Characteristics Of GaAs Two-photon Response Photodetector And Optimization Of Its Electrodes

The measurements of ultrashort laser pulses at two important optical wavelengths of 1.31μm and 1.55μm are becoming increasingly important for electro-optic sampling techniques and high-speed optical communication systems. The wavelength range of two-photon response of gallium arsenide(GaAs) just covers the two wavelengths, therefore, the study of GaAs two-photon response photodetectors has practical significance. However, the main physical mechanism of the photodetectors has been unclear so far, and the responsivity of the photodetector still needs to be improved. Based on the above problems, the research work is as follows:In order to explore the main physical mechanism of GaAs two-photon response photodetectors, we developed the GaAs material into a hemispherical photodetector with a radius of 3 mm and a bottom of(001) plane, and studied the two-photon response characteristics of the photodetector. Under the irradiation of 1.55μm continuous wave(CW) solid laser, the photocurrent generated in the photodetector dependent on the incident optical power exhibits quadratic nonlinearity, the dependence of the photocurrent on biased voltage doesn’t present a tendency of saturation but exhibits quadratic nonlinearity as well. Furthermore, the relationship between the photocurrent and the azimuth is in agreement with the theory of the electric field induced optical rectification effect. These results indicate that the double-frequency absorption(DFA) is responsible for the physical mechanisms of the GaAs two-photon response photodetector. Meanwhile, according to the quadratic dependence of the photocurrent on bias and the anisotropy of photocurrent, we can conclude that the electric field induced DFA plays an important role in the two-photon response. Moreover, due to the surface electric field of GaAs, when the electrode fixed at the bottom center of the hemispherical photodetector is negatively charged, the photocurrent is quite larger while the dark current much smaller than those in the case of the central electrode positively charged, which result in a bigger photo-to-dark-current ratio. In addition, under the same conditions, such as the same illumination and the same biased voltage, the photocurrent of the photodetector with the electrode structure of a tip-sector and a bow pad is also much greater than that of the photodetector with the homocentric inner-disk and outer-ring electrodes. Therefore, the enhancement of photocurrent can be attributed to the electric field distribution and electric field intensity in the active region of the two-photon response photodetector.In order to improve the responsivity of the photodetector significantly, we further optimized the electrode structure based on the optimized hemispherical optical structure. We designed interdigital electrodes and tip-group electrodes. The electric field distributions in the hemisphere GaAs photodetector samples for different electrode structures were simulated by ANSYS software. The simulation results demonstrate that electric field intensity of the active region become stronger gradually with the finger spacing of interdigital electrodes decreasing and the tip numbers of tip-group electrodes increasing, severally. Moreover, the tip-group electrodes can induce a stronger electric field compared to the interdigital electrodes at the same bias. We further fabricated the designed electrodes with different sizes on the sheet GaAs samples by evaporation and photolithographic techniques. The response characteristics of sheet GaAs photodetectors with different electrode structures operating at the 1.55 μm CW solid laser were studied. We can draw the conclusion that the interdigital electrode structure with a finger spacing of 30μm is better according to the results that photocurrent and photo-to-dark-current ratio dependent on bias voltage, respectively. In addition, for the tip-group electrodes, the performances of photocurrent and photo-to-dark-current ratio severally dependent on bias voltage are gradually improved with increasing the tip numbers, which is consistent with the simulation results. More excellent tip-group electrodes are expected to be fabricated on the hemispherical GaAs sample, so as to achieve a higher responsivity by optimizations of both optical and electrical structures.