Photoluminescence from photochemically etched silicon

The photoluminescence (PL) of photochemically etched silicon is studied. In the photochemical etching process, an n-type silicon wafer is immersed in an etchant solution of hydrofluoric acid (HF) and H2O2. A low-power visible laser (typically He–Ne) is used to illuminate the samples. The etching process occurs through the photogeneration of carriers. Although no electrodes are used in this etching method, the final samples show PL similar to electrochemically etched porous silicon. The samples were prepared using (1 0 0) n-type silicon with a resistivity of 1.0–5.5 Ω cm. An He–Ne laser with 20 mW of maximum power output was used and the spot radius (on the samples) was varied from 1 to 4 mm. A strong emission in the red-yellow optical region can be present in the final samples depending on the HF:H2O2 concentration ratio, etching time and laser intensity of the etching process. The PL spectra excited with the monochromated output of an Xe light source as excitation is studied. The peak wavelength of the PL intensity shifts to the blue region of the spectrum when increasing the laser intensity. Quantum confinement can explain this blue shifting if smaller silicon nanocrystallites are formed with higher laser intensity. The peak PL intensity also decreases when increasing the laser intensity, although a “threshold” condition must be reached to have measurable PL. Each sample also exhibits a shifting in peak PL wavelength when varying the PL excitation wavelength. The corresponding dependence and the variations of the PL intensity are studied. Other experimental conditions are discussed.