K. Remashan

Effect of N2O Plasma Treatment on the Performance of ZnO TFTs

Postfabrication rapid thermal annealing (RTA) and subsequent nitrous oxide (N 2 O) plasma treatment improved the performance of zinc oxide (ZnO) thin-film transistors (TFTs) in terms of off current and on/off current ratio by almost 2 orders of magnitude. The off current of 2 × 10 -8 A and on/off current ratio of 3 × 10 3 obtained after RTA were improved to 10 -10 A and 10 5 , respectively, by the subsequent N 2 O plasma treatment. X-ray photoelectron spectroscopy analysis of the TFT samples showed that the RTA-treated ZnO surface had more oxygen vacancies as compared to as-deposited samples, and the oxygen vacancies at the surface of RTA-treated ZnO were reduced by subsequent N 2 O plasma treatment. The reduction of oxygen vacancies at the top region of the ZnO channel is the cause of better off current and on/off current ratio of the TFTs.

High Performance MOCVD-Grown ZnO Thin-Film Transistor with a Thin MgZnO Layer at Channel/Gate Insulator Interface

ZnO thin-film transistors TFTs with and without a thin MgZnO layer at the channel/gate insulator interface were fabricated using glass substrates. Both ZnO and MgZnO films were grown by metal organic chemical vapor deposition MOCVD . The ZnO TFTs employing the MgZnO layer exhibit high performance with a field-effect mobility FE of 9.1 cm2/V s, a subthreshold slope S of 0.38 V/dec, an on/off current ratio of 2.3 108, and a turn-on voltage of −2.75 V. This is the best performance reported to date for ZnO TFTs that are realized on glass substrates with MOCVD-grown channel layers. The ZnO TFTs without the MgZnO layer, on the other hand, exhibit poor performance, and the FE, S, on/off current ratio, and turn-on voltage of these devices are 2.3 cm2/V s, 0.78 V/dec, 6.4 107, and −6.75 V, respectively. The superior performance of ZnO TFTs with the MgZnO layer is attributed to the larger grains in the ZnO film.

Impact of Hydrogenation of ZnO TFTs by Plasma-Deposited Silicon Nitride Gate Dielectric

Plasma-enhanced chemical vapor deposition grown silicon nitride gate insulator with high refractive index of 2.39 was employed as the source of hydrogen to hydrogenate zinc oxide (ZnO) thin-film transistors (TFTs) with bottom-gate configuration. The hydrogenated TFTs exhibited a field-effect mobility of 7.8 cm2/Vmiddots, an on/off current ratio of 106, and a subthreshold slope of 1.2 V/dec. In comparison, TFTs using silicon nitrides with lower refractive indices of 2.26, 1.92, and 1.80 showed relatively poor performance. Dynamic secondary ion mass spectroscopy study showed that the amount of hydrogen present in the ZnO TFT structures using high refractive index silicon nitride gate dielectric is higher than that in the TFT samples using low-refractive index silicon nitride, which indicate the evidence of hydrogenation of ZnO TFTs by high refractive index silicon nitride gate dielectric. The enhanced performance of the hydrogenated TFTs is attributed to the passivation of ZnO/dielectric interface states and doping of the channel by hydrogenation effect.

Stable gallium arsenide MIS capacitors and MIS field effect transistors by (NH/sub 4/)/sub 2/S/sub x/ treatment and hydrogenation using plasma deposited silicon nitride gate insulator

The beneficial effects of sulfur passivation of gallium arsenide (GaAs) surface by (NH/sub 4/)/sub 2/S/sub x/ chemical treatment and by hydrogenation of the insulator-GaAs interface using the plasma-enhanced chemical vapor-deposited (PECVD) silicon nitride gate dielectric film as the source of hydrogen are illustrated by fabricating Al/PECVD silicon nitride/n-GaAs MIS capacitors and metal insulator semiconductor field effect transistors (MISFET). Post metallization annealing (PMA) at temperatures in the range 450-550/spl deg/C is shown to be the key process for achieving midgap interface state density below 10/sup 11//cm/sup 2//eV and maximum incremental transconductance, which is about 75% of the theoretical maximum limit. MIS capacitors are fabricated on (NH/sub 4/)/sub 2/S/sub x/ treated GaAs substrate using gate dielectrics such as PECVD SiO/sub 2/ and silicon oxynitride to demonstrate that the PMA is less effective with these dielectrics because of their lower hydrogen content. The small signal AC transconductance, g/sub ms/ measurements on MISFETs fabricated using silicon nitride, have shown that the low-frequency degradation of g/sub ms/ is almost absent in the devices fabricated on (NH/sub 4/)/sub 2/S/sub x/-treated GaAs substrates and subjected to PMA. The drain current stability in these devices is demonstrated to be excellent, with an initial drift of only 2% of the starting value. The dual role of silicon nitride layer, namely, protection against loss of sulfur and an excellent source of hydrogen for additional surface passivation along with sulfur is demonstrated by comparing the transconductance of MISFETs fabricated on GaAs substrates annealed without the nitride cap after the (NH/sub 4/)/sub 2/S/sub x/ treatment.

ZnO-based thin film transistors having high refractive index silicon nitride gate

The effect of properties of silicon nitride films on the electrical performance of zinc oxide (ZnO) thin film transistors (TFTs) has been investigated by utilizing silicon nitride films having refractive indices of 2.45 and 1.85. The ZnO TFTs having a silicon nitride with a high refractive index of 2.45 exhibited a field effect mobility of 8cm2∕Vs, on/off current ratio of 106, and subthreshold slope of 0.9V/decade. On the other hand, TFTs having a silicon nitride with a low refractive index of 1.85 showed a field effect mobility of 0.5cm2∕Vs, on/off current ratio of less than 102, and subthreshold slope of 19V/decade. The improved device performance was ascribed to a better interface between ZnO and high refractive index silicon nitride, and hydrogenation of the ZnO channel with the hydrogen originated from the high refractive index silicon nitride.

Effect of Rapid Thermal Annealing on the Electrical Characteristics of ZnO Thin-Film Transistors

Thin-film transistors (TFTs) with a bottom-gate configuration were fabricated with an RF magnetron sputtered undoped zinc oxide (ZnO) channel layer and plasma-enhanced chemical vapor deposition (PECVD) grown silicon nitride as a gate dielectric. Postfabrication rapid thermal annealing (RTA) and subsequent nitrous oxide (N2O) plasma treatment were employed to improve the performance of ZnO TFTs in terms of on-current and on/off current ratio. The RTA treatment increases the on-current of the TFT significantly, but it also increases its off-current. The off-current of 2×10-8 A and on/off current ratio of 3×103 obtained after the RTA treatment were improved to 10-10 A and 105, respectively, by the subsequent N2O plasma treatment. The better device performance can be attributed to the reduction of oxygen vacancies at the top region of the channel due to oxygen incorporation from the N2O plasma. X-ray photoelectron spectroscopy (XPS) analysis of the TFT samples showed that the RTA-treated ZnO surface has more oxygen vacancies than as-deposited samples, which results in the increased drain current. The XPS study also showed that the subsequent N2O plasma treatment reduces oxygen vacancies only at the surface of ZnO so that the better off-current and on/off current ratio can be obtained.

Effects of octa decyl thiol (ODT) treatment on the gallium arsenide surface and interface state density

Abstract The ability of Octa Decyl Thiol (ODT) compound in passivating Gallium Arsenide (GaAs) surfaces is studied by comparing the XPS spectra on the ODT treated and untreated GaAs samples and by comparing the electrical characteristics of Schottky diodes and MIS capacitors fabricated on them. A detailed study on MIS capacitors and an evaluation of the interface state density ( D it ) has clearly demonstrated that even though the ODT treatment reduces the interface state density in the upper half of the band gap more than an order of magnitude compared to untreated samples, it is not effective in reducing the midgap D it values. From the XPS spectra taken on the ODT treated GaAs surfaces, this is attributed to an island-like ODT coating and to the inability of this sulphur compound treatment to remove the native oxide present on the GaAs surface.

Enhanced Performance of MOCVD ZnO TFTs on Glass Substrates with Nitrogen-Rich Silicon Nitride Gate Dielectric

Thin-film transistors (TFTs) on glass substrates were fabricated using ZnO, grown by a metallorganic chemical vapor deposition (MOCVD) technique, with N- and Si-rich silicon nitrides as gate dielectrics. This is a report on MOCVD-grown ZnO TFTs that use silicon nitride as gate dielectrics. The ZnO TFTs using N-rich silicon nitride exhibited a field-effect mobility of 6.5 cm 2 /v s, a subthreshold slope of 0.8 V/decade, an on/off current ratio of 10 8 , and a threshold voltage of 2.05 V. The performance of these TFTs is better than that of TFTs employing Si-rich silicon nitride. This enhanced device performance can be attributed to a larger average grain size of 126 nm observed in the ZnO film grown on the N-rich silicon nitride compared to an average grain size of 69 nm for the ZnO film grown on Si-rich silicon nitride.

Inductively coupled plasma etching of GaN using BCl3/Cl2 chemistry and photoluminescence studies of the etched samples

Inductively coupled plasma (ICP) etching of GaN is investigated using BCl3 /Cl2 chemistry. The maximum etch rate is observed when the percentage of Cl2 in the BCl3 /Cl2 gas mixture is about 80-100%. From photoluminescence (PL) study of the etched GaN samples, we found that the ICP etching creates non-radiative surface recombination states and it has been observed that the creation of surface states is a minimum when the Cl2 in the BCl3 /Cl2 mixture is about 90-100%. The atomic force microscope (AFM) study shows that the etching does not make the surface rough and the root mean square (rms) roughness of the etched surface is about 3-5 nm.

Improved Characteristics of Metal Organic Chemical Vapor Deposition-Grown ZnO Thin-Film Transistors by Controlling VI/II Ratio of ZnO Film Growth and Using a Modified Thin-Film Transistor Layer Structure

Thin-film transistors (TFTs) were fabricated on glass substrates using ZnO channel layers grown with two VI/II (oxygen/diethylzinc) ratios of 25,000 and 100,000 by metal organic chemical vapor deposition (MOCVD). The ZnO TFTs employing the channel grown with a VI/II ratio of 25,000 exhibit a field-effect mobility (µFE) of 4.3 cm2 V-1 s-1, a subthreshold slope (SS) of 1.3 V/dec, and an on/off current ratio of 9×107. On the other hand, ZnO TFTs using the channel grown with a higher VI/II ratio of 100,000 exhibit a µFE of 2.1 cm2 V-1 s-1, an SS of 0.64 V/dec, and an on/off current ratio of 1×108. The improvement in SS and the reduction in µFE are respectively attributed to the lower growth rate and smaller grain size of the ZnO film. The ZnO TFTs fabricated by using the higher-VI/II-ratio-grown ZnO layer together with a thin MOCVD-grown MgZnO layer at the channel/gate insulator interface exhibit high performance, and their µFE, SS, and on/off current ratio are 8.9 cm2 V-1 s-1, 0.42 V/dec, and 3×108, respectively. This is the best performance reported to date for ZnO TFTs that are realized on glass substrates with MOCVD-grown channel layers. The superior performance of ZnO TFTs with the MgZnO layer is ascribed to the larger grain size of the ZnO film and the better channel/gate insulator interface.