Modulation speed of an efficient porous silicon light emitting device

Trends in the efficiency and small signal modulation behavior of porous silicon light emitting diodes (LEDs) are reported for devices formed by the anodization of bulk silicon p-n junctions. As the average size of the silicon skeleton is decreased, the external electroluminescence (EL) efficiency increases from 0.001% to 0.18% and there is a corresponding blue shift in the EL peak from 776 to 633 nm. An associated tenfold increase is observed in the photoluminescence efficiency while the diode resistance, at 2 V, increases from 3x10(3) to 1x10(6) Omega. Under small signal pulsed operation, the voltage dependence of the rising edge of the EL is well described by a carrier mobility of 3x10(-4) cm(2) s(-1) V-1 which is independent of the average size of the luminescent regions of the silicon nanostructure. The falling edge of the EL transient is dominated by radiative recombination of quantum confined excitons. The modulation speed is found to be limited by a combination of carrier mobility in the silicon wires and radiative recombination processes. Evidence of charge trapping and discharge is found in an EL overshoot phenomenon. The major application of this type of porous silicon LED, with modulation speeds below 1 MHz, appears to be for displays integrated with circuitry rather than for optical interconnection. [S0021-8979(99)07717-8].