Hiroaki Saitoh

High-Speed Growth of High-Quality 4H-SiC Bulk by Solution Growth Using Si-Cr Based Melt

High-speed solution growth using Si-Cr based melt has been performed on on-axis 4H-SiC(0001) at a high temperature of about 2000°C. The maximum growth rate for one-hour growth reaches to 1120 m/h, while the typical growth rate of growth for 2h is about 500 m/h. A large crystal that is about 25 mm in diameter and 1650 m in thickness can be obtained by growth for 5h. The crystal quality is confirmed to be homogeneous by X-ray diffraction and X-ray topography, because FWHM is less than 30 arcsec. Etch pit density of the threading dislocations in the grown crystal is 103-104 cm-2, and that of basal plane dislocation is 2×102-3×103 cm-2. Resistivity of the crystals grown by the solution growth is comparable to those of crystals grown by physical vapor transport technique.

Low-concentration deep traps in 4H-SiC grown with high growth rate by chemical vapor deposition

Epitaxial growth was performed on 8° off-axis 4H-SiC(0001) by horizontal hot-wall chemical vapor deposition (CVD) in a SiH4-C3H8-H2 system at 1550°C. High growth rates of 10–13 µm/h were attained by growth with a high SiH4 flow rate of 4.0 sccm. A mirror like surface was obtained by employing an optimum C/Si ratio and by an improved process of initial growth. The epilayers grown with an optimum C/Si ratio showed high purity in the low 1013 cm-3 range (n-type) and low trap concentrations in the low 1011 cm-3 range. Minority carrier lifetime was investigated by a differential microwave-detected photoconductance decay (µ-PCD) measurement. A long carrier lifetime of approximately 2 µs was obtained for a 50-µm-thick epilayer

Molten KOH Etching with Na2O2 Additive for Dislocation Revelation in 4H-SiC Epilayers and Substrates

A novel etching solution using molten KOH with Na2O2 additive (KN etching) for dislocation revelation in 4H-SiC epilayers and substrates has been proposed. Threading screw and edge dislocations (TSDs and TEDs) have been clearly revealed as hexagonal etch pits differing in pit size, and basal plane dislocations (BPDs) as seashell-shaped pits. KN etching has provided a solution to the problem that KOH etching is not effective for dislocation identification in n+-4H-SiC. The influences of SiC off-axis angles, carrier concentrations, and growth techniques on the effectiveness of KN etching have also been investigated. It has been shown that KN etching is applicable to SiC epilayers and substrates with any off-axis angle from 0 to 8° and electron concentrations from 1015 to 1019 cm-3.

Interface Properties of SiO2/4H-SiC(0001) with Large Off-Angles Formed by N2O Oxidation

In this study, we have investigated N2O oxidation of various off-angled 4H-SiC (0001) epilayers and characterized the properties of MOS interfaces. The oxide thickness almost linearly increases with increasing off-angle. Oxidation on highly off-angled (0001) 4H-SiC is faster than that on 8o off-axis (0001). The off-angle dependence of Dit is very small for the MOS capacitors in the off-angle range from 8o to 30o. The depth profiles of carbon and nitrogen atoms near the MOS interface on 15o off-axis 4H-SiC(0001) are similar to those on 8o off-axis (0001).

4H-SiC epitaxial growth on SiC substrates with various off-angles

Chemical vapor deposition of 4H-SiC on (0001) substrates with various off-angles from 1o to 45o has been investigated. On large-off-angled (15o-45o) substrates, very smooth surface morphology is obtained in the wide range of C/Si ratio. The micropipe dissociation during epitaxial growth is observed on 4o-45o off-angled substrates with a low C/Si ratio. The incorporation of nitrogen was dramatically suppressed by increasing C/Si ratio irrespective of substrate’s off-angle.

Diffusion of Transition Metals in 4H-SiC and Trials of Impurity Gettering

The diffusion of transition metals in 4H-SiC has been investigated by secondary ion mass spectrometry using epilayers and substrates implanted with titanium (Ti), chromium (Cr), iron (Fe), or nickel (Ni). Implanted Cr, Fe, and Ni atoms diffuse by subsequent Ar annealing at 1780 °C in n-type 4H-SiC epilayers. In n+-type substrates, the diffusivities of Ti, Cr, and Fe are almost negligible, while only Ni diffuses. By the helium implantation following the implantation of transition metals, no diffusion of Ti, Cr and Fe is observed in epilayers. The diffusion of transition metals in SiC is discussed based on the results of first-principles calculation.

Characterization of Surface Defects of Highly N-Doped 4H-SiC Substrates that Produce Dislocations in the Epitaxial Layer

The structures of defects that form different types of etch pits on highly N-doped 4H-SiC substrates, that were produced by a sublimation method, after molten KOH etching were characterized. It was found that most of the dislocations in the epitaxial layer originated from defects at the surface of substrate whose etch pit structures were clearly different from the conventional structures. The etch pits were classified into drop, oval, round and caterpillar pits. The drop and oval pits were concluded to be formed by the deformation of conventional etch pits. Round pits were concluded to originate from half loop dislocations and were transformed to complex dislocations by epitaxial growth. Analysis by transmission electron microscopy measurement indicates that slipped edge dislocations (or screw dislocations) on the basal plane form caterpillar pits.

Dislocation Revelation in Highly Doped N-Type 4H-SiC by Molten KOH Etching with Na2O2 Additive

We have proposed a new wet etching recipe using molten KOH and Na2O2 as the etchant (“KN etching”) for dislocation revelation in highly doped n-type 4H-SiC (n+-4H-SiC). Threading screw dislocations (TSDs) and threading edge dislocations (TEDs) have been clearly revealed as hexagonal etch pits differing in pit sizes, and basal plane dislocations (BPDs) as seashell-shaped pits. This new etching recipe has provided a solution to the problem that conventional KOH etching is not effective for dislocation identification in 4H-SiC if the electron concentration is high (>mid-1018 cm-3). We have investigated the effect of SiC off-cut angle on KN etching and it has been shown that the “KN etching” is applicable for the n+-SiC substrate with off-angle from 0o to 8o.

Dislocation Analysis in Highly Doped n-Type 4H-SiC by Using Electron Beam Induced Current and KOH+Na2O2 Etching

Dislocations in highly doped n-type 4H-SiC (n+-SiC, n>1019 cm-3) substrate have been studied by means of electron beam induced current (EBIC). Ni/n-SiC/n+-SiC/Al structure was fabricated in order to simultaneously observe the dislocations in n-SiC epilayer and n+-SiC substrate. We have found that dark dots in the EBIC image correspond to threading screw dislocations (TSDs) and threading edge dislocations (TEDs) with the former being relatively darker. Short dark lines along off-cut are attributed to basal plane dislocations (BPDs) in the epilayer; and the randomly oriented long dark lines are caused by the BPDs in the substrate. The classification of the dislocations by EBIC has been examined by wet etching in KOH+Na2O2.

A simultaneous observation of dislocations in 4H-SiC epilayer and n+-substrate by using electron beam induced current

With a new structure of Ni/n-SiC/n+-SiC/Al, we have achieved a simultaneous observation of the dislocations in n-SiC epilayer and n+-SiC substrate by electron beam induced current (EBIC). The EBIC images were compared to the results of a depth-controlled wet etching in KOH+Na2O2. It has been found that each type of dislocations has its own signature in EBIC images in terms of the darkness, shape and orientation of the dark contrast. By changing the accelerating voltage of the electron beam, we can also observe the depth dependent presence of each type of dislocations and where and how the dislocation conversion happens.