Amitesh Shrivastava

Cathodoluminescence study of the properties of stacking faults in 4H-SiC homoepitaxial layers

In-grown stacking faults in n-type 4H-SiC epitaxial layers have been investigated by real-color cathodoluminescence imaging and spectroscopy carried out at room and liquid helium temperatures. Stacking faults with 8H stacking order were observed, as well as double layer and multilayer 3C-SiC structures and a defect with an excitonic band gap at 2.635 eV. It was found that 8H stacking faults and triangular surface defects can be generated from similar nucleation sources. Time-resolved measurements reveal that compared to defect-free regions, the carrier lifetimes are severely reduced by the presence of stacking faults corresponding to triangular surface defects and three-dimensional 3C-SiC inclusions.

Recombination processes controlling the carrier lifetime in n−4H–SiC epilayers with low Z1/2 concentrations

The dominant recombination processes controlling the carrier lifetime in n-type 4H–SiC epitaxial layers grown with low concentrations of the Z1/2 defect (the dominant bulk lifetime killer), where Z1/2 no longer determines the lifetime, have been investigated by studying the variation in the carrier lifetime with temperature. The temperature dependent lifetimes were obtained primarily by low-injection photoluminescence decay for several low-Z1/2 epilayers over a wide temperature range. The results were fitted to simulations of the temperature dependent recombination rate, where bulk, surface and interface recombination was considered. No significant contribution from other bulk defects was observed, and upper limits to the bulk recombination rate were found to be consistent with the low Z1/2 concentrations measured in these materials. There was also no significant contribution from carrier capture at the epilayer/substrate interface, which is consistent with behavior expected at low injection for low-doped...

Investigation of Triangular Defects in 4H-SiC 4° Off Cut (0001) Si Face Epilayers Grown by CVD

Triangular defects and inverted pyramid type defects formed during homoepitaxial growth on 4H-SiC Si face, 4° off-cut towards [11-20] direction have been investigated. Growth parameters responsible for triangular defect formation were identified and optimized for its reduction in this study. It was found that although the high temperature reduces the density of inverted pyramid type defects, it is not the only remedy for reducing their density and cleanliness of susceptor along with the initial growth condition plays a major role in the formation of these defects.

Long Carrier Lifetime in 4H-SiC Epilayers Using Chlorinated Precursors

In this work we report the measurement of minority carrier lifetimes using the time resolved photoluminescence technique. It was found that 4H-SiC homo-epilayers grown using chlorine-based precursors have longer carrier lifetimes if used in conjunction with a tantalum carbide coated (TaC-coated) graphite susceptor rather than a SiC-coated graphite susceptor. Longer carrier lifetimes were obtained by optimal combinations of precursor gases and susceptor type. The controllable variation in lifetime from 250 ns to 9.9 s was demonstrated.

Propagation of stacking faults from surface damage in SiC PiN diodes

The propagation of stacking faults (SF) in SiC PiN diodes under forward bias was studied by the electron beam induced current mode of scanning electron microscopy. The primary SF nucleation sites were confirmed to be pre-existing basal plane dislocations (BPD). Damage to the diode surface can also cause SF propagation in the device. Hence, in addition to the elimination of BPDs in the active layer of the diode, avoidance of surface damage by paying careful attention to device processing and testing is also important for fabricating stable SiC PiN diodes.

Why Are Only Some Basal Plane Dislocations Converted to Threading Edge Dislocations During SiC Epitaxy

Dislocations were tracked from 4H-SiC epilayer to the substrate by a new method based on combination of molten KOH etching and Reactive Ion Etching. It was found that basal plane dislocations (BPDs) with dislocation lines parallel (or approximately parallel) to the off-cut direction might propagate as BPDs into the epilayer, while those with dislocation lines forming large angles (>10º) with the off-cut direction will get converted to threading edge dislocations (TEDs). A model is proposed to explain the observations.

Temperature Dependence of the Carrier Lifetime in 4H-SiC Epilayers

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.

Inverted Pyramid Defects in 4H-SiC Epilayers

In this work we identified the nucleation sites of inverted pyramid defects in 4H-SiC epilayers using AFM and KOH etching and proposed a mechanism for its formation. Partial dislocations, bounding the stacking faults, mostly aligned along the <11-20> directions, were found at the base of the inverted pyramid defects. It is shown that the basal plane dislocations, serve as nucleation centers for stacking faults, and eventually the formation of inverted pyramid defects. A geometrical model is formulated to explain the formation mechanism of inverted pyramid defects.

Variations in the Measured Carrier Lifetimes of n- 4H-SiC Epilayers

The effects of measurement technique and measurement conditions (injection level, temperature) on the measured carrier lifetimes in n- 4H-SiC epilayers are investigated. For three optical measurement techniques, it is shown that the high and low injection lifetimes can vary dramatically. Differences in the lifetime for varying injection level and temperature are approached both experimentally and via carrier dynamics simulations, assuming Z1/Z2 as the dominant defect. Reasonable agreement between measured and calculated behavior is obtained, as is insight into the recombination kinetics associated with the lifetime limiting defect.

Identification of nucleation sites and formation mechanism of inverted pyramids in 4H-SiC epilayers

Growth of high quality epilayers on low off-cut (≤4°) 4H-SiC substrates is essential for the fabrication of high performance power devices. Morphological defects such as “inverted pyramids” are device killers and hence knowing their origin and developing methods to eliminate them are essential. The nucleation sites and formation mechanism of the inverted pyramid defects, investigated using atomic force microscopy and KOH etching, are reported in this study. Partial dislocations, bounding the stacking faults, mostly aligned along the ⟨11–20⟩ directions, were found at the base of the inverted pyramid defects. It is shown that in addition to the basal plane dislocations, localized defects, attributed to clusters of impurities, serve as nucleation centers for stacking faults, and eventually the formation of inverted pyramid defects. A geometrical model is formulated to explain the formation mechanism of inverted pyramid defects.