Masatomo Honjo

High-Temperature Growth of GaN Single Crystals Using Li-Added Na-Flux Method

The Na-flux method is a promising for fabricating GaN crystals with high quality. In our previous study, we found that the surface morphology and transparency of these crystals were improved by raising the growth temperature. Increasing the threshold pressure of nitrogen for GaN growth, however, made GaN growth at high temperatures difficult. In this study, we attempted to grow GaN crystals by the Na-flux method with the addition of Li to the flux, which promoted the solubility of nitrogen in the flux. As a result, the threshold pressure of nitrogen for GaN growth decreased, and GaN crystals with high crystallinity were grown at 900 °C. In addition, we found that the crystallinity of the grown GaN crystals was improved and the concentration of impurities in the grown GaN crystals was decreased by raising the growth temperature.

Crack-free GaN substrates grown by the Na-flux method with a sapphire dissolution technique

GaN wafers are generally fabricated by separating a foreign substrate from a GaN layer using thermal stress; however, thermal stress also leads to the cracking of the GaN layer. In this study, we first succeeded in dissolving a sapphire substrate just after Na-flux growth by successively changing the flux content for GaN growth (Ga–Na–C) to that for dissolving sapphire (Ga–Na–C–Li) at the considered growth temperature. Hence, no thermal stress was induced in the grown GaN crystals, resulting in a crack-free GaN substrate. We concluded that this process is a good candidate technique for supplying free-standing GaN substrates.

Control of GaN crystal habit by solution stirring in the Na-flux method

In our previous study, we succeeded in fabricating low-curvature GaN wafers with low dislocation density by the Na-flux coalescence growth technique. However, the crystals consisted of many pyramidal grains with facets, leading to an increase in the oxygen concentration in the crystal, an increase in the lattice constant, and blackening. In this study, we attempted to improve the crystal habit of the GaN crystals by employing a solution-stirring technique in the coalescence growth on multipoint seeds. Scanning electron microscope images indicated that the c-face area became larger by increasing the stirring rate and growth period. We concluded that solution stirring in the Na-flux coalescence growth technique is an effective approach to improve the crystal habit and uniformize the lattice constant of GaN crystals.

Influence of Nitrogen Doping on the LaAlO Film Properties

Introduction A gate insulator film with a large band gap and a high dielectric constant is required for high power field effect transistors (FET) using wide band gap semiconductors, such as SiC, GaN, and diamond. The gate insulating material must have a wider band gap than SiC (4H-SiC: 3.2 eV), GaN (3.4 eV), and diamond (5.5 eV) semiconductors. Furthermore, a high dielectric constant (high-k) and low space charge are needed to obtain a high performance FET. Stable operation in severe environments and suppression of the gate leakage current are also important. LaAlO, as a compound of La2O3 and Al2O3, offers advantages such as a high dielectric constant of 25–27 and amorphous structure at a high temperature of 850 °C. However, moisture absorption is a serious problem for oxide films containing La. We investigated nitrogen doping of the LaAlO film (LaAlON) as a way of improving its water resistance. This paper reports the influence of nitrogen doping on electrical and optical properties of LaAlON (N: 4%) films.

Characterization of Lanthanoid and Aluminum Based Oxide Film for Wide Bandgap Semiconductors

A gate insulator film with a wide bandgap and a high dielectric constants required to achieve high power field effect transistor (FET) using wide bandgap semiconductors such as SiC and diamond. We can achieve to suppress the gate leakage current and the charge shifts by using the AlSiO film. We found that the leakage current of Nitrogen-doped AlSiO film can be suppressed in high temperature region compared with AlSiO film. In addition, we attempted to study Yttrium aluminate (YAlO) and Lanthanum aluminate (LaAlO) as a gate insulator film. Since the Y (+3) and La(+3) have the same valence as Al (+3), it is expected that there is an advantage that Y, La doping generates less dangling bonds in the Al2O3 film. We can achieve to form aluminum based oxide film with higher dielectric constant by doping Lanthanoid atoms such as Y and La. The optimized YAlO and LaAlO films were applied to SiC-MIS structure. The Lanthanoid and Aluminum based oxide films can readily be applicable to wide bandgap semiconductor devices.

Properties of LaAlO Film after Waterless Process Using Organic Solvent Containing Anhydrous Hydrofluoric Acid

Lanthanum (La)-based oxide films have been studied as high-k (high dielectric constant) gate dielectrics. However, moisture absorption is a serious problem for oxide films containing La. We have attempted to use waterless solutions instead of water-based solutions to remove high-k films to suppress the moisture absorption of the lanthanum aluminate (LaAlO) film. We report the effect of an anhydrous hydrofluoric acid (AHF) and isopropyl alcohol (IPA) mixed solution as an etching solution and hydrofluoro-ether (HFE) as a rising solution on the properties of LaAlO films. We have succeeded in suppressing the moisture absorption of LaAlO films by using waterless solutions for a front end of line (FEOL) process. In addition, the selectivity (LaAlO/SiO2), the etching ratio of LaAlO to SiO2, was improved using this process. It is considered that this technology will be useful for the next-generation devices with lanthanum-based oxide films.

Properties of LaAlO Film after Waterless Process using Organic Solvent containing Anhydrous HF

Introduction To achieve high performance and low stand-by power applications, the integration of a high-k dielectric film is necessary for LSI devices 22nm technology node and beyond. Recently, Lanthanum based oxide films such as La2O3 and LaAlO have been studied as a high-k gate dielectric. Minimum equivalent oxide thickness (EOT) value of 0.5 nm has been achieved by means of direct oxidation of La deposited on Si . Especially, LaAlO, as a compound of La2O3 and Al2O3, has some advantages such as a high dielectric constant of 25-27, high band gap of 7 eV and amorphous structure up to a high temperature of 850 °C[1]. However, the moisture absorption is a serious problem for oxide films containing La. [2] A water based solution such as diluted HF (DHF) is used to remove residual high-k film and post-dry etching residues in FEOL process. Therefore, the removal and post water rinse process caused the damage in the side of the LaAlO gate dielectric, as shown in Fig 1. In this study, we have attempted to use waterless solutions in the removal wet process for suppress the damage of the LaAlO film. We report the effect of the anhydrous HF (AHF) and Isopropyl Alcohol (IPA) mixture as etching solution, and hydrofluoro ethers (HFE) as rising solution.