The microelectronics industry has been driven by the ever increasing demand for small, high-performance, low-power consumption microelectronic devices. Microelectronics manufacturers can, for example, fabricate microprocessor transistors with dimensions on the order of 50 nm. However, there is a relatively large delay in transmitting digital information across these microprocessors and, in general, transmitting digital information between computational and storage devices located on the same circuit board. Although copper and aluminum wire interconnects have traditionally been used to carry digital information, increasing the number of electronic interconnects to satisfy the number of connections needed by an exponentially growing number of nanoscale electronic devices has been insufficient. Unlike transistors, for which performance improves with scaling, the delay caused by a corresponding increase in the number of electronic interconnects has increased and has become a substantial bottleneck in the speed of digital circuits.
Optical interconnects using optical fibers and polymer waveguides have been proposed as alternatives to electronic interconnects. For example, a single fiber optic cable can carry terabits per second of digital information encoded in channels or different wavelengths of light with a capacity of about 1000 times greater than transmitting the same information using electrical cables. The term “light” is not limited to electromagnetic radiation with wavelengths that lie in the visible portion of the electromagnetic spectrum but is also used to refer to electromagnetic radiation with wavelengths outside the visible portion, such as the infrared and ultraviolet portions, and can be used to refer to both classical and quantum electromagnetic radiation. Semiconductor lasers and light-emitting diodes (“LEDs”) are two commonly used light sources for optical communication. However, the configuration and operation of these sources are fundamentally different and the difference in performance and cost can be significant factors in determining which source to use. In general, semiconductor lasers and LEDs employ semiconductor materials, but the primary difference is in the manner of operation and in the internal structures that control their operation. The following is a general and brief description of structural and operational similarities and differences between LEDs and semiconductor lasers.
An LED is a semiconductor p-i-n junction diode that emits incoherent narrow-spectrum light when electrically biased in the forward direction. This effect is a form of electroluminescence. FIG. 1 shows a schematic representation and cross-sectional view of an LED 100. The LED 100 comprises an intrinsic or undoped region 102 sandwiched between a p-type semiconductor region 104 and an n-type semiconductor region 106. Electrodes 108 and 110 are connected to the regions 104 and 106, respectively. Regions 104 and 106 can be wider (direct or indirect) electronic bandgap semiconductors while the region 102 can be a narrower, direct bandgap semiconductor, thus forming a double heterostructure p-i-n junction. The p-type region 104 is doped with impurity or electron acceptor atoms having fewer electrons than the atoms they replace in the semiconductor compound, which creates holes or vacant electronic energy states in the valance band of the p-type region 104. On the other hand, the n-type region 106 is doped with impurities or electron donar atoms that donate electrons to the semiconductor, which leaves extra electrons in the electronic energy states of the conduction band of the n-type region 106. A depletion regions forms in region 102 forms as a result of the difference in chemical potential between the p-type and n-type semiconductor regions 104 and 106. This built-in potential difference is an equilibrium condition that impedes the flow of electrons and holes between the p- and n-type regions 104 and 106. This potential difference must be overcome before current can flow through the diode.
FIGS. 2A-2B show electronic energy band diagrams for the regions 102, 104 and 106. In FIGS. 2A-2B, heavily shaded regions, such as region 202, represent mostly filled electronic energy states and lightly shaded regions, such as region 204, represent mostly vacant electronic energy states called “holes” which act like positive charge carriers. Electrons and holes are called “charge carriers.” Electron donor impurities create electronic states near the conduction band while electron acceptors create electronic states near the valence band. Thus, as shown in FIG. 2A, the valance and conduction bands associated with the p-doped region 104 are higher in electronic energy than the valance and conduction bands associated with n-doped region 106. Depending on the size of the band gap energies associated with the regions 102, 104, and 106, some electrons can be thermally excited into mostly empty conduction bands as indicated by regions 206 and 208. At zero bias, the region 102 has a relatively low concentration of electrons in the conduction band and an equal number of holes in the valance band. FIG. 2A also reveals steep conduction and valance bands associated with the region 102 which prevent holes and electrons from migrating from the neighboring p- and n-doped regions 104 and 106, respectively. However, when a forward bias is applied to the LED 100, electrons are injected into the n-doped region 106 and holes are injected into the p-doped region 104. Thus, the electronic energy band diagram changes accordingly as shown in FIG. 2B. The steep potential associated with the region 102 flattens. Electrons are injected into the conduction band of the region 102 from the n-type region 106, while holes are injected into the valance band of the region 102 from the p-type region 104. Note the number of electrons and holes remains the same. The relatively higher electronic bandgap energies of the regions 104 and 106 serve to confine the injected carriers to the intrinsic region 102. Electrons spontaneously recombine from the bottom of the conduction band 210 with holes in the top of the valance band 212 in a radiative process called “electron-hole recombination” or “recombination,” emitting photons of light with an energy:E=hv≧Eg where h is Plank's constant, and v is the frequency of the light emitted. As long as an appropriate voltage is applied in the same forward bias direction, population inversion is maintained, electrons and holes flow through the diode and spontaneously recombine at the junction 102, and light is emitted with the frequency v in nearly all directions from the LED 100.
A semiconductor laser, on the other hand, includes a gain medium, a pump, and feedback that can be created by placing the gain medium in a laser cavity. FIG. 3 shows a schematic representation and cross-sectional view of a semiconductor diode laser 300. The gain medium of the laser 300 comprises an intrinsic region with one or more quantum wells 302 sandwiched between a p-type region 304 and an n-type region 306, as described above with reference to the LED 100. The laser 300 also includes a cavity formed by a fully reflective mirror 308 and a partially reflective mirror 310. The mirrors 308 and 310 provide the feedback needed to produce a coherent beam of light. Pumping the gain medium causes carriers to be injected into the conduction band and holes into the valance band in process called “population inversion,” However, unlike the LED, the spontaneous emission of light with frequency v is reflected back into the gain medium by the mirrors 308 and 310. The light produced by the spontaneous emission stimulates the emission of more light with frequency v, and the stimulated emissions stimulates the emission of even more light with frequency v. The light is not absorbed and continues to build-up by bouncing hark and forth between the mirrors 308 and 310 with substantially the same direction, wavelength, and phase and constructively interferes to produce an amplified coherent beam of light with frequency v that leaks out of the partially reflective mirror 310 substantially parallel to the junction 302. Semiconductor lasers can be configured with layers having different refractive indexes to create Bragg reflectors that emit light perpendicular to the junction, such as vertical cavity surface emitting lasers (“VCSELs”).
In general, LEDs emit light via spontaneous electron-hole recombination. In contrast, semiconductor lasers emit light primarily via stimulated electron-hole recombination, which is accomplished by having light already propagating through the gain medium. As a result, lasers can be modulated at much higher speeds than typical LEDs because the electron-hole recombination rate is enhanced by stimulated emission, while in LEDs, spontaneous electron-hole recombination is a much slower process. On the other hand, pre-populating the laser cavity with photons has an energy cost measured in terms of the laser threshold. LEDs do not have a threshold and can be operated at a lower input power and simpler drive circuitry.
The relatively high cost of semiconductor lasers used in optoelectronic devices is, however, a factor limiting their widespread use. For example, in many high-speed optical communication systems, the highest cost element is the laser. VCSELs are an example of a laser that can be used in optical communication links of less than about 300m. Although VCSELs are reliable and efficient and are capable of modulation rates in excess of 10 Gb/s, the cost of implementing VCSELs in computational and communications devices can be prohibitive.
LEDs may provide a reliable inexpensive alternative to lasers, because typical LEDs cost about 1000 times less than high-speed VCSELs. However, when comparing the performance of LEDs to lasers, LEDs have a number of serious limitations including modulation speeds of less than 800 Mb/s, wide spectral linewidth of approximately 30 nm of full-width-at-half-maximum, low efficiency, and a lambertian radiation pattern.