Invited; Ternary amorphous oxide semiconductor material toward 3D-integrated ferroelectric devices

Interest in transistor-based ferroelectric memory (FeFET) using ferroelectric HfO2[1] as a candidate for nextgeneration memory devices has been growing, and FeFETs with a three-dimensional stacked structure (3DFeFET) have been proposed[2]. Recently, amorphous oxide semiconductors (AOS) such as In-Ga-Zn-O have been mentioned as a candidate channel material, and it is expected to suppress the characteristic degradation caused by the formation of interface layers, which is a problem with Si-based materials [3]. deposition (ALD) technology is required to apply AOS to 3D-FeFETs. Conventional AOS are mainly quaternary, and have been designed for display applications that require low-temperature deposition. Please click Download on the upper right corner to see the full abstract

Oxide thin film transistors for flexible devices

Much attention has been gathered to flexible devices which will surely change our life style drastically. There are many kinds of flexible devices such as flexible display or medical chart. In order to realize the flexible devices, oxide thin film is one of the promising material. Because oxide film has several features which are not observed in conventional silicon materials. They are low fabrication temperature, high electrical performance or unique optical properties. To realize flexible devices with oxide thin film, several key issues should be discussed. In this talk, we will introduce several new techniques which are now being developed in our laboratory. We study the fabrication method of high performance oxide thin film transistors by using solution processed InZnO. High mobility and highly reliable TFT was demonstrated using spin coating method. In this technique, there was a problem of larger fluctuation of the performance. To solve this problem, we introduced wet annealing after the TFT fabrication and achieved very low fluctuation of the electrical performance such as mobility of threshold voltage. We apply this solution processed InZnO to logic circuit such as invertor or ring oscillators. We could demonstrate clear invertor operation or high frequency circuit operations. We demonstrate ELA on a-IGZO TFTs passivated with a hybrid passivation layer (Fig.1). The hybrid passivation layer, based on polysilsesquioxane (PSQ), is transparent and fabricated by solution process. The PSQ passivated a-IGZO TFTs has a bottom gate top contact structure. The channel used is a 70 nm thick a-IGZO (2217) deposited at room temperature by RF magnetron sputtering. Highly doped n-type Si with 100 nm thermally oxidized SiO2 layer were used as the gate and gate insulator, respectively. A stack of 80 nm Mo and 20 nm Pt deposited by RF magnetron sputtering were used as source/drain electrodes. PSQ passivated TFTs were subjected to either 248 nm KrF ELA or 308 nm XeCl ELA at room temperature and atmospheric pressure. KrF ELA was performed under ambient atmosphere while XeCl ELA was performed under N2 environment. Note that ELA was performed after the passivation coating process. Since the PSQ passivation is transparent, we expect that the incident beam will be absorbed throughout the channel. Irradiating Me 100 samples with 90-110 mJ/cm2 XeCl ELA and Me 60/Ph 40 samples with 80 mJ/cm2 KrF ELA greatly improved the transfer characteristics and mobility (~13-18 cm2/Vs) (Fig.2). Please click Additional Files below to see the full abstract

Solution processed ultrawide bandgap insulator to semiconductor conversion of amorphous gallium oxide via fermi level control

Silicon and more recently wide bandgap (WBG) semiconductor materials have dominated the integrated circuit (IC) and thin-film transistor (TFT) space, respectively. For instance, Si technology is widely used in complementary metal oxide semiconductor (CMOS) architectures and as conventional channel material in TFTs in its amorphous and polycrystalline phases. On the other hand, WBG semiconductors such as amorphous InGaZnO have been recently poised to replace a-Si as the dominant TFT channel material especially in modern displays for their superior mobility, transparency, and low temperature processability. Nevertheless, a shift towards ultrawide bandgap (UWBG) semiconductors which have bandgaps (Eg) larger than 4.0 eV unlocks additional properties such as higher breakdown voltage, excellent transparency at wider wavelength range, and harsh environment resilience [1]. Please click Download on the upper right corner to see the full abstract

Photomechanical modification of ZnS microcrystal to enhance electroluminescence by ultrashort-pulse laser processing

A ZnS microcrystal was treated with an ultrashort-pulse laser and applied to an inorganic electroluminescence (EL) phosphor. We found that the emission intensity of the EL phosphor was increased by laser-induced photomechanical modification. The pulse duration dependence of the emission enhancement and structural analysis by scanning electron microscopy indicated that the structural modification was induced inside the ZnS microcrystal, although a mechanical grinding would induce the structural modification mainly on the crystal surface. The results suggested a new way of enhancing the emission of inorganic EL devices

The Influence of Ga–OH Bond at Initial GaN Surface on the Electrical Characteristics of SiO2/GaN Interface

Herein, the influence of the Ga–OH bond at the GaN surface on the electrical characteristics of the SiO2/GaN metal-oxide semiconductor structure is investigated. The GaN surface is modified by three different surface treatments (O2 annealing, wet annealing, and ultraviolet [UV]/O3 treatment). The Ga–OH bond is evaluated by X-ray photoelectron spectroscopy and characterized by capacitance–voltage (CV) measurements and a positive bias stress test. Increasing the ratio of Ga–OH bonds at the SiO2/GaN interface decreases the net fixed charge at the SiO2/GaN interface in the CV measurements and increases the voltage shift in the stress test. Therefore, the Ga–OH bond at the SiO2/GaN interface develops a negative charge and behaves as an electron trap. The undesirable influence of the Ga–OH-related traps is reduced by low-temperature annealing

Analysis of thermoelectric properties of amorphous InGaZnO thin film by controlling carrier concentration

We have investigated the thermoelectric properties of amorphous InGaZnO (a-IGZO) thin films optimized by adjusting the carrier concentration. The a-IGZO films were produced under various oxygen flow ratios. The Seebeck coefficient and the electrical conductivity were measured from 100 to 400 K. We found that the power factor (PF) at 300 K had a maximum value of 82 × 10−6 W/mK2, where the carrier density was 7.7 × 1019 cm−3. Moreover, the obtained data was analyzed by fitting the percolation model. Theoretical analysis revealed that the Fermi level was located approximately above the potential barrier when the PF became maximal. The thermoelectric properties were controlled by the relationship between the position of Fermi level and the height of potential energy barriers