Tetsuya Takami

In-plane orientation and coincidence site lattice relation of (001) Bi2Sr2CaCu2Ox formed on (001) MgO

Abstract A (001) Bi 2 Sr 2 CaCu 2 O x thin film fabricated on a (001) MgO substrate has been found to consist of two kinds of domain with in-plane orientation of Bi 2 Sr 2 CaCu 2 O x [100]∥MgO [510] and Bi 2 Sr 2 CaCu 2 O x [100] ∥MgO [5 1 0]. It is shown that near coincidence site lattice theory supports the occurence of this in-plane orientation. The dependence of the in-plane orientation on the substrate temperature was also investigated.

TDDB Measurement of Gate SiO2 on 4H-SiC Formed by Chemical Vapor Deposition

The reliability of CVD gate oxide was investigated by CCS-TDDB measurement and compared with thermally grown gate oxide. Although the QBD of thermal oxide becomes smaller for the larger oxide area, the QBD of CVD oxide is almost independent of the investigated gate oxide area. The QBD at F = 50% of CVD oxide, 3 C/cm2, is two orders of magnitude larger for the area of 1.96×10-3 cm2 at 1 mA/cm2 compared to that of thermal oxide. More than 80% of the CVD oxide breakdown occurs at the field oxide edge and more than 70% of the thermal oxide breakdown in the inner gate area. These results suggest that the lifetime of CVD oxide is hardly influenced by the quality of SiC, while the defects and/or impurities in SiC affect the lifetime of thermally grown oxide.

Realization of low on-resistance 4H-SiC power MOSFETs by using retrograde profile in P-body

Inversion-type 4H-SiC power MOSFETs using p-body implanted with retrograde profiles have been fabricated. The Al concentration at the p-body surface (Nas) is varied in the range from 5×1015 to 2×1018 cm-3. The MOSFETs show normally-off characteristics. While the Ron is 3 cm2 at Eox = (Vg-Vth)/dox ≅ 3 MV/cm for the MOSFET with the Nas of 2×1018 cm-3, the Ron is reduced by a decrease in the Nas and 26 mcm2 is attained for the device with the Nas of 5×1015 cm-3. The inversion channel mobility and threshold voltage are improved with a decrease in the Nas. By modifying the structural parameter of the MOSFET, a still smaller Ron of 7 mcm2 is achieved with a blocking voltage of 1.3 kV.

(01n)-Oriented BiSrCaCuO Thin Films Formed on CeO2 Buffer Layers

With the use of RF magnetron sputtering, (01n)-oriented BiSrCaCuO superconducting films were first formed using MgO(110) substrates with CeO2(110) buffer layers. The film obtained using the just MgO(110) substrate had (01n)- and (0n)-preferred orientations with the c-axis tilted 45° against the surface of the CeO2 buffer layer. Moreover, with the use of an off-oriented MgO(110) substrate, a uniquely oriented BiSrCaCuO film with the c-axis not perpendicular to the substrate surface was obtained without twin boundaries.

BiSrCaCuO Thin Film Grown on SrTiO3 Substrate with Off-Oriented (110) Surface

A Bi2(Sr, Ca)3Cu2Ox thin film with the (117) orientation was formed on a slightly off-oriented (110) SrTiO3 substrate by single-target sputtering. The (110) substrate off-oriented by 5° is considered to have limited film growth in one direction. From RHEED and cross-sectional TEM observations, it was found that the c-axis of the film tilts against the substrate surface by 46-47° and the SrTiO3(110) plane by 41-42°. Also, the surface morphology observed by SEM has been much improved.

Fabrication and Performance of 1.2 kV, 12.9 mΩcm2 4H-SiC Epilayer Channel MOSFET

4H-SiC epilayer channel MOSFETs are fabricated. The MOSFETs have an n- epilayer channel which improves the surface where the MOS channel is formed. By the optimization of the epilayer channel and the MOSFET cell structure, an ON-resistance of 12.9 mcm2 is obtained at VG = 12 V (Eox = 2.9 MV/cm). A normally-OFF operation and stable avalanche breakdown is obtained at the drain voltage larger than 1.2 kV. Both the ON-resistance and the breakdown voltage increase slightly with an increase in temperature. This behavior is favorable for high power operation. By the evaluation of the control MOSFETs with n+ implanted channel, the resistivity of the MOS channel is estimated. The MOS channel resistivity is proportional to the channel length and it corresponds to an effective channel mobility of about 20 cm2/Vs.

45° grain boundary junctions in (001)-oriented BiSrCaCuO films

In this study, 45°-rotated [001] tilt boundary junctions were fabricated by controlling the in-plane epitaxy using an MgO seed layer grown on an SrTiO3(001) substrate. The in-plane preferential orientation of the BiSrCaCuO film (2223 composition) grown on the MgO seed layer on the SrTiO3(001) substrate of BiSrCaCuO [100], [010]//MgO[100]//SrTiO3[100] was found using X-ray diffraction. The 45°-rotated [001] tilt boundary junction in the BiSrCaCuO film exhibited RSJ-like behavior. With the use of these junctions, a dc SQUID was fabricated and operated at temperatures up to 65 K.

Artificial Grain Boundary Junctions in BiSrCaCuO Thin Films with (11n) and (001) Orientation

Artificial grain boundary junctions in sputtered BiSrCaCuO films on SrTiO3 (110) substrates have been fabricated by controlling the orientation of the films using a MgO buffer layer. A grain boundary with large twist misorientation is formed along the edge of the MgO buffer layer that is placed perpendicular to the [001] direction of the SrTiO3 substrate. The grain boundary junctions thus obtained exhibited clear Shapiro steps in response to microwave irradiation (15.7 GHz). This indicates that the artificial grain boundaries obtained by controlling the orientation of the BiSrCaCuO films behave as a weak link.

Bi2(Sr, Ca)3Cu2Ox and Bi2(Sr, Ca)4Cu3Ox Thin Films with (11n) Orientation

As-grown Bi2(Sr, Ca)3Cu2Ox and Bi2(Sr, Ca)4Cu3Ox thin films with the (11n) orientation were formed on SrTiO3(110) substrates by consecutive sputtering. RHEED patterns and XRD analysis indicated that the c-axis of the Bi2(Sr, Ca)3 Cu2Ox and Bi2(Sr, Ca)4Cu3Ox films are tilted against the substrate surface by approximately 41° and 45°, respectively. The Bi2(Sr, Ca)3Cu2O3 film was found to have mainly a (117) orientation, whereas the Bi2(Sr, Ca)4Cu3Ox film is considered to have mainly a (1110) orientation.

Dependence of Crystal Orientation of BiSrCaCuO Thin Films on Off-Angles of Vicinal SrTiO3 (110) Surfaces

Crystal orientation of BiSrCaCuO(11n) films depends on off-angles of vicinal SrTiO3(110) surfaces. Surfaces of SrTiO3(110) were cut 2°, 5° and 10° toward the [100] direction, and BiSrCaCuO thin films, which had a 2212 phase, were formed on these vicinal surfaces by RF magnetron sputtering. C-axes almost parallel to the [100] and [010] directions of the substrate were observed when the 2°- and 10°-off substrates were used, but only the c-axis parallel to the [010] was observed when the 5°-off substrate was used. These results suggest that there is an appropriate off-angle to obtain BiSrCaCuO thin films with the c-axis almost parallel to [010] only.