Naho Itagaki

Combinatorial approach to thin-film transistors using multicomponent semiconductor channels: An application to amorphous oxide semiconductors in In–Ga–Zn–O system

A combinatorial approach was applied to thin-film transistors (TFTs) using amorphous In–Ga–Zn–O semiconductor channels. A large number of TFTs, having n-type channels with different chemical compositions, were fabricated simultaneously on a substrate. A systematic relation was clarified among the compositional ratio of In:Ga:Zn, oxygen partial pressure in film deposition atmosphere, and TFT characteristics. The results provide an experimental basis to understand the roles of each metallic element in the In–Ga–Zn–O system. This information leads to a guideline to tune the metallic compositions for required TFT specifications.

Materials, Devices, and Circuits of Transparent Amorphous-Oxide Semiconductor

This paper presents the following recent investigations of transparent amorphous-oxide semiconductors (TAOS) from materials to devices and circuits. 1) Composition of metals in TAOS are widely explored with the aim of seeking semiconductors suitable for the channel layers of thin-film transistors (TFTs) composing backplanes for flat-panel displays. It is found in combinatorial approaches to the materials exploration that indium-based ternary TAOS (In-X-O) and their TFTs show the properties and the performance as good as those of the most popular material of amorphous In-Ga-Zn-O (alpha-IGZO) when X = Zn or Ge. 2) Defects and impurities in TAOS are investigated by theoretical approaches. The first-principle calculation of the electron states reveals that charge-neutral oxygen vacancy or interstitial forms the density of states around mid-gap level and does not generate carriers directly, while hydrogen impurity raises the Fermi level beyond the conduction-band minimum and acts as a donor in TAOS. 3) Device structures of TAOS-TFTs are also investigated extensively for better performance and stability. It is found in channel-etch type TFTs with bottom-gate inverse-stagger structures that the TFT characteristics and stability are significantly improved by chemically removing the back-channel layer in a wet-etching process. Coplanar homojunction (CH) structure is proposed as a novel device structure where conductive alpha-IGZO regions work as the source and drain electrodes to the channel region of semiconductor alpha-IGZO. The CH TFTs show excellent characteristics and stability, with low series resistance without any difficulty in making good electrical contact between metals and TAOS. 4) Circuits using TAOS-TFTs are demonstrated. A ring oscillator composed of fifteen-stage inverters with a buffer circuit operates as designed by circuit simulation with a TFT model for hydrogenated amorphous Si TFTs. Pixel circuits composed of three TFTs and one transparent capacitor successfully drive organic light-emission diode cells without a planarization layer on a 2-in diagonal panel having 176 times144 times 3 pixels.

Highly Conducting and Very Thin ZnO:Al Films with ZnO Buffer Layer Fabricated by Solid Phase Crystallization from Amorphous Phase

We propose a novel method of oxide crystal growth via atomic-additive mediated amorphization. By utilizing this method, solid-phase crystallization (SPC) of ZnO from amorphous phase has been successfully demonstrated via nitrogen atom mediation. The resultant SPC-ZnO films are highly orientated and the crystallinity is higher than that of the films prepared by conventional sputtering. By using the SPC-ZnO as a buffer layer, the resistivity of ZnO:Al (AZO) films is drastically decreased. 20-nm-thick AZO films with a resistivity of 5×10-4 Ω cm and an optical transmittance higher than 80% in a wide wavelength range of 400–2500 nm have been obtained.

Electron-temperature dependence of nitrogen dissociation in 915 MHz ECR plasma

Abstract The nitrogen molecular dissociation level in 915 MHz ECR plasma was evaluated as a function of the electron temperature by optical emission spectroscopy. The intensity ratio of N line to N2 line, which provides information on changes in the degree of dissociation of nitrogen molecules, was found to increase monotonously from 0.01 to 0.07 with increasing the electron temperature from 2 to 7 eV. Furthermore, it was confirmed that increasing the electron density and/or the introduction of argon in the discharge significantly enhanced the nitrogen molecular dissociation level.

Model for thickness dependence of mobility and concentration in highly conductive zinc oxide

Abstract. The dependences of the 294 and 10 K mobility μ and volume carrier concentration n on thickness (d=25 to 147 nm) are examined in aluminum-doped zinc oxide (AZO). Two AZO layers are grown at each thickness, one with and one without a 20-nm-thick ZnON buffer layer. Plots of the 10 K sheet concentration ns versus d for buffered (B) and unbuffered (UB) samples give straight lines of similar slope, n=8.36×1020 and 8.32×1020  cm−3, but different x-axis intercepts, δd=−4 and +13  nm, respectively. Plots of ns versus d at 294 K produce substantially the same results. Plots of μ versus d can be well fitted with the equation μ(d)=μ(∞)/[1+d*/(d−δd)], where d* is the thickness for which μ(∞) is reduced by a factor 2. For the B and UB samples, d*=7 and 23 nm, respectively, showing the efficacy of the ZnON buffer. Finally, from n and μ(∞) we can use degenerate electron scattering theory to calculate bulk donor and acceptor concentrations of 1.23×1021  cm−3 and 1.95×1020  cm−3, respectively, and Drude theory to predict a plasmonic resonance at 1.34 μm. The latter is confirmed by reflectance measurements.

Simple method of improving harvest by nonthermal air plasma irradiation of seeds of Arabidopsis thaliana (L.)

We have studied the effects of air nonthermal plasma irradiation of seeds of Arabidopsis thaliana (L.) on their growth from the beginning of cultivation to their harvest. Three minute plasma irradiation of dry seeds resulted in growth acceleration in all the growth stages. Compared with the control, the plasma irradiation led to an 11% shorter harvest period, a 56% increase in total seed weight, a 12% increase in each seed weight, and a 39% increase in seed number.

Characterization of the energy flux toward the substrate during magnetron sputter deposition of ZnO thin films

The characterization of energy influxes from plasma to substrate during sputter deposition of ZnO films is presented and discussed. Measurements were carried out in a triple rf magnetron sputter deposition system using calorimetric probes in various Ar/N2 and Ar/H2 mixtures at typical substrate positions. By variation of the probe bias the different contributions originating from the kinetic energy of charge carriers, the recombination of charge carriers (electrons and ions) at the surface as well as the contributions from the impact of neutral sputtered particles and subsequent film growth were determined. Radial scans in the substrate plane were recorded to obtain information about inhomogeneities in the total energy influx.The results show that the crystallinity reaches its optimum at that Ar/N2 ratio where the influence of the bombarding ions reaches its lowest value, indicating the destructive character of ion impact. Radial measurements indicate the influence of the magnetic field on the homogeneity of the energy influx caused by the superposition of the three (balanced) magnetic configurations. The superposition leads to an ?unbalanced character? resulting in a lowering of the electron trapping. The admixture of H2 leads to a drastic increase in the energy influx due to molecule formation at the (substrate/probe) surface.

Novel fabrication method for ZnO films via nitrogen-mediated crystallization

High quality ZnO films have been obtained by utilizing buffer layers fabricated via nitrogen mediated crystallization (NMC), where sputtering method is employed for preparation of both buffer layers and ZnO films. The crystal grain size of ZnO:Al (AZO) films with NMC-buffer layers is about 3 times larger than that of conventional films, which is considered to be due to the low nuclei density of NMC-buffer layers. As a result, the resistivity of AZO films drastically reduces from 4.76 m Ωcm for the conventional films to 0.48 m Ωcm for our films when the total film thickness is 20 nm. The NMC buffer layers also improve the spatial distribution of the resistivity, which indicates that the crystallinity at the initial stage of deposition govern the properties of AZO films. Furthermore, we have succeeded in epitaxial growth of ZnO films, whose FWHM of the rocking curve of (002) peak is as narrow as 0.061°, on c-plane sapphire substrates by using the NMC method. From these results, we conclude that our method described here is full of promise for fabrication of ZnO-based materials.

Plasma parameter measurements and deposition of a-Si:H thin films in pulsed ECR plasma.

Abstract Pulse modulation of the incident microwave power was investigated for the control of plasma parameters and improvement of the films deposited in ECR plasma. Measurements of the temporal variation of plasma parameters in the pulse-modulated ECR plasma indicate that the electron density varies little in time while the electron temperature decreases rapidly within 10 μs after the discharge. The results suggest that pulse modulation where the power-off period is approximately 10 μs enables reduction of the mean electron temperature while keeping a high electron density. Furthermore, we have prepared hydrogenated amorphous silicon films by the ECR plasma CVD method. It was found that the ratio of photoconductivity to dark conductivity of the film was improved from 103 to 2.5×105 without heating the substrate and the deposition rate was 14 A/s by pulse-modulated ECR plasma.

Effects of Nitrogen on Crystal Growth of Sputter-Deposited ZnO Films for Transparent Conducting Oxide

We have studied the effects of the N2 gas flow rate on the surface morphology of ZnO films deposited by the sputtering of a ZnO target using Ar/N2. Height-height correlation function (HHCF) analysis indicates that introducing a small amount of N2 (<5 sccm) to the sputtering atmosphere enhances adatom migration, leading to a larger grain size in the ZnO films associated with an increase in the lateral correlation length. The HHCF analysis also reveals that films deposited with and without N2 exhibit a self-affine fractal surface structure. We demonstrate that utilizing such ZnO films deposited using Ar/N2 as buffer layers, the crystallinity of ZnO:Al (AZO) films on the buffer layers can be greatly improved. The electrical resistivity of 100-nm-thick AZO films decreases from 1.8×10-3 to 4.0×10-4 Ωcm by utilizing a ZnO buffer layers prepared at N2 flow rate of 5 sccm.