Sang Jin Han

Photobias Instability of High Performance Solution Processed Amorphous Zinc Tin Oxide Transistors

The effects of the annealing temperature on the structural and chemical properties of soluble-processed zinc-tin-oxide (ZTO) films were examined by transmission electron microscopy, atomic force microscopy, high resolution X-ray reflectivity, and X-ray photoelectron spectroscopy. The density and purity of the resulting ZTO channel layer increased with increasing annealing temperature, whereas the oxygen vacancy defect density decreased. As a result, the device performance of soluble ZTO thin film transistors (TFTs) was improved at higher annealing temperature. Although the 300 °C-annealed ZTO TFT showed a marginal field-effect mobility (μFE) and high threshold voltage (Vth) of 0.1 cm(2)/(V s) and 7.3 V, respectively, the 500 °C-annealed device exhibited a reasonably high μFE, low subthreshold gate swing (SS), Vth, and Ion/off of 6.0 cm(2)/(V s), 0.28 V/decade, 0.58 V, and 4.0 × 10(7), respectively. The effects of dark negative bias stress (NBS) and negative bias illumination stress (NBIS) on the degradation of transfer characteristics of ZTO TFTs were also investigated. The instability of Vth values of the ZTO TFTs under NBS and NBIS conditions was suppressed with increasing annealing temperature. To better understand the charge trapping mechanism, the dynamics of Vth shift with NBS and NBIS time for all ZTO TFTs was analyzed on the basis of the stretched exponential relaxation. The negative Vth shift for each transistor was accelerated under NBIS conditions compared to NBS, which resulted in a higher dispersion parameter and smaller relaxation time for NBIS degradation. The relaxation time for NBS and NBIS instability increased with increasing annealing temperature, which is discussed on the basis of the transition mechanism of oxygen vacancy defects.

Impact of the cation composition on the electrical performance of solution-processed zinc tin oxide thin-film transistors.

This study examined the structural, chemical, and electrical properties of solution-processed (Zn,Sn)O3 (ZTO) films with various Sn/[Zn+Sn] ratios for potential applications to large-area flat panel displays. ZTO films with a Zn-rich composition had a polycrystalline wurtzite structure. On the other hand, the Sn-rich ZTO films exhibited a rutile structure, where the Zn atom was speculated to replace the Sn site, thereby acting as an acceptor. In the intermediate composition regions (Sn/[Zn+Sn] ratio from 0.28 to 0.48), the ZTO films had an amorphous structure, even after annealing at 450 °C. The electrical transport properties and photobias stability of ZTO thin film transistors (TFTs) were also examined according to the Sn/[Zn+Sn] ratio. The optimal transport property of ZTO TFT was observed for the device with an amorphous structure at a Sn/[Zn+Sn] ratio of 0.48. The mobility, threshold voltage, subthreshold swing, and on/off current ratio were 4.3 cm(2)/(V s), 0 V, 0.4 V/decade, and 4.1 × 10(7), respectively. In contrast, the device performance for the ZTO TFTs with either a higher or lower Sn concentration suffered from low mobility and a high off-state current, respectively. The photoelectrical stress measurements showed that the photobias stability of the ZTO TFTs was improved substantially when the ZTO semiconducting films had a lower oxygen vacancy concentration and an amorphous structure. The relevant rationale is discussed based on the phototransition and subsequent migration mechanism from neutral to positively charged oxygen vacancies.

Effect of sputter power on the photobias stability of zinc-tin-oxide field-effect transistors

This study examined the effect of sputtering power on the performance of zinc-tin-oxide field-effect transistors and the stability under photobias stress. Large improvements in the saturation mobility and subthreshold swing were found in devices fabricated at higher sputtering powers; 13.80 cm2/V·s, 0.33 V/decade at a power of 400 W compared with 2.70 cm2/V·s, 1.19 V/decade at a power of 50 W. The threshold voltage shift under negative bias illumination stress (NBIS) for the device fabricated at a power of 400 W shows superior properties (−2.41 V) compared with that (−5.56 V) of the device fabricated at 50 W. The improvements in electrical performance and NBIS stability were attributed to the formation of a denser film and the reduced dielectric/channel interfacial trap densities due to the more energetic bombardment used under high power sputtering conditions.

The Anomalous Effect of Oxygen Ratio on the Mobility and Photobias Stability of Sputtered Zinc–Tin–Oxide Transistors

The oxygen ratio-dependent device performance and reliability of zinc tin oxide (ZTO) thin-film transistors (TFTs) were examined. ZTO TFTs fabricated under pure Ar conditions exhibited a high saturation mobility of 21.5 cm²/Vs, low subthreshold gate swing of 0.34 V/decade, and high I_ON/OFF ratio (10⁹), whereas modest mobility of 7.3 cm²/Vs was obtained for the ZTO TFTs prepared at an oxygen ratio [R = O₂/(Ar + O₂)] of 0.3. The photobias stability (AV_th ≈-2.0 V) of the ZTO TFTs under the Ar only condition was much better than that (AV_th ≈-5.9 V) of the device at R of 0.3, even though their channel layers contained the larger density of oxygen vacancies. This abnormal behavior was attributed to the compressive stress of ZTO films retarding the phototransition of oxygen vacancies.

Dynamics of negative bias thermal stress-induced threshold voltage shifts in indium zinc oxide transistors: impact of the crystalline structure on the activation energy barrier

The kinetics of the negative bias thermal stress (NBTS)-induced Vth variations of indium zinc oxide (IZO) transistors with different crystallographic qualities were examined based on the stretched-exponential formalism. A poly-crystalline IZO device had a 0.64?eV lower activation barrier energy than an amorphous IZO device under NBTS conditions. This was attributed to the difference in the migration energy barrier between poly-crystalline and amorphous IZO films. For the recovery process, however, the activation energy barriers (?0.75?eV) were independent of the crystal structure. A plausible microscopic mechanism to account for the experimental results is proposed.