Low-injection minority carrier lifetimes ͑MCLs͒ and deep trap spectra have been investigated in n − 4H-SiC epilayers of varying layer thicknesses, in order to enable the separation of bulk lifetimes from surface recombination effects. From the linear dependence of the inverse bulk MCL on the concentration of Z1/Z2 defects and from the behavior of the deep trap spectra in 4H-SiC p-i-n diodes under forward bias, we conclude that it is Z1/Z2 alone that controls the MCL in this material.

Transmission electron microscopy and KOH etching were used to determine the structure of the carrot defect in 4H-SiC epilayers. The defect consists of two intersecting planar faults on prismatic {11¯00} and basal {0001} planes. Both faults are connected by a stair-rod dislocation with Burgers vector 1∕n [101¯0] with n>3 at the crossover. A Frank-partial dislocation with b=1∕12[44¯03] terminates the basal fault.

Forward voltage instability, or Vf drift, has confounded high voltage SiC device makers for the last several years. The SiC community has recognized that the root cause of Vf drift in bipolar SiC devices is the expansion of basal plane dislocations (BPDs) into Shockley Stacking Faults (SFs) within device regions that experience conductivity modulation. In this presentation, we detail relatively simple procedures that reduce the density of Vf drift inducing BPDs in epilayers to <10 cm-2 and permit the fabrication of bipolar SiC devices with very good Vf stability. The first low BPD technique employs a selective etch of the substrate prior to epilayer growth to create a near on-axis surface where BPDs intersect the substrate surface. The second low BPD technique employs lithographic and dry etch patterning of the substrate prior to epilayer growth. Both processes impede the propagation of BPDs into epilayers by preferentially converting BPDs into threading edge dislocations (TEDs) during the initial stages of epilayer growth. With these techniques, we routinely achieve Vf stability yields of up to 90% in devices with active areas from 0.006 to 1 cm2, implying that the utility of the processes is not limited by device size.

The driving force of stacking-fault expansion in SiC p-i-n diodes was investigated using optical emission microscopy and transmission electron microscopy. The stacking-fault expansion and properties of the partial dislocations were inconsistent with any stress as the driving force. A thermodynamic free energy difference between the perfect and a faulted structure is suggested as a plausible driving force in the tested diodes, indicating that hexagonal polytypes of silicon carbide are metastable at room temperature.

Articles you may be interested inA method to determine fault vectors in 4H-SiC from stacking sequences observed on high resolution transmission electron microscopy images

The growth of large diameter silicon carbide (SiC) crystals produced by the physical vapor transport (PVT) method is outlined. Methods to increase the crystal diameters, and to turn these large diameter crystals into substrates that are ready for the epitaxial growth of SiC or other non homogeneous epitaxial layers are discussed. We review the present status of 150 mm and 200 mm substrate quality at Cree, Inc. in terms of crystallinity, dislocation density as well as the final substrate surface quality.

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Synchrotron white beam x-ray topography (SWBXT) and Nomarski optical microscopy (NOM) have been used to characterize 4H-SiC epilayers and to study the character of triangular inclusions therein. 4H-SiC substrates misoriented by a range of angles from (0001), as well as (1 100) and (112 0) oriented substrates were used. For epilayers grown on substrates misoriented by 3.5 ~ from (0001) toward <1120>, the triangular inclusions were identified as consisting of two 3C-SiC structural configurations which are related to each other by a 180 ~ rotation about the [111] axis. The epitaxial relationships between the 3C inclusions and the 4H-SiC epilayers (or substrates) were also determined. No evidence was found for the nucleation of 3C-SiC inclusions at superscrew dislocations (along the [0001] axis) in the 4H-SiC substrates. Increasing the offaxis angle of the substrates from 3.5 to 6.5 ~ was found to greatly suppress the formation of the triangular inclusions. In the case of substrates misoriented by 8.0 ~ from (0001) toward <1120>, the triangular inclusions were virtually eliminated. The crystalline quality of 4H-SiC epilayers grown on the substrates misoriented by 8.0 ~ from (0001) was very good. For the (1100) and (1120) samples, there is no indication of 3C-SiC inclusions in the epilayers. Possible formation mechanisms and the morphology of 3C-SiC inclusions are discussed.