Formation of a broad photoluminescence band from <inline-formula><math display="inline" overflow="scroll"><msup><mi mathvariant="normal">Si</mi><mo>+</mo></msup></math></inline-formula>-implanted <inline-formula><math display="inline" overflow="scroll"><mrow><mi mathvariant="normal">Si</mi><msub><mi mathvariant="normal">O</mi><mn>2</mn></msub></mrow></math></inline-fo