Ke Chen, Deji Akinwande, Seth Bank and Yaguo Wang
Transport properties, such as mobility, diffusion coefficient and diffusion length, of photoexcited carriers in electronic materials directly determine the performance of optoelectronics devices like photo-detector, phototransistor and photovoltaic. Effective measurement of these transport properties is the fisrt step for characterization of electronic materials, scientific study of carrier transport physics, and engineering design of opto-electronic devices.In this work, we present a novel optical technique developed from ultrafast pumpprobe spectroscopy to measure the photo-excited carrier diffusion coefficient, diffusion length and mobility in both traditional semiconductor and the newly developing 2D material. In the method, an objective lens is applied to image a photo mask (grating) onto the sample plane. The pump and probe beams overlap at the grating, being diffracted and then re-overlap at the sample again, with their intensities modulated into a grating-shape. The modulated pump generates carrier density grating in the sample, and the transmission of the modulated probe monitors the decay of the carrier density grating, which contains information of both carrier recombination and carrier diffusion dynamics. Carrier recombination lifetime Tr can be easily acquired from a conventional experiment without grating. A model based on diffusion equation and the relation between differential transmission and carrier density is derived and applied to extract ambipolar diffusion coefficient Da and diffusion length La . In our method, not the weak diffraction1 but the strong transmission change of the probe beam is collected as signal. Only two measurements (with and without grating) needs to be performed to retrieve transport parameter such as carrier lifetime and diffusion coefficient. And unlike the electrical measuring techniques2,3 which require electrode deposition or optical mask-modulation approach4 , the sample here is free of any undesired contact. The above features make our technique sensitive, simple, and truly non-destructive. The validity of our technique has been demonstrated by successful measurements on carrier diffusion coefficients of GaAs quantum well and graphene. We believe the technique has a great potential in measuring and studying the transport property of various electronic materials.