ABSTRACT
The present investigation deals with the effect of a BaTiO3 (BTO) dielectric layer on the performance of MoS2/p-Si heterojunction photodetectors. The MoS2/p-Si junction demonstrates a responsivity of â¼80 A W-1 and detectivity of â¼1012 Jones. The inclusion of a dielectric BTO layer significantly enhances the performance of MoS2/p-Si photodetectors, leading to a remarkable improvement with a very high responsivity of â¼603 A W-1 and detectivity of â¼1013 Jones. The I-V characteristics of the MoS2/p-Si and MoS2/BTO/p-Si junctions under illumination can be understood by considering their respective energy band diagrams. This addition alters the energy band alignment, leading to higher conduction band offset and valence band offset values. The large photocurrent in forward bias in the MoS2/BTO/p-Si junction may be attributed to the presence of photogenerated electrons in the depletion region of BTO. BTO exhibits characteristics such as a long carrier diffusion length and low recombination rates, contributing to a reduction in carrier recombination within the photodetector for which the photocurrent of the MoS2/BTO/p-Si heterojunction can be improved significantly. The enhanced performance of the MoS2/BTO/p-Si junction, characterized by higher responsivity and detectivity, underscores the potential of this heterojunction for advanced photodetection applications, suggesting promising avenues for further research and development in the field of photodetectors. A comparative study with the available literature reveals that the excellent responsivity of â¼603 A W-1 and detectivity of â¼1013 Jones of the presently studied MoS2/BTO/p-Si heterojunction appear highly promising for various futuristic device applications.
ABSTRACT
Hybrid devices consisting of graphene or transition metal dichalcogenides (TMDs) and semiconductor quantum dots (QDs) were widely studied for potential photodetector and photovoltaic applications, while for photodetector applications, high internal quantum efficiency (IQE) is required for photovoltaic applications and enhanced carrier diffusion length is also desirable. Here, we reported the electrical measurements on hybrid field-effect optoelectronic devices consisting of compact QD monolayer at controlled separations from single-layer graphene, and the structure is characterized by high IQE and large enhancement of minority carrier diffusion length. While the IQE ranges from 10.2% to 18.2% depending on QD-graphene separation, ds, the carrier diffusion length, LD, estimated from scanning photocurrent microscopy (SPCM) measurements, could be enhanced by a factor of 5-8 as compared to that of pristine graphene. IQE and LD could be tuned by varying back gate voltage and controlling the extent of charge separation from the proximal QD layer due to photoexcitation. The obtained IQE values were remarkably high, considering that only a single QD layer was used, and the parameters could be further enhanced in such devices significantly by stacking multiple layers of QDs. Our results could have significant implications for utilizing these hybrid devices as photodetectors and active photovoltaic materials with high efficiency.
ABSTRACT
The effect of thickness of Ga doped ZnO (GZO) layer on the performance of GZO/p-Si heterojunctions fabricated by reactive co-sputtering of Zn-GaAs target is investigated. GZO films were deposited at 375 °C with 0.5% GaAs area coverage of Zn target and 5% O2 in sputtering atmosphere. X-ray diffraction and X-ray photoelectron spectroscopy show that c-axis orientation of crystallites, Ga/Zn ratio and oxygen related defects depend substantially on the thickness of films. The 200-350 nm thick GZO films display low carrier concentration â¼1017 cm-3, which increases to >1020 cm-3 for thicker films. The diodes fabricated with >500 nm thick GZO layers display non-rectifying behaviour, while those fabricated with 200-350 nm thick GZO layers display nearly ideal rectification with diode factors of 1.5-2.5, along with, turn-on voltage â¼1 V, reverse saturation current â¼10-5 A, barrier height â¼0.4 eV and series resistance â¼200 Ω. The drastically improved diode performance is attributed to small Ga/Zn ratio (â¼0.01) and extremely low dopant activation (â¼0.3%), owing to diffusion and non-substitutional incorporation of Ga in thin GZO layers, which cause self-adjustment of doping concentration. These factors, together with c-axis orientation and chemisorbed oxygen at grain boundaries, facilitate ideal diode characteristics, not reported earlier for GZO/p-Si heterojunctions.
ABSTRACT
p-Type nc-Si (p-nc-Si) films have been optimized under low growth temperature (â¼180 °C) and low power (â¼30 W) parametric conditions in 13.56 MHz RF-PECVD. At elevated gas pressure (p), the growth rate enhances; however, the optical band gap reduces. At optimum p = 2.5 Torr, a p-nc-Si window-layer possessing high crystallinity, large grain size, high electrical conductivity, wide optical band gap and a preferred ã220ã crystallographic orientation of the nanocrystallites is obtained. Single p-i-n junction nc-Si solar cells in superstrate configuration have been realized with reasonably acceptable conversion efficiency, η â¼ 7.05%. The preferred ã220ã oriented (Iã220ã/Iã111ã â¼ 1.68) highly crystalline (XC â¼ 86%) p-nc-Si window-layer minimizes the lattice mismatch at the p/i-junction, facilitates the growth of proper crystallinity in the i-nc-Si absorber layer from its incubation stage during its sequential growth over the window layer and ensures low recombination losses for conduction of charge carriers along the vertical direction at the p/i-interface. Further improvement in cell efficiency sensitively depends on proper optimization and future ungradation of the i-nc-Si absorber layer, and the single junction nc-Si cell could play a significant role as an integral part of premium all-Si tandem structure solar cells.
ABSTRACT
Hydrogenation of the ZnO:Ga network has been chosen as a promising avenue to further upgrade the optoelectronic and structural properties of the films. With an optimum incorporation of hydrogen at a low substrate temperature (TS = 100 °C) in RF magnetron sputtering plasma, the ZnO:Ga:H film, with a large crystallite size (â¼17 nm) and improved crystallinity along the optimally preferred c-axis orientation with respect to both the ã100ã (Iã002ã/Iã100ã â¼ 74) and ã103ã (Iã002ã/Iã103ã â¼ 10) directions, attains a high electrical conductivity (σ â¼ 1.5 × 10(3)) and â¼90% visible range optical transmission that yields a wide optical band gap of â¼3.78 eV. The dominant c-axis orientation of the ZnO crystals exhibits a distinct UV luminescence band at â¼340 nm that arises as a result of the typical exciton emission or near-band-edge emission, which occurs due to the recombination of photo-generated electrons and holes in the valence band or in traps near the valence band. Vacancies created by the out diffusion of oxygen from the network induces the growth along the ã103ã crystallographic orientation. With the introduction of an optimum amount of hydrogen into the network, the VO peak (OII) in the O 1s XPS spectrum significantly reduces in intensity while the Zn-OH peak (OIII) increases, indicating enhanced surface absorption of O species, which causes the improvement of c-axis orientation. The increase in the conductivity has been attributed to the centers assigned to isolated hydrogen atoms in the anti-bonding sites (ABO) or bond-centered sites of O-Zn bonds (BC), and Zn vacancies passivated by one or two hydrogen atoms. Hydrogen-induced dopant-like defects in the film and the associated large amount of tensile stress developed within the network has been correlated to the high conductivity and the wide band gap of the ZnO:Ga:H film due to the decreased repulsion between the O 2p and the Zn 4s bands and the Burstein-Moss effect as a consequence of the increased carrier concentration. Highly conducting and transparent c-axis oriented ZnO:Ga:H films grown by a device compatible process at a low TS are extremely useful for various stacked layer thin film devices, including solar cells.