The valence band offset value of 2.3 ± 0.2 eV at epitaxial NiO/Al2O3 heterojunction is determined from photoelectron spectroscopy experiments. Pulsed laser deposited thin film of NiO on Al2O3 substrate is epitaxially grown along [111] direction with two domain structures, which are in-plane rotated by 60° with respect to each other. Observation of Pendellosung oscillations around Bragg peak confirms high interfacial and crystalline quality of NiO layer deposited on Al2O3 substrate. Surface related feature in Ni 2p3/2 core level spectra along with oxygen K-edge soft X-ray absorption spectroscopy results indicates that the initial growth of NiO on Al2O3 substrate is in the form of islands, which merge to form NiO layer for the larger coverage. The value of conduction band offset is also evaluated from the measured values of band gaps of NiO and Al2O3 layers. A type-I band alignment at NiO and Al2O3 heterojunction is also obtained. The determined values of band offsets can be useful in heterojunction based light emitting devices.
High-quality, continuous transition-metal
dichalcogenide (TMD)
thin films with large-area coverage are the prerequisites for practical
device applications. To address the growing demand, here we report
high photoresponse and high detectivity of WS2 nanosheet/Si
p–n heterojunction diodes for potential application in UV–visible
broadband photodetection based on the wafer-scale deposition of WS2 nanosheets. Monolayer equivalent efficiency, i.e., high responsivity
and high detectivity, has been achieved in these devices utilizing
its nanotextured morphology. Nanotexturization in the surface morphology
increased the effective area for absorption as well as contributed
to quantization, leading to superior device performance. RF sputtering
followed by high-temperature annealing in the sulfur-rich environment
has been adopted to achieve stoichiometric WS2 films with
high crystalline quality. The valence band offset for the WS2/Si heterointerface has been determined to be 0.48 ± 0.2 eV,
for the band alignment of heterojunction devices. The fabricated n-WS2/p-Si junction diode displayed excellent rectifying characteristics
(rectification ratio of ∼630) with a low leakage current (∼1
× 10–7 A). The photodiode exhibits a superior
photo-to-dark current ratio (∼1200) and very high photoresponsivity
(>4 A/W) under the reverse bias condition. Moreover, a high detectivity
of ∼4.8 × 1012 Jones with a linear dynamic
range (LDR) of 62 dB at −3 V has also been achieved. A reasonably
fast response time, of approximately a few milliseconds of the fabricated
photodiode, has made them excellent candidates for large-area photodetectors,
operational for the broadband spectral range (300–800 nm).
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