Bong Seob Yang

Role of ZrO2 incorporation in the suppression of negative bias illumination- induced instability in Zn-Sn-O thin film transistors

Thin film transistors (TFTs) with In and Ga-free multicomponent Zn–Sn–Zr–O (ZTZO) channel layers were fabricated using the cosputtering approach. The incorporation of ZrO2 into the Zn–Sn–O (ZTO) films increased the contact resistance, which led to the degradation of the transport properties. In contrast, the threshold voltage shift under negative bias illumination stress (NBIS) was largely improved from −12.5 V (ZTO device) to −4.2 V (ZTZO device). This improvement was attributed to the reduction in the oxygen vacancy defects in the ZTZO film, suggesting that the photoinduced transition from VO to VO2+ was responsible for the NBIS-induced instability.

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.

Improvement of the photo-bias stability of the Zn–Sn–O field effect transistors by an ozone treatment

Highly improved negative bias illumination stress stability was achieved in a Zn–Sn–O field effect transistor after an ozone (O3) treatment. The untreated ZTO FET exhibited a huge negative threshold voltage shift of 4.2 V but the O3 treated device exhibited superior stability under NBIS conditions: the Vth value of the O3 treated ZTO FET for 600 s showed almost no change (ΔVth = −0.07 V) under the same NBIS. The improvement in NBIS stability of the O3 treated ZTO FETs was attributed to the lower oxygen vacancy concentration and retarded desorption of adsorbed oxygen under photon irradiation by the O3 treatment.

Anomalous behavior of negative bias illumination stress instability in an indium zinc oxide transistor: A cation combinatorial approach

This study examined the effects of the indium fraction in indium zinc oxide (IZO) on the performance and stability of IZO thin film transistors (TFTs). The field-effect mobility and sub-threshold swing were much improved with increasing In fraction; 41.0 cm2/Vs and 0.2 V/decade, respectively, at 85 at. % In, compared to 1.1 cm2/Vs and 2.4 V/decade of ZnO TFTs. In contrast, a local minimum negative bias illumination stress instability was observed near 73–77 at. % In. This behavior was explained by a poly-crystalline to amorphous phase transition in IZO thin films.

The reason for the increased threshold switching voltage of SiO2 doped Ge2Sb2Te5 thin films for phase change random access memory

This study examined the threshold switching voltage (VT) of 150 nm thick SiO2 doped Ge2Sb2Te5 (SGST) films for phase change random access memory applications. The VT of the SGST films increased from ∼0.9 V (for GST) to ∼1.5 V with increasing SiO2 content. The optical band gap and Urbach edge of the SGST films were similar regardless of the SiO2 concentration. The dielectric constant decreased by ∼37% and the electrical resistivity increased by ∼19%. The increase in VT of SGST films is associated with an effective increase in electric field and the decreased generation rate caused by impact ionization.

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.

Improvement of electrical and optical properties of molybdenum oxide thin films by ultralow pressure sputtering method

In this work, we investigated the structural, electrical and optical properties of molybdenum oxide thin films deposited by the reactive dc magnetron sputtering method. The molybdenum oxide films were prepared at sputtering pressures ranging from 6.7 × 10−1 to 6.7 × 10−2 Pa. In order to promote their electrical conductivity, all the deposited MoOx films were annealed in Ar ambient at 450 °C for 8 h. The resistivity of the MoOx films varied from 10−4 to 10−2 Ω cm depending on the O2 content in the sputtering ambient. The lowering of the resistivity of the MoO2 films was mainly attributed to the formation of a monoclinic MoO2 polycrystalline phase. As the sputtering pressure decreased, the content of monoclinic polycrystalline MoO2 phase increased, resulting in low resistivity films. The formation of the dominant MoO2 phase at lower sputtering pressures was attributed to the stress induced crystallization. The post-deposition annealed (PDA) MoOx film, deposited at an ultralow sputtering pressure (6.7 × 10−2 Pa) and O2 content of 40%, had an atomic ratio of O to Mo ≈ 2.85 and was highly transparent and conductive: the transmittance in the visible wavelength range of 400–500 nm was about 73% and the resistivity was 1.05 × 10−3 Ω cm. This result is superior to those of MoOx films epitaxially grown by the pulse laser deposition method.In this work, we investigated the structural, electrical and optical properties of molybdenum oxide thin films deposited by the reactive dc magnetron sputtering method. The molybdenum oxide films were prepared at sputtering pressures ranging from 6.7 × 10−1 to 6.7 × 10−2 Pa. In order to promote their electrical conductivity, all the deposited MoOx films were annealed in Ar ambient at 450 °C for 8 h. The resistivity of the MoOx films varied from 10−4 to 10−2 Ω cm depending on the O2 content in the sputtering ambient. The lowering of the resistivity of the MoO2 films was mainly attributed to the formation of a monoclinic MoO2 polycrystalline phase. As the sputtering pressure decreased, the content of monoclinic polycrystalline MoO2 phase increased, resulting in low resistivity films. The formation of the dominant MoO2 phase at lower sputtering pressures was attributed to the stress induced crystallization. The post-deposition annealed (PDA) MoOx film, deposited at an ultralow sputtering pressure (6.7 × 10−2...

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.

Comprehensive Studies on the Carrier Transporting Property and Photo-Bias Instability of Sputtered Zinc Tin Oxide Thin Film Transistors

This study examined the effects of the chamber pressure, radio frequency power and oxygen flow ratio during channel deposition on the performance and photobias stability of zinc tin oxide (ZTO) thin film transistors (TFTs). The densification of the ZTO thin film allowed the improvement in the field-effect mobility and the suppression in the negative bias illumination stress (NBIS) instability of the resulting TFTs simultaneously, irrespective of the specific process condition. The porosity in the ZTO channel layer was shown to prevent the effective intercalation of the Sn 5s orbital and, thus, deteriorate the field-effect mobility. Furthermore, the increased effective surface area in the porous ZTO film adversely affected the NBIS stability of the resulting TFTs because the porosity-related surface states and oxygen vacancy defects provide the hole trapping centers and the delocalized electron free carrier, respectively. Therefore, the densification of ZTO channel layer is a key factor for the high mobility and good photobias stability of the TFTs. This concept can be applicable for any metal-oxide-TFTs.

Improving the Performance of Tin Oxide Thin-Film Transistors by Using Ultralow Pressure Sputtering

Thin-film transistors (TFTs) are fabricated with a tin oxide channel deposited by using ultralow pressure sputtering (ULPS). The effect of sputtering pressure on the device performance of the tin oxide TFTs was investigated. The TFTs with tin oxide channel deposited by conventional sputtering pressure did not show a promising performance. However, the saturation mobility (μ sat ) and the threshold voltage (V th ) of the ULPS-deposited SnO x TFTs were improved to ~3.9 cm 2 /V s and ~0.6 V, respectively. The better device performance of the ULPS-deposited SnO x TFT was attributed to the reduced free electron density (~10 17 /cm 3 ) resulting from the formation of a nanocrystalline phase.