| dc.description.abstract | The vast array of optical and electrical capabilities of graphene (Gr) and analogous
two dimensional (2D) van der Waals (vdW) materials have piqued scientists’ interest for
long. Stacking these materials to form hybrid heterostructures provide a versatile platform
to explore a variety of fundamental physical phenomena such as light-matter interactions,
lattice-strain engineering, quantum phenomenon, electron-phonon interactions and
so on. The recent progress in Gr-based photonic devices, especially with transition metal
dichalcogenides (TMDCs), shows their true potential in optoelectronics with remarkably
strong light absorbing capabilities and high photo-responsivity. However, despite extensive
investigation of device applications, like solar cells, photodetectors, ultra fast lasers,
sensors, quantum technologies etc, understanding of underlying physical mechanisms is
still in its incipient stages. This thesis encompasses a study of optoelectronic responses
in optimized ultra-low noise Gr-TMDC hybrid field effect transistors (FETs).
Here, we first explore the interlayer charge transfer mechanisms in Gr-TMDC heterostructure
by engineering the spectral dependence of photoresponse. We show that
the Gr-TMDC FETs exhibit a large photoresponse not only for visible photons, which
have energy (E_{Ph}) greater than the bandgap (E_g) of the TMDC (i.e., E_{Ph} > E_g), but
also for NIR photons, which have E_{Ph}< E_g, where both follow the photogating effect.
This sub-band gap photoresponse is attributed to mid-gap states present in the TMDC
layer. Moreover, we study the bidirectional charge transfer in the optically excited state
using both excitation sources together (i.e., visible and NIR photons). We demonstrate
that the excess electrons in graphene by absorbing the NIR photons, back transfer from
graphene to TMDC. Using visible and NIR light pulses in various sequences, we show that
the bidirectional interlayer charge transfer can be controlled. This leads to a new NIR
photodetection mechanism, which we characterize with several parameters. With this,
we demonstrate a broadband photodetection with high responsivity in Gr-MoSe_2 hybrid
structure, which makes it one of the best MoSe2-based photodetectors reported so far.
We have then optimized the graphene FETs for ultra-low noise device configuration
by systematic investigation and elimination of various low frequency noise contributions.
We demonstrate that hBN encapsulation of graphene devices with graphite as a
back gate eliminates the low frequency noise contribution from environment and Si/SiO_2
substrate. Also, using Hall bar geometry with large channel to metal-contact distance
(W_d > 1.2 μm) makes the noise from channel region dominate over the contact noise.
We report the carrier density fluctuations to be a dominant source in channel noise, due
to trapping-detrapping of charges from hBN trap/defect states, which limit the device
performance. We provide a solution to further reduce the noise magnitude by introducing
a TMDC layer underneath graphene, which screens the trap states located in the hBN
layer. This configuration provides extremely low noise magnitude of ∼ 5.2×10^{−9} μm^2 Hz^{−1}
at room temperature, which is the lowest reported value for single layer graphene FETs
so far.
Finally, we study the light-matter interactions in Gr-TMDC heterostructure with ultra
low noise device configuration, and probe the bulk originated photoresponse via fourprobe
measurements. In addition to overall dominated photogating effect, we report
an unexpected negative photo-resistance in a gate voltage window at low temperatures.
Wavelength dependence of photoresponse reveals that the primary photo-absorption occurs
in the TMDC layer, which is detected by the graphene channel. We demonstrate
that the Hall carrier density does not change as a result of optical illumination for unconventional
negative photo-resistance, indicating absence of the interlayer charge transfer
mechanism. Moreover, this negative photo-resistance has shown channel to channel variation,
indicating strong dependence on coupling between the Gr-TMDC layers, and it
vanishes at higher temperatures (> 100 K). All these observations suggest the presence
of a sensitive photodetection mechanism (possibly energy transfer) which competes with
the dominating photogating effect. | en_US |
