Silver Cold Welding Integration and Back Reflector Performance for Optoelectronic Applications

Abstract

Metal back reflectors enhance photodetector efficiency by enabling photon recycling, where unabsorbed photons reflect back through the active layer. Gold has been the standard material despite substantial optical losses in the visible spectrum due to interband transitions. Silver exhibits superior reflectance exceeding 95% across visible to infrared wavelengths but suffers from oxidation susceptibility that has limited its implementation. Here we show that direct silver wafer bonding using Ti/Pt/Ag stacks achieves reliable heterogeneous integration while maintaining exceptional optical properties. The bonding occurs at 200 °C through atomic interdiffusion, with platinum acting as an effective diffusion barrier. Silver reflectors maintain low roughness below 1.68 nm and over 95% reflectance after thermal processing at 350 °C. Transfer matrix calculations reveal that silver reduces parasitic metal absorption by up to 60 percentage points compared to gold in the blue spectrum. This translates to quantum efficiency improvement for InGaN photodetectors reaching a maximum of 18% at 40° incidence angle and consistent advantages across all wavelengths. These findings establish silver direct bonding as a superior alternative for photodetectors and thermophotovoltaic devices.