The two-dimensional/three-dimensional van der Waals heterostructures provide novel optoelectronic properties for the next-generation of information devices. Herein, MoS2/Ge heterojunction avalanche photodetectors are readily obtained. The device with an Ag electrode at MoS2 side exhibits more stable rectification characteristics than that with an Au electrode. The rectification radio greater than 103 and a significant avalanche breakdown are observed in the device. The responsivity of 170 and 4 A/W and the maximum gain of 320 and 13 are obtained under 532 and 1550 nm illumination, respectively. Such photoelectric properties are attributed to the carrier multiplication at a Ge/MoS2 junction due to an avalanche breakdown. The mechanism is confirmed by the Sentaurus TCAD-simulated I-V characteristics.
Special flask-shaped Au grating-Ge nanowire arrays are used to improve the performance of a Ge photodetector in the infrared optical communication band. The responsivity of the device with alternate Au grating-Ge nanowire arrays reaches as high as 0.75 and 0.62 A/W at 1310 and 1550 nm, respectively, indicating a nearly 100% increment compared to a device without a grating structure. This enhancement is attributed to the excitation of the surface plasmon polaritons, which simultaneously enhance the inter-band transition absorption and the internal photoemission of carriers. Moreover, the photoresponsivity of the dual-band plasmon-enhanced device is remarkably asymmetrical with regard to the voltage polarity, and the asymmetric ratios are about 4:1 and 3:1 at 1310 and 1550 nm, respectively. Band energy theory indicates that this bias-dependent responsivity originates from the asymmetrical distribution of hot electrons between the two electrodes and the mobility difference between electrons and holes in Ge. These results provide a valuable guideline for achieving a high performance dual-band near infrared photodetector, and the results demonstrate the potential of this approach for developing next-generation optoelectronic devices.
A Ge metal–semiconductor–metal photodetector covered with asymmetric
H
f
S
e
2
contact geometries has been proposed to realize high-performance unbiased photodetection at 1550 nm. At -1 V bias, the responsivity of this device shows a 71% improvement compared to the device without
H
f
S
e
2
. Moreover, the responsivity and detectivity of this device at zero bias can reach to 71.2 mA/W and
3.27
×
10
10
Jones, respectively. Furthermore, the fall time of this device is 2.2 µs and 53% shorter than the device without
H
f
S
e
2
. This work provides a feasible way to develop unbiased Ge-based photodetectors in the near-IR communications band.
We propose an infrared-sensitive negative differential transconductance (NDT) phototransistor based on a graphene/WS2/Au double junction with a SiO2/Ge gate. By changing the drain bias, diverse field-effect characteristics can be achieved. Typical p-type and n-type behavior is obtained under negative and positive drain bias, respectively. And NDT behavior is observed in the transfer curves under positive drain bias. It is believed to originate from competition between the top and bottom channel currents in stepped layers of WS2 at different gate voltages. Moreover, this phototransistor shows a gate-modulated rectification ratio of 0.03 to 88.3. In optoelectronic experiments, the phototransistor exhibits a responsivity of 2.76 A/W under visible light at 532 nm. By contrast, an interesting negative responsivity of −29.5 µA/W is obtained and the NDT vanishes under illumination by infrared light at 1550 nm. A complementary inverter based on two proposed devices of the same structure is constructed. The maximum voltage gain of the complementary inverter reaches 0.79 at a supply voltage of 1.5 V. These results demonstrate a new method of realizing next-generation two- and three-dimensional electronic and optoelectronic multifunctional devices.
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