very good stability at room temperature over a long period of time. The effect of gas sensing temperature and concentration of gas has also been observed.