Palladium diselenide (PdSe2) has been discovered
as
an intriguing two-dimensional (2D) semiconductor for its unique pentagonal
crystalline structure, electrical and optical anisotropy, thickness-modulated
band gap, robust air stability, and high carrier mobility, demonstrating
great potential in field-effect transistors (FETs), photodetectors,
and thermoelectric devices. Controlling the carrier polarity and electrical
contact Schottky barriers is of great significance for realizing high-performance
PdSe2 optoelectronic devices. Here, by combining work-function
measurement and electrical transport, we observe a thickness-modulated
carrier polarity transition in layered PdSe2 FETs from
n-type unipolar to intrinsic ambipolar and p-type unipolar behavior
by simply varying the number of layers of PdSe2. Due to
the weak Fermi-level pinning in few-layer PdSe2, n-type
and ambipolar FETs are achieved by contacting PdSe2 to
Sc, Ti, and Pd electrodes, respectively. The Schottky barrier heights
are determined by temperature-dependent transport, showing a 15 meV
barrier for the Sc contact at the electron side and 62 and 93 meV
barriers for Ti and Pd contacts, both at the hole side, respectively.
We further demonstrate ozone-treatment-induced hole doping in PdSe2, which effectively converts the PdSe2 FETs from
n-type to p-type. The doping is robust in the ambient environment
over 3 months. In-plane homojunction on a PdSe2 flake is
realized by selective ozone doping, demonstrating typical diode rectifying
behavior. The ability to control the carrier polarity and Schottky
barriers in layered PdSe2 makes this 2D semiconductor highly
feasible for complementary metal-oxide semiconductor device integration
and high-performance photodetectors based on atomically thin p–n
junctions.
Broad-bandgap semiconductor-based solar-blind ultraviolet
(SBUV)
photodetectors have attracted considerable research interest because
of their broad applications in missile plume tracking, flame detectors,
environmental monitoring, and optical communications due to their
solar-blind nature and high sensitivity with low background radiation.
Owing to its high light absorption coefficient, abundance, and wide
tunable bandgap of 2–2.6 eV, tin disulfide (SnS2) has emerged as one of the most promising compounds for application
in UV–visible optoelectronic devices. However, SnS2 UV detectors have some undesirable properties such as slow response
speed, high current noise level, and low specific detectivity. This
study reports a metal mirror-enhanced Ta0.01W0.99Se2/SnS2 (TWS) van der Waals heterodiode-based
SBUV photodetector with an ultrahigh photoresponsivity (R) of ∼1.85 × 104 AW–1 and
a fast speed with rising time (τr) of 3.3 μs
and decay time (τd) of 3.4 μs. Notably, the
TWS heterodiode device exhibits a significantly low noise equivalent
power of ∼1.02 × 10–18 W Hz–1/2 and a high specific detectivity of ∼3.65 × 1014 cm Hz1/2 W–1. This study provides an
alternative method for designing fast-speed SBUV photodetectors with
enormous potential in applications.
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