Light-Emitting Porous Silicon: Materials Science, Properties, and Device Applications

Silicon-based. light-emitting devices (LED's) should find numerous uses in optoelectronics. For example, the integration of silicon LED's with silicon microelectronics could lead to reliable and inexpensive optical displays and optical interconnects. Until recently, however, it had not been possible to obtain efficient room-temperature luminescence from silicon. The demonstration in 1990 that a form of silicon called "porous" can emit bright photoluminescence in the red region of the spectrum triggered worldwide research efforts aimed at establishing the mechanisms for the unexpected luminescence and at fabricating efficient and durable LED's. In less than five years, significant progress has been achieved on both fronts. LED's emitting throughout the visible spectrum have been demonstrated. and the best measured external electroluminescence efficiency has risen from lop5% to better than 0.01% at room temperature. The photoluminescence efficiency of the best samples is near 10% at room temperature, and lightemitting porous silicon (LEPSi) that luminesces from the blue part of the spectrum to the infrared beyond 1.5 pm has been produced. In this article, we first review why silicon is a poor light emitter and then define porous silicon and its main properties. We then focus on the properties of the three luminescence bands ("red," "blue," and "infrared") and present the results of femtosecond time-resolved optical measurements. Next, we report progress toward the fabrication of LED's and discuss some specific device structures. Finally, we outline what is necessary for commercial LEPSi LED's to become a reality and report on experimental results that suggest the possible integration of LEPSi with standard microelectronic devices.