Special correlation of photoluminescent peak of porous silicon with its resistivity

Porous silicon (PS) layers were formed on p-type textured Si wafers of orientation 〈1 0 0〉 and 1 Ω cm resistivity electrochemically in 1:2 solution of HF:C2H5OH at 25 °C at a constant current density of 20 mA/cm2 for etching durations (te) from 20 s to 20 min. The wafers had an Al ohmic contact on the back surface. The photoluminescent spectra of the PS layers were measured under excitation radiation of 405 nm wavelength. All PS layers formed above the etching duration te ⩾ 2 min were found to be photoluminescent with PL peak lying between 626 and 675 nm wavelengths. The PL intensity increased with increase in te, for 2 < te < 8 min and decreased with te after having exhibited maximal value for te = 8 min. The thickness of the PS layers was found to exhibit similar behavior with te showing a maximal value for te = 8 min. The contacts on the same PS layers were made over an area of 1 cm2 by vacuum evaporated Pd metal layers and the current voltage (I–V) characteristics were measured in the Pd/PS/Si/Al sandwich configuration at different temperatures in 20–40 °C range. The resistances of the PS layers were determined from the decrease of the slopes of their forward I–V characteristics. The resistivities of PS layers were obtained using the measured resistance data and thickness values. The resistivity values were much greater than the intrinsic resistivity of monocrystalline silicon. The resistivity increased with etching time and attained a maximum value of 2.88 × 1014 Ω cm for te = 8 min and then decreased for further increase in te. The PS layers behaved as intrinsic semiconductor layers and the values of the band gap Eg and the corresponding wavelengths (λg) determined from the resistivity (ρ) matched extremely well with the peak PL emission wavelengths. The most resistive PS layer was found to be the most efficient PL emitter. Resistances of PS layers decreased with temperature. However, the values of activation energies obtained from resistance vs. temperature data of different PS samples were much smaller than the Eg/2 values.

► Effect of etching duration on series resistance and PL intensity of PS layers. ► Photoluminescent porous silicon layers behave as intrinsic semiconductor. ► PL intensity increases with PS layer thickness and the resistivity. ► The thickest PS layer is the most resistive and also the most efficient PL emitter.