fig5: Variation of response time and photocurrent autocorrelation amplitude with laser fluence. The results were obtained at a gate voltage of 0 V.

Mentions: The dependence of response time and autocorrelation amplitude on laser fluence for the MGM detector is shown in Figure 5. In this measurement, the laser fluence was varied from 12 to 115 μJ/cm2. The gate voltage was set to 0 V. The response time of the photodetector increases slightly with increasing laser fluence. Hence, Auger recombination(37) can be excluded as the dominant recombination mechanism. It would show the opposite behavior. Other recombination mechanisms such as plasmon emission(38) or recombination due to intravalley and intervalley optical phonon scattering(39) are more likely. Taking into account an estimated density of photogenerated carriers of about 1012 cm–2 at the lower laser fluence limit and about 1013 cm–2 at the higher laser fluence limit in our experiment, we find qualitative agreement with recent theoretical predictions of the recombination time due to optical phonon scattering.(39) We conclude that response times in the linear regime (at lower laser fluence) should be even shorter than the values obtained in our experiment. Furthermore, we observe the autocorrelation amplitude to correlate with the response time (see Figure 5). The longer the response time, the more carriers accumulate in the high-field region and the stronger the saturation. Since the autocorrelation amplitude depends strongly on the degree of saturation, it follows the response time of the photodetector. This interpretation is also consistent with the variation of response time and autocorrelation amplitude with respect to gate voltage (see Figures 3 and 4a). An increasing gate voltage results into a shortening of the response time and accordingly to a reduction of saturation and autocorrelation amplitude.

Urich A, Unterrainer K, Mueller T - Nano Lett. (2011)

Bottom Line: Here, we present measurements of the intrinsic response time of metal-graphene-metal photodetectors with monolayer graphene using an optical correlation technique with ultrashort laser pulses.We obtain a response time of 2.1 ps that is mainly given by the short lifetime of the photogenerated carriers.This time translates into a bandwidth of ∼262 GHz.

Affiliation: Institute of Photonics, Vienna University of Technology, Gusshausstrasse 27-29, 1040 Vienna, Austria. alexander.urich@tuwien.ac.at

Abstract: Graphene-based photodetectors are promising new devices for high-speed optoelectronic applications. However, despite recent efforts it is not clear what determines the ultimate speed limit of these devices. Here, we present measurements of the intrinsic response time of metal-graphene-metal photodetectors with monolayer graphene using an optical correlation technique with ultrashort laser pulses. We obtain a response time of 2.1 ps that is mainly given by the short lifetime of the photogenerated carriers. This time translates into a bandwidth of ∼262 GHz. Moreover, we investigate the dependence of the response time on gate voltage and illumination laser power.