Temperature dependence of Ge/Si avalanche photodiodes

Abstract

By combining Si (with a low k-value, <0.1) and Ge (with a high absorption in the infrared range), Ge/Si photodetectors have been demonstrated with high performances [1–2]. Since data-com transceivers are typically operated up to 70°C [3], it is important to characterize the temperature dependence of the photodetectors and to reduce the temperature dependence. However, very little work has been done for analyzing the temperature dependence of Ge/Si SACM APDs. In this paper, we present the characterization of normal-incidence Ge/Si SACM APDs from 10°C to 60°C. Fig. 1 (a) shows the cross section of the present normal-incidence SACM Ge/Si APD, which is the same as that in Ref. [1]. In our experiment, the APD sample has a diameter D=30µm and operated at the wavelength λ=1300nm. Fig. 2 shows the measured dark currents (see the dotted curves) at different temperatures as the bias voltage increases. The I-V curves for the case with an optical illumination of P= −20dBm are also shown in the same figure to give a comparison (see the dashed curves). In our experiment, the temperature ranged from −73°C to 27°C. As the temperature decreases, not only the dark current level changes but also the breakdown voltage decreases. Here the breakdown voltage Vbd is defined as the voltage where the dark current is 100µA. The temperature dependence of the breakdown voltage Vbd is then shown in the inset. One sees that the voltage Vbd increases with the temperature. This is due to the temperature dependence of phonon scattering [4]. Fig. 33 shows the dark current Idark (in logarithmic scale) versus 1/(kT) for different bias voltages. The dark current increases by nearly a factor 2 every 10 °C when Vbias= −15V, which is similar to that for a Ge-Si PIN photodiode shown in Ref. [3]. We have also measured the dark current from 200K–300K. Fig. 4 shows the activation energy extracted from dark current versus temperature, using the relation Id∞T2exp(-Ea /kT). The activation energy decreases as bias voltage increases. This can be explained as follows. At higher bias voltages, the depletion layer becomes wider and consequently the G-R in the space charge region increases. Thus, the dominant source of the dark current becomes the G-R current, which is less sensitive to the temperature.