Design, fabrication, and modeling of high-speed metal-semiconductor-metal (MSM) photodetectors with indium-tin-oxide (ITO) and Ti/Pt/Au contacts

Abstract

Metal-semiconductor-metal photodetectors have been fabricated on undoped epitaxial GaAs material with gold and indium-tin-oxide interdigitated contacts. In both cases, various electrode configurations were laid out with combinations of finger spacings and finger widths ranging from 1 to 3 micrometers and detector cross-sections of 25 X 25, 50 X 50 and 100 X 100 micrometers <SUP>2</SUP>. Frequency response measurements were carried out up to 20 GHz using a high-speed electro-optic modulator combined with a DC-operated laser diode and an ultrafast photodetector for system calibration. This frequency domain technique ensures accurate measurement of true analog bandwidth compared to time-domain techniques which can easily lead to an overestimation of photodetector bandwidth. Photodetector responsivity has been plotted as a function of bias voltage. We note that for similar devices, Ti/Pt/Au contact MSMs require lower bias voltages before they reach their saturation bandwidth than ITO contact MSMs. For a 100 X 100 micrometers <SUP>2</SUP> ITO MSM with a 2 micrometers finger width and a 2 micrometers finger spacing, the 3 dB bandwidth was measured to be 4 GHz at 10 V bias. By comparison, similar gold contact MSMs exhibit 3 dB bandwidths in excess of 12 GHz. The difference in speed is partly explained by the higher device parasitics of the ITO MSMs, as confirmed by S<SUB>11</SUB> measurements made on both types of device. The S<SUB>11</SUB> data was also used to extract the MSM equivalent circuit parameters for a high-frequency MSM model. Similar measurements on other electrode configurations show that as expected, the speed of ITO MSMs become considerably higher as device size is decreased, until the limit where transmit-time effects start to dominate the overall performance.