SiGe/Si resonant cavity photodetector

The smaller absorption coefficient of silicon as compared to direct band-gap III-V compound materials necessitates the use of thick (typically > 5 pm) absorption regions in silicon photodiodes. Although high responsivities can be achieved with the thick absorbing layers, the associated transit times limit bandwidths to 90% [2]. This has made it possible to realize a Sibased resonant-cavity photodiode. The photodiode structure was grown by solid source molecular beam epitaxy. The asymmetric mirror consisted of 40, n-doped (1x1018) G@.gSio.~/Si periods grown on a silicon substrate. The thicknesses of the Geo.30Si0.70 and Si layers were 22581 and 570 81, respectively. The mirror was followed by a 1.03 pm-thick intrinsic silicon absorbing layer. On top of this was grown a 0.2 pm, p-doped (1~10~8) Si layer followed by a 200 81 p+ Si contact layer. The photodetector structure is shown in Fig.1. The simulated and measured reflectivity spectra of the bottom GeSi/Si mirror are shown in Fig. 2. The quantum efficiency of the resonant cavity photodetector was measured by comparing it with a calibrated silicon photodiode. The quantum efficiency curve is shown in Fig. 3. The peak quantum efficiency was found to be 89%. At resonance, essentially all the absorption takes place in the 1.25 pm long cavity. In a conventional photodiode structure, even with a perfect antireflection coating, the absorbing layer would have to be at least 8 times thicker (- 10 pm) to achieve the same responsivity and there would be a comparable decrease in the bandwidth. The calculated, transit-time-limited bandwidth of the resonant-cavity photodiode is greater than 25 GHz, Work is also in progress on novel GexSil-,/Si mirrors and photodetectors that will be resonant at two distinct wavelengths [3]. The dual mirror structure essentially consists of a quarter wavelength asymmetric mirror with additional quarter wavelength layers of Si inserted at appropriate points in the structure to modulate the reflected phase. The measured reflectivity spectrum of a dual wavelength asymmetric mirror is shown in Fig. 4. The two reflectivity peaks are close to 632 nm and 780 nm. A cavity which will be resonant at both the peak wavelengths can be grown on top of this mirror to provide twin-peak resonant cavity photodetectors. Such devices can be used for wavelength-division-multiplexing applications and integrated noise filters.