Ultrafast Uni-Traveling Carrier Photodiodes with GaAs_0.5Sb_0.5/In_0.53Ga_0.47As Type-II Hybrid Absorbers for High-Power Operation at THz Frequencies

We demonstrate a novel type of ultrafast photodiode (PD), which offers high-power performance in the THz regime. The incorporation of a type-II GaAs_0.5Sb_0.5 (p)/In_0.53Ga_0.47As (i) hybrid absorber in the InP-based uni-traveling carrier photodiode (UTC-PD) structure leads to an improvement in the responsivity, due to the narrowing of the bandgap and enhancement of the photo-absorption process at the type-II interface between the GaAs_0.5Sb_0.5 and In_0.53Ga_0.47As absorption layers. Furthermore, the current blocking effect, which is usually one of the major factors limiting the output power of UTC-PDs, can be minimized due to the high-excess energy of the photo-generated electrons injected from the GaAs_0.5Sb_0.5 layer to the InP-based collector layer. The flip-chip bonding packaged device with an active diameter of 3 μm shows a moderate responsivity (0.11 A/W) along with a record wide 3-dB optical-to-electrical bandwidth at 0.33 THz, among all those reported for long wavelength (1.3-1.55 μm) PDs. A 13-mA saturation current and a continuous wave output power as high as -3 dBm are successfully demonstrated at an operating frequency of 0.32 THz under an optical signal with a sinusoidal envelope and a ∼63% modulation depth for PD excitation.