Hidekazu Tsuchida

Reduction of traps and improvement of carrier lifetime in 4H-SiC epilayers by ion implantation

The authors report a significant reduction in deep level defects and improvement of carrier lifetime in 4H-SiC material after carrying out carbon or silicon ion implantation into the shallow surface layer of 250nm and subsequent annealing at 1600°C or higher temperature. Reduction of Z1∕2 and EH6∕7 traps from 3×1013cm−3 to below the detection limit (5×1011cm−3) was observed by deep level transient spectroscopy in the material 4μm underneath the implanted layer. Minority carrier lifetime almost doubled in the implanted samples compared to the unimplanted samples. The authors propose that the implanted layer acts as a source of carbon interstitials which indiffuse during annealing and accelerate annealing out of grown-in defects in the layer underneath the implanted region.

Structural analysis and reduction of in-grown stacking faults in 4H–SiC epilayers

We investigated the structure of in-grown stacking faults in the 4H–SiC(0001) epilayers. The in-grown stacking faults nucleate near the substrate/epilayer interface and expand the area with increasing epilayer thickness in a triangular shape. From transmission electron microscope observation, the formation of 1c of 8H polytype was confirmed in the in-grown stacking fault area. We also investigated the dependence of in-grown stacking fault density on the epitaxial growth rate, growth temperature, and substrate surface preparation.

Enhanced annealing of the Z1∕2 defect in 4H–SiC epilayers

The authors investigated the application of the carbon-implantation/annealing method for the annealing of the main lifetime limiting defect Z1∕2 in thick 4H–SiC epilayers. Examination of different implantation doses and annealing temperatures showed that finding the optimum conditions is crucial for obtaining thick layers with carrier trap concentration below 1011cm−3 in the whole 100μm epilayer. The carrier lifetime increased from less than 200ns to over 1μs at room temperature in the samples annealed with the carbon-implanted layer. The thick 4H–SiC epilayers after the application of the carbon-implantation/annealing were confirmed to be applicable for fabrication of high-voltage bipolar devices and resulted in improved conductivity modulation. Possible annealing mechanisms are discussed in detail making a comparison between annealing of as-grown material and irradiated material.

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.

Evaluation of Free Carrier Lifetime and Deep Levels of the Thick 4H-SiC Epilayers

Correlations between the free carrier lifetime of thick, lightly-doped n-type 4H-SiC epilayers and some deep levels (the Z1/2 center, the EH6/7 center and the D1 center) were investigated. Concentrations of the Z1/2 center and the EH6/7 center correlated with the measured carrier lifetime to some extent. We have also compared the free carrier lifetime measured by time-resolved photoluminescence (TRPL) measurement and microwave-detected photoconductive decay (μ-PCD) measurement. The carrier lifetime measured by both technique was in close agreement. The carrier lifetime measured by LTPL increased at an elevated temperature of 500 K.

Annealing effects on single Shockley faults in 4H-SiC

We investigated the annealing effect on single Shockley faults (SSFs) in the SiC epitaxial layers by photoluminescence mapping in combination with high-power laser illumination. Comparing before and after annealing at 350–550°C, it became obvious that annealing results in the shrinking of the faulted area of SSFs. When high-power laser illumination is performed again on the same area annealed at 550°C, the right-angled triangular SSFs reformed into exactly the same features as those before annealing, but the isosceles triangular SSFs did not reform. The annealing temperature to start shrinking the faulted area differs according to the type of SSF.

Development of 4H--SiC Epitaxial Growth Technique Achieving High Growth Rate and Large-Area Uniformity

A vertical hot-wall epi-reactor that makes it possible to simultaneously achieve a high growth rate and large-area uniformity has been developed. A maximum growth rate of 250 µm/h is achieved with a mirror-like morphology at 1650 °C. Under a modified epi-reactor setup, a thickness uniformity of 1.1% and a doping uniformity of 6.7% for a 65-mm-radius area are achieved while maintaining a high growth rate of 79 µm/h. A low doping concentration of ~1×1013 cm-3 is obtained for a 50-mm-radius area. The low-temperature photoluminescence (LTPL) spectrum shows the predominance of free exciton peaks with only few impurity-related peaks and the L1 peak below detection limit. The deep level transient spectroscopy (DLTS) measurement for an epilayer grown at 80 µm/h shows low trap concentrations of Z1/2: 1.2×1012 and EH6/7: 6.3×1011 cm-3. A 280-µm-thick epilayer with a RMS roughness of 0.2 nm and a carrier lifetime of ~1 µs is obtained.

Evaluation of long carrier lifetimes in thick 4H silicon carbide epitaxial layers

The carrier lifetime of ∼265 μm thick n-type 4H silicon carbide epilayers prepared using the carbon-implantation/annealing method was evaluated. An extraordinarily long minority carrier lifetime of 18.5 μs and a high injection lifetime of 19.2 μs were evaluated from time-resolved photoluminescence and microwave photoconductivity decay measurements, respectively. Based on the relationship between the epilayer thickness and carrier lifetime, the influence of surface recombination on the carrier lifetime was eliminated, and the bulk lifetime and hole diffusion constant were discussed.

INFRARED SPECTROSCOPY OF HYDRIDES ON THE 6H-SIC SURFACE

Hydrides on the 6H-SiC(0001) and (0001) surfaces before and after hydrogen annealing were investigated by infrared attenuated total reflection spectroscopy. The absorption bands of CH2 and CH3 were observed from the 6H-SiC(0001) and (0001) surfaces before hydrogen annealing. After hydrogen annealing, a sharp C–H stretching vibration attributable to a monohydride appeared on the 6H-SiC(0001) surface, in contrast to a sharp Si–H stretching vibration which can be observed on the 6H-SiC(0001) surface.