The effects of AlGaAs cap layers on the DC and speed performance of GaAs metal-semiconductor-metal photodetectors

The effects of AlGaAs cap layers for GaAs metal-semiconductor-metal photodetectors (MSM-PD's) have been investigated. By changing the thickness of the AlGaAs cap layer (i.e. 30 nm, 60 nm and 90 nm), both DC and speed performance are analyzed and discussed. Meanwhile, two cap structures with abrupt and graded hetero-junction, respectively, were also grown for comparison. The results of this work show that adding a 30 nm-thick AlGaAs cap layer can greatly suppress the low frequency internal gain and lead to improved linearity. At 5 V bias, the rise time and the full-width at half maximum (FWHM) of such 100 μm × 100 μm devices are measured to be 28 ps and 39 ps, respectively, corresponding to a bandwidth exceeding 10 GHz. The devices with a thicker cap layer, however, have significant internal gain and lower speed response due to the formation of a thicker barrier blocking the photocarriers to reach the electrodes.