Design On Electrode Configurations And Study On Nonlinear Photoresponse Characteristics For GaAs Photodetectors

The measurements of ultrashort laser pulses at1.31μm and1.55μm arebecoming increasingly important for electrooptical sampling techniques and thecharacterization of high-speed optical communication systems. The most importantsemiconductors of GaAs crystals are commonly used in optoelectronics. The bandgapof GaAs crystal is1.43eV corresponding to a wavelength range of0.88-1.73μm intwo-photon response(TPR), which covers the wavelengths of1.31μm and1.55μmcommonly used in electrooptical sampling techniques and optical communicationsystems. So, the TPR detector of GaAs will play an important role in optoelectronicsand optical communications.In this paper, the theories of two-photon absorption (TPA), the double-frequencyabsorption (DFA) and the electric field induced double-frequency absorption areelaborated in detail. We designed two different electrode configurations based onGaAs sample, and the distributions of electric field for the two different electrodeconfigurations were simulated by ANSYS software. The simulation results show thatthe electric field for the needle electrode situated at the region sensitive to TPR is thestrongest. Since the electric field induced DFA is strongly dependent on the DCelectric field, this will be helpful for us to carry out the experimental research work.We deduced the changes of second order nonlinear susceptibility for GaAs crystalunder the disturbance of surface electric field, and the symmetry of GaAs is reducedfrom43m group to mm2group. We studied the nonlinear photoresponsecharacteristics of the Cr-doped semi-insulating (SI-) GaAs photodetectors with two types of aluminum electrode configurations operating at1.56μm laser. The quadraticdependence of the photocurrent on the optical power was investigated under a fixedbiased voltage; also, the quadratic dependence of the photocurrent on the bias voltagewas investigated under a fixed incident optical power. These investigated results areproved to be dominantly responsible for the nonlinear photoresponse of the sample.With the further investigation on the nonlinear photoresponse of the sample, one canget the conclusion that the DFA induced by the surface electric field or the DCelectric field applied to the SI-GaAs has an effect on the native DFA in SI-GaAs. Inaddition, when the electrode situated at the region sensitive to TPR was biased at theconstant voltage but with two opposite polarities, the variation of the photocurrent isin reverse to that of the dark current. Specifically, for the hemispherical SI-GaAssample, biased at the same voltage, when the central electrode is negatively charged,the direction of the applied electric field in the vicinity of the bottom center isapproximately considered to be the same as that of the surface electric field inside thesemiconductor, which consequently builds up the depletion-electric field and enlargesthe depletion region, and the equivalent resistor of the sample is larger. As a result,the photocurrent tends to be larger while the dark current is smaller. However, whenthe central electrode is positively charged, the applied electric field in the vicinity ofthe bottom center is against the direction of the surface electric field, which thusdecreases the depletion-electric field and reduces the depletion region, and theequivalent resistor of the sample is smaller. Then the photocurrent tends to be smallerand the dark current is larger. Such experimental investigated results agree well withthe theories of the surface band-bending and the surface electric field.