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

Electric Field- And Current-Induced Electroforming Modes in NbOx

Electroforming is used to initiate the memristive response in metal/oxide/metal devices by creating a filamentary conduction path in the oxide film. Here, we use a simple photoresist-based detection technique to map the spatial distribution of conductive filaments formed in Nb/NbOx/Pt devices, and correlate these with current-voltage characteristics and in situ thermoreflectance measurements to identify distinct modes of electroforming in low- and high-conductivity NbOx films. In low-conductivity films, the filaments are randomly distributed within the oxide film, consistent with a field-induced weakest-link mechanism, while in high-conductivity films they are concentrated in the center of the film. In the latter case, the current-voltage characteristics and in situ thermoreflectance imaging show that electroforming is associated with current bifurcation into regions of low and high current density. This is supported by finite element modeling of the current distribution and shown to be consistent with predictions of a simple core-shell model of the current distribution. These results clearly demonstrate two distinct modes of electroforming in the same material system and show that the dominant mode depends on the conductivity of the film, with field-induced electroforming dominant in low-conductivity films and current bifurcation-induced electroforming dominant in high-conductivity films.This work was partly funded by the Australian Research Council (ARC) and Varian Semiconductor Equipment/ Applied Materials through an ARC Linkage Project Grant: LP150100693

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

Evaluate Fixed Charge and Oxide-Trapped Charge on SiO2/GaN Metal-Oxide-Semiconductor Structure Before and After Postannealing

The electrical properties of SiO2/GaN metal-oxide-semiconductor capacitors with different SiO2 thicknesses are evaluated before and after postmetallization annealing (PMA). The distribution of charges in bulk SiO2 and the SiO2/GaN interface is estimated by analyzing the electrical properties. It is revealed that the net effective charges (Nf) tend to increase with decreasing SiO2 thickness before PMA. This indicates that the negative bulk charges are present uniformly, and the positive interface charges exist locally near the interface. In the sample after PMA, Nf tends to decrease with decreasing SiO2 thickness at the thin SiO2 region (<25 nm). This indicates that the distribution width of positive charges becomes larger by applying PMA. Therefore, PMA widens the distribution of interface positive charges, although it is effective in reducing the amount of Nf

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

Optimizing the thermoelectric performance of InGaZnO thin films depending on crystallinity via hydrogen incorporation

The thermoelectric power factor of amorphous (a-IGZO), c-axis aligned crystalline (c-IGZO) and (cc-IGZO) crystal-embedded c-axis aligned crystalline InGaZnO were optimized by performing hydrogen incorporation via post-annealing. Highest power factor was achieved for a-IGZO when annealing with pure N2 was used, while adding 4% H2 in the N2 atmosphere significantly enhanced those of c-IGZO and cc-IGZO. However, the cc-IGZO samples displayed weaker properties compared to both a-IGZO and c-IGZO regardless of the annealing conditions. The presence of H2 was effective in passivating oxygen-related defects for both the c-IGZO and cc-IGZO samples, which translated to better electron transport while maintaining a respectable Seebeck coefficient. On the other hand, the formation of a high amount of oxygen vacancies from annealing with pure N2 was likely responsible not only for the good electrical conductivity of a-IGZO, but also for the relatively good Seebeck coefficients. Establishing the post-annealing conditions to maximize the thermoelectric properties depending on crystallinity paves the way for future commercial transparent IGZO thermoelectric devices