Modulation speed of an efficient porous silicon light emitting device

Trends in the efficiency and small signal modulation behavior of porous sil icon light emitting diodes (LEDs) are reported for devices formed by the an odization of bulk silicon p-n junctions. As the average size of the silicon skeleton is decreased, the external electroluminescence (EL) efficiency in creases 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, increa ses 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 car rier mobility of 3x10(-4) cm(2) s(-1) V-1 which is independent of the avera ge size of the luminescent regions of the silicon nanostructure. The fallin g edge of the EL transient is dominated by radiative recombination of quant um confined excitons. The modulation speed is found to be limited by a comb ination of carrier mobility in the silicon wires and radiative recombinatio n processes. Evidence of charge trapping and discharge is found in an EL ov ershoot phenomenon. The major application of this type of porous silicon LE D, with modulation speeds below 1 MHz, appears to be for displays integrate d with circuitry rather than for optical interconnection. [S0021-8979(99)07 717-8].