SPECIAL TOPIC — Photodetector: Materials, physics, and applications
Ultrathin optical interference in a system composed of absorbing material and metal reflector has attracted extensive attention due to its potential application in realizing highly efficient optical absorption by using extremely thin semiconductor material. In this paper, we study the physics behind the high absorption of ultrathin film from the viewpoint of destructive interference and admittance matching, particularly addressing the phase evolution by light propagation and interface reflection. The physical manipulations of the ultrathin interference effect by controlling the substrate material and semiconductor material/thickness are examined. We introduce typical two-dimensional materials–i.e., MoS2 and WSe2–as the absorbing layer with thickness below 10 nm, which exhibits~90% absorption in a large range of incident angle (0°~70°). According to the ultrathin interference mechanism, we propose the ultrathin (<20 nm) MoS2/WSe2 heterojunction for photovoltaic application and carefully examine the detailed optoelectronic responses by coupled multiphysics simulation. By comparing the same cells on SiO2 substrate, both the short-circuit current density (up to 20 mA/cm2) and the photoelectric conversion efficiency (up to 9.5%) are found to be increased by~200%.
We demonstrate a reconfigurable black phosphorus electrical field transistor, which is van der Waals heterostructured with few-layer graphene and hexagonal boron nitride flakes. Varied homojunctions could be realized by controlling both source-drain and top-gate voltages. With the spatially resolved scanning photocurrent microscopy technique, photovoltaic photocurrents originated from the band-bending regions are observed, confirming nine different configurations for each set of fixed voltages. In addition, as a phototransistor, high responsivity (~800 mA/W) and fast response speed (~230 μs) are obtained from the device. The reconfigurable van der Waals heterostructured transistors may offer a promising structure towards electrically tunable black phosphorus-based optoelectronic devices.
Gallium oxide (Ga2O3) thin films were deposited on a-Al2O3 (1120) substrates by pulsed laser deposition (PLD) with different oxygen pressures at 650℃. By reducing the oxygen pressure, mixed-phase Ga2O3 films with α and β phases can be obtained, and on the basis of this, mixed-phase Ga2O3 thin film solar-blind photodetectors (SBPDs) were prepared. Comparing the responsivities of the mixed-phase Ga2O3 SBPDs and the single β-Ga2O3 SBPDs at a bias voltage of 25 V, it is found that the former has a maximum responsivity of approximately 12 A/W, which is approximately two orders of magnitude larger than that of the latter. This result shows that the mixed-phase structure of Ga2O3 thin films can be used to prepare high-responsivity SBPDs. Moreover, the cause of this phenomenon was investigated, which will provide a feasible way to improve the responsivity of Ga2O3 thin film SBPDs.
Flexible electronic devices have attracted much attention due to their practical and commercial value. Integration of thin films with soft substrate is an effective way to fabricate flexible electronic devices. Ga2O3 thin films deposited directly on soft substrates would be amorphous mostly. However, the thickness of the thin film obtained by mechanical exfoliation method is difficult to control and the edge of the film is fragile and easy to be damaged. In this work, we fabricated free-standing Ga2O3 thin films using the water-soluble perovskite Sr3Al2O6 as a sacrificial buffer layer. The obtained Ga2O3 thin films were polycrystalline. The thickness and dimension of the films were controllable. A flexible Ga2O3 solar-blind UV photodetector was fabricated by transferring the free-standing Ga2O3 film on a flexible polyethylene terephthalate substrate. The results displayed that the photoelectric performances of the flexible Ga2O3 photodetector were not sensitive to bending of the device. The free-standing Ga2O3 thin films synthesized through the method described here can be transferred to any substrates or integrated with other thin films to fabricate electronic devices.
We report on the transition of photovoltaic and photoconductive operation modes of the amorphous Ga2O3-based solar-blind photodetectors in metal-semiconductor-metal (MSM) configurations. The conversion from Ohmic to Schottky contacts at Ti/Ga2O3 interface is realized by tuning the conductivity of amorphous Ga2O3 films with delicate control of oxygen flux in the sputtering process. The abundant donor-like oxygen vacancies distributed near the Ti/Ga2O3 interface fascinate the tunneling process across the barrier and result in the formation of Ohmic contacts. As a consequence, the serious sub-gap absorption and persistent photoconductivity (PPC) effect degrades the performance of the photoconductive detectors. In contrast, the photovoltaic device with a Schottky contact exhibits an ultra-low dark current less than 1 pA, a high detectivity of 9.82×1012 cm·Hz1/2·W-1, a fast response time of 243.9 μs, and a high ultraviolet C (UVC)-to-ultraviolet A (UVA) rejection ratio of 103. The promoting performance is attributed primarily to the reduction of the sub-gap states and the resultant suppression of PPC effect. With simple architecture, low fabrication cost, and easy fusion with modern high-speed integrated circuitry, these results provide a cost-effective way to realize high performance solar-blind photodetectors towards versatile practical applications.
Tin sulfide quantum dots (SnS2 QDs) are n-type wide band gap semiconductor. They exhibit a high optical absorption coefficient and strong photoconductive property in the ultraviolet and visible regions. Therefore, they have been found to have many potential applications, such as gas sensors, resistors, photodetectors, photocatalysts, and solar cells. However, the existing preparation methods for SnS2 QDs are complicated and require a high temperature and high pressure environments; hence they are unsuitable for large-scale industrial production. An effective method for the preparation of monodispersed SnS2 QDs at normal temperature and pressure will be discussed in this paper. The method is facile, green, and low-cost. In this work, the structure, morphology, optical, electrical, and photoelectric properties of SnS2 QDs are studied. The synthesized SnS2 QDs are homogeneous in size and exhibit good photoelectric performance. A photoelectric detector based on the SnS2 QDs is fabricated and its J-V and C-V characteristics are also studied. The detector responds under λ=365 nm light irradiation and reverse bias voltage. Its detectivity approximately stabilizes at 1011 Jones at room temperature. These results show the possible use of SnS2 QDs in photodetectors.
Transition metal dichalcogenides (TMDCs) belong to a subgroup of two-dimensional (2D) materials which usually possess thickness-dependent band structures and semiconducting properties. Therefore, for TMDCs to be widely used in electronic and optoelectronic applications, two critical issues need to be addressed, which are thickness-controllable fabrication and doping modulation of TMDCs. In this work, we successfully obtained monolayer WS2 and achieved its efficient doping by chemical vapor deposition and chemical doping, respectively. The n- and p-type dopings of the monolayer WS2 were achieved by drop coating electron donor and acceptor solutions of triphenylphosphine (PPh3) and gold chloride (AuCl3), respectively, on the surface, which donates and captures electrons to/from the WS2 surface through charge transfer, respectively. Both doping effects were investigated in terms of the electrical properties of the fabricated field effect transistors. After chemical doping, the calculated mobility and density of electrons/holes are around 74.6/39.5 cm2·V-1·s-1 and 1.0×1012/4.2×1011 cm-2, respectively. Moreover, we fabricated a lateral WS2 p-n homojunction consisting of non-doped n-type and p-doped p-type regions, which showed great potential for photodetection with a response time of 1.5 s and responsivity of 5.8 A/W at VG=0 V and VD=1 V under 532 nm light illumination.
Ni/β-Ga2O3 lateral Schottky barrier diodes (SBDs) were fabricated on a Sn-doped quasi-degenerate n+-Ga2O3 (201) bulk substrate. The resultant diodes with an area of 7.85×10-5 cm2 exhibited excellent rectifying characteristics with an ideality factor of 1.21, a forward current density (J) of 127.4 A/cm2 at 1.4 V, a specific on-state resistance (Ron, sp) of 1.54 mΩ·cm2, and an ultra-high on/off ratio of 2.1×1011 at ±1 V. Due to a small depletion region in the highly-doped substrate, a breakdown feature was observed at -23 V, which corresponded to a breakdown field of 2.1 MV/cm and a power figure-of-merit (VB2/Ron) of 3.4×105 W/cm2. Forward current–voltage characteristics were described well by the thermionic emission theory while thermionic field emission and trap-assisted tunneling were the dominant transport mechanisms at low and high reverse biases, respectively, which was a result of the contribution of deep-level traps at the metal–semiconductor interface. The presence of interfacial traps also caused the difference in Schottky barrier heights of 1.31 eV and 1.64 eV respectively determined by current–voltage and capacitance–voltage characteristics. With reduced trapping effect and incorporation of drift layers, the β-Ga2O3 SBDs could further provide promising materials for delivering both high current output and high breakdown voltage.
Two-dimensional transition metal dichalcogenides (TMDs) provide fertile ground to study the interplay between dimensionality and electronic properties because they exhibit a variety of electronic phases, such as semiconducting, superconducting, charge density waves (CDW) states, and other unconventional physical properties. Compared with other classical TMDs, such as Mott insulator 1T-TaS2 or superconducting 2H-NbSe2, bulk 2H-TaSe2 has been a canonical system and a touchstone for modeling the CDW measurement with a less complex phase diagram. In contrast to ordinary semiconductors that have only single-particle excitations, CDW can have collective excitation and carry current in a collective fashion. However, manipulating this collective condensation of these intriguing systems for device applications has not been explored. Here, the CDW-induced collective driven of non-equilibrium carriers in a field-effect transistor has been demonstrated for the sensitive photodetection at the highly-pursuit terahertz band. We show that the 2H-TaSe2-based photodetector exhibits a fast photoresponse, as short as 14 μs, and a responsivity of over 27 V/W at room temperature. The fast response time, relative high responsivity and ease of fabrication of these devices yields a new prospect of exploring CDW condensate in TMDs with the aim of overcoming the existing limitations for a variety of practical applications at THz spectral range.
Cadmium sulfide quantum dots (CdS QDs) are widely used in solar cells, light emitting diodes, photocatalysis, and biological imaging because of their unique optical and electrical properties. However, there are some drawbacks in existing preparation techniques for CdS QDs, such as protection of inert gas, lengthy reaction time, high reaction temperature, poor crystallinity, and non-uniform particle size distribution. In this study, we prepared CdS QDs by liquid phase synthesis under ambient room temperature and atmospheric pressure using sodium alkyl sulfonate, CdCl2, and Na2S as capping agent, cadmium, and sulfur sources respectively. This technique offers facile preparation, efficient reaction, low-cost, and controllable particle size. The as-prepared CdS QDs exhibited good crystallinity, excellent monodispersity, and uniform particle size. The responsivity of CdS QDs-based photodetector is greater than 0.3 μA/W, which makes them suitable for use as ultra-violet (UV) detectors.
Perovskite photoconductor-type photodetector with metal-semiconductor-metal (MSM) structure is a basic device for photodetection applications. However, the role of electrode interlayer in MSM-type perovskite devices is less investigated compared to that of the pin diode structure. Here, a systematic investigation on the influence of phenyl-C61-butyric acid methyl ester (PCBM) and indene-C60 bisadduct (ICBA) interfacial layers for MSM perovskite photodetectors is reported. It is found that the fullerene-based interlayer significantly enhances the photocurrent of the MSM photodetectors. On one hand, the PCBM interlayer is more suitable for CH3NH3PbI3 photodetector, with the responsivity two times higher than that of the device with ICBA interlayer. The ICBA layer, on the other hand, becomes more effective when the band gap of perovskite is enlarged with bromine composition, denoted as CH3NH3Pb(I1-xBrx)3 (0 ≤ x ≤ 1). It is further found that the specific detectivity of photodetectors with ICBA interlayer becomes even higher than those with PCBM when the bromine compositional percentage reaches 0.6 (x > 0.6).