Jerzy Kanicki

Bias‐stress‐induced stretched‐exponential time dependence of charge injection and trapping in amorphous thin‐film transistors

The threshold voltage instabilities in nitride/oxide dual gate dielectric hydrogenated amorphous silicon (a‐Si:H) thin‐film transistors are investigated as a function of stress time, stress temperature, and stress bias. The obtained results are explained with a multiple trapping model rather than weak bond breaking model. In our model, the injected carriers from the a‐Si:H channel first thermalize in a broad distribution of localized band‐tail states located at the a‐Si:H/aSiNx:H interface and in the a‐SiNx:H transitional layer close to the interface, then move to deeper energies in amorphous silicon nitride at longer stress times, larger stress electric fields, or higher stress temperatures. The obtained bias‐stress‐temperature induced threshold voltage shifts are accurately modeled with a stretched‐exponential stress time dependence where the stretched‐exponent β cannot be related to the β=TST/T0 but rather to β≂TST/T0*−β0 for TST≤80 °C; for TST≥80 °C, the β is stress temperature independent. We have al...

Thin-film organic polymer phototransistors

We have studied the electrical performance of organic polymer thin-film transistors (OP-TFTs) under steady-state white-light illumination, as well as the performance of these devices as photodetectors. The off-state drain current of the OP-TFT is significantly increased due to the illumination, while a smaller relative effect is observed on the drain current in the strong-accumulation regime. The illumination effectively decreases the threshold voltage of the device and increases the apparent subthreshold swing, while the field-effect mobility of the charge carriers in the polymer channel is unchanged. We have observed full recovery of our devices after the illumination is removed at room temperature. These observations are explained in terms of the photogeneration of excitons due to the absorbed photons. The photogenerated excitons subsequently diffuse and dissociate into free charge carriers, thereby enhancing the carrier density in the channel of the device. We have found broadband responsivities of approximately 0.7 mA/W for devices biased in the strong-accumulation regime and gate-to-source voltage-independent photosensitivities of approximately 10/sup 3/ for devices in the off-state. We also determine, for the first time, the flatband voltage of these devices to be about -2.3 V.

Two-dimensional numerical simulation of radio frequency sputter amorphous In-Ga-Zn-O thin-film transistors

We reported on a two-dimensional simulation of electrical properties of the radio frequency (rf) sputter amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs). The a-IGZO TFT used in this work has the following performance: field-effect mobility (μeff) of ∼12 cm2/V s, threshold voltage (Vth) of ∼1.15 V, subthreshold swing (S) of ∼0.13 V/dec, and on/off ratio over 1010. To accurately simulate the measured transistor electrical properties, the density-of-states model is developed. The donorlike states are also proposed to be associated with the oxygen vacancy in a-IGZO. The experimental and calculated results show that the rf sputter a-IGZO TFT has a very sharp conduction band-tail slope distribution (Ea=13 meV) and Ti ohmic-like source/drain contacts with a specific contact resistance lower than 2.7×10−3 Ω cm2.

Current-source a-Si:H thin-film transistor circuit for active-matrix organic light-emitting displays

In this letter, we describe a four thin-film-transistor (TFT) circuit based on hydrogenated amorphous silicon (a-Si:H) technology. This circuit can provide a constant output current level and can be automatically adjusted for TFT threshold voltage variations. The experimental results indicated that, for TFT threshold voltage shift as large as /spl sim/3 V, the output current variations can be less than 1 and 5% for high (/spl ges/0.5 /spl mu/A) and low (/spl les/0.1 /spl mu/A) current levels, respectively. This circuit can potentially be used for the active-matrix organic light-emitting displays (AM-OLEDs).

Electrically active point defects in amorphous silicon nitride: An illumination and charge injection study

We observe a strong correlation between changes in the density of paramagnetic silicon ‘‘dangling‐bond’’ centers and changes in the space‐charge density in amorphous silicon nitride films subjected alternately to illumination and both positive‐ and negative‐charge injection. We demonstrate that ultraviolet illumination annihilates space charge and creates stable paramagnetic centers in silicon nitride. These centers can be passivated with a 1‐h anneal at 250 °C. Our results provide the first direct experimental evidence associating a specific point defect with the trapping phenomena in amorphous silicon nitride. We also demonstrate both directly and for the first time the amphoteric nature of the silicon nitride dangling‐bond center. Furthermore, our ability to cycle the defect between its paramagnetic neutral state and both its charged diamagnetic states suggests that the optical generation of dangling bonds in amorphous silicon nitride involves no complex structural rearrangement, but simply changes in ...

Density of States of a-InGaZnO From Temperature-Dependent Field-Effect Studies

Temperature-dependent field-effect measurements were performed on radio-frequency sputtered amorphous In-Ga-Zn-O thin film transistors (TFTs). We studied the effect of temperature on the TFT electrical properties. We observed that the field-effect mobility (mu) increases and the threshold voltage (V T) shifts negatively with temperature, while the current on-off ratio and subthreshold slope (S) remain almost unchanged. We also observed that the TFT drain current (ID) is thermally activated, and the relation between the prefactor (ID0) and activation energy (E a) obeys the Meyer-Neldel rule. The density of localized gap states (DOS) was then calculated by using a self-consistent method based on the experimentally obtained E a. The result shows good agreement with the DOS distribution calculated from SPICE simulations.

ELECTRICAL INSTABILITY OF HYDROGENATED AMORPHOUS SILICON THIN-FILM TRANSISTORS FOR ACTIVE-MATRIX LIQUID-CRYSTAL DISPLAYS

We investigated the threshold voltage shifts (ΔVT) of inverted-staggered hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) induced by steady-state (dc) and pulsed (ac) gate bias-temperature-stress (BTS) conditions. Our study showed that, for an equivalent effective-stress-time, ΔVT has an apparent pulse-width dependence under negative BTS conditions–the narrower the pulse width, the smaller the ΔVT. This gate-bias pulse-width dependence is explained by an effective-carrier-concentration model, which relates ΔVT for negative pulsed gate-bias stress to the concentration of mobile carriers accumulated in the conduction channel along the a-Si:H/gate insulator interface. In addition, our investigation of the methodology of a-Si:H TFT electrical reliability evaluation indicates that, instead of steady-state BTS, pulsed BTS should be used to build the database needed to extrapolate ΔVT induced by a long-term display operation. Using these experimental results, we have shown that a-Si:H TFTs have a satisfactory electrical reliability for a long-term active-matrix liquid-crystal display (AMLCD) operation.

Gate dielectric and contact effects in hydrogenated amorphous silicon‐silicon nitride thin‐film transistors

The characteristics of glow‐discharge hydrogenated amorphous silicon‐silicon nitride (a‐Si:H/a‐SiNx:H) thin‐film transistors (TFTs) are reported for various deposition conditions. TFTs incorporating a N‐rich nitride gate dielectric, a‐SiN1.6:H, are superior to a‐Si:H TFTs with a Si‐rich gate nitride, a‐SiN1.2:H. In particular, the N‐rich gate nitride TFTs show considerably less interface or near‐interface charging during operation, improved stability, and a higher field‐effect mobility. The average field‐effect mobility μFE is found to be 0.27 and 0.41 cm2/V s for the Si‐ and N‐rich gate nitride TFTs, respectively. A further improvement in mobility, μFE =0.61 cm2/V s, is achieved by increasing the N‐rich gate nitride deposition temperature from 250 to 450 °C. These results suggest that N‐rich a‐SiNx:H, deposited at elevated temperatures, yields a more abrupt or ‘‘cleaner’’ a‐SiNx:H/a‐Si:H interface. We also show, for the first time, that using n+ μc‐Si:H source‐drain contacts in place of n+ a‐Si:H improve...

Performance of thin hydrogenated amorphous silicon thin-film transistors

In this paper we have analyzed the influence of the mask channel length (LM) on the performance of the 55‐nm‐hydrogenated amorphous silicon (a‐Si:H) thin‐film transistors (TFTs), incorporating nitrogen‐rich hydrogenated amorphous silicon nitride gate dielectric and phosphorus‐doped microcrystalline silicon (n+μc‐Si:H) source/drain (S/D) contacts. In our TFTs the n+μc‐Si:H S/D contacts have a specific contact resistance around or below 0.5 Ω cm2. We have shown that in our TFTs a field‐effect mobility and threshold voltage are dependent on LM, and this dependence is most likely due to the influence of the S/D contact series resistance on TFTs characteristics. Finally, we have demonstrated that if the mask channel length is extended by a ΔL (which is a distance from the S/D via edge at which the electron injection/collection is taking place) the field‐effect mobility and threshold voltage are independent of the channel length. In such a case μFE, VT, and ON/OFF current ratio around 0.76 cm2/V s, 2.5 V, and 1...

High-efficiency organic polymer light-emitting heterostructure devices on flexible plastic substrates

In this letter, we describe a high-efficiency organic polymer light-emitting heterostructure device with aluminum cathode fabricated on a thin, flexible plastic substrate. The device consists of a hole transporting (amine-fluorene) and an emissive (benzothiadiazole-fluorene) conjugated organic polymer layers. The best heterostructure device has a green light emission and a maximum luminance higher than 2000 cd/m2. Device shows a maximum emission of ∼56.2 cd/A and, accordingly, a maximum luminous and external quantum efficiency of ∼9.0 lm/W and ∼15%, respectively. This organic light-emitting diode performance is acceptable for flat panel display applications.