Suguru Funasaki

XRD Investigation of the Crystalline Quality of Sn Doped β-Ga2O3 Films Deposited by the RF Magnetron Sputtering Method

For the purpose of increasing the conductivity of β-Ga2O3 films, Sn doping in the β-Ga2O3 films has been explored using co-sputtering. Growth of β-Ga2O3 was confirmed by the XRD pattern for the undoped sample. However, it is shown that the Ga2O3 phase is transformed from the β to the γ phase by Sn doping, because of the increase of the phase transition temperature from the γ to the β phase. To improve the crystalline quality, additional annealing at 900°C for 60 min is performed to the Sn doped film. The XRD peaks corresponding with β-Ga2O3 could be confirmed after the additional annealing.

Electric properties of carbon-doped n-type β-FeSi2/p-type Si heterojunction diodes

Motoki Takahara*, Tarek M. Mostafa, Ryuji Baba, Suguru Funasaki, Mahmoud Shaban, Nathaporn Promros, and Tsuyoshi Yoshitake 1Department of Applied Science for Electronics and Materials, Kyushu University, Kasuga, Fukuoka 816-8580, Japan 2Department of Electrical Engineering, Aswan Faculty of Engineering, Aswan University, Aswan 81542, Egypt 3Department of Physics, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang, Chalongkrung Road, Bangkok 10520, Thailand

Epitaxial growth of n-type β-FeSi thin films on p-type Si(111) substrates by radio-frequency magnetron sputtering and rectifying action of heterojunctions

Tarek M. Mostafa, Motoki Takahara, Ryuji Baba, Suguru Funasaki, Mahmoud Shaban*, Nathaporn Promros**, Aki Tominaga, Maiko Nishibori, and Tsuyoshi Yoshitake*** 1Department of Applied Science for Electronics and Materials, Kyushu University, Kasuga, Fukuoka 816-8580, Japan 2Department of Electrical Engineering, Aswan Faculty of Engineering, Aswan University, Aswan 81542, Egypt 3Department of Physics, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand 4Research Center for Synchrotron Light Applications, Kyushu University, Kasuga, Fukuoka 816-8580, Japan

Electrical Characteristics of n-Type Nanocrystalline FeSi2/Intrinsic Si/p-Type Si Heterojunctions Prepared by Facing-Targets Direct-Current Sputtering

n-Type nanocrystalline FeSi2/intrinsic Si/p-type Si heterojunctions were prepared by FTDCS. In order to estimate their diode parameters such as ideality factor, barrier height and series resistance, their current-voltage characteristics were measured in the temperature range from 300 to 77 K and analyzed on the basis of thermionic emission theory and Cheungs method. Based on thermionic emission theory, the ideality factor was calculated from the slope of the linear part from the forward lnJ-V characteristics. The barrier height was calculated once the saturation current density was derived from the straight line intercept of lnJ-V plot at a zero voltage. The obtained results exhibit an increase of ideality factor and a decrease of barrier height at low temperatures, which might be owing to inhomogeneity of material and non-uniformity of charge at the interface. Based on Cheungs method, the ideality factor and barrier height were estimated from y-axis intercept of dV/d (lnJ)J plot and y-axis intercept of H(J)J plot, respectively. The series resistance was analyzed from the slopes of dV/d (lnJ)J and H(J)J plots. The values of ideality factor and barrier height obtained from this method are in agreement with those obtained from the thermionic emission theory. The obtained series resistances from dV/d (lnJ)J and H(J)J plots, which were approximately equal to each others, were increased as the temperature decreased. This result should be owing to the increased ideality factor and remarkably reduced carrier concentrations at low temperatures.

Current Transport Mechanism of n-Type Nanocrystalline FeSi2/Intrinsic Si/p-Type Si Heterojunctions Fabricated by Facing-Targets Direct-Current Sputtering

n-Type nanocrystalline FeSi2/intrinsic Si/p-type Si heterojunctions were successfully fabricated by FTDCS and their forward current-voltage characteristics at low temperatures were analyzed on the basis of thermionic emission theory. The analysis of J-V characteristics exhibits an increase in the ideality factor and a decrease in the barrier height at low temperatures. The values of ideality factor were estimated to be 2.26 at 300 K and 9.29 at 77 K. The temperature dependent ideality factortogether with the constant value of parameter A indicated that a trap assisted multi-step tunneling process is the dominant carrier transport mechanism in this heterojunction. At high voltages, the current transport mechanism is dominated by SCLC process.

Temperature Dependent Current-Voltage Characteristics of n-Type Nanocrystalline-FeSi2/p-Type Si Heterojunctions Fabricated by Pulsed Laser Deposition

n-Type NC-FeSi2/p-type Si heterojunctions were successfully fabricated by PLD, and their forward current-voltage characteristics were analyzed on the basis of thermionic emission theory (TE) in the temperature range from 300 down to 77 K. With a decrease in the temperature, the ideality factor was increased while the zero-bias barrier height was decreased. The calculated values of ideality factor and barrier height were 3.07 and 0.63 eV at 300 K and 10.75 and 0.23 eV at 77 K. The large value of ideality factor indicated that a tunneling process contributes to the carrier transport mechanisms in the NC-FeSi2 films. The series resistance, which was estimated by Cheungs method, was strongly dependent on temperature. At 300 K, the value of series resistance was 12.44 Ω and it was dramatically enhanced to be 1.71× 105 Ω at 77 K.

Characterizations of mesa structural near-infrared n-type nanocrystalline-FeSi2/p-type si heterojunction photodiodes at low temperatures

In order to reduce the parasitic capacitance, mesa structural n-type NC-FeSi2/p-type Si heterojunctions were fabricated by photolithography. Their current-voltage characteristics were measured in the dark and under illumination using a 1.31 μm laser in the temperature range of 60 300 K. Their junction capacitance density was evidently reduced as compared with that of the normal structural diodes. The dark current was markedly reduced with a decrease in the temperature. At 60 K, a rectifying current ratio in the dark became more than five orders of magnitude at ±1V. The ratio of the photocurrent to the dark current was dramatically enhanced to be approximately two orders of magnitude, and the detectivity was calculated to be 1.5 × 1011 cmHz1/2/W at-1V. The obtained results showed a remarkable improvement in the device performance as compared with those at 300 K.

Improvement of the Crystalline Quality of β-Ga2O3 Films by High-Temperature Annealing

For the purpose of improving the crystalline quality of undoped and Si doped β-Ga2O3 films, high temperature annealing at 900°C was performed. The crystalline quality of the films investigated using scanning electron microscopy and X-ray diffraction. Also the conductivity of the films is compared before and after the annealing. After the 900°C annealing, the XRD peaks intensity corresponding to β-Ga2O3 is increased. This result indicates that the crystalline quality improves by the high temperature annealing.