We report on the optimum geometric design of In 0.53 Ga 0.47 As metal-semiconductor-metal ͑MSM͒ photodetectors for maximizing external quantum efficiency with transparent indium tin oxide ͑ITO͒-based interdigitated fingers. Both ITO finger and conventional metal finger MSM photodetectors were fabricated to verify the simulated external quantum efficiency and good agreement between experimental and simulated results was obtained. In all cases, the ITO provided higher efficiencies, particularly at short finger gap spacings.There has been strong recent interest in transparent conducting oxides ͑TCOs͒ for optical device applications. Among the various possible TCOs, indium tin oxide ͑ITO͒ is commonly used as a contact for transparent electrodes due to its excellent properties in transmittance, electrical conductivity, substrate adhesion, hardness, and chemical inertness. 1-8 A variety of ITO deposition techniques have been reported, including electron-beam evaporation, 9 dc or radio frequency ͑rf͒ sputtering, 10 reactive ion plating, 11 chemical vapor deposition, 12 and pulsed laser deposition. 13 ITO has been used for interdigitated finger electrodes on metalsemiconductor-metal photodetectors ͑MSM͒, improving the photon responsivity by minimizing blocking of incoming light by the finger electrodes. 14 However, studies of the optimal geometric design of transparent ITO finger MSM photodetectors have not been reported to date. The optimized designs employed for the more conventional metal-based interdigitated fingers 15,16 are not suitable for the ITObased transparent contacts due to the absence of shadowing effects in the latter case.In this paper, we present a study of the effects of using ITO as the interdigitated electrodes for InGaAs-based MSM photodectors in terms of both external quantum efficiency and response time. InGaAs-based MSM photodectors with conventional metal and transparent ITO interdigitated electrodes were fabricated with various configurations to verify the simulation results.
ExperimentalThe InGaAs MSM photodetector structure consisted of 150 Å of InAlAs as a Schottky enhancement layer, a 230 Å In͑Ga,Al͒As graded layer, a 1.0 m InGaAs absorption layer, and a 0.3 m InAlAs buffer layer grown on a semi-insulating InP/Fe substrate with a molecular beam epitaxy system ͑MBE͒. 17 The device fabrication started with a blanket deposition of SiN x using a low-power plasma-enhanced chemical vapor deposition ͑PECVD͒ system. Buffered oxide etch ͑BOE͒-based wet chemical etching was used for opening the interdigitated finger contact window. A 200 Å Ti/1800 Å Au deposition with an electron-beam evaporator was used for the contacts to the conventional-metal-based interdigitated electrode devices. For the devices with transparent ITO fingers, 2000 Å ITO was deposited with a plasma-assisted rf sputtering system using Ar/3% O 2 discharges at a pressure of 4 mTorr and 125 W of rf power ͑13.56 MHz͒. The sputtering target for the ITO was a composite target consisting of 90% In 2 O 3 and 10% SnO 2 . The testing contact...