Progress Toward Crystalline-Silicon-Based Light-Emitting Diodes
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from all competing nonradiative processes, and rr is that limited by all radiative processes. This simple expression shows that there are two ways to realize high radiative efficiency: we must either remove all nonradiative processes (i.e., make rnr very long) or generate a very fast radiative process (make rr very short). The intrinsic radiative efficiency of an undoped defect-free semiconductor is determined by its bandstructure and, in particular, whether its bandgap is direct or indirect. If direct (e.g., GaAs or InP), then electron-hole radiative recombination is usually a fast (TF ~ ns) and efficient process. If indirect (e.g., Si or GaP), then to conserve momentum, the electron and hole must interact with specific phonons to recombine. This three-particle process is a much slower process (rr ~ ms) and hence competes poorly with nonradiative processes and is inefficient. The Auger effect is also a three-particle process, but a nonradiative one whereby the recombination energy of the electron-hole pair is transferred to another electron or hole. That then thermalizes rapidly by phonon emission. Another important origin of nonradiative recombination in all semiconductors is their surfaces. The net effect of these surfaces is often described by a surface recombination velocity (SRV), where
The semiconductor silicon is the dominant material in microelectronics and is one of the best-studied materials known to humanity. Its inability to emit light efficiently is therefore well documented. Nevertheless, a "holy grail" of semiconductor materials research has for decades been the realization of an efficient Si light-emitting diode (LED). Such a device would enable optoelectronic circuitry to be based entirely on silicon and would revolutionize VLSI technology since the other required Si-based devices (detectors, waveguides, modulators, etc.) have already been demonstrated. Although this holy grail has proved elusive, the 1990s have heralded greatly renewed interest and optimism in the development of such devices for both the visible and near-infrared spectral ranges. Dramatic progress is at last being made. This review focuses, in a somewhat chronological manner, on the progress of specific approaches to realizing crystalline structures of high radiative efficiency, and the materials constraints involved.
one another for excited carriers. Each process can be described by the effect it has on minority carrier lifetime T; if its recombination route is fast enough it will control that lifetime. The internal radiative efficiency of a semiconductor can be given by
Luminescence Efficiency of Semiconductors Semiconductors can be excited to emit light in a variety of ways, but in each case a high concentration of electron-hole pairs must be generated. Whether or not light is subsequently emitted efficiently then depends on the complex interplay between the various ways in which those excited carriers can lose energy within the material. Table I lists some common radiative and nonradiative processes that occur in semicondu
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