A semiconductor element comprising: a substrate; a semiconductor element structure provided on an upper surface of the substrate; and a metal film provided on a lower surface of the substrate, wherein the metal film contains nanoparticles, and characterised in that a combination of the substrate and the nanoparticles is selected from a group consisting of (i) a combination of a substrate made of an oxide and nanoparticles made of a material selected from a group consisting of silicon oxide, aluminum oxide, zirconium oxide, titanium oxide, zinc oxide, magnesium oxide, gallium oxide, tantalum oxide, niobium oxide, bismuth oxide, yttrium oxide, iridium oxide, indium oxide, tin oxide, nickel oxide, hafnium oxide, ITO, IZO, AZO, GZO, and FTO, (ii) a combination of a substrate made of a nitride and nanoparticles made of a material selected from a group consisting of silicon nitride, aluminum nitride, zirconium nitride, titanium nitride, zinc nitride, magnesium nitride, gallium nitride, tantalum nitride, niobium nitride, bismuth nitride, yttrium nitride, iridium nitride, indium nitride, tin nitride, nickel nitride, and hafnium nitride, (iii) a combination of a substrate made of a carbide and nanoparticles made of a material selected from a group consisting of silicon carbide, zirconium carbide, titanium carbide, tantalum carbide, niobium carbide, yttrium carbide, and hafnium carbide. a substrate; a semiconductor element structure provided on an upper surface of the substrate; and a metal film provided on a lower surface of the substrate, wherein the metal film contains nanoparticles, and characterised in that a combination of the substrate and the nanoparticles is selected from a group consisting of (i) a combination of a substrate made of an oxide and nanoparticles made of a material selected from a group consisting of silicon oxide, aluminum oxide, zirconium oxide, titanium oxide, zinc oxide, magnesium oxide, gallium oxide, tantalum oxide, niobium oxide, bismuth oxide, yttrium oxide, iridium oxide, indium oxide, tin oxide, nickel oxide, hafnium oxide, ITO, IZO, AZO, GZO, and FTO, (ii) a combination of a substrate made of a nitride and nanoparticles made of a material selected from a group consisting of silicon nitride, aluminum nitride, zirconium nitride, titanium nitride, zinc nitride, magnesium nitride, gallium nitride, tantalum nitride, niobium nitride, bismuth nitride, yttrium nitride, iridium nitride, indium nitride, tin nitride, nickel nitride, and hafnium nitride, (iii) a combination of a substrate made of a carbide and nanoparticles made of a material selected from a group consisting of silicon carbide, zirconium carbide, titanium carbide, tantalum carbide, niobium carbide, yttrium carbide, and hafnium carbide. (i) a combination of a substrate made of an oxide and nanoparticles made of a material selected from a group consisting of silicon oxide, aluminum oxide, zirconium oxide, titanium oxide, zinc oxide, magnesium oxide, gallium oxide, tantalum oxide, niobium oxide, bismuth oxide, yttrium oxide, iridium oxide, indium oxide, tin oxide, nickel oxide, hafnium oxide, ITO, IZO, AZO, GZO, and FTO, (ii) a combination of a substrate made of a nitride and nanoparticles made of a material selected from a group consisting of silicon nitride, aluminum nitride, zirconium nitride, titanium nitride, zinc nitride, magnesium nitride, gallium nitride, tantalum nitride, niobium nitride, bismuth nitride, yttrium nitride, iridium nitride, indium nitride, tin nitride, nickel nitride, and hafnium nitride, (iii) a combination of a substrate made of a carbide and nanoparticles made of a material selected from a group consisting of silicon carbide, zirconium carbide, titanium carbide, tantalum carbide, niobium carbide, yttrium carbide, and hafnium carbide.