Toshitake Nakata

The effects of N + dose in implantation into 6H-SiC epilayers

Ion implantation of nitrogen (N) into p-type 6H-SiC {0001} epilayers was investigated as a function of implant dose. Lattice damage induced by implantation was characterized by Rutherford backscattering spectroscopy and Raman scattering. The damage severely increased when the implant dose exceeds 1 x 1015 cm-2, and amorphous layers were formed at doses higher than 4 x 1015 cm-2. By high-temperature annealing at 1500°C, relatively high electrical activation ratios (≈50%) can be obtained in the case of low-dose implantation (<1 x 1015cm−2). However, the electrical activation showed sharp decrease with increasing implant dose, which may be caused by the residual damage in implanted layers.

Aluminum and boron ion implantations into 6H-SiC epilayers

Aluminum and boron ion implantations into n-type 6H-SiC epilayers have been systematically investigated. Redistribution of implanted atoms during high-temperature annealing at 1500°C is negligibly small. The critical implant dose for amorphization is estimated to be 1 × 1015 cm−2 for Al+ implantation and 5 × 1015 cm-2 for B+ implantation. By Al+ implantation followed with 1500°C-annealing, p-type layers with a sheet resistance of 22 kΩ/— can be obtained. B+ implantation results in the formation of highly resistive layers, which may be attributed to the deep B acceptor level.

Formation of semi‐insulating 6H‐SiC layers by vanadium ion implantations

Vanadium ion (V+) implantation has been successfully applied to the formation of semiinsulating 6H‐SiC layers. The resistivity of V+‐implanted layers strongly depended on the conduction type of initial 6H‐SiC crystals. The resistivity at room temperature exceeded 1012 Ω cm and 106 Ω cm for p‐ and n‐type samples, respectively. Compensation mechanism is discussed based on the temperature dependence of resistivity. This technique will be useful for device isolation, edge termination, and reduction of parasitic impedance of SiC devices.

Recrystallization of MeV Si implanted 6H‐SiC

Microstructures of recrystallized layers in 8 MeV Si3+ ion implanted 6‐H‐SiC (0001) wafers have been characterized by means of transmission electron microscopy. Epitaxial recrystallization of buried amorphous layers was observed at annealing temperature as low as 1000 °C. Layer‐by‐layer epitaxy of 6H‐SiC initially occurred and it was changed to columnar growth when layer‐by‐layer growth exceeded 100 nm in thickness. From the microdiffraction analysis, it was found that the columnar regions are defected 6H‐SiC with crystal orientations different from the substrate. In addition to 6H‐SiC, epitaxial 3C‐SiC was also confirmed in the recrystallized layer. Based on these results, we have proposed a structure model of the recrystallized layer in which stacking faults in the columnar regions are induced by mismatched connections between the columnar and layered 6H‐SiC regions.

Simultaneous mass-analyzed positive and negative low-energy ion beam deposition apparatus

Abstract We have developed a new concept low-energy ion beam deposition apparatus. This machine can generate mass-analyzed very low energy ion beams with positive and negative charges at the same time. It is possible to deposit these ions not only simultaneously but also alternatively. The name is Taotron and the nickname is PANDA (Positive And Negative-ions Deposition Apparatus). The aiming specifications of this apparatus are: (1) available positive ions are H, B, C, N, O, Si, Fe, etc. and negative ions are H, B, C, O, Si etc., (2) ion energy range covers 10 eV to 20 keV, (3) typical ion beam current is ≥ 10 μA (for 10 eV oxygen ions with both charges) and the beam size is ≥ 10 mm ⊘, (4) the base pressure of the deposition chamber is in the order of 10 −8 Pa and the pressure during deposition is in the order of 10 −6 Pa.

Amorphization and solid phase epitaxy of high-energy ion implanted 6H-SiC

Abstract We have investigated microstructures of damaged and recrystallized layers in MeV-ion implanted 6H-SiC (0001) wafers by means of cross-sectional transmission electron microscopy. The substrate surfaces were implanted at 160°C with 1 × 1017/cm2 8 MeV Si3+ ions using a tandem accelerator. A buried amorphous layer was formed ranging from ∼ 1.6 μm to ∼ 3.4 μm in depth. The amorphous/crystalline transition regions consisted of many stacking faults perpendicular to the [0001] direction, and their density increased toward the amorphous region. The amorphous layer regrew epitaxially from the undamaged substrate at an annealing temperature of ∼ 1000°C. This epitaxial 6H-SiC layer changed to columnar 6H-SiC with crystal orientations different from the substrate. In addition to these crystalline 6H-SiC, the existence of polycrystalline 3C-SiC was confirmed in the middle part of the recrystallized layer.

Fabrication of SiC Blue LEDs Using Off-Oriented Substrates

SiC blue LEDs were fabricated by the dipping technique, using 6H-SiC substrates whose (000)C face varied toward the [110] or [100] direction. The surface morphology of the epitaxial layers on the off-oriented substrates was a striped pattern perpendicular to the off-axis direction. By using off-oriented substrates, the degradation of SiC blue LEDs can be greatly reduced. High-efficiency blue LEDs (12 mcd at 20 mA) were fabricated by increasing the amount of Al dopant in the n-type layer.

Analysis of Heat-Treated 6H-SiC(0001) Surface Using Scanning Tunneling Microscopy

A scanning tunneling microscopy in ultrahigh vacuum (UHV) has been used to study hexagonal (6H) silicon carbide (0001)Si face prepared by heat treatment. √3 x √3, 6 x 6 and 3 x 3 reconstructions were observed above 1100°C. With increasing heat-treatment temperature, surface structure changed drastically and step heights decreased to that of a double layer. The 6 x 6 and 3 x 3 reconstructions can be explained as the structure of a graphite layer on the SiC surface and as the structure proposed by Kaplan, respectively.

FORMATION OF AN OHMIC ELECTRODE IN SIC USING A PULSED LASER IRRADIATION METHOD

Abstract We have succeeded in the formation of a Ni electrode on n-type silicon carbide (6HSiC) at room temperature using the pulsed laser doping (PLD) method. Characteristics of the PLD method for the formation of an ohmic electrode in SiC are shown to be as follows; (1) The formation of an ohmic electrode in SiC at room temperature was achieved. (2) Metal dopants are doped to a depth of about 150 nm from the surface. (3) The interface between the laser doped region and the SiC substrate was found to be an abrupt junction. Furthermore, we have attempted to form the ohmic contact by metal atoms doping with laser irradiation into a bulk SiC through a mask during thermal evaporation of the metal. This technique is successful for the formation of dot electrodes 0.8 mm in diameter on the SiC substrate.

Mapping of Vanadium-Related Luminescence on SiC Wafer at Room Temperature

The intensity distribution of vanadium-related luminescence is investigated on a 4H SiC wafer. Two-dimensional mapping of the vanadium-related line on the sample at room temperature agrees with the defect image obtained by optical microscopy: the luminescence intensity becomes high around the cracks. The intensity profile measurement at 4.2 K indicates that not only the vanadium-related line but the donor-acceptor pair emission increases the intensity around the crack. This result leads us to suggest that microdefects acting as killer centers for minority carriers are gettered by the cracks, resulting in the formation of a denuded zone around them.