Resonant-cavity-enhanced (RCE) multiple-quantum-well (MQW) photodetectors increase light absorption-and therefore efficiency-by sandwiching the detector itself between two mirrors. Light that is not at first absorbed by the detector is repeatedly reflected back into the absorption region. Such devices made of silicon (Si) and containing Bragg reflectors made of layers of Si and either silicon dioxide (SiO2) or silicon-germanium (SiGe) operate well, but are limited to wavelengths shorter than 1.1 µm.
One way to move detector response to the longer 1.3- to 1.55-µm telecommunications wavelengths is to change the material-in this case, to make the MQW out of SiGe/Si. However, such structures of the proper thickness cannot be grown on either SiO2 or SiGe Bragg reflectors, eliminating the possibility of using a Bragg reflector as the detector cavity`s bottom mirror (the one sandwiched between the detector and the substrate).
But Bragg reflection is not the only solution for an imbedded mirror. Scientists at the Chinese Academy of Sciences (Beijing, China) have made a SiGe/Si RCE photodetector by using a separation-by-implanted-oxygen (SIMOX) wafer as a substrate. Used in the semiconductor industry for making certain kinds of integrated circuits, SIMOX wafers contain an implanted buried layer of SiO2 topped with Si. As it turns out, the buried oxide layer can be used as a bottom mirror. Starting with an n-type SIMOX substrate, the researchers grew a MQW region consisting of 20 layers of Si0.65Ge0.35 of 8-nm thickness and 19 layers of Si of 32-nm thickness, followed by a 200-nm layer of intrinsic Si and a 100-nm p-Si top layer. In the last step, a SiO2/Si Bragg reflector is deposited to form a top mirror. The calculated reflectivity of the buried SiO2 mirror was 80% at 1.3 µm, while the measured reflectivity of the top Bragg reflector was 65%. The microcavity was made to be resonant at 1.3 µm.
The device reaches a peak responsivity of 10.2 mA/W at 1.285 µm and a responsivity of 6.5 mA/W at 1.3 µm. Its external quantum efficiency (QE) at 1.3 µm as a function of applied reverse bias rises to 3.5% at 16 V (see figure). An almost-constant responsivity between 3 and 8 V is a result of saturation, while the sharp rise above 8 V occurs as avalanche multiplication kicks in. The 3.5% external QE is three-times higher than that of a conventional SiGe p-i-n photodetector with a Ge content of 0.5. For more information, contact Cheng Li at email@example.com.