Cu 2 ZnSnS 4 (CZTS) has been extended to the field of photodetection owing to its outstanding optoelectronic properties. However, the existence of the ineluctable defects in CZTS semiconductors affects and even determines the optoelectric processes including carrier generation, relaxation, transfer, and recombination. Moreover, photoresponse correlated to the defects in CZTS photodetectors has not well been documented and the possible physics mechanism is still unexplored. High-performance and self-powered PN heterojunction photodetectors are built from Cu 2 ZnSnS 4 and CdS films. The devices exhibit a steady rectifying behavior and a prominent photovoltaic effect. The peak values of responsivity and detectivity are 220 mA W −1 and 2.69 × 10 10 Jones, respectively. A very fast response speed with rising and decay times of up to 18 and 19 μs and an ultrahigh photoswitching ratio beyond 10 4 are demonstrated in these photodetectors. An abnormal dependence of the light response parameters on the incident power and temperature is found in these devices. This anomaly is explained by the formation of the defects and/or defect dipoles, which are evidenced by the temperature dependence of the photocurrent, the dependence of the capacitance on the bias voltage at different temperatures, and the derivative of capacitance with temperature. KEYWORDS: photodetectors, Cu 2 ZnSnS 4 /CdS heterojunctions, self-powered properties, abnormal photoresponse, dependence of capacitance on the bias voltage
Van
der Waals heteroepitaxial (vdWE) has been intensely developed
and considered as the most promising heteroepitaxial technique for
growth of high-quality nanoscale films, covalent-bonded semiconductor
films, and heterostructures to create the next-generation flexible
electronic/optoelectronic devices because of its nature of relief
of the strict requirement of lattice matching and its elegant exfoliation
and transfer to any substrates of interest. However, application of
the vdWE route in growing metastable and artificial materials and
structures is still absent. In this work, by using the molecular beam
epitaxy (MBE), epitaxy of metastable γ-phase nanometer-thick
MnTe thin films is achieved on two-dimensional mica substrates through
attentive control of its growth kinetics. The good crystallinity of
vdWE γ-MnTe thin films is shown by a low half peak width value
of about 0.19° for 50 nm-thick epitaxial films. Moreover, a structure
with perfect lattice, a wide E
g
opt of ∼3.26 eV with the direct electron transition structure,
a broad absorption region from ultraviolet (UV) to near-infrared (NIR),
and an ultrahigh absorption coefficient beyond 106 cm–1 in the UV region are found in vdWE nanometer-thick
γ-MnTe thin films. The photodetectors with vdWE γ-MnTe/mica
systems exhibit a highly sensitive broadband detecting from the UV
to the NIR region. The detectors show an outstanding UV response with
a high responsivity of 526 A W–1 and specific detectivity
of 2.46 × 1012 Jones and show fast photoresponse speeds
(τrising = 1.9 ms and τdecay = 1.7
ms) under a 375 nm laser illumination that indicate a great potential
in flexible UV and broad spectrum detection of the photodetectors
with vdWE nanometer-thick γ-MnTe/mica.
The effective mass mismatch of a conduction band electron in a semiconductor. superlattice renormaiizes the electron confining potential. Taking this effect into account we calculate the absorption coefficient of the intraband optical transition in an InGaAs/InAlAs superlattice. We find that the electron effective mass mismatch causes a notable broadening of spectrum linewidth, which can well account for the experimental result.
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