A semiconductor light emitting device comprising: a substrate; and a semiconductor structure disposed on the substrate, the semiconductor structure comprising a III-nitride light emitting layer disposed between an n-type region and a p-type region; wherein: the substrate comprises a first sidewall and a second sidewall; the first sidewall and the second sidewall are disposed at different angles relative to a major surface of the semiconductor structure; the first sidewall forms a right angle with the major surface of the semiconductor structure; and a reflective layer is present over the first sidewall, wherein light is extracted from the substrate through the second sidewall, characterized in that the substrate and the semiconductor structure are disposed in a cavity in a mount, wherein the cavity is shaped to accommodate the substrate and the semiconductor structure, and wherein the second sidewall is parallel with a top surface of the mount, and further comprising: a wavelength converting layer disposed over the second sidewall; and a reflective layer present on a top surface of the mount, surrounding the wavelength converting layer, wherein the reflective layer is made from a heat conducting material, such that heat is conducted away from the substrate and the semiconductor structure and/or the wavelength converting layer. a substrate; and a semiconductor structure disposed on the substrate, the semiconductor structure comprising a III-nitride light emitting layer disposed between an n-type region and a p-type region; wherein: the substrate comprises a first sidewall and a second sidewall; the first sidewall and the second sidewall are disposed at different angles relative to a major surface of the semiconductor structure; the first sidewall forms a right angle with the major surface of the semiconductor structure; and a reflective layer is present over the first sidewall, wherein light is extracted from the substrate through the second sidewall, the substrate comprises a first sidewall and a second sidewall; the first sidewall and the second sidewall are disposed at different angles relative to a major surface of the semiconductor structure; the first sidewall forms a right angle with the major surface of the semiconductor structure; and a reflective layer is present over the first sidewall, wherein light is extracted from the substrate through the second sidewall, characterized in that the substrate and the semiconductor structure are disposed in a cavity in a mount, wherein the cavity is shaped to accommodate the substrate and the semiconductor structure, and wherein the second sidewall is parallel with a top surface of the mount, and further comprising: a wavelength converting layer disposed over the second sidewall; and a reflective layer present on a top surface of the mount, surrounding the wavelength converting layer, wherein the reflective layer is made from a heat conducting material, such that heat is conducted away from the substrate and the semiconductor structure and/or the wavelength converting layer. the substrate and the semiconductor structure are disposed in a cavity in a mount, wherein the cavity is shaped to accommodate the substrate and the semiconductor structure, and wherein the second sidewall is parallel with a top surface of the mount, and further comprising: a wavelength converting layer disposed over the second sidewall; and a reflective layer present on a top surface of the mount, surrounding the wavelength converting layer, wherein the reflective layer is made from a heat conducting material, such that heat is conducted away from the substrate and the semiconductor structure and/or the wavelength converting layer. a wavelength converting layer disposed over the second sidewall; and a reflective layer present on a top surface of the mount, surrounding the wavelength converting layer, wherein the reflective layer is made from a heat conducting material, such that heat is conducted away from the substrate and the semiconductor structure and/or the wavelength converting layer.