Tamotsu Jikimoto

Epitaxial growth of thick 4H–SiC layers in a vertical radiant-heating reactor

A vertical radiant-heating reactor has been developed for thick silicon carbide (SiC) epitaxial growth, in which the susceptor and substrates are heated by radiation from the hot wall. The benefit of the heating and sample-holding method is demonstrated by improvements in the curvature of crystal bending and FWHM of X-ray ω-rocking curves followed by epitaxial growth. The typical growth rate is 13 16 μm/h at 1530 1550 C at the susceptor top under reduced pressure as low as 50 70 mbar. Low background doping at low 10 13 cm 3 (N d - N a ) was achieved, and some of the 4H SiC epilayers exhibited a high resistivity. We also succeeded in growing a 4H SiC epilayer over 240 μm-thick with minimal surface roughness. Little sigh of impurities was observed by low-temperature photoluminescence (LTPL), and no impurities (Al, B, Ti, V and Cr) exceeding I × 10 14 cm 3 were found by secondary ion mass spectroscopy (SIMS) for a 150 μm-thick 4H SiC epilayer. Thickness and doping uniformity along the gas flow of 5% and 11%, respectively, were obtained for 2-in substrates. Molten KOH etching analysis revealed that some of the micropipes were dissociated into closed core screw dislocations during epitaxial growth. The electrical performance of high-voltage devices was also demonstrated.

Structural Transformation of Screw Dislocations via Thick 4H-SiC Epitaxial Growth

In this paper, we studied the structural transformation of screw dislocations through gas-phase 4H-SiC epitaxial growth. We confirmed based on the numbers and features of etch pits on a surface after KOH treatment that some micropipes were closed in the epitaxial layer, and were divided into several elementary screw dislocations. We discuss the magnitude of Burgers vectors of micropipes in 4H-SiC substrates in relation to the number of elementary screw dislocations generated by micropipe closing. A depth analysis further revealed that most micropipe closings took place in the initial stage of epitaxial growth.

A 4.15 kV 9.07-m/spl Omega//spl middot/cm/sup 2/ 4H-SiC Schottky-barrier diode using Mo contact annealed at high temperature

In this letter, we report the fabrication of high-voltage and low-loss 4H-SiC Schottky-barrier diodes (SBDs) with a performance close to the theoretical limit using a Mo contact annealed at high-temperature. High-temperature annealing for the Mo contact was found to be effective in controlling the Schottky-barrier height at 1.2-1.3 eV without degradation of n-factor and reverse characteristics. We successfully obtained a 1-mm/sup 2/ Mo-4H-SiC SBD with a breakdown voltage (V/sub b/) of 4.15 kV and a specific on resistance (R/sub on/) of 9.07 m/spl Omega//spl middot/cm/sup 2/, achieving a best V/sub b//sup 2//R/sub on/ value of 1898 MW/cm/sup 2/. We also obtained a 9-mm/sup 2/ Mo-4H-SiC SBD with V/sub b/ of 4.40 kV and R/sub on/ of 12.20 m/spl Omega//spl middot/cm/sup 2/.

Influence of 4H-SiC Growth Conditions on Micropipe Dissociation

In this study, we investigated the influence of 4H–SiC growth conditions on micropipe dissociation. The C/Si ratio of the reactant gases for chemical vapor deposition (CVD) epitaxial growth has a major influence on the probability of micropipe dissociation. A high probability of micropipe dissociation of over 98% was successfully obtained at a low C/Si ratio. We also investigated the surface morphology of 4H–SiC epilayers around dissociated and continuous micropipes grown under different C/Si growth conditions.

Improvement in Electrical Properties of 4H-SiC Epilayers by Micropipe Dissociation

In this study, the effect of micropipe dissociation via 4H-SiC vapor phase epitaxy on the electrical properties of the epilayer was investigated. Ni/4H-SiC Schottky barrier diodes (SBDs) were fabricated on a 21-µm-thick epilayer, and their reverse blocking characteristics were examined. Some micropipes dissociated into closed core screw dislocations during epitaxial growth and some of them propagated into the epilayer. The SBDs that included a propagated micropipe showed poor reverse blocking performance, as reported previously, while the breakdown voltages were below -400 V. On the other hand, the SBDs that included a dissociated micropipe withstood breakdown up to -1000 V, where the leakage current density was 10-2 to 10-6 A/cm2. This demonstrated that a great improvement of the electrical properties could be achieved by the structural transformation of micropipes into closed core screw dislocations.

Interface States in Abrupt SiO2/4H- and 6HSiC(0001) from First-Principles: Effects of Si Dangling Bonds, C Dangling Bonds and C Clusters

Abstract. First-principles calculations of abrupt SiO2/4H-SiC and 6H-SiC(0001) interfaces have been performed. The calculated interface states, which originate from dangling bonds of interface Si atoms, lie 1.8 eV above the valence band edge of the SiO2/4H-SiC and the SiO2/6H-SiC interfaces. These states vanish when Si dangling bonds are saturated with hydrogen atoms. In the carbon cluster model, interface states appear not only at the conduction band edge but also at the valence band edge.

4H-SiC Epitaxial Growth for High-Power Devices

4H-SiC epilayers are grown in a vertical hot-wall reactor with an inner susceptor configuration. Reduction of micropipe density can be achieved by chemic al vapor deposition (CVD) growth using SiH4 and C3H8 as source gases. This technique involves dissociation of micropipes into elementary screw dislocations by growth of a micr opipe stop (MS) layer. The most important parameter to control micropipe dissociation was found to be C/Si ratios of the source gases. A high probability of micropipe dissociation is obtai ned at a relatively low C/Si ratio. Meanwhile we have succeeded in closing more than 99.6% of m icropipes in a commercial 4H-SiC substrate by a single growth run. Low-doped active layers ar grown at a relatively high C/Si ratio onto MS layers without coalescing of elementary sc rew dislocations. A large Schottky barrier diode (SBD) with a diameter of 11.2 mm φ was fabricated using this technique. We also discuss growth of very thick 4H-SiC epilayers at a high growth rate and other i ssu s.

Improvement in Electrical Performance of Schottky Contacts for High-Voltage Diode

We investigated the effect of high temperature annealing on the Schottky barrier height (Fb) and the ideality factor (n-factor) of a Mo contact. In a Mo contact, the Fb increased and the leakage current decreased by annealing at 600oC, while no increase in n-factor and forward excess current owing to the high temperature annealing was observed. The Schottky barrier diode with Mo contact annealed at 600oC showed a blocking-voltage (Vb) of 4.15 kV and a specific on resistance (Ron) of 9.07 mWcm2, achieving a high Vb 2/Ron value of 1898 MW/cm2.

Conditions for Micropipe Dissociation by 4H-SiC CVD Growth

4H-SiC epilayers are grown by chemical vapor deposition (CVD) unde r a various C/Si ratio of source gases. At a relatively low C/Si ratio, micropipes are dissociated into several closed-core screw dislocations in a high probability. Correspondingly, line-shaped surface depressions are generated at a relatively low C/Si ratio. Thus, based on the morphological observation on the epilayers grown under a various C/Si ratio, conditions and mechanisms for micropipe dissociation have been discussed. Introduction SiC has excellent properties and it is suitable for high power and low-loss applications. In these days, some prototype SiC devices with a very high reverse blocking vol ta e have been reported [1]. Moreover commercial SiC diodes have recently been produced [2]. However, it is still difficult to fabricate large SiC devices with a high yield. It is understood that a high defect density in SiC substrates and epilayers is the main cause of thi s problem. Thus a reduction of the defects in substrates and epilayers is required. The micropipe is a major defect in SiC substrates, and it was known that micropipes in substrates propagate t o th epilayer by conventional chemical vapor deposition (CVD). Thus, liquid phase epitaxy ( LPE) was expected to be a micropipe reduction method [3]. Recently we found tha t some micropipes were dissociated into several closed-core screw dislocations during CVD growth [4]. On this occasion, we succeeded in finding a controlling method of micropipe disso ciation by adjusting the C/Si ratio in reactant gases during growth [5, 6]. In this paper, we conside r the r lationship between micropipe dissociation and growth conditions based on the surface morphology.

Vibrational study on the carbonization of Si surface

Abstract The initial carbonization of Si (100) and (111) in the temperature range 600–1200°C, under atmospheric pressure, was investigated by probing changes of the surface Si–H vibrational modes using FTIR-ATR. When the samples were exposed to C 3 H 8 with H 2 at 1000°C, without cooling down after in-situ etching, both Si–H vibrational modes associated with carbonization and Si–H vibrational modes indicating a H-terminated clean Si surface were observed simultaneously. This indicates that initial carbonization occurred as an island growth mode. When the samples were exposed to C 3 H 8 with H 2 and heated up to the temperature range 600–1200°C, with a cooling down process after in-situ etching, Si–H vibrational modes associated with carbonization with broad spectral feature were obtained in the temperature range 1050–1200°C. This indicates that the carbonized surfaces were microscopically rough.