Papers by Author: Kok Keong Lew

Abstract: The effect of various types of in-grown stacking faults and threading screw/edge type dislocations on carrier lifetime and diffusion lengths in 4H-SiC epitaxial films was investigated through cathodoluminescence decays and charge collection efficiencies of electron beam induced current signals at specific defects sites. Most stacking faults yielded ~40% reduction in the carrier lifetime. Moreover, drastic lifetime reductions were observed in regions containing surface triangular defects and bulk 3C polytype inclusions. Dislocations of both types serve as efficient recombination centers, though stronger reduction in diffusion lengths was observed in the vicinity of screw type dislocations.

Abstract: The temperature dependence of the carrier lifetime was measured in n-type 4H-SiC epilayers of varying Z1/2 deep defect concentrations and layer thicknesses in order to investigate the recombination processes controlling the carrier lifetime in low- Z1/2 material. The results indicate that in more recently grown layers with lower deep defect concentrations, surface recombination tends to dominate over carrier capture by other bulk defects. Low-injection lifetime measurements were also found to provide a convenient method to assess the surface band bending and surface trap density in samples with a significant surface recombination rate.

Abstract: This paper reports a 4H-SiC single photo avalanche diode (SPAD) operating at the solar blind wavelength of 280 nm. The SPAD has an avalanche breakdown voltage of 114V. At 90% and 95% of the breakdown voltage, the SPAD shows a low dark current of 57.2fA and 159fA, respectively. The quantum efficiency of 29.8% at 280nm and <0.007% at 400nm indicates a high UV-to-visible rejection ratio of >4300. Single photon counting measurement at 280nm shows that a single photon detection efficiency of 2.83% with a low dark count rate of 22kHz is achieved at the avalanche breakdown voltage of 116.8V.

Abstract: X-ray diffraction (XRD) rocking curves were mapped across 4H-SiC, 3-inch, 8° off-cut substrates prior to and after epitaxial growth, where a pattern of slightly higher defectivity region was clearly seen. This same pattern was apparent in both cross-polarization images of the epiwafers and microwave photoconductivity decay (μ-PCD) lifetime maps of the epilayers, where the latter shows the lifetime in the high defectivity regions had drastically decreased. Within the short lifetime regions, electron trap concentrations were similar to that as in the long lifetime regions as determined by deep level transient spectroscopy; however, the extended defect density was significantly higher. Consequently, high spatial resolution XRD can be a valuable tool in preselecting substrates for epitaxial growth to produce low defect density material with long injected carrier lifetimes.

Abstract: The evolution of basal plane dislocations (BPDs) in 4H-SiC epitaxy during its growth is investigated by using two types of interrupted growth in conjunction with ultraviolet photoluminescence (UVPL) imaging of the dislocations. For the first, each epitaxial growth was stopped after 10-20 μm and a UVPL map was collected. For the second, changing the gas flow interrupted the growth and the BPDs were imaged at the end. The first sequence made it possible to track the formation of half-loop arrays and show that they arise from BPDs that glide perpendicular to the offcut direction. For both types, each interruption causes between 30 – 50% of the BPDs to be converted to threading edge dislocations (TEDs). This result suggests that using interrupted growth may be an alternate method to producing epitaxial layers with low BPD concentration.

Abstract: Epitaxial growth of 3-in, 4° off-axis 4H SiC with addition of HCl has been presented. Good surface morphology with a low defect density has been obtained, even for epi thickness of 38 µm. Comprehensive characterization techniques conducted on the epi material obtained in this process have independently confirmed the high purity and low density of crystalline imperfections. Low temperature PL displays clear free exciton I77 recombination while no L1 line is discernable. DLTS measurements have confirmed a low concentration of Z1/2 and EH6/7 below or in the range of 1011 cm-3. Time resolved PL at room temperature performed on a 38 µm thick epi wafer gives long carrier lifetime in the range of 1.5 to above 5 µsec. PiN diodes with diode area up to 25 mm2 have demonstrated blocking voltages above 900V, with a max electric field of above 2.5 MV/cm.

Abstract: Hydrogen etching of 4H-SiC has been performed in a hot-wall chemical vapor deposition reactor to reduce surface damage and to create a bilayer-stepped surface morphology, optimal for initiation of growth on 4H-SiC substrates offcut 4° and 8° towards the <11-20> direction. To understand how step bunching evolves during the ramp to growth temperature, samples were etched ending at temperatures from 1400 to 1580°C under 0, 2 or 10 sccm of propane (C3H8) addition to hydrogen. Initial exploratory growth of 5 μm thick epilayers on the 4° etched surfaces are also discussed. Atomic force microscopy (AFM) and Nomarski microscopy were employed to investigate changes in the surface morphology. The 8° substrates subjected to H2-C3H8 etching up to growth temperature routinely exhibited bilayer steps. However, when the 4° substrates were etched with a 10 sccm C3H8 flow, considerable step bunching was observed. At 1450°C, with a 10 sccm of C3H8 flow (partial pressure is 1.25x10-5 bar), step bunching started with the formation of ribbon-like steps. Progression to higher temperature etches have shown the coalescence of the ribbons into larger macro-steps up to 30 nm in height. Etching 4° substrates under 2 sccm of C3H8 (partial pressure is 2.5x10-6 bar) or in pure H2 up to 1500°C results in minimal step bunching.

Abstract: A set of three 4H-SiC wafers with manufacturer specified micropipe density of 0-5 cm-2 were characterized by x-ray diffraction (XRD) maps before and after final chemical-mechanical polish. After final polish, the wafers were also investigated with atomic force microscopy, radius of curvature measurements and cross-polarization (x-pol) mapping. It was found that there was largely a lack of correlation between the XRD and x-pol maps, which strongly suggests that x-pol is insensitive to crystalline imperfections to which XRD is sensitive.

Abstract: Real-time analysis of downstream nitrogen process-gas flows during 4H-SiC growth is reported. A Hiden Analytical HPR-20 quadrupole mass-spectrometer (QMS) was used to measure the process gas composition in the gas-stream of a hot-wall chemical vapor deposition (CVD) reactor. Using the 28 amu peak, it was found that the nitrogen partial pressure measured by the mass spectrometer directly correlates to the expected partial pressure of nitrogen in the process cell based on input flows. Two staircase doping samples were grown to track doping variations. The nitrogen mass flow was varied and corresponded to doping levels ranging from 1x1015 cm-3 to 8x1018 cm-3. Electron and nitrogen concentrations in the epilayers were measured by capacitancevoltage (CV) profiling and secondary ion mass spectrometry (SIMS), respectively. These efforts show real-time QMS monitoring is effective during growth for determining relative changes in nitrogen concentration in the gas flow, and thus, the level of nitrogen incorporation into the growing layer.