A physical vapor transport growth system comprising: a growth chamber (60a,78) charged with SiC source material (61,71,91) and a SiC seed crystal (63,72) in spaced relation; and an envelope that is at least partially gas-permeable disposed in the growth chamber (60a,78) and separating the growth chamber (60a,78) into a source compartment that includes the SiC source material (61,71,91) and a crystallization compartment that includes the SiC seed crystal (63,72), said gas-permeable envelope formed of a material that is reactive to vapor generated by sublimation growth of a SiC single crystal (64,73) on the SiC seed crystal (63,72) in the crystallization compartment to produce a C-bearing vapor that acts as an additional source of C during the growth of the SiC single crystal (64,73) on the SiC seed (63,72) crystal, wherein the envelope is comprised of: a sleeve that surrounds sides of the SiC seed crystal and the growing SiC single crystal(64,73); and a gas-permeable membrane disposed between the SiC source material (71,91) and a surface of the SiC seed crystal that faces the SiC source material (71,91), wherein the gas-permeable membrane is made of porous graphite having a density between 0.6 and 1.4 g/cm 3 and a porosity between 30% and 70%, characterized in that the graphite forming the gas-permeable membrane is comprised of graphite grains, each of which has a maximum dimension between 100 and 500 microns, and all of the sublimated SiC source material that condenses on the SiC seed crystal and forms the SiC single crystal (64,73) diffuses through the gas-permeable membrane from the source compartment into the crystallization compartment. a growth chamber (60a,78) charged with SiC source material (61,71,91) and a SiC seed crystal (63,72) in spaced relation; and an envelope that is at least partially gas-permeable disposed in the growth chamber (60a,78) and separating the growth chamber (60a,78) into a source compartment that includes the SiC source material (61,71,91) and a crystallization compartment that includes the SiC seed crystal (63,72), said gas-permeable envelope formed of a material that is reactive to vapor generated by sublimation growth of a SiC single crystal (64,73) on the SiC seed crystal (63,72) in the crystallization compartment to produce a C-bearing vapor that acts as an additional source of C during the growth of the SiC single crystal (64,73) on the SiC seed (63,72) crystal, wherein the envelope is comprised of: a sleeve that surrounds sides of the SiC seed crystal and the growing SiC single crystal(64,73); and a gas-permeable membrane disposed between the SiC source material (71,91) and a surface of the SiC seed crystal that faces the SiC source material (71,91), wherein the gas-permeable membrane is made of porous graphite having a density between 0.6 and 1.4 g/cm 3 and a porosity between 30% and 70%, characterized in that the graphite forming the gas-permeable membrane is comprised of graphite grains, each of which has a maximum dimension between 100 and 500 microns, and all of the sublimated SiC source material that condenses on the SiC seed crystal and forms the SiC single crystal (64,73) diffuses through the gas-permeable membrane from the source compartment into the crystallization compartment. a sleeve that surrounds sides of the SiC seed crystal and the growing SiC single crystal(64,73); and a gas-permeable membrane disposed between the SiC source material (71,91) and a surface of the SiC seed crystal that faces the SiC source material (71,91), wherein the gas-permeable membrane is made of porous graphite having a density between 0.6 and 1.4 g/cm 3 and a porosity between 30% and 70%, characterized in that the graphite forming the gas-permeable membrane is comprised of graphite grains, each of which has a maximum dimension between 100 and 500 microns, and all of the sublimated SiC source material that condenses on the SiC seed crystal and forms the SiC single crystal (64,73) diffuses through the gas-permeable membrane from the source compartment into the crystallization compartment.