Microwave photonics (MWP) seamlessly integrates microwave technology with photonics, harnessing the strengths of both to enable long-distance transmission and efficient processing of microwave signals. By encoding microwave signals onto optical signals and utilizing optical fibers for transmission, MWP offers significant benefits such as large bandwidth, low loss, and strong anti-interference capabilities. This technology finds wide application in various fields, including long-distance communication, radar array systems, and radio frequency (RF) signal processing. Balanced photodiodes (BPDs) are essential components in analog photonics transmission links, effectively mitigating relative intensity noise from laser sources and amplified spontaneous emission noise from erbium-doped fiber amplifiers (EDFAs). Therefore, the development of high-power and high-speed balanced photodiodes is crucial for achieving high link gain, low noise, and a large spurious-free dynamic range in microwave photonics transmission links. Traditional PIN photodetectors suffer from limitations in bandwidth and output power due to the slow drift velocity of holes and serious space-charge effects. Uni-traveling carrier photodetectors (UTC-PDs) have been demonstrated to overcome these limitations by separating the absorption and drift regions. In UTC-PDs, light is absorbed in the heavily doped P-type absorption layer, generating electron-hole pairs. The photo-generated holes are then collected by a metal contact via dielectric relaxation, allowing only electrons with high mobility to drift to the collection layer. In this study, a high-speed and high-power modified uni-traveling carrier balanced photodiode is demonstrated through flip-chip bonding on a diamond submount.

The epitaxial layer structure of the balanced photodiode is optimized. A 50 nm thick P-type doping charge layer is introduced to regulate the electric field in the drift layer, enabling electron overshoot. Additionally, a stepped doping undepleted absorption layer, with a thickness of 120 nm, is adopted to generate a stepped potential distribution, thereby accelerating the diffusion of electrons. Furthermore, a 40 nm depleted absorption layer is implemented to provide a high electric field, alleviating the accumulation of carriers and mitigating carrier blocking at the interface between the absorption layer and the drift layer. InGaAsP quaternary layers are utilized to smooth the band discontinuity and mitigate carrier blocking. Simultaneously, a cliff layer is incorporated to enhance the electric field across the heterojunction interface. In the fabrication process, the active region is defined by double mesa structures, which are dry-etched. Metal stacks are employed to form good ohmic contacts with InGaAs and InP contact layers, respectively. Subsequently, a 255 nm thick SiO₂ layer is deposited on the back of the polished InP substrate as an anti-reflective (AR) coating. Finally, the wafer is diced into 1.0 mm×1.3 mm chips. To enhance heat dissipation, the diced chips are flip-chip bonded onto diamond submounts with high thermal conductivity employing the FineTech FINEPLACER? pico system.

Lumerical 3D model simulations are conducted to analyze the energy band diagram and frequency response of the high-speed and high-power modified uni-traveling carrier balanced photodiode, validating the feasibility of the optimization scheme. Subsequently, to verify the performance of the balanced photodiode, we test and analyze the characteristics of the device-including dark current, responsiveness, frequency response, and saturated output power. The fabricated back-illuminated balanced photodiodes exhibit approximately 200 nA dark current [Fig. 3(a)] and a responsivity of 0.12 A/W at a -3 V bias voltage. Utilizing a vector network analyzer with a scanning frequency range from DC to 67 GHz, the S11 parameters of the device are measured. Combined with the equivalent circuit model of the balanced photodetector, this facilitates the extraction of the device’s physical parameters to analyze the resistor capacitance (RC) limited bandwidth and transit time-limited bandwidth. The junction capacitances of one-side devices with diameters of 4, 6, 8, and 10 μm are 3.8, 8.5, 15.2, and 23.7 fF, respectively. Additionally, the parasitic capacitance is approximately 33.0 fF [Fig. 3(c)]. Finally, to measure the frequency response of the balanced photodetector devices, we establish a test system (Fig. 4). The 3 dB bandwidths at differential mode are 52, 42, and 40 GHz, corresponding to diameters of 4, 6, and 8 μm, respectively (Fig. 5). Notably, RF output powers of 14.0 dBm at 47 GHz and 8.0 dBm at 50 GHz are achieved (Fig. 6).

A back-illuminated modified uni-traveling carrier balanced photodetector is proposed. The introduction of a 50 nm thick P-type doping charge layer regulates the electric field in the drift layer, enabling electrons to overshoot and effectively improving the bandwidth of the balanced photodetector. Simultaneously, flip-chip bonding technology is developed to achieve heterogeneous integration of the balanced photodetector chip with a high thermal conductivity diamond substrate, effectively reducing the core temperature of the device and increasing the output power. The back-illuminated balanced photodiodes exhibit ~200 nA dark current and 0.12 A/W responsivity at a -3 V bias voltage. The 3 dB bandwidths at differential mode are measured at 52, 42, and 40, with diameters of 4, 6, and 8 μm, respectively. Notably, RF output powers of 14.0 dBm at 47 GHz and 8.0 dBm at 50 GHz are achieved. While the proposed back-illuminated modified uni-traveling carrier balanced photodetector has shown performance improvement around 50 GHz, it is noted that lower coplanar waveguide (CPW) inductance and reduced parasitic capacitance are still needed to further increase output power at high frequencies.