High-speed InGaAs-based vertical Schottky barrier photodetectors

High-speed photodetectors that possess good sensitivity are required for high-speed fiber optic communication receivers, optical data links, and sampling systems [1]-[2]. The vertical Schottky barrier (VSB) photodetector is capable of achieving extremely high bandwidths [3] and high responsivities [4]-[5]. To minimize the difficulty of coupling optical radiation into the device and to avoid complex and expensive packaging techniques, front-illumination is typically employed [3]-[7]. A transparent and low-resistivity Schottky contact material is required in the VSB in order to simultaneously obtain high responsivity and high bandwidth. Previously, a 10 nm thick layer of semi-transparent Au on n(-)-InAlAs has been used to realize 0.42 A/W InGaAs-based VSB photodetectors [7], and a 100 nm thick film of transparent indium-tin-oxide (ITO) on n(-)-GaAs has been used to realize 0.40 A/W GaAs-based VSB photodetectors [3], [6]. Although the 10 nm thick layer of Au is semi-transparent, the opacity of Au limits the responsivity. Therefore, it is important to investigate the potential of transparent conductors as an electrode material for VSB photodetectors. In this work, we report on the design, fabrication, and characterization of an InGaAs-based VSB photodetector that uses a lattice-matched n(-)-InAlAs Schottky barrier height enhancement layer in conjunction with a 320 nm-thick film of ITO to form a high quality transparent Schottky contact. The Schottky barrier height of ITO on n(-)-InAlAs was determined to be 0.68 eV through the use of Norde plots and I-V measurements. The thin film of ITO and the device passivation layer of silicon nitride, together, form an odd-multiple quarter-wave transformer to 1.31 mu m perpendicularly incident light. The devices exhibited very low dark current densities, high responsivities, and high 3-dB bandwidths. A dark current per unit area of 8.87.10(-5) A/cm(-2) at an applied bias of 5 V was obtained. The responsivity for all the devices tested ranged from 0.55 to 0.59 A/W at a wavelength of 1.31 mu m. The 15 mu m diameter devices exhibited 3-dB bandwidths of 19 and 25 GHz in response to 1.55 mu m illumination with an applied bias of 5 and 10 V, respectively.