In this study, we
report highly efficient and flexible photosensors with GaN nanowires
(NWs) horizontally embedded in a graphene sandwich structure fabricated
on polyethylene terephthalate. GaN NWs and the graphene sandwich structure
are used as light-absorbing media and the channel for carrier movement,
respectively. To form uniform high-quality crystalline GaN NWs on
Si(111) substrates, the initial nucleation behavior of the NWs was
manipulated by applying the new growth technique of Ga predeposition.
High-resolution transmission electron microscopic images obtained
along the vertical direction of GaN NWs showed that stacking faults,
typically observed in Si-based (In,Ga)As NWs, were rare. Consequently,
narrow and strong optical emission was observed from the GaN NWs at
wavelengths of 365.12 nm at 300 K. The photocurrent and photoresponsivity
of the flexible photosensor with 802 nm long GaN NWs horizontally
embedded in the graphene sandwich channel were measured as 9.17 mA
and 91.70 A/W, respectively, at the light intensity of 100 mW/cm2, which are much higher than those previously reported. The
high optical-to-electrical conversion characteristics of our flexible
photosensors are attributed to the increase in the effective interface
between the light-absorbing media and the carrier channel by the horizontal
distribution of the GaN NWs within the graphene sandwich structure.
After 200 cyclic-bending test of the GaN NW photosensor at the strain
of 3%, the photoresponsivity under strain was measured as 89.04 A/W
at 100 mW/cm2, corresponding to 97.1% of the photoresponsivity
obtained before bending. The photosensor proposed in this study is
relatively simple in device design and fabrication, and it requires
no sophisticated nanostructural design to minimize the resistance
to metal contacts.
We report the fast response characteristics of flexible
ultraviolet photosensors with GaN nanowires (NWs) and a graphene channel.
The GaN NWs used as light-absorbing media are horizontally and randomly
embedded in a graphene sandwich structure in which the number of bottom
graphene layers is varied from zero to three and the top is a fixed
single layer of graphene. In the response curve of the photosensor
with a double-layer bottom graphene, as obtained under pulsed illumination
with a pulse width of 50 ms and a duty cycle of 50%, the rise and
decay times were measured as 24.1 ± 0.1 and 28.2 ± 0.1 ms,
respectively. The eye-crossing percentage was evaluated as 52.1%,
indicating no substantial distortion of the duty cycle and no pulse
symmetry problem. The rise and decay times estimated from an equivalent
circuit analysis represented by resistances and capacitances agree
well with the measured values. When the device was under the bending
condition, the rise and decay times of the photosensor were comparable
to those in the unbent state.
Group III-nitride
light-emitting diodes (LEDs) fabricated on sapphire
substrates typically suffer from insufficient heat dissipation, largely
due to the low thermal conductivities (TCs) of their epitaxial layers
and substrates. In the current work, we significantly improved the
heat-dissipation characteristics of an InGaN/GaN quantum-well (QW)
green LED by using hexagonal boron nitride (hBN) as a heat-transfer
medium. Multiple-layer hBN with an average thickness of 11 nm was
attached to the back of an InGaN/GaN-QW LED (hBN-LED). As a reference,
an LED without the hBN (Ref-LED) was also prepared. After injecting
current, heat-transfer characteristics inside each LED were analyzed
by measuring temperature distribution throughout the LED as a function
of time. For both LED chips, the maximum temperature was measured
on the edge n-type electrode brightly shining fabricated on an n-type
GaN cladding layer and the minimum temperature was measured at the
relatively dark-contrast top surface between the p-type electrodes.
The hBN-LED took 6 s to reach its maximum temperature (136.1 °C),
whereas the Ref-LED took considerably longer, specifically 11 s. After
being switched off, the hBN-LED took 35 s to cool down to 37.5 °C
and the Ref-LED took much longer, specifically 265 s. These results
confirmed the considerable contribution of the attached hBN to the
transfer and dissipation of heat in the LED. The spatial heat-transfer
and distribution characteristics along the vertical direction of each
LED were theoretically analyzed by carrying out simulations based
on the TCs, thicknesses, and thermal resistances of the materials
used in the chips. The results of these simulations agreed well with
the experimental results.
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