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Posted: Nov 02, 2014
InAs nanowire phototransistors based on the majority-carrier-dominated photodetection mechanism
(Nanowerk News) Due to the large surface-to-volume ratio and shorter effective conductive channel, many kinds of semiconductor nanowires have been explored for photodetection in the past decade to pursue a higher photosensitivity and a faster transport speed. However, the short photocarrier lifetime, small light-sensing area and weak optical absorption of nanowires greatly limit the device performance.
The photogating effect of semiconductor nanostructures – as a promising approach to improve the responsivity – has been successfully and efficiently applied to graphene based phototransistors recently. Photocarriers located near the conductive channel form a strong local electric field to regulate the channel conductance through capacitive coupling. However, researchers have been lacking ways of realizing this effect in one-dimensional nanowire materials for highly efficient photodetection.
Now, a research team based at the Shanghai Institute of Technical Physics (SITP), has designed core/shell-like InAs nanowire, and the as-fabricated device shows an excellent anomalous photoresponse due to the photogating effect.
The as-grown core/shell-like n-type InAs nanowires have a self-assembled 'photogating layer' near the nanowire surface. Its main function is trapping electrons generated from the core and in return the electrons maintaining trapped in the photogating layer form a built-in electric field to modulate the core conductance under optical illumination.
In this design, majority carriers, i.e. electrons, are contributing to the photocurrents. The whole channel of the transistor, as the effective photosensitive region, responds to the light signal. The fabricated devices exhibit anomalous high photoconductive gain of -105 and fast response time 12 ms.
The researchers realized the conversion of photogate polarity by modulating the back-gate voltages. After light exposure, a long time keeping negative/positive photogate is formed by capturing electrons/holes in the photogating layer under positive/negative back-gate voltage modulation. The device can be used as a light-induced current amplifier when the back-gate voltage is negative.
The device also shows excellent performance in both air and vacuum environments. The photodetection free from environmental impacts makes the device more widely applicable.