Andrei Sazonov

Low-Temperature Materials and Thin Film Transistors for Flexible Electronics

This paper addresses the low-temperature deposition processes and electronic properties of silicon based thin film semiconductors and dielectrics to enable the fabrication of mechanically flexible electronic devices on plastic substrates. Device quality amorphous hydrogenated silicon (a-Si:H), nanocrystalline silicon (nc-Si), and amorphous silicon nitride (a-SiN/sub x/) films and thin film transistors (TFTs) were made using existing industrial plasma deposition equipment at the process temperatures as low as 75/spl deg/C and 120/spl deg/C. The a-Si:H TFTs fabricated at 120/spl deg/C demonstrate performance similar to their high-temperature counterparts, including the field effect mobility (/spl mu//sub FE/) of 0.8 cm/sup 2/V/sup -1/s/sup -1/, the threshold voltage (V/sub T/) of 4.5 V, and the subthreshold slope of 0.5 V/dec, and can be used in active matrix (AM) displays including organic light emitting diode (OLED) displays. The a-Si:H TFTs fabricated at 75/spl deg/C exhibit /spl mu//sub FE/ of 0.6 cm/sup 2/V/sup -1/s/sup -1/, and V/sub T/ of 4 V. It is shown that further improvement in TFT performance can be achieved by using n/sup +/ nc-Si contact layers and plasma treatments of the interface between the gate dielectric and the channel layer. The results demonstrate that with appropriate process optimization, the large area thin film Si technology suits well the fabrication of electronic devices on low-cost plastic substrates.

Directly deposited nanocrystalline silicon thin-film transistors with ultra high mobilities

The authors report ultrahigh mobility nanocrystalline silicon thin-film transistors directly deposited by radio-frequency plasma enhanced chemical vapor deposition at 150°C. The transistors show maximum effective field effect mobilities of 450cm2∕Vs for electrons and 100cm2∕Vs for holes at room temperature. The authors argue that the key factor in their results is the reduction of the oxygen content, which acts as an accidental donor.

Amorphous silicon detector and thin film transistor technology for large area imaging of X-rays

This paper will review amorphous silicon imaging technology in terms of the detector operating principles, electrical and optoelectronic characteristics, and stability. Also, issues pertinent to thin film transistor stability will be presented along with optimization of materials and processing conditions for reduced V/sub T/-shift and leakage current. Selected results are shown for X-ray and optical detectors, thin film transistors, and integrated X-ray pixel structures. Extension of the current fabrication processes to low (<100/spl deg/C) temperature, enabling fabrication of thin film electronics on flexible (polymer) substrates, will also be discussed along with preliminary results.

Backplane Requirements for Active Matrix Organic Light Emitting Diode Displays

Organic light emitting diode (OLED) displays are a serious competitor to liquid crystal displays in view of their superior picture quality, higher contrast, faster on/off response, thinner profile, and high power efficiency. For large area and/or high-resolution applications, an active matrix OLED (AMOLED) addressing scheme is vital. The active matrix backplane can be made with amorphous silicon (a-Si), polysilicon, or organic technology, all of which suffer from threshold voltage shift and/or mismatch problems, causing temporal or spatial variations in the OLED brightness. In addition, the efficiency of the OLED itself degrades over time. Despite these shortcomings, there has been considerable progress in development of AMOLED displays using circuit solutions engineered to provide stable and uniform brightness. Indeed the design of AMOLED pixel circuits, particularly in low-mobility TFT technologies such as a-Si, is challenging due to the stringent requirements of timing, current matching, and low voltage operation. While circuit solutions are necessary, they are not sufficient. Process improvements to enhance TFT performance are becoming inevitable. This paper will review pertinent material requirements of AMOLED backplanes along with design considerations that address pixel architecture, contact resistance, and more importantly, the threshold voltage stability and associated gate overdrive voltage. In particular, we address the question of whether conventional PECVD can be deployed for high mobility and high stability TFTs, and if micro-/nano-crystalline silicon could provide the solution.

Absence of defect state creation in nanocrystalline silicon thin film transistors deduced from constant current stress measurements

The authors discuss time and temperature dependences of the shift in threshold voltage (Delta V-T) of nanocrystalline silicon (nc-Si) thin film transistors (TFTs) stressed at constant drain currents. In contrast to the behavior of the hydrogenated amorphous silicon (a-Si:H) counterpart, a weak temperature dependence of Delta V-T was observed. The results follow the charge trapping model and the predicted stretched-exponential time dependence that saturates at prolonged stress times. In addition, Delta V-T does not fit into the thermalization energy concept that was developed based on the defect state creation model for a-Si:H TFTs. The results indicate absence of defect state creation in nc-Si TFTs.

Stability of nanocrystalline silicon bottom-gate thin film transistors with silicon nitride gate dielectric

We report on the stability of nanocrystalline silicon (nc-Si) bottom-gate (BG) thin film transistors (TFTs) with various compositions of hydrogenated amorphous silicon nitride (a-SiNx:H) gate dielectric. TFTs with nitrogen-rich nitride exhibit higher output transconductance, threshold voltage stability, and effective field effect mobility (μFE) than the devices with silicon-rich gate dielectric. For example, μFE drops from 0.75to0.2cm2∕Vs when the gate dielectric composition [N]∕[Si] changes from 1.3 to 1. The corresponding threshold voltages (VT) are 4 and −2V. Following 5h electrical stress tests, the shift in threshold voltage (ΔVT) is larger for dielectrics with lower [N]∕[Si] content, regardless of the operating regime. Indeed, ΔVT in the saturation regime is considerably less and correlates with the charge concentration in the channel, i.e., ΔVT in saturation is about 2∕3 of that in the linear regime. Relaxation tests on the stressed TFTs show that the charge trapping is the instability mechanism in...

Amorphous silicon nitride deposited at 120 °C for organic light emitting display-thin film transistor arrays on plastic substrates

Nitrogen-rich amorphous silicon nitride (a-SiNx:H) films with [N]/[Si] ratios ranging from 1.4 to 1.7 were deposited by a 13.56 MHz plasma-enhanced chemical vapor deposition method at a temperature of 120 °C. The films’ composition, dielectric constant, electrical resistivity, and breakdown voltage were evaluated. The electrical properties of a-SiNx:H films with a [N]/[Si] ratio of more than 1.6 are superior to their lower N-content counterparts. Amorphous silicon thin film transistors (TFTs) that incorporate a-SiNx:H dielectrics were fabricated on glass and plastic substrates at a maximum processing temperature of 120 °C. The TFTs exhibit effective field effect mobility of 0.5–0.8 cm2/V s, an ON current of ∼10−5 A, an ON/OFF ratio of more than 106 and a subthreshold slope of 0.5 V/dec. The performance of the transistors seems to be compatible with application of them in active–matrix organic light emitting displays.

Leakage current mechanisms in top-gate nanocrystalline silicon thin film transistors

The leakage current in the top-gate nanocrystalline silicon (nc-Si:H) thin film transistors was examined at various temperatures in an attempt to deduce the underlying off-state conduction mechanisms. Under high gate bias, the leakage current can be attributed to the thermal emission of trapped carriers at the midgap grain boundary states at low drain bias, while the behavior is reminiscent of the Poole–Frenkel emission in the drain depletion region at high drain bias. In contrast, Ohmic conduction through the bulk nc-Si:H channel layer seems to be the dominant mechanism of the leakage current under low gate bias.

Thin film imaging technology on glass and plastic

Hydrogenated amorphous silicon (a-Si:H) technology offers a viable technological alternative for improved imaging of optical signals and high energy radiation. This paper reviews X-ray imaging technology in terms of detector operating principles, including optoelectronic characteristics, and fabrication process issues related to pixel (Schottky diode detector plus thin film transistor) integration. Recent results which describe the extension of the current fabrication processes to low (/spl sim/120/spl deg/C) temperature are also presented. The low temperature processing enables fabrication of thin electronics on flexible (polymer) substrates.

Effect of threshold voltage instability on field effect mobility in thin film transistors deduced from constant current measurements

The field effect (FE) mobility of thin film transistors is normally extracted using static measurement methods, which inherently rely on the assumption that the device remains stable during the measurement duration. However, these devices, particularly those based on emerging materials, can show large instability during the measurement, typically exhibiting hysteresis in the static characteristics. This letter looks at the effect of threshold voltage shift in FE mobility extracted using the conventional method, and introduces an alternative and more accurate technique of measuring device characteristics. The technique decouples the effect of transient phenomena, thus permitting extraction of the true device FE mobility, which turns out to be either over or underestimated depending on the magnitude and direction of threshold voltage shift.