Effect of a-Si1-xGex Bonding Layer with Different Ge Compositions on the Performance of InGaAs/Si Avalanche Photodiode

Avalanche Photodiode(APD) is a highly sensitive photodetector for fiber-optic transmission systems. InGaAs/InP APD is difficult to be operated at near-room-temperature with low noise due to the high k value(0.4 similar to 0.5)of InP material. The combination of InGaAs material with Si material is an ideal solution for the fabrication of APD ascribed to the low k value(0.02)of Si material. Nevertheless,high-quality InGaAs film is difficult to be grown on Si material using epitaxial growth techniques. This attributes to the fact that the lattice mismatch between InGaAs and Si is as high as 7.74 %. Semiconductor Interlayer Bonding(SIB) based on the amorphous material can eliminate the effect of mismatched lattices between InGaAs and Si. However,the effect of amorphous semiconductor material on the performance of InGaAs/ Si APD is still unclear. In this work,an a-Si1-xGex bonding layer is inserted at InGaAs/Si bonded interface to isolate the lattices between InGaAs and Si. The recombination rate,energy band,tunneling,and charge accumulation of InGaAs/ Si APD are simulated. It shows that as the bandgap increases,The dark and optical current rise first and then fall. This is consistent with the trend of the recombination rate. Although the recombination rate in the a-SiGe layer shows a downward trend,the dark current of the entire device is not seriously affected becausethe thickness of the a-SiGe bonding layer is thin. A gap is formed between the optical and dark current curves,which is different from the conventional APD. It is noticeable that such a gap is beneficial to reduce the device noise due to the low dark current of the device. At this point,the InGaAs/Si APD with extremely low dark current (similar to 10(-10) A@ 95% breakdown voltage) is obtained at room temperature. In addition. as the bandgap increases,the electron concentration in the a-SiGe layer does not change drastically, while the hole concentration in the a-SiGe layer decreases significantly, leading to a decrease in the recombination rate in the a-SiGe layer. This results from the decrease of the hole concentration in the a-SiGe layer,which can be ascribed to the hole accumulation effect at the a-SiGe/ Si bonded interface and the holes tunneling directly from the Si layer to the InGaAs layer. When the bandgap is 1.02 eV and 1.15 eV,since the valence band wells form at the a-SiGe layer,holes accumulate at the valence band of the a-SiGe layer. The barrier appears when the bandgap increases to 1.3 eV, besides,as the bandgap increases,the barrier increases. One feature is that the hole tunneling rate at the aSiGe/Si bonded interface shows a downward trend,resulting in the decrease of holes tunneling from the Si layer into the InGaAs/a-SiGe bonded interface. This leads to decreasing the hole concentration at the InGaAs/ a-SiGe bonded interface. Moreover,as the bandgap increases,the gain rises and then falls. This is consistent with the optical current. The highest gain of similar to 30 is achieved for the APD with the a-SiGe layer with the bandgap of 1.3 eV. The 3dB- bandwidth of the device decreases with the increase in the bandgap of a-SiGe layer. This is attributed to the fact that as the bandgap increases,the hole velocity in the InGaAs layer first decreases and then slightly rises, while the electron velocity in the InGaAs layer decreases significantly. Finally,as the bandgap increases,the GBP increases first and then decreases. This is consistent with the change of the gain. The maximum GBP reaches similar to 60 GHz when the bandgap of a-SiGe layer is set to be 1.3 eV,which is close to the traditional InGaAs/InP APD. This indicates that the InGaAs/Si APD not onlykeep high performance, but can also achieve a low dark current at room temperature. This work may guide the fabrication of low-noise APD which works in near-infrared region.